Arid regions of the world are generally noted for their low primary productivity which is due to a combination of low, unpredictable water supply and low soil nutrient concentrations (Smith et al. 1997). The effects of low levels of precipitation (< 250 mm per year) are often compounded by the highly variable timing of its occurrence (Noy-Meir 1973). In addition to limiting biomass production, dry conditions lead to slow decomposition rates, and the combination of these factors leads to low levels of soil nitrogen and organic matter (Burke 1989).
The high spatial and temporal heterogeneity of soil minerals in arid systems complicates the effects of the low resources levels. Spatially, soil nitrogen may vary over a scale of cm (Jackson & Caldwell 1993). Litter accumulation and enhanced decomposition for example, leads to soil N levels usually being higher under the canopy of aridland shrub species (Garcia-Moya & McKell 1970; Hook et al. 1991) than in intershrub spaces. Another biotic factor that may affect local soil nutrient concentrations are large mammals such as pronghorn antelope (Antilocapra americana), elk (Cervus canadensis) and mule deer (Odocoileus hemionus) in the south-western United States. Urine and faeces create patches with high concentrations of available N (Stillwell & Woodmansee 1981), as does the decomposition of dead animals. However, the effects of large mammals on soil nutrient heterogeneity and, particularly, plant responses to mammal-generated nutrient patches are less well characterized in arid systems than in temperate grasslands and forests (McNaughton et al. 1988; Pastor et al. 1993; Steinauer & Collins 1995).
Temporal variability in aridland soil nutrient supply may be generated by snowmelt and unpredictable precipitation – factors that temporarily increase mineralization rates by soil microbes (Burke 1989; Bowman 1992; Gallardo & Schlesinger 1992). These relatively brief, localized nutrient pulses can be an important component of the total nutrient supply over the growing season for plants (Campbell & Grime 1989). Plants in habitats with high temporal and spatial heterogeneity in soil nutrient availability have several mechanisms to exploit ephemeral nutrient pulses. Morphologically, plants may rapidly increase fine root growth in areas of high nutrient concentrations (Jackson & Caldwell 1989; Caldwell et al. 1991). Physiologically, plants may increase root uptake kinetics to exploit microsite enrichment (Jackson et al. 1990). However, the ability of plant roots to respond to nutrient pulses often depends on overall plant demand for nutrients (Jackson & Caldwell 1989; Bilbrough & Caldwell 1997), the seasonal timing of nutrient pulses (Gebauer & Ehleringer 2000) and the availability of other resources (e.g. light, Bilbrough & Caldwell 1995).
Large shrubs in desert ecosystems have a large influence on community structure. For example, they may modify the surrounding microclimate by dampening temperature variation, raising humidity, decreasing wind speed and reducing irradiance under their canopies (Valiente-Banuet & Ezcurra 1991; Forseth et al. 2001). Shrubs may also increase the concentration of soil nutrients (Gutierrez et al. 1993) and modify soil moisture levels (Caldwell et al. 1998) in their immediate area. Because of the effects of shrubs on the surrounding microhabitat, interactions between shrubs and plants of other growth forms may range from competition to facilitation (Fowler 1986; Callaway 1997; Holzapfel & Mahall 1999). Tielbörger & Kadmon (1997, 2000) found both facilitation and asymmetric competition between shrubs and annual plants, depending upon the year, species and life history stage. Holmgren et al. (1997) hypothesized that facilitation between shrubs and associated plants will occur if the benefits from higher water or nutrient levels near shrubs outweigh the disadvantages of reduced light under their canopy. Thus, facilitation between s hrubs and associated plants may occur during particularly dry years (Bertness & Callaway 1994) whereas, during times of higher resources, associated plants may experience competitive interactions resulting in decreased performance.
Numerous studies have investigated interactions between large shrubs and associated plant species by examining spatial relationships (Eccles et al. 1999), seedling establishment and growth (Tielbörger & Kadmon 1995; Casper 1996). Fewer studies have examined the physiological mechanisms underlying the observed changes in plant performance (Shumway 2000; Forseth et al. 2001). Even fewer studies have combined a third factor in the investigation of shrub–smaller plant interactions – that of nutrient pulses. Due to the multiple factors (both biotic and abiotic) and the complex interactions that may result, integration of a variety of these factors is important in complex systems in order to understand observed patterns in plant performance. The study reported here takes a manipulative approach to examine the effects of microhabitat location on N pulse use by a long-lived herbaceous perennial, Cryptantha flava. We experimentally manipulated soil N around selected study plants by mimicking a one-time pulse of urea-derived N, such as that generated by Mule deer excretions.
The specific hypotheses tested were as follows.
- 1Plants in open microsites will respond positively to a pulse of N through rapid uptake and incorporation of N into leaf photosynthetic structures. This, in turn will lead to significantly greater growth and reproduction in these plants than in plants not receiving a pulse of N.
- 2Due to asymmetric competition for light with larger shrubs, individuals of C. flava located under shrub canopies will not respond positively to soil N pulses. Due to high year-to-year variation in precipitation in this arid system, we designed the study to extend over 3 years to look at interactive effects of precipitation and nutrient supply. Below average precipitation between the 2nd and 3rd year of the study allowed us to test a further, post hoc, hypothesis.
- 3The effects of N pulse use will be contingent upon seasonal precipitation, with low water availability limiting or eliminating the positive response of plants in the open to N. As a corollary to this, we were also able to test the predictions of Holmgren et al. (1997) and Bertness & Callaway (1994) that shrubs will ameliorate physical conditions for plants associated with them in drier years and act as competitors during average and above average soil moisture conditions.