Ecological relationships between invasive grasses, native grasses, and wildfire

Grasslands and savannas are ecosystems that require frequent wildfires, or modern vegetation clearing methods, to maintain a low density of trees and shrubs. Invasive grasses have become more common in such ecosystems, and although invasive grasses are known to have harmful ecological effects, the direct and indirect relationships between invasive grasses, native grasses, and wildfires in such ecosystems are poorly understood. We studied the effects of wildfire on invasive and native grasses and the effects of invasive grasses on wildfire characteristics in the southern Great Plains. We also studied the immediate impacts of a wildfire on a population of an endangered grass species in a sky island in west Texas.

We found that invasive grasses are one of the most important factors in determining fuel loads and fire temperatures in degraded grasslands and savannas of the southern Great Plains (Chapter 2). However, the changes to the wildfires caused by invasive grasses did not cause negative effects on native species that were growing in a grassland dominated by the invasive grass Bothriochloa ischaemum (King Ranch bluestem, yellow bluestem); native herbaceous plants, especially native perennial grasses, increased where B. ischaemum experienced high mortality in summer and fall prescribed fires (Chapter 1). Lastly, the population size of the rare grass Festuca ligulata was substantially smaller immediately post-fire than it was two years before, but fire severity did not appear to cause the decline.

These studies can inform land management decisions. Summer or fall prescribed fires in B. ischaemum-dominated ecosystems can cause high mortality of the invasive grass. Seeding native plants after summer or fall prescribed fires may not be necessary if native herbaceous species are present, even if they are not abundant, because native species are not negatively affected by the high fire temperatures caused by B. ischaemum. Finally, the effects of fire severity on the rare grass F. ligulata may not be detectable immediately after a fire, especially because, like with all rare species, relationships are harder to detect with small population sizes. Continued monitoring will be necessary to determine if the population will rebound.