Neighborhood Crowding Research Articles

Neighborhood structure, more than soil nutrients, influences net tree interactions among different functional types in a temperate forest

Tree-tree interactions are fundamental in shaping forest community dynamics, driven by factors such as nutrient availability and species composition. While recent research underscores the role of functional traits in influencing tree sensitivity to neighborhood interactions, a clear consensus is lacking on how these traits interact with biotic and abiotic factors. Based on a multi-level modeling approach, this study examines how specific leaf area (SLA), potential maximum tree height (Hmax), and wood density (WD) mediate the combined effects of soil nutrients and neighborhood biotic factors on net tree interactions among 4,210 individual stems for 13 species in a 30-ha temperate forest in northeastern China. The findings indicate that while functional traits independently account for a small portion of the variation in net tree interaction intensity (NIntC), they significantly mediate the effects of neighborhood crowding, neighborhood structure, and soil nutrients on NIntC. Specifically, only the response of growth NIntC of species with low-Hmax to available phosphorus (AP) was consistent with the stress gradient hypothesis. Conspecific neighborhood crowding consistently increases competitive NIntC, whereas heterospecific neighborhood crowding significantly facilitates survival NIntC. Moreover, the positive impact of heterospecific crowding on survival NIntC is significantly greater for high-Hmax species compared to low-Hmax species. Neighborhoods with higher tree species diversity, greater size heterogeneity, and more uniform distribution increase the facilitative NIntC of high-SLA or high-WD species but intensify the competitive NIntC of low-SLA or low-WD species. Our results also indicate that the relative importance of neighborhood structure exceeds that of soil nutrients in explaining variation in NIntC. When environmental conditions are difficult to change, adjusting the forest stand structure may be a more efficient way to regulate the intensity of interactions between trees and improve forest productivity.

Disturbance history, neighborhood crowding and soil conditions jointly shape tree growth in temperate forests.

Understanding how different mechanisms act and interact in shaping communities and ecosystems is essential to better predict their future with global change. Disturbance legacy, abiotic conditions, and biotic interactions can simultaneously influence tree growth, but it remains unclear what are their relative contributions and whether they have additive or interactive effects. We examined the separate and joint effects of disturbance intensity, soil conditions, and neighborhood crowding on tree growth in 10 temperate forests in northeast China. We found that disturbance was the strongest driver of tree growth, followed by neighbors and soil. Specifically, trees grew slower with decreasing initial disturbance intensity, but with increasing neighborhood crowding, soil pH and soil total phosphorus. Interestingly, the decrease in tree growth with increasing soil pH and soil phosphorus was steeper with high initial disturbance intensity. Testing the role of species traits, we showed that fast-growing species exhibited greater maximum tree size, but lower wood density and specific leaf area. Species with lower wood density grew faster with increasing initial disturbance intensity, while species with higher specific leaf area suffered less from neighbors in areas with high initial disturbance intensity. Our study suggests that accounting for both individual and interactive effects of multiple drivers is crucial to better predict forest dynamics.

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.

Height–diameter allometry for a dominant palm to improve understanding of carbon and forest dynamics in forests of Puerto Rico

AbstractTropical forests play a major role in the global carbon cycle but their diversity and structural complexity challenge our ability to accurately estimate carbon stocks and dynamics. Palms, in particular, are prominent components of many tropical forests that have unique anatomical, physiological, and allometric differences from dicot trees, which impede accurate estimates of their aboveground biomass (AGB) and population dynamics. We focused on improving height estimates and, ultimately, AGB estimates for a highly abundant palm in Puerto Rico, Prestoea acuminata. Based on field measurements of 1003 individuals, we found a strong relationship between stem height and diameter. We also found some evidence that height–diameter allometry of P. acuminata is mediated by various sources of environmental heterogeneity including slope and neighborhood crowding. We then examined variability in AGB estimates derived from three models developed to estimate palm AGB. Finally, we applied our novel height:diameter allometric model to hindcast dynamics of P. acuminata in the Luquillo Forest Dynamics Plot during a 27‐year period (1989–2016) of post‐hurricane recovery. Overall, our study provides improved estimates of AGB in wet forests of Puerto Rico and will facilitate novel insights to the dynamics of palms in tropical forests.Abstract in Spanish is available with online material.

Intraspecific variation in fruit production of African mahogany (Khaya anthotheca) in a semi‐deciduous East African rainforest

AbstractRestoring valuable timber trees is a necessary part of regaining many ecosystem services and products provided by degraded tropical forests. However, knowledge of seed production, a regeneration prerequisite, is limited for many tropical timber tree species. We studied the fruit production of African mahogany (Khaya anthotheca) in a semi‐deciduous rainforest in East Africa, to provide information for ecological restoration programs. First, we quantified population variability and synchrony in fruit production. Then, we assessed how reproductive status and output were influenced by tree size, neighbourhood crowding, exchangeable cations, liana presence, and crown exposure. We found fruit production at the population level was weakly synchronised, with high interannual variability at the population and individual levels. Tree size determined reproductive status. Individual quantity of fruit produced increased with tree size but decreased with neighbourhood crowding. Our results indicate that K. anthotheca does not meet the criteria for mast fruiting. Our findings suggest that reproductive output in mahogany is regulated by tree size and competition. In addition to active protection, thinning to reduce competition around seed trees may boost fruit production at the individual tree level to provide seed for forest enrichment.

Sapling recruitment does not compensate for increased tree mortality under climate change

Sapling recruitment within established stands plays an important role in the long-term functionality of forest ecosystems. However, the specific impacts of climate change on sapling recruitment and its underlying mechanisms remain unclear. We hypothesized that sapling recruitment is directly affected by climate change and indirectly through the responses of tree mortality and growth to climate change as well as their induced modification of neighbourhood conditions. We employed a dataset of 173 permanent sample plots with individual stems that were spatially mapped and monitored at 5-year intervals from 1986 to 2010 in the central boreal forests of North America. By constructing structural equation models (SEMs), we examined how sapling recruitment probability may, directly and indirectly, respond to climate change through the mortality and growth of established trees, neighbourhood crowding, and functional and phylogenetic dissimilarities. We used the middle calendar year of a 5-year census period to represent recent systematic temporal trends in climate change. Our SEMs revealed that sapling recruitment probability did not respond significantly directly to temporal climate change in the studied forests, where atmospheric CO2 concentration, air temperature, and climatic water availability simultaneously increased temporally. However, the temporally increased mortality and growth of neighbourhood established trees (all stems except for new recruits) exerted indirect negative effects on sapling recruitment probability. Furthermore, temporally increased neighbourhood crowding had an indirect adverse effect, while temporally increased neighbourhood shade tolerance and phylogenetic dissimilarities had weak indirect positive effects on sapling recruitment probability. Collectively, temporal climate change had a significant total negative effect on sapling recruitment probability. Our analysis demonstrated multiple mechanisms that simultaneously regulated the effect of temporal climate change on sapling recruitment. Our results emphasized that sapling recruitment does not compensate for increased tree mortality. We suggest that even so-called favourable climate change could threaten the long-term functionality of boreal forest ecosystems.

Evaluating Effects of Remotely Sensed Neighborhood Crowding and Depth-to-Water on Tree Height Growth

Previous research on the effects of neighborhood crowding and soil moisture on tree height growth have been limited by time-consuming and sometimes inaccurate ground-based measurements of tree height. Recent developments in unmanned aerial vehicles (UAVs) allow detailed 3D point clouds of the canopy surface to be generated at relatively low cost. Using UAV-derived point clouds, we obtained height measurements of 4386 trees for the years 2019 and 2021. We also calculated four neighborhood crowding indices and a topography-based moisture index (depth-to-water) for these trees. Using initial tree height, neighborhood crowding indices and the depth-to-water index, we developed Bayesian hierarchical models to predict height growth for three tree species (Picea glauca (white spruce), Populus tremoluides (trembling aspen) and Pinus contorta (lodgepole pine)) across different stands. Bayes-R2 values of the final models were highest for white spruce (35%) followed by trembling aspen (28%) and lodgepole pine (25%). Model outputs showed that the effect of crowding and depth-to-water on height growth are limited and species-dependent, adding a maximum of 7% to the Bayes-R2 metric. Comparing different neighborhood crowding indices revealed that no index is clearly superior to others across all three species, as different neighborhood crowding indices resulted in only minor differences in model performance. While height growth can be partially explained by aerially derived neighborhood crowding indices and the depth-to-water index, future studies should focus on identifying relevant site characteristics to predict tree growth with greater accuracy.

Ontogeny influences tree growth response to soil fertility and neighbourhood crowding in an old-growth temperate forest.

Abiotic and biotic factors simultaneously affect tree growth and thus shape community structure and dynamics. In particular, trees of different size classes show different growth responses to soil nutrients and neighbourhood crowding, but our understanding of how species' joint responses to these factors vary between size classes remains limited in multi-storied temperate forests. Here, we investigated size class differences in tree growth response to soil gradients and neighbourhood crowding in an old-growth temperate forest. We combined growth data over 15 years from 38 902 individuals of 42 tree species with trait data in a 25-ha temperate forest plot in northeast China. We built hierarchical Bayesian models of tree growth to examine the effects of soil gradients and neighbourhood crowding between size classes and canopy types. We found that soil and neighbours mainly acted separately in shaping tree growth in small and large trees. Soil total nitrogen and phosphorus increased tree growth in small trees, in particular of understorey species, but not in large trees. Neighbours reduced tree growth in both tree size classes, with stronger effects on large than small trees, and on canopy than understorey species. Furthermore, small trees with higher specific leaf area grew faster in fertile soils, and small trees with less seed mass grew faster in crowded environments. Large trees with higher specific leaf area, specific root length and less seed mass grew faster in crowded environments, while these traits had limited influence on tree growth response to soil gradients. Our study highlights the importance of size class in modulating the response of tree growth to soil and neighbours, and the differential role of species canopy types and functional traits in capturing these effects in large vs. small trees.

Spatial models can improve the experimental design of field-based transplant gardens by preventing bias due to neighborhood crowding.

Field-based transplant gardens, including common and reciprocal garden experiments, are a powerful tool for studying genetic variation and gene-by-environment interactions. These experiments assume that individuals within the garden represent independent replicates growing in a homogenous environment. Plant neighborhood interactions are pervasive across plant populations and could violate assumptions of transplant garden experiments. We demonstrate how spatially explicit models for plant-plant interactions can provide novel insights on genotypes' performance in field-transplant garden designs. We used individual-based models, based on data from a sagebrush (Artemisia spp.) common garden, to simulate the impact of spatial plant-plant interactions on between-group differences in plant growth. We found that planting densities within the range of those used in many common gardens can bias experimental outcomes. Our results demonstrate that higher planting densities can lead to inflated group differences and may confound genotypes' competitive ability and genetically underpinned variation. Synthesis. We propose that spatially explicit models can help avoid biased results by informing the design and analysis of field-based transplant garden experiments. Alternately, including neighborhood effects in post hoc analyses of transplant garden experiments is likely to provide novel insights into the roles of biotic factors and density dependence in genetic differentiation.

Functionally and phylogenetically diverse boreal forests promote sapling recruitment

Sapling recruitment in established forest stands is critical to long-term ecosystem functioning but can be constrained due to competition from increased neighbourhood crowding. Tree species diversity has the potential to ameliorate negative crowding effects; however, the mechanisms underlying the relationship between diversity and sapling recruitment remain unclear. We used data from spatially mapped individual stems, which were repeatedly measured at 5-year intervals between 1986 and 2010 from 173 permanent plots in Canada. We analyzed individual sapling recruitment probabilities over 5-year census intervals as a function of neighbourhood crowding index, stand age, neighbourhood diversity from shade tolerance dissimilarity and phylogenetic dissimilarity, and their interactions. We show that sapling recruitment probability on average decreases with neighbourhood crowding but increases with stand age and neighbourhood shade tolerance and phylogenetic dissimilarities. The positive effect of shade tolerance dissimilarity was strongest in young stands, while phylogenetic dissimilarity had a positive effect only in older stands. Moreover, sapling recruitment probability in neighbourhoods with high shade tolerance and phylogenetic dissimilarities was less negatively affected by crowding than in more similar neighbourhoods. Our study highlights the mechanisms that promote sapling recruitment in diverse neighbourhoods are context-dependent, with niche complementary being more important in young stands and the feedback from pests and pathogens playing a greater role in old boreal stands. Our results provide direct evidence that neighbourhood diversity helps ameliorate the negative neighbourhood crowding effect on sapling recruitment, particularly in old-growth boreal forests. The mixtures of broadleaves and conifers increase sapling recruitment and are particularly important in the late-successional stages of boreal forests.

Neighborhood dissimilarity consistently attenuates competition stress on tree growth under altered water availability in a natural boreal forest

Tree growth regulates forest productivity and carbon sequestration. Although there has been evidence showing positive effects of neighborhood diversity on tree growth, few have experimentally demonstrated how tree growth may respond to neighborhood diversity in natural forests under altered water availability, which is anticipated to occur more frequently with ongoing global climate change. We examined three-year individual tree basal area growth under the water alteration treatments of reduced (-25% throughfall during the growing season), ambient and added (+25% throughfall during the growing season) rainfall in the broadleaf stands, conifer stands and their mixtures of a young natural boreal forest. We quantified neighborhood drought tolerance dissimilarity, phylogeny dissimilarity and neighborhood crowding based on individual tree sizes and the distance between them. We found that the tree growth on average did not change significantly with water alteration but decreased with neighborhood crowding, with the negative crowding effect being more pronounced in water reduction treatment than in ambient and water addition treatments. Higher neighborhood dissimilarity of drought tolerance and phylogeny promoted tree growth consistently across all water conditions. Moreover, the positive effects of neighborhood dissimilarity of drought tolerance and phylogeny became more pronounced with increasing neighborhood crowding. Our results suggest that promoting species mixtures in boreal forests may sustain higher boreal forest productivity under changing water availability.

Multimodal Multi-Objective Optimization Based on Local Optimal Neighborhood Crowding Distance Differential Evolution Algorithm

Multimodal Multi-Objective Optimization Based on Local Optimal Neighborhood Crowding Distance Differential Evolution Algorithm

Tree growth response to soil nutrients and neighborhood crowding varies between mycorrhizal types in an old-growth temperate forest.

Forest dynamics are shaped by both abiotic and biotic factors. Trees associating with different types of mycorrhizal fungi differ in nutrient use and dominate in contrasting environments, but it remains unclear whether they exhibit differential growth responses to local abiotic and biotic gradients where they co-occur. We used 9-year tree census data in a 25-ha old-growth temperate forest in Northeast China to examine differences in tree growth response to soil nutrients and neighborhood crowding between tree species associating with arbuscular mycorrhizal (AM), ectomycorrhizal (EM), and dual-mycorrhizal (AEM) fungi. In addition, we tested the role of individual-level vs species-level leaf traits in capturing differences in tree growth response to soil nutrients and neighborhood crowding across mycorrhizal types. Across 25 species, soil nutrients decreased AM tree growth, while neighborhood crowding reduced both AM and EM tree growth, and neither soil nor neighbors impacted AEM tree growth. Across mycorrhizal types, individual-level traits were stronger predictors of tree growth than species-level traits. However, most traits poorly mediated tree growth response to soil nutrients and neighborhood crowding. Our findings indicate that mycorrhizal types strongly shape differences in tree growth response to local soil and crowding gradients, and suggest that including plant-mycorrhizae associations in future work offers great potential to improve our understanding of forest dynamics.

Temporal trait plasticity predicts the growth of tropical trees

AbstractQuestionsLinking tree growth dynamics to functional traits and neighborhood conditions that are measured at a single time point gives limited insights into the direction of forest structural and functional change. We used trait and neighborhood data collected at different time points for each individual tree of 15 Ficus species to test how trait (temporal trait plasticity) and neighborhood crowding change over time, and to test how their temporal change affects individual growth rate. We asked the following questions: (i) how do traits, neighborhood crowding, and growth of individual trees change over time; (ii) are functional traits and neighborhood crowding temporally consistent in their association with growth rate of individuals; and (iii) do temporal trait plasticity and changes in neighborhood crowding better predict the relative growth rate of individuals compared to using only a single snapshot of traits and neighborhood crowding?LocationXishuangbanna Tropical Seasonal Rainforest, southwest China.MethodsWe collected traits (specific leaf area [SLA], leaf area (LA), leaf dry matter content [LDMC], leaf chlorophyll, leaf thickness, and leaf succulence) at two time points (2010 and 2017) for 472 individuals of 15 Ficus species. We used linear mixed‐effect models to test the effect of temporal trait plasticity and neighborhood crowding on the relative growth rate of individuals.ResultsWe found that the temporal change in trait values predicts the growth rate of individuals better compared to static trait values in the initial and final censuses. We found significant temporal changes in individual traits suggesting a shift in ecological strategies from being acquisitive to conservative. A difference in neighborhood crowding between the two census years was also observed, indicating that the neighborhood effect on growth might also change over time.ConclusionsOur results in general highlight the need to consider the temporal dimension of traits and biotic interactions, as our results suggest that growth–trait relationships may vary between time points, allowing us to understand the demographic response of species to temporal environmental change through functional traits.

Topography and Traits Modulate Tree Performance and Drought Response in a Tropical Forest

Predicting drought responses of individual trees in tropical forests remains challenging, in part because trees experience drought differently depending on their position in spatially heterogeneous environments. Specifically, topography and the competitive environment can influence the severity of water stress experienced by individual trees, leading to individual-level variation in drought impacts. A drought in 2015 in Puerto Rico provided the opportunity to assess how drought response varies with topography and neighborhood crowding in a tropical forest. In this study, we integrated 3 years of annual census data from the El Yunque Chronosequence plots with measurements of functional traits and LiDAR-derived metrics of microsite topography. We fit hierarchical Bayesian models to examine how drought, microtopography, and neighborhood crowding influence individual tree growth and survival, and the role functional traits play in mediating species’ responses to these drivers. We found that while growth was lower during the drought year, drought had no effect on survival, suggesting that these forests are fairly resilient to a single-year drought. However, growth response to drought, as well as average growth and survival, varied with topography: tree growth in valley-like microsites was more negatively affected by drought, and survival was lower on steeper slopes while growth was higher in valleys. Neighborhood crowding reduced growth and increased survival, but these effects did not vary between drought/non-drought years. Functional traits provided some insight into mechanisms by which drought and topography affected growth and survival. For example, trees with high specific leaf area grew more slowly on steeper slopes, and high wood density trees were less sensitive to drought. However, the relationships between functional traits and response to drought and topography were weak overall. Species sorting across microtopography may drive observed relationships between average performance, drought response, and topography. Our results suggest that understanding species’ responses to drought requires consideration of the microenvironments in which they grow. Complex interactions between regional climate, topography, and traits underlie individual and species variation in drought response.

Competition influences tree growth, but not mortality, across environmental gradients in Amazonia and tropical Africa.

Competition among trees is an important driver of community structure and dynamics in tropical forests. Neighboring trees may impact an individual tree’s growth rate and probability of mortality, but large‐scale geographic and environmental variation in these competitive effects has yet to be evaluated across the tropical forest biome. We quantified effects of competition on tree‐level basal area growth and mortality for trees ≥10‐cm diameter across 151 ~1‐ha plots in mature tropical forests in Amazonia and tropical Africa by developing nonlinear models that accounted for wood density, tree size, and neighborhood crowding. Using these models, we assessed how water availability (i.e., climatic water deficit) and soil fertility influenced the predicted plot‐level strength of competition (i.e., the extent to which growth is reduced, or mortality is increased, by competition across all individual trees). On both continents, tree basal area growth decreased with wood density and increased with tree size. Growth decreased with neighborhood crowding, which suggests that competition is important. Tree mortality decreased with wood density and generally increased with tree size, but was apparently unaffected by neighborhood crowding. Across plots, variation in the plot‐level strength of competition was most strongly related to plot basal area (i.e., the sum of the basal area of all trees in a plot), with greater reductions in growth occurring in forests with high basal area, but in Amazonia, the strength of competition also varied with plot‐level wood density. In Amazonia, the strength of competition increased with water availability because of the greater basal area of wetter forests, but was only weakly related to soil fertility. In Africa, competition was weakly related to soil fertility and invariant across the shorter water availability gradient. Overall, our results suggest that competition influences the structure and dynamics of tropical forests primarily through effects on individual tree growth rather than mortality and that the strength of competition largely depends on environment‐mediated variation in basal area.

Intraspecific variation in surface water uptake in a perennial desert shrub

Abstract Despite broad recognition that water is a major limiting factor in arid ecosystems, we lack an empirical understanding of how this resource is shared and distributed among neighbouring plants. Intraspecific variability can further contribute to this variation via divergent life‐history traits, including root architecture. We investigated these questions in the shrub Artemisia tridentata and hypothesized that the ability to access and utilize surface water varies among subspecies and cytotypes. We used an isotope tracer to quantify below‐ground zone of influence in A. tridentata, and tested whether spatial neighbourhood characteristics can alter plant water uptake. We introduced deuterium‐enriched water to the soil in plant interspaces in a common garden experiment and measured deuterium composition of plant stems. We then applied spatially explicit models to test for differential water uptake by A. tridentata, including intermingled populations of three subspecies and two ploidy levels. The results suggest that lateral root functioning in A. tridentata is associated with intraspecific identity and ploidy level. Subspecies adapted to habitats with deep soils generally had a smaller horizontal reach, and polyploid cytotypes were associated with greater water uptake compared to their diploid variants. We also found that plant crown volume was a weak predictor of water uptake, and that neighbourhood crowding had no discernable effect on water uptake. Intraspecific variation in lateral root functioning can lead to differential patterns of resource acquisition, an essential process in arid ecosystems in the contexts of changing climate and seasonal patterns of precipitation. Altogether, we found that lateral root development and activity are more strongly related to genetic variability within A. tridentata than to plant size. Our study highlights how intraspecific variation in life strategies is linked to mechanisms of resource acquisition. A free Plain Language Summary can be found within the Supporting Information of this article.

Using aerial canopy data from UAVs to measure the effects of neighbourhood competition on individual tree growth

Unmanned aerial vehicles (UAVs) have opened new opportunities for measuring 3D canopy structure from aerial imagery. Image data collected with a UAV can be processed to generate detailed information on local canopy structure around an individual tree, which may be a useful proxy for the amount of competition that tree experiences from its neighbours. Structural indices of competition traditionally have been derived from ground-based plot data, and it is not clear whether aerial canopy data from a UAV are as effective as ground data for modelling the effects of competition on individual-tree growth. Here, we compare the relative performance of four ground-based competition indices derived from plot data, five canopy-based competition indices derived from UAV data, and one hybrid index that uses both data types, for predicting the radial growth of two northern tree species (white spruce, lodgepole pine). Ground-based and canopy-based competition indices were both represented among the top-performing models for each species, but no single index was unambiguously favoured over all others. Of the ten competition indices we considered, the mean canopy height within 15 m of a subject tree had the strongest performance across both species, including the best performance for lodgepole pine (R2 = 0.29) and third-best performance for white spruce (R2 = 0.42). Models with mean canopy height also revealed interactions between competition and soil moisture, with growth reductions from competition limited to dry sites for white spruce and to mesic sites for lodgepole pine. Although we did not identify a systematic advantage for either ground-based or canopy-based competition indices, indices that were centred on the subject tree tended to perform better than plot-based indices that were not. Overall, our comparison showed that canopy-based metrics such as mean canopy height can be at least as effective as traditional ground-based metrics for measuring the effects of local competition on tree growth. As a new research tool in forest ecology, UAVs thus offer a valuable approach for measuring neighbourhood crowding and its effects on the performance of individual trees.

Drivers of individual tree growth and mortality in an uneven-aged, mixed-species conifer forest

Individual tree growth and mortality drive forest stand dynamics and are universally important metrics of tree success. Studying factors that affect growth and mortality is particularly challenging in mixed-species, uneven-aged systems due to their defining heterogeneity and strong temporal and spatial variability. The goal of this study was to determine the relative importance of individual tree attributes (i.e., species, size, neighborhood crowding, crown position) and environmental characteristics (i.e., soil moisture) in driving tree growth and survival in an uneven-aged, mixed species forest. In particular we tested if the factors regulating growth were the same as those regulating mortality, as is often assumed. Due to its large size and intensive sampling, the 3-ha, stem-mapped plot (established in 1989) at Howland Research Forest in central Maine, USA, allowed us to address additional questions regarding the influence of sapling crowding, neighbor species identity, and legacies of past disturbance. Growth and survival of over 3000 plot trees was assessed after 25 years and modeled using multiple linear regression (growth) and binary logistic regression (survival). As expected, species, neighborhood crowding, and tree diameter were top predictors of growth and survival. Specifically, growth and survival decreased with greater crowding, and increased with larger diameters. We also found that the identity of neighbors influenced focal tree growth: growth generally improved in neighborhoods comprised of species different from that of the focal. However, this general finding did not hold for all species: eastern hemlock grew better in hemlock neighborhoods, and northern white-cedar showed no response related to neighbor identities. In contrast to growth, neighborhood identity was not related to survival. Crowding from saplings did not explain any additional variability in growth; however, unexpectedly, individuals with greater sapling crowding were more likely to survive. For both growth and survival, we found an interaction between crowding and soil moisture, suggesting that within a single stand, individual success can be limited by both excess and insufficient moisture. We found no relationship between neighborhood cut stumps (legacy of past disturbance) and recent growth or survival. These results highlight the many variables driving growth and survival in uneven-aged, mixed-species forests. The top predictors for growth were identical to those for survival; however, other predictors differed in their relative importance. Given the recent emphasis on promoting uneven-aged, mixed-species forests, we suggest that studies addressing a full range of predictors of individual tree success are necessary to better manage and maintain these complex systems.

Fruit production is influenced by tree size and size-asymmetric crowding in a wet tropical forest.

In tropical forest communities, seedling recruitment can be limited by the number of fruit produced by adults. Fruit production tends to be highly unequal among trees of the same species, which may be due to environmental factors. We observed fruit production for ~2,000 trees of 17 species across 3 years in a wet tropical forest in Costa Rica. Fruit production was modeled as a function of tree size, nutrient availability, and neighborhood crowding. Following model selection, tree size and neighborhood crowding predicted both the probability of reproduction and the number of fruit produced. Nutrient availability only predicted only the probability of reproduction. In all species, larger trees were more likely to be reproductive and produce more fruit. In addition, number of fruit was strongly negatively related to presence of larger neighboring trees in 13 species; presence of all neighboring trees had a weak‐to‐moderate negative influence on reproductive status in 16 species. Among various metrics of soil nutrient availability, only sum of base cations was positively associated with reproductive status, and for only four species. Synthesis Overall, these results suggest that direct influences on fruit production tend to be mediated through tree size and crowding from neighboring trees, rather than soil nutrients. However, we found variation in the effects of neighbors and nutrients among species; mechanistic studies of allocation to fruit production are needed to explain these differences.