Douglas-fir and Western Hemlock Responses to Extreme Heat

The present project uses the 2021 Pacific Northwest Heat Dome as a case study for evaluating the relationships between microclimate metrics and the growth & water use of trees at the HJ Andrews Experimental Forest. We do so while exploring the applications and limitations of statistical tools to process and analyze high-frequency dendrometry data. 2021 was undoubtedly the hottest year recorded at the HJA, and as record-breaking weather events like the Heat Dome are likely to increase in the future, it is increasingly important to understand how they affect the ability of plants and ecosystems to function.
Focusing on instrumented young and old Douglas-firs as well as old western hemlocks, we find evidence of both resilience and vulnerability. When comparing microclimate metrics across a 7-year record of high-frequency data, high temperature and VPD spikes from the Heat Dome coincided with immediate stem contraction for all 11 sampled trees within the same time period. Modeled assessments of Douglas-firs within our sample reveal that this led to a statistically significant decline in basal area increment for most trees lasting at least two weeks after the event in a way that was uncharacteristic of typical mid-summer decreases in growth rate. The extent of growth recovery after 2021 is more variable; basal area increment in subsequent years (2022-2025) has mostly remained low despite a return to more typical climatic conditions, suggesting possible carry-over effects that are also related to ongoing drought. These observations are consistent between assessments based on dendrometry or dendrochronology, but are statistically insignificant. We suggest that future research continues to build upon this growing body of work at the intersection of climate, canopy ecology, and phenology subcomponents. Further incorporating fine-scale data is sure to increase global understanding of biotic interactions and the disturbance and climate regimes that shape them.