Robust projections and consequences of an expanding bimodal growing season in the western United States

Abstract

Plant growth is restricted to times of the year when actual evapotranspiration (AET) is greater than zero because AET requires both the presence of water in the soil and temperatures warm enough to allow transpiration. Locations where water rather than temperature limits plant growth, such as semi‐arid areas of the southwestern United States, often have a bimodal growing season, such that distinct AET (growth) peaks occur in the spring and late summer, with a period of very limited plant growth occurring during the intervening summer months. We hypothesized that future warming will increase the zone containing bimodal growth seasons, likely resulting in significant changes in the competitive relationships between plant species that differ in their tolerance of a bimodal seasonality. This will likely alter plant distributions. Using climate projections to drive a water balance model, we mapped geographic regions within the continental United States projected to experience bimodal growing seasons in the future. The area containing bimodal seasonality increased under all 13 general circulation models (GCMs) and two representative concentration pathways (RCPs) examined. This robust result (seen in all alternative futures examined) nevertheless showed considerable variability depending on the GCM examined. The bimodal zone was projected to increase 13%–212% (49,000–792,000 km2) by the late 21st century relative to 1981–2010 estimates. Climate futures that contained the greatest temperature increases and greatest precipitation decreases projected the greatest expansion in the bimodal zone. For plant species that depend on relatively long, consistent time periods that are favorable to growth each year, the projected shift in seasonality may be an acute disturbance that could cause widespread mortality. These changes will likely have cascading ecological and management implications, including changes in the dominant plant life history strategies that occur in affected areas.