Drought, invasive grasses and fire cycles – implications for ecosystem resilience in drylands
Extreme climatic events appear to be increasing in their frequency in the western USA over the last 100 years. Whether and how such events promote vegetation change remains an unanswered question. While managers generally expect drought to be correlated with the occurrence of fire, and in arid systems for invasive grasses and fire occurrence to be related, it is not clear whether or not this is the case.
Long term data sets are important for helping managers anticipate vegetation responses to drought including the possible occurrence of fire, and drought/fire/invasion interactions. In addition to long term observations of vegetation change, and patterns of fire occurrence, manipulative field experiments, and controlled laboratory or greenhouse studies or trait analyses can aid in predicting species responses to environmental stressors including drought and fire.
In this presentation, we evaluate some patterns of vegetation change in response to climate variation, the presence of exotic grasses and fire. I contrast an exotic annual grass system in the Great Basin with a Hawaiian dry forest system invaded by perennial grasses. I evaluate the role of fire and drought in patterns of vegetation change and demonstrate challenges and strengths in our ability to accurately predict vegetation change
In Great Basin ecosystems, summer lightning and hence natural ignitions are common. My colleagues analysed remote sensing data on fire and vegetation occurrence and found strong support for a grass:fire cycle: fres burn more frequently in sites dominated by the exotic annual grass, Bromus tectorum compared to native shrubland vegetation. However, fire/invasion/climate relationships were not strong and fires occurred more frequently after high rainfall years than in association with severe drought. Ultimately then high rainfall years set the stage for fire. If severe drought occured for several years in a row, it is likely that fire frequencies would decline because of depletion and lack of replenishment of the annual grass seedbank, By contrast in Hawaiian dryland ecosystems exotic perennial grasses predominate and fire occurrence does not appear to be correlated with climate events because natural ignition sources are limited and management limits access to protected areas during extreme drought. Such droughts do induce vegetation change by causing a decline in native woody species, and a decline in non-drought tolerant exotics. This allows invasion by more drought tolerant and potentially more detrimental exotics. Thus drought causes dramatic long term change without fire via rearrangement of grass dominance.