What mediates tree mortality during drought in the southern Sierra Nevada?

Abstract

Severe drought has the potential to cause selective mortality within a forest, thereby inducing shifts in forest species composition. The southern Sierra Nevada foothills and mountains of California have experienced extensive forest dieback due to drought stress and insect outbreak. We used high-fidelity imaging spectroscopy (HiFIS) and light detection and ranging (LiDAR) from the Carnegie Airborne Observatory (CAO) to estimate the effect of forest dieback on species composition in response to drought stress in Sequoia National Park. Our aims were (1) to quantify site-specific conditions that mediate tree mortality along an elevation gradient in the southern Sierra Nevada Mountains, (2) to assess where mortality events have a greater probability of occurring, and (3) to estimate which tree species have a greater likelihood of mortality along the elevation gradient. A series of statistical models were generated to classify species composition and identify tree mortality, and the influences of different environmental factors were spatially quantified and analyzed to assess where mortality events have a greater likelihood of occurring. A higher probability of mortality was observed in the lower portion of the elevation gradient, on southwest- and west-facing slopes, in areas with shallow soils, on shallower slopes, and at greater distances from water. All of these factors are related to site water balance throughout the landscape. Our results also suggest that mortality is species-specific along the elevation gradient, mainly affecting Pinus ponderosa and Pinus lambertiana at lower elevations. Selective mortality within the forest may drive long-term shifts in community composition along the elevation gradient.

Forest structure and climate mediate drought‐induced tree mortality in forests of the Sierra Nevada,USA

“…Among the drought's many effects was the initiation of a severe tree mortality event (Paz‐Kagan et al. , Preisler et al. , Young et al.…”

“…Mortality triggered by the 2012–2016 drought was concentrated in pines (particularly ponderosa pine), drier locations (generally lower elevations and latitudes), and larger trees (Paz‐Kagan et al. , Preisler et al. , Young et al.…”

“…The 2012-2016 California drought may have been the most extreme in the last 1,000 yr (Griffin andAnchukaitis 2014, Robeson 2015), and an example of a "hot" drought where low levels of precipitation and high temperatures combine to increase drought intensity and amplify the ecological consequences (Overpeck 2013, Allen et al 2015). Among the drought's many effects was the initiation of a severe tree mortality event (Paz-Kagan et al 2017, Preisler et al 2017, Young et al 2017, Fettig et al 2019. By December 2017, it was estimated that 129 million trees had died across the state, with the highest concentration along the west slope of the southern and central Sierra Nevada (USDA 2017).…”

Forest Drought Resistance at Large Geographic Scales

“…Requisite to the identification of drought refugia is the resolution of spatial and temporal indicators of drought‐induced biological response. Early efforts linking meteorology with forest response have included spatially explicit but temporally static relationships (Paz‐Kagan et al, ; Young et al, ) or temporally dynamic but spatially limited relationships (Bell et al, ; Chen et al, ). The simultaneous examination of vegetation response to drought through both time and space requires a robust indicator of forest physiological response to drought.…”

Fragmentation, forest structure, and topography modulate impacts of drought in a tropical forest landscape

“…, Guarín and Taylor , Paz‐Kagan et al. ). A small number of studies in tropical forests have found more severe drought impacts in drier topographic positions (Nishimua et al.…”