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Many recent studies have analysed plant species responses to environmental change, but interactive effects of global change drivers and how they are modulated by biotic interactions are still poorly understood. In a mesocosm experiment, we studied the interactive effects of nitrogen (N) fertilization and drought events on plant growth and how these effects are shaped by competitive interactions, using a segetal plant community typical of the lowlands of central Europe (composed of Lilium bulbiferum (segetal species) and Secale cereale (crop species)). We expected that N fertilization increases the drought sensitivity of Lilium (negative interaction effect), and that these effects are shaped by interspecific competition with Secale. Secale and Lilium showed opposing responses to N fertilization (second year of the experiment): Whilst Secale aboveground and belowground biomass almost doubled with N fertilization, Lilium aboveground and belowground biomass showed no response or decreased, respectively, providing Secale with a competitive advantage. Lilium aboveground tissue dieback (as a proxy for growth vigour) was 22% in N and 35% in drought treatments (control: 6%), but reached 91% when combining these treatments. Increasing Lilium tissue dieback was strongly related to decreasing belowground (root) biomass, caused by both negative direct effects of combined treatments (N fertilization + drought), and negative indirect effects acting via treatment-induced increase in Secale biomass. Our results demonstrate that competitive interactions can shape the effects of global change drivers on plant growth. This knowledge in turn could be important for plant species conservation, particularly in the face of ongoing shifts in environmental conditions.

Forest restoration increases global forest area and ecosystem services such as primary productivity and carbon storage. How tree species functional composition impacts the provisioning of these services as forests develop is sparsely studied. We used 10-year data from 478 plots with 191,200 trees in a forest biodiversity experiment in subtropical China to assess the relationship between community productivity and community-weighted mean (CWM) or functional diversity (FD) values of 38 functional traits. We found that effects of FD values on productivity became larger than effects of CWM values after 7 years of forest development and that the FD values also became more reliable predictors of productivity than the CWM values. In contrast to CWM, FD values consistently increased productivity across ten different species-pool subsets. Our results imply that to promote productivity in the long term it is imperative for forest restoration projects to plant multispecies communities with large functional diversity.

Though competition operates distinctly above and below ground, competition with near neighbours explained more variation in crown and root system morphology of young tropical trees than did abiotic conditions. Comparisons of above- and below-ground tree architecture and variation remain scarce in the literature of competition dynamics in forests. Trees compete above and below ground with neighbours, but the extent of such a neighbourhood and the relative importance of above- or below-ground neighbours in determining tree growth rates remain poorly understood. We combined terrestrial laser scans and a root excavation campaign in the tropical planted Sardinilla forest experiment with annual diameter-at-breast height measurements to test how the crowns and root systems of five tropical tree species (N = 128) varied with biotic neighbourhood and abiotic environmental characteristics. Crown and root system extent were compared, and the Hegyi competition index calculated at three spatial scales was regressed on focal tree biomass. We identified which neighbourhood and environmental variables explained most of the variation in mean crown and root system extent and compactness (a measure of morphological homogeneity) at the plot level and for species separately. Root systems were 2.6 ± 1.1 times wider than crowns on average and focal tree biomass was the best predicted by competition with near neighbours. At the plot level, more variation in crown and root system traits was explained by biotic than abiotic characteristics (65.8 and 26.0%, respectively). More intense competition in the near neighbourhood reduced crown extent of one study species (Anacardium excelsum) and root system extent of 4 out of 5 study species, and it also led to more compact crowns and root systems of Tabebuia rosea. After accounting for spatial autocorrelation in focal tree location, soil nitrogen concentration was not found to significantly affect the studied morphological traits. These results emphasize the role of competition in determining tree morphology generally, while also supporting the claim that competition operates distinctly above and below ground.

With progressing climate change, increasing weather extremes will endanger tree regeneration. Canopy openings provide light for tree establishment, but also reduce the microclimatic buffering effect of forests. Thus, disturbances can have both positive and negative impacts on tree regeneration. In 2015, three years before an extreme drought episode hit Central Europe, we established a manipulation experiment with a factorial block design in European beech (Fagus sylvatica L.)-dominated forests. At five sites located in southeastern Germany, we conducted three censuses of tree regeneration after implementing two different canopy disturbances (aggregated and distributed canopy openings), and four deadwood treatments (retaining downed, standing, downed + standing deadwood and removing all deadwood), as well as in one untreated control plot. In addition, we measured understory light levels and recorded local air temperature and humidity over five years. We (i) tested the effects of experimental disturbance and deadwood treatments on regeneration and (ii) identified the drivers of regeneration density as well as seedling species and structural diversity. Regeneration density increased over time. Aggregated canopy openings supported species and structural diversity, but reduced regeneration density. Tree regeneration was positively associated with understory light levels, while maximum vapor pressure deficit influenced tree regeneration negatively. Deadwood and browsing impacts on regeneration varied and were inconclusive. Our study indicates that despite the drought episode regeneration in beech-dominated forests persisted under moderately disturbed canopies. However, the positive effect of increased light availability on tree regeneration might have been offset by harsher microclimate after canopies have been disturbed.

Forest liming is often used as a mitigation tool to counteract soil acidification that is caused by atmospheric deposition. However, our understanding of the long-term effects of liming on forest communities is still incomplete. We examine the effects of repeated liming on the herb layer of spruce stands in the Ore Mountains in East Germany. The area is characterised by acidic bedrock and profound soil acidification caused by high levels of anthropogenic sulphur deposition in the late twentieth century. Lime has been applied as part of forest management since 1988. We created a stratified sample in which we recorded vegetation releves covering spruce stands without liming (L0) and with one to five lime applications (L1–L5). We found a positive correlation between liming intensity and alpha diversity. Most of the indicator species were identified for spruce stands with high liming intensity (L2–L5), but no indicators were found for unlimed stands. From the third liming, the impact of lime application on species diversity and composition saturated. Several of the species promoted by liming are typical for mature stands of the natural vegetation but had declined in the twentieth century. Thus, a slight trend towards the restoration of the natural vegetation can be seen after liming. However, this is contrasted with a strong increase in species that are disturbance indicators (i.e. species of nitrogen-rich soils and open habitats). Continuation of liming as part of regular forestry management will cause these effects to establish permanently over large areas. Based on these findings, we make suggestions to adapt forest management strategies.

Climate change and atmospheric deposition of nitrogen affect biodiversity patterns and functions of forest ecosystems worldwide. Many studies have quantified tree growth responses to single global change drivers, but less is known about the interaction effects of these drivers at the plant and ecosystem level. In the present study, we conducted a full-factorial greenhouse experiment to analyse single and combined effects of nitrogen fertilization (N treatment) and drought (D treatment) on 16 morphological and chemical response variables (including tissue δ13C signatures) of one-year-old Fagus sylvatica seedlings originating from eight different seed families from the Cantabrian Mountains (NW Spain). Drought exerted the strongest effect on response variables, reflected by decreasing biomass production and increasing tissue δ13C signatures. However, D and N treatments interacted for some of the response variables, indicating that N fertilization has the potential to strengthen the negative effects of drought (with both antagonistic and amplifying interactions). For example, combined effects of N and D treatments caused a sevenfold increase of necrotic leaf biomass. We hypothesize that increasing drought sensitivity was mainly attributable to a significant reduction of the root biomass in combined N and D treatments, limiting the plants’ capability to satisfy their water demands. Significant seed family effects and interactions of seed family with N and D treatments across response variables suggest a high within-population genetic variability. In conclusion, our findings indicated a high drought sensitivity of Cantabrian beech populations, but also interaction effects of N and D on growth responses of beech seedlings.

In the original publication of the article, the Table 2 column heading that reads “L6” should read “SB” and the ORCID iD of the fourth author was missed in the original publication and included in this correction.

We investigated long-term responses (since 1850) of Fagus sylvatica (Luxembourg; central Europe) to shifts in temperature, precipitation, and nitrogen deposition by analyzing diameter at breast height (DBH) increment, basal area increment (BAI), and tree-ring stable isotopes (δ13C, δ15N). We compared stands on soils with contrasting water supply (Regosols and Cambisols with an available water capacity of ca. 40 and 170 mm, respectively) and of two different age classes (ca. 60 vs. 200 years). All stands showed a peak in DBH increment in the decade 1978–1987, but a decline in increment growth in subsequent decades. In addition, BAI declined in mature stands in the last two decades. Decreasing increment rates were attributable to an increasing drought limitation of stands, mainly induced by increasing temperatures in the last two decades. Contrary to our expectations, stands on Cambisols showed a similar susceptibility to shifts in temperature and precipitation as stands on Regosols, suggesting a strong adaptation of the respective ecotypes grown at dryer sites. This result was in line with long-term trends for tree-ring δ13C signatures, which did not differ significantly between stands on Cambisols and Regosols. Climate impacts on tree-ring δ15N signatures were low. High spring precipitation and temperatures caused increasing and decreasing δ15N values, respectively, but only in mature stands on Cambisols. Stands on Regosols tended to have lower δ15N values than stands on Cambisols. Decreasing δ15N values in recent decades suggest an increasing impact of allochthonous N loads with isotopically lighter N. We investigated long-term responses (since 1850) of Fagus sylvatica (Luxembourg; central Europe) to shifts in temperature, precipitation, and nitrogen deposition by analyzing diameter at breast height (DBH) increment, basal area increment (BAI), and tree-ring stable isotopes (δ13C, δ15N). We compared stands on soils with contrasting water supply (Regosols and Cambisols with an available water capacity of ca. 40 and 170 mm, respectively) and of two different age classes (ca. 60 vs. 200 years). All stands showed a peak in DBH increment in the decade 1978–1987, but a decline in increment growth in subsequent decades. In addition, BAI declined in mature stands in the last two decades. Decreasing increment rates were attributable to an increasing drought limitation of stands, mainly induced by increasing temperatures in the last two decades. Contrary to our expectations, stands on Cambisols showed a similar susceptibility to shifts in temperature and precipitation as stands on Regosols, suggesting a strong adaptation of the respective ecotypes grown at dryer sites. This result was in line with long-term trends for tree-ring δ13C signatures, which did not differ significantly between stands on Cambisols and Regosols. Climate impacts on tree-ring δ15N signatures were low. High spring precipitation and temperatures caused increasing and decreasing δ15N values, respectively, but only in mature stands on Cambisols. Stands on Regosols tended to have lower δ15N values than stands on Cambisols. Decreasing δ15N values in recent decades suggest an increasing impact of allochthonous N loads with isotopically lighter N.

We investigated the response of the perennialgrass Molinia caerulea (L.) Moench to combinedeffects of fertilization (N, P) and drought events. Wehypothesized that N fertilization increases, anddrought decreases productivity, but that N additionstrengthens negative effects caused by drought.Methods Within a full-factorial 2-year greenhouseexperiment we measured biomass productivity andallocation, tissue nutrient concentrations and nitrogenallocation patterns using 15N as a tracer.Results N fertilization caused a strong increase inproductivity, but effects of drought were almostinsignificant. However, we found strongly interrelated,non-additive effects of fertilization and drought,expressed by a strong increase of necrotic tissue.Dead aboveground biomass showed the highestvalues for N and 15N.Conclusions Accelerated productivity of abovegroundtissue under N fertilization resulted in increased evaporativedemands and thus higher drought susceptibility.In addition 15N allocation patterns showed thatfertilization-drought treatments disenabled plants’control of their N allocation. Molinia was unable towithdraw leaf N during the dieback of abovegroundtissue. Due to the lack of an adaptive strategy to thecombined effects of fertilization and drought, increasingsummer drought may weaken the competitiveperformance of species with traits comparable tothose of Molinia in N-fertilized environments. We investigated the response of the perennialgrass Molinia caerulea (L.) Moench to combinedeffects of fertilization (N, P) and drought events. Wehypothesized that N fertilization increases, anddrought decreases productivity, but that N additionstrengthens negative effects caused by drought.Methods Within a full-factorial 2-year greenhouseexperiment we measured biomass productivity andallocation, tissue nutrient concentrations and nitrogenallocation patterns using 15N as a tracer.Results N fertilization caused a strong increase inproductivity, but effects of drought were almostinsignificant. However, we found strongly interrelated,non-additive effects of fertilization and drought,expressed by a strong increase of necrotic tissue.Dead aboveground biomass showed the highestvalues for N and 15N.Conclusions Accelerated productivity of abovegroundtissue under N fertilization resulted in increased evaporativedemands and thus higher drought susceptibility.In addition 15N allocation patterns showed thatfertilization-drought treatments disenabled plants’control of their N allocation. Molinia was unable towithdraw leaf N during the dieback of abovegroundtissue. Due to the lack of an adaptive strategy to thecombined effects of fertilization and drought, increasingsummer drought may weaken the competitiveperformance of species with traits comparable tothose of Molinia in N-fertilized environments.

The specialized fauna of peat bogs declines strongly both in species numbers and population density and becomes fragmented because of anthropogenic land use changes. We investigated 15 populations of the stenotopic ground beetle species Agonum ericeti in south Sweden to address the question of reduced genetic variability in populations inhabiting smaller habitat patches. Our results reveal a generally low differentiation and a significant positive relationship between habitat size and allelic richness The specialized fauna of peat bogs declines strongly both in species numbers and population density and becomes fragmented because of anthropogenic land use changes. We investigated 15 populations of the stenotopic ground beetle species Agonum ericeti in south Sweden to address the question of reduced genetic variability in populations inhabiting smaller habitat patches. Our results reveal a generally low differentiation and a significant positive relationship between habitat size and allelic richness.

In the present study we analyzed the combined effects of management (grazing, mowing, prescribed burning, sod-cutting) and atmospheric deposition on N and P budgets of heathland ecosystems (Lüneburger Heide nature reserve; N Germany). We hypothesize that management measures such as grazing and mowing can accelerate a deposition-induced imbalance of N and P pools as a result of a disproportionally high output of P. We analyzed management and deposition affected input-output flows of N and P and related them to changes in the nutritional status of Calluna vulgaris 5 years after treatment application. We found that grazing and mowing caused the highest net loss of P due to high P concentrations in the aboveground biomass. In contrast, prescribed burning only slightly affected P pools, as P remained in the system due to ash deposition. Management-mediated effects on N and P pools were mirrored in the nutritional status of Calluna vulgaris: at the grazed and mown sites, the P content of current season's shoots significantly decreased within 5 years after treatments, whereas the N content remained unchanged. We conclude that grazing and mowing can accelerate declining availability of P and, thus, accelerate a deposition-induced shift from N- to P-limited plant growth in the medium term. In the face of ongoing atmospheric N loads management schemes need to combine high- and low-intensity measures to maintain both a diverse structure and balanced nutrient budgets in the long term. © 2009 Springer Science+Business Media, LLC. In the present study we analyzed the combined effects of management (grazing, mowing, prescribed burning, sod-cutting) and atmospheric deposition on N and P budgets of heathland ecosystems (Lüneburger Heide nature reserve; N Germany). We hypothesize that management measures such as grazing and mowing can accelerate a deposition-induced imbalance of N and P pools as a result of a disproportionally high output of P. We analyzed management and deposition affected inputoutput flows of N and P and related them to changes in the nutritional status of Calluna vulgaris 5 years after treatment application. We found that grazing and mowing caused the highest net loss of P due to high P concentrations in the aboveground biomass. In contrast, prescribed burning only slightly affected P pools, as P remained in the system due to ash deposition. Management-mediated effects on N and P pools were mirrored in the nutritional status of Calluna vulgaris: at the grazed and mown sites, the P content of current season’s shoots significantly decreased within 5 years after treatments, whereas the N content remained unchanged. We conclude that grazing and mowing can accelerate declining availability of P and, thus, accelerate a deposition-induced shift from N- to P-limited plant growth in the medium term. In the face of ongoing atmospheric N loads management schemes need to combine high- and low-intensity measures to maintain both a diverse structure and balanced nutrient budgets in the long term.

The diversity, spatial patterns and temporal dynamics of dead wood were examined within the near-natural beech forests (Fagus sylvatica) of Serrahn (North-eastern Germany). Data were collected in an 8 ha sample plot and in two permanent plots (0.36 and 0.25 ha) that had been established at the end of the 1960s. The mean volume of dead wood was 94 m(3) ha(-1), amounting to 14% of the total volume of all trees. The dead wood displayed a large variation in dead wood type, tree size and decay class. Standing dead wood accounted for about one-third of the total dead wood volume. The densities of standing dead trees were about 10% of the densities of the living trees over a wide range of diameters. The overall spatial distribution of dead trees exhibited a random pattern. Among the different dead wood types, standing entire dead trees and uprooted trees deviated from this pattern by displaying a significantly aggregated pattern. In the permanent plots a high mortality of overstorey trees was observed (1.3% year(-1)) and the average amount of dead wood increased greatly from 2.9 to 111.6 m(3) ha(-1) over the 35-year observation period. The near-natural beech forests of Serrahn have experienced a long period of low human interference. Nevertheless, our results suggest that the structure and dynamics of dead wood are strongly affected by the last major disturbance events that took place at the end of the Middle Ages. Information about the forest history is therefore a basic requirement when interpreting the results obtained in near-natural forests.

The effects of soil disturbance caused by the uprooting of a single or a few canopy trees on species richness and composition of vascular plant species and bryophytes were examined in a temperate beech forest (Fagus sylvatica) in northern Germany. We recorded the vegetation in 57 pairs of disturbed and adjacent undisturbed plots and established a chronosequence of mound ages to study the effect of time since microsite formation on plant species richness and composition. We found significant differences in plant species richness and composition between disturbed and adjacent undisturbed plots. Species richness of both vascular plants and bryophytes was higher in the disturbed than in the undisturbed plots, but these differences were more pronounced for bryophytes. We suggest that three main factors are responsible for this differential response. The availability of microsites on the forest floor that are suitable for the recruitment of bryophytes is lower than for vascular plants. Establishment of bryophytes in disturbed microsites is favoured by a greater abundance of propagules in the close vicinity and in the soil of the disturbed microsites, as well as by a greater variety of regeneration strategies in bryophytes than in vascular plants. Time since mound formation was a major factor determining plant species richness and composition. A significant decrease in the mean number of species was found from young mounds to intermediate and old mounds. However, differences were observed between vascular plants and bryophytes in the course of changes through time in species richness and composition. A large number of exclusive and infrequent vascular plant species was observed on young mounds, among them several disturbance specialists. We suggest that the establishment of many vascular plant species was infrequent and short-lived due to unfavourable light conditions and a low abundance of propagules. By contrast, the development of a litter layer was the main reason for the decreased mean number of bryophytes on old mounds. Our study supports the view that groups of species differing in important life history traits exhibit different responses to soil disturbance.

Heathland management is an important tool with which to modify ecosystem impacts caused by atmospheric nutrient deposition. Since changes in nutrient availability as a result of management measures affect the outcomes of heathland succession and species competition, studies on this issue are important from both a nature conservation and management point of view. This study reports the effects of prescribed burning on nutrient availability in dry heathland soils and the nutrient content of the two competing heathland species Calluna vulgaris and Deschampsia flexuosa, with particular reference to N and P. We hypothesise that winter prescribed burning leads to additional N availability, which enhances the importance of P in the context of nutrient limitation in heathland ecosystems. In the nature reserve ``Lueneburg Heath'' (NW Germany) we examined the availability of nutrients in the humus horizons and in the leachate as well as the relevant C:element ratios in Calluna and Deschampsia before and after a burning experiment. Our results show that prescribed burning resulted in drastically increased NH4+ availability in the O-horizon. We observed only short-term effects (for NO3-, PO43-, Mg) and insignificant effects on the availability of other nutrients (K, Ca). As a consequence of an increased nutrient availability in the humus horizons and a limited nutrient uptake by plants after burning, leaching increased significantly for N, Ca, K, and Mg after burning treatment. No significant changes were found in the foliar C:N ratios for either species after prescribed burning, although Deschampsia showed an increased deficiency for all the other nutrients, particularly for P, as expressed by increased foliar C:P and N:P ratios. By contrast, the nutrient content of Calluna did not change significantly, suggesting that prescribed burning favours the competitive capacity of Calluna as against Deschampsia. We assume that water shortage as a result of changes in the microclimate was mainly responsible for the deterioration of the nutrient content of Deschampsia. This gives Calluna a competitive advantage, enabling it to out-compete Deschampsia on burned heathlands, with respect to the key factor P-limitation. Heathland management is an importanttool with which to modify ecosystem impactscaused by atmospheric nutrient deposition. Sincechanges in nutrient availability as a result ofmanagement measures affect the outcomes ofheathland succession and species competition,studies on this issue are important from both anature conservation and management point ofview. This study reports the effects of prescribedburning on nutrient availability in dry heathlandsoils and the nutrient content of the two competingheathland species Calluna vulgaris andDeschampsia flexuosa, with particular referenceto N and P. We hypothesise that winter prescribedburning leads to additional N availability,which enhances the importance of P in the contextof nutrient limitation in heathland ecosystems.In the nature reserve ‘‘Lueneburg Heath’’(NW Germany) we examined the availability ofnutrients in the humus horizons and in theleachate as well as the relevant C:element ratiosin Calluna and Deschampsia before and after aburning experiment. Our results show thatprescribed burning resulted in drastically increasedNH4+ availability in the O-horizon. Weobserved only short-term effects (for NO3 – , PO43–,Mg) and insignificant effects on the availability ofother nutrients (K, Ca). As a consequence of anincreased nutrient availability in the humus horizonsand a limited nutrient uptake by plants afterburning, leaching increased significantly for N,Ca, K, and Mg after burning treatment. Nosignificant changes were found in the foliar C:Nratios for either species after prescribed burning,although Deschampsia showed an increased deficiencyfor all the other nutrients, particularly forP, as expressed by increased foliar C:P and N:Pratios. By contrast, the nutrient content of Callunadid not change significantly, suggesting that prescribedburning favours the competitive capacityof Calluna as against Deschampsia. We assumethat water shortage as a result of changes in themicroclimate was mainly responsible for thedeterioration of the nutrient content ofDeschampsia. This gives Calluna a competitiveadvantage, enabling it to out-competeDeschampsia on burned heathlands, with respectto the key factor P-limitation.

The endozoochorous dispersal of vascular plant species by red deer (Cervus elaphus) was investigated in three forest areas of Schleswig-Holstein and Lower Saxony. The primary aim of this study was to determine of which plant species and in what numbers diaspores are dispersed by red deer, an animal with eminently large home ranges during a single day as well as over the year. A second aim was to examine the significance of endozoochory for plant species composition and diversity in forests. Special emphasis was placed on whether there are differences between the three study sites and in the course of the vegetation period. Freshly deposited faecal pellet groups were collected in the forests on five dates between May and November 2002 and viable seed content was determined by greenhouse germination (seedling emergence method). A total of 28,009 individuals representing 59 vascular plant species were detected in 220 dung samples. Red deer appears thus as an important vector for endozoochorous dispersal of plants. The number of seedlings found in the faecal pellets differs between plant species and research areas as well as in the course of the vegetation period. Dung seed content is largely dominated by one species, Urtica dioica. Grouping the species found in dung according to their habitat preference showed that mainly species that occur in forests as well as in open vegetation and non-forest species were dispersed endozoochorously, while species occurring mainly under a closed canopy were present in the excrements only in low numbers.