Understory Research Articles

Segmentation of conifer tree crowns from terrestrial laser scanning point clouds in mixed stands of Scots pine and Norway spruce

Terrestrial laser scanning of conifer tree crowns is challenged by occlusion problems causing sparse point clouds for many trees. Automatic segmentation of conifer tree crowns from sparse point clouds is a task that has only recently been addressed and not solved in a way that all trees can be segmented automatically without assignment errors. We developed a new segmentation algorithm that is based on region growing from seeds in voxelized 3D laser point clouds. In our data, field measured tree positions and diameters were available as input data to estimate crown cores as seeds for the region growing. In other applications, these seeds can be derived from the laser point cloud. Segmentation success was judged visually in the 3D voxel clouds for 1294 tree crowns of Norway spruce and Scots pine on 24 plots in six mixed species stands. Only about half of the tree crowns had only minor or no segmentation errors allowing to fit concentric crown models. Segmentation errors were most often caused by unsegmented neighbors at the edge of the sample plots. Wrong assignments of crown parts were also more frequent in dense groups of trees and for understory trees. For some trees, point clouds were too sparse to describe the crown. Segmentation success rates were considerably higher for dominant trees in the plot center. Despite the incomplete automatic segmentation of tree crowns, metrics describing crown size and crown shape could be derived for a large number of sample trees. A description of the irregular shape of tree crowns was not possible for most trees due to the sparse point clouds in the upper crown of most trees.

Canopy arthropod responses to repeated canopy opening in a wet tropical forest

AbstractWith predictions of increased frequency of intense hurricanes, it is increasingly crucial to understand how biotic and abiotic components of forests will be affected. This study describes canopy arthropod responses to repeated experimental and natural canopy opening at the Luquillo Experimental Forest Long‐term Ecological Research Site (LTER) in Puerto Rico. The canopy trimming experiment (CTE1) treatments were started in 2004, and a second trimming (CTE2) was conducted in 2014, to study effects of increased hurricane frequency at the site. Paired disturbed plots with canopy trimmed (trim) and undisturbed plots with no trimming (no trim) were replicated in three experimental blocks. Arthropods were sampled by bagging branches on seven representative early and late successional overstory and understory tree species annually from 2004 to 2009 for CTE1 and 2015 to 2019 for CTE2. In addition to the experimental manipulation, the CTE site was disturbed by Hurricane Maria (Category 4) in September 2017, providing an additional natural canopy opening to the experiment. We evaluated the effect of the second experimental trimming, compared canopy arthropod responses to the three canopy‐opening events, and compared the effects of experimental trimming and natural canopy opening by Hurricane Maria. The second experimental canopy trimming produced canopy arthropod responses consistent with hurricane disturbances, with sap‐sucking herbivores increasing in abundance on the trimmed plots, whereas other functional groups generally declined in abundance in disturbed plots. Responses to the first and second trimmings were generally similar. However, Hurricane Maria exacerbated the responses, indicating the likely effect of increased hurricane frequency and intensity.

Abundance of white lace lerp psyllids on understorey and canopy river red gums and relationships with foliar sugars and tannins

Abstract1. Trees present herbivorous insects with the greatest diversity of resources of any plant growth form. Both ontogeny and shading can alter suitability for arboreal insect herbivores.2. We conducted a longitudinal study of tagged ‘mature’ (>12 months old) Eucalyptus camaldulensis leaves to compare the suitability of understorey and canopy trees for the leaf senescence‐inducing psyllid, Cardiaspina albitextura. We quantified sugars and tannins as possible predictors of nymphal abundance.3. Canopy leaves hosted double the number of nymphs as understorey leaves. Variation among individual trees (understorey and canopy) was the most important source of heterogeneity explaining psyllid abundance, although relative leaf age significantly influenced oviposition on canopy leaves. The diversity of foliar sugars was higher among canopy leaves than among understorey leaves. There was significant between‐tree diversity in total hydrolysable tannins (HTs) and total condensed tannins (CTs) among understorey trees but not among canopy trees. Heterogeneity among understorey and canopy trees was explained by greater diversity of ellagitannins (HTs) than of CTs.4. Shading is detrimental to the survival of nymphs on both host types, but sugars are unlikely to explain variation in suitability. Vescalagin (an ellagitannin) was negatively correlated with the abundance of nymphs on both host types.

Different-Sized Vessels of Quercus variabilis Blume Respond Diversely to Six-Year Canopy and Understory N Addition in a Warm-Temperate Transitional Zone

Nitrogen is a necessary macroelement in plant growth and is usually considered a limiting factor in many forest ecosystems. Increasing N deposition has been reported to affect tree growth. However, the effects still remain controversial due to variable N fertilization methods used. In order to study the realistic responses of tree growth to increasing N deposition, we investigated effects of canopy and understory N addition on tree-ring growth and vessel traits of Quercus variabilis Blume. Since 2013, 50 kg N ha−1 year was applied monthly from April to December to either the canopy (CN) or understory (UN) of trees in a warm-temperate forest in Central China. During 2013–2018, tree-ring growth and vessel-related traits (mean vessel area, theoretical xylem hydraulic conductivity (KH), relative ratio of KH, etc.) were analyzed. Tree rings were negatively impacted by both CN and UN treatments, but only the effect of UN was significant. Neither CN nor UN significantly impacted the detected vessel traits. However, some diverging influencing trends were still showed in some vessel traits. Both CN and UN treatments positively affected the percentage of annual total vessel area and vessel density, with the effect of UN on vessel density being more severe. All the detected vessel traits of the large vessels formed at the beginning of the tree-ring responded positively to CN, whereas the opposite response to UN was showed on mean vessel area and the relative ratio of KH. All these diverging responses in different vessel traits likely reflected the compensation and trade-off between maximizing growth and adapting to CN and UN treatments. Six-year long N addition negatively and positively affected tree-ring growth and vessel traits of Q. variabilis in Central China, respectively. UN treatment could not fully simulate the real effect on tree growth, especially on the hydraulic architecture.

Drivers of forest change in the Greater Yellowstone Ecosystem

AbstractQuestionsGlobal climate change is predicted to cause widespread shifts in the distribution and composition of forests, particularly in mountain environments where climate exerts strong controls on tree community arrangement. The upslope movement of vegetation has been observed in association with warming temperatures and is especially evident in ecotones—the transition zones between vegetation types. We explored the role of drought and tree mortality on recent changes in high‐elevation forests.LocationGreater Yellowstone Ecosystem, USA.MethodsWe established 19 forest demography plots along an elevational gradient spanning dominant high‐elevation vegetation types.ResultsTree establishment dates indicated the upslope movement of Pinus albicaulis (whitebark pine) treeline and ecotone shift from meadow to forest starting in the 1950s. An expansion of the growing season likely contributed to the upward expansion of the treeline. Comparisons between overstory and understory tree composition suggested ongoing succession in the absence of fire at lower elevations, namely the replacement of Pinus contorta (lodgepole pine) by Abies lasiocarpa (subalpine fir). P. contorta seedlings were distributed at higher elevations than overstory trees of the same species, suggesting some potential for upslope movement with warming conditions; P. albicaulis seedlings, conversely, were distributed throughout all elevations of the transect. Significant tree mortality occurred in Pinus spp. and disproportionately affected P. albicaulis, as a result of a regional Dendroctonus ponderosae (mountain pine beetle) outbreak (2008–2012). Mortality events were strongly associated with drier than average conditions 2–3 years prior to tree death.ConclusionRising sensitivity to arid conditions in the mid‐20th century amid already dense, aging forests appears to have increased susceptibility to beetle‐induced mortality during the most recent drought. Tree species in the study area responded individually to global change stressors, which acted on these forests in complex ways and led to both ecotone shifts and stability. This work highlights the interplay between succession, forest disturbances and climate‐related growth responses in driving forest compositional change in subalpine and treeline environments.

Local-scale structuration of species-habitat association in the semi-deciduous tropical rainforest in eastern Cameroon

Tropical forests are hyperdiverse, and although broad-scale variation in diversity can be reasonably well explained, local-scale variation in diversity is harder to understand. It has been revealed that species may show species-habitats associations. However, how the assembled species are associated with local habitats and how community-habitat associations vary regarding species abundance and class size remains unclear. Here, we analyzed the community-habitat associations by applying the multivariate regression trees (MRT) technique on data of 22,064 trees distributed across 30 1-ha plots varying in topography and soil conditions in a semi-deciduous tropical rainforest of Cameroon. Further, the indicator species analysis was used to determine the most important species for structuring assemblages.The MRT divides the whole tree community inventoried into four habitats types: Habitat 1 (H1) sandy loamy, acidic soil with a low Na concentration (pH ≤ 6.245 and [Na] < 4.535 cmol kg−1); H2 equal to neutral sandy soil with a small Na concentration (7 ≤ pH > 6.245 and [Na] < 4.535 cmol kg−1); H3 equal to Loamy sandy soil with average Na concentration (4.535 ≤ [Na] < 6.243 cmol kg−1) and; H4 equal to Loamy sandy soil with high nutrients concentration ([Na] ≥ 6.243 cmol kg−1). Coincidentally, the same habitat types were also generated for the understorey and large trees classes taken separately, although a difference was somehow recorded for the small stems class. Among investigated environmental variables, soil Na content and, to a lesser extent, soil pH were identified as the primary responsible for the first split for habitat types for understorey and large trees classes. By contrast, the most determinant for discrimination habitat types for small stems was soil cation exchange capacity, followed by soil moisture content. Overall, 29 species were found as indicators characterizing the four habitat types for the whole tree community, with index values varying between 33% and 99 %. But specifically, the number of indicators species for large trees was 28. Intriguingly, some indicator species within the large trees class changed to either non-indicator or intermediate indicator species in the understorey trees class that was best represented by as less as 16 species. A comparable species richness was recorded across all plots of the studied area; however, significant variability was found regarding species abundance, with habitats H1 and H4 with higher average abundance values. This study highlights that soil variables adding to topographical habitat filtering were important in shaping the composition of local species.

Stem girth changes in response to soil water potential in lowland dipterocarp forest in Borneo: An individualistic time-series analysis.

Time-series data offer a way of investigating the causes driving ecological processes as phenomena. To test for possible differences in water relations between species of different forest structural guilds at Danum (Sabah, NE Borneo), daily stem girth increments (gthi), of 18 trees across six species were regressed individually on soil moisture potential (SMP) and temperature (TEMP), accounting for temporal autocorrelation (in GLS-arima models), and compared between a wet and a dry period. The best-fitting significant variables were SMP the day before and TEMP the same day. The first resulted in a mix of positive and negative coefficients, the second largely positive ones. An adjustment for dry-period showers was applied. Interactions were stronger in dry than wet period. Negative relationships for overstorey trees can be interpreted in a reversed causal sense: fast transporting stems depleted soil water and lowered SMP. Positive relationships for understorey trees meant they took up most water at high SMP. The unexpected negative relationships for these small trees may have been due to their roots accessing deeper water supplies (if SMP was inversely related to that of the surface layer), and this was influenced by competition with larger neighbour trees. A tree-soil flux dynamics manifold may have been operating. Patterns of mean diurnal girth variation were more consistent among species, and time-series coefficients were negatively related to their maxima. Expected differences in response to SMP in the wet and dry periods did not clearly support a previous hypothesis differentiating drought and non-drought tolerant understorey guilds. Trees within species showed highly individual responses when tree size was standardized. Data on individual root systems and SMP at several depths are needed to get closer to the mechanisms that underlie the tree-soil water phenomena in these tropical forests. Neighborhood stochasticity importantly creates varying local environments experienced by individual trees.

Regularly Planted Rather Than Natural Understory of Norway Spruce (Picea abies H. Karst.) Contributes to the Individual Stability of Canopy Silver Birch (Betula pendula Roth.)

Forest plantations, particularly high-density planted stands, are considered to be more prone to wind damage compared to naturally regenerated stands. The wind resistance (mechanical stability) of plantations can, however, be improved by close-to-natural management, for example, combining pioneer and shade-tolerant species. Presumably, the stability of such stands would be enhanced by the reduced competition of canopy trees and stronger root contacts provided by understory trees, which depend on spatial distribution. In the hemiboreal forest zone, silver birch (Betula pendula Roth.) and Norway spruce (Picea abies (L.) H.Karst.) form such a combination naturally. In this study, the static tree-pulling tests were performed to estimate the mechanical stability of canopy silver birch growing with random Norway spruce understory in naturally regenerated (post-clear-cut) and regularly planted bi-species mixed stands. The regular mixing of the high-density bi-species stand significantly improved the loading resistance of canopy silver birch compared to the naturally regenerated stands of similar composition and age. Such an effect might be related to the stratification of the canopy space between pioneer birch and shade-tolerant spruce, which improved the individual stability of the canopy trees. Further, a regular rooting network of the planted stands likely contributed to the stability by reducing weak spots. Accordingly, the wind resistance of trees in regularly planted bi-species stands might be improved, avoiding additional management.

Estimating height-diameter relations for structure groups in the natural forests of Northeastern China

Tree height-diameter (H-D) models are essential for estimating tree volume and forest production. The development of such models can be especially challenging in multi-species natural forests. We present new generalized mixed-effects models based on a set of 456 sample plots and 77,908 individual tree measurements. Tree species were assigned to one of four structure groups according to their life forms, regeneration modes and stand structure characteristics: (1) Understory tree species with an L-shaped D distribution (UL); (2) Sub-canopy tree species with an L-shaped D distribution (SL); (3) Sub-canopy tree species with a bell-shaped D distribution (SB); and (4) Canopy tree species with a bell-shaped D distribution (CB). A distance-dependent competition index (DCI) was included in the base model of the structure groups, and a mixed-effects model for each group with tree species as random-effect. The four models represent unique allometric relationships for the four structure groups. Model accuracy was improved for all groups after adding DCI. Two sets of mixed-effects H-D models (with or without DCI) are presented to improve the estimates of productivity and carbon stock in the temperate natural forests of Northeastern China. The general approach adopted in this study may serve as an example for similar studies in other multi-species and multi-layered forest stands.

Ecosystem Carbon Stocks and Their Annual Sequestration Rate in Mature Forest Stands on the Mineral Soils of Estonia

Mature forest ecosystems are the most considerable reservoir of organic carbon (OC) among terrestrial ecosystems. The effect of soil type on aboveground OC stocks and their annual increases (AI) of overstorey tree, understorey tree and ground vegetation layers in Estonian forest phytocoenoses with mature stands on mineral soils were studied. The study enfolds nine mineral soil groups, which are characterized by their phytocoenoses composition, soil cover properties and tree stands’ taxation data. An assemblage of soil and plant cover or plant–soil system is the main focus point in explaining causal and quantitative sides of ecosystems functioning. Surface densities of OC stocks in aboveground phytomass of forests varied significantly in the range of 52–100 Mg OC ha−1. High AI or productivity (4.8–5.5 Mg OC ha−1 year−1) is a characteristic of forest ecosystems formed on leached, eluviated and pseudopodzolic soils. Forest ecosystem ground vegetation, which is an important ecological indicator, fulfils vacant ecological niches with herbs and/or mosses (up to 0.50 Mg OC ha−1). The variation of ecosystem OC stocks and their AI by soil type should be taken into account in regional OC stocks and its annual increase estimations.

Ecological and genomic vulnerability to climate change across native populations of Robusta coffee (Coffea canephora).

The assessment of population vulnerability under climate change is crucial for planning conservation as well as for ensuring food security. Coffea canephora is, in its native habitat, an understorey tree that is mainly distributed in the lowland rainforests of tropical Africa. Also known as Robusta, its commercial value constitutes a significant revenue for many human populations in tropical countries. Comparing ecological and genomic vulnerabilities within the species' native range can provide valuable insights about habitat loss and the species' adaptive potential, allowing to identify genotypes that may act as a resource for varietal improvement. By applying species distribution models, we assessed ecological vulnerability as the decrease in climatic suitability under future climatic conditions from 492 occurrences. We then quantified genomic vulnerability (or risk of maladaptation) as the allelic composition change required to keep pace with predicted climate change. Genomic vulnerability was estimated from genomic environmental correlations throughout the native range. Suitable habitat was predicted to diminish to half its size by 2050, with populations near coastlines and around the Congo River being the most vulnerable. Whole-genome sequencing revealed 165 candidate SNPs associated with climatic adaptation in C. canephora, which were located in genes involved in plant response to biotic and abiotic stressors. Genomic vulnerability was higher for populations in West Africa and in the region at the border between DRC and Uganda. Despite an overall low correlation between genomic and ecological vulnerability at broad scale, these two components of vulnerability overlap spatially in ways that may become damaging. Genomic vulnerability was estimated to be 23% higher in populations where habitat will be lost in 2050 compared to regions where habitat will remain suitable. These results highlight how ecological and genomic vulnerabilities are relevant when planning on how to cope with climate change regarding an economically important species.

Optimizing aerial imagery collection and processing parameters for drone‐based individual tree mapping in structurally complex conifer forests

Abstract Recent advances in remotely piloted aerial systems (‘drones’) and imagery processing enable individual tree mapping in forests across broad areas with low‐cost equipment and minimal ground‐based data collection. One such method involves collecting many partially overlapping aerial photos, processing them using ‘structure from motion’ (SfM) photogrammetry to create a digital 3D representation and using the 3D model to detect individual trees. SfM‐based forest mapping involves myriad decisions surrounding methods and parameters for imagery acquisition and processing, but it is unclear how these individual decisions or their combinations impact the quality of the resulting forest inventories. We collected and processed drone imagery of a moderate‐density, structurally complex mixed‐conifer stand. We tested 22 imagery collection methods (altering flight altitude, camera pitch and image overlap), 12 imagery processing parameterizations (image resolutions and depth map filtering intensities) and 286 tree detection methods (algorithms and their parameterizations) to create 7,568 tree maps. We compared these maps to a 3.23‐ha ground reference map of 1,775 trees &gt;5 m tall that we created using traditional field survey methods. The accuracy of individual tree detection (ITD) and the resulting tree maps was generally maximized by collecting imagery at high altitude (120 m) with at least 90% image‐to‐image overlap, photogrammetrically processing images into a canopy height model (CHM) with a twofold upscaling (coarsening) step and detecting trees from the CHM using a variable window filter after applying a moving window mean smooth to the CHM. Using this combination of methods, we mapped trees with an accuracy exceeding expectations for structurally complex forests (for canopy‐dominant trees &gt;10 m tall, sensitivity = 0.69 and precision = 0.90). Remotely measured tree heights corresponded to ground‐measured heights with R2 = 0.95. Accuracy was higher for taller trees and lower for understorey trees and would likely be higher in less dense and less structurally complex stands. Our results may guide others wishing to efficiently produce broad‐extent individual tree maps of conifer forests without investing substantial time tailoring imagery acquisition and processing parameters. The resulting tree maps create opportunities for addressing previously intractable ecological questions and informing forest management.

Sinks for plant surplus carbon explain several ecological phenomena

Plants engage in many processes and relationships that appear to be wasteful of the high-energy compounds that they produce through carbon fixation and photosynthesis. For example, living trees keep leafless tree stumps alive (i.e. respiring) and support shaded understory trees by sharing carbohydrates through root grafts or mycorrhizal fungal networks. Plants exude a diverse array of organic compounds from their roots and leaves, which support abundant rhizosphere and phyllosphere microbiomes. Some plants release substantial amounts of sugar via extra-floral nectaries, which enrich throughfall and alter lichen communities beneath the canopy. Large amounts of photosynthetically fixed carbon are transferred to root associates such as mycorrhizal fungi and N-fixing micro-organisms. Plants also respire fixed C through an alternative pathway that does not generate ATP. Rates of each of these processes appear to be highest when plants are growing under mild-to-moderate deficiencies of nutrients or water. During this stage of deficiency, aboveground plant growth is curtailed more than photosynthesis, causing leaves to produce surplus carbohydrates. Each of the above phenomena provide a sink for these surplus carbohydrates, thereby preventing feedback inhibition of photosynthesis, and perpetuating the influx of C. Because these processes incur little cost to the source plant, they need not provide a benefit beyond the removal of surplus carbohydrates.

Relationship between Canopy Structure and Community Structure of the Understory Trees in a Beech Forest in Japan

Understory trees occupy a spatially heterogeneous light environment owing to light interception by patchily distributed canopy leaves. We examined the spatial distribution of canopy leaves and the spatial structure of the understory tree community (height &lt; 5 m) and their relationships in a beech forest in Nagano, Japan. We measured the canopy leaf area index (LAI) at 10 m intervals (n = 81) in a permanent research plot (1 ha). We established a circular subplot centered on each LAI measurement point, and determined the species composition and the aboveground net primary production of wood (ANPPW) of the understory tree community by using tree size data from an open database in the Monitoring Sites 1000 project. There was a significant negative correlation between canopy LAI and the ANPPW of understory trees and a significant positive correlation between the ANPPW of understory and understory tree density. The dominant species of understory trees differed between subplots with high and low LAI values. Our results suggest that niche differentiation allows trees in the understory community to make use of various light conditions, thereby enhancing the primary productivity of the entire community.

The influence of microhabitat factors on the regeneration and species composition of understory woody plants in Pinus tabuliformis plantations on the Loess Plateau

Chinese pine (Pinus tabuliformis) is one of the main tree species for water and soil conservation on the Loess Plateau. However, most of these plantations face a series of ecological problems, such as low diversity of understory plants and difficulty in natural regeneration. Our study focused on the microhabitat factors that affected the species composition of understory woody plants and the distribution and growth of regeneration in Chinese pine plantations. Plots of 50 × 50 m were randomly established in three typical mature plantations, and then each plot was divided into 100 5 × 5 m microplots to investigate the characteristics of canopy trees, Chinese pine regeneration, other understory woody plants, the light environment, litter and soil. The involved microhabitat factors jointly explained 37.68% of the variation in the species composition of understory woody plants, among which soil total phosphorus (2.66%) and available potassium (2.19%) were the main factors. In the regeneration layer, Chinese pine regeneration (7,261 stems·ha−1, 124.6 cm), Quercus liaotungensis (3,114 stems·ha−1, 117.0 cm) and Betula platyphylla (953 stems·ha−1, 305.7 cm) were considered the canopy replacement species with the most potential. Additionally, we found that the regeneration density was significantly affected by soil total nitrogen, available potassium and shrub density, while the differences in regeneration height growth mainly resulted from the density of other understory trees and shrubs, soil total phosphorus and diffuse non-interceptance. The density of saplings and juvenile trees of Chinese pine was more affected by microhabitat factors than that of seedlings, but the effect of the microenvironment on the former two was less different, indicating that the sapling stage may be the critical period for successful recruitment of the plantations. Therefore, gap creation or thinning is suggested to promote the growth of saplings and juvenile trees, thus increasing the opportunity for them to enter the canopy. At the stand scale, soil resources and interspecies interactions were the dominant microhabitat factors influencing the regeneration of Chinese pine, which inspired us to deeply explore the vegetation-soil-microbial systems and the relationship between regeneration and their neighbors in the future. Moreover, managers should pay more attention to the survival and growth of saplings and juvenile trees to promote the sustainable development of Chinese pine plantations. We believe that these plantations will be transformed into coniferous and broad-leaved mixed forests with high stability and species diversity (e.g., P. tabuliformis × B. platyphylla or P. tabuliformis × Q. liaotungensis).

Metabarcoding of canopy arthropods reveals negative impacts of forestry insecticides on community structure across multiple taxa

Abstract Insecticides used to combat outbreaks of forest defoliators can adversely affect non‐target arthropods. Forestry insecticides typically suppress Lepidoptera larvae which are the cornerstone of the canopy community of deciduous oak forests. The abrupt removal of this dominant component of the food web could have far‐reaching implications for forest ecosystems, yet it is rarely investigated in practice owing to several methodological shortcomings. The taxonomic impediment and the biased nature of arthropod sampling techniques particularly impede the assessment of insecticide impacts on diverse communities. To tackle this issue, we propose an experimental approach combining sampling by pyrethrum knockdown and species determination via DNA metabarcoding, using community subsampling to derive estimates of species abundances. We applied this protocol to investigate the short‐term effects of the insecticides diflubenzuron (DFB) and Bacillus thuringiensis var. kurstaki (BTK) on canopy‐dwelling arthropod communities in German oak woodlands. Our approach allowed us to detect most of the diversity and integrate species abundances in our analyses. By classifying arthropod species into assemblages based on their expected sensitivity rather than coarse taxonomic groupings, we could unveil substantial effects of DFB across multiple taxa 5 weeks after application. Although strong effects on single species appear related to direct toxicity, substantial impacts of DFB on parasitoids and xylophagous beetles suggest that anti‐defoliator treatments can have previously unsuspected indirect effects on some components of forest arthropod communities. The impacts of BTK on community structure were consistent with that of DFB though much weaker. Synthesis and applications. Comparing diversity patterns in the arthropod communities of sprayed and unsprayed oak canopies, our results show that selective insecticides can alter species diversity in presumably non‐sensitive taxa. Even though the ecological significance of these impacts has yet to be assessed in an operational setting, their existence calls for increased regulatory scrutiny on indirect effects. As community approaches become more attainable with the rapid development of DNA metabarcoding, we suggest the inclusion of community‐level end points as regulatory requirements for the approval of forest‐use insecticides.

Leaf trait expression varies with tree size and ecological strategy in a subtropical forest

Abstract How the dramatic vertical environmental gradients in closed canopy forests shape intraspecific variation in the functional traits of tree species and their ecological strategies is not well understood. In an Asian subtropical forest, we tested the hypothesis that, because species’ maximum height and shade tolerance determine the lifetime environmental variation of a tree, they should be correlated with the magnitude of intraspecific variation in leaf traits and how strongly it depends on tree size. We collected data on three vertical environmental variables, tree size and four leaf traits of 3,880 trees of 24 species in a subtropical forest. Air temperature and relative humidity displayed linear, and insolation displayed nonlinear, variation with height above‐ground. Intraspecific trait variation (ITV) varied significantly among species, for all but one understorey tree species, and also varied with tree size for at least one trait. Many trait‐size relationships were nonlinear with inflection points near the height where insolation dramatically increased. While ITV did not correlate with species’ maximum height nor shade tolerance, the amount of ITV explained by size (SDTV) and the rate of change in trait expression with size did. Greater ITV is thought to be associated with greater environmental heterogeneity, and yet strong evidence supporting this has not always been found. Our findings shed new light on how trait plasticity is phenotypically integrated with tree species’ ecological strategy by pointing to the importance of accounting for tree size, since SDTV, rather than ITV, was associated with strategy variation in maximum height and shade tolerance. Our study improves understanding of tree size's effect on leaf trait expression, and implies that SDTV is not only a key mechanism promoting interspecific variation in tree stature, contributing to species coexistence via vertical niche partitioning, but is also likely to influence the effects of climate change on forests by constraining tree responses to vertical environmental gradients. A free Plain Language Summary can be found within the Supporting Information of this article.

Does natural stand dynamics generate an aggregated pattern of canopy openness? A case study in Abies-Picea-Fagus old-growth forests

The action of many agents causing the mortality of overstory trees may potentially induce the formation of an aggregated distribution of canopy gaps. In this study we tested the hypothesis that natural stand dynamics generates an aggregated pattern of canopy openness in old-growth forests formed by Fagus sylvatica L., Abies alba Mill. and Picea abies (L.) H. Karst. We compared canopy openness and its spatial heterogeneity in five stands in the Western Carpathians (Central Europe) and three stands in the Dinaric Mountains (Southeast Europe). The stands were between 4.48 and 9.24 ha in size. In each stand we took hemispheric photographs in a regular 20 × 20 m grid in the leafless period to minimize coverage by the understory. Tree species and the dbh of live trees of dbh ≥7 cm were recorded on circular plots with a radius of 7 m centered at the grid points. At the stage of picture processing, understory trees of dbh <25 cm and polar coordinates recorded during field measurements were removed from the pictures, and for every grid point the local canopy openness of the overstory layer was determined in the sky region defined by a zenith angle of 15°. We characterized the spatial pattern of canopy openness by using variograms and Moran’s I coefficients and tested the spatial dependence in the distribution patterns of grid points characterized by different levels of canopy openness. Depending on the stand, mean canopy openness varied between 17.5 and 41.0%, with the greatest values recorded in the Carpathian stands with a considerable proportion of Norway spruce and a more severe disturbance regime. Relationships between canopy openness and the overall number, basal area or volume of trees forming the overstory were strongly modified by variation in the density and species percentage in the mid-canopy zone. In all the stands small-scale variation determined for inter-point distances of 20 m accounted for the majority of the total variation in canopy heterogeneity and ranged between 61 and 100%. We found a tendency to form random patterns of canopy openness in the Dinaric stands, which were characterized by a greater basal area and probably also lower frequency of severe disturbances, and aggregated patterns in the Carpathian stands, which were characterized by a smaller basal area driven by more severe disturbances. The revealed spatial dependence in canopy openness may suggest that in the studied ecosystem canopy gaps are not only a legacy of the mortality of canopy trees but also self-organizing structural elements which, under a more severe disturbance regime, can affect the mortality rate in their neighbourhood. Nonetheless, the percentage of spatially structured variability in canopy openness observable at scales larger than the grid spacing used in our study was insignificant (83% on average). The close-to-random pattern of canopy heterogeneity may weaken the spatio-temporal synchronization of the juvenile growth, maturation and senescence of neighbouring trees and counteract the formation of coarse-grained patch mosaics.

Towards low vegetation identification: A new method for tree crown segmentation from LiDAR data based on a symmetrical structure detection algorithm (SSD)

Obtaining low vegetation data is important in order to quantify the structural characteristics of a forest. Dense three-dimensional (3D) laser scanning data can provide information on the vertical profile of a forest. However, most studies have focused on the dominant and subdominant layers of the forest, while few studies have tried to delineate the low vegetation. To address this issue, we propose a framework for individual tree crown (ITC) segmentation from laser data that focuses on both overstory and understory trees. The framework includes 1) a new algorithm (SSD) for 3D ITC segmentation of dominant trees, by detecting the symmetrical structure of the trees, and 2) removing points of dominant trees and mean shift clustering of the low vegetation. The framework was tested on a boreal forest in Sweden and the performance was compared 1) between plots with different stem density levels, vertical complexities, and tree species composition, and 2) using airborne laser scanning (ALS) data, terrestrial laser scanning (TLS) data, and merged ALS and TLS data (ALS + TLS data). The proposed framework achieved detection rates of 0.87 (ALS + TLS), 0.86 (TLS), and 0.76 (ALS) when validated with field-inventory data (of trees with a diameter at breast height ≥ 4 cm). When validating the estimated number of understory trees by visual interpretation, the framework achieved 19%, 21%, and 39% root-mean-square error values with ALS + TLS, TLS, and ALS data, respectively. These results show that the SSD algorithm can successfully separate laser points of overstory and understory trees, ensuring the detection and segmentation of low vegetation in forest. The proposed framework can be used with both ALS and TLS data, and achieve ITC segmentation for forests with various structural attributes. The results also illustrate the potential of using ALS data to delineate low vegetation.

Complementary allometric model of understory tree biomass in the semi-deciduous rainforest of Cameroon

Forest understorey contains remarkable plant diversity, contributing to the heterogeneous environments for other biotic gatherings and soil supplement aggregations. Nonetheless, the biomass of the understorey vegetation is neglected because of the lack of appropriate allometric equations without which there are uncertainties in biomass estimation. This study was aimed at developing multispecies allometric equations that will be used to estimate the aboveground biomass of different compartments (trunk, crown, and leaves) of understorey trees in the semi-deciduous rainforest of Cameroon. Understorey tree diameter (1.0-10.0 cm), height, and crown diameter were measured on 1023 trees as biomass predictors. The results showed that the fit of the model improves with more predictive variables, four of which were considered in all studied compartments (trunk, crown, and leaves). Existing specific and pantropic allometries based on diameter tend to overestimate the aboveground biomass of understorey trees when compared to the allometry developed in this study. This study highlighted the importance of a specific aboveground biomass allometric equation for understorey trees. Furthermore, the multispecies allometric equation developed for understorey trees complements those recently developed for overstorey trees, thereby contributing to the total aboveground woody biomass estimation in semi-deciduous forest in the Congo Basin.