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

Abstract

Midstory hardwoods are traditionally removed to restore longleaf pine on fire‐excluded savannas. However, recent evidence demonstrating midstory hardwood facilitation on longleaf pine seedling survival has brought this practice into question on xeric sites. Also, midstory hardwoods could facilitate longleaf pine seedling establishment, as hardwood litter may conceal seeds from seed predators or improve micro‐environmental conditions for seedling establishment. However, little is known about these potential mechanisms. In this study, we tracked longleaf pine seed depredation and germination in artificially seeded plots (11 seeds/m2) in a factorial design fully crossing hardwood retention or removal with vertebrate seed predator access or exclusion in the Sandhills Ecoregion of North Carolina, U.S.A. Seed depredation averaged 78% across treatments and was greatest in unexcluded plots. Hardwood retention did not affect seed depredation. Longleaf pine averaged 3.6 germinants/4 m2 across treatments, and was six times more abundant where vertebrates had been excluded. Hardwood removal had a strong positive effect on seedling germination, likely due to the removal of litter, but only when vertebrates were excluded. Our results indicated midstory hardwoods are not facilitating longleaf pine seedling establishment. Nevertheless, our results indicated that hardwood removal may not increase longleaf pine seedling establishment, as seed depredation diminished the effectiveness of hardwood removal under mast seed availability. Collectively, these results demonstrate the underlying complexity of the longleaf pine ecosystem, and suggest that planting may need to be part of the restoration strategy on sites where seed depredation limits longleaf pine natural regeneration.

Tree encroachment impacts on seed predator selection and seedling establishment in degraded pine woodlands

“…Also, our results indicate that any potential facilitative effects on seedling establishment created by midstorey retention are not affecting germinant density for either species (Wahlenberg, 1946; Louise Loudermilk et al, 2016; Prévosto et al, 2020). Similarly, neither species’ germinant density was statistically improved by proximity to Aristida stricta cover, as has been noted in previous studies (Miller et al, 2019; Willis et al, 2019). However, Aristida stricta cover had a biologically relevant positive effect on germination for both species, indicating that Aristida stricta was not impeding germination.…”

“…We suspect the selection for smaller‐seeded species was likely influenced by the local seed predator population. Although seed predator identity was not recorded in this study, small‐mammal abundance has been shown to be extremely low in frequently burned pine woodlands (Sasmal et al, 2017), and circumstantial evidence suggests that invertebrates are acting as the primary seed predators at our stand (Willis et al, 2019). Based on this assumption, the low occurrence of Quercus nigra granivory may be explained by the principles of optimal foraging theory, as foraging invertebrates may have avoided acorns due to their thick seed coat or size (Rey et al, 2002; Lichti et al, 2017).…”

“…It is well known that granivores can limit seed availability for Pinus palustris (Boyer, 1964; Croker & Boyer, 1975). However, much of what is known about how seed depredation impacts tree recruitment in pine woodlands comes from studies that have isolated Pinus palustris from sympatric tree species (Boyer, 1964; Nolte & Barnett, 2000; Willis et al, 2019). Consequently, it is currently unknown how an influx of seeds from encroaching tree species will affect seed predator preference and how that may influence seedling layer species composition.…”

“…Other research has shown that small‐mammal granivory increases in areas with extensive vegetation cover (Ostfield et al, 1997; Manson & Stiles, 1998; Brown & Kottler, 2004; Orrock et al, 2004). Conversely, arthropods have been shown to be effective seed predators in frequently burned stands (Stuhler & Orrock, 2016) or in areas where midstorey vegetation has been removed (Willis et al, 2019). Thus, tree species seed availability may be indirectly affected by the density and size of the encroaching vegetation.…”

“…In areas where encroachment has occurred, the existence of a dense forest floor could constrain the establishment of smaller‐seeded species (Westoby et al, 2002; Varner et al, 2005). Increases in midstorey and canopy density could also exclude Aristida stricta from the understorey, which has been negatively associated with Pinus taeda invasion (Fill et al, 2017), but positively associated with Pinus palustris seedling establishment (Willis et al, 2019; Miller et al, 2019). Currently, it is unknown whether Aristida stricta inhibits Pinus taeda seedling establishment.…”