RESEARCH NOTES

By Dr. Jeffery B. Cannon and Brandon T. Rutledge

HOW DO HURRICANES SHAPE

longleaf pine ecosystems?

Photos courtesy of The Jones Center at Ichauway

Disturbance in longleaf pine forests

Readers of this publication are familiar with the importance of low-intensity fire for healthy and biodiverse longleaf pine (Pinus palustris) forests. A century of research reveals fire’s principal place in the cycle of plant death and birth that supports the high diversity and open structure of pine woodlands1 .

As a coastal species, longleaf pine endures less frequent, but potentially catastrophic winds from hurricanes. Like fire, wind can be an important force that shapes the development and change within a longleaf forest. High-intensity winds can cause elevated levels of mortality, release resource-starved saplings from their taller competitors, shift dominance among understory plants, and even alter the frequency or intensity of future fires2 .

It is difficult to overstate the intensity of hurricane impacts on coastal forests. Winds from Hurricane Michael (2018) affected a substantial portion of longleaf pine habitat, causing mortality rates that rose from 15% on the outer edges of the storm to 88% near its center3. Even more dramatically, Hurricane Katrina (2005) toppled enough trees to set back an entire year’s worth of the nation’s forest growth4 . Longleaf as a wind-resistant species

Climate forecasts of the southeastern U.S forewarn of increased drought, heightened risk of wildfire, and intense hurricanes that pose elevated risk to forests5. Recent research proposes longleaf pine as a candidate for more drought-resistant forestry6. With its thick bark, resinous needles, and brevity of vulnerable growth stages, longleaf pine has long been known as a fire-resistant species7. But could longleaf pine also fare better in a world with more extreme hurricanes?

Pines are important to the ecology and economy of the Southeast, and longleaf pine occurs in hurricane-prone areas throughout its range. Nevertheless, little research rigorously tests wind resistance of the species. Several post-hurricane studies have documented fewer numbers of longleaf pine killed or damaged by severe winds compared to other species. Drawing conclusions from these studies is a challenge as they do not account for other important wind risk factors such as soil type and tree size.

Apples-to-oranges comparisons of mortality can complicate conclusions about wind risk. To illustrate, slash pine is thought to be more vulnerable to wind compared to longleaf pine, but the species naturally occurs on wetter soils which can accentuate

Model prediction indicating the relationship between tree size (DBH) and estimated probability of treefall across six soil types for four pine species and two oak species groups. Soil types include excessively drained (drainage class 1) to very poorly-drained (drainage class 6). Damage following Hurricane Michael (2018) and subsequent salvage harvesting

its vulnerability. To truly judge the stability of pines, research should account for the natural range of tree sizes and the soils on which they occur.

Putting longleaf to the test

After destructively striking the Florida Panhandle in October 2018, Hurricane Michael continued further inland and toppled trees at the Jones Center at Ichauway—a research and conservation center with 18,000 acres of longleaf pine woodlands in southwest Georgia. Though damage was not as severe inland, the moderate damage sustained was ideal for disentangling the effects of tree species, size, and soil type.

Immediately after the storm, we visited over 250 plots that were set up as part of a long-term forest monitoring program and recorded the fate of 3,500 trees. We collected data on tree species, size, and soil, and combined them into an analysis to unravel their individual effects.

Our recent article in the journal Forest Ecology and Management8 reports the full results of the study, but four patterns were clear from the analysis. • Large pines are more vulnerable to wind than small pines, but the sensitivity to size varies by species. Large individuals of shortleaf pine are especially at-risk. • Pines are more vulnerable to wind on wet soils than on dry soils, but the sensitivity to soil moisture varied by species. Slash pine and loblolly pine are more sensitive in wet soils than both longleaf and shortleaf pine. • Longleaf pine was wind resistant across a broad range of sizes and soil types compared to other pine species. • Oaks typically associated with wetter soils (laurel oak, live oak, water oak) were more vulnerable to winds when they were on dry soils, while the oaks typically associated with drier soils (southern red oak, post oak, turkey oak) were more vulnerable when they were on wetter soils.

An overarching theme of our study is that complex interactions make a straightforward ranking of wind resistance by tree species difficult. Instead, a full suite of factors such as tree species, size, and soil characteristics should be considered in assessing wind risk. It is widely recognized that frequent fire assembled, and continues to shape, longleaf pine systems by limiting the presence of fire intolerant species within fire-prone areas. Our research supports the idea that—like fire—hurricane winds may also act as a natural filter sharing a role in determining which tree species occur where on the landscape. Land managers can use knowledge of how hurricane vulnerability changes with tree species, soils, and topography when planning management and restoration activities.

Assessing the damage from Hurricane Michael

Upcoming studies on wind damage in longleaf pine Although we are learning more about the wind resistance of longleaf pine, many questions remain about the role hurricanes play in the ecology of longleaf pine woodlands. Several upcoming and ongoing projects involving the Jones Center and partners hope to explore this important topic further:

• How do roots, stems, and crowns contribute to wind vulnerability in southeastern pines? We are conducting tree winching studies to experimentally measure torque needed to topple southeastern pines. Combined with detailed studies of crown architecture, these data will allow us to predict the vulnerability of trees at varying wind speeds.

• What risk do hurricanes pose to forests of the southeast? By combining simulations of hurricane-force winds with national forest inventory data, we aim to measure the expected annual risk that hurricanes pose to forests of the southeast.

• How does wind damage interact with prescribed fire regimes? Though fire-resistant, frequent prescribed fire can leave scars and alter growth of longleaf pine. Do patterns of burning and scorch change susceptibility to wind damage? References 1. Mattoon, W. R. 1922. Longleaf pine. Page United States

Department of Agriculture Bulletin No. 1061. 2. Cannon, J. B., C. J. Peterson, J. J. O’Brien, and J. S.

Brewer. 2017. A review and classification of interactions between forest disturbance from wind and fire. Forest Ecology and Management 406:381–390. 3. Zampieri, N. E., S. Pau, and D. K. Okamoto. 2020. The impact of Hurricane Michael on longleaf pine habitats in

Florida. Nature Scientific Reports 10:1–11. 4. Chambers, J. Q., J. I. Fisher, H. Zeng, E. L. Chapman,

D.B. Baker, and G. C. Hurtt. 2007. Hurricane

Katrina’s carbon footprint. Science 318:2. 5. Mitchell, R. J., Y. Liu, J. J. O’Brien, K. J. Elliott, G. Starr,

C. F. Miniat, and J. K. Hiers. 2014. Future climate and fire interactions in the southeastern region of the

United States. Forest Ecology and Management 327:316–326. 6. Mclaughlin, D. L., D. A. Kaplan, and M. J. Cohen. 2013.

Managing forests for increased regional water yield in the southeastern U.S. coastal plain. Journal of the

American Water Resources Association 49:953–965. 7. Wang, G. G., L. S. Pile, B. O. Knapp, and H. Hu. 2016.

Longleaf pine adaptation to fire: is early height growth pattern critical to fire survival? Page 614 p.

Proceedings of the 18th biennial southern silvicultural research conference. 8. Rutledge, B. T., J. B. Cannon, R. K. McIntyre, A. M.

Holland, and S. B. Jack. 2021. Tree, stand, and landscape factors contributing to hurricane damage in a coastal plain forest: post-hurricane assessment in a longleaf pine landscape. Forest Ecology and

Management 481:118724.