Mike A. Battaglia

SPATIAL DISTRIBUTION OF OVERSTORY RETENTION INFLUENCES RESOURCES AND GROWTH OF LONGLEAF PINE SEEDLINGS

Increasingly, overstory retention is being used in forests traditionally managed for single-cohort structure. One rationale for retention is that residual stand structure better resembles the complex structure of forests after natural disturbance, helping to perpetuate ecosystem fuctions dependent on that structure. The benefits of retention come at the cost of reduced survival and growth of regeneration because of competition with residual trees. We argue that inhibition of regeneration depends not only on the number and size of residual trees, but also on their spatial arrangement, which ranges from dispersed to aggregated. We use a model of competition at the scale of seedlings to hypothesize that maximum stand-level resource availability, seedling growth, and seedling survival occur with aggregate retention, rather than dispersed retention, even with constant residual basal area. We test our hypothesis with a silvicultural experiment in longleaf pine (Pinus palustris) in Georgia, USA. Replicated treatments included an uncut control, dispersed retention, small-aggregate retention, and large-aggregate retention. We measured light, soil nitrogen, soil moisture, and growth of longleaf pine seedlings across the full range of overstory conditions in each treatment. Postharvest basal areas in the cut treatments were similar. Gap light index increased from the control to large-aggregate retention, as did nitrogen availability, measured on exchange resins. Nitrogen mineralization did not differ among treatments, nor did soil moisture or temperature. Seedling biomass increment increased significantly from the control to large-aggregate retention. Survival did not differ among treatments. We argue that these results are a consequence of exponential relationships between overstory competition intensity, resource availability, and seedling growth. Given this relationship, resources and seedling growth are low across a wide range of decreasing overstory competitor abundance but increase exponentially only at very low competitor abundance. This seedling-scale model translates into maximum stand scale resource availability and seedling growth with large-aggregate retention, compared to dispersed retention, because the probability of a seedling occupying a site free of overstory competition is greater with the former. Our research shows that one can improve competitive environnments for regeneration by manipulating spatial distribution of residual trees without sacrificing the ecological benefits of overstory retention.

Density‐dependent vulnerability of forest ecosystems to drought

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. 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. 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. 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.