Perennial streams in the Desert LCC support riparian trees such as cottonwood (Populus spp) and box elder (Acer negundo) that are critical components of habitat for riparian obligate birds and other wildlife species (Webb et al. 2007). Trees, snags, and fallen woody debris provide nesting and foraging sites for a variety of riparian animals (Bateman et al. 2008, Smith et al. 2012). Riparian trees require occasional floods to create space suitable for germination and are dependent on accessible groundwater for growth and survival (Lytle and Merritt 2004). Studies along the Middle Rio Grande in New Mexico have shown that rates of woody debris accumulation are also influenced by hydrology because floods physically remove debris or accelerate its decay (Molles et al. 1998). Changes in streamflow, resulting from increased water demand or climate change, could therefore influence the availability of trees, snags, and woody debris for birds and other wildlife species.
In addition to flooding, wildfire influences the structure of riparian forests. Along flow-restricted streams such as the Middle Rio Grande in central New Mexico, high-severity wildfires are fueled by an understory of abundant woody debris and vegetative growth. Following high-severity fire, cottonwoods and other trees are topkilled, but up to 80% resprout. Post-wildfire forests are thus characterized by numerous snags, resprouted woody plants, and, after snags fall to the ground, large amounts of woody debris (Smith et al. 2009).
Rivers in the Desert LCC differ from one another in flow characteristics, levels of regulation, and vulnerability to wildfire; characteristics that will be influenced by climate change (Seager et al. 2007, Mortiz et al. 2012). An understanding of how changes in streamflow and wildfire frequency will affect structure of live and dead woody vegetation is needed for managers to assess the vulnerability of riparian obligate species to climate change. We are developing stochastic transition models for cottonwood trees and snags along the Middle Rio Grande by modifying Lytle and Merritt’s (2004) stage-structured cottonwood population model (Figure 1). By incorporating influences of flood and wildfire into stage transition rates, we can project future tree and snag density under current conditions and under climate change scenarios. We propose further modifications of this model to create an applied science tool that can be used to model vegetation structure, wildlife habitat, and wildfire risk along streams throughout the Southern Rockies LCC.
This project was co-funded by multiple Landscape Conservation Cooperatives: Desert LCC and the Southern Rockies LCC.
