DOI: 10.25338/b8bp74
title: Data for: Effectiveness of forest density reduction treatments for increasing drought resistance of ponderosa pine growth

description: 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 &gt; c. 1100 mm and tree diameter at breast height
 &lt; 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.
description: Please see the accompanying manuscript for detailed data
 collection and processing methods. Briefly, we
 collected tree cores from live and dead ponderosa pine trees in a series
 of paired "treated" and "untreated" forest stands
 along a precipitation gradient. Treatments included prescribed fire and/or
 mechanical thinning intended to reduce stand density and increase
 resilience to droughts and fires. We measured ring widths and assigned
 each ring to a year following standard dendroecological methods. We
 detrended the ring width series by dividing ring widths by a modified
 negative exponential function fitted to the raw ring widths. We summarized
 both raw and detrended ring widths across three periods –
 "baseline" (1970-1999), "pre-drought" (2000-2010), and
 "drought" (2013-2014) – by computing the mean. We also measured
 the diameter at breast height of all cored trees and recorded any evidence
 of insect pest presence. Finally, we tallied the number of trees (DBH
 &gt; 7.6 cm) and saplings occurring with 12.6 m radius plots where
 cored trees were located.
description: All data files are .csv format and can be opened by standard
 spreadsheet software or read by common scripting languages (e.g., the
 read.csv function in R).