Forest recovery following extreme drought in California,USA: natural patterns and effects of pre‐drought management

Abstract

Rising temperatures and more frequent and severe droughts are driving increases in tree mortality in forests around the globe. However, in many cases, the likely trajectories of forest recovery following drought‐related mortality are poorly understood. In many fire‐suppressed western U.S. forests, management is applied to reverse densification and restore natural forest structure and species composition, but it is unclear how such management affects post‐mortality recovery. We addressed these uncertainties by examining forest stands that experienced mortality during the severe drought of 2012–2016 in California, USA. We surveyed post‐drought vegetation along a gradient of overstory mortality severity in paired treated (mechanically thinned or prescribed‐burned) and untreated areas in the Sierra Nevada. Treatment substantially reduced tree density, particularly in smaller tree size classes, and these effects persisted through severe drought‐related overstory mortality. However, even in treated areas with severe mortality (>67% basal area mortality), the combined density of residual (surviving) trees (mean 44 trees/ha) and saplings (mean 189 saplings/ha) frequently (86% of plots) fell within or exceeded the natural range of variation (NRV) of tree density, suggesting little need for reforestation intervention to increase density. Residual tree densities in untreated high‐mortality plots were significantly higher (mean 192 trees/ha and 506 saplings/ha), and 96% of these plots met or exceeded the NRV. Treatment disproportionately removed shade‐tolerant conifer species, while mortality in the drought event was concentrated in pines (Pinus ponderosa and P. lambertiana); as a consequence, the residual trees, saplings, and seedlings in treated areas, particularly those that had experienced moderate or high drought‐related mortality, were more heavily dominated by broadleaf (“hardwood”) trees (particularly Quercus kelloggii and Q. chrysolepis). In contrast, residual trees and regeneration in untreated stands were heavily dominated by shade‐tolerant conifer species (Abies concolor and Calocedrus decurrens), suggesting a need for future treatment. Because increased dominance of hardwoods brings benefits for plant and animal diversity and stand resilience, the ecological advantages of mechanical thinning and prescribed fire treatments may, depending on the management perspective, extend even to stands that ultimately experience high drought‐related mortality following treatment.

Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?

“…When beetle populations are less than epidemic such as at higher elevations, during moderate droughts, or early in severe droughts, previous fire and its associated reduced density may be neutral or ameliorating for conifer mortality. Our sugar pine results may indicate a tipping point beyond which the combination of extreme water stress from drought, bark beetle outbreaks, and fire result in increasingly high rates of tree mortality (Nesmith et al 2015), and subsequent forest structural changes outside the natural range of variation (Young et al 2020). These results suggest cautious low‐intensity and small (i.e., stand) scale prescribed burning, as it is often applied by managers, may only benefit forests under short duration drought stress while contributing to higher mortality in red fir and sugar pine during prolonged and exceptional droughts.…”

Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?

“…When beetle populations are less than epidemic such as at higher elevations, during moderate droughts, or early in severe droughts, previous fire and its associated reduced density may be neutral or ameliorating for conifer mortality. Our sugar pine results may indicate a tipping point beyond which the combination of extreme water stress from drought, bark beetle outbreaks, and fire result in increasingly high rates of tree mortality (Nesmith et al 2015), and subsequent forest structural changes outside the natural range of variation (Young et al 2020). These results suggest cautious low-intensity and small (i.e., stand) scale prescribed burning, as it is often applied by managers, may only benefit forests under short duration drought stress while contributing to higher mortality in red fir and sugar pine during prolonged and Treatments using only thinning consistently reduced mortality of large (> 75 cm DBH) trees across species, albeit with different effect sizes.…”

Do fuel treatments decrease forest mortality or increase streamflow? A case study from the Sierra Nevada (USA)

“…This drought was notable for near‐record low precipitation levels coinciding with record high temperatures (Shukla, Safeeq, AghaKouchak, Guan, & Funk, 2015). The ‘hot drought’ had many consequences on both the hydrology (Bales et al, 2018; Goulden & Bales, 2019) and ecology (Roberts, Burnett, Tietz, & Veloz, 2019; Young et al, 2020) of the southern Sierra Nevada. Snowpack and streamflow in the KREW were at record lows and included the first recorded summer cessation of streamflow in P301 in 2013.…”

“…et al, 2018;Goulden& Bales, 2019) and ecology(Roberts, Burnett, Tietz, & Veloz, 2019;Young et al, 2020) of the southern Sierra Nevada. Snowpack and streamflow in the KREW were at record lows and included the first recorded summer cessation of streamflow in P301 in 2013.…”