Delayed tree mortality and Chinese tallow (Triadica sebifera) population explosion in a Louisiana bottomland hardwood forest following Hurricane Katrina

Abstract

Assessing long-term effects of hurricane damage in bottomland hardwood forests is important to detect any permanent, long lasting changes to the forest. Two 75×75m plots were established in a Louisiana bottomland hardwood forest in 2004 and all adult trees were measured. The plots were resurveyed in 2006 after Hurricane Katrina passed over the plots in 2005, and in 2011, to gain an understanding of mortality over time. Species composition, mortality and biomass change were assessed over the study period from 2004 to 2011. Sweetgum, water oak, and laurel oak were the most important overstory species in 2004, and American holly and American hornbeam were the most important understory species. In the more damaged plot, there was a shift in dominant species as 63 new Chinese tallow individuals recruited into the adult class (>10cm) between 2004 and 2011. Chinese tallow is an invasive tree species that often out-competes native species in bottomland hardwood forests. Annual mortality in the bottomland hardwood forest plots between 2004 and 2011 was 6% per year, 11% per year from 2004 to 2006 (representing direct hurricane mortality), and 5% between 2006 and 2011 (delayed tree mortality). Approximately 53% of the total biomass (188,000kg) was lost between 2004 and 2011. A plot in a cypress tupelo forest was added in 2006 and very little damage or mortality was observed. This study revealed that delayed mortality to hurricane-damaged trees is a significant factor in the long-term dynamics of bottomland hardwood forests and represents an amplification of the effects of the hurricane over time. The fact that direct and delayed mortality is different by species indicates that the measurement only of direct mortality can lead to false conclusions about which species are resistant to hurricanes. Hurricane damage opened up new habitat for invasion by Chinese tallow which grew prolifically in highly damaged, low elevation, wet areas, indicating that large disturbances are an important factor in accelerating the population expansion of this invasive species. The high mortality and low recruitment of some species into the sapling and adult layers and the corresponding expansion of Chinese tallow indicates that species composition will differ from pre-hurricane composition for some time in the future.