Mesophication of upland oak forests: Implications of species-specific differences in leaf litter decomposition rates and fuelbed composition

Abstract

Without periodic fire, historically open-canopied, oak-dominated upland woodlands of the central and eastern United States are shifting to closed-canopied forests with increased abundance of shade-tolerant and typically fire-sensitive species. Once established, these encroaching species (i.e., mesophytes) are hypothesized to initiate a positive feedback termed mesophication where mesophytes perpetuate conditions that foster their own proliferation at the expense of oaks (Quercus spp.). One potential mechanism of mesophication is reduced fuel loads through faster decomposition rates of mesophyte leaf litter, as leaf litter is the primary fuel in closed canopy forests. To better understand how different tree species impact fuel loads, we compared initial leaf litter chemistry and one-year (2016–2017) decomposition rates of four non-oak species exhibiting increased abundance in the region and/or relatively high sapling/midstory abundance relative to the overstory on our sites (red maple [Acer rubrum], sugar maple [A. saccharum], American beech [Fagus grandifolia], and hickory [Carya spp.]) and three oak species (black oak [Q. velutina], chestnut oak [Q. montana], white oak [Q. alba]) in an upland oak forest in north-central Kentucky. We also evaluated fuelbed mass, composition, and bulk density beneath overstory (20–60 cm DBH) individuals of each species following leaf fall in December 2016. Except for American beech (66% mass remaining), we found non-oak leaf litter, especially that of red and sugar maple, decomposed the fastest (45 and 48% mass remaining), and oak leaf litter decomposed the slowest (54–64% mass remaining). Further, although total leaf litter fuel loads in December were similar beneath individuals of different tree species, fuel composition differed. Under non-oak crowns, the proportion of leaf litter from non-oaks was 22–35%, while under oak crowns, the proportion of leaf litter from non-oaks was only 10–12%. This suggests that individual trees of non-oaks are impacting fuel composition beneath their own crowns despite continued oak dominance at the stand scale. Considering differences between species in both leaf litter inputs based on allometric equations and decomposition rates, modeled leaf litter fuel loads in a forest composed entirely of red maple, sugar maple, or Carya spp. were ∼20% lower than an oak forest after one year of decomposition. While limited to the species examined in this study, these findings confirm that the leaf litter of non-oaks, excluding American beech, decomposes more rapidly than oak and that individual non-oak trees alter fuelbed composition beneath their crowns, suggesting that oak woodlands will become less-flammable with increasing mesophytic dominance.