OOS 33-2 - Primary production responses to extreme changes in North American Monsoon precipitation move up an elevation gradient through time

Background/Question/Methods
Primary production in dryland ecosystems is limited by water availability and projected to be strongly affected by future shifts in seasonal precipitation. Warm-season precipitation derived from the North American Monsoon contributes 40% of annual precipitation to dryland ecosystems in the southwestern U.S. and is projected to become more variable. However, there is large uncertainty on whether this variability will be expressed as either extreme wet or dry years and how primary production will respond across widespread elevation gradients in this region. We experimentally imposed extreme drought and heavy rainfall from 2016 – 2020, during which ambient warm-season precipitation declined to reach historic lows, to understand production sensitivity of dominant plant functional types along a 1,000 m elevation gradient.

Results/Conclusions
We found that the effects of monsoon precipitation extremes varied by plant functional type, with responses dependent on the number of treatment years that occurred across sites along the elevation gradient. C₄ perennial grasses were most responsive to precipitation manipulation treatments, followed by C₃ perennial grasses and annuals, while perennial forbs and shrubs had no responses. Primary production reductions due to extreme drought were generally stronger or occurred earlier at low elevation sites, while high elevation sites were initially less responsive to extreme drought, and all sites showed delayed responses to multi-year water addition. We found that the sensitivity of C₃ perennial grass production differed for extreme dry and wet years compared to ambient precipitation at one site, but other sites and plant functional types had similar sensitivities to the different treatment types. The upward advance of primary production responsiveness from single- to multi-year extreme changes in warm-season precipitation suggests more immediate shifts in functional composition and carbon cycling at low elevation, while high elevation ecosystems may become less resistant as the effects of extreme precipitation compound through time.