Data for: Effectiveness of forest density reduction treatments for increasing drought resistance of ponderosa pine growth

As the climate changes, it is increasingly important to understand how
forests will respond to drought and how forest management can influence
those outcomes. In many forests that have become unnaturally dense,
“restoration treatments,” which decrease stand density using fire and/or
mechanical thinning, are generally associated with reduced mortality
during drought. However, the effects of such treatments on tree growth
during drought are less clear. Previous studies have yielded apparently
contradictory results, which may stem from differences in underlying
aridity or drought intensity across studies. To address this uncertainty,
we studied growth of ponderosa pine (Pinus ponderosa) in paired treated
and untreated areas before and during the extreme California drought of
2012–2016. Our study spanned gradients in climate and tree size and found
that density reduction treatments could completely ameliorate
drought-driven declines in growth under some contexts, specifically in
more mesic areas and in medium-sized trees (e.g., normal annual
precipitation > c. 1100 mm and tree diameter at breast height
< c. 65 cm). Treatments were much less effective in ameliorating
drought-associated growth declines in the most water-limited sites and
largest trees, consistent with underlying ecophysiology. In medium-sized
trees and wetter sites, growth of trees in untreated stands decreased by
more than 15% during drought, while treatment-associated increases in
growth of 25% or more persisted during the drought. Trees that ultimately
died due to drought showed greater growth reductions during drought
relative to trees that survived. Our results suggest that density
reduction treatments can increase tree resistance to water stress, and
they highlight an important pathway for treatments to influence carbon
sequestration and other ecosystem services beyond mitigating tree
mortality.