Climate-induced tree growth decline and mortality in Oregon

The frequency and scale of drought-related tree mortality events have increased in recent years, particularly in the western United States. The unprecedented 2012-2016 drought in California, USA is estimated to have caused the demise of 129 million trees. Progressively increasing temperatures and seasonal shifts causing reductions in snowpack and low summer stream flows in Oregon, USA suggest that the recent observed levels of tree mortality in California could foreshadow future events for forests in Oregon. We examined the effects of climate on tree growth and mortality based on mortality survey, dendrochronological, and meteorological data from an EPA network of permanent field sites in western Oregon. Each site was instrumented with a meteorological station and below-ground sensors to record air temperature, precipitation, VPD, soil temperature and available soil moisture for the period 1996-present. Our findings indicate that tree mortality rates in Oregon have remained at historic levels < 1% annually, but the risk of future mortality is increasing as indicated by declining growth trends in the most recent decades. Tree growth decline and mortality in the most recent decades are more closely associated with soil temperature and soil available water than air temperature and precipitation. Climatic conditions may be changing faster than previously thought as soil temperatures at our sites have been increasing at ~1ºC/decade which is three times that of air temperature. The rate and duration of tree growth decline varies by species and site. Less-drought tolerant species such as western hemlock at lower elevations surrounding the Willamette Valley show the greatest decline and are most at risk of future climate-induced mortality. Growth rates of more drought-tolerant species including Douglas-fir and the true firs are also declining in western Oregon and are also at risk of future mortality under continuing trends towards more severe droughts.

Impact/Purpose

The drought events in the Pacific Northwest (PNW) in the two most recent decades were unprecedented in the last 800 to 1200 years and climate models predict the extended drought conditions will occur more frequently as the region continues to warm in the 21st century. These changes are already affecting sensitive forested ecosystems, as evidenced by the demise of 129 million trees in California during the unprecedented 2012-2016 drought. Progressively increasing temperatures and seasonal shifts causing reductions in snowpack and low summer stream flows in Oregon, USA suggest that the recent observed levels of tree mortality in California could foreshadow future events for forests in Oregon. PESD scientists examined the effects of climate on growth and mortality of conifer species using dendrochronological and meteorological data collected at five mature forest stands in western Oregon, USA. PESD scientists were first to show that: 1) tree growth rates are declining in response to increasing soil temperature and decreasing soil moisture across the region; 2) tree mortality events at low elevations in the Willamette Valley are more closely associated with soil moisture availability than with air temperature and precipitation; 3) less drought-tolerant tree species such as western hemlock and true firs are at greatest risk of future mortality under continuing trends towards more severe droughts. Our work is important for filling in the gaps of knowledge in understanding the complex interactions of temperature, water, and biotic disturbance agents on conifer forests in the PNW under climate change scenarios. While mortality rates have not changed from historic levels < 1% annually, trees will experience greater risk of growth decline and mortality, resulting in decreased forest health and condition.

Citation

Lee, E Henry, P. Beedlow, AND S. Cline. Climate-induced tree growth decline and mortality in Oregon. Forest Health Conference, Corvallis, OR, OR, February 26, 2020.