Surveying the Damage

Prior to Hurricane Katrina, the last inventory of Mississippi’s forests had been taken in 1994. In the late 1990s, the national FIA program started using an annualized inventory system; Mississippi was one of the last Southern States to move to that process from the older 5-year cycle. “Before Katrina, there had been so many changes with the impacts of the forest industry’s disposal of their land and the onset of timber investment management organizations and real estate investment trusts,” says Burkman. “We didn’t really have a very good idea of what we had in Mississippi. Katrina provided the impetus to get started.”

As soon as possible after Katrina, the Mississippi inventory began as a large cooperative effort, with a sample of 5,500 plots. Unique to the Mississippi inventory was the measurement of downed-woody material, which provides data that can be fed into fire fuel models. Normally, field crews collect data from 1/16 of the FIA plots on a large number of additional forest health factors, including soil characteristics, vegetation structure, and diversity of all vascular plant species—and lichen diversity as a measure of climate change and air quality. For the post- Katrina inventory, the crews focused solely on basic forest inventory and tree characteristics in order to establish a new baseline. When the next inventory begins in 2009, the other factors will be measured also. Resource analyst Sonja Oswalt, who leads FIA efforts in Mississippi, is completing the Mississippi forest inventory report, which will include analysis of Katrina’s impacts.

Results on the Ground

Mississippi Counties Hancock, Pearl River, Harrison, Jackson, Stone, and George took a pummeling from Hurricane Katrina. Oswalt and Pat Glass, MIFI director of operations, jointly analyzed and reported data for these six counties. Based on a sample of 1,349 plots, they found that 83 percent of measured plots sustained damage that ranged from minor to intensive. Only 34 percent of merchantable live trees (about 50 trees per acre) showed damage. Windthrow was the most common damage type, and damage levels were highest in oak-gum-cypress stands, where impacts were seen on 40 percent of basal area (the total cross-sectional area of the trees in a stand).

Sonja Oswalt and FIA resource analyst Christopher Oswalt took a special interest in how stand-level factors such as the size, density, and species of trees present influence the likelihood of damage. They also wanted to compare the initial estimates made using remote sensing to the data they collected on the ground. In an article published earlier this year, the Oswalts state that tree species and diameter at breast height consistently affected the probability of trees suffering at least some windrelated damage in each zone of damage.

The Oswalts used the hurricane path and damage zones developed by Jacobs in his initial damage assessment. Zone numbers rose from one to five based on distance from landfall. The percentage of FIA survey plots with damage decreased as distance from landfall increased, with the exception of zone 5, the most western area, which was most likely also impacted by spinoff tornado activity. Analysis revealed that the damage figures Jacobs came up with using remotely sensed data were comparable with the Oswalts’ on-the-ground inventory. Hardwood forests sustained more damage from Hurricane Katrina than softwoods, probably due to the dominance of hardwoods in forest composition rather than susceptibility to damage. The Oswalts found that, in softwoods, stand spacing and tree height were more important than species type for determining potential breakage.

The Oswalts caution, however, that trying to reduce the vulnerability of forests to hurricanes using management techniques is complicated by many variables. “We are not attempting to make recommendations for managing for a random wind event like a hurricane or tornado,” says Sonja Oswalt. “While our data show that height and diameter play a role in the probability of damage, we were unable to successfully make predictions using those variables.”

Partnerships Count

When storms subside, it helps to have partners to assess damage, come up with options, and plan for the future. After Katrina, 97 people showed up to survey in Mississippi over the course of 687 days. FIA field crews from SRS and the Northern Research Station and the States of Mississippi, South Carolina, Georgia, North Carolina, Texas, and Arkansas sampled plots. Burkman said of the huge effort, “They didn’t have to come, but they came.” That response contributed significantly to what we know and continue to learn about the impacts of hurricanes on forests.

For more information:
Bill Burkman at 865–862–2073 or
bburkman@fs.fed.us

Sonja Oswalt at 865–862–2058 or
soswalt@fs.fed.us