The path back: oaks (Quercusspp.) facilitate longleaf pine (Pinus palustris) seedling establishment in xeric sites

Abstract

Mitchell. 2016. The path back: oaks (Quercus spp.) facilitate longleaf pine (Pinus palustris) seedling establishment in xeric sites. Ecosphere 7(6):e01361. 10.1002/ecs2.1361Abstract. Understanding 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.

Global change stressors alter resources and shift plant interactions from facilitation to competition over time

“…In contrast, pines growing with uninvaded vegetation positively responded to increasing soil moisture in all years. This phenomenon of offsetting abiotic stress has been reported for native overstory oaks, which facilitated pine seedlings in longleaf pine sandhills habitat in Florida (McGuire et al 2001, Loudermilk et al 2016. However, to our knowledge, this is the first study to show that an aggressive nonnative plant species can facilitate a native species by ameliorating abiotic stress (see review by Rodriguez 2006).…”

“…In the presence of the invader, pine survival was independent of soil moisture (2014 and 2015) or negatively related to soil moisture (2016)(2017)(2018). Especially when pines were young (2014 and 2015), and thus most susceptible to mortality (Loudermilk et al 2016), their survival was completely decoupled from soil moisture availability in the presence of the invader. native species facilitation of invaders (Maron and Connors 1996, Cavieres et al 2005, Brooker et al 2008, Flory and Bauer 2014, McIntire and Fajardo 2014.…”

The role of understory phenology and productivity in the carbon dynamics of longleaf pine savannas

“…Oak components have been shown to increase climate resilience on xeric sites by facilitating longleaf pine regeneration (Loudermilk et al. ). We also showed that differences in site productivity during drought were mainly governed by differences in photosynthetic activity, such that the xeric site had similar levels of R eco over the range of PDSI but could not maintain similar levels of GEP, compared to the mesic site.…”

Seed depredation negates the benefits of midstory hardwood removal on longleaf pine seedling establishment

“…The traditional paradigm of regenerating longleaf pine on fire‐excluded sites considers competing vegetation and the organic layer as inhibiting factors to seedling establishment (Croker & Boyer ; Boyer ; Mitchell et al ). However, evidence from xeric sites suggests that midstory hardwoods can facilitate longleaf pine regeneration (Wahlenberg ; Loudermilk et al ). Our results indicate that midstory hardwood retention had no effect on longleaf pine seedling germination, as germinant density was equivalent with and without midstory retention in plots with seed predator access.…”