Bigcone Douglas-fir (Pseudotsuga macrocarpa; BCDF) is a rare conifer endemic to a few coastal mountain ranges in the montane chaparral and woodlands ecoregion of southern California. BCDF is listed as ‘Near threatened’ by the IUCN Red List and this status may be in decline considering recent large fires and unprecedented drought conditions throughout its range. Within its narrow distribution, there may be areas of ‘hydrologic refugia’ or sites that have been relatively buffered from recent fire and drought impacts. Identification of these areas is crucial for conservation of BCDF. However, current distribution and status of BCDF stands is difficult to evaluate owing to its often remote and steep habitat. The objectives of our research were to: (1) map the location and status of BCDF stands (2) assess impacts of recent drought on BCDF health; and (3) test hypotheses about potential climate change refugia for BCDF within Los Padres National Forest (LPNF) to inform conservation and restoration decision-making for this species. Satellite imagery was used to identify stands and evaluate stand health. Time series of a remotely-sensed vegetation health index--Enhanced Vegetation Index (EVI)--were used to assess the response of BCDF stands to recent fires and droughts.
Results/Conclusions
Our process detected and mapped > 2000 BCDF stands across LPNF, which were ground-truthed with field campaigns in the summers of 2017 and 2018. We found that all BCDF stands (burned and unburned) experienced declining health during the drought. However, there was considerable variation in drought response among stands. Stands that burned in a large fire prior to the drought (Zaca Fire, 2007) showed a relatively dampened drought response compared to unburned stands (P < 0.05). However, burned sites had not fully recovered before the onset of the drought and have yet to return to pre-fire conditions even 10+ years after the fire (difference from historical baseline; P < 0.05). Within unburned stands, response to the historic drought varied significantly suggesting that some stands were buffered by local availability of water. This suggests that all BCDF stands in LPNF are experiencing some form of degradation but the level of degradation depends on interactions between recent fires and drought, as well as topographic effects on water availability. The results justify the focus on BCDF by the USDA Forest Service as high conservation priorities and a better understanding of the topographic drivers of water availability may be important to these efforts.