UC Irvine

Climate change is impacting forests in multiple interacting ways such as changing fire regimes and hotter droughts linked to drought induced forest die-off. These first order climate change impacts can interact with subsequent disturbances by producing amplifying feedbacks that make subsequent disturbances more severe or dampening feedbacks that make subsequent disturbances less severe.

In Chapter 1 we tested the common assumption that that recent tree mortality will not alter die-off severity during subsequent droughts. by comparing die-off in semi-arid conifer forest stands in California that were exposed to a single drought and two sequential droughts. We found that recent tree morality reduces die-off severity in semi-arid conifer forests exposed to subsequent drought.

In Chapter 2 we tested how a history of prescribed fire and wildfire with varying severities changes forest cover and water use and forest drought vulnerability. Forests with recent fire history had reduced tree cover, increased shrub cover, and decreased water use (ET), with the greatest changes due to wildfires and high severity fires. These decreases in tree cover and ET led to decreased forest die-off severity compared to similar forests that had not recently experienced fire.

In Chapter 3, we compared the importance of potential forest drought vulnerability risk factors and modeled how forest die-off risk is changing using a simple statistical approach. We found that drought exposure, proximity to the climate water limit, and forest density were generally the most important predictors of drought vulnerability. Forests without recent disturbance, especially those at moderate elevations with relatively high tree cover and where water use and annual precipitation are similar in magnitude currently have the highest drought vulnerability.

As forests continue to experience disturbances linked to climate change, dampening effects will impose a transient, and perhaps long-term, constraint on the impact of drought. Human actions such as prescribed fire and perhaps other management actions, can produce decreases in tree cover and water that in turn increase forest resistance to drought. Implementing natural climate solutions that consider both climate change amplified risks and dampening feedbacks, will improve the likelihood that climate mitigation projects provide long-term climate mitigation.