Tree-Ring Evidence of Forest Management Moderating Drought Responses: Implications for Dry, Coniferous Forests in the Southwestern United States

Abstract

Drought, coupled with rising temperatures, is an emerging threat to many forest types across the globe. At least to a degree, we expect management actions that reduce competition (e.g., thinning, prescribed fire, or both) to improve growth of residual trees during drought. The influences of management actions and drought on individual tree growth may be measured with high precision using tree-rings. Here, we summarize treering-based assessments of the effectiveness of thinning and prescribed fire as drought adaptation tools, with special consideration for how these findings might apply to dry coniferous forests in the southwestern United States. The existing literature suggests that thinning treatments generally improve individual tree growth responses to drought, though the literature specific to southwestern coniferous forests is sparse. Assessments from studies beyond the southwestern United States indicate treatment effectiveness varies by thinning intensity, timing of the drought relative to treatments, and individualistic species responses. Several large-scale studies appear to conflict on specifics of how site aridity influences sensitivity to drought following thinning. Prescribed fire effects in the absence of thinning has received much less attention in terms of subsequent drought response. There are limitations for using tree-ring data to estimate drought responses (e.g., difficulties scaling up observations to stand-and landscape-levels). However, tree-rings describe an important dimension of drought effects for individual trees, and when coupled with additional information, such as stable isotopes, aid our understanding of key physiological mechanisms that underlie forest drought response.

Assessing the effects of forest biomass reductions on forest health and streamflow

“…In overgrown forests with water‐limited vegetation, the former two changes can be beneficial. Lower forest biomass may reduce competition for water and increase transpiration amongst the remaining vegetation; increasing forest health and reducing vulnerability to drought and climate change (Sohn et al, 2016; Tague & Moritz, 2019; van Mantgem et al, 2020). Reductions in biomass may also decrease total forest water use, increasing the amount of water available for streamflow and downstream uses (Brown et al, 2005; Stednick, 1996).…”

Drought responses in floodplain forests obtained by remotely sensed and tree‐ring data

“…Given this richness in species and microtopography, it is not surprising that remotely sensed water budgets are not necessarily reflected at the scale of individual sample trees. Using tree rings to assess forest responses to meteorological drought presents several limitations, including difficulty scaling, survivor and large‐tree bias, and variations in species composition (Gleason et al, 2017; van Mantgem et al, 2020). In addition, individuals of all species are not distributed equally across sites and topography, so that we could not obtain a record independent of existing tree‐site relationships.…”

Restoration and fuel hazard reduction result in equivalent reductions in crown fire behavior in dry conifer forests

“…Climate change presents an additional risk to the continued ecological function of these ecosystems by enhancing tree stress and sensitivity to biotic disturbances (Weed et al, 2013 ), increasing the potential for wildfires to occur under extreme weather conditions resulting in more severe fires (Abatzoglou & Williams, 2016 ; Khorshidi et al, 2020 ; Parks & Abatzoglou, 2020 ; Westerling, 2016 ), and limiting the opportunity for post‐fire fire regeneration and recovery (Coop et al, 2020 ; Haffey et al, 2018 ; Rodman, Veblen, Battaglia, et al, 2020 ; Rodman, Veblen, Chapman, et al, 2020 ; Stevens‐Rumann et al, 2018 ). These changes to forest structures and climate are not only associated with reductions to biodiversity and ecosystem resistance and resilience (Graham et al, 2019 , Hessburg et al, 2019 , Latif et al, 2020 , van Mantgem et al, 2020 ), but contribute to highly visible societal and economic costs in the form of smoke impacts on human health and the loss of life and property due to uncontrolled wildfire occurring in areas that are increasingly urbanized (Caggiano et al, 2020 ; Radeloff et al, 2018 ; Schweizer et al, 2019 ).…”

“…Tree density reduction through mechanical treatments or silvicultural practices is a major management strategy used to address the linked ecological and social concerns associated with altered forest structure, wildfire behavior, and an actively changing climate (Kalies & Yocom Kent, 2016 ; Peterson et al, 2005 ; Stephens et al, 2021 ). Although the primary objective of such treatments is typically the reduction of potential fire behavior, additional considerations include the reduction of drought stress, harvesting of commercial products, shifting stands and landscapes towards the historical range of variability, improving wildlife habitat, and increasing resistance and resilience to disturbance (Addington et al, 2018 ; Crotteau & Keyes, 2020 ; Hessburg et al, 2015 ; Reynolds et al, 2013 ; van Mantgem et al, 2020 ). The scientific basis for reducing potential fire behavior through the direct manipulation of the fuel complex derives from a basic understanding of the biophysical factors that, in conjunction with fire weather and topography, influence fire behavior (Graham et al, 2004 ).…”