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.

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“…Gazol and Camarero (2022) conclude that since summer temperatures and the frequency of droughts are rising, more forests will become vulnerable to compound climate extremes and tree mortality will become a more important driver of forest dynamics. Vegetation responses to heatwave and drought will also be mediated strongly by local site factors linked to soil depth, texture and water-holding capacity (McGrath et al 2023).…”

Climate change or tree disease: challenges for diagnosing causes of forest die-off

“…1) associated with extreme climatic conditions, including a heatwavecompounded drought (Matusick et al 2013). The die-off was most common on sites containing rocky soils with low water-holding capacity, at high elevations, on steep slopes surrounding rock outcrops, in xeric areas (Brouwers et al 2013;Andrew et al 2016), with shallow bedrock and thin soils (McGrath et al 2023), and most severe on sites experiencing a long-term precipitation decline (Matusick et al 2018). Forest vegetation surrounding rock outcrops experiences waterlogging during precipitation events, and excessive soil drying during drought (Withers 2000;Schut et al 2014).…”