Ecological effects of experimental drought and prescribed fire in a southern California coastal grassland

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Abstract

How drought and fire disturbance influence different levels of biological organization is poorly understood but essential for robust predictions of the effects of environmental change. During a year of severe drought, we conducted a prescribed fire in a Mediterranean-type coastal grassland near Irvine, California. In the weeks following the fire we experimentally manipulated rainfall in burned and unburned portions of the grassland to determine how fire and drought interact to influence leaf physiological performance, community composition, aboveground net primary productivity (ANPP) and component fluxes of ecosystem CO2 exchange and evapotranspiration (ET). Fire increased leaf photosynthesis (Anet) and transpiration (T) of the native perennial bunchgrass, Nassella pulchra and the non-native annual grass, Bromus diandrus but did not influence ANPP or net ecosystem CO2 exchange (NEE). Surprisingly, drought only weakly influenced Anet and T of both species but strongly influenced ANPP and NEE. We conclude that despite increasing experimental drought severity, prescribed fire influenced leaf CO2 and H2O exchange but had little effect on the component fluxes of ecosystem CO2 exchange. The differential effects of prescribed fire on leaf and ecosystem processes with increasingly severe drought highlight the challenge of predicting the responses of biological systems to disturbance and resource limitation.

Highlights

► We manipulated drought severity and fire in a Mediterranean-type grassland. ► We measured physiological, community, and ecosystem responses to drought and fire. ► Fire and increasing drought severity did not interact to influence biological processes. ► Physiological responses are modulated by community shifts to influence ecosystem function.

Introduction

Predicting biophysical phenomena across scales is a critical organizing theme in ecological research (Ehleringer and Field, 1993, Enquist et al., 2003, Jarvis and McNaughton, 1986, Lavorel and Garnier, 2002, Suding et al., 2008, Valentini et al., 1995). If larger scale phenomena are the collective dynamics of processes at smaller scales, it is essential to understand how patterns at one scale relate to patterns at other scales (Levin, 1992). For example, responses to resource availability and disturbance are theorized to scale across levels of ecological organization (Lavorel and Garnier, 2002). However, resource alteration and disturbance may drive species composition shifts and non-additive productivity responses related to resource mediated thresholds in recruitment and mortality (Smith et al., 2009). The existence of such thresholds may create scenarios in which the responses of individuals to resource availability and disturbance may not translate to ecosystem responses without consideration of these modulating effects at the community level (Suding et al., 2008).

California’s Mediterranean-type grasslands are characterized by year-to-year rainfall variability and frequent fire, providing a tractable system in which to examine the effects of resource availability and disturbance across scales of biological organization (Dukes et al., 2005). In southern California, future warming associated with anthropogenic atmospheric CO2 increases may be accompanied by declines in winter season precipitation (Seager and Vecchi, 2010) and droughts of increasing severity (Bell et al., 2004, Cayan et al., 2010). Rainfall controls the productivity of California grasslands (Dukes et al., 2005, Harpole et al., 2007, Henry et al., 2006), with productivity varying widely between dry and wet years (Chou et al., 2008). In addition to influencing productivity, rainfall also influences community composition through shifts in the abundance of native and non-native grasses (Seabloom et al., 2003) due to differences in life history and functional rooting depths (Everard et al., 2010).

Against the backdrop of year-to-year rainfall variability, Mediterranean-type grasslands are strongly influenced by fire (Bartolome et al., 2004, George et al., 1992, Hatch et al., 1999, Henry et al., 2006, Noy-Meir, 1995). Episodic releases of soil nutrients associated with fire is likely to favor non-native annual grasses over native perennial bunchgrasses, due to the greater physiological responses (Ignace et al., 2007) and earlier plant phenology (Cleland et al., 2006) of the non-native grasses. While the functional impacts of water limitation on Mediterranean-type and other semi-arid grasslands are well studied (English et al., 2005, Harpole et al., 2007, Potts et al., 2006, St. Clair et al., 2009, Suttle et al., 2007), fire effects in these systems are less understood (Scheintaub et al., 2009).

The balance of CO2 uptake by plants and release of CO2 by plants and soil microbes is a critical variable in understanding ecosystem function (Baldocchi, 2008). Expressed as the difference between of CO2 assimilation by photosynthesis (gross ecosystem exchange; GEE) and the release of CO2 by plant and soil microbial respiration (ecosystem respiration; Re), net ecosystem CO2 exchange (NEE) integrates leaf and community processes (Flanagan et al., 2002, Jenerette et al., 2008, Potts et al., 2006). Likewise, by combining evaporation from soil and canopy surfaces with transpiration from stomata, evapotranspiration (ET) is critical in understanding the role of ecosystems in linking the movement of water between the land surface and the atmosphere. Used in a variety of experimental settings, chamber-based measurements have demonstrated the effects of seasonal rainfall (Harpole et al., 2007, Patrick et al., 2007), rainfall timing (Potts et al., 2006) and fire (Prater and DeLucia, 2006) on CO2 exchange and ET.

The objectives of this research were to determine how fire disturbance and drought influence native and non-native grass physiological performance and, in turn, how these responses influence plant community composition and ecosystem CO2 exchange and ET. To address this objective, we conducted a prescribed fire in a Mediterranean-type grassland in southern California during a growing season characterized by severe drought. In burned and unburned portions of the grassland, we constructed a rainfall manipulation experiment which had the effect of ameliorating drought conditions in plots which received supplemental rainfall while increasing drought severity in rainfall reduction plots. We predicted that prescribed fire and reduced drought severity would interact to influence the physiological performance and canopy cover of the non-native annual grass Bromus diandrus. Specifically, we predicted that the positive effects of prescribed fire on the physiological performance and canopy cover of B. diandrus would decline with increasing drought severity. In contrast, we predicted the physiological performance and abundance of the more extensively-rooted native perennial grass, Nassella pulchra, would be less responsive to prescribed fire and drought. At the canopy scale, we predicted that fire and drought-mediated shifts in physiological performance translated through changes in the canopy cover of B. diandrus and N. pulchra, would predict ecosystem CO2 assimilation and evapotranspiration.

Section snippets

Site description

Field work was conducted on the Irvine Ranch Conservancy (IRC; 33° 44′ N, 117° 42′ W, elev. 365 m), 5 km north of Irvine, California on a northwest facing slope (<10%), having soils of fine-loamy, mixed, thermic Typic Palexeralfs sandy loam (California Soil Resource Lab, http://casoilresource.lawr.ucdavis.edu) during the 2006–2007 growing season. The IRC has a Mediterranean climate of warm, dry summers and cool, wet winters. Based on the nearest long-term weather station, mean annual temperature

Results

On January 17, prior to the prescribed fire and the initiation of the rainfall manipulation, volumetric soil moisture (θ) was not significantly different between plots slated for burned and unburned treatments (mean ± SD = 8.48% ± 0.99 and 9.08% ± 0.73 respectively). Following the prescribed fire and the initiation of the rainfall manipulation, mean growing season θ did not differ between burned and unburned plots, but declined in response to increasing drought severity (Fig. 2A). Consistent with the

Discussion

In contrast with our first prediction, we found little evidence to show that drought severity interacts with low intensity, prescribed fire to affect leaf physiological performance or ecosystem CO2 exchange in a California Mediterranean-type grassland. This result was surprising given the general expectation that disturbance and resource alteration would interact to influence biological processes across scales. For example, Smith et al. (2009) recently proposed a hierarchical response framework

Acknowledgments

We gratefully acknowledge I. Carbone, A. Fellows and J. Smith for their contribution to the construction of our rainfall manipulation experiment. In addition, J. Smith, E. Hernandez, R. Shirk, L. Larios, J. Cheng, D. Hseuh and M. Murdock assisted with data collection in the field. N. English, E. Hamerlynck and H. Riessen provided helpful comments on an earlier draft of this manuscript. We acknowledge the cooperation of the Irvine Ranch Conservancy, the Irvine Company, the Nature Conservancy,

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