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Resource limitations influence plant community properties, such as structure and species distribution, as well as ecosystem functions, such as productivity and nutrient cycling. Although multiple-factor limitations have long been recognized, few studies have investigated individual plant or community responses to such limitations (Chapin et al. 1987; Field et al. 1992; Shaw et al. 2002; Ho et al. 2004). This lack of information may be related to the large number of treatments needed to evaluate responses to both single resource limitations and their interactions with other factors. Additionally, individual species within a community may be limited by different resources, making community level and individual plant responses in species-rich communities difficult to interpret (Aerts & Chapin 2000; Drenovsky & Richards 2004). Despite these challenges, such insight is essential for understanding how individual species and communities are impacted following natural and anthropogenic perturbations, as these are inherently multifactorial (Chapin & Shaver 1985; Press et al. 1998; Shaver et al. 1998; Shaw et al. 2002).
Desert plant communities are excellent model systems in which to investigate multiple resource limitations to growth due to their simple structure, their low, but highly variable, annual productivity and their limited, heterogeneous supply of soil resources. Productivity patterns in deserts are thought to be driven mainly by low and infrequent precipitation inputs (Noy-Meir 1973; Hadley & Szarek 1981). In addition to the direct effect of precipitation on plant growth, low and variable water supply reduces soil nutrient availability by limiting the weathering of parent material, organic matter production and mineralization (Fisher et al. 1987; Burke 1989; Schlesinger 1997). Studies in several desert systems have shown that N or P can limit growth once water limitations are removed (Smith et al. 1997; Drenovsky & Richards 2004). Additionally, large temporal and spatial heterogeneity of soil resources compounds the effects of low soil nutrient supply on productivity. For example, soil inorganic N concentrations can vary three- to tenfold within a site through the season (Burke 1989; Ryel et al. 1996; Xie & Steinberger 2001; Z. Aanderud et al., unpublished data). Significant spatial variation has also been reported for N, P, Ca, Mg, pH and organic matter in a range of desert systems (Jackson & Caldwell 1993; Schlesinger et al. 1996; Cross & Schlesinger 1999).
While most research has focused on water limitations in deserts and how this interacts with low soil N availability, there is evidence that the availability of other soil resources may limit productivity in deserts, particularly in highly saline-alkaline systems. Topographic gradients across interior drainage basins in arid western North America are paralleled by gradients of decreasing soil fertility and increasing salinity, pH and toxic ion concentrations (e.g. Na, B) at lower positions. Thus, at lower topographic positions, productivity and diversity decline and, eventually, monospecific stands develop at the most stressful sites (West 1983; Donovan & Richards 2000), probably as a combined result of several factors. Increasing salinity not only increases plant nutrient requirements, but it can interfere with uptake of nutrient cations such as K+, Ca2+ and Mg2+ (Flowers et al. 1977; Marschner 1995; Drenovsky & Richards 2003). Likewise, increasing alkalinity is expected to further reduce N availability through volatilization of mineralized and to decrease P, Ca and Mg solubility (Lajtha & Schlesinger 1988; Schlesinger & Peterjohn 1991; Lambers et al. 1998).
Evidence for multiple resource limitations in saline-alkaline deserts is, however, inconsistent. In a survey of widespread Great Basin and Mojave Desert species that establish on the lower portions of the topographic gradients (including Atriplex parryi, the species that we focus on here), Snyder et al. (2004) found that water addition alone had minimal effects on productivity or on physiological functions such as photosynthesis. When soil organic matter and nutrients are available, water addition should increase mineralization and nutrient mobility (Birch 1960; Fisher et al. 1987; Cui & Caldwell 1997), but no changes in leaf nutrient concentrations were observed. It was therefore suggested that plant growth was co-limited by soil nutrient availability. Other work supports the prediction that low nutrient availability regulates plant growth response to water in such environments. Along gradients of increasing soil salinity and alkalinity in the Great Basin and Mojave, both growth and leaf Ca and Mg concentrations of dominant shrubs, including Sarcobatus vermiculatus and A. parryi, declined in parallel with soil Ca and Mg availability (Richards 1994; Donovan et al. 1997). Although N and P are expected to be limiting in deserts, no relationship was observed between growth and leaf N or P concentration. Macro- and micronutrient additions at one site, however, demonstrated that S. vermiculatus growth was stimulated by N (Drenovsky 2002), but not by P, Ca or Mg, supporting earlier experimental work in deserts (see Smith et al. 1997 for review).
The discord between these different approaches to determining resource limitations in deserts could be due to methodology. For example, nutrient addition may not significantly increase soil fertility if nutrients are fixed by soil chemical reactions (Chapin et al. 1986). Without dose-response curves it is not possible to determine if a lack of response is because the resource was not limiting or because supply rates were insufficient to overcome limitations or soil fixation. Of more fundamental importance, however, is the need to consider how multiple limitations may interact to influence plant response. Current models of allocation predict that multiple resources will be limiting, but differences in acquisition costs and absolute demand for resources should interact to influence plant response to changes in resource supply. In these models, small increases in resources that are in high demand, such as N, which is required in large quantities, result in significant re-allocation to other functions (Bloom et al. 1985; Gleeson & Tilman 1992; Gleeson & Good 2003). The magnitude of this re-allocation should therefore influence plant response to other resources or resource manipulation.
We determined whether single or multiple resources (water, N, P, Mg and Ca) limit productivity of A. parryi in the saltbrush scrub community it dominates. We had two specific objectives. The first was to determine experimentally which soil resources limit productivity of A. parryi and to evaluate if these limitations differed along a soil stress gradient or between life stages (reproductive adults vs. pre-reproductive juveniles). We applied resources in combination at different supply rates in two A. parryi stands, one at a high-stress site and the other at a more species-diverse, low-stress site. We then developed and applied a conceptual model of plant response to resource addition to evaluate patterns of nutrient limitations (Fig. 1). Treatments were applied to adult plants at both the low and high stress sites and to juvenile plants at the high stress site because juvenile plants may be less tolerant of soil stress (Donovan & Ehleringer 1994; Richards 1994). The second objective was to investigate how resource limitations interact to influence plant growth and function. We used a full factorial approach to investigate how resource limitations identified in the first experiment interact to influence plant growth and function (biomass allocation, gas exchange, water relations) at the high-stress end of the gradient.
Figure 1. Conceptual model for determining nutrient limitations. Nutrient limitations are inferred if both plant biomass and nutrient concentration increase following resource addition. Other permutations of the response variables include sufficient supply (biomass increases but nutrient concentrations remain constant), dilution (biomass increases but nutrient concentration declines) and luxury consumption (nutrient accumulates but no biomass increase occurs). (Model modified from Timmer and Morrow 1984).
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