Abstract
Climate change is impacting forested ecosystems worldwide, particularly in the Northern Hemisphere where warming has increased at a faster rate than the rest of the globe. As climate warms, trembling aspen (Populus tremuloides) is expected to become more successful in northern boreal forests because of its current presence in drier areas of North America. However, large-scale productivity decline of aspen has recently been documented throughout the United States and Canada as a result of drought and insect outbreaks. We used tree ring measurements (basal area increment (BAI) and stable carbon isotopes (δ13C)) and remote sensing indices of vegetation productivity (NDVI) to study the impact of climate and damage by the aspen epidermal leaf miner (Phyllocnistis populiella) on aspen productivity and physiology in interior Alaska. We found that productivity decreased with greater leaf mining and was not sensitive to growing season (GS) moisture availability. Although productivity decreased during high leaf mining years, it recovered to pre-outbreak levels during years of low insect damage, suggesting a degree of resilience to P. populiella mining. Climate and leaf mining interacted to influence tree ring δ13C, with greater leaf mining resulting in decreased δ13C when GS moisture availability was low. We also found that NDVI was negatively associated with leaf mining, and positively correlated with BAI and the δ13C decrease corresponding to mining. This suggests that NDVI is capturing not only variations in productivity, but also changes in physiology associated with P. populiella. Overall, these findings indicate that the indirect effects of P. populiella mining have a larger impact on aspen productivity and physiology than climate under current conditions, and is essential to consider when assessing growth, physiology and NDVI trends in interior Alaska.
Highlights
Global temperatures have increased at an unprecedented rate in recent decades (IPCC 2013) contributing to increased drought stress and tree productivity decline in forested areas worldwide (Choat et al 2012, Williams et al 2013, Allen et al 2015)
Based on the trees sampled for herbivore damage, the greatest variability in leaf mining at the tree and stand level was during years of low mining, yet mining was still minimal across all leaves (i.e. 1.93 ± 3.66, mean % leaf mining ± SD for all trees surveyed in 2013)
Tree productivity is sensitive to leaf mining Our findings indicate that during P. populiella infestation, insect damage negatively impacts aspen radial growth in interior Alaska, which is in agreement with studies in the southern boreal forest and aspen parkland on aspen growth during insect outbreaks (Hogg et al 2002, 2005, 2008, Chen et al 2018, Itter et al 2019)
Summary
Global temperatures have increased at an unprecedented rate in recent decades (IPCC 2013) contributing to increased drought stress and tree productivity decline in forested areas worldwide (Choat et al 2012, Williams et al 2013, Allen et al 2015). Many insect species experience shortened life cycles and increased survival in response to higher temperatures and longer growing seasons (GSs) (Dale et al 2001, Régnière et al 2012), resulting in severe and pervasive outbreaks that have led to widespread dieback of boreal trees (Berg et al 2006, Soja et al 2007, Kurz et al 2008, Kautz et al 2017) Incidence of both drought and insect infestation are expected to increase under continued climate change (Hinzman et al 2005, Gray 2008, Bentz et al 2009, Williamson et al 2009), and they may interact to influence tree productivity and mortality, more so than insects or drought alone (Gaylord et al 2013, Anderegg et al 2015)
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