The Ecological Classification of Coastal Wet Longleaf Pine (Pinus palustris) of Florida from Reference Conditions

Abstract

Tropical storms, fire, and urbanization have produced a heavily fragmented forested landscape along Florida's Gulf coast. The longleaf pine forest, one of the most threatened ecosystems in the US, makes up a major part of this fragmented landscape. These three disturbance regimes have produced a mosaic of differently-aged pine patches of single or two cohort structures along this coastline. The major focus of our study was to determine reference ecosystem conditions by assessing the soil biochemical properties, overstory stand structure, and understory plant species richness along a patch-derived 110-year chronosequence in order to accurately evaluate on-going longleaf pine restoration projects. This ecological dataset was also used to classify each reference patch as mesic flatwoods, wet flatwoods, or wet savanna. All of the reference locations were found to have similar soil types with no significant differences in their soil biogeochemistry. Mean diameter-at-breast height (DBH), tree height, and patch basal area increased as mean patch age increased. Stand growth reached a plateau around 80 -90 years. Shrub cover was significantly higher in the matureaged patches (86 -110 years) than in the young (6 -10 years) or mid-aged (17 -52 years) patches, despite prescribed fire. Plant species diversity as indicated by the Shannon-Wiener index decreased with patch age. Soil biogeochemical properties, forest structure, and understory species composition were effective for ecologically classifying our pine patches as 55% mesic flatwoods, 20% wet flatwoods, and 25% wet savanna. Florida's Gulf coastal wet longleaf pine flatwoods attain a structural and plant species equilibrium between 80 -90 years.

Soil nitrogen dynamics as an indicator for longleaf pine restoration

“…Ecological restoration requires repairing the functions within soils as well as the structure of forest ecosystems (Johnston & Crossley ; Harris ; Heneghan et al ). For longleaf pine ( Pinus palustris Mill) ecosystems to be resilient (self‐recovering) to degradation, they need to have a vigorous cycling of nutrients for biomass accumulation (Vitousek & Reiners ; Holling ; Johnston & Crossley ; McCaskill ). Ecosystem resilience is considered as a major goal of restoration efforts and has been described in terms of nitrogen retention or the ability of an ecosystem to limit nitrogen loss (Odum ; Vitousek & Reiners ; Pandey et al ; Shade et al ).…”

“…In restoration ecology literature, there are many references to restoring the soils of ecosystems (Hobbs & Harris ; Johnston & Crossley ; Harris ; Banning et al ; McCaskill & Jose ; Piche & Kelting ). The relationship between changes in stand growth rates and the level of inorganic nitrogen turnover can be used to identify a steady‐state restoration threshold when stand growth rates have slowed, and when the available supply of ammonium exceeds nitrate in maturing forests (Vitousek & Reiners ; Nave et al ).…”

“…Combining the pattern of forest development with the process of nutrient cycling is essential in identifying any meaningful restoration threshold (Johnston & Crossley ; Harris ; Bestelmeyer ; Shade et al ). Research results from restoration projects conducted within pine forests of the southeastern United States linking stand productivity with soil nitrogen cycling have mostly focused on the conversion of loblolly ( Pinus taeda ) or slash ( Pinus elliotti ) pine plantations to longleaf pine savannas (Van Lear et al ; McCaskill ; Jose et al ; McCaskill & Jose ; Foote et al ). Restoration research conducted in other ecotypes is also found in the literature (Vance & Entry ; Stanturf et al ; Banning et al ; Piche & Kelting ).…”

“…Restoration research conducted in other ecotypes is also found in the literature (Vance & Entry ; Stanturf et al ; Banning et al ; Piche & Kelting ). However, studies concerning soil nitrogen dynamics in facultative‐wetland habitats, some of the most threatened ecosystems in the coastal portions of the southern United States, are few (Harms et al ; McCaskill ; McCaskill & Jose ). An earlier longleaf pine study found that nitrogen mineralization rates declined as the moisture gradient increased, assessing soils from xeric sandhills through mesic flatwoods to wet pine savannas, but the levels of tree biomass, total soil carbon and nitrogen, and microbial biomass exhibited reverse trends (Wilson et al ).…”

“…Our restoration study centered on identifying the links between changes in wood accumulation rates and soil nitrogen transformations along a 110‐year patch‐derived longleaf pine chronosequence to discover this restoration trajectory and a point of restored condition (benchmark) along the trajectory (Hobbs & Harris ; Johnston & Crossley ; Bestelmeyer ; Shade et al ; Piche & Kelting ). This was accomplished through the analysis of soil biogeochemical data (net ammonification [NH 4 + ] and nitrification [NO 3 − ], abundances of ammonium‐oxidizing bacteria [AOB] and nitrite‐oxidizing bacteria [NOB], soil fungal [C FB ] and microbial [C MB ] biomass [carbon]), collected from 36 longleaf pine reference patches and six control (untreated) plots from the restoration site (Foster & Tilman ; Aravena et al ; McCaskill ; Jose et al ; Walker et al ; McCaskill & Jose ). These analyses were used for identifying restoration benchmarks (restored conditions) for coastal wet longleaf pine flatwoods (Christensen & MacAller ; Johnston & Crossley ; Chapman et al ; Harris ; McCaskill & Jose ).…”