The topic of past novel ecosystems has never gained much traction in the semi-arid lands of western North America. Complexities in physiography, surface geology, and climate lead to plenty of opportunities for unique plant communities, often localized but sometimes widespread. For example, extensive areas of hypersaline substrate, the Mancos Shale, blanket the west slope of the Colorado Rockies and also outcrop in broad bands on adjacent lowlands of the Colorado Plateau. In the uplands, high soil salinity discourages conifer establishment and instead favors persistent stands of deciduous and clonal aspen and Gambel oak (Quercus gambelii), which can avoid the stringent water requirements of seedling recruitment and dispense of salt by shedding leaves annually. In the adjacent lowlands are relatively unique desertscrub plant communities rich in endemics and dominated by blackbrush (Coleogyne ramosissima) on migrating sandsheets and halophytes that grow on the more clayey soils. Given this inherent complexity and heterogeneity of natural vegetation today, what could actually be considered novel ecosystems in the past and present?
Results/Conclusions
In western North America, fossil packrat middens provide unique taxonomic and spatial resolution in the reconstruction of local plant communities and regional ecosystems. The transition from glacial to interglacial resulted in broadscale changes in growing season temperature (5-10ºC), and seasonal precipitation driven in part by large latitudinal displacements (~10º) in the average position of the westerlies in both winter and summer. During the last glacial period, thornscrub or succulent (e.g., Carnegiea gigantea) species restricted far to the south of the U.S.-Mexico border or to arid patches in more mesic vegetation spread north in the Holocene to form the modern Sonoran Desert. In the context of the Quaternary, the modern Sonoran Desert, typified by diverse shrubs and cacti interspersed with an abundance of seasonally bare soil, could be thought of as a novel Holocene ecosystem riddled with unfulfilled niches and highly vulnerable to new invasions.
For the last 100 years, European and African grass invasions have been driving ‘grassification’ of the Sonoran Desert and other desert shrublands, with a vigorous grass-fire cycle generating novel, fast-spreading, and self-sustaining ecosystems that are barely understood and nearly impossible to manage using existing knowledge and methods. Most research about grassification hasbeen focused on biodiversity impacts and control methods, and not on a realistic and urgent prioritization of the science needed to support societal adaptation to novelties in fire risk, ecosystem processes, and ecological services.