Temperate Forest Tree Species Research Articles

The synergistic effect of hydraulic and thermal impairments accounts for the severe crown damage in Fraxinus mandshurica seedlings following the combined drought-heatwave stress

Drought combined with extreme heatwaves has been increasingly identified as the important trigger of worldwide tree mortality in the context of climate change; nonetheless, our understanding of the potential hydraulic and thermal impairments of hot droughts to trees and the subsequent post-recovery process remains limited. To investigate the response of tree water and carbon relations to drought, heatwave, and combined drought-heatwave stresses, three-year-old potted seedlings of Fraxinus mandshurica Rupr., a dominant tree species in temperate forests of northeast China, were grown under well-watered and drought-stressed conditions and exposed to a rapid, acute heatwave treatment. During the heatwave treatment with a maximum temperature exceeding 40 °C for two days, the leaf temperature of drought-stressed seedlings was, on average, 5 °C higher than that of well-watered counterparts due to less effective evaporative cooling, indicating that soil water availability influenced leaf thermoregulatory capacity during hot extremes. Consistently, more pronounced crown damage, as shown by 13 % irreversible leaf scorch, was found in seedlings under the drought-heatwave treatment relative to sole heatwave treatment, alongside the more severe stem xylem embolism and leaf electrolyte leakage. While the heatwave treatment accelerated the depletion of non-structural carbohydrates in drought-stressed seedlings, the increase of branch soluble sugar concentration in response to heatwave might be related to the requirement for maintaining hydraulic functioning via osmoregulation under high dehydration risk. The coordination between leaf stomatal conductance and total non-structural carbohydrate content during the post-heatwave recovery phase implied that plant-water relations and carbon physiology were closely coupled in coping with hot droughts. This study highlights that, under scenarios of aggravating drought co-occurring with heatwaves, tree seedlings could face a high risk of crown decline in relation to the synergistically increased hydraulic and thermal impairments.

Mycorrhizal symbiosis pathway and edaphic fertility frame root economics space among tree species.

The root economics space (RES) is multidimensional and largely shaped by belowground biotic and abiotic influences. However, how root-fungal symbioses and edaphic fertility drive this complexity remains unclear. Here, we measured absorptive root traits of 112 tree species in temperate and subtropical forests of China, including traits linked to functional differences between arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) hosts. Our data, from known mycorrhizal tree species, revealed a 'fungal-symbiosis' dimension distinguishing AM from ECM species. This divergence likely resulted from the contrasting mycorrhizal evolutionary development of AM vs ECM associations. Increased root tissue cortical space facilitates AM symbiosis, whereas increased root branching favours ECM symbiosis. Irrespective of mycorrhizal type, a 'root-lifespan' dimension reflecting aspects of root construction cost and defence was controlled by variation in specific root length and root tissue density, which was fully independent of root nitrogen content. Within this function-based RES, we observed a substantial covariation of axes with soil phosphorus and nitrate levels, highlighting the role played by these two axes in nutrient acquisition and conservation. Overall, our findings demonstrate the importance of evolved mycorrhizal symbiosis pathway and edaphic fertility in framing the RES, and provide theoretical and mechanistic insights into the complexity of root economics.

Herbivore‐induced branching increases sapling survival in temperate forest canopy gaps

Abstract Tree architectures reflect the main abiotic and biotic selection pressures determining tree growth and survival. Studies have shown that trees growing in herbivore‐dominated ecosystems, such as savannas, develop denser, more divaricate ‘cage’‐like architectures in response to chronic browsing pressure (also known as ‘brown world’ architectures). In contrast, trees growing under resource‐limited conditions develop traits that allow them to better compete for abiotic resources (referred to as ‘green world’ architectures); for example, light limitation can induce trees to form more pole‐like structures that allow them to grow faster in height. In temperate forests, abiotic drivers have traditionally been assumed to be the main factors affecting plant architecture. However, gap dynamics typically characterize old‐growth temperate forests, where disturbances such as storms create gaps with temporary high‐light availability, but also attracting intense mammalian browsing. Under such conditions, one would expect tree saplings to exhibit high architectural plasticity enabling them to develop a ‘cage’ in response to high herbivore pressure, but when herbivore pressure declines switch to a more pole‐like structure that allows them to grow quickly in height and compete for light. We tested the ability of five dominant European tree species to induce a ‘cage’ architecture (hereafter referred to as ‘cage plasticity’) under contrasting light and herbivory regimes in a full‐factorial experiment in Białowieża Forest. We planted 720 young tree saplings in six replicated blocks in which we manipulated ungulate browsing (exclosure vs. control) and light levels (gap vs. closed forest). Two out of the five species (hornbeam Carpinus betulus and lime Tilia cordata) were able to develop a cage when exposed to ungulate herbivores but only under high‐light conditions. These two species had the highest survival rates in the herbivory treatment over a 10‐year period. Synthesis. Cage plasticity in temperate forest tree species may reflect an overlooked adaptation to mammal herbivory originating in a past when large mammals exerted strong selection pressures in European ecosystems. Our findings challenge the view of closed‐canopy forest communities being solely driven by abiotic drivers and call for a re‐evaluation of the importance of large herbivores in shaping the functional traits of temperate tree species.

Tree-to-Shrub Shift Benefits the Survival of Quercus mongolica Fisch. ex Ledeb. at the Xeric Timberline

Woody species are either trees or shrubs, with the exception of approximately 9.2% “trub” species exhibiting both tree and shrub growth forms. Little is known thus far about the ecological importance of plant growth-form plasticity under a drying climate. Quercus mongolica Fisch. ex Ledeb., a dominant tree species of temperate forests in East Asia, typically shows tree-to-shrub growth-form shifts at the xeric timberline and is suitable to test whether growth-form plasticity can physiologically benefit plant drought acclimation. We quantified the tree architecture, drought stress, physiological drought tolerance, and nonstructural carbohydrates of Q. mongolica trees and shrubs on the opposite slope of the same mountain at the xeric timberline in July 2018. Compared with Q. mongolica trees on shady slopes, the shrubs on sunny slopes had less available water to use and were more severely threatened by drought. Moreover, the shrubs had stronger tolerance to drought but still tended to have less nonstructural carbohydrate accumulation. The tree-to-shrub shift effectively benefits the survival of Q. mongolica under a dry climate and may strongly contribute to forest dynamics and even fire regimes under climate drying, especially for sensitive ecosystems such as the xeric timberline, which will be vulnerable under future climate change scenarios.

PINUS SYLVESTRIS L. REGENERATION FROM SEED TREES IN VACCINIOSA AND MYRTILLOSA

Scots pine (Pinus sylvestris L.) is an economically important tree species in temperate forests, including Latvia. Purposeful selection work has resulted in the provision of faster-growing planting material for forest planting. Natural restoration initially requires less investment, it has several disadvantages. When using this method, it must be considered that the forest stands will have an uneven species composition, the age and height of the trees will differ. The study was carried out in young naturally regenerated stands in Vacciniosa and Myrtillosa forest site type and compared with planted stands The aim of the study was to evaluate the natural regeneration and last 5 year tree height increment of Scots pine regenerated from the seed trees in the Vacciniosa and Myrtillosa, compared to planted stands, as well as to assess the impact of tree health on pine height increment. In total 10 young stands were surveyed, 66 sample plots were established and 704 trees were measured. The area of the temporary sample plot was 25 m2 with a radius of 2.82 m. Comparing the annual height increases of Scots pine, both in naturally regenerated and planted young stands in Vacciniosa and Myrtillosa, it was found that there are statistically significant differences between forest types, stand origin type, as well as in existing stands in the last 5 years by years (p <0.05). The most significant damage to young pine stands is caused by needle cast, ungulates, and large pine weevil.

Heat-stress response of photosystem II in five ecologically important tree species of European temperate forests

Heat-stress response of photosystem II in five ecologically important tree species of European temperate forests

Mapping temperate forest tree species using dense Sentinel-2 time series

Precise information on tree species composition is critical for forest management and conservation, but mapping tree species with satellite data over large areas is still a challenge. Since 2017, Sentinel-2A/B provide multi-spectral time series with global coverage at an unprecedented spatial and temporal resolution. This is a new opportunity for mapping tree species over large areas that has not yet been fully explored. Because of the high spatial and temporal resolution, Sentinel-2 time series improve the characterization of vegetation phenology and canopy structure, parameters that are intrinsically linked to tree species. The objective of this study was to test the utility of a Sentinel-2 time-series based approach for mapping tree species in a temperate forest region in Central Europe. Using stand-wise forest inventory data for single species stands we assess how well main and minor tree species can be mapped, and if the addition of environmental variables and spatial texture metrics improves the classification accuracy. Our time series approach utilizes all available Sentinel-2 observations and an ensemble of radial basis convolution filters to build cloud-free 5-day time series for each spectral band. The time series are then used as input features to classify seventeen tree species.Our results show the potential of Sentinel-2 time-series based classification, but they also show the challenges associated with mapping a diverse portfolio of tree species. Accuracy of the nine main species, with an area proportion greater than 0.5%, ranged between 98.9% and 66.8%, which is promising for a large area. Adding detailed environmental data and texture metrics to the spectral model only marginally increased the accuracy of a few minor tree species. Overall, the eight minor tree species with area proportions less than 0.5% were most strongly affected by classification errors. Although the absolute mapped area of minor species correlated well with the estimated reference area, the small class areas of minor species lead to high classification errors in relative terms. Mapping minor tree species is challenging for statistical reasons (i.e., class imbalance, small sample size and class variance). Using all available Sentinel-2 data allows building dense time series at high spatial resolution that are mandatory for improved tree species mapping. We were able to show that the spectral time series is the prime explanatory information, even when complementing our analyses with texture information and various environmental data. The results suggest that with the applied data harmonization approach precise regional tree species mapping is feasible.

Ectomycorrhizal Inoculation Enhances the Salt Tolerance of Quercus mongolica Seedlings.

Salt stress harms the growth and development of plants, and the degree of soil salinization in North China is becoming increasingly severe. Ectomycorrhiza (ECM) is a symbiotic system formed by fungi and plants that can improve the growth and salt tolerance of plants. No studies to date have examined the salt tolerance of Quercus mongolica, a typical ectomycorrhizal tree species of temperate forests in the northern hemisphere. Here, we inoculated Q. mongolica with two ectomycorrhizal fungi (Gomphidius viscidus; Suillus luteus) under NaCl stress to characterize the effects of ECM. The results showed that the symbiotic relationship of Q. mongolica with G. viscidus was more stable than that with S. luteus. The cross-sectional area of roots increased after inoculation with the two types of ectomycorrhizal fungi. Compared with the control group, plant height, soluble sugar content, and soluble protein content of leaves were 1.62, 2.41, and 2.04 times higher in the G. viscidus group, respectively. Chlorophyll (Chl) content, stomatal conductance (Gs), and intracellular CO2 concentration (Ci) were significantly higher in Q. mongolica inoculated with ectomycorrhizal fungi than in the control, but differences in the net photosynthetic rate (Pn), transpiration rate (Tr), and photosystem II maximum photochemical efficiency (Fv/Fm) were lower. The relative conductivity of Q. mongolica inoculated with the two ectomycorrhizal fungi was consistently lower than that of non-mycorrhizal seedlings, with the effect of G. viscidus more pronounced than that of S. luteus. The malondialdehyde (MDA) content showed a similar pattern. Peroxidase (POD) and catylase (CAT) levels in mycorrhizal seedlings were generally higher than those of non-mycorrhizal seedlings under normal conditions, and were significantly higher than those of non-mycorrhizal seedlings on the 36th and 48th day after salt treatment, respectively. Overall, the results indicated that the salt tolerance of Q. mongolica seedlings was improved by ectomycorrhizal inoculation.

Approaches of <I>De novo</i> Assembly, Annotation and RNA-Seq Profiling in Forest Tree Improvement: A Review

Transcriptome analysis (RNA-seq) using next generation sequencing has allowed the researchers to simultaneously identify gene expression dynamics and differential gene expressions. This technique has a better approach to understand the biological functions of plants/trees. RNA-seq also gives information that how genes are regulated and the assembled sequence data help in designing biomarkers for unknown or new species. In forestry species, it has enabled the study of how gene expression changes in several trees due to stress or other factors. Therefore, RNAseq analysis plays a key role in advancing genomic and molecular biology research in forest trees. Due to lack of deep genomic background of forest trees, transcriptomes for most species need to be assembled de novo i.e., without reference. In the present review, we have reported and assembled reference RNA-seq data available for tropical and temperate forest tree species for the first time. This article also focuses on the terminologies and steps used for whole transcriptome analysis system, since from the collection of plant material, to extraction of total RNA, library preparation, transcripts, coding sequences, unigenes, functional annotation, gene ontology, differential expressed genes (DEGs), gene expression values and functional metabolic pathways analysis.

Ecological genetics of Juglans nigra: Differences in early growth patterns of natural populations.

Many boreal and temperate forest tree species distributed across large geographic ranges are composed of populations adapted to the climate they inhabit. Forestry provenance studies and common gardens provide evidence of local adaptation to climate when associations between fitness traits and the populations' home climates are observed. Most studies that evaluate tree height as a fitness trait do so at a specific point in time. In this study, we elucidate differences in early growth patterns in black walnut (Juglans nigra L.) populations by modeling height growth from seed up to age 11. The data comprise tree height measurements between ages 2 and 11 for 52 natural populations of black walnut collected through its geographic range and planted in one or more of 3 common gardens. We use the Chapman–Richards growth model in a mixed effects framework and test whether populations differ in growth patterns by incorporating populations' home climate into the model. In addition, we evaluate differences in populations' absolute growth and relative growth based on the fitted model. Models indicated that populations from warmer climates had the highest cumulative growth through time, with differences in average tree height between populations from home climates with a mean annual temperature (MAT) of 13°C and of 7°C estimated to be as high as 80% at age 3. Populations from warmer climates were also estimated to have higher and earlier maximum absolute growth rate than populations from colder climates. In addition, populations from warm climates were predicted to have higher relative growth rates at any given tree size. Results indicate that natural selection may shape early growth patterns of populations within a tree species, suggesting that fast early growth rates are likely selected for in relatively mild environments where competition rather than tolerance to environmental stressors becomes the dominant selection pressure.

Tree species rather than type of mycorrhizal association drive inorganic and organic nitrogen acquisition in tree–tree interactions

Mycorrhizal fungi play an important role for the nitrogen (N) supply of trees. The influence of different mycorrhizal types on N acquisition in tree-tree interactions is, however, not well understood, particularly with regard to the competition for growth-limiting N. We studied the effect of competition between temperate forest tree species on their inorganic and organic N acquisition in relation to their mycorrhizal type (i.e., arbuscular mycorrhiza or ectomycorrhiza). In a field experiment, we quantified net N uptake capacity from inorganic and organic N sources using 15N/13C stable isotopes for arbuscular mycorrhizal tree species (i.e., Acer pseudoplatanus L., Fraxinus excelsior L., and Prunus avium L.) as well as ectomycorrhizal tree species (i.e., Carpinus betulus L., Fagus sylvatica L., and Tilia platyphyllos Scop.). All species were grown in intra- and interspecific competition (i.e., monoculture or mixture). Our results showed that N sources were not used complementarily depending on a species' mycorrhizal association, but their uptake rather depended on the competitor, indicating species-specific effects. Generally, ammonium was preferred over glutamine and glutamine over nitrate. In conclusion, our findings suggest that the inorganic and organic N acquisition of the studied temperate tree species is less regulated by mycorrhizal association but rather by the availability of specific N sources in the soil as well as the competitive environment of different tree species.

Sensitivity and threshold dynamics of Pinus strobus and Quercus spp. in response to experimental and naturally occurring severe droughts.

Increased drought frequency and severity are a pervasive global threat, yet the capacity of mesic temperate forests to maintain resilience in response to drought remains poorly understood. We deployed a throughfall removal experiment to simulate a once in a century drought in New Hampshire, USA, which coupled with the region-wide 2016 drought, intensified moisture stress beyond that experienced in the lifetimes of our study trees. To assess the sensitivity and threshold dynamics of two dominant northeastern tree genera (Quercus and Pinus), we monitored sap flux density (Js), leaf water potential and gas exchange, growth and intrinsic water-use efficiency (iWUE) for one pretreatment year (2015) and two treatment years (2016-17). Results showed that Js in pine (Pinus strobus L.) declined abruptly at a soil moisture threshold of 0.15m3m-3, whereas oak's (Quercus rubra L. and Quercus velutina Lam.) threshold was 0.11 m3m-3-a finding consistent with pine's more isohydric strategy. Nevertheless, once oaks' moisture threshold was surpassed, Js declined abruptly, suggesting that while oaks are well adapted to moderate drought, they are highly susceptible to extreme drought. The radial growth reduction in response to the 2016 drought was more than twice as great for pine as for oaks (50 vs 18%, respectively). Despite relatively high precipitation in 2017, the oaks' growth continued to decline (low recovery), whereas pine showed neutral (treatment) or improved (control) growth. The iWUE increased in 2016 for both treatment and control pines, but only in treatment oaks. Notably, pines exhibited a significant linear relationship between iWUE and precipitation across years, whereas the oaks only showed a response during the driest conditions, further underscoring the different sensitivity thresholds for these species. Our results provide new insights into how interactions between temperate forest tree species' contrasting physiologies and soil moisture thresholds influence their responses and resilience to extreme drought.

Higher temperatures increase growth rates of Rocky Mountain montane tree seedlings

AbstractRecent observational studies report weak or flat temperature − growth relationships for many tree species in temperate forests. In contrast, distribution limits of trees are strongly shaped by temperature, and studies show marked short‐term temperature effects on leaf‐level ecophysiology. To better determine the effects of warming on trees, we planted one‐year‐old seedlings of one lower montane (ponderosa pine), two upper montane (quaking aspen and lodgepole pine), and one subalpine tree species (subalpine fir) in in situ experimental gardens on an elevation gradient in the Rocky Mountains (USA) which span a 6°C range in temperature but have approximately uniform precipitation. Seedlings were lightly watered the first three growing seasons to facilitate establishment, and growth and survivorship were followed for four years. We expected a trade‐off between growth and survivorship, as seedlings in high temperatures grow faster (e.g., with a longer growing season), but have higher mortality from heat stress. Compared to the coldest site, aspen (+256% wider, +337% taller), ponderosa pine (+234% wider, 270% taller), and lodgepole pine (+235% wider, 283% taller) all had strikingly higher cumulative diameter and height growth in the warmest site by the end of the study. Linear models of cumulative and annual growth in the montane species showed strong, positive relationships with growing‐season temperature, but no significant relationships with growing‐season precipitation. In contrast, growth of subalpine fir did not vary significantly with temperature, but increased slightly with higher growing‐season precipitation. Accelerated growth did not come at the expense of survivorship in the montane species: cumulative four‐year survivorship of the montane species remained robust (71.4–94.4%) in high temperatures, but caused complete mortality of subalpine fir. As long as precipitation remains adequate, these results indicate that warming is likely to strongly increase growth in seedlings of montane species with only modest decreases in survivorship despite higher evapotranspiration, especially in cooler and wetter portions of their current distributions where hydric stress is low. In contrast, warming may negatively affect seedling growth and survival in hotter and drier areas of the Rockies, and warming of +3–6°C may endanger the persistence of subalpine fir over much of its current distribution.

Overyielding in young tree communities does not support the stress‐gradient hypothesis and is favoured by functional diversity and higher water availability

Abstract Biodiversity effects on productivity and other ecosystem functions are strongly dependent on climate and resource availability. Based on the stress‐gradient hypothesis, under conditions of greater abiotic stress, diversity effects on plant performance are intensified due to the increased relative importance of positive plant interactions. However, whether this hypothesis is consistently applicable in forest systems remains unclear. A field trial was established to test the stress‐gradient hypothesis and examine diversity effects on above‐ground biomass production of young trees in mixtures exposed to different water availability. Six native tree species of northern temperate forests (Acer saccharum, Betula papyrifera, Larix laricina, Picea glauca, Pinus strobus and Quercus rubra) were planted as monocultures and as mixtures of two, four and six species. For five growing seasons, four replicates of each community were exposed to conditions of either low‐ or high‐water availability created by rainfall exclusion and weekly irrigation, respectively. Growth‐years 4 and 5 were significantly different when the climatic water balance of the growing seasons was compared. We tested the effects of functional diversity on: (a) total growth of mixtures under low‐ and high‐water availability, and (b) annual growth in years 4 (higher water availability, 2017) and 5 (lower water availability, 2018). Annual growth of most species in both years was greater under high‐ versus low‐water availability. Functional diversity had a significant positive effect on total biomass production and annual growth, and this effect was more strongly expressed under high‐water availability. Functional diversity effects on annual growth did not differ between years 4 and 5 regardless of their climatic water balance. Functional and species identity were key to understanding productivity responses to mixture and treatment effects. Synthesis. Contrary to the stress‐gradient hypothesis, the positive effects of functional diversity on productivity were enhanced by high‐water availability and were independent of seasonal water balance.

Aboveground herbivores drive stronger plant species-specific feedback than belowground fungi to regulate tree community assembly.

Ectomycorrhizal (EcM) tree species often become more dominant than arbuscular mycorrhizal (AM) tree species in temperate forests, but they generally coexist. Theory predicts that ecological feedback mediated by aboveground herbivory and/or belowground microbes could explain these dominance/coexistence patterns. An experimental test of how aboveground/belowground organisms associated with AM/EcM trees mediate ecological feedbacks has been lacking at the community-level. By establishing AM and EcM tree sapling assemblages in mesocosms and then introducing seedlings of each type in areciprocal planting experiment, we compared seedling performance under varying sapling species (conspecifics, heterospecifics within the same and different mycorrhizal types), using traits that reflect either aboveground herbivory-mediated feedback or belowground fungal-mediated feedback or both. When examining seedling traits that reflect aboveground herbivory-mediated feedbacks (i.e., foliar damage), AM plants tended toexperience less foliar damage and EcM plants more damage under conspecific versus heterospecific saplings within the same mycorrhizal types, and aboveground herbivory-mediated feedback was species-specific rather than mycorrhizal type-specific. Conversely, when examining traits that reflect belowground fungal-mediated feedbacks, both AM and EcM plant species often exhibited mycorrhizal type-specific feedbacks (e.g., greater aboveground biomass under the same versus different mycorrhizal-type saplings) rather than species-specific feedbacks. Furthermore, tree species affected by herbivory-mediated feedback were less affected by belowground feedback, indicating that the relative importance of the feedbacks varied among plant species. Analysis of plant-associated organisms verified that the feedback outcomes corresponded with species accumulation of belowground fungi (but not of abovegroundherbivores). Thus, aboveground herbivores drive stronger plant species-specific feedback than belowground fungi to regulate temperate tree diversity.

Effect of tree mixture on Collembola diversity and community structure in temperate broadleaf and coniferous forests

Springtails (Collembola) are the most abundant arthropods in terrestrial ecosystems and, are considered as key indicators of organic matter turnover and soil functioning. Mixture of tree species are often regarded as a mean to improve tree growth, soil fertility and biodiversity.We compared α-diversity, taxonomic β-diversity and functional diversity of Collembola of mixed forest stands to pure stands in two forest sites, a mountain and a lowland site composed of a coniferous and a deciduous species for effect on. We choose sessile oak (Quercus petraea) and Scot pine (Pinus sylvestris) in lowland, and beech (Fagus sylvatica) and silver fir (Abies alba) in mountain stands.In total 41 species Collembola were identified. We showed that richness and abundance in mixed stands were in between those found in the pure stands, with a more pronounced response of the soil fauna in lowland compared to mountain. In the lowland, Shannon diversity index followed the same pattern, and we found species richness from 6.3 to 11.7 mean species, and 4400 to 9000 ind.m−2, dominated by epedaphic group. In the mountain, we found species richness from 7 to 9 mean species, and 6600 to 103,000 ind.m−2, dominated by euedaphic group.Among the 12 soil and litter characteristics, many differs between sites and/or stand type. The best predictors of the model explaining differences in mean Collembola were litter chemical composition including the lignin to N ratio and C to N ratio. Soil characteristics, such as humus index, organic layer thickness or pH, was also a good predictors for some life-forms and one or the other site.In addition, mixture modified Collembola community structure with some species found only in the pure stands. Jaccard similarity index showed that mixture, even composed of different tree species, homogenized Collembola community structure.We conclude that mixture of tree species in temperate forests can locally increase Collembola diversity, but this management should not be generalized to maximize the β-diversity.

Mapping forest tree species in high resolution UAV-based RGB-imagery by means of convolutional neural networks

The use of unmanned aerial vehicles (UAVs) in vegetation remote sensing allows a time-flexible and cost-effective acquisition of very high-resolution imagery. Still, current methods for the mapping of forest tree species do not exploit the respective, rich spatial information. Here, we assessed the potential of convolutional neural networks (CNNs) and very high-resolution RGB imagery from UAVs for the mapping of tree species in temperate forests. We used multicopter UAVs to obtain very high-resolution (<2 cm) RGB imagery over 51 ha of temperate forests in the Southern Black Forest region, and the Hainich National Park in Germany. To fully harness the end-to-end learning capabilities of CNNs, we used a semantic segmentation approach (U-net) that concurrently segments and classifies tree species from imagery. With a diverse dataset in terms of study areas, site conditions, illumination properties, and phenology, we accurately mapped nine tree species, three genus-level classes, deadwood, and forest floor (mean F1-score 0.73). A larger tile size during CNN training negatively affected the model accuracies for underrepresented classes. Additional height information from normalized digital surface models slightly increased the model accuracy but increased computational complexity and data requirements. A coarser spatial resolution substantially reduced the model accuracy (mean F1-score of 0.26 at 32 cm resolution). Our results highlight the key role that UAVs can play in the mapping of forest tree species, given that air- and spaceborne remote sensing currently does not provide comparable spatial resolutions. The end-to-end learning capability of CNNs makes extensive preprocessing partly obsolete. The use of large and diverse datasets facilitate a high degree of generalization of the CNN, thus fostering transferability. The synergy of high-resolution UAV imagery and CNN provide a fast and flexible yet accurate means of mapping forest tree species.

Responses of functional traits to seven-year nitrogen addition in two tree species: coordination of hydraulics, gas exchange and carbon reserves.

Atmospheric nitrogen (N) deposition has been observed to impact plant structure and functional traits in terrestrial ecosystems. Although the effect of N deposition on plant water use has been well-evaluated in laboratories and in experimental forests, the linkages between water and carbon relations under N deposition are unclear. Here, we report on hydraulics, gas exchange and carbon reserves of two broad-leaved tree species (Quercus mongolica and Fraxinus mandshurica) in mature temperate forests after a seven-year experiment with different levels of N addition (control (CK), low (23kgNha-1yr-1), medium (46kgNha-1yr-1) and high (69kgNha-1yr-1)). We investigated variation in hydraulic traits (xylem-specific hydraulic conductivity (Ks), native percentage loss of conductivity (PLC) and leaf water potential), xylem anatomy (vessel diameter and density), gas exchange (maximum net photosynthesis rate and stomatal conductance) and carbon reserves (soluble sugars, starch and total nonstructural carbohydrates (NSC)) with different N addition levels. We found that medium N addition significantly increased Ks and vessel diameter compared to control, but accompanied increasing PLC and decreasing leaf water potential, suggesting that N addition results in a greater hydraulic efficiency and higher risk of embolism. N addition promoted photosynthetic capacity via increasing foliar N concentration but did not change stomatal conductance. In addition, we found increase in foliar soluble sugar concentration and decrease in starch concentration with N addition, and positive correlations between hydraulic traits (vessel diameter and PLC) and soluble sugars. These coupled responses of tree hydraulics and carbon metabolism are consistent with a regulatory role of carbohydrates in maintaining hydraulic integrity. Our study provides an important insight into the relationship of plant water transport and carbon dynamics under increasing N deposition.

Predicting litter decomposition rate for temperate forest tree species by the relative contribution of green leaf and litter traits in the Indian Himalayas region

We examined eight green leaf traits (leaf area, specific leaf area, leaf dry matter, leaf water content, leaf carbon, nitrogen, phosphorous, and chlorophyll) and eight litter traits (cellulose, lignin, litter carbon, nitrogen, phosphorous, lignin: nitrogen, carbon: nitrogen and ash content) and the litter decomposition rates of 10 tree species from a temperate forest ecosystem. Amongst all green leaf and litter traits tested, cellulose content, lignin content and specific leaf area were identified as the best predictors of litter decomposition, with percentage contributions to litter decomposition of 40%, 36% and 8%, respectively. The results indicated that litter decomposition in the temperate forest ecosystem studied was not dependent solely on litter traits, as green leaf traits also contributed to nutrient cycling. Individual green leaf traits of living species and dead leaf characteristics both influenced the fate of leaves, affecting decomposition and nutrient and carbon cycling in the forest ecosystem. For a better overall understanding of the decomposition mechanisms operating in a temperate forest, both green leaf traits and litter traits thus need to be considered.

Effects of insufficient chilling on budburst and growth of six temperate forest tree species in Ontario

Insufficient winter chilling related to rising temperatures has raised questions about its potential effects on budburst timing (growth initiation) and subsequent tree growth. We quantified budburst timing and growth of seedlings of six Ontario temperate forest tree species after exposure to different levels of winter chilling using controlled forcing experiments. Based on chilling–forcing relationships, chilling requirements for the species tested ranged from 1000 to 1500 weighted chilling hours. Our findings showed that the species’ chilling needs ranked from highest to lowest were yellow birch (Betula alleghaniensis), eastern larch (Larix laricina), eastern white pine (Pinus strobus), red pine (Pinus resinosa), red spruce (Picea rubens), and eastern white cedar (Thuja occidentalis). Species that needed more chilling to release dormancy generally required less cumulative forcing for budburst. The variations in chilling–forcing relationships among species suggest that they would respond differently to temperature changes, resulting in year to year variation in their relative phenology. Insufficient chilling increased cumulative forcing needed for budburst but only extreme chilling deficiency (< 700 weighted chilling hours) caused abnormal budburst and growth, including reduced leaf and shoot expansion, early and erratic budburst in lower branches and stems, and even lack of budburst after 7 months in optimal growing conditions. Overall, our results suggest that based on climate warming projected for this century, insufficient winter chilling is not likely to occur in Ontario temperate forests in the foreseeable future.