Understory response to oak decline in an upland oak-hickory forest of Oklahoma

Background/Question/Methods

Considered to be the most widespread problem affecting oak forests, oak decline has greatly altered the structure and composition of temperate hardwood forests.  The purpose of this study was to determine the magnitude of changes in understory species composition, species richness and biomass in upland oak forests during five years following total canopy mortality over large areas due to oak decline.  The study area was in the Cross Timbers; a vegetation type characterized as a mosaic of upland oak forest, savanna and prairie communities.  It is an ecotone between eastern forests and western grasslands.  Natural disturbances, such as fire, drought, wind and ice storms that create gaps by killing one or two trees, are relatively common in the Cross Timbers.  Large catastrophic disturbances are rare but may play an important role in maintaining the mosaic of vegetation types.  We measured species composition, species ground cover, species richness and biomass of the understory in non-decline and decline areas immediately after canopy mortality in 2008 and again in 2013.

Results/Conclusions

The effects of total canopy mortality due to oak decline had very large effects on understory species composition, species richness and biomass.  The response was similar across five years post-disturbance and varied by plant functional group.  Graminoid cover increased 2–4 fold, species richness increased 60–80% and biomass increased 4–10 fold.   Forb species richness increased 1.7–2.0 fold.  Biological diversity, composition and biomass of legumes and woody plants remained unchanged following oak decline.  The dramatic increase in diversity and composition of graminoid species and diversity of forb species may benefit wildlife including white-tailed deer.  Although the catastrophic disturbance over large areas caused by oak decline is rare in the Cross Timbers, it may be a mechanism for maintaining species diversity in this vegetation ecotone between eastern forests and western grasslands.  Most of the woody species in these forests resprout following disturbance, therefore their continued presence was more certain than many species in other functional groups.  This may explain the lack of a strong response to opening the canopy.  Although graminoid species increased dramatically in the opening the woody plants may eventually recover and grow into a forest canopy similar to the one before oak decline.