Aboveground and belowground communities interact directly and indirectly, predominantly via feedbacks mediated by plants. These interactions can have important implications for how ecosystems respond to environmental changes, especially as they may influence ecosystem processes such as primary production, decomposition and nutrient cycling. We conducted a greenhouse experiment using 2-year old white spruce seedlings to investigate linkages between spruce budworm (SBW, a lepidopteran defoliator of spruce and fir) and soil nematode communities.
We used three SBW density treatments (low, high and no-SBW control) and a native nematode community to observe effects of defoliation on herbivores, plants and nematode community responses. Second instar SBW larvae were added in mid-April 2015 and the experiment was carried through to SBW pupation and moth emergence (beginning mid-May 2015). At the end of the experiment, SBW were collected and measured (survival, weight, wing-length), plant physiological responses were measured (above- and belowground biomass, adventitious shoot production, photosynthetic activity) and nematodes were sampled, counted and sorted to trophic groups.
Results/Conclusions
SBW density had significant effects on adventitious shoot production, root growth, total defoliation, and back-feeding (consumption of older foliage classes). Greater SBW survival was observed in the low vs. high density treatments and both moth dry weight and female wing length (a surrogate for reproductive capacity) were lower in the high density treatment, although SBW density had no effect on SBW sex ratio or development time. SBW density treatments had no effects on nematode community or trophic group abundances, with the only significant nematode response observed between root herbivorous nematode abundances and needle biomass remaining following defoliation.
Plant responses to defoliation were in line with the principle of functional balance, as reduced foliage biomass and photosynthetic capacity were mirrored by reduced root growth, balancing resource acquisition and allocation. Following defoliation, seedlings allocated resources to adventitious shoot production to compensate for defoliation, which would allow for photosynthesis through the remainder of the growing season once the seedling was released from herbivore pressure. SBW showed density-dependent impacts to survival, weight and female wing-length, demonstrating that density-induced mortality was insufficient to negate impacts to larval performance, likely a consequence of feeding on poorer quality foliage. Finally, nematodes appear relatively resilient to one year of even intense herbivory, although there is an apparent link between root herbivore abundances and needle biomass, possibly indicating a connection between photosynthetic resources available to root tissues following defoliation.