The Stoddard Plots — Six Decades of Change in Forest Structure

A recent publication predicted that old-field pinelands, much like those of Red Hills region, will ultimately lose their pine component and become closed canopy hardwood forests, even if treated with frequent fire. We are challenging that notion by revisiting data from the Stoddard Plots on Tall Timbers Research Station, to see how forest structure has changed over time under a frequent fire regime.

1-year interval Stoddard Plot being burned in March.

The Stoddard Plots are half-acre plots set up by Herbert Stoddard in 1960, two years after the establishment of Tall Timbers, with the goal of determining how different fire return intervals influence the vegetation and soils of old-field pinelands. Upon their establishment, Stoddard measured the species and diameters of trees greater than 4 cm diameter at breast height. Former Tall Timbers’ ecologist Sharon Hermann measured them again in 1994, and the Fire Ecology Lab measured them in 2011.

Figure 1 shows our results, with square symbols representing the average basal area for each category of trees and error bars showing standard deviation among the three replicate plots per fire treatment. The round symbols show data from the Pebble Hill plots in native longleaf pine-wiregrass communities burned at the same intervals.

Figure 1. Basal area at breast height per unit area of trees in different categories in the Tall Timbers Research Station Stoddard Plots (square symbols) burned at different fire return intervals since 1960, and in the Pebble Hill Fire Plots burned at different intervals since 2005. Error bars indicate standard deviation among three replicate plots.

In the 1-year fire interval (annually burned plots), there was no notable increase in off-site hardwoods (such as water oak and sweet gum that historically were in wetter environments) or in native upland hardwoods (like southern red oak and mockernut hickory that were historically in uplands). Loblolly pine decreased some, and shortleaf pine increased slightly.

In the 2-year interval plots, the hardwoods once again did not significantly increase, loblolly pine decreased, and shortleaf pine increased substantially.

In the 3-year interval plots, hardwoods similarly did not increase over time, loblolly pine stayed about the same, and shortleaf pine increased a great deal.

In the unburned plots, off-site hardwood increased dramatically, and other categories of trees remained about the same.

Total tree basal area increased with increasing fire return interval, corresponding to greater differences between old-field and native sites.

This evidence from the Stoddard Plots, which have not had mechanical or chemical treatment since their establishment 60 years ago, suggests that fire return intervals of 1-3 years prevent conversion to hardwood forests, although shortleaf pine is likely to increase over time. Shortleaf pine is a native, upland, fire-resistant pine species that provides pine needles as fuel for fires, so it is not likely to contribute to the plots transitioning to a fire-resistant community type.

Other management approaches, including thinning hardwoods, may be needed in more open woodlands, wetter areas, and other special cases. However, these results underscore the necessity and effectiveness of frequent fire for maintaining old-field pine communities for decades — without a trend toward hardwood dominance.

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