Precipitation timing and grazer performance in a tallgrass prairie

Changes in precipitation amount and variability have the potential to alter the structure and function of grasslands, but we know little about how changes in the timing of precipitation might aff ect grasslands. Here, we analyze long-term records from a tallgrass prairie to show that shifts in the timing of precipitation during the growing season have little eff ect on primary productivity or grass reproduction, but can greatly aff ect grazer performance. While greater late-season precipitation increases the weight gain of adult and young bison, greater mid-season precipitation decreases their weight gain. In addition, calving rates are lower after years with greater mid-season precipitation and higher after years with greater lateseason precipitation. As well-timed drought can actually increase grazer weight gain and reproduction, it will be necessary to generate predictions of within-season distribution of precipitation to successfully forecast future grazer performance. Oikos 000: 001–008, 2012 doi: 10.1111/j.1600-0706.2012.20400.x © 2012 Th e Authors. Oikos © 2012 Nordic Society Oikos Subject Editor: Joseph Bailey. Accepted 11 April 2012