Density‐dependent vulnerability of forest ecosystems to drought

Abstract

1. Climate models predict increasing drought intensity and frequency for many regions, which may have negative consequences for tree recruitment, growth and mortality, as well as forest ecosystem services. Furthermore, practical strategies for minimizing vulnerability to drought are limited. Tree population density, a metric of tree abundance in a given area, is a primary driver of competitive intensity among trees, which influences tree growth and mortality. Manipulating tree population density may be a mechanism for moderating drought-induced stress and growth reductions, although the relationship between tree population density and tree drought vulnerability remains poorly quantified, especially across climatic gradients. 2. In this study, we examined three long-term forest ecosystem experiments in two widely distributed North American pine species, ponderosa pine Pinus ponderosa (Lawson & C. Lawson) and red pine Pinus resinosa (Aiton), to better elucidate the relationship between tree population density, growth and drought. These experiments span a broad latitude and aridity range and include tree population density treatments that have been purposefully maintained for several decades. We investigated how tree population density influenced resistance (growth during drought) and resilience (growth after drought compared to pre-drought growth) of stand-level growth during and after documented drought events. 3. Our results show that relative tree population density was negatively related to drought resistance and resilience, indicating that trees growing at lower densities were less vulnerable to drought. This result was apparent in all three forest ecosystems, and was consistent across species, stand age and drought intensity. 4. Synthesis and applications. Our results highlighted that managing pine forest ecosystems at low tree population density represents a promising adaptive strategy for reducing the adverse impacts of drought on forest growth in coming decades. Nonetheless, the broader applicability of our findings to other types of forest ecosystems merits additional investigation.

Lagged mortality among tree species four years after an exceptional drought in east Texas

“…One caveat to this is that mean predicted loblolly weather mortality was consistently underestimated each year (i.e., observations were 29-66% greater than predictions across all years; Appendix S1: Fig. Recent tree-ring studies of pine species across the U.S. highlight the effectiveness of thinning and reduced stand densities at mitigating water stress (D'Amato et al 2013, Bottero et al 2017, Gleason et al 2017) lending support to our observations in east Texas. Despite this, observed mean loblolly pine weather mortality and standard errors were consistently lower than other species.…”

Detecting the fingerprint of drought across Europe’s forests: do carbon isotope ratios and stem growth rates tell similar stories?

“…This would again suggest that critical to understanding the effects of drought on growth is factoring in not just the occurrence, but also the timing, duration and intensity of drought events, as well as the local adaptation or preconditioning of trees to drought. In addition to this, another factor that could have contributed to the strong drought response in Romania is the high basal area of the stands at this site (Table 1), which previous work has shown can modulate the impact of drought on forest growth by exacerbating belowground competition for water (Bottero et al 2017). …”

The roles of competition and climate in tree growth variation in northern boreal old‐growth forests

“…A number of studies have demonstrated the interactions between competition and climate variables (Bottero et al., ), but the mechanisms are still poorly understood (Grant, Tague, & Allen, ; Thomas & Waring, ). In our analyses, only the summer soil moisture deficit for P. sylvestris interacted with competition, and so that stronger tree–tree competition ameliorated the negative effects of summer drought.…”