Semiarid Grassland Community Research Articles

Grazing alters the relationships between species diversity and biomass during community succession in a semiarid grassland

The relationships between biodiversity and ecosystem functions (BEF) are crucial for ecosystem management. However, little is known about how grazing affects BEF relationships in the context of ecological succession. Here, using a 5-year experiment in a semiarid grassland of the Loess Plateau, China, we mainly focused on how grazing affected the relationships between plant species diversity and aboveground biomass (AGB) and explored the underlying mechanisms behind the relationships. In addition, we compared the plant dynamics of community composition and structure under no-grazing and grazing treatments during succession. We found that the plant species diversity-AGB relationship shifted from a negative-linear pattern in no-grazing to a humped-back model in grazing during plant community succession, suggesting that grazing could regulate dominant species and alter the availability of light resources to suppress competitive exclusion during succession. In addition, changes in annual plants over time played crucial roles in the BEF relationships. The increase in annual Salsola collina in this study, which also alters multiple mechanisms of plant interaction, had a significant effect on the negative-linear relationship both with and without grazing. On average, compared to no-grazing treatment, grazing significantly decreased the plant community density (39.53 %), cover (16.97 %), height (7.85 %), and AGB (9.35 %), but increased plant diversity, including species richness and the Shannon–Wiener index, and especially dramatically enhanced the Shannon–Wiener index (ranging from 1.55 to 2.13). These results underline the close association between grazing and the dynamics of plant communities in semiarid grasslands during succession. In particular, our findings further reveal grazing-dependent relationships between diversity and AGB, which have significant implications for the management and biodiversity conservation measures of semiarid grassland ecosystems.

Leaf Photosynthetic and Functional Traits of Grassland Dominant Species in Response to Nutrient Addition on the Chinese Loess Plateau

Leaf photosynthetic and functional traits of dominant species are important for understanding grassland community dynamics under imbalanced nitrogen (N) and phosphorus (P) inputs. Here, the effects of N (N0, N50, and N100, corresponding to 0, 50, and 100 kg ha-1 yr-1, respectively) or/and P additions (P0, P40, and P80, corresponding to 0, 40, and 80 kg ha-1 yr-1) on photosynthetic characteristics and leaf economic traits of three dominant species (two grasses: Bothriochloa ischaemum and Stipa bungeana; a leguminous subshrub: Lespedeza davurica) were investigated in a semiarid grassland community on the Loess Plateau of China. Results showed that, after a three-year N addition, all three species had higher specific leaf area (SLA), leaf chlorophyll content (SPAD value), maximum net photosynthetic rate (PNmax), and leaf instantaneous water use efficiency (WUE), while also having a lower leaf dry matter content (LDMC). The two grasses, B. ischaemum and S. bungeana, showed greater increases in PNmax and SLA than the subshrub L. davurica. P addition alone had no noticeable effect on the PNmax of the two grasses while it significantly increased the PNmax of L. davurica. There was an evident synergetic effect of the addition of N and P combined on photosynthetic traits and most leaf economic traits in the three species. All species had relatively high PNmax and SLA under the addition of N50 combined with P40. Overall, this study suggests that N and P addition shifted leaf economic traits towards a greater light harvesting ability and, thus, elevated photosynthesis in the three dominant species of a semiarid grassland community, and this was achieved by species-specific responses in leaf functional traits. These results may provide insights into grassland restoration and the assessment of community development in the context of atmospheric N deposition and intensive agricultural fertilization.

The contribution of understory and Compositae species to canopy storage capacity of semiarid grassland communities on the Loess Plateau

AbstractCanopy rainfall interception reduces the effectiveness of limited rainfall resources in arid and semiarid regions. To clarify the effects of species composition and community vertical structure on canopy storage capacity, the artificial wetting and simulated rainfall methods were used to determine the species (C) and community (Cmax) water storage, respectively, in three typical grassland communities on the semiarid Loess Plateau. Results indicated that water storage capacity of 31 grassland species ranged from 0.47 to 3.73 g g−1, with an average of 1.60 g g−1, and leaf storage capacity was significantly higher than the stem. Plant storage capacity was correlated with leaf traits (including leaf fresh weight, length and area). Canopy storage capacity of three grassland communities (Bothriochloa ischaemum, Artemisia gmelinii and Lespedeza davurica) ranged from 0.77 to 1.09 mm and were positively correlated with community leaf area index (LAI), coverage and leaf dry biomass. Leaf and stem water storage accounted for 57.6% and 42.4% of the total community, respectively. The water storage of Compositae, Gramineae, Leguminosae and other species accounted for 35.5%, 27.0%, 23.4% and 14.1%, respectively. The plants in the community with heights of >40, 20–40 and 0–20 cm contributed 5.8%, 30.3% and 63.9% of the water storage, respectively. The field observed canopy storage capacity was significantly higher than the estimated value through scaling up the individual storage data to the community, revealing complex community structure and species composition may increase actual rainfall interception. These results highlighted the species at 0–20 cm height was the key component affecting canopy water storage, and Compositae species may intercept more rainfall within the community, which imply that reducing the understory or Compositae species composition of grassland community would be helpful for increasing limited rainfall partitioning to soil in the drought area.

Soil microbial community structure is unaltered by grazing intensity and plant species richness in a temperate grassland steppe in northern China

Grazing intensity influences the productivity and sustainability of grassland systems through modifying aboveground biomass, soil microbes, and nutrient dynamics. Although heavy grazing is a key factor associated with grassland degradation in China and alleviating grazing intensity can assist grassland vegetation restoration, the influence of variable grazing intensity on belowground processes is less understood. We examined the effects of four grazing intensities (0, 1.00, 1.43 and 2.33 sheep-units ha−1 year−1) applied during five growing seasons on soil microbial biomass and community structure of a grassland ecosystem in Hebei Province, China. Within each grazing intensity, vegetation and soil were sampled in three patch types: low and high plant species richness and one composed of the dominant species (i.e., Leymus chinensis [Trin.], Tzvel. and Carex duriuscula C. A. Mey.). We sought to clarify whether soil microbial communities are influenced by an interaction between grazing intensity and patch type. Grazing significantly (P < 0.05) decreased soil microbial biomass and increased the ratio of bacterial-to-fungal biomass relative to the non-grazed control, but differences were not significant among the grazing intensities. Patch type did not significantly impact microbial biomass (P = 0.12), but the interaction between patch type and grazing intensity was significant (P < 0.05). The highest microbial biomass value (19.20 ± 0.51 μg · g−1) was found in non-grazed, low diversity patches. In addition, principal components analysis revealed that grazing intensity explained 25.42 and 54.53% of the variation among bacterial and fungi communities, respectively. Grazing, regardless of intensity, also resulted in bacterial communities distinct from the non-grazed control. Redundancy analysis also revealed that soil pH and organic carbon had the strongest influence on bacterial community structure, soil electrical conductivity and NO3−-N primarily influenced fungi community structure. Collectively our results indicate that microbial communities in semiarid grasslands can remain structurally stable even when subjected to persistent environmental change imposed by grazing pressure.

Elevated CO2, warming, N addition, and increased precipitation affect different aspects of the arbuscular mycorrhizal fungal community

The functional diversity of arbuscular mycorrhizal fungi (AMF) affects the resistance and resilience of plant communities to environmental stress. However, considerable uncertainty remains regarding how the complex interactions among elevated atmospheric CO2 (eCO2), nitrogen deposition (eN), precipitation (eP), and warming (eT) affect AMF communities. These global change factors (GCFs) do not occur in isolation, and their interactions likely affect AMF community structure and assembly processes. In this study, the interactive effects of these four GCFs on AMF communities were explored using an open-top chamber field experiment in a semiarid grassland. Elevated CO2, eN, eT, eP, and their interactions did not affect AMF biomass. The relative abundance of Paraglomus increased with N addition across treatment combinations, whereas that of Glomus decreased with N addition, especially combined with eT and eCO2. Precipitation, temperature (T), and N affected AMF phylogenetic α-diversity, and the three-way interaction among CO2, T, and N affected taxonomic and phylogenetic α-diversity. N addition significantly affected the composition of AMF communities. Both variable selection and dispersal limitation played major roles in shaping AMF communities, whereas homogeneous selection and homogenizing dispersal had little effect on AMF community assembly. The contribution of variable selection decreased under eCO2, eN and eT but not under eP. The contribution of dispersal limitation decreased under eCO2, eT, and eP but increased under eN. The assembly of AMF communities under the sixteen GCF combinations was strongly affected by dispersal limitation, variable selection and ecological drift. Elevated CO2, warming, N addition, and increased precipitation affected different aspects of AMF communities. The interactive effects of the four GCFs on AMF communities were limited. Overall, the results of this study suggest that AMF communities in semiarid grasslands can resist changes in global climate.

Responses of soil respiration to rainfall depth and frequency in semiarid grassland communities

AbstractClimate change is increasing the extreme precipitation depth and frequency, which may cause a strong response of ecological processes in drought regions. To investigate how rainfall depth and frequency alter soil respiration (SR), a rainfall simulation experiment was conducted in grassland communities dominated by Artemisia gmelinii and Lespedeza davurica in Loess Plateau of China. SR rate (Rs), soil temperature (Ts) and soil volumetric water content (Sv) were monitored before and after the rainfall treatments, that is, four depths (5, 10, 20 and 40 mm) and three frequencies (40 mm × 1, 20 mm × 2 and 10 mm × 4) during the growing season (June to September). Results indicated that the response magnitude of Rs increased with rainfall depths, reaching the maximum under 40 mm, and the increments were tightly related to rainfall frequency and community type. The increase and mean value of Rs in A. gmelinii community were significantly higher than those in L. davurica and bare land under same rainfall depth. L. davurica community was more sensitive to rainfall lower than 10 mm, whereas it had weaker but longer response under rainfall larger than 10 mm compared with A. gmelinii community. Successive rainfall events dampened the pulse effect of Rs but generated more cumulative CO2 emission in vegetation communities. Ts and Sv varied significantly with rainfall depth and co‐regulated SR. These findings implied more CO2 will be released from soil in the semiarid grasslands under extreme and successive rainfalls and emphasized the importance of species impact on SR for soil carbon evaluation under future rainfall regimes.

Effects of glyphosate on soil fungal communities: A field study

The driving forces behind many soil processes are microorganisms and they are able to respond immediately to environmental changes. The soil microbial community impacts on many soil properties. More than one-third of the terrestrial ecosystems are semiarid. However, a limited number of studies have been conducted to characterize soil fungal communities in semiarid grasslands, in particular those of agricultural fields. The aim of this study was to explore changes in the diversity and structure of soil fungal communities in semiarid grasslands, after different doses of glyphosate were applied under field conditions. Changes in soil fungal communities were examined using different approaches including culturing, calcofluor white stain and denaturing gradient gel electrophoresis (DGGE). The different approaches complement each other, revealing different aspects of the effect of glyphosate on soil fungal communities. We demonstrated a negative effect of glyphosate on soil fungal biomass at high doses and an early and transitory stimulatory effect on soil fungal biomass. We also found a negative effect of glyphosate on the species richness of cultivable fungi and changes in the molecular structure of soil fungal communities after double doses or long-term glyphosate application. In summary, our findings demonstrate an overall negative effect of glyphosate on soil fungal communities.

Direct effects of nitrogen addition on seed germination of eight semi-arid grassland species.

Seed germination plays an important role in mediating plant species composition of grassland communities under nitrogen (N) enrichment. Shifts of plant community structure with N‐enhanced deposition in terrestrial ecosystems have occurred globally. Despite numerous studies about the effects of enhanced N deposition on mature plant communities, few studies have focused on seed germination. Using a laboratory experiment, we report the effects of five N concentrations, including 0, 5, 10, 20, and 40 mM N (NH4NO3) on seed germination of eight semi‐arid grassland species. Results showed that low N concentrations (5‐ and 20‐mM N) promoted mean final germination proportion of all eight species by 4.4% and 6.4%, but high concentrations (40 mM N) had no effect. The mean germination rate was decreased 2.1% and 5.1% by higher N concentration (20‐ and 40‐mM N) levels, but germination start time showed the opposite trend, delayed by 0.7, 0.9, and 1.8 d for the 10, 20, and 40 mM N treatments. Final germination proportion, mean germination rate, and germination start time were significantly different among species in response to N concentration treatments. The final germination proportion of Allium tenuissimum and Chenopodium glaucum were suppressed by increased N concentration, whereas it increased for Potentilla bifurca, Plantago asiatica, and Setaria viridis. Our findings provide novel insights into N deposition‐induced species loss based on seed germination factors in semi‐arid grassland communities.

Identification of suites of traits that explains drought resistance and phenological patterns of plants in a semi-arid grassland community

Grassland ecosystems are comprised of plants that occupy a wide array of phenological niches and vary considerably in their ability to resist the stress of seasonal soil-water deficits. Yet, the link between plant drought resistance and phenology remains unclear in perennial grassland ecosystems. To evaluate the role of soil water availability and plant drought tolerance in driving phenology, we measured leaf hydraulic conductance (Ksat), resistance to hydraulic failure (P50), leaf gas exchange, plant and soil water stable isotope ratios (δ18O), and several phenology metrics on ten perennial herbaceous species in mixed-grass prairie. The interaction between P50 and δ18O of xylem water explained 67% of differences in phenology, with lower P50 values associated with later season activity, but only among shallow-rooted species. In addition, stomatal control and high water-use efficiency also contributed to the late flowering and late senescence strategies of plants that had low P50 values and relied upon shallow soil water. Alternatively, plants with deeper roots did not possess drought-tolerant leaves, but had high hydraulic efficiency, contributing to their ability to efficiently move water longer distances while maintaining leaf water potential at relatively high values. The suites of traits that characterize these contrasting strategies provide a mechanistic link between phenology and plant-water relations; thus, these traits could help predict grassland community responses to changes in water availability, both temporally and vertically within the soil profile.

Woody plant encroachment restructures bird communities in semiarid grasslands

The abundance and distribution of woody plants have increased in grassland ecosystems worldwide. Robust generalizations about the consequences of this transformative process on animal communities have been elusive, especially in semiarid regions where populations of many species have declined. We evaluated how distributions and species richness of breeding birds responded to woody plant encroachment by using spatial variation in woody cover as a proxy for the temporal process by which grasslands transform into shrub savannas. Specifically, we surveyed breeding birds and vegetation on 140 10-ha plots in semiarid grasslands that spanned the gradient of cover by Prosopis (mesquite), a genus of shrubs that has proliferated in semiarid grasslands worldwide. We used a multispecies occupancy model to characterize distributions of breeding bird species along the encroachment gradient. Distributions of 29 of 35 species changed markedly in response to encroachment, with distributions of most obligate grassland species contracting and most facultative grassland species expanding. Species richness increased sharply as cover of woody plants increased and peaked at ∼22% cover; this increase was driven by recruitment of generalist and shrub-associated species, many of which are common at regional scales. Lastly, we identified thresholds of woody cover where distributions contracted or expanded markedly, which provide targets for conservation and restoration efforts. Our results highlight the importance of understanding species-specific responses to woody plant encroachment as the basis for explaining community-level patterns because increases in diversity at local scales might ultimately reduce diversity at broader scales as grassland specialists are displaced.

The effects of abiotic factors in South African semi-arid grassland communities on Seriphium plumosum L density and canopy size.

Most studies suggest that multiple factors are responsible for woody plant encroachment, but are inconclusive on its causes. Woody plant encroachment is modified by local patterns of disturbance, topography, soil texture and fertility, and their relative importance and interaction strength varies among locations. We used grassland communities, Carletonville Dolomite Grassland (CDG), Rand Highveld Grassland (RHG) and abiotic factors (Soil Organic Carbon (SOC), Total nitrogen (TN), Phosphorus (P), Potassium (K), Sodium (Na), Calcium (Ca), Magnesium (Mg) and pH) to investigate habitat preferences of Seriphium plumosum L. in South African semi-arid grassland. Grassland community had a significant effect on S. plumosum density, canopy size, and on components of soil fertility (P< 0.05). Slope position had a significant effect on S. plumosum density and canopy size (P< 0.05). Seriphium plumosum density was significantly lower (23 plants/50 m2±2.39 (SE)) in CDG than in RHG (40 plants/50 m2±4.19). By contrast, S. plumosum canopy size was significantly higher (1.39 m2±0.11) at CDG than at the RHG (1.06 m2±0.08). The interaction of grassland community and slope position had a significant effect on S. plumosum density and soil sodium content (P< 0.05). Bottom slope regions in RHG (59.00 plants/50 m2±8.62) had higher S. plumosum density than bottom slope regions (19.75 plants/50 m2±3.01) in CDG. Soil sodium content was similar in CDG slopes and higher than in RHG slopes. Seriphium plumosum density was positively related to components of soil fertility; P (r2 = 0.1270; P 0.0036), K (r2 = 0.0786; P 0.0237), Na (r2 = 0.0686; P 0.0350), Ca (r2 = 0.0681; P 0.0358), and SOC content (r2 = 0.0669; P 0.0374). However, Seriphium plumosum canopy size did not show any relationship with components of soil fertility. This study revealed that S. plumosum density in grassland communities increased with increasing soil texture and fertility.

Small rainfall pulses affected leaf photosynthesis rather than biomass production of dominant species in semiarid grassland community on Loess Plateau of China.

In the semiarid region Loess Plateau of China, rainfall events, typically characterised as pulses, affect photosynthesis and plant community characteristics. The response of dominant species and grassland community to rainfall pulses was evaluated through a simulation experiment with five pulse sizes (0, 5, 10, 20 and 30mm) in the semiarid Loess Plateau of China in June and August of 2013. The study was conducted in a natural grassland community dominated by Bothrichloa ischaemum (L.)Keng and Lespedeza davurica (Lax.) Schindl. In June, the leaf photosynthetic rate (Pn), transpiration rate, stomatal conductance, intercellular CO2 concentration of both species and soil water content increased rapidly after rainfall pulses. B. ischaemum was more sensitive to the pulses and responded significantly to 5mm rainfall, whereas L. davurica responded significantly only to rainfall events greater than 5mm. The magnitude and duration of the photosynthetic responses of the two species to rainfall pulse gradually increased with rainfall sizes. The maximum Pn of B. ischaemum appeared on the third day under 30mm rainfall, whereas for L. davurica it appeared on the second day under 20mm rainfall. Soil water storage (0-50cm) was significantly affected under 10, 20 and 30mm rainfall. Only large pulses (20, 30mm) increased community biomass production by 21.3 and 27.6% respectively. In August, the effect of rainfall on the maximum Pn and community characteristics was generally not significant. Rainfall pulses affected leaf photosynthesis because of a complex interplay between rainfall size, species and season, but might not induce a positive community-level feedback under changing rainfall patterns.

Plant assemblage composition and soil P concentration differentially affect communities of AM and total fungi in a semi-arid grassland.

Adding inorganic P- and N-fixing legumes to semi-arid grasslands can increase forage yield, but soil nutrient concentrations and plant cover may also interact to modify soil fungal populations, impacting short- and long-term forage production. We tested the effect of plant assemblage (seven native grasses, seven native grasses + the domesticated N-fixing legume Medicago sativa, seven native grasses + the native N-fixing legume Dalea purpurea or the introduced grass Bromus biebersteinii + M. sativa) and soil P concentration (addition of 0 or 200 P2O5 kg ha(-1) at sowing) on the diversity and community structure of arbuscular mycorrhizal (AM) fungi and total fungi over two consecutive years, using 454-pyrosequencing of 18S rDNA and ITS amplicons. Treatment effects were stronger in the wet year (2008) than the dry year (2009). The presence of an N-fixing legume with native grasses generally increased AM fungal diversity, while the interaction between soil P concentration and plant assemblage modified total fungal community structure in 2008. Excluding interannual variations, which are likely driven by moisture and plant productivity, AM fungal communities in semi-arid grasslands appear to be primarily affected by plant assemblage composition, while the composition of other fungi is more closely linked to soil P.

Interactions of soil water content heterogeneity and species diversity patterns in semi-arid steppes on the Loess Plateau of China

• Coverage, biomass were significantly increased with surface soil water contents. • Plant diversity were closely correlated to soil water content of 0–10 cm soil layer. • Soil water content affect species richness and then affect belowground biomass. • Species diversity is an indicator of community responses to soil water content. Soil water is a major driving force for plant community succession in semi-arid area. Many studies have focused on the relationships of species diversity–productivity, but few studies have paid attentions to the effects of soil water content heterogeneity on the plant species diversity in the semi-arid loess regions. To determine relationship of soil water content heterogeneity and plant community structure properties a semi-arid steppe on the Loess Plateau, we conducted a gradient analysis of soil water content variation and above- and below-ground properties of plant communities. Results showed that community coverage, above- and below-ground biomass were significantly and positively related to the surface soil water contents (0–5 cm). Plant diversity (Shannon index and Richness index) were closely correlated to soil water content at the soil depth of 0–10 cm. But plant height, litter biomass and root/shoot ratio were not related to soil water content. These results showed that there is an positive interaction effects for plant diversity and soil water content in the semi-arid grassland communities. Our observations indicate that change of plant species diversity is also an important community responses to soil water content heterogeneity in the semi-arid grassland ecosystem.

Differences in plant cover and species composition of semiarid grassland communities of central Mexico and its effects on net ecosystem exchange

Abstract. Changes in land use across the semiarid grasslands of northern Mexico have driven a decline of plant cover and alteration of plant species composition. A number of different plant communities have resulted from these changes. Their implications, however, on the carbon (C) cycle and regional carbon balance are still poorly understood. Here, we examined the effects of plant cover loss and changes in species composition on net ecosystem CO2 exchange (NEE) and their biotic and abiotic controls. NEE was measured in five representative plant community types within a semiarid grassland by temporarily enclosing the entire aboveground ecosystem using a chamber method (i.e., geodesic dome). Sites included an oat crop (crop), a moderately grazed grassland (moderate grazing), a 28 yr-old grazing exclosure (exclosure), an overgrazed site with low perennial grass cover (overgrazed), and an overgrazed site presenting shrub encroachment (shrub encroachment). For natural vegetation, rates of standardized daytime NEE for sites with a high plant cover (exclosure and moderate grazing) were similar (P &gt; 0.05) as compared to sites with low plant cover (overgrazed and shrub encroachment). However, yearly total nighttime NEE (carbon loss) was more than double (P &lt; 0.05) for sites with high plant cover compared to sites with low cover, resulting to slight C sinks for the low plant cover sites, and neutral or sources for the high plant cover sites as accounted by daytime and nighttime NEE annual balance. Differences in plant cover and its associated biomass defined the sensitivity to environmental controls. Thus, daytime NEE in low plant cover sites reached light compensation points at lower photosynthetic photon flux density than those from high plant cover sites. Differences in species composition did not influence NEE rates even though there were transient or permanent changes in C3 vs. C4 functional groups. Our results allowed the detection of the large variability and contribution of different plant communities to regional C balance in patchy landscapes. Identification of the role of landscape patches in the regional C balance as either sinks or sources may provide tools allowing land use management strategies that could favor C uptake in patchy landscapes.

Soil microbial responses to warming and increased precipitation and their implications for ecosystem C cycling

A better understanding of soil microbial ecology is critical to gaining an understanding of terrestrial carbon (C) cycle-climate change feedbacks. However, current knowledge limits our ability to predict microbial community dynamics in the face of multiple global change drivers and their implications for respiratory loss of soil carbon. Whether microorganisms will acclimate to climate warming and ameliorate predicted respiratory C losses is still debated. It also remains unclear how precipitation, another important climate change driver, will interact with warming to affect microorganisms and their regulation of respiratory C loss. We explore the dynamics of microorganisms and their contributions to respiratory C loss using a 4-year (2006-2009) field experiment in a semi-arid grassland with increased temperature and precipitation in a full factorial design. We found no response of mass-specific (per unit microbial biomass C) heterotrophic respiration to warming, suggesting that respiratory C loss is directly from microbial growth rather than total physiological respiratory responses to warming. Increased precipitation did stimulate both microbial biomass and mass-specific respiration, both of which make large contributions to respiratory loss of soil carbon. Taken together, these results suggest that, in semi-arid grasslands, soil moisture and related substrate availability may inhibit physiological respiratory responses to warming (where soil moisture was significantly lower), while they are not inhibited under elevated precipitation. Although we found no total physiological response to warming, warming increased bacterial C utilization (measured by BIOLOG EcoPlates) and increased bacterial oxidation of carbohydrates and phenols. Non-metric multidimensional scaling analysis as well as ANOVA testing showed that warming or increased precipitation did not change microbial community structure, which could suggest that microbial communities in semi-arid grasslands are already adapted to fluctuating climatic conditions. In summary, our results support the idea that microbial responses to climate change are multifaceted and, even with no large shifts in community structure, microbial mediation of soil carbon loss could still occur under future climate scenarios.

Effects of community- and neighborhood-scale spatial patterns on semi-arid perennial grassland community dynamics

Although plant spatial patterns strongly influence community-structuring processes, few empirical studies have addressed pattern effects on perennial community dynamics. We tested the effects of community- and neighborhood-scale patterns in experimental semi-arid grassland communities comprising the stronger competitor crested wheatgrass (Agropyron cristatum) and the weaker competitor Snake River wheatgrass (Elymus wawawaiensis). Treatments consisted of community-scale patterns (Poisson random, regular, and aggregated) and neighborhood-scale patterns (Poisson random, small, and large aggregations) applied to 6.25-m(2) plots, with aggregations generated through simulated realizations of Neyman-Scott cluster processes. Two years of data were collected on aboveground biomass of both species, and variability in light (photosynthetically active radiation; PAR) was also quantified. We found that plant performance was strongly affected by community-scale spatial patterns and time, with additional effects of neighborhood-scale pattern in certain treatments. Mean biomass and relative growth rates of both species were highest in plots with community-scale regularity and random neighborhoods, suggesting a strong effect of pattern on competition that was magnified for the weaker competitor E. wawawaiensis, especially in the second year. There were also significant effects of treatment and time on variability of PAR, supporting past research on the importance of canopy patterns for light distribution near the soil surface. We observed more variable light environments in plots with community-scale aggregation, and variability also increased in the second year. Our research provides new information on the effects of plant patterns on community dynamics, with particular relevance for semi-arid perennial grasslands.

Above- and belowground responses to nitrogen addition in a Chihuahuan Desert grassland

Increased available soil nitrogen can increase biomass, lower species richness, alter soil chemistry and modify community structure in herbaceous ecosystems worldwide. Although increased nitrogen availability typically increases aboveground production and decreases species richness in mesic systems, the impacts of nitrogen additions on semiarid ecosystems remain unclear. To determine how a semiarid grassland responds to increased nitrogen availability, we examined plant community structure and above- and belowground net primary production in response to long-term nitrogen addition in a desert grassland in central New Mexico, USA. Plots were fertilized annually (10 g N m(-2)) since 1995 and NPP measured from 2004 to 2009. Differences in aboveground NPP between fertilized and control treatments occurred in 2004 following a prescribed fire and in 2006 when precipitation was double the long-term average during the summer monsoon. Presumably, nitrogen only became limiting once drought stress was alleviated. Belowground NPP was also related to precipitation, and greatest root growth occurred the year following the wettest summer, decreasing gradually thereafter. Belowground production was unrelated to aboveground production within years and unrelated to nitrogen enrichment. Species richness changed between years in response to seasonal precipitation variability, but was not altered by nitrogen addition. Community structure did respond to nitrogen fertilization primarily through increased abundance of two dominant perennial grasses. These results were contrary to most nitrogen addition studies that find increased biomass and decreased species richness with nitrogen fertilization. Therefore, factors other than nitrogen deposition, such as fire or drought, may play a stronger role in shaping semiarid grassland communities than soil fertility.

Absence of a Grass/Fire Cycle in a Semiarid Grassland: Response to Prescribed Fire and Grazing

Many nonnative invasive grasses alter fire regimes to their own benefit and the detriment of native organisms. In southern Arizona the nonnative Lehmann lovegrass (Eragrostis lehmanniana Nees) dominates many semiarid grasslands where native grasses were abundant. Managers are wary of using prescribed fire in this fire-prone community partly due to the perceived effects of a grass/fire cycle. However, examples of the grass/fire cycle originate in ecosystems where native plants are less fire-tolerant than grasses and the invasive plant does not mimic the physiognomy of the native community. We investigate the effects of prescribed fire and livestock grazing on a semiarid grassland community dominated by a nonnative invasive grass. Lehmann lovegrass does not appear to alter the fire regime of semiarid grasslands to the detriment of native plants. Prescribed fire reduced the abundance of Lehmann lovegrass for 1 to 2 yr while increasing abundance of native grasses, herbaceous dicotyledons and fall richness, and diversity. Effects of livestock grazing were less transformative than the effects of fire in this long-grazed area, but grazing negatively affected native plants as did the combination of prescribed fire and livestock grazing. Although Lehmann lovegrass produces more fuel than native plants, fire frequency in semiarid grasslands appears to be limited by the paucity of above-average precipitation, which constrains high fuel loads. In addition, many native grasses tolerate high temperatures produced by Lehmann lovegrass fires. Consistent with previous research, fire does not promote the spread of Lehmann lovegrass, and more importantly human alteration of the fire regime is greater than the nominal effects of Lehmann lovegrass introduction on the fire regime.

Runoff and erosional responses to a drought‐induced shift in a desert grassland community composition

This study investigates how drought‐induced change in semiarid grassland community affected runoff and sediment yield in a small watershed in southeast Arizona, USA. Three distinct periods in ecosystem composition and associated runoff and sediment yield were identified according to dominant species: native bunchgrass (1974–2005), forbs (2006), and the invasive grass, Eragrostis lehmanniana (2007–2009). Precipitation, runoff, and sediment yield for each period were analyzed and compared at watershed and plot scales. Average watershed annual sediment yield was 0.16 t ha−1 yr−1. Despite similarities in precipitation characteristics, decline in plant canopy cover during the transition period of 2006 caused watershed sediment yield to increase 23‐fold to 1.64 t ha−1 yr−1 comparing with preceding period under native bunchgrasses (0.06 t ha−1 yr−1) or succeeding period under E. lehmanniana (0.06 t ha−1 yr−1). In contrast, measurements on small runoff plots on the hillslopes of the same watershed showed a significant increase in sediment discharge that continued after E. lehmanniana replaced native grasses. Together, these findings suggest alteration in plant community increased sediment yield but that hydrological responses to this event differ at watershed and plot scales, highlighting the geomorphological controls at the watershed scale that determine sediment transport efficiency and storage. Resolving these scalar issues will help identify critical landform features needed to preserve watershed integrity under changing climate conditions.