Landscape analysis of post-burn succession in a Great Basin pinyon-juniper woodland.

摘要

Plant succession following fire is complex as the post-fire landscape is heterogeneous with respect to propagule availability and environmental conditions. Environmental conditions are influenced by variation in abiotic factors such as topography, climate, and soil. However, important biotic legacy effects associated with site history are difficult to quantify. This study examines the driving factors underlying plant community composition six years following a wildland fire in Wall Canyon of the Toiyabe Range of the central Great Basin. It also examines the direct and indirect influences of various abiotic and biotic factors on the distribution and abundance of two species of particular interest, Bromus tectorum and Artemisia tridentata ssp. vaseyana.;Direct and indirect ordination methods were used to describe post-fire plant community composition according to underlying gradients reflecting the physical environment, pre-burn vegetation structure, anthropogenic disturbances, and proximity to potential seed sources. The strongest abiotic influences on plant community composition included solar radiation, likely reflecting a soil moisture gradient, and soil pH. Biotic influences on community composition, in order of importance, were proximity of unburned patch edge within the burn, pre-burn canopy cover and distance from the burn perimeter. Each of these predictors favored species with particular life history traits. Increased solar radiation favored drought avoiding species such as Bromus tectorum and Descurania sophia. High values of soil pH, possibly indicating areas of high burn severity or reduced soil development, favored annual, widely dispersing species such as Lactuca serriola and Salsola tragus. A few native species were favored by simultaneous increases in pre-burn canopy cover and influence of unburned patches that could provide seed sources: Astragalus purshii, Cryptantha torreyana, and Senecio multibalatus. Increased distance from the burn perimeter favored widely dispersing species such as Crepis acuminata and Gnaphalium palutre. Abiotic controls accounted for 11.3% of the variance in species composition and biotic interactions accounted for 7.1% with an interaction of 1.4%. Plant species distributions following fire in Wall Canyon were largely driven by abiotic factors, especially soil moisture, with biotic factors also playing a significant role in determining the availability of plant propagules.;Principal components regression models of B. tectorum and A. tridentata ssp. vaseyana distributions revealed species-specific environmental preferences. B. tectorum preferred xeric sites with gentler slopes, lower pre-burn canopy cover, more acidic soils, and higher concentrations of ammonium (R2 adj = 0.153). A. tridentata ssp. vaseyana preferred more mesic sites with deeper soils (R2 adj = 0.296). Structural equation models identified causal pathways predicting the abundance of A. tridentata ssp. vaseyana that included a positive effect of soil depth and a negative effect of pre-burn canopy cover. Direct pathways predicting the abundance of B. tectorum included a positive effect of solar radiation. Structural equation models also identified negative direct effects of topographic convergence index, TCI, and percent coarse fragment on pre-burn canopy cover.;Plant community development following fire in pinyon-juniper woodlands represents the interaction of the three successional mechanisms proposed by Connell and Slatyer (1977): stress tolerance, facilitation, and inhibition. Annual species such as B. tectorum tolerate the environmental stress of low soil moisture and impose stress on other plant species. B. tectorum uses available soil moisture before the growing season of native plant species, inhibiting the establishment of natives. Shrub species, once established, modify the physical environment by increasing available soil moisture and thereby facilitating the establishment of Pinus monophylla and Juniperus osteosperma. Without the prior establishment of shrubs, soil moisture is too low for these tree species to establish. Once established, these arborescent species inhibit the establishment of understory species through increased canopy cover, extensive rooting systems, and domination of available soil moisture.;Although abiotic factors strongly influence species distributions following fire, pre-burn vegetation modifies these conditions. My findings indicate that effective restoration must consider the pre-burn conditions of the system. Pre-fire restoration efforts should focus on woodlands whose understory vegetation has not been completely reduced by increasing tree canopy cover and that can therefore recover following fire. Post-fire restoration efforts should focus on sites that are dry, have high pre-burn canopy cover, or are at greater distances from unburned patches as these conditions collectively favored invasion of non-native species.