Restoring ecosystem functions and services is increasingly a management priority. Many studies in grasslands indicate higher levels of biodiversity increase individual ecosystem functions, and at times, multi-functionality. Whether this relationship translates to other ecosystems is of interest to groups wanting to plant forests for carbon sequestration. In carbon plantings, monocultures are common, but diverse tree-plantings may offer additional benefits including higher levels of carbon sequestration and restoration of other valued ecosystem services. Through a 5-year field-based experiment in Western Australia, we explored how native tree diversity simultaneously affects three ecosystem functions including: carbon sequestration, invasion resistance, and nutrient cycling. In 2010, we planted trees in treatments that increased in species, structural, and functional group richness. Treatments included species of different heights (structural diversity) and soil-based resource acquisition-traits (functional diversity). From 2011-14 we measured carbon sequestration, weed invasion (weed cover), and soil nutrient availability (N, P). We determined: (1) if species, structural, or functional group diversity increased multifunctionality compared with commonly utilized low-diversity plantings, and (2) if all three ecosystem functions could be maximized simultaneously in any diversity treatment.
Results/Conclusions
Our results indicate that after four years, more diverse carbon plantings did not maximize all desired ecosystem services simultaneously. Rather, within this timeframe, both carbon sequestration and nutrient uptake from soils were similar across all diversity treatments regardless of species, structural, or functional group diversity. In contrast, weed invasion increased as species and functional group diversity (and marginally, structural diversity) increased, indicating that in this system there were ecosystem dis-services of more diverse carbon plantings. This result does not align with the common finding in grasslands that increased diversity results in higher levels of multiple ecosystem functions. It does, however, provide information for managers aiming to restore multi-species forests for the purpose of sequestering carbon. In particular, while increasing tree diversity did not increase total carbon sequestration, it also did not reduce it. This highlights that adding diversity to carbon plantings did not reduce the efficacy of these plantings. Our results also suggest that plantings containing nitrogen-fixing tree species accumulated woody biomass more quickly than treatments without nitrogen-fixers, a finding supported by meta-analyses and other field-based studies. Finally, although more diverse forests had more weed cover, this weedy understory appeared to be affected by forest structure. Thus, the ongoing maturation of tree canopy and resulting reduction of understory light may ultimately mitigate this dis-service.