Hindsight is 20/20 for the 2012-2015 California drought: Tree-rings forecasted widespread forest mortality decades prior to the event

Abstract:

The recent increase in the frequency of transformational droughts and large-scale forest mortality events across western North America. Long-term forecasts of these mortality events could reduce their threat to the sustainability of ecosystem services provided by intact forests. However, few forecasting techniques have been demonstrated at time scales exceeding a few years, which is too short for landscape scale management actions to be undertaken. Here, we provide a retrospective view on potential forecasting methods for determining the likelihood of drought-related mortality by investigating tree-ring responses of ponderosa pine (Pinus ponderosa) from the southern Sierra Nevada Mountains leading up to the 2012-2015 California drought and associated outbreak of western pine beetle (Dendroctonus brevicomis). Tree-ring growth and carbon isotope discrimination (∆¹³C) were strongly correlated with remote sensing observations of vegetation moisture deficit over time, which have been shown to forecast mortality for two to three years in advance. Increasing sensitivity (i.e., increased explained variation) of tree-growth and ∆¹³C vs. Palmer Drought Severity Index (PDSI) during the 20^th century presaged forest mortality. More importantly, growth and ∆¹³C sensitivity to PDSI plateaued over the recent ca. 30 years despite continued warming and increased drought severity. We contend this shift in system response indicated that forests had crossed a vulnerability tipping point and this phenomenon, if proven true, could be used to forecast susceptibility to drought and virulent pests by beyond 10 years. With the clarity of hindsight, we conclude that the recognition of decades-long early warning signals could have provided the needed information for government and private forest managers to mitigate the devastation that has afflicted conifer forests across much of California during and after the 2012-2015 severe drought.