Fire History and Oak-Cynipid Community Structure

In the Spring of 2018, I initiated a long term research project to study the effects of fire history on oak-cynipid community structure at Archbold Biological Station (ABS) in Lake Placid, Florida. ABS is a leader in the study of fire management practices on forest community structure. For more than 40 years, the two dominant plant ecosystems at ABS, the scrubby flatwoods and sand-pine scrub (Photos 1 and 2) have been subjected to scheduled burning. Records have been kept on not only the date of each burn but the intensity of each fire (at a resolution of < 10 m2).

photo of flatwoods
Photo 1. Scrubby flatwoods 3 years post burn at Archbold Biological Station.
photo of sand-pine scrub
Photo 2. Sand-pine scrub, five years post burn at Archbold Biological Station.
photo of Warren (Abe) Abrahamson
Photo 3. Warren (Abe) Abrahamson inspecting oaks for galls at ABS

In February of 2018, my colleague Dr. Warren Abrahamson (Bucknell University, retired; Photo 3) and I conducted an intensive census of cynipid gall wasps on four oak tree species in the three different ecosystems subjected to different burn histories (scrubby flatwoods, sand pine scrub, and ridge sandhill). Replicate sites were burned 2-3 years ago (recent burn), 5-7 years ago (intermediate burn) and > 19 years ago (not burned since fire management was initiated). We inspected 20 trees per tree species at each site and recorded the numbers of galls per cynipid species.

Among all stands and oaks, we found 25 species of cynipids and there was very little species overlap among oak species, even within the same stand. A few representative cynipid species are shown in Figure 1.

photo of cynipids
Figure 1. From left to right, top to bottom: Amphibolips murata, Belonocnema quercusvirens, Neuroterus quercusverrucarum, Disholcaspis quercusvirens, Dipteran pinecone gall, Sphaeroterus carolina, Andricus cinnomomeus, Amphibolips murata
Figure 2. The relationship between time-since-fire (< 3 years, 3-7 years, and ≥ 19 years) and oak species (A) mean gall abundance per tree and (C) species richness. Means ± SE are reported.

Gall abundance (number per tree) increased with time-since-fire and was correlated with tree height, suggesting that available host material may be a primary driver of cynipid recovery. Within three years of a fire, 16 of the 25 cynipid species were detected among the stands, and by seven years since fire, all but two species could be detected. Overall, species richness and diversity reached an asymptote by 3-7 years (i.e., the intermediate time-since-fire category) (Fig. 2).

Given how quickly the cynipid assemblage recovers after a fire, frequent fires at ABS are unlikely to negatively impact these insects. However, in smaller or more isolated scrub-oak fragments, recovery could be much slower. A manuscript from this work is currently in preparation.

An important next step in our understanding of the ecology of this system would be an investigation of the role of fragment isolation, size and burn history and intensity on cynipid species richness, abundance and population-genetic structure. Landscape features, such as proximity to unburned habitat, the types of unburned habitat within the surrounding landscape, and the amount of edge, may all be important factors to consider as well. We also recommend that future studies with this system explore how the above factors influence multi-trophic interactions.