Landscape‐scale restoration minimizes tree growth vulnerability to 21^stcentury drought in a dry forest

Abstract

Increasing aridity is a challenge for forest managers and reducing stand density to minimize competition is a recognized strategy to mitigate drought impacts on growth. In many dry forests, the most widespread and common forest management programs currently being implemented focus on restoration of historical stand structures, primarily to minimize fire risk and enhance watershed function. The implications of these restoration projects for drought vulnerability are not well understood. Here, we examined how planned restoration treatments in the Four Forests Restoration Initiative, the largest forest restoration project in the United States, would alter landscape‐scale patterns of forest growth and drought vulnerability throughout the 21st century. Using drought–growth relationships developed within the landscape, we considered a suite of climate and treatment scenarios and estimated average forest growth and the proportion of years with extremely low growth as a measure of vulnerability to long‐term decline. Climatic shifts projected for this landscape include higher temperatures and shifting seasonal precipitation that promotes lower soil moisture availability in the early growing season and greater hot‐dry stress, conditions negatively associated with tree growth. However, drought severity and the magnitude of future growth declines were moderated by the thinning treatments. Compared to historical conditions, proportional growth in mid‐century declines by ~40% if thinning ceases or continues at the status quo pace. By comparison, proportional growth declines by only 20% if the Four Forest Restoration Initiative treatments are fully implemented, and <10% if stands are thinned even more intensively than currently planned. Furthermore, restoration treatments resulted in dramatically fewer years with extremely low growth in the future, a recognized precursor to forest decline and eventual tree mortality. Benefits from density reduction for mitigating drought‐induced growth declines are more apparent in mid‐century and under RCP4.5 than under RCP8.5 at the end of the century. Future climate is inherently uncertain, and our results only reflect the climate projections from the representative suite of models examined. Nevertheless, these results indicate that forest restoration projects designed for other objectives also have substantial benefits for minimizing future drought vulnerability in dry forests and provide additional incentive to accelerate the pace of restoration.

Deforestation triggering irreversible transition in Amazon hydrological cycle

“…From 2004, large areas of tropical forests were set aside for conservation, and expansion of plantation was banned within Amazonian region (Assunção et al 2015), which had greatly slowed down deforestation over the period 2004-2015 (Nobre et al 2016). Large scale forest conservation and restoration is considered positive to mitigate climate change (Cook-Patton et al 2021) and maintain hydrological and ecosystem services (Bradford et al 2021, Sankey et al 2021, which is also the last chance to sustain the role of Amazon in global carbon cycle and avoid global environmental catastrophe (Lovejoy and Nobre 2019).…”

Thinning increases forest resiliency during unprecedented drought

“…However, the benefits are significant during drought years and will be especially important in addressing climate change impacts in the coming years. Hotter and drier droughts are projected to increase and will likely trigger further impacts from insects and diseases in forested ecosystems 12 , 38 – 40 . While the main goal of the regional forest restoration efforts in Arizona and other western states is to reduce catastrophic wildfire impacts 22 , 26 , 27 , our results highlight the benefits of the restoration thinning treatment during exceptional and extreme drought periods.…”

Dead again: predictions of repeat tree die-off under hotter droughts confirm mortality thresholds for a dryland conifer species

“…Yet significant uncertainties remain about when and where these droughts will ultimately occur, and which trees are the most likely to die following drought. Thus, managing for forest and woodland persistence in a hotter future will require the capacity to respond to extreme climate events as they are developing, and to rapidly implement targeted interventions that increase ecosystem resiliency to drought or aid in the recovery of ecosystems following droughts (Bradford et al 2018, 2020a, Redmond et al 2019.…”