Geophysics reveals forest vulnerability to drought

Abstract

Drought and heat-induced forest die-off are being increasingly reported across the planet. As vulnerable areas tend to have thin soils and poor water holding capacities, quantification of soil depth thresholds, relative to drought intensity, has global implications for identifying forest areas at risk. Measuring soil depth at forest stand or regional scales is, however, difficult. Our aim was to quantify soil thickness across drought impacted forest stands using geophysics. We asked whether impacted sites had shallow soils and whether soil thickness was associated with drought effects and forest structure. Electrical resistivity measurements were conducted at three sites in the Northern Jarrah Forest, Western Australia, which experienced die-off during a 2010 drought and subsequent 2011 heatwave. Multispectral imaging quantified stand structure and vegetation cover. Geophysics identified shallow bedrock in the centre of all drought sites. Soil thicknesses correlated well with stand structure and cover, consistent with increasing water limitation in thinner soils. Smaller cover by trees and shrubs, more ground cover and shorter canopies were observed in soils <20 m thick and were more likely in soils <12 m thick, while treeless areas had thin soils, <2 m thick. The apparent resilience of this forest to long-term drying looks to be due to the region's deep soils. With predicted future drying it is expected die-off patches will expand outwards and new die-off patches will emerge over thin soils. Geophysics can identify areas of forest vulnerable to future drought events, suggesting the potential for landscape-scale mapping of drought vulnerability via airborne methods.