Restoring multiple woodland ecosystem functions through diverse carbon plantings

Background/Question/Methods

The restoration of ecosystem functions and services is increasingly a management priority.  As such, throughout Australia, groups are interested in planting trees not just to restore historical woodlands, but also for carbon sequestration. Deciding which trees to plant is pivotal to project success.  Monoculture plantings are common, however, diverse tree plantings may offer a number of benefits including higher levels of carbon sequestration and restoration of additional valued ecosystem functions and services. Through a field-based experiment in Western Australia, we are exploring how native tree diversity simultaneously affects three valued ecosystem functions including: carbon sequestration, invasion resistance, and nutrient cycling. We chose species based on resource acquisition-traits including root structure and microbial associations, which potentially affect species’ contributions to all three target functions. In 2010, we planted trees in treatments that increased in functional group richness, and in 2011, measured a carbon sequestration-index (summed tree height), weed invasion (weed cover), and soil nutrient availability (nitrate, ammonium, and phosphate). We used data to determine: (1) if high functional group diversity in carbon plantings might increase ecosystem functioning compared with commonly utilized low-diversity plantings, and (2) if all three ecosystem functions could be maximized simultaneously in any one planting treatment.

Results/Conclusions

After one year, carbon sequestration was significantly greater in stands containing the single functional group of deep-rooted trees compared with stands containing greater functional diversity. Total weed invasion was equal across treatments, although the cover of capeweed (Arctotheca calendula), a pest in Australian pastures, was greatest in low-diversity stands. There was no difference in nutrient availability among treatments. Our results indicate that functional group diversity based on resource-acquisition traits did not influence levels of target functions; rather, other traits appeared to play a large role. For example, rapid tree-growth maximized carbon sequestration. Additionally, light availability appeared to influence invasion resistance with minimum capeweed invasion occurring where light levels were high. Ultimately, low-diversity treatments strongly influenced functioning via the presence of rapidly-growing trees that resulted in low-light levels. As stand development continues, other traits including the nutrient-acquisition traits originally targeted in our experimental design, may increasingly influence ecosystem processes. Our results additionally indicate that at this early stage in our study, the functions of carbon sequestration and invasion resistance could not be maximized simultaneously. In particular, the low-diversity treatment maximized carbon sequestration, while minimizing invasion resistance to capeweed. Such functional tradeoffs, if they persist, may influence planting composition in carbon-sequestration projects.