Fire, climate and changing forests
A changing climate implies potential transformations in plant demography, communities, and disturbances such as wildfire and insect outbreaks. How do these dynamics play out in terrestrial ecosystems across scales of space and time? “Vegetation type conversion” (VTC) is a term used to describe abrupt and long-lasting changes in vegetation structure and composition due to various kinds of perturbations. For example, it has long been observed that fire-adapted ecosystems such as the California chaparral shrublands are readily replaced by non-native annual grasses when humans increase fire frequency. Similar effects are being observed in Sonoran Desert plant communities, where invasion by non-native buffelgrass (Cenchrus ciliare) exposes a fire-sensitive plant community to high-intensity fires, which are lethal to the long-lived iconic Sonoran flora such as Saguaro cactus (Carnegiea gigantea). While type conversions following severe disturbance have been observed for some time, the mechanisms that underlay such changes are only recently coming to light. Threshold behavior is of particular concern: as various climate indices pass critical values, some ecosystems may be reaching points of no return where recovery to the earlier state is no longer possible. A recent publication in the Proceedings of the National Academy of Sciences, U.S., has now revealed that increased fire severity impairs post-fire forest regeneration, setting the stage for potential large-scale type conversion of western conifer forests.