Fieldwork Letters from the Gulf Coastal Plain: Dendrotempestology

 Feb 22, 2016    by Clay Tucker

Dendrotempestology (it’s a mouthful I know!) is the study of the effects of hurricanes on trees. When people hear this, they normally spout something like, “Well, hurricanes kill the trees! Duh!” I quickly attempt to note that though the trees surrounding their houses may suffer substantial damage, many ecosystems are adapted to these disturbances and can respond positively to the damage. Many of these ecosystems occur very close to coastlines around the Gulf of Mexico. Abundant and skilled field research can connect the scientists, land-users, and ecology of the ecosystem.

Specifically, my research uses methods in dendrochronology, the study of tree rings, to demonstrate the effects on tree growth from hurricane high winds and storm surge. High winds defoliate trees of their leaves, and storm surge adds an unusual amount of saltwater to the water table, thus increasing water evaporation and mineral accumulation in the soil. Before I can explain my fieldwork, I think it is useful to describe two principles underlying how this research works. The Principle of Limiting Factors states that the most limited environmental condition controls the growth of an organism. For example, a tree growing in a desert is highly limited by water resources, thus it can only grow as much as it receives water. The Principle of Site Selection maximizes an environmental signal that can be observed in a tree. In other words, you wouldn’t look for evidence of hurricanes in Arizona because Arizona doesn’t experience vast effects from hurricanes. The closer one can get to the effecting environmental condition, the higher the chances of seeing evidence of that environmental effect in the biosphere.

Photo: GBNERR is a great example of places that show stress from saltwater.

My research takes place in coastal Mississippi, a place that receives plenty of rainfall, nice warm temperatures, few days of freezing, and plentiful soil reconditioning from river input. This means that limiting factors are few and far between, and trees are usually very “happy” because they receive all the nutrients that they need. However, the closer we get to the Gulf of Mexico, the more likely we are to see the effects of saltwater inundation as noted above. Approximately a mile from the coast is one of my research sites in Grand Bay National Estuarine Research Reserve (GBNERR, see map). A photo of my site included here shows the extremely evident ecotone that exists in GBNERR: slash pine (Pinus elliottii) trees grow adjacent to tidally influenced salt marshes. You may also notice a tapering effect in the images where the trees nearest to the marsh are much smaller and shorter than those further inland. These trees are not younger; instead the growth of these trees is stunted by the effects of saltwater! An image of two tree cross-sections is also included here to show these effects: the smaller cross-section is actually much older, but because it grows in near-saltwater conditions, it cannot grow as much as those trees further inland.

Fieldwork in dendrochronology is extremely enjoyable and rewarding because it requires a level of simply spending time with trees and noticing their differences. Trees are often chosen for their aged appearance and for their sensitivity to climate. Therefore, one’s senses must be in tune to the standing environment to understand what the trees experience so that it can be described later in words. A good field scientist can spend a few hours in a study site and produce a general natural history of the environment just from sensory clues. An area that experiences frequent fire will have lots of young sedges and grasses, trees will be fire-scarred, the soil will be black from organic accumulation, sunlight will easily reach the ground, and many of the surviving species will show evidence of resilience to the fire, such as high resin content, thick, porous bark, roots with multiple rhizomes, and will quickly take advantage of new resources.

Summer is often the most convenient time to do fieldwork because classes aren’t in session and meetings are usually scheduled at other times. In July 2015, a few colleagues accompanied me to GBNERR-of course, the weather was hot and humid and bugs were plentiful, but we managed to do ample fieldwork despite the sunburn and itching. Each day started with a 2-mile trek from the research center to the edge of the marsh. Usually the gnats and mosquitos would meet us about halfway there. As the loud hum of our ATV settled, nothing could be heard but the rustling of leaves in the wind, one of my favorite sounds. It wouldn’t take me long to pick a tree to start with, and I quickly removed my increment borer from its sheath and began to extract a core. (Instead of cutting down a tree to see its rings, we can take a small core from the tree in two or three places. This is done using a hollow increment borer that screws into the tree with a handle, and the extractor spoon is used to pull out a core two millimeters wide and as long as half of the diameter of the tree. This can be a strenuous activity, but precision here is crucial to future lab work.) One of my colleagues took GPS measurements of each tree, writing down information such as species and diameter, and often taking notes that I screamed between breaths while turning the increment borer, “There’s a fire scar on this tree,” “Write down that this tree is dominant in the canopy!” Another was measuring the diameter, transferring my cores to straws for preservation, and handing me WD40 for ease-of-use with the increment borer. I often received the chance to teach my colleagues something about the area we were working, “This tree was struck by lightning which caused a small fire. That’s why this area doesn’t have many bushes or tall grasses,” “This tree died in the last year. You can tell because it still has lots of small branches. It’s on the edge of the marsh, so it probably died because of saltwater intrusion.”

Photo: Ecotone in coastal Mississippi between salt marsh and pine savanna. Photo: Clay Tucker

I am lucky to have been able to be in the field as much as I have. I do my best to travel to my and other colleagues’ research sites as much as possible. Connecting with the land is beneficial to all people. Those who inhabit these areas on a daily basis often recognize the issues associated with that area, but their local ecological knowledge isn’t always recorded or quantified. During my academic courses and literature reviews, I certainly get my fair chance to learn and read about processes that occur on Earth, but experiencing those processes firsthand - coring the tree, feeling the soil, listening to the leaves - increases my understanding of the system I am studying, and makes me a better scientist.

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Trees nearest to the coast suggest substantial stress from saltwater. The coastal (top) cross section is much older than the inland (bottom) cross-section. Photo: Clay Tucker