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Evaluating impacts of southern pine beetle on pitch pine forest dynamics in a newly invaded region

Thesis by Molly Heuss.

Abstract: Southern pine beetle (SPB; Dendroctonus frontalis Zimmerman), a native insect that has historically affected pine ecosystems in the southeastern U.S., has recently expanded northward causing extensive tree mortality in pitch pine (Pinus rigida) and pitch pine-oak (Quercus spp.) forests across much of eastern Long Island, NY. Given the historic lack of SPB within these fire-dependent ecosystems, little is known regarding its impacts to forest composition, forest structure, or fuel loading. This study examined the short-term effects of SPB-induced tree mortality on the structure, composition, and fuel loading of pitch pine and pitch pine-oak communities to inform management recommendations and projections of future forest conditions and fire hazard.

Overstory pine basal area declined following SPB infestation and infestation suppression management, particularly in pitch pine forests. These treatments did not impact the density or composition of seedlings and saplings, with hardwood species, including scarlet oak (Quercus coccinea), scrub oak (Quercus ilicifolia), and black gum (Nyssa sylvatica), making up the majority of species in this layer and pine representing <6% of stems. Likelihood of herbivory was influenced partly by species, with pitch pine less likely to be browsed than white oak and scarlet oak. SPB infestation significantly increased the snag component of both forest types, which largely became downed coarse woody debris (CWD) following suppression management. Treatments did not significantly influence understory species assemblages. Understory communities in pitch pine stands were characterized by Vaccinium angustifolium prior to SPB or suppression management, with these disturbances leading to an increase in the diversity of understory communities. In contrast, infestation decreased variation in understory species assemblages in pine-oak forests and encouraged regeneration of pitch pine and scarlet oak, while suppression increased diversity largely through increases in disturbance-adapted species, such as Smilax rotundifolia. SPB infestation decreased the biomass of live fuels and subsequently increased loading of dead fuels in both forest cover types. Suppression management felled preexisting and SPB-generated snags, especially in pitch pine forests, transforming vertical fuels into horizontal CWD.

Collectively, results indicate SPB could functionally eliminate pitch pine without additional management intervention to maintain this species. Suppression efforts to reduce SPB impacts may accelerate succession towards hardwood dominance, particularly in pine-oak stand, leading to dramatic shifts in forest conditions across the Long Island Pine Barrens. SPB and suppression management significantly increase dead fuel loading and felling of snags during suppression served to decrease the density of ladder fuels effectively decreasing the risk of crowning. However, heavy CWD loading may also promote volatile fire behavior. Therefore, forest managers must weigh the expected potential impacts of SPB relative to changes to fuel structure and composition generated by suppression management activities. Our results demonstrate short-term effects of SPB and suppression management. Given the limited experience with SPB in these forests and the results of this study, further research on fire behavior effects and patterns of stand development over the long-term are needed.

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