Abstract

Eastern hemlock (Tsuga canadensis (L.) Carrière) is a foundation species in eastern North American forests, providing critical habitats for a number of species. Hemlocks are experiencing widespread decline due to the spread of hemlock woolly adelgid (HWA: Adelges tsugae Order Hemiptera) into their range, potentially resulting in the functional disappearance of hemlocks from eastern forests. Hemlock dieback can lead to cascading effects on associated ecosystems, including below-ground, mycorrhizal fungal communities. Ectomycorrhizal fungi (EM), which are mutualistic with many tree species and provide nutrients to plant hosts, are known to colonize hemlock and neighboring tree species at lower rates following HWA infection. This study investigated the effect of hemlock decline from HWA infestation on mycorrhizal communities. Hemlock health surveys were conducted in healthy (Carl Sandburg Home National Historic Site – CARL) and declining (Warren Wilson College – WWC) stands in western North Carolina, and trees were paired between stands based on diameter. In each stand, northern red oak (Quercus rubra L.) “bait” seedlings were planted within a meter of “host” hemlock trees in early summer and allowed to grow for eight weeks, when they were harvested. Seedling growth and dry biomass were recorded at harvest and roots were sampled for mycorrhizal colonization rates. Different mycorrhizal morphotypes were collected from seedling roots for DNA extraction to compare mycorrhizal community assemblages between the two stands. Mycorrhizal inoculation rate (percentage of total number of root tips on a seedling that were colonized by EM) and growth in seedling height were significantly greater in the healthy hemlock stand (CARL) relative to the declining stand (WWC), suggesting that healthy hemlock stands are more favorable for oak seedling growth than declining stands. DNA barcoding determined that a greater proportion of seedlings grown in a healthy stand were inoculated with EM taxa, indicating that EM assemblages differ between a healthy and declining hemlock stand. Root:shoot ratio decreased significantly with increasing mycorrhizal colonization driven by significant decreases in root biomass. In declining stands, mycorrhizal inoculation was lower and the mycorrhizal community was different, resulting in differential growth in the declining stand relative to the healthy stand. We conclude EM communities differ between a healthy and declining stand and that changes in EM communities following hemlock dieback may affect the growth of replacement species.

How to Cite