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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 6: 1227 - 1242

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.06.011

Plant Ecology

Exploring the Impact of Thinning Operations on Forest Ecosystems in Tropical and Temperate Regions Worldwide: A Comprehensive Review

  • JOSHI Rajeev , 1, * ,
  • K. C. Jibesh Kumar 2 ,
  • DHAKAL Purna Prasad 3 ,
  • DEVKOTA Utpal 1
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  • 1. College of Natural Resource Management, Faculty of Forestry, Agriculture and Forestry University, Udayapur, Koshi 56310, Nepal
  • 2. Connecting People to Nature (CPN) Consulting Ltd., Surkhet, Karnali 21701, Nepal
  • 3. Ministry of Forest, Environment and Soil Conservation, Rukum, Lumbini 22004, Nepal
* JOSHI Rajeev, E-mail:

Received date: 2022-08-30

  Accepted date: 2023-01-20

  Online published: 2023-10-23

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Abstract

Thinning is a major tending operation conducted in a forest stand to obtain various objectives, including forest products and ecosystem services. The impact of thinning in tropical and temperate forests is one of the least studied subjects. Therefore, this study aims to look for such studies in the tropical and temperate regions and find out the trend in the response of the remaining trees regarding tree growth and development, climate resilience, and other services. The Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) framework and Population, Intervention, Comparison, and Outcome measures (PICO) tools were used to select the important papers according to defined objectives and then data extraction and analysis. Extracted data was grouped, pooled, and sorted to see if there were any temporal or spatial trends or patterns in the variables studied, and the reviews' findings were detailed. The impacts on the growth and yield of a forest or stand from thinning have been widely studied throughout the world. However, there are still some uncertainties regarding species-specific responses. This review also underscores the need for studies on the effects of thinning on other ecosystem services, including non-wood forest products, biodiversity, social functions, and tradeoffs between different ecosystem services. The study stresses the importance of long-term trials for forest ecosystem monitoring. Furthermore, analysis on the impact of thinning on other ecosystem services except growth and yield discovered that the few researches conducted to date, especially as regards to biodiversity and provisioning functions, must focus on a large number of ecosystem services without only concentrating on the components described in past studies. This will help us to develop our understanding of different ecosystem services and their responses after thinning and enable us to analyze the trade-offs between them. While managing forests, we cannot maximize all the services at the same time. However, with the best available knowledge on tradeoffs between different outcomes, we can optimize the benefits.

Key words： thinning; growth and yield; ecosystem services; PRISMA; PICO

Cite this article

JOSHI Rajeev , K. C. Jibesh Kumar , DHAKAL Purna Prasad , DEVKOTA Utpal . Exploring the Impact of Thinning Operations on Forest Ecosystems in Tropical and Temperate Regions Worldwide: A Comprehensive Review[J]. Journal of Resources and Ecology, 2023 , 14(6) : 1227 -1242 . DOI: 10.5814/j.issn.1674-764x.2023.06.011

1 Introduction

Different forest management interferences and disturbances are ongoing in a forest stand, and trees respond to those disturbances differently (Park et al., 2014). Thinning is a major and effective tending operation in a forest stand for achieving various objectives and getting forest products in a dynamic manner with a high return (Messier et al., 2019; Benz et al., 2020). Thinning is frequently used to increase the value and benefits of commercial timber production (Soares et al., 2017; Li et al., 2018), reduce competition in forest stands by increasing light availability, and perform ecological functions such as promoting regeneration and succession (Hale, 2003; Stone et al., 1999; Leonardsson et al., 2015). Thinning interventions also play an important role in the part of even-aged forest management for the efficient intermediate financial return and profitable production of timber products through reduction in rotation age (Cameron, 2002; Vitkova and Dhubháin, 2013; Curtis et al., 2018; Shephard et al., 2021). Thinning operations are used in trees to promote more growing space for the remaining trees, increase the production of usable timber and wood products by removing diseased, stressed, forked, and promising trees, and reduce competition for above and belowground resources, including water (Pukkala, 2014; Hynynen et al., 2015; Ramage et al., 2017; Ashton and Kelty, 2018; Gavinet et al., 2020). It is generally understood to stimulate the diameter growth of the remaining stems but has little if any effect on height growth (Rodríguez-Calcerrada et al., 2011; Sein, 2012; Naing et al., 2015). Many studies have shown that the diameter growth in the remaining trees is incited by thinnings (Peltola et al., 2002), while height growth is poorly documented (Harrington and Reukema, 1983; Hébert et al., 2016; Weng et al., 2020). Several pieces of research have shown that lowering the stand density does not cause a reduction in volume increment for many species (Condés et al., 2013). These studies mainly concentrated on short and long-term responses to integrate thinning responses into growth and yield models. Pre-commercial thinning is executed in early stand development, before or around the closure of the canopy, and before the trees attain merchantable size (Lindh and Muir, 2004). Commercial thinning disturbances can occur at any time later in the stand’s development (Olson et al., 2014). Various studies of commercial and pre-commercial thinning provide an unpredictable picture of long-term understory response (Lindh and Muir, 2004). The commercial thinning succeeding canopy closure results in larger effects on understory cover and composition than pre-commercial thinning (Thysell and Carey, 2001; Reicis et al., 2020).
Gathering knowledge about thinning as an adaptive management tool and improvement of its application can guide the reestablishment of ecological processes that contribute to the maintenance of restored forest ecosystems (Ellison et al., 2020). Conversely, intense thinning can facilitate the establishment of species that hamper succession, such as invasive and weed species (Kueffer et al., 2010; Jones et al., 2015; Podadera et al., 2015). Although there has been an increase in the use of thinning in ecological restoration (Dwyer and Mason, 2018), some studies have addressed its effects on high-diversity tropical forests undergoing restoration (Podadera et al., 2015). In such cases, thinning disturbances can reduce competition and readjust stand structure. Thinning increases carbon sequestration, regeneration, understory density, understory cover, and tree growth (Gavinet et al., 2015; Brown et al., 2019). Thinning disturbances can be viewed as a tool for influencing forest growth and development in South Asia and Central Europe (Dieler et al., 2017). Contradictory claims and counterclaims dealing with the effect of thinning on forest growth and development are abundant (Zeide, 2001). This relationship is typically found to be difficult to generalize due to differences in origin, tree species, stand density, site quality, stand age, and thinning methods (Skovsgaard and Vanclay, 2008). Therefore, regional and local level studies are mandatory for thinning impact assessment in geographically distinct forest areas. Various growth responses to different thinning operations have long been a research topic of interest in the forestry sector, and the correct choice of thinning regime to apply in a forest stand is one of the most imperative decisions in silviculture (Zeide, 2001; Gardiner and Quine, 2000). Many researchers assessed thinning effects over a short interval of time (<5 years after disturbance), whereas few others considered longer time scales, i.e., eight or more years after thinning (Dwyer and Mason, 2018; Olson et al., 2014; Podadera et al., 2015). Though developing longer and longitudinal studies is difficult, the resulting data allows avoidance of conclusions based on temporary responses and provides long-term and threshold information (Schreiber et al., 2004). Numerous traditional thinning experiments were initially designed to study growth and yield principles but nowadays provide a significant source of information to satisfy the present demand for knowledge about aspects such as the effect of thinning on structural diversity, carbon sequestration, and growth response to extreme droughts (Berndes et al., 2016; Stupak and Raulund- Rasmussen, 2016).
Studies on thinning and its response to tree growth and increment with several associated attributes are frequent, which is often higher for temperate regions than for tropical regions. The research databases are very few in the case of tropical regions. The main objective of thinning is to regulate the stand density by reducing competition among trees and focusing on growth in a smaller number of trees (Martin-Benito et al., 2010). The effect of thinning on the size of trees and/or future crops is of greater importance than its effect on yield and growth of the stand, particularly for species of the tropical forests (Wright, 2005). An estimate of the size and volume of wood a forest contains is essential in managing a forest for commercial timber production. Such an estimate is also important for determining the biomass of the forest, the amount of carbon storage, fuel sources, etc. However, the effects of thinning on stand growth and the balance between tree size and stand yield are also important to measure. This study aims to look for such studies in the tropical and temperate regions and find out the trend in the response of the forest ecosystem after thinning and highlight its importance to quantify the effects of thinning operations.

2 Materials and methods

2.1 PRISMA framework

The “Preferred Reporting Items for Systematic Review and Meta-Analysis” (PRISMA) framework and “Population, Intervention, Comparison, and Outcome measures” (PICO) tools were used for the selection of the reviewed papers, data extraction, and analysis. The PRISMA framework developed by Moher et al. (2009) has been used to report a systematic review of “Response of the Remaining Trees to Thinning Operations in Tropical and Temperate Forests across the Globe”. Paper reviews and analyses were presented clearly and extensively by using the PRISMA framework (Liberati et al., 2009; Papageorgiou et al., 2014), involving four steps, namely: Search Strategy/Identification, Selection Criteria/Screening, Quality Assessment/Eligibility, and Data Extraction/Inclusion. Various keywords, such as “impact” and “of”, “forest management” or “tending operation”, “silviculture”, “silvicultural operation”, “thinning”, “harvesting” and “on” and “remaining crop” or “growth” or “stand structure” or “increment” or “tropical forest” or “temperate forest”, etc., have been used for searching the relevant scientific literature. Similarly, online databases including “Web of Science, Scopus and search engines like Google Scholar, Researchgate” have been used to search the relevant literature and the digital library of the Goettingen University has also been used for this purpose. Journal articles, review papers, proceedings, and conference papers were searched for the systematic review. Master’s dissertation, report, and book were not considered for the search. The collected literature includes papers from 1970 to 2021, which are presented in this study. After obtaining the literature, they have been screened on the basis of relevance in terms of title, objectives, and results, respectively, for the detailed study. The number of citations assigned to the research article denotes the character and impact of the research (Aksnes et al., 2019). Therefore, this has been taken into consideration when finalizing the articles for review. The final collection of literature has been used for extracting the necessary data and results of the studies conducted. At this phase, PICO elements are used, which helps to acquire a response to the reviewed queries and provides a basis for including studies for systematic review (Aromataris and Pearson, 2014). The results obtained are presented in bar diagram, flow-chart, and tabular form, and a detailed description is depicted in the result section. The findings acquired through the conducted data analysis are summarized in a narrative synthesis in the conclusion and recommendation section.

2.2 Quality assessment

2.2.1 Relevance of articles

Through the combinations of keywords and use of databases mentioned above, a total of 330 articles were obtained in the identification phase. According to the relevancy of the literature, a total of 98 articles remained for the further screening process. Together with the additional 32 identified documents from additional sources (Google Scholar, Research Gate, and JSTFOR), a total of 130 articles were considered for screening. The three-step process was applied to assess the significance of the retrieved articles and filter them for final inclusion (Bettany-Saltikov, 2010). In the first step, titles were assessed, and if the titles had any information related to the key indicators, the articles were included. The advantage of going through the titles was that the articles that did not meet the rationale of the research were dropped within a relatively short time (Lange, 2014). After the first screening in each batch of databases, a total of 105 articles remained. Since three databases were used for searching the documents, there were 25 duplicate records of the documents. After removing the duplicates, 105 documents remained to be screened in the second phase. The ‘Abstracts’ of the articles were reviewed. During the review, 42 documents were found irrelevant to the study objectives. 63 documents remained for the third stage of screening. In the third and final stage of screening, an attempt was made to read 63 full-text articles. 23 articles had no full text access, and six articles had no relevant outcomes. Finally, 34 articles were included in the data/information extraction and analysis process.
Fig. 1 PRISMA Flow diagram for literature selection process

Note: n=0 means no any studies included where meta-analysis was conducted.

2.2.2 Citation as an indication of literature quality

The quality of the research or its impact can be measured by the number of citations that have been assigned to the research articles (Aksnes et al., 2019). Hence, it is also considered as one of the bibliometric indicators to assess the quality of research outputs globally in the education sector (Jarwal et al., 2009). The included reviewed studies have been cited a maximum of 250 times. Citations have not been mentioned in four studies, and two articles from 2021 have had no citations. However, these papers were included based on their relevance to the study. On average, the included studies have been cited 67.7 times. The lists of reviewed studies with the number of citations are given in Annex 1.

2.3 Data extraction

A data extraction module was designed to identify relevant data or information from the included 34 sources for qualitative synthesis. Each included item has been screened for relevant information and recorded in the form. At this stage, PICO elements (population, intervention, comparison intervention, and outcome measures) were used. The PICO elements help to find answers to the reviewed questions and form the basis for the inclusion of the studies for the systematic review (Aromataris and Pearson, 2014). Accordingly, only those studies were identified for final inclusion which were conducted in the correct population (forest or stand), had used interventions (different types of thinning) of interest, and had the predetermined and relevant outcomes (growth, yield, increment, and ecological outcomes).
This data extraction matrix eliminated the need to read all the papers completely. It consisted of information about the papers’ title, author, year of publication, abstract, type of study, methodology applied, management system, forest composition, types of thinning, positive and negative impacts, constraints, opportunities, recommendation, reference, and other information. However, potential subjectivity can be observed in this process as it was carried out by the researcher alone.

2.4 Data analysis

Extracted data was grouped, pooled, or sorted to see if there were any temporal or spatial trends or patterns in the variables being studied. Comparisons and categorization of the variables were made concerning the countries. General characteristics of the included studies were also quantified, and data analysis was done with the help of Microsoft Excel 2010 and SPSS Software. The Microsoft Excel spreadsheet was also used to draw bar diagrams, flow-charts, graphs, tables, and interpret data.

2.5 General information of selected literatures

The included literature for review dates back from 1970 to 2021. We noted a significant increase in publications after 2001 (Annex 1). The highest number of articles reviewed (n = 9) belongs to the USA. The oldest plots were more than a century old and were monitored to assess the impact of different silvicultural operations on crops. Three articles are review articles, which are based on research in different regions of the world, from tropical to temperate regions. European countries came second in the position, with the six articles studied being Finland, France, Italy, and Luthiana (Fig. 2).
Fig. 2 Spatial distribution of selected articles for this study

2.6 Methodology adopted by selected literatures

31 out of 34 papers adopted quantitative methods (refer to Annex 1), and these are field experiments that required a certain period of time to assess the impact of thinning methods on forest ecosystem. Another three are review-based literature, which provides a solid foundation for locating additional related literature and comprehending the overall effects of thinning and other silvicultural treatments on forest crops. There are no articles that adopt qualitative methods. It may be possible because researchers preferred field-based methods over qualitative approaches to determine the impact of thinning.

3 Results and discussion

3.1 Impact of thinning on remaining crops

Most of the studies on thinning impacts provide general information regarding tree growth, yield, and size distribution, and some studies also include characteristics related to wood quality. However, the ecological effects of thinning have also been assessed in this study with aspects such as climate resilience, biodiversity, recreation, non-wood forest products, and others. Thinning intervention has increased the availability of belowground resources such as water and nutrients that can limit understory plant abundance or richness (Webster et al., 2018; Jose et al., 2019). Thinning disturbances are controlled to increase rates of tree growth and development, although the heaviest thinning exhibits the highest rates of growth and development (Franklin et al., 2002). There is a positive effect recorded on growth and yield after thinning, with a strong relationship identified with the total yield of the stands (Zeide, 2001). However, a negative effect was recorded in total volume growth (Niinimäki et al., 2012). The effect of thinning from below on stand volume growth is strongly site dependent, but heavy thinning usually leads to a reduction in volume and density. Unlike some commercial thinning in mature and older forest stands, pre-commercial thinning failed to initiate an understory cohort of shade-tolerant conifer species (Bose et al., 2018; Gauthier and Tremblay, 2019). From the effect of early and heavy thinning, a strong positive increment in quadratic mean diameter has been noted. Moreover, high thinning with high intensity at early ages was found to have a strong positive increment in the dominant diameter of forest stands (Table 1). Heavy thinning intensity has no effect on mean height, and heavy low thinning has no effect on dominant height. However, there is some positive relationship between heavy high thinning and dominant height. There are mixed impacts observed in total biomass production in forests. In addition, a positive relationship is observed between wood density and wood quality, and a strong positive effect is observed on stem form.
Table 1 Impact of thinning on remaining crops
Types Impact on Effects
Growth Positive
Yield Strong positive
Volume growth Negative
Height No effects
Growth and yield Diameter Strong positive
Biomass Mixed
Stem form Strong positive
Wood density Positive
Wood quality Positive
Wind resistance Positive
Snow resistance Positive
Climate resilience Drought resistance Positive
Water use efficiency Positive
Carbon storage Negative
Nutrient availability and cycle Positive
Understory vegetation Positive
Biodiversity Mixed
Other impacts Recreation Positive
Non-wood forest products Positive
Regeneration Strong positive
Water yield and runoff No effects
Resistance to wind is increased by the application of light intensity-low thinning at an early age for short-rotation crops. However, the same short-rotation crop decreases its resistance when heavy intensity-low thinning is applied lately. The results showed decreased horizontal canopy heterogeneity by lowering the incidence of storm-related canopy gaps (Rahman et al., 2022). In contrast, resistance to snow is increased by heavy intensity-low thinning applied at an early age. Substantial numbers of trees in undulating land unthinned areas toppled as a result of snow loads in the winter of 1996, an effect that has been detected in other states (Tian, 2002). However, heavy intensity-high thinning decreases the stability of stands against snow. An increase in drought resistance, and thereby tree growth recovery, is recorded in short-rotation crops with the application of heavy thinning. In addition, there is no conclusive effect noted on the intrinsic water use efficiency. Heavy thinning has reduced the total litter quantity in forest grounds, thereby decreasing carbon storage in forests. In addition, reduced carbon stocks have been observed from the application of high-intensity thinning operations. Although there are no definite effects stated on decomposition and soil carbon fluctuations. Within biodiversity, species diversity, deadwood occurrence, and structural diversity were dealt with. A positive relationship has been reported between understory vegetation and early and high-intensity thinning. However, structural diversity has decreased with the application of heavy and low thinning operations. In addition, there is no significant impact observed on deadwood availability after thinning. Regarding recreation value, the accessibility and visibility aspects of recreation have been positively impacted by thinning operations; however, the amenity aspect has decreased with the application of thinning. The impact of thinning on non-wood forest products (NWFPs) is not researched much. However, increased production of NWFPs depends on site productivity, and an increase in site productivity has been observed from thinning interventions. Therefore, it is expected to have positive effects on the production of NWFPs, particularly on berries and mushrooms. More access to light creates a favorable environment for regeneration. However, there is no effect that has been observed on water yield and runoff after conducting thinning operations in the catchment. A general impact of thinning on remaining crops is presented in the following Table 1, and a detailed description is provided in the following subheadings.

3.1.1 Impact of different types of thinning studied

Fifty percent of studies focused on thinning as a normal silvicultural operation (n=17) instead of its variability. Seven studies (twenty percent) focused on finding out the impacts of variable thinning intensities. Most of them described three intensities of thinning, such as low, medium, and high thinning intensity. Among them, three papers (n = 3) considered the response of one to two intensities of thinning on remaining crops. The impact of low thinning operations has been dealt with under twelve percent (n = 4) of the literature. In comparison, half (n = 2) of that deals with the impact of high thinning, mechanical thinning, and early thinning operations (Fig. 3). At present, high thinning, whether light (D-grade) or heavy (E-grade), has a largely historical and psychological interest except to control species composition (Attocchi, 2015). In France, where high thinning was practiced for centuries (it is even known as French thinning), it was basically traditional if slightly embellished high grading (Zeide, 2001). What is left after this controversy is the understanding that in even-aged stands, bigger trees grow faster than smaller ones only on average (Knoke, 2012). The thinning needs to exercise judgment and remove some slow-growing and poorly formed dominant trees ("wolf" trees and those with flat tops or excessive seed production), while leaving in a few promising intermediates to keep density uniform (Stanturf et al., 2017). It differs from Hartig’s approach in that our anticipation of natural thinning is substantially longer (Zeide, 2001).
Fig. 3 Different types of thinning included in the study

3.1.2 Studies on different species composition

About half of the literature (16 out of 34) is concerned with determining the effects of thinning on single species, while the other thirteen studies are concerned with the effects on mixed species composition, and the remaining four studies are concerned with the effects on groups of similar species, which included groups of pine (n = 2) and oak (n = 2) species. Mixed forest includes the highest number of studies (n = 13), among others. Within this category, the highest number of studies is on diverse forests (n = 6). Under it, one study each was conducted on mixed coniferous forest, mixed hardwood forest, and other forests. The second highest number of studies was conducted in Pine (n = 5), followed by Eucalyptus (n = 4), Douglas-fir (Pseudotsuga menziesii) (n =3), Scot pine (Pinus sylvestris) (n =3), Acacia (n =2), and others (Fig. 4).
Fig. 4 Number of studies conducted to find thinning impact on different species composition

3.1.3 Impact of thinning on different aspects studied by literatures

About 65% (22 out of 34) of articles studied the impact of thinning on the growth of forest crops. Water yield and runoff aspects have been covered by four studies, followed by biomass (n = 3), climate (n = 3), biodiversity (n = 2), height and diameter (2), and regeneration (2). Other responses of thinning were observed in drought stress, stem form, tree composition, water use efficiency, yield, wood density, survival, light, and nutrient availability (Fig. 5). In the plantation of some fast-growing exotic species, thinning is non-essential for biomass, but for large-sized timber with higher quality, at least one thinning is required (Ramachandra et al., 2016). Many reforestation programs in Southeast Asia have given urgency to the planting of trees but did not pay much attention to tending the stands after establishment, thereby leaving them unmanaged for a long time (Kröger, 2014; Marhaento et al., 2021). Acacia mangium, widely grown in South-East Asia, has the huge potential to produce both pulpwood and timber products (Nambiar et al., 2015). However, to yield large-sized trees suitable for plywood, thinning is mandatory (Peng et al., 2018). Thinning seems applicable where restored forests exceed reference ecosystems in biomass (Götmark, 2013; Martin et al., 2013; Suganuma and Durigan, 2015) and develop tree size-class distributions slightly different from reference areas (Oliveira et al., 2019). Under such conditions, competition can be extreme and succession arrested, which may decrease ecological sustainability (Prévosto et al., 2012). In such cases, thinning disturbances can reduce competition and readjust stand structure so as to be more similar to the structure of reference forests.
Fig. 5 Number of studies on different aspects of impact of thinning
The various growth models developed based on two thinning experiments in Scots pine stands in Central Spain predicted less impact from the climate under low levels of competition, emphasizing the appropriateness of heavy thinning to mitigate the negative impacts of climate change (Fernández-de-Uña et al., 2015). Intrinsic water use efficiency (WUEi) estimated through stable carbon isotope ratios in tree rings has frequently been used to study the effect of thinning on tree response to drought, although many of the researchers reported no differences in WUEi between thinned and un-thinned forest stands (Sohn et al., 2016; Ammer, 2017). Fernández-de-Uña et al. (2015) reported a similar result for Scots pine based on data from two thinning experiments in Central Spain, suggesting that the thinning operations trigger structural changes to the trees rather than leaf-level efficiency. In the study by Giuggiola et al. (2013), it showed an increment of leaf area to sapwood area ratio linked with lower competition for water after thinning in a xeric Scots pine forest in Switzerland. However, Giuggiola et al. (2016) report changes in the isotopic ratio between thinned and un-thinned forest stands, showing less water stress in heavily thinned stands at the same site. These various opposing results might be due to species inconsistency since different responses to drought have been reported among Scots pine provenances (Taeger et al., 2013; Steckel et al., 2020). Therefore, while defining thinning schedules to mitigate the impact of drought, consideration must be given to provenance, although more research into provenance-competition-drought resistance relationships is required (del Río Gaztelurrutia et al., 2017). It is found that climate change is projected to reduce the growth and survival of Scots pine throughout Europe, except for the northern part, where the growth of Scots pine stands is expected to increase with climate change due to higher temperatures and a longer growing season (Gonzalez et al., 2017).

3.2 Management of tropical forests through silvicultural systems

The development of silvicultural systems in tropical forests dates back to the 19th century, when the first attempts were made to introduce management plans for teak (Tectona grandis) in Indonesia and India (Lamprecht, 1989; Dawkins and Philip, 1998). Since then, complete silvicultural systems have been developed not only in Asia but also in other tropical forests worldwide. These silvicultural systems have been classified into two main groups: monocyclic and polycyclic systems. Silvicultural systems may also be classified into shelterwoods, efficient for shade-intolerant species, and selection systems, more appropriate for shade-tolerant species (Ashton and Hall, 2011). In the monocyclic systems, timber extraction happens only once, and the subsequent cutting cycle depends solely on the regeneration growth, which takes 60 to 100 years to attain harvestable volume. But in a polycyclic silvicultural system (PSS), cutting cycles are more frequent than the rotation period since not all harvestable trees are removed at once and the remaining ones ensure that the interval until the next cutting cycle is shorter (Schöngart, 2010).
Tropical forest tree species differ evidently in their tolerance of shade and in their ability to respond to changes in irradiance. The responses of species to variation in irradiance can be studied by growth and development analysis using shade house gaps created in the forest (Popma and Bongers, 1991). Likewise, the light response curves change in the rates of transpiration and photosynthesis and are measured promptly in the same seedling under different irradiances (Kwesiga and Grace, 1986). The latter approach records the rapid responses of existing leaves and photosynthetic apparatus to changes in light, usually diffuse and neutral in spectral composition (Pearcy, 2007). Growth analysis can be conducted in shade houses or in the forest by creating canopy openings of different sizes, effectively using natural shade where light quality varies with irradiance (Agyeman et al., 2011).

3.3 Impact on growth and yield

Thinning interventions provide more growing space for the remaining trees, increasing the production of wood. Yield depends on the growing space, including other factors such as species, site productive capacity, and thinning types. Growth response to thinning operations is a much-studied topic in forestry, and the correct choice of the variability of thinning to apply in a forest is one of the most crucial decisions one should take for forest management (Zeide, 2001). The effects of thinning on retained crops and their functions and growth were studied in numerous research studies by comparing various thinning operations and control stands. In practice, the effect on the size of future crops of important species has got to be of primary importance rather than its effect on total growth and yield. Thinning has comprehensive impacts on stand structure, reducing forest fire risk and promoting forest growth (Moghli et al., 2022). Proper spacing and thinning can reduce overcrowding and relieve tree stress. This helps to maintain the health and vigor of the forest. Thinning can reduce fire hazards, generate revenue, and increase the value of remaining trees (Charnley et al., 2017). However, it is important to assess the effects on stand growth and the change between tree size and overall yield (Pye, 1988). There are different components that fall under growth and yield, including volume, yield, height, diameter, and wood quality of stands. Late and heavy thinning operations have a negative impact on volume growth. Zeide (2001) suggests that forest growth may or may not be increased by eliminating some trees, and whether volume growth is reduced by heavy thinning are questions that have been addressed by studies for years. The relationship varies according to species and depends on age and conditions of the site (Pretzsch, 2009); thus, it is important to measure this variation for each tree species. Juodvalkis et al. (2005) demonstrated that a significant increase in volume increment could be achieved only with thinning in young forest stands. However, studies have also shown that early thinning in temperate forests usually results in mechanical damage to growing stock, on an average comprising 10%-15% of the remaining trees (Vasiliauskas, 2001).
The impact on yield from different intensities of thinning has been observed in some pine thinning experiments where low thinning has been applied (Nilsson et al., 2010). A reduction in total volume was observed in all of those studies, and this was also found for other methods, including high thinning. To understand the impact of different thinning intensities on yield, Nilson et al. (2010) found greater yield losses with few heavy thinnings than with frequent light thinnings. The study suggests the relationship between thinning intensity and total yield remains similar among different site fertilities (del Río et al., 2017). However, at richer sites, the differences in yield seem to be higher between different thinning intensities (Mäkinen and Isomäki, 2004). According to del Rio et al. (2017), optimum height growth generally occurs at about 70% stocking; thinning applied to this density will promote both diameter and height growth as compared to control plots. Thinning heavily generally increases diameter growth and reduces height growth. Since very heavy thinning tend to increase taper, delay pruning, stimulate branching, and as a result reduce overall tree quality. Therefore, generally, thinning interventions are not desirable because they reduce a significant amount of stock density. Generally, 60% stocking is recommended as a useful compromise between height and diameter growth, and stem quality should not be adversely affected (Allen and Marquis, 1970). Thinning according to tree social status (size) has found a significantly lower difference between thinning intensities in larger trees compared to smaller trees (Mäkinen and Isomäki, 2004). On the other hand, according to Peura et al. (2018), tree size mainly affects response time to tree growth rather than the growth itself, such that shorter response times in large trees. Mäkinen and Isomäki (2004) reported that the effect of thinning decreased with decreasing relative size, although, in comparison, basal area growth in larger trees was less than in smaller trees. In addition, the best diameter growth is observed at medium diameters (Pukkala et al., 1998). Medeiros et al. (2017) found that thinning has no effect on the height growth of dominant trees in Eucalyptus stands. On the other hand, the mean diameter and the diameter distribution were affected. del Rio et al. (2017) combined the thinning studies between the 19th and 20th centuries for Central Europe and other regions. He summarized that the tree height has least responded to the variability of heavy intensity thinning out of tree parameters. However, trees in open areas were shorter than equivalent forest trees (Hemery, 2005).

3.4 Climate resilience

Thinning is presumed to increase access to solar radiation and provide water and nutrients to the remaining trees (Candel-Pérez et al., 2018). Thinning in an oak forest increased tree growth due to the increase of soil moisture as a result of a reduction in interception and transpiration loss (Bréda et al., 1995). A study by Weng et al. (2007) assessed the impact of heavy thinning in a Japanese cedar (Cryptomeria japonica) plantation, and the result was an increase in soil water and temperature in a severe summer drought. Intensive thinning increases evapotranspiration of understory vegetation, particularly in extreme drought conditions, while overstory transpiration loss of Pinus ponderosa is low due to stomatal closure (Simonin et al., 2007). These results highlight the importance of climate as an important variable affected by thinning and which thereby modifies the environment for remaining trees. A thinning regime is closely associated with the prime tree characteristics that determine the proneness of a tree to wind and snow damage. The prime tree attributes important for stability are height, height and diameter ratio (h/d), and crown length, and other secondary characteristics like taper, diameter, crown eccentricity, stem inclination, or root architecture also affect tree stability (Coutts and Grace, 1995). Additionally, the relative importance of these attributes differs depending on whether the risk of damage is from wind or snow; tree height is often the variable most closely related to wind damage (Schmidt et al., 2010). In the first few years after thinning, the susceptibility to wind and snow damage increases due to the retained trees having not attained the appropriate developmental stage to maintain stability. This effect is important when high-intensity thinning is applied in dense and young stands with slenderness and small crown ratios (Cameron, 2002). A similar situation was observed in a thinning in Spain where greater losses were caused by heavy snow, where crops had been thinned with high intensity and high thinning five years previously in comparison to control plots (del Río et al., 2017). Likewise, greater snow damage has been reported in Sweden after applying high thinning. Consequently, to prevent damage in the future, the initial thinning is important to apply at a young stand age, which improves tree stability from the early growth stages (Cameron, 2002). In the future, climate change will lead to more unfreezing of soils in temperate regions, including Norden. Hence, storm damage is estimated to increase in the winter. Therefore, forest owners are recommended to apply early and relatively severe thinning to reduce the probability of storm damage later (Keskitalo et al., 2016).
The impact of increased temperature and drought occurrences due to climate change should also be considered in decision making. These impacts reduce the growth and survival of shade demander species, whereas the growth of light demander species is predicted to increase due to increasing warming and a longer growing season. The thinning is one of the adaptation techniques adopted to reduce the forest stand’s vulnerability to climate change (Spittlehouse and Stewart, 2003). Thinning improves the growth of the retained trees by reducing competition for above-ground and below-ground resources, including water access. After thinning, there will be more soil moisture due to lower interception and evapo-transpiration as well as increased nutrient availability (Gebhardt et al., 2014). Recently, thinning is being considered as a mitigation measure for extreme drought, though the findings differ as per the species, sites, and thinning types (Ammer, 2017). Similar to the growth response to thinning, the growth response to drought depends on factors including age, intensity, type, and frequency of thinning (Sohn et al., 2016). With the aim of understanding the thinning impacts on drought tolerance of crops, a recent study by Sohn et al. (2016) compared the impacts of four long-term thinning experiments in Germany with varying sites, age and intensity of thinning, and time difference between thinning operations. Findings suggest that thinning recovers and improves radial growth after drought occurrence, but does not quite affect drought resistance. Climate change scenarios envisage an increase in the intensity and recurrence of droughts (Doblas-Miranda et al., 2017; Andivia et al., 2018). Among the several adaptive measures to reduce the vulnerability of forest stands to climate change, the improvement of stand density through thinning is one of the most essential silvicultural treatments (Ammer, 2016; Ameztegui et al., 2017). After applying thinning operations in stands, there may be more water availability in soils due to lower interception and water consumption through transpiration (Domec et al., 2012; Gebhardt et al., 2014). The positive impact of thinning operations as regards mitigating the extreme effects of drought has recently been stated for several species, though the results may vary according to the sites, species, and other silvicultural treatments and regimes (Ammer, 2016; Sohn et al., 2016). Growth responses to thinning disturbances mainly depend on the thinning regime, that is, the age at the first thinning, intensity, type, and frequency of thinning; the thinned stand may also depend on the resistance to drought (del Río et al., 2017; Bosela et al., 2021). The growth recovery was found to be significant after the initial thinning and also in the latest and heavily thinned plots.
Effects of thinning on nutrient cycling include less litter production as a result of reduced litter mass in the forest floor. In the short term, this effect can be negated due to canopy closure after some years of thinning (Roig et al., 2005). However, in the long term, nutrient concentration is reduced in green foliage, and the nutrient budget balance may be disturbed (Jonard et al., 2006). Nevertheless, Nave et al. (2010) stressed the consideration of the kind of harvest as it has a greater effect on the final nutrient budget of the forest floor. A study by Bravo-Oviedo et al. (2015) addressed the long-term thinning impacts on soil condition in a natural forest. The study did, however, record similar carbon and nitrogen levels, as well as the bulk density of mineral soil.

3.5 Impacts on biodiversity and recreational services

Thinning operations impact stand composition and structure. Within biodiversity, species diversity, deadwood occurrence, and structural diversity are often considered in evaluating the effectiveness of silvicultural techniques (Lafond et al., 2015). Greater availability of light, water, and nutrients to the outstanding trees after thinning facilitates the growth of diverse understory vegetation, mainly consisting of pioneer taxa, including aggressive indigenous as well as exotic species. In comparison, frequent thinning promotes the appearance of mono-dominancy in understory vegetation; however, no effect is recorded on canopy cover after light thinning (Ares et al., 2010). Thus, the impact depends on thinning intensity, rotation age, and species composition before thinning. Consequently, a study in Spain by Torras and Saura (2008) recorded a positive effect on shrub and tree species richness and shrub dominance after thinning. Deadwood is habitat for small fauna and decomposers and is also important for forest ecosystem functioning. Almost all managed forests have a limited deadwood distribution. Thinning interventions reduce woody plant mortality, and existing dead trees and logs are usually removed from managed areas (Mäkinen and Isomäki, 2004). Several studies have been conducted to estimate coarse woody debris in forests (Rouvinen et al., 2002; Montes and Caellas, 2006; Herrero et al., 2010). However, research into the effect of thinning on deadwood quantity and decay processes is required. Thinning modifies the structural diversity by reducing stand density with the selective tree removal of trees of particular social status, which depends on the applied thinning type. Some studies have reported that the low thinning reduces variation in diameter distribution and increases the abundance of matured and over-matured trees (Dhakal, 2021). The opposite can be expected when applying high thinning operations. The homogeneity of tree size has been observed in many thinned stands. Thinning increases the homogeneity of tree sizes and it might also reduce the range and standard deviation of tree heights (Barbeito et al., 2009). Generally, managed forests and parks with more open space and more light are preferred for recreational use (Hale et al., 2019). Therefore, for recreational purposes, the creation of larger gaps and understory thinning to provide better views inside the forests is important (Jankovska et al., 2014). Thinning operations can improve recreational preferences by enabling accessibility and maintaining in-between stand densities (Jensen and Skovsgaard, 2009). It is preferred to follow long rotations as disturbances hampering recreational values are reduced (Holgen et al., 2000).

4 Conclusions and recommendation

This review of literature related to the impact of thinning on remaining crops highlights several important questions that still need to be addressed regarding thinning impacts, particularly those aspects that were not conventionally considered in the objectives of the thinning intervention. The impacts on the growth and yield of a forest or stand from thinning have been widely studied throughout the world. The species-specific relationship between density and growth depends on site quality and varies among different areas. Additionally, future environmental change (climate, land use, and other changes in the environment) may have positive or negative impacts on growth depending on the region and will modify this relationship. Therefore, management standards may need to be adapted in order to exploit the benefits that environmental change seems to provide under different site conditions. For instance, thinning can increase height at poor site conditions (including drier sites), which will be widespread due to the change in climate. This review on the impact of thinning on other ecosystem services except growth and yield discovered that the few researches conducted to date, especially as regards to biodiversity and provisioning functions, Studies in the future must focus on a large number of ecosystem services without only concentrating on the components described in past studies. This will help us to develop our understanding of different ecosystem services and their responses after thinning, and enable us to analyze the trade-offs between them. While managing forests, we cannot maximize all the services at the same time; however, we can optimize the benefits with the best knowledge available on tradeoffs between different outcomes. Therefore, it is important to detect the foremost trade-offs. In addition to the usual bargain between yield and timber quality, thinning can lead to other trade-offs such as those between wood quality and biodiversity or climate change resilience, which should be studied. In the same way, bargain between adaptations versus mitigation of climate change is important for thinning. The study underscores the importance of long-term thinning trials for long-term forest ecosystem monitoring. In order to maintain a sound knowledge base for science, training, and demonstration purposes, new thinning experiments are needed to cover a broad range of ecosystem services under different site conditions. Large experiments providing a high number of potential alternatives related to thinning at an early age, thinning type, intensity, and rotation age need to be conducted to compare different thinning schedules. Such trials can provide the basis to assess the long-term impacts of thinning on different ecosystem services.

Annex 1 Details of included literatures

S. N Title Author Year of
publication		 Number of
Citation		 Type of study Area
1 Thinning temporarily stimulates tree regeneration in a restored tropical forest Oliveira et al. 2021 0 Quantitative Brazil
2 Thinning from below: Effects on height of dominant trees and diameter distribution in Eucalyptus stands Medeiros et al. 2017 7 Quantitative Brazil
3 A review of thinning effects on Scots pine stands: From growth and yield to new challenges under global change del Rio et al. 2017 54 Review Tropical to Temperate
4 Effects of early thinning regime and tree status on the radial growth and wood density of Scots pine Peltola et al. 2007 8 Quantitative Finland
5 The effects of high intensity thinning on yield Hamilton 1981 NA Quantitative ND*
6 Effects of thinning on growth of six tree species in north-temperate forests of Lithuania Juodvalkis et al. 2005 75 Quantitative Luthiania
7 Fifteen-year growth patterns after thinning a ponderosa- Jeffrey pine plantation in northeastern California Oliver 1979 NA Quantitative USA
8 The effects of thinning intensity and tree size on the growth response to annual climate in Cedrus atlantica: A linear mixed modeling approach Guillemot et al. 2015 24 Quantitative France
9 The effect of heavy or “free growth” thinning on oak (Quercus petraea and Q. robur) Kerr 1996 62 Quantitative Britain
10 Effect of thinning on height and diameter growth of Oak & Yellow-Poplar saplings Allen and Marquis 1970 NA Quantitative USA
11 Response of a clonal teak plantation to thinning and prunning in Java, Indonesia Budiadi et al. 2017 19 Quantitative Indonesia
12 Response of unmanaged Acacia mangium plantations to delayed thinning in north-east Thailand Kamo et al. 2009 8 Quantitative Thailand
13 Thinning and growth: A full turnaround Zeide 2001 160 Review ND*
14 Thinning intensity and growth of Norway spruce stands in Finland Makinen and Isomaki 2004 123 Quantitative Finland
15 Thinning intensity and growth of Scots pine stands in Finland Makinen and Isomaki 2004 250 Quantitative Finland
16 Growth effects of thinning operation in an umbrella pine (Pinus pinea L.) stand in central Italy Monaco et al 2013 NA Quantitative Italy
17 Effect of thinning on Pericopsis elata (Harms) Meeuwen (Fabaceae) found in forest plantations in the east and south regions of Cameroon Betti et al. 2021 0 Quantitative Cameroon
18 Thinning regimes and initial spacing for Eucalyptus plantations in Brazil Ferraz et al. 2018 13 Quantitative Brazil
19 Understory vegetation in young Douglas-fir forests: does thinning help restore old-growth composition? Lindh and
Muir		 2004 125 Quantitative USA
20 Individual tree growth response to variable-density thinning in coastal Pacific Northwest forests Roberts and
Harrington		 2008 95 Quantitative USA
21 Overstorey and juvenile response to thinning and drought in a jarrah (Eucalyptus marginata Donn ex Sm.) forest of southwestern Australia Qiu et al. 2012 11 Quantitative Australia
22 The response in water yield to the thinning of Pinus radiata, Pinus patula and Eucalyptus grandis plantations Lesch and Scott 1997 43 Quantitative South Africa
23 Above-ground biomass recovery following logging and thinning over 46 years in an Australian tropical forest Hu et al. 2020 6 Quantitative Australia
24 Biodiversity response to intensive biomass production from forest thinning in north American forests Verschuyl et al. 2011 194 Quantitative USA
25 Growth responses to thinning, pruning and fertilizer application in Eucalyptus plantations: A review of their production ecology and interactions. Forrester 2013 53 Quantitative ND*
26 The response of light, water, and nutrient availability to pre-commercial thinning in dry inland Douglas-fir forests Chase et al. 2016 45 Quantitative USA
27 Potential of forest thinning to mitigate drought stress: A meta-analysis Sohn et al. 2016 192 Review Tropical to Temperate
28 Growth and stem form responses of plantation-grown Acacia melanoxylon (R. Br.) to form pruning and nurse-crop thinning Medhurst et al. 2002 26 Quantative Australia
29 Growth response of Acacia koa trees to thinning, grass control, and phosphorus fertilization in a secondary forest in Hawaii Scowcroft et al. 2006 23 Quantitative USA
30 Initial tree regeneration responses to fire and thinning treatments in a Sierra Nevada mixed-conifer forest, USA Zald et al. 2008 78 Quantitative USA
31 Modelling the impacts of various thinning intensities on tree growth and survival in a mixed species eucalypt forest in central Gippsland, Victoria, Australia Kariuki 2008 20 Quantitative Australia
32 Response of climate-growth relationships and water use efficiency to thinning in a Pinus nigra afforestation Martín-Benito et al. 2010 112 Quantitative China
33 Biodiversity response to intensive biomass production from forest thinning in North American forests—A meta-analysis Verschuyl et al. 2011 138 Review USA
34 Runoff responses to forest thinning at plot and catchment scales in a headwater catchment draining Japanese cypress forest Dung et al. 2012 68 Quantitative Japan

Note: *ND: Not Defined.

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