Impacts of mega drought in fire-prone montane forests and implications for forest management.

In the wake of severe drought coupled with altered disturbance regimes across the Sierra Nevada, understanding current forest conditions and pace of forest change is critical for managing for human and resource protection goals. Widespread tree mortality and heavy accumulations of dead and downed fuels pose immediate risks to human safety, wildlife habitat, cultural resources, and functioning forest ecosystems. We combine field-based forest monitoring, terrestrial and airborne light detection and ranging, and satellite data in statistical modeling frameworks that improve our understanding of conditions leading to tree mortality and the subsequent effects of forest disturbance across multiple spatial scales within Yosemite National Park to inform management planning, prioritization, and implementation. Despite drought mortality and landscape scale fires of increased severity, we observed relatively high basal area across the Park, particularly in low and mid montane forests. Parkwide, 32.8% of basal area was dead, with mortality varying by species and forest type and strongly associated with higher total stand basal area and lower elevations. Change in satellite-derived Normalized Difference Vegetation Index over time also closely tracked mortality, demonstrating its utility as an indicator of mortality across larger spatial scales and longer timescales. While mortality may shift live basal area towards the historic range of variability, the resulting forest conditions are unlikely to reflect resilient structure and composition. The continuity of standing and downed dead fuels, loss of large-diameter trees, and mismatched species composition of mortality and regeneration suggests shifting fire risk and forest composition. Fire history also strongly influenced components of forest structure and succession that drive subsequent fire risk and fire behavior and are therefore characteristics that can be manipulated through fire management. For example, duff depth and ladder fuels decreased with total times burned and increased with time since fire. Characterizing current conditions of an iconic landscape after the compounding effects of mega-drought and fire offers insights into which biophysical settings might be resilient to future episodic events and how proactive management of forest structure through fire and fuels reduction might be moderated to enhance the potential for survival and regeneration. Incorporating a suite of technologies and data sources also adds value to standard forest monitoring across a large spatial extent and improves managers' ability to focus actions and interventions according to current mortality trends. • High basal area remains across the Park, particularly in low and mid montane forests. • Parkwide mortality was 32.8% of basal area, strongly associated with higher total stand basal area and lower elevations. • Mortality tracked with NDVI change, an indicator of forest condition across larger spatial scales and longer timescales. • Fire history influenced elements of forest structure driving fire risk and behavior that fire management can manipulate. • Dead fuel continuity, large-diameter tree loss, and changes in recruitment hint shifting fire risk and forest composition. [ABSTRACT FROM AUTHOR]