Abstract
AbstractUnderstanding plant–plant facilitation is critical for predicting how plant community function will respond to changing disturbance and climate. In longleaf pine (Pinus palustris Mill.) ecosystems of the southeastern United States, understanding processes that affect pine reproduction is imperative for conservation efforts that aim to maintain ecosystem resilience across its wide geographic range and edaphic gradients. Variation in wildland fire and plant–plant interactions may be overlooked in “coarse filter” restoration management, where actions are often prescribed over a variety of ecological conditions with an assumed outcome. For example, hardwood reduction techniques are commonly deemed necessary for ecological restoration of longleaf pine ecosystems, as hardwoods are presumed competitors with longleaf pine seedlings. Natural regeneration dynamics are difficult to test experimentally given the infrequent and irregular mast seed events of the longleaf pine. Using a long‐term, large‐scale restoration experiment and a long‐term monitoring data site at Eglin Air Force Base, Florida (USA), this study explores the influence of native fire‐intolerant oaks on longleaf regeneration. We test for historical observations of hardwood facilitation against the null hypothesis of competitive exclusion. Our results provide evidence of hardwood facilitation on newly germinated longleaf pine seedlings (<2 yr old) after two mast seeding events (1996, 2011). Using regression‐tree and Kaplan–Meier survival analyses, we found that deciduous oak midstory density was the most significant variable associated with longleaf pine seedling survival rates in the first 2 yr after germination. We found that as few as 43 oak midstory stems ha−1 were sufficient to facilitate seedling survival, but as many as 1400 stems ha−1 maintained facilitation without competitive exclusion of seedlings. We found that 1.5‐yr‐old pine seedlings were more moisture stressed under more open canopy conditions when compared to those immediately adjacent to a midstory oak canopy. Recognition that deciduous oaks are important facilitators of longleaf seedling establishment on xeric sites represents a significant departure from conventional wisdom and current management practices that has largely focused on competitive exclusion. This points to a critical role of a deciduous oak midstory of moderate densities for long‐term ecosystem resilience in xeric longleaf pine ecosystems in light of climate uncertainty.
Highlights
There has been a renewed focus on the role of positive interactions among species, such as facilitation in influencing community dynamics (e.g., Bruno et al 2003)
Recognition that deciduous oaks are important facilitators of longleaf seedling establishment represents a significant departure from conventional wisdom and current management practices, which largely focus on competitive limitation of longleaf regeneration by broadleaved species
Since our study was able to capture the influence of historic midstory reduction treatments on a longleaf pine mast seeding in 1996, this study provides a unique perspective on ecosystem resilience and recovery not explored in the current conservation management paradigm
Summary
There has been a renewed focus on the role of positive interactions among species, such as facilitation in influencing community dynamics (e.g., Bruno et al 2003). In the face of climate uncertainty, with potentially hotter temperatures, longer growing seasons, extended droughts (Cook et al 2015), and more variable rainfall predicted ( in the southeastern United States, sensu Mitchell et al 2014), understanding key processes that are responsible for maintaining foundation and keystone species will be critical for managing native biodiversity and restoring ecosystems (e.g., Ross et al 2010) These ecosystem processes, which include wildland fire and plant–plant interactions, may be overlooked in “coarse filter” restoration management, where actions are prescribed over a wide range of conditions with an assumed ecological outcome. The conventional wisdom of removing midstory oaks—those typically less than 16 cm dbh—has the potential to affect the ecosystem’s resiliency in response to future novel climates by removing these native species that at least partly interact with pines through facilitation (e.g., Fig. 1, McGuire et al 2001, Pecot et al 2007)
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