Natural Precipitation Gradient Research Articles

Variations in water use strategies of Tamarix ramosissima at coppice dunes along a precipitation gradient in desert regions of northwest China.

The precipitation pattern has changed significantly in arid desert areas, yet it is not clear how the water use strategies of Tamarix ramosissima Ledeb. on coppice dunes along a natural precipitation gradient are affected. In this study, the hydrogen and oxygen isotope compositions of xylem water, soil water, precipitation, and groundwater were measured by stable isotope techniques in Huocheng, Mosuowan, and Tazhong. Additionally, the water use strategies of natural precipitation gradient were investigated in conjunction with the MixSIAR model. The results indicated that the water sources of T. ramosissima exhibited significant variation from semi-arid to hyper-arid areas. In semi-arid areas, T. ramosissima mainly absorbed shallow, shallow-middle, and middle soil water; however, T. ramosissima shifted its primary water sources to middle and deep soil water in arid areas. In hyper-arid areas, it mainly utilized deep soil water and groundwater. In contrast, the water source contribution rate of T. ramosissima exhibited relative uniformity across each layer in an arid area. Notably, in hyper-arid areas, the proportion of groundwater by T. ramosissima was significantly high, reaching 60.2%. This is due to the relatively shallow groundwater supplementing the deep soil water content in the area. In conclusion, the proportion of shallow soil water decreased by 14.7% for T. ramosissima from semi-arid to hyper-arid areas, illustrating the occurrence of a gradual shift in potential water sources utilized by T. ramosissima from shallow to deep soil water and groundwater. Therefore, T. ramosissima on coppice dunes shows flexible water use strategies in relation to precipitation and groundwater, reflecting its strong environmental adaptability. The findings hold significant implications for the conservation of water resources and vegetation restoration in arid areas.

Moss species and precipitation mediate experimental warming stimulation of growing season N2 fixation in subarctic tundra.

Climate change in high latitude regions leads to both higher temperatures and more precipitation but their combined effects on terrestrial ecosystem processes are poorly understood. In nitrogen (N) limited and often moss-dominated tundra and boreal ecosystems, moss-associated N2 fixation is an important process that provides new N. We tested whether high mean annual precipitation enhanced experimental warming effects on growing season N2 fixation in three common arctic-boreal moss species adapted to different moisture conditions and evaluated their N contribution to the landscape level. We measured insitu N2 fixation rates in Hylocomium splendens, Pleurozium schreberi and Sphagnum spp. from June to September in subarctic tundra in Sweden. We exposed mosses occurring along a natural precipitation gradient (mean annual precipitation: 571-1155 mm) to 8 years of experimental summer warming using open-top chambers before our measurements. We modelled species-specific seasonal N input to the ecosystem at the colony and landscape level. Higher mean annual precipitation clearly increased N2 fixation, especially during peak growing season and in feather mosses. For Sphagnum-associated N2 fixation, high mean annual precipitation reversed a small negative warming response. By contrast, in the dry-adapted feather moss species higher mean annual precipitation led to negative warming effects. Modelled total growing season N inputs for Sphagnum spp. colonies were two to three times that of feather mosses at an area basis. However, at the landscape level where feather mosses were more abundant, they contributed 50% more N than Sphagnum. The discrepancy between modelled estimates of species-specific N input via N2 fixation at the moss core versus ecosystem scale, exemplify how moss cover is essential for evaluating impact of altered N2 fixation. Importantly, combined effects of warming and higher mean annual precipitation may not lead to similar responses across moss species, which could affect moss fitness and their abilities to buffer environmental changes.

Soil microbial gene abundance rather than diversity and network complexity predominantly determines soil multifunctionality in Tibetan alpine grasslands along a precipitation gradient

Abstract The relationship between biodiversity and ecosystem functioning has mainly focused on plant communities, with comparably little known about soil microbial‐driven ecosystem functions. Climate change severely threatens soil microbial roles, but how soil microbial communities determine soil multifunctionality under climate change is poorly understood. Here, we evaluated the effects of diverse bacterial and fungal properties, including microbial gene abundance, diversity and network complexity, on soil multifunctionality (nine soil functions) across a 3000 km transect along a natural precipitation gradient in Tibetan alpine grasslands. Variation partitioning analyses were performed to disentangle the relative importance of bacterial and fungal properties to the variation of soil multifunctionality. Moreover, structural equation modelling was adopted to explore the influencing pathways of precipitation‐induced changes in plant and edaphic factors to soil microbial properties and, consequently, soil multifunctionality. Soil multifunctionality was positively associated with bacterial and fungal gene abundance, diversity and network complexity. Microbial gene abundance was the more important driver influencing soil multifunctionality than microbial diversity and network complexity. In addition, microbial gene abundance was mainly determined by precipitation‐induced changes in soil pH. Meanwhile, the effects of bacterial properties on soil multifunctionality were much larger than those of fungi. Soil multifunctionality was closely associated with different bacterial (cellulolysis, ligninolysis, nitrogen reduction, denitrification and nitrate fixation etc.) and fungal (soil saprotrophs, arbuscular mycorrhizal and plant pathogens etc.) functional guilds, which exert vital regulations on an array of soil biogeochemical cycling processes. Our results provide the large‐scale evidence of the relative contribution of soil microbial gene abundance, diversity and network complexity to the variation of soil multifunctionality in alpine grasslands with changing precipitation, which is pivotal for understanding microbial roles in modulating and predicting soil multifunctionality under future precipitation changes. Read the free Plain Language Summary for this article on the Journal blog.

Effects of a natural precipitation gradient on fish and macroinvertebrate assemblages in coastal streams.

Anthropogenic climate change is expected to increase the aridity of many regions of the world. Surface water ecosystems are particularly vulnerable to changes in the water-cycle and may suffer adverse impacts in affected regions. To enhance our understanding of how freshwater communities will respond to predicted shifts in water-cycle dynamics, we employed a space for time approach along a natural precipitation gradient on the Texas Coastal Prairie. In the spring of 2017, we conducted surveys of 10 USGS-gauged, wadeable streams spanning a semi-arid to sub-humid rainfall gradient; we measured nutrients, water chemistry, habitat characteristics, benthic macroinvertebrates, and fish communities. Fish diversity correlated positively with precipitation and was negatively correlated with conductivity. Macroinvertebrate diversity peaked within the middle of the gradient. Semi-arid fish and invertebrate communities were dominated by euryhaline and live-bearing taxa. Sub-humid communities contained environmentally sensitive trichopterans and ephemeropterans as well as a variety of predatory fish which may impose top-down controls on primary consumers. These results warn that aridification coincides with the loss of competitive and environmentally sensitive taxa which could yield less desirable community states.

Variations in water use strategies of sand-binding vegetation along a precipitation gradient in sandy regions, northern China

Water is the most important factor limiting the growth of sand-binding vegetation (SBV) in desert ecosystems. This study explored the water sources and water use strategies of SBV under a natural precipitation gradient. Six typical SBV species were selected, comprising three tree species, Pinus sylvestris, Haloxylon ammodendron and Populus euphratica, and three shrub species, Artemesia ordosica, Caragana korshinshii and Tmarix ramosissima. The stable isotope values (δD, δ18O) of xylem water and potential water sources (precipitation, soil water, and groundwater) of these species under a precipitation gradient in the sandy regions of northern China were measured. In addition, the relative contributions of the different potential water sources to water use by SBV were analyzed using the MixSIAR model. The results indicated that the Local Meteoric Water Lines (LMWLs) of the six SBV species were located on the right side of the Global Meteoric Water Line (GMWL), with the degree of deviation gradually increasing with a decrease in precipitation. The results showed that T. ramosissima, H. ammodendron and P. euphratica mainly used groundwater (GW), whereas P. sylvestris, A. ordosica and C. korshinshii mainly used soil water. Among those species using soil water, P. sylvestris and A. ordosica mainly used shallow soil water (20–80 cm). P. sylvestris and C. korshinshii showed the highest soil water utilization rate of 14%, whereas P. euphratica showed the lowest soil water utilization rate of 2%. The different SBV species showed significantly different soil water utilization rates due to the influence of plant root distribution (p < 0.001). Among the six SBV species, trees showed a higher GW utilization rate compared to shrubs at 65.74% and 47.29%, respectively, whereas shrubs showed a higher deep soil water utilization rate (140–200 cm) compared to trees at 65.74% and 47.29%, respectively. The precipitation utilization rates amongst the six SBV species were highest in spring, followed by autumn and summer. The utilization of GW by SBV will be intensified in the future due to climate change. However, the stability of T. ramosissima may pose a threat due to its low utilization rate of GW.

Mosses modify effects of warmer and wetter conditions on tree seedlings at the alpine treeline.

Climate warming enables tree seedling establishment beyond the current alpine treeline, but to achieve this, seedlings have to establish within existing tundra vegetation. In tundra, mosses are a prominent feature, known to regulate soil temperature and moisture through their physical structure and associated water retention capacity. Moss presence and species identity might therefore modify the impact of increases in temperature and precipitation on tree seedling establishment at the arctic-alpine treeline. We followed Betula pubescens and Pinus sylvestris seedling survival and growth during three growing seasons in the field. Tree seedlings were transplanted along a natural precipitation gradient at the subarctic-alpine treeline in northern Sweden, into plots dominated by each of three common moss species and exposed to combinations of moss removal and experimental warming by open-top chambers (OTCs). Independent of climate, the presence of feather moss, but not Sphagnum, strongly supressed survival of both tree species. Positive effects of warming and precipitation on survival and growth of B. pubescens seedlings occurred in the absence of mosses and as expected, this was partly dependent on moss species. P. sylvestris survival was greatest at high precipitation, and this effect was more pronounced in Sphagnum than in feather moss plots irrespective of whether the mosses had been removed or not. Moss presence did not reduce the effects of OTCs on soil temperature. Mosses therefore modified seedling response to climate through other mechanisms, such as altered competition or nutrient availability. We conclude that both moss presence and species identity pose a strong control on seedling establishment at the alpine treeline, and that in some cases mosses weaken climate-change effects on seedling establishment. Changes in moss abundance and species composition therefore have the potential to hamper treeline expansion induced by climate warming.

Physiological and anatomical trait variability of dominant C4 grasses

Climate variability is a key driver of physiological responses in common grass species in grasslands of North America. Differences in microanatomical traits among coexisting species may influence physiological responses to climate variability over large geographic scales. The goal of this research was to determine leaf-level physiological and microanatomical trait variability among four dominant C4 grass species across a natural precipitation gradient. Physiological traits were observed to vary significantly across the gradient with greater variability than microanatomical traits. Microanatomical traits were shown to predict physiological responses in A. gerardii and P. virgatum, but the nature of the relationships varied between species. These results illustrate that microanatomical and physiological traits vary across a precipitation gradient, there are clear linkages between microanatomy and physiology in grass species, and this evidence underscores the need for further investigation using phylogenetically diverse assemblages.

Scale dependent responses of pine reproductive traits to experimental and natural precipitation gradients

Drought considerably affects physiological and biochemical processes in trees and thus also reproduction. However, little is known about shifts in reproductive strategies in relation to changes in water availability.We investigated cone and seed traits as well as needle and twig growth traits in pine trees along a natural precipitation gradient (290 mm–750 mm) and in a long-term irrigation experiment to understand reproductive responses to water availability. Combining multiple traits, we found that cone trait richness and divergence were positively correlated with water availability. This suggests drought-induced environmental filtering resulting in a reduced set of cone trait combinations. In contrast, seed trait diversity indices were negatively correlated with water availability, suggesting that pine trees tend to maximize seed trait dissimilarity in multiple dimensions to efficiently utilize scarce resources. Trade-offs between reproduction and vegetative growth were observed along the transect but not in the irrigation experiment, which may be a result of different drought severity between the two study regions.Our results highlight the importance of considering a broad aridity gradient and different spatial-temporal scales when studying plant reproductive responses to precipitation change. Furthermore, the present study indicates that quantifying complementary components of trait diversity allows to interpret ecologically complex changes in trait distribution in multiple dimensions.

Ecological mechanisms underlying soil bacterial responses to rainfall along a steep natural precipitation gradient.

Changes in the structure and function of soil microbial communities can drive substantial ecosystem feedbacks to altered precipitation. However, the ecological mechanisms underlying community responses to environmental change are not well understood. We used an 18-month soil reciprocal transplant experiment along a steep precipitation gradient to quantify how changes in rainfall affected bacterial community structure. We also conducted an enhanced dispersal treatment to ask whether higher immigration rates of taxa from the surrounding environment would accelerate community responses to climate change. Finally, we addressed how the composition of soil bacteria communities was related to the functional response of soil respiration to moisture in these treatments. Bacterial community structure (OTU abundance) and function (respiration rates) changed little in response to manipulation of either rainfall environment or dispersal rates. Although most bacteria were ecological generalists, a subset of specialist taxa, over 40% of which were Actinobacteria, tended to be more abundant in the rainfall environment that matched their original conditions. Bacteria community composition was an important predictor of the respiration response to moisture. Thus, the high compositional resistance of microbial communities dictated respiration responses to altered rainfall in this system.

Exacerbated nitrogen limitation ends transient stimulation of grassland productivity by increased precipitation

AbstractGiven that plant growth is often water‐limited in grasslands, it has been proposed that projected increases in precipitation could increase plant productivity and carbon sequestration. However, the existing evidence for this hypothesis comes primarily from observational studies along natural precipitation gradients or from short‐term manipulative experiments. It remains unclear whether long‐term increased precipitation persistently stimulates grassland productivity. In the world's largest remaining temperate grassland, we found that experimentally increased precipitation enhanced net primary production, soil‐available nitrogen and foliar nitrogen concentrations during the first six years, but it ceased to do so in the following four years, unless nitrogen was simultaneously added with water. The 15N enrichment of plant and soil nitrogen pools in later years indicates increased nitrogen losses, which exacerbated nitrogen limitation and ended the stimulation of productivity by increased precipitation. Changes in species abundance might have contributed little to the changes in water treatment effects. Our study demonstrates that the long‐term response of grassland ecosystems to increased precipitation will be mediated by nitrogen availability. Our results also point to a shift from co‐limitation by water and nitrogen early to perhaps limitation by nitrogen only later in this temperate grassland, highlighting significant variations in the type of resource limitation induced by climate change.

Parasitism of a native Hawaiian stream fish by an introduced nematode increases with declining precipitation across a natural rainfall gradient

AbstractClimate change can promote disease emergence if shifting conditions favour infection of native fauna by introduced parasites and pathogens. In Hawai'i, climate warming is predicted to reduce net precipitation and surface flow in streams, which in turn could increase parasitism of native stream fishes by non‐native parasites. In this study, we utilised a natural precipitation gradient across the Hamakua coast on the island of Hawai'i to assess the relationship between precipitation and infection of Awaous stamineus, a native amphidromous goby, by the introduced nematode Camallanus cotti. We found that the abundance, intensity and prevalence of C. cotti in A. stamineus increased with declining rainfall. Our results also show that parasitism tracks precipitation patterns across the archipelago and that parasitism increases even with moderate decreases in rainfall. As the Hamakua coast precipitation gradient represents a proxy for predicted climate‐driven reductions in precipitation, these findings suggest that infection of native Hawaiian fishes by introduced parasites will increase if climate conditions change as expected. Our findings also suggest that parasitism may be exacerbated by other factors that reduce surface flow, including water extraction for agricultural and urban uses. If so, then adaptive management of minimum flow standards in Hawai'i and elsewhere could improve the well‐being of at‐risk native fishes by alleviating parasitism under current and future climate conditions.

Below‐ground competition drives the self‐thinning process of Stipa purpurea populations in northern Tibet

AbstractQuestionsAbove‐ground, below‐ground and individual mass–density relationships for perennial herbs were examined along a natural precipitation gradient in northern Tibet. We asked: (1) how do the self‐thinning exponents respond to variation in precipitation; and (2) what mechanisms drive the observed population self‐thinning?LocationThe alpine grassland of northern Tibet.MethodsForty‐seven fenced sites along a precipitation gradient were established and surveyed in 2011 and 2012. Data (geographic coordinates, elevation, and vegetation information) were collected for Stipa purpurea populations at each site. Population self‐thinning exponents were estimated using reduced major axis regression.ResultsThe self‐thinning exponents for below‐ground (−1.27, −0.47) and individual biomass (−1.26, −0.46) increased with increasing mean annual precipitation, but those for above‐ground biomass decreased with precipitation (0.18, −0.25). Soil resources (moisture and nutrients) are a more important constraining factor for below‐ground components than light is for above‐ground components. Root competition for below‐ground resources dominated in S. purpurea population self‐thinning. The driving force of density regulation changed from above‐ground competition to below‐ground competition with increased drought stress. Our results indicate that an increased root/shoot ratio was linked to enhanced below‐ground competition and weakened above‐ground competition. Our study further confirmed the hypothesis that plant populations in different environments exhibit different biomass allocation patterns, which, in turn, leads to different biomass–density relationships.ConclusionsOur study revealed the mechanisms of population self‐thinning for perennial herbs in the extreme environment of northern Tibet, where below‐ground processes play a critical role in regulating population self‐thinning. Our study also advances understanding of the interactions between above‐ and below‐ground processes, providing baseline knowledge useful for local grassland management.

Contrasting trait responses in plant communities to experimental and geographic variation in precipitation

• Patterns of precipitation are likely to change significantly in the coming century, with important but poorly understood consequences for plant communities. Experimental and correlative studies may provide insight into expected changes, but little research has addressed the degree of concordance between these approaches. • We synthesized results from four experimental water addition studies with a correlative analysis of community changes across a large natural precipitation gradient in the United States. We investigated whether community composition, summarized with plant functional traits, responded similarly to increasing precipitation among studies and sites. • In field experiments, increased precipitation favored species with small seed size, short leaf life span and high leaf nitrogen (N) concentration. However, with increasing precipitation along the natural gradient, community composition shifted towards species with higher mean seed mass, longer leaf life span and lower leaf N concentrations. • The differences in temporal and spatial scale of experimental manipulations and natural gradients may explain these contrasting results. Our results highlight the complexity of responses to climate change, and suggest that transient dynamics may not reflect long-term shifts in functional diversity and community composition. We propose a model of community change that incorporates these differences between short- and long-term responses to climate change.

Biomass, Litter, and Soil Respiration Along a Precipitation Gradient in Southern Great Plains, USA

Knowledge of how ecosystem carbon (C) processes respond to variations in precipitation is crucial for assessing impacts of climate change on terrestrial ecosystems. In this study, we examined variations of shoot and root biomass, standing and surface litter, soil respiration, and soil C content along a natural precipitation gradient from 430 to 1200 mm in the southern Great Plains, USA. Our results show that shoot biomass and soil respiration increased linearly with mean annual precipitation (MAP), whereas root biomass and soil C content remained relatively constant along the precipitation gradient. Consequently, the root/shoot ratio linearly decreased with MAP. However, patterns of standing, surface, and total litter mass followed quadratic relationships with MAP along the gradient, likely resulting from counterbalance between litter production and decomposition. Those linear/ quadratic equations describing variations of ecosystem C processes with precipitation could be useful for model development, parameterization, and validation at landscape and regional scales to improve predictions of C dynamics in grasslands in response to climate change. Our results indicated that precipitation is an important driver in shaping ecosystem functioning as reflected in vegetation production, litter mass, and soil respiration in grassland ecosystems.

Spatiotemporal dynamics of aboveground primary productivity along a precipitation gradient in Chinese temperate grassland

Investigating the spatial and temporal variance in productivity along natural precipitation gradients is one of the most efficient approaches to improve understanding of how ecosystems respond to climate change. In this paper, by using the natural precipitation gradient of the Inner Mongolian Plateau from east to west determined by relatively long-term observations, we analyzed the temporal and spatial dynamics of aboveground net primary productivity (ANPP) of the temperate grasslands covering this region. Across this grassland transect, ANPP increased exponentially with the increase of mean annual precipitation (MAP) (ANPP=24.47e 0.005MAP , R 2 =0.48). Values for the three vegetation types desert steppe, typical steppe, and meadow steppe were: 60.86 gm -2 a -1 , 167.14 gm -2 a -1 and 288.73 gm -2 a -1 respectively. By contrast, temperature had negative effects on ANPP. The moisture index (K), which takes into account both precipitation and temperature could explain the spatial variance of ANPP better than MAP alone (ANPP=2020.34K 1.24 , R 2 =0.57). Temporally, we found that the inter-annual variation in ANPP (calculated as the coefficient of variation, CV) got greater with the increase of aridity. However, this trend was not correlated with the inter-annual variation of precipitation. For all of the three vegetation types, ANPP had greater inter-annual variation than annual precipitation (PPT). Their difference (ANPP CV/PPT CV) was greatest in desert steppe and least in meadow steppe. Our results suggest that in more arid regions, grasslands not only have lower productivity, but also higher inter-annual variation of production. Climate change may have significant effects on the productivity through changes in precipitation pattern, vegetation growth potential, and species diversity. © Science in China Press 2007.

Carbon and nitrogen dynamics across a natural precipitation gradient in Patagonia, Argentina

Abstract.Both ecosystem carbon gain and nutrient availability are largely constrained by the magnitude and seasonality of precipitation in arid and semi‐arid ecosystems. We investigated the role of precipitation on ecosystem processes along an International Geosphere Biosphere Programme (IGBP) transect in temperate South America. The transect consists of a contiguous precipitation gradient in the southern region of Argentinean Patagonia (44–45° S), from 100 mm to 800 mm mean annual precipitation (MAP) and vegetation ranging from desert scrub to closed canopy forest. Gravimetric soil water content tracked changes in seasonal and annual precipitation, with a linear increase in soil water content with increasing MAP. Above‐ground net primary production (ANPP) increased linearly along the gradient of precipitation (ANPP =– 31.2 + 0.52 MAP,r2= 0.84,p= 0.028), supporting the relationship that carbon assimilation is largely controlled by available water in these sites, and was in general agreement with regional models of ANPP and rainfall. However, inorganic soil nitrogen was also highly linearly correlated with both MAP ([N] = 0.19 MAP – 32,r2= 0.96,p= 0.003) and ANPP (ANPP = 2.6 [Ninorganic]+59.4,r2= 0.79,p= 0.042), suggesting a direct control of precipitation on nitrogen turnover and an interaction with nitrogen availability in controlling carbon gain. The asynchrony of precipitation and changes in dominant vegetation may play important roles in determining the carbon‐nitrogen interactions along this rainfall gradient.

Carbon and nitrogen dynamics across a natural precipitation gradient in Patagonia, Argentina

Abstract Both ecosystem carbon gain and nutrient availability are largely constrained by the magnitude and seasonality of precipitation in arid and semi-arid ecosystems. We investigated the role of precipitation on ecosystem processes along an International Geosphere Biosphere Programme (IGBP) transect in temperate South America. The transect consists of a contiguous precipitation gradient in the southern region of Argentinean Patagonia (44–45° S), from 100 mm to 800 mm mean annual precipitation (MAP) and vegetation ranging from desert scrub to closed canopy forest. Gravimetric soil water content tracked changes in seasonal and annual precipitation, with a linear increase in soil water content with increasing MAP. Above-ground net primary production (ANPP) increased linearly along the gradient of precipitation (ANPP = − 31.2 + 0.52 MAP, r2 = 0.84, p = 0.028), supporting the relationship that carbon assimilation is largely controlled by available water in these sites, and was in general agreement with regi...

THE EFFECTS OF PRODUCTIVITY, HERBIVORY, AND PLANT SPECIES TURNOVER IN GRASSLAND FOOD WEBS

In order to evaluate the patterns and processes of food web structure in grasslands, we compiled data from published studies on the relationship between precipitation (which is a predictor of primary productivity in grasslands), plant and herbivore standing crop biomass, and the results of large-herbivore exclosure experiments on plant abundance and composition. Three informative patterns emerged: (1) both producer and herbivore biomass increase across a natural precipitation gradient; (2) the relative effect of large herbivores on plant biomass, inferred from exclosure studies, decreases with increasing precipitation; and (3) the effect of herbivores on changes in plant species composition increases with precipitation. Simple resource-controlled and consumer-controlled food chain models can explain different subsets of these patterns. However, models of heterogeneous food webs that incorporate differences among species within trophic levels, and compositional turnover within such trophic levels, are consistent with all of the reviewed patterns. We suggest that this compositional turnover of plant species, combined with the interactive controlling effects of consumers and resources, may help to explain why studies performed in different areas, and with different methodologies, often draw different conclusions about the patterns and structuring processes in grassland ecosystems.

The Effects of Productivity, Herbivory, and Plant Species Turnover in Grassland Food Webs

In order to evaluate the patterns and processes of food web structure in grasslands, we compiled data from published studies on the relationship between precipitation (which is a predictor of primary productivity in grasslands), plant and herbivore standing crop biomass, and the results of large-herbivore exclosure experiments on plant abundance and composition. Three informative patterns emerged: (1) both producer and herbivore biomass increase across a natural precipitation gradient; (2) the relative effect of large herbivores on plant biomass, inferred from exclosure studies, decreases with increasing precipitation; and (3) the effect of herbivores on changes in plant species composition increases with precipitation. Simple resource-controlled and consumer-controlled food chain models can explain different subsets of these patterns. However, models of heterogeneous food webs that incorporate differences among species within trophic levels, and compositional turnover within such trophic levels, are consistent with all of the reviewed patterns. We suggest that this compositional turnover of plant species, combined with the interactive controlling effects of consumers and resources, may help to explain why studies performed in different areas, and with different methodologies, often draw different conclusions about the patterns and structuring processes in grassland ecosystems.

Climate and the U.S. distribution of C4 grass subfamilies and decarboxylation variants of C4 photosynthesis

I compared the C(4) grass flora and climatic records for 32 sites in the United States. Consistent with previous studies, I found that the proportion of the grass flora that uses the NADP malic enzyme (NADP-ME) variant of C(4) photosynthesis greatly increases with increasing annual precipitation, while the proportion using the NAD malic enzyme (NAD-ME) variant (and also the less common phosphoenolpyruvate carboxykinase [PCK] variant) decreases. However the association of grass subfamilies with annual precipitation was even stronger than for the C(4) decarboxylation variants. Analysis of the patterns of distribution by partial correlation analysis showed that the correlations between the frequency of various C(4) types and rainfall were solely due to the association of the C(4) types with particular grass subfamilies. In contrast, there was a strong correlation of the frequency of the different subfamilies with annual precipitation that was independent of the influence of the different C(4) variants. It therefore appears that other, as yet unidentified, characteristics that differ among grass subfamilies may be responsible for their differences in distribution across natural precipitation gradients.