Abstract
Wildland fires occur with increasing frequency in southwestern riparian forests, yet little is known about the effects of fire on populations of native and exotic vegetation. From 2003 to 2006, we monitored recovering woody vegetation in wildfire sites in the bosque (riparian forest) along the Middle Rio Grande of central New Mexico, USA. To examine recovery potential, we estimated densities of native Rio Grande cottonwood (Populus deltoides ssp. wislizenii) and exotic saltcedar (Tamarix ramosissima) in burned plots and estimated mortality rates of resprouts. Exotic vegetation density was higher in most burned sampling plots than in paired unburned plots. Native shrub density was lower in most burned sampling plots than in unburned plots, although the effect sizes were small. Native tree density did not differ between burned and unburned plots, despite relatively high mortality of cottonwood resprouts. All observed cottonwood mortality occurred between 2003 and 2004, following the hottest and driest summer of the study. Cottonwood resprouts had the lowest apparent mortality rate at the site with the lowest depth to ground-water. Saltcedar resprout mortality was low at all wildfire sites, consistent with previous studies showing that this species is less sensitive to drought and better adapted to wildfire than cottonwood and other native species. Our observations suggest that, in the absence of ideal hydrologic and climatic conditions, fire can reduce cottonwood density in the bosque and promote the spread of saltcedar. Increasingly xeric conditions predicted under most climate change scenarios could result in greater recovery of exotic saltcedar over native vegetation.
Highlights
Riparian forests of the southwestern US have been identified as areas vital to the protection of regional biodiversity and are characterized by woody vegetation adapted to periodic floods (Busch and Smith 1995)
Wildland fires occur with increasing frequency in southwestern riparian forests, yet little is known about the effects of fire on populations of native and exotic vegetation
Saltcedar resprout mortality was low at all wildfire sites, consistent with previous studies showing that this species is less sensitive to drought and better adapted to wildfire than cottonwood and other native species
Summary
Riparian forests of the southwestern US have been identified as areas vital to the protection of regional biodiversity and are characterized by woody vegetation adapted to periodic floods (Busch and Smith 1995). There is, variation in sprouting probability, resprout condition, and resprout survival among native species and between native and exotic species (Busch and Smith 1993, Gom and Rood 1999, Ellis 2001, Stromberg et al 2009) that may result in altered riparian forest structure and composition following wildfire. Climatic factors such as temperature and precipitation influence the hydrology of a site and the physiological condition of woody vegetation (Cleverly et al 1997, Scott et al 1999, Snyder and Williams 2000) Because abiotic factors such as hydrology and climate affect riparian forest structure and composition, their variation and the nature of their effects must be considered when predicting post-fire population trajectories of native and exotic species. We reviewed predicted effects of climate change on hydrology in the southwestern US and combined this information with our results to describe potential recovery scenarios of native and exotic vegetation
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