PS 53-81
Burning and canopy openness vs. common mycorrhizal networks: Which is more important in the first season for upland oaks?
One of the most endangered ecosystems in North America is the open oak woodland/savannah, which occupies only 0.02% of its original area before fire suppression began. A contributing factor to this decline is the failure of oak species to regenerate at a rate that adequately replaces mortality. Upland oaks evolved to benefit from the relatively frequent fires and open canopy of the historically fire maintained habitat of the xeric uplands. They devote a majority of resources to root growth in the early stages of life and re-sprout quickly after top-kill events such as fire. In other systems, connections to conspecific adult trees through common mycorrhizal networks (CMNs) have been shown to reduce seedling mortality. What is more important in the first year of oak seedling growth, CMNs or burning and canopy openness? In a field experiment, we measured relative growth rate (RGR) of height and basal diameter of oak seedlings either connected or not connected to an adult CMN at four sites: Open/burned, open/unburned, closed/burned and closed/unburned. For each seedling, we also measured overstory canopy openness, distance from ridge-top, %sand, slope, and aspect.
Results/Conclusions
Canopy openness and a history of frequent fire were more important than inclusion in a common mycorrhizal network to explain an increase in basal diameter growth in the first growing season for Quercus falcata and Quercus alba in the xeric uplands of north Mississippi. Seedlings not connected to a CMN showed greater height growth than did those that were connected. This might be due to seedlings that were connected to the network devoting more of their resources to root growth because the CMN is much larger than the network of mycorrhizae supported by the seedling alone. Seedlings not connected to the CMN may need to devote more to height in the first season. Finally, seedlings closer to the nearest ridge-top increased in diameter at a greater rate than did those further down slope. Altogether, these results suggest that fire and canopy openness may affect different aspects of oak seedling performance compared to CMNs in this system. Analysis of belowground biomass, above and belowground biomass ratios, mycorrhizal colonization, and seedling specific canopy openness data will help to support or refute these claims.