Moisture Stress Indicators in Giant Sequoia Groves in the Southern Sierra Nevada of California, USA

Abstract

Giant sequoia [Sequoiadendron giganteum (Lindl.) J. Buchholz] trees and their ecosystems are unique natural treasures in the Sierra Nevada, California, where most groves are federally managed for biodiversity, perpetuation of the species, and aesthetic, recreational, ecological, and scientific values. Increasing temperatures during the next several decades may create conditions unfavorable for these giant sequoias. Therefore, it is necessary to develop effective management systems to preserve the health of these giant sequoia groves. This study used a topographic wetness index (TWI) as the indicator of soil moisture conditions to evaluate the vulnerability of giant sequoia groves to soil moisture stress and focused on evaluating TWI distributions among all 70 sequoia groves to assess their vulnerability to soil moisture stress. The TWI values were derived using a 10-m digital elevation model and compared with soil, geology, slope, aspect, and elevation at the sequoia groves to understand the vulnerability of the groves to soil moisture stress. The TWI values were also compared with snow cover persistence derived from 12 yr of MODIS snow cover products. In addition, satellite soil moisture products were used to compare the dry and wet periods predicted by snow cover persistence. Results showed that the groves located at higher elevation are less vulnerable unless the TWI across the groves is low. For the large number of groves with elevations mainly in the 1800-to 2100-m range, the TWI distributions can serve as a first-order indicator of relative vulnerability. Further, this analysis showed that areas with milder slopes and more converging area (higher TWI), plus longer snow cover persistence, should be less susceptible to low summer soil moisture than areas having steeper slopes, more diverging topography (lower TWI), and earlier snowmelt. This analysis can be used to highlight groves that are potentially more vulnerable, particularly when considering TWI, snow cover persistence, and satellite soil moisture together.

Assessing multi-year-drought vulnerability in dense Mediterranean-climate forests using water-balance-based indicators

“…While subsurface mapping of grove areas would be ideal, lacking those data we consider the relation of topographic features (e.g. TWI and curvature) to potential subsidies of water draining from shallow soil and groundwater (Hoylman et al, 2019;Ray, 2016). Curvature is closely related to surface flow on hillslopes (Bogaart and Troch, 2006;Talebi et al, 2008), soil moisture (Famiglietti et al, 1998), and groundwater level ; and we included the upslope curvature (mean curvature of upslope contribution area; Freeman, 1991;Kadirhodjaev et al, 2018), with a positive value for upwardly convex surface and a negative value for concave surface.…”

“…Another indicator of available soil moisture based on landscape surface characteristics is Topographic Wetness Index (TWI) (Western et al, 1999). For example, sequoia groves have higher TWI than other forested areas (Ray, 2016;, further suggesting they may receive water subsidies from outside to support their large water demand.…”

Future vulnerability mapping based on response to extreme climate events: Dieback thresholds in an endemic California oak

“…Water stress associated with this drought has been linked to chaparral species mortality and changes in stand structure (Venturas et al, 2016). Ray (2016) found increased vulnerability among giant sequoia (Sequoiadendron giganteum). Nearly 10.6 million ha of California forests experienced loss in canopy water content in a variety of forest types including coastal redwood, pinyon-juniper, lodgepole pine, red fir and black oak (Asner et al, 2016).…”