Individual Tree Growth Research Articles

Fire return intervals and recruitment affect population growth rate of canopy trees in tall open forest in humid savanna

AbstractSavannas are the major biome in tropical regions of the globe, defined as sparsely wooded regions with a continuous herbaceous layer of mainly C4 grasses where rainfall is distinctly seasonal. Fire is a common feature of most savannas. The largest protected areas of savannas are found in sparsely populated monsoonal northcentral Australia with strong annual wet and dry seasons. The most common vegetation type is relatively intact, tall (<15 m), open forests where Eucalyptus canopy trees form the basic structure. Over the past half century, traditional indigenous fire regimes were largely replaced by contemporary fires where individual trees may experience fire as often as 3 out of 5 years. The potential for long‐term persistence of the canopy tree populations is an open question. A stage‐based population model of the canopy trees was previously developed to address this question, drawing on data from three decades of experimental field studies wherein the survival, growth, and reproduction of individual marked trees were recorded under different seasonal fires and understory types to produce transition matrices among eight life history stages, and used to calculate population growth rates (λ). Here, we apply that model to determine how λ varies across a range of fire return intervals from 1 to 12 years for both early and late dry season fires, in two different understory types. We also explore the sensitivity of λ to two key life history parameters: recruitment and seedling survival. Minimum fire return intervals of 2–5 years were generally required for λ ≥1 that would allow populations to persist; these were shorter with stochastic year‐to‐year timing of fires and with higher recruitment rates. Uniquely, under certain conditions, there was also a maximum fire return interval above which λ <1, creating a “window” of fire return intervals that allowed canopy tree populations to persist. Mechanisms underpinning results as well as implications for savanna structure, alternate states, cyclical dynamics, future research, and management by fire are discussed.

Predicting the Effects of Ozone on Long-Term Growth of Aspen Trees Using Response Functions Developed From Seedlings Grown in Field Chambers.

Tropospheric ozone (O3) is among the most pervasive and harmful air pollutants known to affect ecosystems. In the United States, the Environmental Protection Agency and other agencies are tasked with protecting plants and ecosystems from harmful O3 exposures. Controlled exposure experiments conducted in field open-top chambers (OTCs) with small tree seedlings have been used to estimate empirical models of tree growth in response to O3 exposure for more than 16 species. While this experimental method makes it possible to obtain detailed exposure-response data, it remains uncertain whether predictions of empirical models parameterized using those data are sufficiently accurate when applied to trees grown in uncontrolled natural environments for long periods. We used O3 exposure-response relationships developed from several OTC studies of trembling aspen (Populus tremuloides Michx.) seedlings to predict the growth of the same species in the Aspen FACE "free-air" O3 exposure experiment in Rhinelander, Wisconsin, over 11 years. We acquired individual tree growth data and hourly O3 exposure from the ambient and elevated O3 plots in the Aspen FACE experiment, computed annual exposure using the same metrics of O3 exposure as were used in the OTC seedling experiments, and generated predictions of growth in the Aspen FACE exposures. A simple empirical model parameterized using the OTC seedling data accurately predicted the percent above-ground biomass loss due to O3 exposure in the Aspen FACE trees for all 11 years. In the Aspen FACE experiment, the effect of O3 exposure was established in early years and continued to be observed in later years without worsening. Our study suggests that O3 exposure-response relationships obtained from OTC seedling studies can be used to make inferences about effects on larger trees. These results imply that researchers can use these relationships with confidence when estimating risks of O3 pollution across the United States.

Precommercial Thinning but Not Commercial Thinning Increases the Merchantable and Large Sawlog Volume Production of Lodgepole Pine on Fair Sites

Abstract The effect of thinning on stand structure and volume production is well studied. However, the effect of combinations of precommercial (PCT) and commercial thinning (CT) on the production of different log grades and differential growth responses is less understood, although it is crucial for forest managers to choose an optimal management regime. Starting with actual operational field data from a lodgepole pine (Pinus contorta var.latifolia) PCT trial on fair quality sites (site index: 16–18 m at 50 years), we used an individual tree growth model to forecast development over the entire rotation. We found thinning changed the structure (density, quadratic mean diameter) of the stands but not the total volume production. However, in terms of merchantable volume and large sawlog volume, stands with only PCT provided the largest volume, more than either CT or a combination of PCT and CT, or the unthinned control. Moreover, our study findings also showed that, by doing only PCT, stands reached maximum merchantable or large sawlog mean annual increment earlier than other thinned or unthinned stands. This indicates that PCT might shorten the rotation length and contribute to an enhanced supply of timber. Study Implications: The effect of commercial thinning (CT) on stand structure and volume production is well studied. However, the effect of combinations of precommercial (PCT) and commercial thinning (CT) on the production of different log grades and differential growth responses is less studied, although it is crucial for forest managers to choose a management option. Starting with actual operational field data from a lodgepole pine (Pinus contorta var.latifolia) PCT trial in a fair-quality site (site index [SI]: 16–18), we used an individual tree growth model to develop the stands over the entire rotation. We found thinning changed the structure (density, quadratic mean diameter) of the stands but not the total volume production. However, in terms of merchantable volume and large sawlog volume, stands with only PCT provided the largest volume, more than either CT or a combination of PCT and CT, or the unthinned control. Moreover, our study findings also showed that, by doing only PCT, stands reached maximum merchantable or large sawlog MAI earlier than other thinned or unthinned stands. This indicates that PCT might shorten the rotation length and contribute to the continuous supply of timber production.

A new approach to monitor the life cycle of urban street tree canopies

One common measure to mitigate the impacts of rising temperatures within cities is increasing the amount of vegetation, especially urban trees and their canopies. To maximize the associated benefits of urban trees in a given city or neighborhood, it is important that the canopy of newly planted trees matches or exceeds the canopy of the trees that are lost. However, cities often use cross-sectional data to make decisions about the number of trees that should be planted without considering an individual tree’s growth and life span. In this study, individual tree growth and longevity data were used to portray the current and future conditions of street trees in six neighborhoods near Downtown Los Angeles and to identify locations in those neighborhoods that are in urgent need of interventions for tree planting. The results of this study indicate that, under the typical tree mortality scenario, near half of the current tree canopy will disappear by 2050, and on average, approximately one percent of the existing street trees will be lost due to aging each year. The approach used in this study shows how cities can monitor the age and mortality rate of the urban tree canopy over time. This approach informs tree planting campaigns that will preserve the long-term vitality and size of urban tree canopies and contribute to the resiliency of cities in a warmer future climate.

Local neighborhood affects stem rehydration under drought: evidence from mixtures of European beech with two different conifers.

Mixed-species forests are, for multiple reasons, promising options for forest management in Central Europe. However, the extent to which interspecific competition affects tree hydrological processes is not clear. High-resolution dendrometers capture subdaily variations in stem diameter; they can simultaneously monitor stem growth (irreversible changes in diameter) and water status (reversible changes) of individual trees. Using the information on water status, we aimed to assess potential effects of tree species mixture, expressed as local neighborhood identity, on night-time rehydration and water stress. We deployed 112 sensors in pure and mixed forest stands of European beech, Norway spruce and Douglas fir on four sites in the northwestern Germany, measuring stem diameter in 10-min intervals for a period of four years (2019-2022). In a mixture distribution model, we used environmental variables, namely soil matric potential, atmospheric vapor pressure deficit, temperature, precipitation and neighborhood identity to explain night-time rehydration, measured as the daily minimum tree water deficit (TWDmin). TWDmin was used as a daily indicator of water stress and the daily occurrence of sufficient water supply, allowing for stem growth (potential growth). We found that species and neighborhood identity affected night-time rehydration, but the impacts varied depending on soil water availability. While there was no effect at high water availability, increasing drought revealed species-specific patterns. Beech improved night-time rehydration in mixture with Douglas fir, but not in mixture with spruce. Douglas fir, however, only improved rehydration at a smaller share of beech in the neighborhood, while beech dominance tended to reverse this effect. Spruce was adversely affected when mixed with beech. At species level and under dry conditions, we found that night-time rehydration was reduced in all species, but beech had a greater capacity to rehydrate under high to moderate soil water availability than the conifers, even under high atmospheric water demand. Our study gives new insights into neighborhood effects on tree water status and highlights the importance of species-specific characteristics for tree-water relations in mixed-species forests. It shows that drought stress of European beech can be reduced by admixing Douglas fir, which may point towards a strategy to adapt beech stands to climate change.

The effect of neighbor species' phylogenetic and trait difference on tree growth in subtropical forests

AbstractQuestionsTo comprehensively understand ecological dynamics within a forest ecosystem, it is vital to explore how surrounding trees influence the growth of individual trees in a community. This study investigates the importance of biotic interactions on tree growth by examining several metrics of competitive interactions and community structure and considering three classes of intrinsic growth rates among the focal individuals: slower, intermediate, and faster‐growing trees. We also separated the focal trees based on their canopy position.LocationBrazilian subtropical forests.MethodsWe assessed various factors related to the focal trees and their neighbors, including differences in traits, neighborhood crowding, phylogenetic distance, and overall trait composition within the community. We then ran linear mixed‐effects models to test how these different metrics influenced the growth rates of the focal trees.ResultsOur results indicate that phylogenetic distance is linked to higher growth. Specific leaf area (SLA), leaf area (LA), and wood density (WD) are significantly related to tree growth. Trees surrounded by neighbors with higher SLA than themselves grow better, particularly smaller trees. Similarly, taller trees with smaller LA than their neighbors grow better. Trees in the intermediary growth class grow better when they have higher WD than their neighbors. Conversely, smaller trees benefit from greater WD difference between the focal trees and their neighbors, while height difference negatively impacts faster‐growing trees. Moreover, communities with higher SLA and WD positively impact the growth of faster‐growing trees.ConclusionsWe conclude that the interactions between trees are mediated by their ecological differences, but the performance and responses to surrounding competitors vary along with their grow class and position within a community. This study has revealed that the tree's intrinsic growth rate mediates the effect of traits and phylogeny of surrounding trees on individual tree growth.

Multi-objective optimization of forest ecosystem services under uncertainty

Forest ecosystems deliver multiple services crucial for human wellbeing and welfare, with complex relationships among forest ecosystem services (FESs). Implementing sustainable forest management (SFM) policy requires insights into trade-offs, conflicts and synergies among key FESs, necessitating decision-support tools for multiple objectives. However, uncertainty in key parameters adds complexity. We formulated a tri-objective optimization problem concentrating on three vital ecosystem services in Nordic boreal forests: timber supply, climate change mitigation, and biodiversity conservation. Furthermore, future timber price uncertainty was integrated into the optimization model. Utilizing the TreeSim individual-tree growth and yield simulator, we simulated forest growth and yields, carbon stocks, and biodiversity values. Pareto-optimal solutions were found for four distinct management strategies using the epsilon-constrain method to solve the optimization problem. These management strategies differed in spatial restrictions on harvest location and timing, including considerations like harvest adjacency, green-up and maximum harvest opening area (MOA). The analysis revealed trade-offs among all three objectives, with variations influenced by management intensity. Furthermore, within the Pareto solutions for each strategy, a compromise solution was identified based on the knee-point method. The study demonstrated nuanced impacts of green-up constraints related to harvest practices, showing minor effects on harvesting, carbon sequestration, and biodiversity conservation when MOA was restricted to 35 hectares or less. However, more substantial impacts were observed with a MOA limited to 60 hectares or in a strategy without spatial restrictions. Our results underscore the importance of considering multiple objectives in forest management and policy under uncertain wood prices, preventing undesired effects from a singular or deterministic approach. This work provides insights into trade-offs and synergies, contributing to strategic planning and policy design.

Can mixing Quercus robur and Quercus petraea with Pinus sylvestris compensate for productivity losses due to climate change?

The climate change scenarios RCP 4.5 and RCP 8.5, with a representative concentration pathway for stabilization of radiative forcing of 4.5 W m−2 and 8.5 W m−2 by 2100, respectively, predict an increase in temperature of 1–4.5° Celsius for Europe and a simultaneous shift in precipitation patterns leading to increased drought frequency and severity. The negative consequences of such changes on tree growth on dry sites or at the dry end of a tree species distribution are well-known, but rarely quantified across large gradients. In this study, the growth of Quercus robur and Quercus petraea (Q. spp.) and Pinus sylvestris in pure and mixed stands was predicted for a historical scenario and the two climate change scenarios RCP 4.5 and RCP 8.5 using the individual tree growth model PrognAus. Predictions were made along an ecological gradient ranging from current mean annual temperatures of 5.5–11.4 °C and with mean annual precipitation sums of 586–929 mm. Initial data for the simulation consisted of 23 triplets established in pure and mixed stands of Q. spp. and P. sylvestris. After doing the simulations until 2100, we fitted a linear mixed model using the predicted volume in the year 2100 as response variable to describe the general trends in the simulation results. Productivity decreased for both Q. spp. and P. sylvestris with increasing temperature, and more so, for the warmer sites of the gradient. P. sylvestris is the more productive tree species in the current climate scenario, but the competitive advantage shifts to Q. spp., which is capable to endure very high negative water potentials, for the more severe climate change scenario. The Q. spp.-P. sylvestris mixture presents an intermediate resilience to increased scenario severity. Enrichment of P. sylvestris stands by creating mixtures with Q. spp., but not the opposite, might be a right silvicultural adaptive strategy, especially at lower latitudes. Tree species mixing can only partly compensate productivity losses due to climate change. This may, however, be possible in combination with other silvicultural adaptation strategies, such as thinning and uneven-aged management.

Pre-Commercial Thinning Increases Tree Size and Reduces Western Gall Rust Infections in Lodgepole Pine

Alberta’s forest industry is predicted to be impacted by a medium-term decline in timber supply. Intensive silviculture tools, such as pre-commercial thinning, have been shown to increase individual tree growth, shorten rotation lengths, and improve stand merchantability in important commercial species such as lodgepole pine. However, lodgepole pine stands are susceptible to western gall rust infections, and thinning at an early stage may increase infection rates. This study collected tree and stand level data from 33 operational harvest origin lodgepole pine stands consisting of 11 stands thinned at age 17–19 years (PCT_18), 11 stands thinned at age 23–25 (PCT_24), and 11 unthinned stands. Approximately 40 years after pre-commercial thinning, merchantable volume is similar in all stands but thinned stands, regardless of timing, had greater individual tree size (~15% higher) compared to unthinned stands. Pre-commercially thinned stands also had a higher potential for commercial thinning since they have lower variability in tree size and longer live crown lengths. In addition, delayed thinning (PCT_24) reduced western gall rust infections and the severity of infections compared to both PCT_18 and unthinned stands. In conclusion, pre-commercial thinning should be considered for lodgepole pine stands in order to address timber supply issues in Alberta.

Urbanization exacerbates climate sensitivity of eastern United States broadleaf trees.

Tree growth is a key mechanism driving carbon sequestration in forest ecosystems. Environmental conditions are important regulators of tree growth that can vary considerably between nearby urban and rural forests. For example, trees growing in cities often experience hotter and drier conditions than their rural counterparts while also being exposed to higher levels of light, pollution, and nutrient inputs. However, the extent to which these intrinsic differences in the growing conditions of trees in urban versus rural forests influence tree growth response to climate is not well known. In this study, we tested for differences in the climate sensitivity of tree growth between urban and rural forests along a latitudinal transect in the eastern United States that included Boston, Massachusetts, New York City, New York, and Baltimore, Maryland. Using dendrochronology analyses of tree cores from 55 white oak trees (Quercus alba), 55 red maple trees (Acer rubrum), and 41 red oak trees (Quercus rubra) we investigated the impacts of heat stress and water stress on the radial growth of individual trees. Across our three-city study, we found that tree growth was more closely correlated with climate stress in the cooler climate cities of Boston and New York than in Baltimore. Furthermore, heat stress was a significant hindrance to tree growth in higher latitudes while the impacts of water stress appeared to be more evenly distributed across latitudes. We also found that the growth of oak trees, but not red maple trees, in the urban sites of Boston and New York City was more adversely impacted by heat stress than their rural counterparts, but we did not see these urban-rural differences in Maryland. Trees provide a wide range of important ecosystem services and increasing tree canopy cover was typically an important component of urban sustainability strategies. In light of our findings that urbanization can influence how tree growth responds to a warming climate, we suggest that municipalities consider these interactions when developing their tree-planting palettes and when estimating the capacity of urban forests to contribute to broader sustainability goals in the future.

Assessing the Relationship between Tree Growth, Crown Size, and Neighboring Tree Species Diversity in Mixed Coniferous and Broad Forests Using Crown Size Competition Indices

Competition among trees for limited resources (e.g., sunlight, water, and nutrients) impacts their growth differently. Crown plays a crucial role in resource access for trees. However, uncertainties persist regarding crown size differences, tree growth, and tree competition levels between coniferous and broadleaf trees in mixed-age forests. We conducted a study on 3008 live trees across 28 plots in Hunan Province to measure individual crown size and tree growth increment. Introducing a new metric, the Crown Size Competition Index (CSCI), we assessed competition pressure among coniferous and broadleaf species in mixed-age forests. We examined the correlation between competition indices and tree growth increment while also investigating the influence of neighboring species diversity on tree competition. Our results revealed a significant negative correlation between the tree growth of Cunninghamia lanceolata (Lamb.) Hook.(CL) and Phoebe bournei (Hemsl.) Yang (PB) and the competition index. Increased competitive pressure on the focal tree corresponded to a decline in the growth of focal tree volume, with a more pronounced impact observed in PB compared to CL. The diversity of neighboring species contributed to variations in competition for the focal tree, with broadleaved species (PB) exerting a greater influence on the focal tree than coniferous species (CL). These findings underscore the competitive potential of PB in mature coniferous stands and advocate for the restoration of stratified mixes in CL broadleaf forests. Furthermore, our findings support the management strategies for the valuable tree species PB.

Accuracy comparison of terrestrial and airborne laser scanning and manual measurements for stem curve-based growth measurements of individual trees

Monitoring forest growth accurately is important for assessing and controlling forest carbon stocks that impact, for example, the atmospheric CO2 concentration and, consequently, the climate change. In prior studies, forest growth monitoring with laser scanning methods has resulted in relatively high errors. However, the contribution of reference measurement error to uncertainty in growth resolution has rarely been analysed, and the reference measurements are usually considered mostly flawless. In this study, a seven-year-long growth of individual trees was estimated using both airborne and terrestrial laser scanning (ALS, TLS) that have emerged as potential candidates for digital forest reference measurements. The growth values were derived for diameter at breast height (DBH) and stem volume between the years 2014 and 2021 using an indirect approach. The values obtained with laser scanning were paired with manual field measurements and also with each other to study pairwise errors. The pairwise comparison showed that even though all the three measurement methods produced good Pearson correlation coefficients for one-time measurements (all above 0.88), the coefficients for growth measurements were significantly lower (0.19–0.44 for DBH and 0.47–0.66 for stem volume). The best correlation and root mean squared deviation (RMSD) for DBH growth (ρ = 0.44, RMSD = 0.98 cm) and stem volume growth (ρ = 0.66, RMSD = 0.052 m3) was observed between the manual field measurements and the ALS-based growth measurement method, in which the tree stem curve was obtained from the 2021 point cloud, and the stem curve was predicted backwards for the year 2014 according to height growth. The ALS method suffered less from outlying values than the TLS-based growth measurement method, in which the growth was computed based on the difference of stem curves derived separately for the years 2014 and 2021. The study showed that observing the stem curve is a potential method for short-period growth monitoring. Using the pairwise comparison results, we further derived estimates for the mean and standard deviation of measurement error of each individual measurement method. For the manual measurements, the standard deviation of error was found to be approximately 0.4 cm for DBH growth and 0.03 m3 for volume growth, which were the lowest of the three methods but not by a large margin. This highlights the need for more accurate reference data as the accuracy of laser scanning-based growth estimation methods continues to approach the accuracy of manual measurements.

Height growth and biomass partitioning during secondary succession differ among forest light strata and successional guilds in a tropical rainforest

In closed‐canopy systems globally, plants exhibit intense competition for light, prioritizing vertical growth to attain elevated positions within the canopy. Light competition is especially intense in tropical rainforests because of their dense shaded stands, and during forest succession because of concomitant changes in vertical light profiles. We evaluated how the height growth of individual tree differs among forest light strata (canopy, sub‐canopy and understorey) and successional guilds (early, mid‐ and late successional species) during secondary succession in a Mexican rainforest. Fourteen secondary forest stands differing in time since agricultural abandonment (1–25 years) were monitored for seven consecutive years. For each stand and census year we estimated relative light intensity (RLI) for each height and categorized trees into forest light strata: understorey (RLI ≦ 33.3%), sub‐canopy (33.3% ≦ RLI ≦ 66.6%) and canopy (RLI ≧ 66.6%), and into successional guilds based on the literature. We estimated two measures of height growth: absolute height growth (HGabs, cm year−1) calculated as the difference in tree height between two consecutive censuses, and biomass partitioning to height growth (HGbp, in kg kg−1 × 100) calculated as the percentage of total aboveground biomass growth partitioned to height growth. Earlier in succession, trees for all strata had greater HGabs and HGbp, resulting in rapid vertical forest development. HGabs was fastest for canopy trees, followed by sub‐canopy and understorey trees. These differences in HGabs among strata, combined with their inter‐specific variation and continuous recruitment of small individuals, lead to a rapid differentiation in tree sizes and increase stand structural heterogeneity. HGbp was greater for understorey and sub‐canopy trees than for canopy trees, reflecting ontogenetic changes in the light competition strategy from growth to persistence. With succession, both HGabs and HGbp decreased, most strongly for canopy trees, probably because of an increased exposure to drought stress. These successional changes stabilize stand size structure and reduce the rate of development.

Future impacts of climate change on black spruce growth and mortality: review and challenges

Black spruce ( Picea mariana (Mill.) B.S.P.) is the dominant conifer species across a large part of North American boreal forests, providing many goods and services essential to human activities, and playing a major climatic role through the global carbon cycle. However, a comprehensive synthesis of the effects of climate change on black spruce has not yet been undertaken. The dynamics of black spruce are influenced by various living (biotic) and non-living (abiotic) factors, as well as their combined effects, which are particularly responsive to changes in climate. Climate change predictions suggest that northern ecosystems will experience the world's most significant impact. Therefore, black spruce is likely to undergo profound disruptions in its growth and mortality rate in the next few decades, resulting in significant changes in forestry and carbon storage. However, these changes will not be uniform throughout the entire distribution of the species. Future changes in temperature and precipitation will create more stress for water availability in the boreal forests of western and central North America than in their eastern counterparts. Thus, significant longitudinal and latitudinal gradients in tree growth and mortality variability are expected throughout the range of the species. This literature review aims to summarise the impacts of climate change on individual tree growth and mortality of this major species. While enhanced black spruce productivity could occur through both increased air temperature and nitrogen mineralisation in the soil, moisture limitation in central and western North America will result in significant growth reduction and mortality events across these regions. Conversely, under the expected climate change scenarios, black spruce forests may be more resilient in eastern North America, where climatic conditions appear more suitable, particularly in their northernmost range. In this review, we identify current research gaps for some disturbances, which should be addressed to better understand the impact of climate change on black spruce. Finally, we identify issues associated with sustainable forest management and the maintenance of black spruce under projected future climate changes.

Climate anomalies and neighbourhood crowding interact in shaping tree growth in old‐growth and selectively logged tropical forests

Abstract Climate extremes and biotic interactions at the neighbourhood scale affect tropical forest dynamics with long‐term consequences for biodiversity, global carbon cycling and climate change mitigation. However, forest disturbance may change crowding intensity, and thus the relative contribution of climate extremes and neighbourhood interactions on tree growth, thereby influencing tropical forest resistance and resilience to climate change. Here, we aim to evaluate the separate and interactive effects of climate and neighbours on tree growth in old‐growth and disturbed tropical forests. We used 30 years of growth measurements for over 300 tropical tree species from 15 forest plots in French Guiana to investigate the separate and interactive effects of climate anomalies (in solar radiation, maximum temperature, vapour pressure deficit and climatic water deficit) and neighbourhood crowding on individual tree growth. Contrasting old‐growth and selectively logged forests, we also examined how disturbance history affects tree growth sensitivity to climate and neighbours. Finally, for the most abundant 100 species, we evaluated the role of 12 functional traits pertaining to water relations, light and carbon use in mediating tree growth sensitivity to climate anomalies, neighbourhood crowding and their interactions. Climate anomalies tied to heat and drought stress and neighbourhood crowding independently reduced tree growth, and showed positive interactive effects which attenuated their separate effects on tree growth. Their separate and interactive effects were stronger in disturbed than undisturbed forests. Fast‐growing species (i.e. higher intrinsic growth rates) were more abundant in disturbed forests and more sensitive to climate anomalies and neighbourhood crowding. Traits related to water relations, light and carbon use captured species sensitivities to different climate anomalies and neighbourhood crowding levels but were weak predictors of their interactions. Synthesis: Our results demonstrate that climate anomalies and neighbourhood crowding can interact to shape tropical tree growth, suggesting that considering the biotic context may improve predictions of tropical forest dynamics facing altered climate regimes. Furthermore, species traits can capture tree growth sensitivity to the separate effects of climate and neighbours, suggesting that better representing leading functional dimensions in tropical tree strategies offers a promising way towards a better understanding of the underlying ecological mechanisms that govern tropical forest dynamics.

Characterizing street trees in three metropolises of central China by using Street View data: From individual trees to landscape mapping

Street trees can benefit urban spaces, and the characterization of them is a basis for evaluating their functions and proper management. In this paper, we speculate that Street View imagery-based measurement favors characterizing street trees from individual tree sizes and growth status to spatial mapping and risk region identification at multiple city levels. Using Wuhan, Changsha, and Nanchang as examples, nearly 6000 sites and over 111,000 Baidu Street View (BSV) images were used to measure tree sizes, growth status, and vertical structure. Together with BSV-derived tree data, we also calculated the street-view greenness index (GVI) and the Normalized Difference Vegetation Index (NDVI) from remote sensing data, and then city-level spatial mapping was produced to identify risk regions for urban afforestation. The results showed that BSV measurement could compare city-level street trees, i.e., Changsha had a 1.2–1.3-fold higher diameter, tree height, and canopy size than the lowest at Nanchang; Changsha had the highest GVI, but the lowest NDVI; while Nanchang had the lowest GVI, but the highest NDVI, showing different significance between street-view and remote-sensing greenness. BSV data could identify urbanization effects, i.e., the tree sizes decreased from the urban center to the edge, and three cities showed the same pattern. By using BSV data, the growth status score was summarized from leaf blade color, tree-supported, dieback, and dead-tree percents, and thereafter, urbanization-induced decreases of growth status scores can be observed from the city center to the suburbs. At the same urban-rural gradient, BSV data found the vertical structure tended to be complex, and the sky and middle GVI decreased while the ground GVI rose in all three cities in general. The BSV data made landscape mapping available, and the map can be used to identify risk areas of big-sized tree conservation, health problems, greenness deficiency, and the over-simplified forest structure within a city or among different cities. Together with BSV-derived street development data, complex associations between tree characteristics and geo-climates and street social development could be decoupled by redundancy ordination. In general, streets with more pedestrians were often accompanied by taller, larger, healthier trees, more complex communities, and an increasing sky/ground ratio of GVI greenery. Our findings highlighted that street-view data could be used to assess street tree characteristics, geographic distribution, and possible associations with other factors, favoring the city-level urban tree inventory from individuals to the landscape for urban tree management and policymaking.

Early influences of tamarack (Larix laricina) on black spruce (Picea mariana) and its immediate environment in plantations

Mixed plantations are garnering increased attention due to their potential to provide a broader array of benefits compared to monocultures. Although numerous studies have indicated promising complementarity between black spruce ( Picea mariana) and tamarack ( Larix laricina), few have delved into individual tree growth interactions to thoroughly assess early growth complementarity. We sampled 119 planted black spruce and their immediate environment to quantify and qualify any differences between two conditions in young plantations: mixed tamarack (“mixtures”) and black spruce monocultures (“monocultures”) within young plantations. We investigated the effect of neighbouring under four perspectives: tree competition, microenvironment, foliar nutrients, and soil nutrients. Our results showed increased values for black spruce foliar nitrogen total concentration, soil pH, and canopy closure in mixtures compared to monocultures. Furthermore, black spruce stem volume was increased by 38.1% in mixture compared to monoculture. Black spruce stem volume was negatively affected (86% decrease) by the combined effect of shrubs and non-crop trees under high competition pressure, despite the plantation being mechanically released in 2017. Collectively, our results suggest that black spruce growing in mixtures holds a greater growth potential than black spruce in monocultures.

Tree regeneration and ontogenetic strategies of northern European hemiboreal forests: transitioning towards closer-to-nature forest management.

Tree ontogeny is the genetic trajectories of regenerative processes in trees, repeating in time and space, including both development and reproduction. Understanding the principles of tree ontogeny is a key priority in emulating natural ecological patterns and processes that fall within the calls for closer-to-nature forest management. By recognizing and respecting the growth and development of individual trees and forest stands, forest managers can implement strategies that align with the inherent dynamics of forest ecosystem. Therefore, this study aims to determine the ontogenetic characteristics of tree regeneration and growth in northern European hemiboreal forests. We applied a three-step process to review i) the ontogenetic characteristics of forest trees, ii) ontogenetic strategies of trees for stand-forming species, and iii) summarise the review findings of points i and ii to propose a conceptual framework for transitioning towards closer-to-nature management of hemiboreal forest trees. To achieve this, we applied the super-organism approach to forest development as a holistic progression towards the establishment of natural stand forming ecosystems. The review showed multiple aspects; first, there are unique growth and development characteristics of individual trees at the pre-generative and generative stages of ontogenesis under full and minimal light conditions. Second, there are four main modes of tree establishment, growth and development related to the light requirements of trees; they were described as ontogenetic strategies of stand-forming tree species: gap colonisers, gap successors, gap fillers and gap competitors. Third, the summary of our analysis of the ontogenetic characteristics of tree regeneration and growth in northern European hemiboreal forests shows that stand-forming species occupy multiple niche positions relative to forest dynamics modes. This study demonstrates the importance of understanding tree ontogeny under the pretext of closer-to-nature forest management, and its potential towards formulating sustainable forest management that emulates the natural dynamics of forest structure. We suggest that scientists and foresters can adapt closer-to-nature management strategies, such as assisted natural regeneration of trees, to improve the vitality of tree communities and overall forest health. The presented approach prioritizes ecological integrity and forest resilience, promoting assisted natural regeneration, and fostering adaptability and connectivity among plant populations in hemiboreal tree communities.

Street Tree Management Strategies for Different Special-Purpose Areas: Using Citizen Science Data

Background and objective: This study investigated the occurrence characteristics of street tree management factors based on different special-purpose areas with emphasis on citizen participation in data collection.Methods: We collected growth and management data of street tree sections and individual trees within sections using a monitoring form developed for street trees in four special-purpose areas in the urban area of Chuncheon. We used a binary logistic regression model to analyze the probability of occurrence for each management factor (pruning traces, sign blocking, disease and pest, felled trees, piled objects, and broken branches).Results: The probability of occurrence of management factors varied by special-purpose area, which was related to the characteristics of each area.Conclusion: This study suggests the need to develop efficient street tree management strategies by prioritizing management factors in consideration of differences in special-purpose areas

Influence of crown and canopy structure on light absorption, light use efficiency, and growth in mixed and pure Pseudotsuga menziesii and Fagus sylvatica forests

Mixed-species forests can provide higher levels of ecosystem functions and services and can be more resistant and resilient in the face of global change. While many studies focus on the growth and yield of mixed forests, fewer have examined the underlying processes. Inter- or intra-specific differences and interactions influence tree- and stand-level light absorption by determining the vertical structure of stratified canopies, stand density, leaf area index, and the size or allometry of trees. While canopy light absorption is a very important process, it is difficult to quantify it for individual species within a mixture and is rarely examined. A detailed tree-level model (MAESTRA) was used in combination with measurements of tree sizes and stand structures to examine effects of mixing on absorbed photosynthetically active radiation (APAR) in 41–63-year-old stands of Pseudotsuga menziesii and Fagus sylvatica at three sites in Bavaria, Germany. The effects of initial stand density on APAR were analysed in 46-year-old P. menziesii stands of a spacing experiment at two sites. At the tree level, mixing increased mean height and leaf area, growth (185% higher) and APAR (85% higher) of P. menziesii at all sites. Mean tree heights and crown sizes of F. sylvatica were larger in mixtures, while recent growth rates and APAR were not significantly different to monocultures. Planting density did not influence mean tree variables (e.g. height, leaf area, crown volume), because any initial spacing effects had been gradually removed by thinning across all treatments. At the stand level, there were no differences in growth, basal area or in the annual growth per annual APAR (light use efficiency, LUE) between monocultures and mixtures. The highest APAR values were observed in P. menziesii monocultures, while the lowest APAR values were observed in F. sylvatica monocultures. While mixing these species may not increase stand-level growth during later phases of development, mixing accelerated initial growth of individual trees and reduced the time to reach target diameters, which are both important aspects in adapting forests to global change.