Fire suppression has led to an increase in fire-sensitive Juniperus spp. throughout many parts of the central and western United States. We studied changes in understory vegetation due to eastern redcedar (Juniperus virginiana) encroachment into the forest midstory of post oak (Quercus stellata) dominated Cross Timbers west of Stillwater in central Oklahoma. We compared vegetation in forest gaps, oak dominated forest without redcedar, at the inner and outer edge of redcedar trees and near redcedar tree trunks (200 plots total). Our objective was to determine the influence of redcedar on its physical environment in terms of changes in plant species composition, vegetation cover and richness along a gradient of increasing midstory redcedar.
Results/Conclusions
Species richness (11 to 6 spp. m-2) and cover (53.3 to 12.7%) declined with proximity to redcedar trunks. While these declines were correlated with both increases in litter mass and decreases in photosynthetic photon flux density (PPFD), regression analysis indicated that richness (R²=0.078) and cover (R²=0.177) were best explained by redcedar litter mass. Partial canonical correspondence analysis (pCCA) revealed two strong canonical axes, one related to litter/light and the other to cover of oak versus redcedar. Tree seedlings and woody vines dominated near redcedar trunks. Forbs, graminoids and oak seedlings were more common in oak-dominated areas without redcedar. Our study indicates that redcedar litter is the main determinant of understory vegetation declines associated with midstory encroachment in fire-suppressed forests. Decreases in herbaceous litter loads, which historically contributed to the accumulation of fuel beds, will have a positive feedback effect for further redcedar encroachment by reducing fire intensity and return interval. Declines in oak recruitment that were related to increasing redcedar abundance may lead to changes in overstory composition.
