Understorey Layer Research Articles

Biodiversity of key soil phylotypes is associated with increased plant richness and productivity following agricultural abandonment and afforestation

Abstract Anthropogenic land use modifications are causing severe degradation of terrestrial ecosystems, and multiple revegetation strategies are emerging globally to counteract the loss of plant richness and productivity. While soil microorganisms are essential for plant community dynamics, the role of soil microbial biodiversity in regulating changes in plant richness and productivity under different revegetation strategies remains unknown. We used multitrophic co‐occurrence networks to identify soil network modules of strongly co‐occurring phylotypes along a 50‐year revegetation chronosequence of agricultural abandonment and afforestation. Soil biodiversity within these modules was related to soil nutrient cycling functions, plant richness and productivity (understorey layer in afforestation), elucidating how these network modules are associated with changes in plant richness and productivity. Plant richness and productivity increased simultaneously following both agricultural abandonment and afforestation. However, the biodiversity of key soil taxa within distinct network modules was associated with these coupled increases through the regulation of different nutrient cycling functions. Key soil phylotypes within the network modules involved in nitrogen (N) cycling correlated with the simultaneous increase in plant richness and productivity following agricultural abandonment. In contrast, those involved in phosphorus (P) and sulphur (S) cycling were linked to the coupled responses of both plant richness and productivity under afforestation. This reflects the divergent microbial mechanisms associated with the coupled increase in plant richness and productivity along the revegetation chronosequence for both agricultural abandonment and afforestation. Synthesis. Our findings provide correlative evidence that the biodiversity of key phylotypes within soil network modules is closely associated with the simultaneous increase in plant richness and productivity following the cessation of agricultural management. We identify key soil taxa, specific to each revegetation strategy, that could serve as potential targets for genomic and cultivation‐based approaches to counteract plant community degradation. Revegetation efforts to enhance plant richness and productivity should focus on soil phylotypes associated with N cycling after agricultural abandonment and those involved in P and S cycling during afforestation.

Three decades of understorey vegetation change in Quercus‐dominated forests as a result of increasing canopy mortality and global change symptoms

AbstractQuestionsThe long‐term response of understorey vegetation to increasing tree mortality has rarely been addressed in resurvey studies. For two Quercus‐dominated forest types, we asked: (a) How did overstorey alterations, induced by canopy mortality, affect understorey diversity and composition? (b) Is there a signal of global change effects on understorey communities? (c) Are these assemblages experiencing a homogenization process?LocationFive sites in Quercus robur (QR) and four sites in Q. petraea (QP) forests, Slovenia.MethodsWe studied changes in vascular plants in the understorey layer from 1992/1993 to 2023 across 45 permanent 20 m × 20 m plots in QR and QP forests, respectively. Vegetation surveys were carried out following the standard Braun‐Blanquet method. We compared original surveys with recent resurveys using multivariate analysis, ecological indicator values (EIV), plant traits and methods that quantify changes in individual species.ResultsSince the early 1990s, tree layer cover decreased from 95% to an average of 55% in QR, whereas it remained relatively high (77%) in QP plots. This resulted in denser understorey vegetation and a significant increase in plot‐level species richness in QR forests, but a slight decrease in QP forests. The extensive loss of canopy cover and disturbance effects in QR forests caused significant changes in species composition. Species turnover in QR was driven by colonization of new disturbance‐tolerant taxa characterized by ruderal traits, whereas the compositional shift in QP was to a greater extent due to species losses. We detected a process of vegetation thermophilization (increase in EIV‐temperature), suggesting an effect of rapid climatic warming. Understorey communities are now more similar to each other than 30 years ago, indicating a decrease in beta‐diversity (floristic homogenization).ConclusionsDespite some common trends, vegetation responses were forest type‐specific. Our study presents evidence of understorey vegetation changes triggered by increased canopy mortality (a strong local driver particularly in QR plots) and also points to the signal of global change symptoms (thermophilization, homogenization), which acted rather independently from the observed decline in tree layer cover.

Linking individualistic growth stability of trees to the complexity of understorey layers

Abstract Tree‐growth stability is crucial in upholding forest ecosystem services. Despite extensive research on the correlation between tree growth and climate, the influence of forest understorey on tree‐growth stability remains understudied. We surveyed forest plots and collected tree‐ring samples along two elevational gradients, ranging from 3600 to 4400 m a.s.l., in the southeastern Tibetan Plateau. An index was developed to quantify individualistic growth stability of trees, and its linkage with the complexity of forest understorey structure was examined. We found that tree‐growth stability is more pronounced in complex forest communities. In forests with higher understorey complexity, the proportion of trees experiencing growth release increased during wet years, while those with growth suppression augmented during dry years. Interestingly, a subset of trees exhibits abnormal growth increases even during the driest years, whereas another subset shows abnormal growth reductions during the wettest years. Furthermore, forests with more complex understorey structures exhibited a higher proportion of trees showcasing these two types of ‘anti‐phase’ growth statuses. Synthesis. Our study underscores the linkage between individualistic growth stability of trees and the complexity of understorey structures. The growth stability of the overstorey layer is conveyed at the expense of individual‐level growth stability and synchrony in forests with complex understorey structures. These findings emphasize the necessity for attention to the interplay between tree growth and understorey community structures when assessing the impacts of future climate change on forest dynamics.

Multiple drivers of functional diversity in temperate forest understories: Climate, soil, and forest structure effects

In macroecology, shifting from coarse- to local-scale explanatory factors is crucial for understanding how global change impacts functional diversity (FD). Plants possess diverse traits allowing them to differentially respond across a spectrum of environmental conditions. We aim to assess how macro- to microclimate, stand-scale measured soil properties, forest structure, and management type, influence forest understorey FD at the macroecological scale.Our study covers Italian forests, using thirteen predictors categorized into climate, soil, forest structure, and management. We analyzed five traits (i.e., specific leaf area, plant size, seed mass, belowground bud bank size, and clonal lateral spread) capturing independent functional dimensions to calculate the standardized effect size of functional diversity (SES-FD) for all traits (multi-trait) and for single traits. Multiple regression models were applied to assess the effect of predictors on SES-FD.We revealed that climate, soil, and forest structure significantly drive SES-FD of specific leaf area, plant size, seed mass, and bud bank. Forest management had a limited effect. However, differences emerged between herbaceous and woody growth forms of the understorey layer, with herbaceous species mainly responding to climate and soil features, while woody species were mainly affected by forest structure.Future warmer and more seasonal climate could reduce the diversity of resource economics, plant size, and persistence strategies of the forest understorey. Soil eutrophication and acidification may impact the diversity of regeneration strategies; canopy closure affects the diversity of above- and belowground traits, with a larger effect on woody species. Multifunctional approaches are vital to disentangle the effect of global changes on functional diversity since independent functional specialization axes are modulated by different drivers.

Use of national forest inventory data to develop stand density driven models for understorey shrubs and overstorey fuel variables and associated temporal dynamics in commercial plantations

Biomass tends to accumulate both in overstorey and understorey layers of forests growing in productive areas. The rapid growth of forest plantations established in productive areas can thus also lead to the creation of fire-prone landscapes. In this study, we assessed the extent to which overstorey stand density determines current values of the fine shrub fuel load (Wshr_G1) in the understorey and also two major canopy fuel characteristics related to crown fire activity -canopy base height (CBH) and canopy bulk density (CBD)- and the temporal dynamics of these. For this purpose, we developed two types of models for the three fuel complex variables: i) extreme response models, to define the upper limit of Wshr_G1 and CBD and the lower limit of CBH, the values of which may depend on stand density; and ii) dynamic models, to estimate the probability of increasing fuel loads and rate of change when a more hazardous situation is predicted. Data were obtained from 8087 plots, measured in the third, fourth and fifth Spanish National Forest Inventories, in plantations dominated by the major commercial species in northern Spain (Pinus pinaster, Pinus radiata, Pinus sylvestris and Eucalyptus globulus). The extreme response models generally explain more than 60% of the observed variability in the three fuel complex variables. According to these models, maximum shrub fuel load and CBD and minimum CBH in stands of the four tree species were limited by stand basal area, with different responses by different species. The dynamics of the fuel complex variables were determined by canopy cover, estimated as a proxy for stand density. The use of a two-step regression approach enabled realistic modelling of the dynamics of the fuel complex variables, allowing for a decrease in the shrub fuel biomass and in CBD between two time points. The dynamic models developed are age-independent and are therefore useful for practical applications because they can be applied to any stand without the need to know the stand age. Overall, the study findings provide insights into the complex relationship between both understorey and overstorey fuels and stand density in commercial plantations. As assessment of forest fuel complex variables is a prerequisite for most fire management activities, the study findings have important implications for forest and fuel management and planning in the region.

Forest grouse response to forestry practices across four decades

Fennoscandivian boreal forests have been logged for centuries, yet only since the second world war has management shifted from selective logging to clear-cutting. In general, this shift equates to fewer stands of uneven age and higher vertical complexity and more stands of even age and lower complexity. Ecological responses to this shift in forestry practice are expected and could be especially significant for umbrella species, those species whose protection affords a sort of trickle-down protection for a host of other species. Two putative umbrella species are the Black Grouse (Lyrurus tetrix) and Western Capercaillie (Tetrao urogallus), large galliform birds of forested habitats of northern Eurasia. We used a 45-year dataset from southeastern Norway to assess how closely each of these species tracked changes in forest cutting class, age, stand density, and in the proportion of deciduous trees. We found that occupancy was hierarchical. The Black Grouse tended to occupy primarily younger (<40 y) forests and secondarily forests with higher cover of deciduous trees (>48%) and, at the understorey level, a dominance of bilberry (Vaccinium myrtilus), whereas the capercaillie tended to occupy primarily forest that had not been logged in many years (cutting class IV or V) and secondarily an understorey layer of bilberry and lower stand density. In general, the Black Grouse tracked change in stand density more closely, whereas the capercaillie restricted occupancy to older forest (>100 y) dominated by conifers. Both species were detected most often when stand density was intermediate, with the densest forest typically avoided. In sum, the Western Capercaillie is likely to be a good umbrella species for old-growth boreal forest, particularly coniferous forest, but the Black Grouse, with its more malleable response to forestry practices, would not, as it occurs in mixed forests that are neither too old nor too young. We conclude that management for healthy Black Grouse populations may require little change from current practice as long as sufficient deciduous cover is retained. By contrast, protection of old-growth or mature forests ultimately will be crucial to persistence of Western Capercaillie populations—the species will occupy cut forest, but it is found in such forests much less often. Such protection may necessitate a shift away from clear-cutting as well as a broader perspective to ensure sufficient old growth is retained across the landscape.

Understorey structural complexity mediated by plantation management as a driver of predation events on potential eucalypt pests

Current forest management aims to reduce the economic and environmental impact of pests on forest ecosystem productivity and to develop sustainable control approaches. The enemies hypothesis states that more diverse plant communities support a greater number and diversity of predators and parasitoids leading to a reduction in pest damage via top-down control. In forests, research on the enemies hypothesis typically focuses on tree species diversity. Here, we modify this hypothesis to determine how structural complexity of the forest understorey, as mediated by management, influences the abundance of predators and predation events on potential forest insect pests. To test this, we studied Eucalyptus sp. plantations managed under two approaches of tree establishment, hypothesising that those managed under the less disturbed system of regrowth (regeneration from clear cut stumps) will have greater vegetation structural complexity, predator abundance and predation events (as an indirect measure of predation rate) than the more disturbed seedling planting system. In six stands of each management type, we measured vegetation understorey layers (ground-, shrub- and sub-canopy cover), predator abundance (insectivorous birds, spider web counts) and predation events (artificial larvae attacks, spider web prey items).Vegetation structural complexity, abundance of insectivorous birds and spider webs, and artificial larvae attacks and web prey items were all greater in the regrowth than in the seedling stands. Path analysis evidenced direct support for our modified enemies hypothesis for birds, the regrowth management with low disturbance levels after clearcutting promotes both predator abundance and predation events via increased vegetation structural complexity. However, for spiders the increase in web abundance and predation events was directly and positively associated with regrowth management, but there was no indirect link via vegetation structural complexity, suggesting other factors driven by forest management are important.Manipulative experiments explicitly exploring the cause-and-effect relationship between predation rates and herbivory rates and consideration of the economic implications of the different approaches are required before changes to management are implemented. Our study agrees with the overarching paradigm in sustainable forest management that promotion of structural complexity will be beneficial to biodiversity, ecosystem function and resilience.

Spatio‐temporal variation in deep soil water use patterns of overstorey and understorey layers in subtropical plantations predicts community assembly

Abstract Deep soil water utilization allows plants to cope with drought stress. However, little is known about the roles of the understorey layers in driving spatio‐temporal variations of deep soil water in forests and how the patterns of deep soil water use among life‐forms contribute to community assembly processes. We assessed the spatio‐temporal patterns and determinants of deep water utilization of tree, shrub and herb layers in subtropical coniferous plantations and investigated associations between deep water use parameters and dominance and richness of understorey vegetation. We found that the understorey layer had a higher reliance on deep soil water in the dry season, a larger seasonal plasticity of deep soil water uptake, but lower spatial variability in deep soil water utilization than the tree layer. We showed that greater reliance of the tree layer on deep soil water was associated with decreased shrub layer diversity, whereas greater reliance of the shrub layer on deep water was associated with increased herb layer diversity. Synthesis. Our results highlight the roles of understorey layers in driving the temporal dynamics of deep soil water in forests and improve our understanding of how deep soil water use patterns among life‐forms shape community assembly in forests.

Plant trait filtering is stronger in the herb layer than in the tree layer in Greek mountain forests

We studied the differentiation among plant communities of deciduous broadleaved and mountain coniferous forests in terms of functional diversity and identity at a regional scale (northern and central Greece). We asked if patterns of functional differentiation among communities are consistent between the overstorey and understorey layers and if they can be influenced by deep past environmental conditions. Functional Richness (FRic) and Functional Dispersion (FDis), as well as their standardized effect sizes, were employed to assess the multivariate functional diversity of the community types. In contrast, single-trait Community Weighted Means (CWMs) were used as surrogates of functional identity. The aforementioned indices were calculated for three datasets, namely all the vascular plant taxa found in individual vegetation plots (total community), all phanerophyte (tree and shrub) taxa (overstorey) and all non-phanerophyte vascular plant taxa (understorey). We found that community types and especially four broad forest types (beech, ravine, pine and oak forests) are well differentiated in terms of functional composition (identity), as indicated by Non-Metric Multidimensional Scaling (NMDS). After conducting an NMDS for the three datasets, functional identity based on the total floristic composition was found to be the best discriminator of the studied communities. However, contrasting patterns were found for some specific traits or their categories between overstorey and understorey layers. The patterns of functional diversity of the community types (based on multivariate indices), revealed by calculating the standardized effect sizes of FRic and FDis based on the richness null model, did not differ substantially from random expectations for most of the studied community types when the dataset of all the vascular plant taxa was analyzed. However, the patterns revealed for the overstorey layer differed from those for the understorey layer. For the latter layer, the clustered structure was revealed in many community types based on the ses.FDis metric. Indications of deep past influence on the functional composition were found for certain community types (i.e. ravine forests) based on single-trait metrics, but no indication of such influence was found based on multivariate indices. Our findings highlight the complementarity and the additive explanatory value of the simultaneous use of single- and multi-trait approaches and their application to different layers in forests.

Thermal differences between juveniles and adults increased over time in European forest trees

Abstract Woody species' requirements and environmental sensitivity change from seedlings to adults, a process referred to as ontogenetic shift. Such shifts can be increased by climate change. To assess the changes in the difference of temperature experienced by seedlings and adults in the context of climate change, it is essential to have reliable climatic data over long periods that capture the thermal conditions experienced by the individuals throughout their life cycle. Here we used a unique cross‐European database of 2,195 pairs of resurveyed forest plots with a mean intercensus time interval of 37 years. We inferred macroclimatic temperature (free‐air conditions above tree canopies—representative of the conditions experienced by adult trees) and microclimatic temperature (representative of the juvenile stage at the forest floor, inferred from the relationship between canopy cover, distance to the coast and below‐canopy temperature) at both surveys. We then address the long‐term, large‐scale and multitaxa dynamics of the difference between the temperatures experienced by adults and juveniles of 25 temperate tree species. We found significant, but species‐specific, variations in the perceived temperature (calculated from presence/absence data) between life stages during both surveys. Additionally, the difference of the temperature experienced by the adult versus juveniles significantly increased between surveys for 8 of 25 species. We found evidence of a relationship between the difference of temperature experienced by juveniles and adults over time and one key functional trait (i.e. leaf area). Together, these results suggest that the temperatures experienced by adults versus juveniles became more decoupled over time for a subset of species, probably due to the combination of climate change and a recorded increase of canopy cover between the surveys resulting in higher rates of macroclimate than microclimate warming. Synthesis. We document warming and canopy‐cover induced changes in the difference of the temperature experienced by juveniles and adults. These findings have implications for forest management adaptation to climate change such as the promotion of tree regeneration by creating suitable species‐specific microclimatic conditions. Such adaptive management will help to mitigate the macroclimate change in the understorey layer.

Rapid thermophilization of understorey plant communities in a 9 year‐long temperate forest experiment

Abstract The vast majority of plant biodiversity associated with temperate forests is harboured by the understorey layer. This layer also plays crucial roles in ecosystem functions such as tree regeneration, nutrient cycling and carbon dynamics. Research using space‐for‐time substitutions and resurveys of vegetation plots has shown that climate warming, changes in forest management and resource availability are key determinants of forest understorey biodiversity change and functioning. However, long‐term experiments are needed to better unravel their complex interactive effects. Here we study the influence of nearly a decade of experimental warming, light addition using fluorescent tubes (as a proxy for management‐driven changes in forest‐floor light levels) and nitrogen input on understorey plant communities of temperate broadleaved forest. Plant communities shifted towards a higher dominance of warm‐adapted species, a process referred to as thermophilization. We detected a marked community shift in all treatments including the control plots, reflecting ongoing ambient environmental changes. This reordering over time was greater than the shift induced by the treatments. Thermophilization was, however, greatest when temperature and/or light availability were enhanced. Communities were also taller in response to warming and increased light availability. Synthesis. Our experiment provides important insights into 9 years of vegetation changes in a temperate forest and how canopy density and forest management can be adapted to limit thermophilization of forest understorey biodiversity under climate change. [Correction added on 27 April 2021, after first online publication: The Synthesis section in the abstract has been updated to reflect the original text supplied.]

Data-driven authoring of large-scale ecosystems

In computer graphics populating a large-scale natural scene with plants in a fashion that both reflects the complex interrelationships and diversity present in real ecosystems and is computationally efficient enough to support iterative authoring remains an open problem. Ecosystem simulations embody many of the botanical influences, such as sunlight, temperature, and moisture, but require hours to complete, while synthesis from statistical distributions tends not to capture fine-scale variety and complexity. Instead, we leverage real-world data and machine learning to derive a canopy height model (CHM) for unseen terrain provided by the user. Trees in the canopy layer are then fitted to the resulting CHM through a constrained iterative process that optimizes for a given distribution of species, and, finally, an understorey layer is synthesised using distributions derived from biome-specific undergrowth simulations. Such a hybrid data-driven approach has the advantage that it incorporates subtle biotic, abiotic, and disturbance factors implicitly encoded in the source data and evidences accepted biological behaviour, such as self-thinning, climatic adaptation, and gap dynamics.

A critical review of fuel accumulation models used in Australian fire management

Various classifications of fuel accumulation models are used to describe the complex temporal relationship between fuel loads and vegetation dynamics. Fuel accumulation models are an important tool in wildfire management as fuel is the only component that can be directly controlled by fire managers. Here we discuss various strengths and limitations of analytical fuel accumulation models that exist in the literature, with a focus on those used in Australia. Early approaches to analytical or continuous models of fuel accumulation centred around the model introduced by Olson in 1963. This model assumes that the rate at which fuel accumulates is determined as a balance between the rate of fuel accession and the rate at which it decays. The Olson model has been shown to provide a reasonable description of litter accumulation, data sparsity issues notwithstanding, but can be of limited use in describing elevated fuels, or in forest stands that exhibit more complex post-disturbance dynamics. Interactions between species and other disturbances have the potential to change the dynamics of fuel accumulation and decay processes. Moreover, post-fire vegetation stands are usually dominated by an understorey layer that eventually senesces as the dominant vegetation grows. Motivated by the critical differences between the models presented, a more general approach featuring vegetation density is proposed. A generic result is presented to indicate how the theoretical predictions of the model are able to emulate patterns of fuel accumulation that have been reported, and that can not be accounted for by the models commonly used in Australia.

Flowering season of vernal herbs is shortened at elevated temperatures with reduced precipitation in early spring

Vernal herbs are exposed to the risk of climate change under spring frost and canopy closure. Although vernal herbs contribute to the biodiversity of the understorey layer in temperate forests, few studies assessed the effect of climate change on the phenology of the herbs. To examine phenological shifts in flowering seasons of vernal herb species caused by climate change, a greenhouse experiment was conducted using four species (Adonis amurensis, Hepatica nobilis var. japonica, Viola phalacrocarpa, and Pulsatilla cernua) under two temperature conditions (ambient or elevated temperature) and two precipitation conditions (convective or reduced precipitation). Experimental warming advanced overall aspects of the flowering timing including the first and last day of flowering. The growth of flowering stalk was also promoted by elevated temperature. Effects of decreased precipitation varied among species, which advanced the last day of the flowering of the later flowering species. Consequently, a decrease in overall flowering period length was observed. These results indicate that overall, climate change results in a shortening of the flowering season of vernal herb species, specifically at a higher temperature and under conditions of less precipitation.

Forest product harvesting in the Eastern Cape, South Africa: Impacts on habitat structure

The Eastern Cape Province harbours 46% of South Africa’s remaining indigenous forest cover, and is one of the country’s poorest and least developed provinces. Forest resources thus represent a vital component of rural livelihoods in this region. Consequently, forest management policies aim to balance the needs of resource users with the ecological integrity of forest ecosystems. In a recent study, forest bird ranges were shown to have declined in the Eastern Cape over the past 20 years, despite increases in forest cover over the same time period, indicating that habitat degradation may be driving forest bird losses. Given that harvesting of forest products represents the primary human disturbance in forests in the Eastern Cape today, insight is needed regarding the link between resource use and habitat modification. We report on effects of harvesting of three key forest products – poles, timber and medicinal bark – on habitat structure at the ground, understorey and canopy layers in indigenous forests in the province. Harvest activities had considerable impacts on habitat structure, depending on the nature and extent of harvesting. Bark and timber harvesting resulted in canopy gaps, whereas pole harvesting reduced tree density, resulting in understorey gaps. Overall, harvest activities increased the frequency of canopy disturbance, and density of understorey layer foliage. Unsustainable bark harvesting practices increased the mortality rate of canopy trees, thereby increasing dead wood availability. By providing insight into human-mediated habitat modification in forests of the Eastern Cape, this study contributes to the development of ecologically informed sustainable resource management policies.&#x0D; Significance:&#x0D; &#x0D; Unregulated harvesting of forest products in state-managed indigenous forests of the Eastern Cape results in habitat modification.&#x0D; The nature and extent of habitat modification is dependent on the type and intensity of resource use, indicating that resource use may be sustainably managed.&#x0D; Timber and medicinal bark harvesting activities result in canopy disturbances, thereby altering natural canopy gap dynamics, with concomitant impacts on understorey habitat structure.&#x0D; Changes in forest habitat structure associated with high levels of resource use are likely to have ramifying effects on forest biodiversity.&#x0D;

Tree recruitment in a drought- and herbivory-stressed oak-beech forest: Implications for future species coexistence

Mixed oak-beech forests are particularly valuable from an ecological and socioeconomic perspective but are increasingly exposed to anthropogenic impacts. We wonder whether anthropogenic change drivers, such as increased drought and ungulate populations affect optimal regeneration niches, species coexistence, and mixed-species forest dynamics. We compared understorey composition, structure and distribution with the overstorey counterpart in an oak-beech forest located at its southernmost European distribution (Central Spain) with increasing drought and ungulate herbivory for the last decades.Results showed that Ilex aquifolium and Prunus avium had greater representation in the recruitment layer as compared to the understorey layer whereas, for the dominant Quercus spp. and Fagus sylvatica, there was a tendency suggesting an expansion of Fagus at the expense of Quercus. Density of tree recruits showed strong differences across microsites. The greatest density was found under tree and under shrub cover whereas the lowest was found under woody debris. By species, Quercus regenerated better in open and shrub microsites whereas Fagus and Ilex regeneration was higher under tree cover. Large patches of shrub cover favoured Prunus avium recruitment, particularly against ungulate herbivory as it was the most heavily browsed species, followed by Quercus, Ilex and Fagus in a descending order. Herbivory occurrence and intensity on tree recruits strongly varied with the microsite, with greater herbivory levels in open microsites. Woody debris on the ground only protected tree recruits from browsing when they were located under tree cover.Overall, fenced areas with no ungulates showed greater tree species coexistence (28% increase in species richness) and recruitment abundance (81% increase) after 7 years of exclusion. We also found a large increase in tree recruit growth and height:diameter ratio (h:d) for all studied species when fenced. Fagus and Prunus avium showed the greatest growth and h:d increase, much greater than those of Quercus and Ilex. The h:d was identified as a reliable indicator of ungulate herbivory and reference values (under null levels of ungulate herbivory) were provided for each species.We conclude that there is an imperative need for adaptive management strategies that address the joint effects of climate change (e.g. increased drought) and ungulate herbivory in mixed oak-beech forests. The provision of different microsites, including patches of shrub cover in open microsites and woody debris under dense tree cover, will facilitate the availability of suitable regeneration niches for most species and will therefore promote future species coexistence.

Forest Understorey Vegetation: Colonization and the Availability and Heterogeneity of Resources

Understorey vegetation comprises a major portion of plant diversity and contributes greatly to nutrient cycling and energy flow. This review examines the mechanisms involved in the response of understorey vegetation to stand development and the overstorey canopy following disturbances. The overall abundance and diversity of the understorey is enhanced with the availability and heterogeneity of light, soil nutrients, soil moisture, and substrates. Vascular plants are positively impacted by the availability and heterogeneity of light and soil nutrients, whereas non-vascular vegetation is more strongly influenced by colonization time, soil moisture, and substrates, and is decreased with a higher proportion of broadleaf overstorey. The availability of resources is a prominent driver toward the abundance and diversity of understorey vegetation, from the stand initiation to stem exclusion stage under a single-species dominated overstorey. However, resource heterogeneity dominates at the later stages of succession under a mixed overstorey. Climate and site conditions modify resource availability and heterogeneity in the understorey layer, but the extent of their influences requires more investigation. Forest management practices (clearcutting and partial harvesting) tend to increase light availability and heterogeneity, which facilitates the abundance and diversity of understorey vascular plants; however, these factors reduce the occurrence of non-vascular plants. Nevertheless, in the landscape context, anthropogenic disturbances homogenize environmental conditions and reduce beta-diversity, as well, the long-term effects of anthropogenic disturbances on understorey vegetation remain unclear, particularly compared with those in primary forests.

Direct and understorey-mediated indirect effects of human-induced environmental changes on litter decomposition in temperate forest

Human-induced environmental changes in temperature, light availability due to forest canopy management, nitrogen deposition, and land-use legacies can alter ecosystem processes such as litter decomposition. These influences can be both direct and indirect via altering the performance of understorey vegetation. To identify the direct and indirect effects of environmental changes on litter decomposition, we performed an experiment with standardised green and rooibos teas. The experiment was conducted in a temperate mixed deciduous forest, and treatments (temperature, light, and nitrogen) were applied to mesocosms filled with ancient and post-agricultural forest soil. Both green tea and rooibos teas were more rapidly decomposed in oligotrophic soil than in eutrophic soil. The direct effects of the treatments on litter decomposition varied among the two litter types, incubation times, and soil fertility groups. Warming and agricultural legacy had a negative direct effect on the decomposition of the green tea in the high soil fertility treatment during the early decomposition stage. In contrast, agricultural legacy had a positive direct effect on the decomposition of rooibos tea. Soil enriched with nitrogen had a negative direct effect on the decomposition of green tea in mesotrophic soil in the early decomposition stage and on rooibos tea in later stage. The indirect effects of the treatments were consistently negative, as treatments (especially the temperature and light treatments in the early decomposition stage) had a positive effect on plant cover, which negatively affected litter decomposition. Our results indicate that warming, increased nitrogen deposition, and land use legacy can directly stimulate the decomposition of labile litter on more fertile soils. Furthermore, warming and increased light had stronger positive direct effects on understorey herbaceous cover, which leads to slower decomposition rates, especially in more fertile soils. Therefore, the indirect effects of environmental changes related to the understorey layer on litter decomposition can be more important than their direct effects, thus should not be overlooked.

Tree regeneration responds more to shade casting by the overstorey and competition in the understorey than to abundance per se

Manipulating the overstorey is the key tool for forest managers to steer natural regeneration. Opening up the canopy does not only create favourable light conditions for tree seedling growth, but also for (competitive) understorey species. Therefore, a thorough understanding of how changes in the abundance of the overstorey and understorey influence tree regeneration is needed to attain successful regeneration.To this end, we used the regional Flemish Forest Inventories, which contain vegetation plots that were surveyed at two times and include large variation in species composition and abundance of both overstorey and understorey layers. These plots were classified into poor and rich forest types, which differ in overstorey and understorey species composition and soil fertility. For each forest type, we first investigated the effect of overstorey abundance and shade-casting ability on the understorey herbaceous vegetation cover and its competitive nature. Then, we modelled how both these strata influence the presence-absence as well as the cover of tree regeneration, using the zero-inflated beta distribution.Our results show that the understorey cover and its competitiveness mainly increase when the abundance and shade-casting ability of the overstorey is reduced. The shade-casting ability of the overstorey and competitiveness of the understorey were more important in determining tree regeneration, especially probability of presence, than the abundance of these layers per se. This was consistent for both forest types, although directions and magnitudes of the effects differed. In predictions mimicking several thinning scenarios we found that in the poor forests, reducing overstorey abundance could lead to an increase in seedling cover, whereas in rich forests, the opposite is true and seedling cover will potentially be reduced. Finally, in a single-species analysis focusing on Quercus, we found a trade-off between sufficiently reducing overstorey abundance, while at the same retaining parent trees as potential seed sources. These findings can be used to guide forest management decisions in order to attain successful forest regeneration in temperate forests.

An assessment of the Tagula Honeyeater Microptilotis vicina, a Data Deficient bird species in a Melanesian endemic hotspot

SummaryWe conducted research into the ‘Data Deficient’ and endemic Tagula Honeyeater Microptilotis vicina of the Louisiade Archipelago, Papua New Guinea. This species was only observed on Sudest and Junet Islands. Islands were visited between October and January in the years from 2012–2014 and in 2016. We conducted the first assessment of spatial and habitat use by this species using radio-tracking in 2016. These findings were also used to inform estimates using traditional population density methods. CTMM package in R was used for home-range estimation for the tracked honeyeaters. Our results supported that members of this species display territoriality during the breeding season, occupying a mean of 2.0 ± 0.6 (SE) ha on Junet Island (n = 5). Whether individuals defended defined territories at other times of the year was not known but re-sightings of marked birds confirmed them to be locally resident. Population estimates ranged between 53,000 and 85,000 individuals. However, more conservative estimates nearing 50,000 individuals were considered prudent given lower population densities observed on parts of the larger Sudest Island (0.64/ha). This species utilised the canopy and understorey layers in a range of habitats from mangroves at sea-level, gardens and regrowth of various ages to cloud forest on the highest point of Sudest Island (∼800 m asl). Dietary observations support that like many closely related species, Tagula Honeyeaters have a broad diet of mostly insects supplemented with nectar and fruit. Observations indicated that this species had life history attributes toward the slower end of the spectrum but similar to other congeners. Vocalisations were more diverse in both structure and complexity than those of suspected close relatives the Mimic Microptilotis analogus and Graceful Microptilotis gracilis Honeyeaters. Morphological measures were similarly different, supporting species level recognition.