Regional Oak Decline Trends Under Periodic Droughts in the Ozark Highlands of Missouri and Arkansas

Abstract

The Ozark highlands of Missouri and Arkansas are dominated by highly stocked oak-hickory forests. Oak decline and escalating mortality have become a concern as upland oak-hickory forests mature. Based on annual FIA plot data from 1999 to 2006 we explored oak decline and mortality trends for major oak species (groups) in this region. Oak decline has elevated cumulative mortality of oak species over this seven year period to 11 and 15 percent in terms of relative density and basal area, respectively. This is 2 to 3 times higher than for non-oak species. Oak decline and the associated mortality have occurred primarily within red oak species, while the white oak group has maintained a relatively stable mortality rate comparable to non-oak species. Cross-correlation analyses indicate that mortality was significantly correlated with growing season Palmer Drought Severity Index (PDSI), although mortality typically lagged 2 to 3 years behind individual drought events. Moreover, based on the past 17 years’ PDSI data, it appears that the cumulative impacts of drought may last up to 10 years. The Ozark highlands experienced severe drought extending from 1998 to 2000 and another milder drought from 2005 to 2006. These drought events triggered the escalation of mortality starting around 2000. Spatially, high red oak mortality (proportional basal area mortality \u3e0.15) occurred mainly in the central areas of the Ozarks. Moderate mortality (proportional basal area mortality of 0.10-0.15) was widely dispersed over most of the Ozark highlands, while low mortality was distributed primarily around the outer perimeter of the Ozarks. In contrast, with white oak and non-oak species, high mortality was rare and moderate mortality was sporadic