The timing of growing season drought and its effects on above- and belowground production in a mesic grassland

As a consequence of climate change, both the timing and amount of precipitation ecosystems receive are expected to be altered. In general, regions that are relatively dry are expected to get drier and the timing of seasonal drought — defined as a prolonged absence or marked deficiency of precipitation — is expected to change. Although drought in general has been extensively studied, particularly in grasslands, we know little about how natural ecosystems will respond to shifts in the timing of growing season drought. In this study I investigated the response of both above- and belowground net primary production (ANPP & BNPP) to reductions in precipitation in a mesic, tallgrass prairie in NE Kansas. Experimental plots were subjected to one of three drought treatments (25% reductions in the average growing season precipitation [GSP]) imposed either in late spring, early summer or mid-summer. A control treatment that received the mean GSP and a wet treatment that received 130% of the mean GSP were included to assess drought responses. In all treatments, I measured soil moisture, soil N and P content, canopy light interception and plant community composition in addition to ANPP and BNPP. I expected that ANPP would be more sensitive to drought than BNPP based on evidence from past studies that have almost always found a positive correlation between precipitation and ANPP, while trends with BNPP are less clear. I also hypothesized that early summer drought would cause the highest reduction in net primary production (ANPP + BNPP), because soil moisture would likely still be high in the late spring from late winter and early spring snow/rain, lessening the effect of reduced precipitation inputs. Moreover, because annual ANPP approaches its maximum by summer, I expected the mid-summer drought to affect NPP the least. I found that without considering timing, a 25% growing season drought reduced ANPP relative to the control by 18-26%, while ANPP in the control and wet treatment was not significantly different. Early summer and mid-summer drought resulted in significant reductions in ANPP (~25%) relative to control plots, but late spring drought did not reduce ANPP significantly despite similar reductions in soil moisture across all treatments. In contrast, neither drought nor wet treatments altered BNPP significantly. Because soil nutrients may increase during drought and plant functional type diversity may buffer productivity responses to drought, I investigated the role these played in determining responses to the treatments imposed. I found that soil nutrients were positively related to ANPP only in the wet treatment; conversely, diversity was negatively related to ANPP in the ambient and drought treatments, but not the wet treatment. I conclude that timing does play an important role in determining ecosystem response to drought with periods of no rain that occur earlier in the year having less of an impact than those that occur later. Furthermore, differences in responses between ANPP and BNPP were striking and need to be accounted for when projecting productivity responses of grasslands to climate change