The effects of deer herbivory and the invasive plant garlic mustard (Alliaria petiolata) on fungal community diversity in a mixed mesophytic forest

Background/Question/Methods

Forest plant communities in the northern United States are increasingly threatened by deer over-browsing and invasive plant species such as Alliaria petiolata (garlic mustard).  Alliaria excretes allelopathic chemicals, including glucosinolates, into the soil that can disrupt important groups of soil fungi, including mutualistic fungi that form mycorrhizal associations with plant roots.  We investigated the effects of Alliaria and deer over-browsing on the community structure of soil fungi, including arbuscular mycorrhizal fungi (AMF), in a 5 year old deer exclusion study.  Six paired plots were established in 2002 where one plot from each pair was fenced to exclude deer.  Beginning in 2006, Alliaria was removed from half of each plot, resulting in a fully crossed design with four treatments (-ALPE4/-DEER, ALPE4/-DEER, -ALPE4/DEER, ALPE4/DEER). In early June and late August 2007, we collected 5 soil cores from each plot to a depth of 5-cm.  These samples were combined into 1 composite sample and used for extraction of soil DNA and molecular analysis (4 treatments x 6 replicates x 2 times = 48 samples).  DNA was used to amplify the 18S rRNA gene or internal transcribed spacer 2 (ITS2) regions for analysis of AMF or saprotrophic/ectomycorrhizal fungal communities respectively.  PCR product was used to generate terminal restriction fragment length polymorphism (TRFLP) profiles of the fungal communities where TRFs were used as operational taxonomic units of the fungal communities. 
Results/Conclusions

We found that both species evenness and Shannon diversity of the saprotrophic/ectomycorrhizal community were significantly higher (P<0.01, P=0.07 respectively) in control plots to which deer had access, suggesting that deer exclusion results in lower levels of fungal species diversity.  Multi-response permutation procedures (MRPP) revealed that sampling time significantly influenced both fungal communities (P<0.01 AMF; P<0.001 saprotrophic/ectomycorrhizal), with communities in early summer significantly different from communities in late summer.  By coupling sequence and indicator species analysis, we found that two Glomus species types were significant indicators of late summer AMF communities.  Both AMF and saprotrophic/ectomycorrhizal communities were significantly different among the plots (P=0.11 AMF; P=0.04) indicating that specific soil conditions had a large influence on these fungal communities.  Additional work will examine the influence of garlic mustard density on fungal community composition.