Survival,growth, and photosynthesis of tree seedlings competing with herbaceous vegetation along a water-light-nitrogen gradient

In herbaceous dominated patches and ecosystems, tree establishment is influenced partly by the ability of woody seedlings to survive and grow in direct competition with herbaceous vegetation. We studied the importance of season long wet and dry spells on the competitive interactions between herbaceous vegetation and oak seedlings along a light and nitrogen gradient in an infertile secondary successional grassland in central North America. We conducted a field experiment in which seedlings of bur oak (Quercus macrocarpa) and northern pin oak (Q. ellipsoidalis) were exposed to two levels of light (full sun and 80% shade), three levels of nitrogen input (0, 5, 15 g m^–1 yr^–1), and three levels of water input (low, medium and high). In addition, seedlings were grown with and without the presence of surrounding herbaceous vegetation under both light and all three water levels. Seedling survival, growth, and rate of photosynthesis were significantly affected by competition with herbaceous vegetation and these effects varied along the multiple resource gradient. Overall, seedling survival of both species was significantly greater in wetter and shaded plots and when surrounding herbaceous vegetation was removed and was lower in nitrogen enriched plots. We found that soil water was significantly affected by varying inputs of water, light, and the presence or absence of herbaceous vegetation, and that seedling survival and rate of photosynthesis were highly correlated with available soil water. Our findings show that the impact of season long wet and dry spells on tree seedling success in grasslands can be affected by light and soil nitrogen availability.     

Variation in the effects of vegetation and litter on recruitment across productivity gradients

1 We tested predictions about how the effect of vegetation and litter on seedling establishment varies among sites and herbaceous community types (sand barrens, prairies, fens). For both vegetation and litter, we also separated direct interactions from indirect interactions and interaction modifications along the gradient.
2 Although the intensity of the effects varied across sites, the direct effects of vegetation or litter alone were consistently facilitative along the productivity gradient. Predominance of facilitative effects may be due to the focus on the seedling establishment phase.
3 However, inclusion of indirect interactions and interaction modifications caused the net effects of both vegetation and litter to become largely negative. While one layer of biomass may be advantageous to ameliorate some moisture stress, the addition of another layer may be disadvantageous if this layer limits light proportionally more than it relieves moisture stress.
4 One exception to this pattern occurred at high productivity when the net effect of vegetation, even in the presence of litter, remained facilitative. The net effect of vegetation was competitive at low productivity and grew increasingly facilitative with productivity. Thus, indirect effects of litter may alter interaction patterns across this gradient.     

Evaluating the Relationship between Competition and Productivity within a Native Grassland

Ideas about how plant competition varies with productivity are rooted in classic theories that predict either increasing (Grime) or invariant (Tilman) competition with increasing productivity. Both predictions have received experimental support, although a decade-old meta-analysis supports neither. Attempts to reconcile the conflicting predictions and evidence include: expanding the theory to include other conditions (e.g. stress gradient hypothesis), development of indices to differentiate either the 'intensity' or 'importance' of competition, a focus on resource supply and demand, and explicit recognition that both growth and survival may exhibit different relationships with productivity. To determine which of these theories accurately predict how competition varies with productivity within a native grassland site, we estimated competitive intensity and relative competitive importance using 22 species across the range of productivity naturally occurring within that site. Plant performance was measured as survival and size with and without neighbours and the local environment was quantified according to variability in standing crop, gross water supply, and net water supply. On average, neighbours weakly facilitated seedling survival, but strongly reduced seedling growth. For both seedling survival and growth, relative competitive importance and competitive intensity declined with some measure of productivity; neighbour effects on survival declined with standing crop, while effects on growth declined with gross water supply. These results add to the growing evidence that plant-plant interactions vary among life history components with different life history components contingent upon separate environmental factors. Although the range of productivity measured in this study was not large, our results do not support the theories of Grime or Tilman. However, our results are consistent with the meta-analysis and parts of other theories, although no single theory is capable of explaining the entirety of these results. This suggests that, at least in moderately productive grasslands, new theory needs to be developed.     

Counterbalancing effects of competition for resources and facilitation against grazing in alpine snowbed communities

Alpine snowbeds are habitats where the major limiting factors for plant growth are herbivory and a small time window for growth due to late snowmelt. Despite these limitations, snowbed vegetation usually forms a dense carpet of palatable plants due to favourable abiotic conditions for plant growth within the short growing season. These environmental characteristics make snowbeds particularly interesting to study the interplay of facilitation and competition. We hypothesised an interplay between resource competition and facilitation against herbivory. Further, we investigated whether these predicted neighbour effects were species‐specific and/or dependent on ontogeny, and whether the balance of positive and negative plant–plant interactions shifted along a snowmelt gradient. We determined the neighbour effects by means of neighbour removal experiments along the snowmelt gradient, and linear mixed model analyses. The results showed that the effects of neighbour removal were weak but generally consistent among species and snowmelt dates, and depended on whether biomass production or survival was considered. Higher total biomass and increased fruiting in removal plots indicated that plants competed for nutrients, water, and light, thereby supporting the hypothesis of prevailing competition for resources in snowbeds. However, the presence of neighbours reduced herbivory and thereby also facilitated survival. For plant growth the facilitative effects against herbivores in snowbeds counterbalanced competition for resources, leading to a weak negative net effect. Overall the neighbour effects were not species‐specific and did not change with snowmelt date. Our finding of counterbalancing effects of competition and facilitation within a plant community is of special theoretical value for species distribution models and can explain the success of models that give primary importance to abiotic factors and tend to overlook interrelations between biotic and abiotic effects on plants.     

Spatial and environmental factors contributing to patterns in arboreal and terrestrial oribatid mite diversity across spatial scales

Recruitment is a crucial event in the plant life cycle that is very sensitive to interaction with established vegetation. Based on a large comparative experiment, we tested the hypothesis that the components of recruitment––emergence time and rate, seedling survival and biomass––differ in response to plant–plant interactions during recruitment. The consequences for the population are predicted with a simple demographic model assessing the response of seed production. In a common garden experiment, we recorded the recruitment of four target species in an individual-based survey protocol. A total of 7,680 seeds were sown within 20 neighbourhoods, consisting of 19 mono-specific herbaceous stands and a control treatment without vegetation. We measured transmitted light, temperature and moisture at soil surface to characterise the environmental conditions within neighbourhoods. The mean height of neighbours controlled temperature buffering and light interception and thus depicted the interaction gradient. Emergence rate and time increased with neighbour height in two of the four target species, while seedling survival and biomass significantly decreased with neighbour height in three and all four target species, respectively. We recorded a shift in seedling neighbour interactions under the tallest neighbours that largely favoured emergence but strongly depressed seedling survival and biomass. The components of recruitment were predicted to differ in their impact on later adult performance. Biomass strongly contributed to predicted seed production in three target species, and emergence had an equal or greater impact on a fourth species. These results confirm the fundamental role of plant–plant interactions in the recruitment of herbaceous species through a complex combination of habitat amelioration, which facilitates emergence and light competition, which in turn limits seedling survival and biomass.     

Effects of resource availability on seedling recruitment in a fire-maintained savanna

The herbaceous ground cover of the longleaf pine ecosystem harbors the highest plant species richness in North America, with up to 50 species per square meter, but the mechanisms that regulate this diversity are not well understood. In this system, variability in seedling recruitment events may best explain the extremely high small-scale species richness and its relationship to soil moisture and system net primary productivity. To understand the potential mechanistic controls on species richness, we used a long-term resource manipulation study across a natural soil moisture gradient to assess environmental controls on seedling recruitment. We considered the availability of resources to be an indicator of seedling safe-site supply, and also manipulated seed availability to examine the relative importance of recruitment limitations on seedling diversity. We found that water availability regulated the number of species in the seedling community regardless of the underlying natural moisture gradient, and that this effect may result from differential responses of seedling guilds to resource availability. Water supply was more important than seed supply in determining seedling establishment, suggesting that appropriate sites for regeneration are a factor limiting seedling success. This is the first study that shows that the episodic supply of microsites for recruitment could influence species richness in the highly threatened and biodiverse longleaf pine savanna.     

Soil nitrogen and carbon heterogeneity in woodlands and grasslands: contrasts between temperate and tropical regions

Aim  Soil resource heterogeneity is linked to several ecological processes including invasion of woody species into grasslands. Studies from the temperate zone have demonstrated greater soil heterogeneity beneath woody vegetation than beneath grasslands. Woody species have a more widespread and coarser root system than herbaceous species, and may have a competitive advantage in relatively heterogeneous soils. We tested the global generality of greater soil heterogeneity beneath woody vegetation.
Location  Global.
Methods  We used data from published literature for soil nitrogen and carbon heterogeneity from paired woodland and grassland sites around the world.
Results  Woodland and grassland soil heterogeneities from paired observations were strongly correlated. There was, however, significant geographical variability in the relationship. Soils were more heterogeneous in woodlands than grasslands in temperate areas, but the opposite was true for tropical habitats. Grassland soils were more heterogeneous at lower than higher latitudes. Woodland soil heterogeneity did not vary with latitude.
Main conclusions  The previously described high soil heterogeneity in woody vegetation compared to grasslands holds only for temperate regions. Consequently, the relationship between soil resource heterogeneity and vegetation type is dependent on the study region. Macroecological studies should test the generality of relationships between soil and vegetation at the global scale.     

Abiotic constraints and biotic resistance control the establishment success and abundance of invasive <Emphasis Type="Italic">Humulus japonicus</Emphasis> in riparian habitats

Dispersal, abiotic and biotic constraints are all involved in explaining the success of invasive plants but how these factors influence the different life stages of an invader remains poorly known. Focusing on highly invaded riparian habitats we asked: (1) how do propagule pressure, resource availability and resident vegetation influence the success of the invasive Asian vine Humulus japonicus at different stages of its life cycle (i.e. seedling, vegetative and flowering) (2) what is the influence of increasing resource availability on the performance and trait plasticity of H. japonicus compared to a functionally similar co-occurring native species? To answer the first question we performed a repeated field survey along the Gardon River (S France) with detailed measurements of distance to the riverbed, soil characteristics, light availability, and resident vegetation cover. To answer the second question we used a greenhouse experiment to compare the biomass and three functional traits of H. japonicus and Galium aparine along a gradient of increasing water and nitrogen availability. Initial germination success was only determined by abiotic constraints, while the role of biotic resistance increased for later stages with establishment success favoured by the interaction of low resident vegetation cover and high soil fertility, and final integrated success favoured by high light availability. H. japonicus performed better and showed higher plasticity in plant height than G. aparine under increased resource availability while their biomass did not differ in the lower part of the resource gradient. Our study demonstrates that by combining field and experimental studies and analysing responses at different life stages we can gain a more complete understanding of how ecological filters shape successful invasions in the course of the life cycle.     

Nutrient and carbon relations in subalpine dwarf shrubs after neighbour removal or fertilization in northern Italy

Two subalpine dwarf-shrub heath communities with differing levels of soil nutrient availability were subjected to a 3-year experimental manipulation, including nutrient addition or removal of one of the two co-dominant species from each community. The main objective of our study was to assess the relative importance of interspecific competition versus nutrient limitation in relation to soil fertility. We also aimed to investigate if and to what extent current-year shoot size, leaf-based rates of net photosynthesis and foliar nutrient status accounted for the observed changes in the aboveground biomass of the shrubs. At the end of the experiment, neighbour removal increased the aboveground biomass of all shrubs, especially in the more fertile community, while fertilization did not. We concluded that: (1) competition is more effective than nutrient limitation in structuring the vegetation of subalpine heathlands; and (2) competition intensity is stronger in the more fertile community. The observed patterns of variations in aboveground biomass were not consistently related to net photosynthetic rates, size of individual shoots and foliar nutrient status. Hence, we also concluded that the growth response of dwarf shrubs to altered environmental conditions is primarily determined by developmental plasticity.     

Ecosystem responses to changes in plant functional type composition: An example from the Patagonian steppe

Abstract. Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.     

三峡水库消落区植被在差异性水淹环境中的分布格局

人工水库修建引发的差异性水文节律是决定消落区植被群落格局的主要因素,高强度水淹环境中水淹胁迫是影响植被的重要因子而低强度水淹环境中物种竞争是影响植被的重要因子。为了探究差异性水淹环境中三峡水库消落区植物的水淹耐受能力及光资源竞争能力(植物株高)对植被群落分布格局的影响,对三峡水库典型消落区不同水淹强度下生长的植被进行了研究,结果表明:(1)典型消落区调查共发现有植物41种,其中高耐淹低竞争能力型植物4种,其生物量在所有物种生物量中的占比达70.99%,低耐淹高竞争能力型植物23种,其生物量占比为28.02%,低耐淹低竞争能力型植物14种,生物量占比不足1%,消落区内无高耐淹高竞争能力型植物物种分布;(2)高耐淹低竞争能力型植物在水淹强度大的消落区区域占优,低耐淹高竞争能力型植物在植物物种竞争压力大的消落区区域占据主导,低耐淹低竞争能力型植物在消落区中仅有零星分布;(3)消落区植被生物量格局随着高程增加呈现出先增加后减少的趋势。研究差异性水淹环境对三峡水库消落区植被分布的影响,可以为深入理解消落区植被分布格局的形成机制和大型水库消落区植被恢复与重建提供理论依据。     

Changes in plant biomass and species composition of alpine <Emphasis Type="Italic">Kobresia</Emphasis> meadows along altitudinal gradient on the Qinghai-Tibetan Plateau

Alpine Kobresia meadows are major vegetation types on the Qinghai-Tibetan Plateau. There is growing concern over their relationships among biodiversity, productivity and environments. Despite the importance of species composition, species richness, the type of different growth forms, and plant biomass structure for Kobresia meadow ecosystems, few studies have been focused on the relationship between biomass and environmental gradient in the Kobresia meadow plant communities, particularly in relation to soil moisture and edaphic gradients. We measured the plant species composition, herbaceous litter, aboveground and belowground biomass in three Kobresia meadow plant communities in Haibei Alpine Meadow Ecosystem Research Station from 2001 to 2004. Community differences in plant species composition were reflected in biomass distribution. The total biomass showed a decrease from 13196.96±719.69 g/m2 in the sedge-dominated K. tibetica swamp to 2869.58±147.52 g/m2 in the forb and sedge dominated K. pygmaea meadow, and to 2153.08±141.95 g/m2 in the forbs and grasses dominated K. humilis along with the increase of altitude. The vertical distribution of belowground biomass is distinct in the three meadow communities, and the belowground biomass at the depth of 0-10 cm in K. tibetica swamp meadow was significantly higher than that in K. humilis and K. pygmaea meadows (P＜0.01). The herbaceous litter in K. tibetica swamp was significantly higher than those in K. pygnaeca and K. humilis meadows. The effects of plant litter are enhanced when ground water and soil moisture levels are raised. The relative importance of litter and vegetation may vary with soil water availability. In the K. tibetica swamp, total biomass was negatively correlated to species richness (P＜0.05); aboveground biomass was positively correlated to soil organic matter, soil moisture, and plant cover (P＜0.05); belowground biomass was positively correlated with soil moisture (P＜0.05). However, in the K. pygnaeca and K. humilis meadow communities, aboveground biomass was positively correlated to soil organic matter and soil total nitrogen (P＜0.05). This suggests that the distribution of biomass coincided with soil moisture and edaphic gradient in alpine meadows.     

Rooting depth,water availability,and vegetation cover along an aridity gradient in Patagonia

Above-and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44–45°S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above-and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at-27), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80–0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2–3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2–3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.     

End-of-season soil water depletion in relation to growth of herbaceous vegetation in a sub-humid Mediterranean dwarf-shrub community on two contrasting soils

Dwarf-shrub communities of Sarcopoterium spinosum dominate large areas of the landscape on hilly, eastern Mediterranean rangelands. Colonisation of new areas depends on the establishment of seedlings that must compete for water with the ubiquitous annual herbaceous species during the spring-winter growing season and also survive the first hot, dry summer. The present study investigated the role of the herbaceous vegetation patches growing between S. spinosum shrubs on the depletion of soil water during the critical transition period between the cool, rainy season and the dry summer. Dense and sparse herbaceous vegetation stands were established in S. spinosum dwarf-shrub communities by differential use of fertiliser on two contrasting soil types – a terra rossa overlying hard limestone where seedling establishment is low and a pale rendzina overlying a soft chalk substrate where seedling establishment is high. Soil water in the main root zone of the herbaceous vegetation between the shrubs was monitored with protected gypsum block sensors permanently placed at two depths (10 and 33 cm). Soil water depletion during the transition from the wet to the dry season was significantly more rapid under dense vegetation only on the terra rossa soil where the herbaceous vegetation also matured more rapidly than on the rendzina soil. However, in both habitats and under both dense and sparse vegetation, soil water depletion during the transition period left very little available water in the rooting zone of the herbaceous vegetation to maintain shrub seedlings throughout the summer. It was concluded that the difference in shrub seedling establishment success in the two habitats mainly reflects the differences in accessibility of water below the rooting zone of the herbaceous vegetation growing on the two contrasting soil types.     

Importance,but not intensity of plant interactions relates to species diversity under the interplay of stress and disturbance

The lack of clarity on how the intensity and importance of plant interactions change under the co‐occurrence of stress and disturbance strongly impedes assessing the relative importance of plant interactions for species diversity. We addressed this issue in subalpine grasslands of the French Pyrenees. A natural soil moisture gradient further experimentally stretched at both ends was used and a mowing disturbance treatment was applied at each position along the soil moisture gradient. Changes in intensity and importance of plant interactions were assessed by a neighbour removal experiment using four target ecotypes. A structural equation modelling approach was used to assess the relative impact of stress, disturbance, the intensity and importance of plant interactions on diversity at both the neighbourhood and community scales. Without mowing, changes in intensity and importance of plant interactions only diverged in the dry part of the soil moisture gradient. The intensity of plant interactions linearly shifted from competition to facilitation with increasing stress, while the importance followed a hump‐shaped relationship. Species diversity components were tightly related to the importance of plant interactions only, both the neighbourhood and community scales. Mowing disturbance strongly reduced the importance of facilitation along the soil moisture gradient, and suppressed the relationship between the importance of plant interactions and diversity components. Together, our results highlight that 1) the importance is the best predictor of variations in species diversity in this subalpine herbaceous system, and 2) that fine‐scale processes such as plant interactions can affect the entire plant communities. Finally, our results suggest that high level of constraints due to co‐occurring stress and disturbance can inhibit the effects of plant interactions on species diversity, highlighting their potential role in regulating diversity and the maintenance/extinction of plant communities. Synthesis How plant interactions change along environmental gradients is an unsolved debate, particularly when both stress and disturbance interact. This lack of clarity explains why the relative impact of plant interactions (intensity and importance) on species diversity has been rarely assessed. Using an experimental approach, we found that the importance of plant interactions highly contributed to variation in species diversity, confirming that neighbourhood scale processes such as plant interactions can affect the entire plant communities. The co‐occurrence of stress and disturbance inhibited the effects of plant interactions, highlighting that plant interactions may regulate drops of diversity and the maintenance/extinction of plant communities.     

Effects of temporal heterogeneity of water supply on the growth of Perilla frutescens depend on plant density

Background and Aims

Plants respond to the spatial and temporal heterogeneity of a resource supply. However, their responses will depend on intraspecific competition for resource acquisition. Although plants are subject to various intensities of intraspecific competition, most studies of resource heterogeneity have been carried out under a single density so that the effects of intraspecific competition on plant responses to resource heterogeneity are largely unknown.

Methods

A growth experiment was performed to investigate plant responses to the temporal heterogeneity of water supply and nutrient levels under multiple plant densities. The annual plant Perilla frutescens was grown using different combinations of frequency of water supply, nutrient level and density, while providing the same total amount of water under all conditions. The effects of the treatments on biomass, allocation to roots and intensity of competition were analysed after 48 d.

Key Results

Biomass and allocation to roots were larger under homogeneous than under heterogeneous water supply, and the effects of water heterogeneity were greater at high density than at low density. The effects of water heterogeneity were greater at high nutrient level than at low level for biomass, while the effects were greater at low nutrient level than high level for allocation to roots. Competition was severer under homogeneous than under heterogeneous water supply.

Conclusions

Competition for water probably makes plants more sensitive to the water heterogeneity. In addition, the intensity of intraspecific competition can be affected by the temporal patterns of water supply. Because both resource heterogeneity and intraspecific competition affect resource acquisition and growth of plants, their interactive effects should be evaluated more carefully under future studies.     

Complex facilitation and competition in a temperate grassland: loss of plant diversity and elevated CO2 have divergent and opposite effects on oak establishment

Encroachment of woody vegetation into grasslands is a widespread phenomenon that alters plant community composition and ecosystem function. Woody encroachment is often the result of fire suppression, but it may also be related to changes in resource availability associated with global environmental change. We tested the relative strength of three important global change factors (CO₂ enrichment, nitrogen deposition, and loss of herbaceous plant diversity) on the first 3 years of bur oak (Quercus macrocarpa) seedling performance in a field experiment in central Minnesota, USA. We found that loss of plant diversity decreased initial oak survival but increased overall oak growth. Conversely, elevated CO₂ increased initial oak seedling survival and reduced overall growth, especially at low levels of diversity. Nitrogen deposition surprisingly had no net effect on survival or growth. The magnitude of these effects indicates that long-term woody encroachment trends may be most strongly associated with those few individuals that survive, but grow much larger in lower diversity patches. Further, while the CO₂ results and the species richness results appear to describe opposing trends, this is due only to the fact that the natural drivers are moving in opposite directions (decreasing species richness and increasing CO₂). Interestingly, the mechanisms that underlie both patterns are very similar, increased CO₂ and increased species richness both increase herbaceous biomass which (1) increases belowground competition for resources and (2) increases facilitation of early plant survival under a more diverse plant canopy; in other words, both competition and facilitation help determine community composition in these grasslands.     

High land‐use intensity exacerbates shifts in grassland vegetation composition after severe experimental drought

Climate change projections anticipate increased frequency and intensity of drought stress, but grassland responses to severe droughts and their potential to recover are poorly understood. In many grasslands, high land‐use intensity has enhanced productivity and promoted resource‐acquisitive species at the expense of resource‐conservative ones. Such changes in plant functional composition could affect the resistance to drought and the recovery after drought of grassland ecosystems with consequences for feed productivity resilience and environmental stewardship. In a 12‐site precipitation exclusion experiment in upland grassland ecosystems across Switzerland, we imposed severe edaphic drought in plots under rainout shelters and compared them with plots under ambient conditions. We used soil water potentials to scale drought stress across sites. Impacts of precipitation exclusion and drought legacy effects were examined along a gradient of land‐use intensity to determine how grasslands resisted to, and recovered after drought. In the year of precipitation exclusion, aboveground net primary productivity (ANPP) in plots under rainout shelters was ?15% to ?56% lower than in control plots. Drought effects on ANPP increased with drought severity, specified as duration of topsoil water potential ψ < ?100 kPa, irrespective of land‐use intensity. In the year after drought, ANPP had completely recovered, but total species diversity had declined by ?10%. Perennial species showed elevated mortality, but species richness of annuals showed a small increase due to enhanced recruitment. In general, the more resource‐acquisitive grasses increased at the expense of the deeper‐rooted forbs after drought, suggesting that community reorganization was driven by competition rather than plant mortality. The negative effects of precipitation exclusion on forbs increased with land‐use intensity. Our study suggests a synergistic impact of land‐use intensification and climate change on grassland vegetation composition, and implies that biomass recovery after drought may occur at the expense of biodiversity maintenance.     

Patterns of diversity in plant and soil microbial communities along a productivity gradient in a Michigan old-field

The relationship between plant diversity and productivity has received much attention in ecology, but the relationship of these factors to soil microbial communities has been little explored. The carbon resources that support soil microbial communities are primarily derived from plants, so it is likely that the soil microbial community should respond to changes in plant diversity or productivity, particularly if the plant community affects the quality or quantity of available carbon. We investigated the relationship of plant diversity and productivity to the composition of the soil microbial community along a topographic gradient in a mid-successional old-field in southwestern Michigan. Soil moisture, soil inorganic N, and plant biomass increased from the top to the base of the slope, while light at ground level decreased along this same gradient. We characterized the changes in resource levels along this gradient using an index of productivity that incorporated light levels, soil N, soil moisture, and plant biomass. Average plant species richness declined with this productivity index and there were associated compositional changes in the plant community along the gradient. The plant community shifted from predominantly low-growing perennial forbs at low productivities to perennial grasses at higher productivities. Although there was variation in the structure of the soil microbial community [as indicated by fatty acid methyl ester (FAME) profiles], changes in the composition of the soil microbial community were not correlated with plant productivity or diversity. However, microbial activity [as indicated by Biolog average well color development and substrate-induced respiration (SIR)] was positively correlated with plant productivity. The similarity between patterns of plant biomass and soil microbial activity suggests that either plant productivity is driving microbial productivity or that limiting resources for each of these two communities co-vary.     

Persistent nitrogen limitation of stream biofilm communities along climate gradients in the Arctic

Climate change is rapidly reshaping Arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in Arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll‐a accumulation. For unamended biofilms, activity and biomass accrual were not closely related to any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition was: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, Arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.