Ant community level response to soil warming and nitrogen addition in temperate forests

Background/Question/Methods

Forest ecosystems are subject to anthropogenic threats such as climate change and nitrogen deposition. This study looks at how community level ant diversity is influenced by projected temperature and nitrogen concentration changes and interactions between the two over three years (2007-2009) with pre-treatment data collected in 2006. The Soil Warming and Nitrogen (SWaN) experiment conducted at the Harvard Forest in Petersham, Massachusetts consists of four treatments (control, warming +N, warming -N, +N only) in 3×3 meter plots with six replicates per treatment in a factorial design. Average soil temperature in the heated plots was elevated 5 °C above ambient by the use of buried heating cables placed at 10 cm depth in the soil and spaced 20 cm apart. In the nitrogen addition plots 50 kg N/ha/yr were added via aqueous solution. Ants were collected annually in July by setting out pitfall traps in the experimental plots for 48 hours on clear days then identified to species in the laboratory. A univariate analysis of variance (ANOVA) tested the effects of treatment, plot nested within treatment, year, and the interaction between year and treatment on ant species richness. A non-parametric multivariate analysis of variance (npMANOVA) tested the effects of the same set of predictors on community level ant incidences.

 

Results/Conclusions

Of the 15 different species of ants identified the most common were  Myrmica punctiventris and Aphaenogaster rudis/picea with 43 occurrences each, and Formica subsericea  with 17 occurrences. According to the npMANOVA, there were not significant differences among ant communities across treatments (F_1,50=1.05, P=0.40), plots nested within treatments (F_1,50=0.01, P=0.75), or a treatment by year interaction (F_1,50=0.01, P=0.65). There were significant differences among ant communities across years (F_1,50=0.10, P=0.004). A permutation test for homogeneity of multivariate dispersions indicated that 2007 was significantly different from 2008 (P<0.001) and 2008 was significantly different from 2009 (P<0.001).  These finding are consistent with the responses of soil microbial biomass and community composition in the same experimental plots, which have shown greater temporal variation relative to variation due to warming or nitrogen addition. These finding are also in line with other warming studies that assess ant community structure at Harvard and Duke Forests, which found that northerly ant species that have less exposure to temperatures at their thermal maxima are more resilient in response to warming.