Contingent productivity responses to more extreme rainfall regimes across a grassland biome

Abstract

Climate models predict, and empirical evidence confirms, that more extreme precipitation regimes are occurring in tandem with warmer atmospheric temperatures. These more extreme rainfall patterns are characterized by increased event size separated by longer within season drought periods and represent novel climatic conditions whose consequences for different ecosystem types are largely unknown. Here, we present results from an experiment in which more extreme rainfall patterns were imposed in three native grassland sites in the Central Plains Region of North America, USA. Along this 600 km precipitation-productivity gradient, there was strong sensitivity of temperate grasslands to more extreme growing season rainfall regimes, with responses of aboveground net primary productivity (ANPP) contingent on mean soil water levels for different grassland types. At the mesic end of the gradient (tallgrass prairie), longer dry intervals between events led to extended periods of below-average soil water content, increased plant water stress and reduced ANPP by 18%. The opposite response occurred at the dry end (semiarid steppe), where a shift to fewer, but larger, events increased periods of above-average soil water content, reduced seasonal plant water stress and resulted in a 30% increase in ANPP. At an intermediate mixed grass prairie site with high plant species richness, ANPP was most sensitive to more extreme rainfall regimes (70% increase). These results highlight the inherent complexity in predicting how terrestrial ecosystems will respond to forecast novel climate conditions as well as the difficulties in extending inferences from single site experiments across biomes. Even with no change in annual precipitation amount, ANPP responses in a relatively uniform physiographic region differed in both magnitude and direction in response to within season changes in rainfall event size/frequency.

Decreased water limitation under elevated CO 2 amplifies potential for forest carbon sinks

“…However, our model predictions can explain the complex responses of controlled field experiments to temporal manipulation of rainfall. Responses of aboveground net primary productivity (ANPP) to experimental repackaging of ambient rainfall into fewer, larger rainfall events depend on site productivity (37). At a less productive experimental site, ANPP increased in response to such an experimental repackaging (38), whereas at a more productive site, ANPP decreased in response to the manipulation (39).…”

It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands

“…Thus, modifications in precipitation patterns with climate change will largely affect ecosystem functioning in drylands [58], although some of these changes may not be necessarily negative (see [59] for a review). For example, increases in above-ground net primary productivity (ANPP) with increases in rainfall variability (less but more intense rainfall events) have been observed in semi-arid steppes from North America [36].…”

“…Climate models project an increase in precipitation variability in drylands, including more extreme rainfall events and intense droughts [34]. Large rainfall pulses infiltrate deeper and last longer than smaller rain events [35], and thus even areas undergoing decreases in annual precipitation could experience increased soil moisture if precipitation becomes more variable but is characterized by larger pulses [36,37]. Nevertheless, there is evidence that climate change will exacerbate the aridity of most drylands worldwide, such as the southwestern US [38], the Mediterranean Basin [39], southern Africa [40], Australia [41], South America [42] and China [33].…”

Enhanced precipitation variability decreases grass- and increases shrub-productivity

“…Aboveground net primary production (ANPP) is the main carbon fixation pathway, and even though its responses to changes in the amount of precipitation have been well studied, knowledge about the effect of precipitation variability on ANPP is rather poor, especially at the interannual to decadal time scales. A few short-term experiments focused on the effect of intra-annual precipitation variance and reported contrasting results, with null or positive effects in arid systems and negative effects in mesic systems (11)(12)(13)(14). A modeling exercise found that increased interannual precipitation variability and temperature in the Tibetan Plateau led to productivity reduction in grasslands (15).…”