Simulating the Impacts of Changes in Precipitation Timing and Intensity on Tree Growth

Erika K. Wise; Matthew P. Dannenberg

doi:10.1029/2022GL100863

The timing and intensity of precipitation varies from year‐to‐year and is expected to change in the future. Assessing the impacts of this moisture delivery variability on tree growth is important both for future forest health and for our interpretation of pre‐instrumental tree‐ring records. Here, we used the Vaganov‐Shashkin model to investigate how changes in precipitation delivery impact tree growth at five sites representing four species in two North American river basins with high precipitation variability but different seasonal cycles. Evenly distributed precipitation increased tree growth in the Lower Sacramento watershed, while the water‐limited South Platte benefited from concentrated precipitation early in the growing season. Although most experimental simulations retained the pattern of high‐ and low‐growth years, tree growth was reduced with fewer, more intense precipitation events, which could affect interpretation of past climate extremes. Under the RCP4.5 scenario, projected warming offset the potential benefits of increased precipitation on tree growth. Plain Language Summary Trees are valued for many different reasons, including their ability to reduce atmospheric carbon dioxide levels and to record climate from past centuries. Tree growth in moisture‐limited environments is strongly affected by the amount and timing of moisture the tree receives, as well as by how gradually or abruptly precipitation reaches the tree. The amount, timing, and rate of precipitation almost certainly changed in the past and are likely to be affected by human‐caused warming. In this study, we used a model that “grows” tree cells based on temperature, precipitation, and other environmental data to experimentally change the amount and timing of precipitation and assess how these changes affect tree growth in California and Colorado, USA. We found that although many of these experiments affected the two basins differently, our experiments with increased precipitation concentration and those that used projections of future climate change consistently caused tree growth to decrease. This is important because it could affect our interpretations of tree‐ring records of past climate (since the same amount of precipitation could cause different amounts of growth depending on how it is distributed) and our future tree growth expectations (since fewer, stronger storms and higher temperatures could reduce growth). Key Points Vaganov‐Shashkin model simulations used to test how variation in timing and intensity of precipitation influences tree growth Identical precipitation totals with different delivery alters tree growth and could affect interpretation of proxy climate records Projected future changes of intensified precipitation and warming temperatures offset growth benefits of increased precipitation totals

Simulating the Impacts of Changes in Precipitation Timing and Intensity on Tree Growth

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