Understorey Species Composition Research Articles

Changes of vegetation in coniferous monocultures in the context of conversion to mixed forests in 30 years – Implications for biodiversity restoration

The understorey vegetation of temperate forests harbours a major proportion of terrestrial biodiversity and fulfills an important role in ecosystem functioning. Over the past decades, temperate forest understoreys were found to change in species diversity and composition due to several anthropogenic and natural drivers. Currently, the conversion and restoration of even-aged coniferous monocultures into more diverse and mixed broad-leaved forests are major objectives of sustainable forest management in Central Europe. This forest conversion alters understorey communities and abiotic site conditions but the underlying patterns and processes are not yet fully understood. Therefore, we investigated changes in the Bavarian Spessart mountains in southwest Germany, where we re-sampled 108 semi-permanent plots from four different coniferous stand types (i.e., Norway spruce, Scots pine, Douglas fir, European larch) about 30 years after the initial assessment. On these plots, we recorded understorey vegetation and forest structure, and derived abiotic site conditions based on ecological indicator values of understorey vegetation, followed by multivariate analysis. We found changes in plant communities that point towards a decrease of soil acidity and a “thermophilization” of forest understoreys. Understorey species richness remained constant, while understorey's Shannon and Simpson diversity increased. The observed changes in forest structure explained the temporal shifts in understorey species composition. The understorey species composition did not experience a significant floristic homogenization since the 1990s. However, plant communities exhibited a reduction in species characteristic of coniferous forests and a simultaneous increase in species associated with broad-leaved forests. The increase of specialist species (closed forests and open sites) may have compensated for the detected decrease in generalist species. We conclude that the forest conversion towards mixed broad-leaved forest in the Spessart mountains of the past decades might have masked homogenization trends that are increasingly reported from Central European forest understoreys.

Topography alters stand structure, carbon stocks and understorey species composition of Cedrela odorata plantation, in a semi-deciduous forest zone, Ghana

Pure plantations established across different topographic positions may respond differently and exhibit varied stand characteristics. However, there is limited information on variability of pure plantation stand attributes along topographic gradients, particularly in West Africa. This study examined the stand structure, carbon stocks and understorey woody species composition of relatively older Cedrela plantations on three topographic positions within the deciduous forest zone of Ghana. About 15, 40 × 40 m plots were surveyed in 23-year-old Cedrela odorata plantations located on hill top, flat terrain and valley bottoms in the Tinte Bepo Forest Reserve, Ghana. Data were analysed using ANCOVA, Bayesian modelling of diameter distribution and correspondence analysis.Diameter at breast height (DBH; p<0.0001), total height (p<0.025), and stem density (p<0.001) were significantly different among the three topographic areas. Trees on the valley bottom had the highest DBH, total height, and biomass carbon while the highest stem density and basal area occurred on the flat terrain. Slope explained 63% of the total variability in the stem density of the C. odorata stand. Bayesian modelling of diameter distribution revealed that the 3-parameter Weibull function better fitted the diameter distribution of the trees on the hill top while the Johnson SB was more appropriate at the flat terrain and the valley bottom. Understorey woody species composition was significantly different among geomorphic positions. Differences in aboveground biomass carbon stocks among topographic positions were not significant.The local topography therefore severely alters plantation stand structure, leading to different distribution models being adopted at different topographic positions. Furthermore, cedrela plantations facilitate climate mitigation, species recruitment and functional/trait diversity differences along topographic gradients. This has implications for large scale plantation development especially on heterogeneous terrains.

Silvicultural management and altitude prevail on soil properties and fungal community in shaping understorey plant communities in a Mediterranean pine forest

Understorey vegetation plays a key role in Mediterranean forest ecosystem functioning. However, we still lack a thorough understanding of the patterns and drivers of understorey composition and diversity. As a result, understoreys are often ignored during assessments of forest functioning under climate change. Here we studied the effect of silvicultural management, topography, soil fungal community composition and soil physical and chemical properties on understorey community composition and diversity. The plant cover and number of individuals of understorey perennial plants, shrubs and non-dominant trees was recorded on 24 plots (paired: control-thinned) in a Mediterranean pine-dominated mountainous area in Northeast Spain. The study area represented a broad thinning intensity gradient (from 0 to 70 % in removed stand basal area) along a 400-m altitudinal range (from 609 m to 1013 m). Our results showed that thinning intensity and topography explained the greatest proportion of the total variance in the understorey species composition, i.e., 18 % and 16 %, respectively. Interestingly, the effects of the silvicultural treatments were significant only when considering the altitudinal effect, so that, the main impacts of thinning on the understorey community composition occurred at low altitudes (between 609 m and 870 m). Moreover, we found a significant decrease in both richness and abundance of understorey species in both the control and thinned plots with increasing altitude, with thinned plots being significantly richer in species compared to the control plots. The difference in the understorey community sensitivity to forest thinning along the altitudinal gradient suggests changes in factors that limit plant growth. Low elevation plots were restrained by light availability while high altitudes plots limited by winter freezing temperature.

Disturbance history drives current compositional and diversity patterns of primary Picea abies (L.) Karst. forest vegetation

Development of primary spruce forests is driven by a series of disturbances, which also have an important influence on the understorey vegetation and its diversity. Early post-disturbance processes have been intensively studied, however, very little is known about the long-term effects of disturbances on the understorey. We quantified disturbance history using dendrochronological methods to investigate its impact on vascular plant diversity and understorey species composition. We sampled 141 plots randomly assigned throughout primary stands located in the zone of natural montane acidophilous forests dominated by Picea abies (L.) Karst. in the Western Carpathians. Dendrochronological, dendrometric, and environmental parameters were related to understorey properties using ordination methods and a Bayesian approach using multilevel linear models (GLMM).Time since the last disturbance (23–260 years ago; mostly windstorms and bark beetle outbreaks) had a significant effect on understorey species composition of the current communities, and it also interacted with disturbance severity to influence species diversity. The effect of disturbances on the understorey was largely mediated by the alteration of stand structure (age, DBH, canopy openness), Vaccinium myrtillus L. cover, and topsoil chemical properties. A period of severe disturbances between 1860 and 1890 resulted in a legacy of our current, relatively homogeneous spruce stands with less diverse sciophilous understorey dominated by V. myrtillus, which is in contrast to heterogeneous stands (in terms of age and spatial structure) driven by small-scale, lower-severity disturbances, which led to an understorey enriched by species with higher demands on light and topsoil quality (higher K concentration and lower C/N ratio).All developmental pathways following disturbances create a unique complex of spatiotemporal understorey variability in the montane spruce forests. Therefore, to preserve their full diversity, disturbances of all severities and sizes should be accepted as natural drivers, both in the field of nature conservation and close-to-nature forestry efforts.

Variation in characteristics and conservation values of plant communities on abandoned agricultural lands with and without fires

AbstractQuestionWhich plant community characteristics most consistently vary with prior land use?LocationOak Openings Preserve, Ohio, USA.MethodsIn 2020, we measured plant communities on 22 sites encompassing three prior land uses: (a) contemporary Quercus forests continuously forested since before the 1930s; (b) former agricultural lands, under cultivation in the 1930s and abandoned 70–80 years before our study and that were unburned; and (c) former agricultural lands also abandoned 70–80 years earlier and that were burned in 2013 prescribed fires. Among the three land uses, we compared 12 plant community variables, categorized into four groups characterizing different sets of community properties (structure, diversity, composition, and conservation values).ResultsA multivariate combination of the 12 community variables differed among all three land uses, each of the four categories of variables differed between at least two land uses, and eight of 12 individual variables differed between at least two land uses. Community structure displayed a gradient of decreasing tree canopy and understorey plant cover and increasing lichen–moss cover from continuously forested to unburned and burned formerly cultivated sites. Understorey plant species richness did not vary with land use, but species diversity was higher on cultivated sites, irrespective of fire history. While all three land uses now contain Quercus tree overstories, each land use exhibited unique understorey species composition. Only formerly cultivated sites contained state‐listed endangered species. Cultivated sites had a lower proportion of native species cover than did continuously forested sites, but native species still comprised over 90% of the plant cover on cultivated sites. Compared with continuous forests, the soil organic layer (O horizon) was five times thinner and loss on ignition in the 0–15 cm mineral soil 49% (unburned) and 66% (burned) lower on previously cultivated sites.ConclusionsCommunity characteristics varied in their sensitivity to land use history, with species composition among the most sensitive to prior land use. A dense sapling layer of Acer rubrum forming in long‐unburned Quercus forests across much of eastern North America was absent in our study on formerly cultivated sites, possibly due to dry, infertile soil. From a restoration and conservation standpoint of the study region's pre‐settlement, frequently burned and open savannas and woodlands, previously cultivated sites may be easier to keep open via prescribed fire. Moreover, perhaps precisely because much of the flora historically developed in open savannas and woodlands which previously cultivated sites on the contemporary landscape most closely mimicked, previously cultivated sites presently contain higher species diversity and more conservation priority species than do continuously forested sites.

Management options for the conversion of allochthonous coniferous forest patches towards more natural species composition in the Vorpommersche Boddenlandschaft National Park, NE Germany

The conversion of coniferous tree plantations towards natural self-regenerating forests is one of the targets of forestry in Europe and particularly desirable in national parks. The aim of this study was to evaluate the effectiveness of different conversion approaches (conducted once) on natural forest development in man-made spruce plantations. We studied the effects of three distinct management options (partial-cutting [PC], clear-cutting with timber removal [CC] and clear-cutting with timber remaining [CCr]) six years after application on tree regeneration (sapling identity, abundance, height, browsing intensity) and understorey species composition in former spruce plantations within the Vorpommersche Boddenlandschaft National Park in Northeastern Germany. An unmanaged spruce [M] and near-natural beech [N] stands were considered as local references. The sites differed significantly in tree regeneration and understorey species composition. The abundance of native tree saplings was highest at site CCr, their height did not differ among sites, while browsing intensity was most pronounced at site CC. Understorey species composition of all management options was equally dissimilar to site N. However, site CCr contained more species that are typically abundant in local beech forests. Clear-cutting of spruce plantations with dead wood remaining on site favoured the natural regeneration of native trees and understorey vegetation compared to clear-cutting with timber removal, partial-cutting and no management. Herbivore browsing detrimentally affected tree regeneration independent of the treatment. Clear-cutting with timber removal promoted the dominance of competitive herbaceous species that may hamper long-term forest regeneration. We conclude that progression of site CCr appears to proceed fastest towards natural forest conditions. Further long-term data is required to derive clear trends regarding time and direction of forest regeneration. Keywords: forest conversion, spruce plantations, beech forest, natural tree regeneration, understorey species composition, deer browsing

Spatial variation and drivers of vegetation structure and composition in coastal freshwater wetlands of subtropical Australia

Coastal freshwater wetlands (CFWs) are among the most understudied wetlands globally and are highly vulnerable to projected climate changes. To address CFW knowledge gaps in south-east Queensland, Australia, we surveyed the floristic composition and structure of wooded CFWs and explored variation in vegetation patterns in relation to selected environmental drivers. Understorey and shrub assemblages were surveyed using a cover-class scale and stem counts for tree species abundance. Vegetation structure attributes (stem density, basal area) were calculated from survey data. Redundancy analysis was used to investigate drivers of vegetation structure and the species composition of each stratum. Vegetation structure patterns were associated with gradients of rainfall, soil moisture, salinity and pH. Understorey species composition was associated with wallum wetland species, native perennial grass and herb species, and vegetation patterns of the canopy. Common CFW species, namely Melaleuca quinquenervia and Eucalyptus tereticornis, dominated tree assemblage variation. Overall, CFW vegetation exhibited strong associations with gradients of salinity, rainfall, groundwater dependence and disturbance. Alterations to key drivers of vegetation pattern with future climate changes are likely to markedly influence the composition, structure and function of CFW vegetation communities. Action is therefore required to maintain CFW vegetation communities and ecological function in these diverse and unique wetland systems.

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.

Cascading effects of canopy mortality drive long‐term changes in understorey diversity in temperate old‐growth forests of Europe

AbstractQuestionsWe investigated the influence of protracted mortality of a dominant canopy tree (Abies alba) on long‐term understorey dynamics. We ask (a) how tree regeneration and understorey species diversity and composition changed over 32 years; and (b) whether the observed changes were mainly driven by mortality of A. alba.LocationThree old‐growth forest reserves dominated by A. alba and Fagus sylvatica in the Dinaric Mountains of Slovenia.MethodTree layer and understorey regeneration and herbs were surveyed in 147 plots across the three forest reserves in 1983 and 2015. Soils were also sampled in 2015. The study period coincides with a protracted period of increased A. alba mortality in the canopy layer associated with anthropogenic emissions.ResultsBetween 1983 and 2015, the decline in canopy layer A. alba caused a recruitment pulse of F. sylvatica regeneration to the subcanopy tree layer across the three reserves. These changes were accompanied by a significant decline in plot level herb species richness. A model‐based analysis of beta‐diversity revealed significant community convergence during the study period, mainly caused by the loss of rare species. Ellenberg values indicate that these changes were mainly driven by loss of understorey light, while an increase in soil pH may have played a role also.ConclusionsThis observational study suggests that the long‐term decline of A. alba resulted in a cascade of processes — widespread F. sylvatica recruitment that impeded penetration of light to the forest floor, and possibly a change in soil conditions due to the decline of coniferous litter. These changes caused a significant loss of herb diversity and homogenization of the understorey community across the three sites. This study sheds light on the potential cascading consequences triggered by episodes of increased tree mortality resulting from global‐change‐type drivers.

Reclamation strategies for mined forest soils and overstorey drive understorey vegetation

Abstract Understorey vegetation accounts for the majority of plant diversity in boreal forest ecosystems and contributes to ecosystem functioning. In restoration of degraded forested ecosystems, however, understorey vegetation is often restored passively, contrasting to clear strategies such as informed species choice and site improvement intervention for overstorey vegetation. The choice of overstorey‐centred restoration strategy may have important consequences for understorey vegetation. We examined the effects of substrate material, overstorey type and time since reclamation (age) on understorey vegetation following reclamation of oil sands mining in Alberta, Canada. We sampled cover, richness, evenness and composition of understorey vegetation at 94 sites of conifer, mixedwood and broadleaf overstorey types on three reclamation substrates (overburden, secondary overburden and tailings sand), with age ranging from 4 to 30 years. Total, woody and non‐woody understorey cover and species richness were the highest on secondary overburden and the lowest on tailings sand, and total cover also decreased with age. Woody cover and richness were the highest under broadleaf overstorey, while non‐woody cover and richness were the lowest under conifer overstorey. Overall species evenness was not significantly affected by substrate type, overstorey type or age, but woody evenness was the highest on secondary overburden and the lowest on tailings sand, and non‐woody evenness showed overstorey‐dependent responses to age. Species composition varied with substrate type, overstorey type and age. Indicator species analysis revealed that tailings sand with conifer overstorey favoured grasses, while overburden and secondary overburden supported a mix of grasses, forbs and shrubs. Synthesis and applications. Our study demonstrates that overstorey‐centred reclamation strategies impact the abundance, diversity and composition of understorey plant communities following oil sands mining. Landforms constructed with secondary overburden substrates and revegetated with mixedwood or broadleaf tree species provide the most favourable habitats for understorey vegetation, while tailings sand provide a poor substrate for understorey species diversity and composition. We therefore recommend utilizing secondary overburden and overburden substrate material during landform construction, and employing revegetation prescriptions that target mixedwood and broadleaf overstorey types to promote productive and diverse understorey plant communities on the reclaimed landscape.

Contrasting understorey species responses to the canopy and root effects of a dominant shrub drive community composition

AbstractQuestionsContrasting understorey species responses to the effects of dominant species might occur within a single community even with neutral community‐level interactions. However, no studies have assessed the contribution of below‐ and above‐ground effects for all species of a community and their consequences for community composition. We tested the following hypotheses: (i) there are contrasting responses of understorey species to the canopy and root effects of a dominant shrub; (ii) the contrasting understorey responses to the canopy and root effects of the shrub are related to community composition.LocationSpecies‐rich sub‐alpine community, eastern Tibet Plateau, China, dominated by Dasiphora fruticosa.MethodsWe used a removal procedure and shade cloth treatment to separate root from canopy effects and quantified the biomass responses of 41 species of the understorey community. Species‐level responses to the root and canopy effects were quantified with the relative interaction index. We conducted multivariate analyses to assess the relative contribution of root and canopy effects to community composition.ResultsWe found contrasting species‐level responses to the canopy and root effects of D. fruticosa that could be grouped into six cluster groups. Dominant effects were positive for the roots and negative for the canopy, with a tendency for a trade‐off between the two. Community‐level effects were less strong and weakly significant, in particular for species richness. Root and canopy effects strongly explained understorey species composition but net shrub effects did not.ConclusionsThis study highlights that communities include species having contrasting responses to both the canopy and root effects of dominant species that importantly explain species composition.

Are germination cues for soil‐stored seed banks different in structurally different fire‐prone communities?

AbstractMany plant species are dependent on soil‐stored seeds for their persistence in fire‐prone systems. Seed germination is often stimulated by fire‐related cues including heat and smoke, but the way these cues promote germination may differ between structurally distinct plant communities with historically different fire regimes. In this study, we examined the effects of heat, smoke and their interactions on the germination of soil‐stored seeds from shrubby woodlands and herbaceous forests in south‐east Australia. The effect of these treatments on species richness, diversity and composition, and species richness and density of germinants within life‐forms (grass, forb and shrub) were assessed. Soils from each community were subjected to low heat (40°C), low heat with smoke, smoke, high heat (80°C), high heat with smoke and untreated (control) before being placed in a glasshouse, where the germinants were identified and counted. Greater species richness was stimulated by high heat treatments and smoke in both communities, a trend driven by shrubs and forbs, rather than grasses. Greater species diversity was stimulated by high heat with smoke in both communities. Greater densities of grass germinants were stimulated by all treatments, except low heat, in both communities. For forbs and shrubs, the effect of treatment depended on community. Compared to the control, low heat with smoke (forbs) and both low heat and low heat with smoke (shrubs) increased densities in the woodland but not in the forest. There were unique species compositions, different from the control, in all treatments in the forest but not in the woodland, where composition in low heat was not different from the control. These results indicate the importance of high soil temperatures and smoke in both communities. In the absence of wildfires, recurring prescribed burns that heat the soil to low temperatures are likely to reduce plant richness, diversity, and density resulting in a change in understorey species composition and structure.

Biodiversity related understorey stability of small peri-urban forest after a 100-year recurrent flood

A late successional and small old peri-urban forest, differing more in species richness than in community quality (due to the presence of ancient forest understorey species), experienced the impact of an exceptionally strong natural disturbance: a 100-year recurring flood. We were able to compare the 1-year old baseline data from permanent plots with the altered species composition after the flood. First, we present a detailed analysis of the baseline biodiversity data as a comparison of the tree and understorey species composition between the late successional and the old forest. We compared the plot community quality aspect—the characteristics of species composition (% geophytes, Illyrian species, presence of vulnerable species, etc.) and the community structure according to biodiversity indicators (S, E, H′). To analyze changes in the understorey after the flood, we calculated species turnover through the Sørensen pairwise dissimilarity index (β_sor) and investigated differences in species survival and post-flood occurrence changes. In our study, the flood did have an impact on understorey communities, and recovery was slower in the case of the species-poor understorey, with more new species entering the existing species composition, among them alien invasive species. Results suggest that high biodiversity in the understorey forms the basis for better disturbance resilience, with lower mortality and a higher nestedness component for recovery in such an understorey. Our results demonstrate how important it is to maintain high biodiversity in order to sustain the ecological resilience of even small and urban forests.

Effects of coarse woody debris on plant and lichen species composition in boreal forests

AbstractQuestionAlthough the importance of coarse woody debris (CWD) for understorey species diversity has been recognized, the relative effects of coarse woody debris decay class and substrate species on understorey species composition have received little attention. We examined how the species composition of understorey vegetation change with CWD decay class and substrate species.LocationBoreal mixed‐wood forests, Ontario, Canada.MethodsTo cover a wide range of CWD decay classes and substrate species, we sampled fire‐origin boreal forest stands that varied in stand age and canopy tree species composition. Vegetation on CWD was sampled by visually estimating percentage cover of each species within a 0.1 m × 0.5 m quadrat, randomly laid lengthwise on top of each sampled CWD log. We also recorded the forest floor vegetation by establishing an adjacent plot of the same size at a distance of 1.0 m in a random direction from the CWD vegetation sample.ResultsMultivariate analysis showed that understorey species composition differed among decay classes and substrate species. A NMDS ordination of understorey species composition revealed a clear separation of decay classes 1 and 2 from higher decay classes, and that decay classes 4 and 5 shared several species with the forest floor. The species composition on the forest floor was completely different from the species composition on CWD decay classes 1, 2 and 3. Two distinct groupings of substrates according to CWD species composition were found: conifer species (Pinus banksiana and Picea spp.) and broad‐leaf species (Betula papyrifera and Populus spp.), with Abies balsamea taking an intermediate position. Indicator species analysis showed distinct understorey species affiliations to substrate species at advanced decay classes. Understorey species composition on the CWD of P. banksiana showed particularly pronounced changes from the dominance of lichens on decay classes 2 and 3 to dominance by mosses and vascular species on decay classes 4 and 5.ConclusionsUnderstorey species composition on CWD not only differed with decay class, but also with CWD substrate species. Conservation strategies should aim at retaining diversity of CWD in terms of both decay classes and species composition in boreal forests.

Differences in early dynamics and effects of slope aspect between naturally regenerated and planted Pinus sylvestris woodland on inland dunes in Poland

Abstract: There is little knowledge of the effects of landform relief on early growth dynamics and competitive interactions of Pinus sylvestris (Scots pine) stands on inland dunes, which could potentially be substantial. The goal of this study was to examine and compare early dynamics (based on growth parameters and properties of the understorey vegetation) and the effects of slope aspect in naturally regenerated and pine-planted woodland on inland dunes in northern Poland. Growth parameters, tree density, and understorey vegetation were monitored on north- and south-facing slopes in a 26.3 ha post-fire area with natural secondary succession and eight even-aged pine-planted stands, 5-34 years old. Clear differences were detected between the woodland types, in both growth parameters of pines with similar ages and effects of slope aspect on the pines. In the natural regeneration area north-facing slopes provided favorable conditions for natural encroachment by pines. Tree density was higher, and the pines were taller and thicker, on north-facing than on south-facing slopes. In contrast, in the pine-planted area pines were larger on south-facing slopes, although growth conditions were less favorable than on north-facing slopes. However, in the pine-planted area the north-facing slopes had significantly higher herb cover, dominated by Deschampsia flexuosa, indicating that even the presence of a relatively low grass species can impede early growth of Scots pine. The understorey species composition differed in the natural regeneration area, being dominated by Calluna vulgaris on north-facing slopes and Corynephorus canescens on south-facing slopes. The results reveal that interactions between landform, natural dynamics, planting practices, and competitive interactions in woodlands on inland dunes are complex, and should be considered in efforts to manage them efficiently.

Forest vegetation in western Romania in relation to climate variables: Does community composition reflect modelled tree species distribution?

European beech (Fagus sylvatica L.) is the prevailing tree species of mesic forests in Central Europe. Increasing summer temperatures and decreasing precipitation, as climate change scenarios predict, may, however, negatively influence beech growth and induce a shift to more thermophilous forest communities. Temperatures as expected in the future for western Central Europe are currently found in parts of western Romania. In light of this climate analogy we investigated forest vegetation as an indicator for future vegetation changes in five regions of western Romania representing a climatic gradient. We related species composition to climate variables and examined if tree and understorey species composition respond similarly to the climatic gradient. We further analysed if tree species occurrences correspond with their modelled distance to the rear niche edge. We found evidence for climatic effects on vegetation composition among regions as well as within deciduous and pine forests, respectively. This underlines that vegetation composition is a useful indicator for environmental change. Tree and understorey species compositions were closely linked showing that community-based characterization of forest stands can provide additional information on tree species suitability along environmental gradients. Both, vegetation composition and a climatic marginality index demonstrate the rear niche edge occurrence of beech in the studied sites of Romania and can predict the site suitability for different tree species. While vegetation surveys indicate Quercus petraea to be associated to moderately mesic forests, the marginality index suggested an inner niche position of sessile oak along the climatic gradient. Phytosociological releves that differentiate between subspecies (or microspecies) of sessile oak with differing habitat requirements should be considered to complement national forest inventories and species distribution maps when modelling rear distribution edges. We conclude that climate driven forest vegetation composition in western Romania is a suitable analogon and may indicate future forest development in western Central Europe.

Variation in plant community composition and vegetation carbon pools a decade following a severe fire season in interior Alaska

AbstractQuestionsHow does fire severity, measured as depth of burn of ground layer fuels, control the regeneration of understorey species across black spruce‐dominated stands varying in pre‐fire organic layer depths? Are successional shifts from evergreen to deciduous understorey vegetation more likely to occur with greater depth of burn? Does a shift in understorey vegetation community towards more deciduous species influence carbon accumulation in vegetation biomass?LocationNorthern boreal forest, interior Alaska.MethodsWe sampled 32 stands in interior Alaska that burned in 2003 and 2004 in which depth of burn had been recorded soon after fire. In 2014 we characterized tree density, understorey vegetation composition, above‐ and below‐ground vegetation carbon pools, and a suite of environmental variables. We used ANOVA and multivariate redundancy analysis models to analyse the dominant controls on vegetation composition and carbon pools.ResultsFire severity was a strong control on post‐fire tree and non‐vascular species composition. Ten years post‐fire, sites that experienced deeper burning of organic layers had a higher abundance of deciduous tree species, fire‐adapted mosses, forbs and graminoids, and lower abundances of evergreen shrubs and Sphagnum mosses. Environmental variables (elevation, soil bulk density and mineral soil pH, moisture and temperature) served as important controls on tree and vascular understorey species composition. We found no evidence that carbon pools associated with recovering vegetation biomass were influenced by landscape position, fire severity or environmental variables.ConclusionsBoth fire frequency and depth of burn are projected to increase with a warmer climate in interior Alaska. These shifts in fire regime are expected to favour the regeneration of deciduous species over conifers, which have the potential to regulate successional pathways and large‐scale trends in albedo, which both ultimately could influence ecosystem–climate feedbacks and dampen future fire cycles. Our results show that understorey species composition is controlled by fire severity, suggesting that more severe burning causes a decline in the self‐replacement of conifer communities across a range of hydrologic settings. Overall, our results suggest a loss of resilience in black spruce forests in the face of a changing fire regime. However, we found no evidence that fire severity influences vegetation carbon pools 10 yrs post‐fire, suggesting that there is less landscape variation in biomass carbon pools than in vegetation composition and succession during early post‐fire succession.

Wild boar rooting intensity determines shifts in understorey composition and functional traits

In recent decades, the European populations of wild boar have grown substantially, as has the impact of this species, owing above all to its rooting activity. Our aim was to investigate the relationships between vascular plant understorey and wild boar rooting intensity. The questions we addressed are: does rooting intensity influence understorey species composition and diversity? Which functional traits are associated with different levels of rooting? We performed a comparative analysis of plant communities in areas with contrasting levels of rooting intensity within a Mediterranean deciduous lowland forest in central Italy. Besides comparing species composition and diversity, we tested the association between species traits and rooting levels through fourth-corner analysis. We found that contrasting levels of rooting were associated to different understorey species composition and evenness, while we observed no significant difference in species richness. In contrast with our expectations, sites with lower rooting returned i) lower evenness values and ii) a higher proportion of species characterized by traits related to resistance or response to herbivory, i.e., spinescence, clonality, endozoochory, underground storage organs, and low height values. Our findings suggest that current vegetation patterns partly depend on the legacy effect of past rooting disturbance, since the areas currently subjected to low rooting intensity were likely to be intensely rooted in the past. These areas may have developed a marked dominance of clonal thorny species that, in turn, inhibited further feeding activities by wild boar.

Above‐ground and below‐ground competition between the willow Salix caprea and its understorey

AbstractQuestionsHas the observed decline in the willow Salix caprea L., one of the most common colonizers of post‐mining sites in Central Europe, contributed to changes in the rest of the plant community on post‐mining sites? How does S. caprea modify its understorey? How is the understorey vegetation affected by above‐ground and below‐ground competition with S. caprea?LocationHeaps after coal mining near Sokolov, Czech Republic (50°14′21″ N, 12°40′45″ E).MethodsThe effects of above‐ground and below‐ground competition with the willow S. caprea on its understorey plant community were studied in un‐reclaimed post‐mining sites. Below‐ground competition was evaluated by comparing (1) frames inserted into the soil that excluded woody roots (frame treatment), (2) frames that initially excluded woody root growth but then allowed regrowth of the roots (open‐frame treatment), and (3) undisturbed soil (no‐frame treatment). These treatments were combined with S. caprea thinning to assess the effect of above‐ground competition.ResultsThree years after the start of the experiment, above‐ground competition from S. caprea (as modified by thinning of the S. caprea canopy) had not affected understorey biomass or species number, but had affected species composition. In contrast, below‐ground competition significantly affected both the above‐ground and below‐ground biomass of the understorey. The above‐ground biomass of the understorey was larger in the frame treatment (which excluded woody roots) than in the other two treatments. The below‐ground biomass of the understorey was larger in the frame than in the open‐frame treatment. Unlike above‐ground competition (light availability), below‐ground competition did not affect understorey species composition.ConclusionsOur results suggest that S. caprea is an important component during plant succession on post‐mining sites because it considerably modifies its understorey plant community. Below‐ground competition is a major reason for the low cover and biomass of the herbaceous understorey in S. caprea stands on post‐mining sites.

Drought impact on carbon and water cycling in a Mediterranean &amp;lt;i&amp;gt;Quercus suber&amp;lt;/i&amp;gt; L. woodland during the extreme drought event in 2012

Abstract. Savannah-type ecosystems account for 26–30% of global gross primary productivity GPP, with water being one of the major driving factors. In Europe, savannah-type woodlands cover an area of about 1.5 million ha. Here, the recent past has shown a significant decrease in precipitation P in winter and spring as well as a decrease in total annual precipitation. Strong effects on local water balance and carbon sink strength have thus been reported due to changes in precipitation regime. The objective of this study is to quantify the impact of the extreme drought event in 2012 on the water balance, gross primary productivity and carbon sink strength of a typical Portuguese cork-oak woodland (montado) compared to the wet year of 2011. Physiological responses of the dominant tree species Quercus suber (L.) are disentangled employing combined photosynthesis and stomatal conductance modelling. Precipitation effectiveness ET/P increased from 86% in 2011 to 122% in the 2012 dry year due to deep soil or groundwater access of the Q. suber trees leaving no water for groundwater replenishment. Understorey and overstorey GPP were strongly reduced, by 53 and 28%, respectively, in 2012 compared to 2011, due to the late onset of the autumn rains in 2011 and an additional severe winter/spring drought. However, the ecosystem was still a carbon sink in both years, but with a 38% reduced sink strength under extreme drought in 2012 compared to 2011. The combined photosynthesis–stomatal conductance model yielded the best results if it was allowed to adjust photosynthetic and stomatal parameters simultaneously. If stomatal response was modelled with the Leuning approach, which allows for a different sensitivity to vapour pressure deficit, the stomatal model parameters were highly coupled. A change in either of the parameters needed to be compensated by the other to guarantee a stable sensitivity of stomatal conductance to assimilation, independent of variations in vapour pressure deficit. The Q. suber trees showed a 37% reduced stomatal conductance during the drought period of 2012 compared to 2011, due to water supply limitations. In response to reduced leaf-internal CO2 availability, the trees strongly reduced the apparent maximum carboxylation rate by 43% in 2012 compared to 2011. Unexpectedly, the optimum temperature Topt of the maximum electron transport rate decreased during the drought period, enhancing the susceptibility of the trees to high temperature stress during the summer. Our results suggest that, if the trend of decreasing annual precipitation and changed precipitation patterns on the Iberian Peninsula continues, sustained effects on local groundwater reservoirs, understorey species composition and tree mortality have to be expected in the long term. To model the effect of drought on the montado ecosystem successfully, variable apparent maximum carboxylation rate Vc,max, stomatal conductance parameter m and vapour pressure deficit sensitivity parameter D0 need to be incorporated into photosynthesis–stomatal conductance modelling.