AUTHOR=Hao Shaorong , Jia Xin , Zhao Hongxian , Li Xinhao , Mu Yanmei , Zha Tianshan , Liu Peng , Bourque Charles P.-A. 

TITLE=Evapotranspiration and its partitioning during and following a mountain pine beetle infestation of a lodgepole pine stand in the interior of British Columbia, Canada

JOURNAL=Frontiers in Forests and Global Change

VOLUME=7

YEAR=2024

URL=https://www.frontiersin.org/journals/forests-and-global-change/articles/10.3389/ffgc.2024.1352853

DOI=10.3389/ffgc.2024.1352853

ISSN=2624-893X

ABSTRACT=<sec id="sec1"><title>Introduction</title><p>Massive tree mortality events in western Canada due to widespread infestation by mountain pine beetle (MPB) are expected to impact local-to-regional evapotranspiration (<italic>ET</italic>) dynamics during and after a disturbance. How ecosystem-level <italic>ET</italic> and its components may vary with canopy-tree mortality (treefall) and subsequent understory recovery remains unclear.</p></sec><sec id="sec2"><title>Methods</title><p>We used 10 years of continuous eddy-covariance and remote-sensing data (2007–2016) and machine-learning models based on <italic>random forest</italic> and <italic>xgboost</italic> to determine forest- and climate-driven effects at temporal scales appropriate for a lodgepole pine-dominated stand following a major, five-year MPB disturbance initiated in the summer of 2006.</p></sec><sec id="sec3"><title>Results</title><p>Total annual <italic>ET</italic> over the 10 years ranged from 207.2 to 384.6 mm, with annual plant transpiration (<italic>T</italic>) contributing to 57 ± 5.4% (mean ± standard deviation) of annual <italic>ET</italic>. Annual <italic>ET</italic> initially declined (2007–2011) and then increased (2011–2016), with <italic>ET</italic> and <italic>T</italic>/<italic>ET</italic> increasing at statistically non-significant rates of approximately 3.2 and 1.2% per year from 2007 to 2016. Air temperature (<italic>T<sub>a</sub></italic>) and vapor pressure deficit (<italic>VPD</italic>) were the most important predictors of seasonal variation in <italic>ET</italic> and <italic>T</italic>/<italic>ET</italic> during the 10-year period, with high <italic>T<sub>a</sub></italic>, <italic>VPD</italic>, and photosynthetically active radiation (<italic>PAR</italic>) causing <italic>ET</italic> and <italic>T</italic>/<italic>ET</italic> to increase. Annual <italic>ET</italic> increased with both increasing spring <italic>T<sub>a</sub></italic> and decreasing <italic>VPD</italic>. Annual <italic>T</italic>/<italic>ET</italic> was shown to increase with increasing <italic>VPD</italic> and decrease with increasing volumetric soil water content at a 5-cm depth (<italic>VWC</italic><sub>5</sub>). Enhanced vegetation index (<italic>EVI</italic>, an indicator of canopy greenness) lagged <italic>T</italic> and overstory tree mortality, whereas previous- and current-year values of <italic>EVI</italic> were shown to be poor predictors of annual <italic>ET</italic> and <italic>T</italic>/<italic>ET</italic>.</p></sec><sec id="sec4"><title>Discussion and conclusions</title><p>These findings suggest that the promotion of climate factors on forest ecosystem-level water vapor fluxes may offset reductions promoted by MPB outbreaks. Climate processes affected water vapor fluxes more than biotic factors, like stand greenness, highlighting the need to include climate-regulatory mechanisms in predictive models of <italic>ET</italic> dynamics during and subsequent to stand disturbance. Climate and forest-greenness effects on water vapor fluxes need to be explored at even longer time scales, e.g., at decadal scales, to capture long-drawn-out trends associated with stand disturbance and its subsequent recovery.</p></sec>