Fire, flood, and climate change: modeling cottonwood populations in western riparian forests

Background/Question/Methods

Along lowland streams in the western United States, reproduction of cottonwood trees (Populus spp.) is dependent on floods that provide substrate for germination space and moisture for seedling survival. The Middle Rio Grande in central New Mexico is heavily regulated by dams and diversions, which have restricted the occurrence of such floods. Frequency of large wildfires has increased here as well due in part to accumulation of litter and debris. To determine the long-term effects of these changes, we examined cottonwood (Populus deltoides ssp. wislizenii) recovery at a site that burned in 2008 in the northern section of our study area, which is rarely burned and occasionally flooded, and at two sites that burned in 2011 in the south section, which is frequently burned and rarely flooded. We incorporated results into a stochastic population model to project changes in abundance of cottonwoods in each section. To include effects of climate change in our model, we obtained projected changes in flood volume and timing based on temperature and precipitation projections from CMIP3 models run under the A2 emissions scenario.

Results/Conclusions

All top-killed cottonwoods had live basal sprouts at the post-fire site in the north section, compared to 53.2% and 21.2% at the south section sites. Cottonwood root sucker density was greater at the north section site (0.09/m²) than at south section sites (0.02/m² and 0.004/m²) as well. We found cottonwood seedlings in an area of the north section that had been burned and flooded but we did not find cottonwood seedlings in the south section. Streamflow projections indicated that floods will be reduced in volume by 22% and occur 19.5 days earlier on average by the end of the century. Population models fitted with these data predicted that, by 2100, abundance of mature cottonwoods will decline by 93% in the north section and 99% in the south section. Our results suggest that continued occurrence of fire, coupled with changes to the flood regime, will hasten the replacement of cottonwoods by drought- and fire-tolerant species along the Middle Rio Grande and other regulated streams.