The other night I watched the movie Twister. I have always hated that movie—it scares the crap out of me. Today I came across a study in the September issue of the Journal of Plant Ecology about just that—tornadoes! So in the spirit of the now almost 15 year old film twister here is a great, natural study about how tornadoes affect re-sprouting and special heterogeneity in the Cross Timbers ecotone. The transition zones between different ecosystems are known as ecotones. The most studied ecotones are ones that involve transitions between woody and non-woody vegetation. You would find these in alpine tree lines, coastal dunes and grasslands. Ecotones can be caused my both abiotic and biotic factors for example maintenance of positive carbon balance and competition, respectively. Myster and Malahy designed a study that was focused on the Cross Timbers ecotone in Oklahoma—the boundary between the eastern deciduous forest and the Great Plains grasslands.
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| Location of Cross Timbers Forests |
The Cross Timbers ecotone was likely structured by several mechanisms including water availability, belowground root competition and burning. Strong winds were also recognized as a major structuring mechanism. There has been little research to date about the effects of tornadoes in structuring the Cross Timbers ecotone. This is surprising given tornadoes are common in some areas of this ecotone. Tornadoes can affect these ecotones in many ways including tree damage, resprouting and many indirect effects including fires, and increasing risks of fungal pathogens as a result of tree damages.
The authors asked many specific research questions about tornadoes and their structuring capabilities. They wanted to know the characteristics among species after tornado damage, the clumping patterns before and after tornado disturbance, and to infer ideas about forest recovery after tornado damage. To answer these questions a natural field survey was designed in which a 100 x 100 m plot was marked after an F1 tornado in November of 2004. This was used as the damaged plot and a control plot was used in an adjacent but unaffected area. In the control plot, location of live trees was noted and the trees were measured.  |
| The Cross Timbers forests |
In the damaged or blow-down plot, each stem was measured and divided into one of three size classes: small, medium, and tall based on dbh. Each stem was also assigned to one of five damage categories: defoliation only, branch damage only, stem snapped 1m or greater above ground, stems snapped 1m or below 1m and completely uprooted. The same trees were sampled again one year later. At this point, trees were assigned to one of five resprouting classes: dead, no resprouting but alive, stem resprouting, at or above 1m and at or below 1m.
In the control plot there were 644 trees, and 474 in the blow-down plot. The mortality rate in the damaged plot was 37% and there were no trees with only lost leaves, or that lived but did not resprout after the tornado. There was a significant negative correlation between damage and resprouting and not between size and resprouting. There was a significant clumping pattern in the control plot but the tornado damages eliminated most clumping in the damaged plot. In the end, what this means is that tornadoes likely have an effect on structuring the Cross Timbers ecotones.
There are some strengths and weaknesses of this study I would like to address. First and foremost, the natural design was great and incorporated a natural manipulation (i.e. tornado). Moreover, the two sites are likely very similar given their location and their site selection choice was based on rigorous criteria.
However, they did only sample one site, so it will be difficult to draw conclusions on other sites based on this data. The authors also had opposing results in comparison to other similar studies which may be due to the nature of the forest. The Cross Timbers forests are not typical in that they do not have a closed canopy. It would have been interesting to see if the damaged plot could regenerate that clumping pattern but that was not within the time constraints of this study.
This study does open our eyes to what needs to be studied. For example hurricanes are drastically understudied in their abilities to shape ecotones. The idea that tornadoes may interact with other factors to influence patterns and structure i.e. fires and tornadoes is a distinct possibility. Other factors such as oak invasion or grass competition may also be key areas of study in the structuring of these ecotones. Importantly this study is a great management tool for the Cross Timbers and ecotones in general. It also makes Helen Hunt and Bill Paxton seem a little less nutty—tornadoes are cool!
Myster & Malahy. 2010. Tornado effects on damage, resprouting and spatial heterogeneity in the Cross Timbers ecotone of Oklahoma, USA. Journal of Plant Ecology. 3(3): 157-163.
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