Diversity and abundance of Lepidoptera and Coleoptera in multiple-species reforestation plantings to offset emissions of carbon dioxide

Multiple-species (floristically diverse) plantings of trees and shrubs in former agricultural landscapes to offset (sequester) emissions of carbon dioxide are a recent component of Australian landscapes. Although their potential to mitigate biodiversity loss is recognised, this ecological function has not been investigated, in particular with respect to insect diversity. Over two summers, we used light trapping to sample Lepidoptera (moths) and Coleoptera (beetles) in multiple-species plantings in two distinct locations in Victoria (plantings of four ages per location) as well as in nearby remnant forest and in pasture. At both locations, we found that plantings had a greater abundance of Lepidoptera than remnants but that the abundance in plantings was comparable to the abundance in pasture. The species richness of Lepidoptera in plantings did not differ significantly from that in remnants but was significantly greater than that in pasture. The abundance and species richness of Coleoptera in plantings was lower than in remnant forests but higher than in pasture. The community composition of Lepidoptera and Coleoptera in plantings was intermediate between that of remnant forest and pasture, i.e. possibly transitional between the two vegetation types. Dissimilarity between all vegetation types was nevertheless high, reflecting that the abundance of individual taxa reflects the influence of temporally and spatially dependent factors, e.g. host plant size and suitability. Greater abundances of grass-feeding Lepidoptera in plantings explained much of the dissimilarity between plantings and remnant forests. Proximity to remnant forest was an important determinant of the community composition of Lepidoptera but not of Coleoptera. Moth forewing length (relevant to vagility which is an indicator of dispersal capacity) appeared less important to proximity relationships than larval host plant specificity, i.e. whether grass or dicot-feeding. The location of sequestration plantings relative to remnant forest as well as their composition (including the persistence of pasture grasses) will determine the attractiveness of the resources provided to insect herbivores and hence the rate at which they are colonised. Plantings near remnant native forest benefit insect diversity but adoption of novel silvicultural practices could hasten the rate at which they become functional mimics of native forests and support more comparable communities of insect. Potential trade-offs between increased establishment costs and more complex carbon accounting might need to be investigated to cost biodiversity credits associated with dual accreditation schemes.