While trees have increased growth in high moisture years, it does not offset the decline in growth in low precipitation years reports the University of Arizona, "Increasing precipitation extremes driving tree growth reductions across Southwest", as reported in ScienceDaily, October 2019.
The study looked at tree rings at 1,300 sites across the US. They found even with no change in average precipitation, the increase in rainfall extremes can cause tree growth decline. The researchers said, "By looking at how tree growth responded to climate in the past, we were able to investigate how changing precipitation extremes are likely to affect our forests."
"The Southwest's iconic ponderosa pine, Douglas-fir and piñon pine were particularly affected, as were the bur oak in the upper Midwest, each seeing significantly reduced growth during dry years that wasn't balanced by the growth spurred by wetter years,” said the researchers. "We hope our study motivates additional research efforts across dryland ecosystems.”