Much has been written about the effects of fire suppression and exclusion on vegetation communities in the southeastern United States. Restoring natural fire regimes to longleaf pine ecosystems is one of the primary management challenges in this region. The most frequently cited and visually obvious changes associated with a loss of growing-season fires is the encroachment of woody vegetation into habitats otherwise dominated by pines and grasses. These vegetation shifts alter the structure of the environment and impact basic ecosystem processes. This may be especially important in small wetlands where much of the food web is rooted in the leaf litter inputs that fall into aquatic environments from trees, shrubs, and grasses. Despite an abundance of work on invertebrate communities and leaf litter in wetland systems, relatively little has been published from wetlands embedded within longleaf pine forests.
Along with coauthors from Virginia Tech and Georgia Southern University, I recently published research in the journal PeerJ examining the effects of changing leaf litter inputs into pine flatwoods wetlands (Chandler et al. 2021). We examined how leaf breakdown and invertebrate communities varied across three species of leaf litter: longleaf pine needles, wiregrass stalks, and black gum leaves. These species all occur naturally in pine flatwoods wetlands, with gum trees being mostly restricted to fire-suppressed wetlands or the deepest parts of fire-maintained wetlands. I started working on this project all the way back in 2015, building off my thesis research that showed a strong effect of vegetation structure on invertebrate communities in the same wetlands (Chandler et al. 2015). These research questions are important from a basic ecology perspective but also have implications for the management of flatwoods salamander who breed in these wetlands and feed primarily on aquatic invertebrates (Whiles et al. 2004).
To measure leaf breakdown and invertebrate communities in different leaf types, we employed leaf packs as our main sampling technique. This widely used method involves filling coarse mesh bags with leaf litter and then submerging them in the wetland for several months. Over that time, invertebrates will colonize the leaf litter, breaking it down as they forage. We placed 72 leaf packs across three wetlands. Importantly, each wetland contained sections of fire-maintained (low canopy, high herbaceous vegetation) and fire-suppressed (high canopy, low herbaceous vegetation) habitats, allowing us to examine both the effects of leaf type and of broad habitat type on the above processes.
After just over 100 days submerged in the wetlands, we removed leaf packs, placing them in ethanol to preserve invertebrates and prevent further decomposition. From that point, the easy work was finished, and it was time to spend many hours in the lab. Invertebrates had to be separated from the remaining leaf material, identified, and counted (over 16,000 total!). Leaf litter was dried and burned in a muffle furnace to estimate the weight that had been lost while in the wetlands. No wonder it took six years to finish this project!
After all this time in the lab, what did we find? Black gum leaves broke down significantly faster than either longleaf pine or wiregrass. Breakdown was slower, on average, in fire-suppressed habitats, but this effect was small compared to differences between species. However, it did appear that the habitat effect on leaf breakdown was strongest in the wetland with the highest canopy cover, perhaps suggesting that this effect is mediated by water temperature. From an invertebrate perspective, leaf type, but not habitat type, impacted the number of invertebrates found within leaf packs relative to the mass of leaf litter remaining. This was true for overall abundance and the abundance of taxa that are important in the diets of larval flatwoods salamanders. In both cases, black gum leaves tended to support the most abundant invertebrate communities followed by wiregrass and longleaf pine. Finally, our analysis of community composition indicated that there were differences in the makeup of invertebrate communities across both leaf and habitat type.
These results provide another example of how fire suppression impacts fire-dependent ecosystems in the southeastern U.S. We documented effects of leaf species and habitat type on litter breakdown and invertebrate communities. Overall, leaf species had stronger effects than habitat type, but it is important to remember that in natural wetlands these two factors are intrinsically linked. In other words, fire suppressed habitats have different vegetation structure and provide different leaf litter inputs into aquatic environments. One of the somewhat surprising results was the lack of effect of habitat type on invertebrate abundance, which contrasted the results from our previous research (Chandler et al. 2015). That study used dipnet surveys to sample for invertebrates instead of leaf packs. Together, these results suggest that overall abundance is similar throughout wetlands, but that spatial configuration and habitat use of invertebrate communities varies. In fire-maintained areas, invertebrates can exploit standing herbaceous vegetation to move up into the water column. This may have important implications for flatwoods salamander larvae because the little information available suggests that they forage in and around standing herbaceous vegetation.
The changes that we documented are important because leaf litter inputs form the foundation of aquatic food webs and contribute to large-scale ecosystem process (i.e., nutrient cycling). These processes are linked to surrounding systems through the annual movement of animals into and out of wetlands. Management in these systems should prioritize maintaining or restoring vegetation structure characteristic of a fire-dependent ecosystem through a combination of mechanical treatments and prescribed fire applied during the growing season or most importantly when wetlands are dry.