Agroforestry systems (AFS) are a land-use system that integrates trees and crops or pastures within the same land-use area to improve food production efficiency and ecosystem sustainability, where the latter includes ecological benefits such as carbon (C) sequestration and biodiversity conservation (Albrecht and Kandji 2003, Nair 2011, Schroth and McNeely 2011). A growing number of studies comparing C stocks between AFS and adjacent croplands indicate that AFS sequester more C in both vegetation biomass and soil compared to agricultural land managed using monoculture cropping (Schoeneberger et al. 2012, Shi et al. 2018). Given this, promoting agroforestry regionally and globally may have a high potential to mitigate climate change by increasing C sequestration. However, more detailed and reliable information is needed to increase the effectiveness of AFS use, where such information includes how much C (stock) can be stored as either tree biomass or soil organic C (SOC), and perhaps more importantly, how quickly this biomass C or SOC accumulates and the time period over which this occurs.

Based on a global dataset of 141 studies, and use of statistical modelling (linear mixed effect modeling and structural equation modeling), we conducted a global meta-analysis (Ma et al. 2020) to address three core questions: (a) does the rate of C accumulation in biomass and soil of AFS change with tree age? (b) how do patterns of C accumulation in AFS change with tree density, tree species diversity, previous land use type, soil sampling depth, and regional climate? and (c) are changes in SOC stock associated with changes in vegetation biomass C?

Our study showed that integrated AFS (trees plus herbland) hold an average of 46.1 Megagram ha^-1 more C in vegetation biomass compared to adjacent cropland- or pasture-based land-uses that lack trees. Additionally, AFS containing multiple tree species were found to store more biomass C and accumulate biomass C faster over time compared to AFS with only a single tree species. We also found that the effect of AFS on SOC stock increased with tree age, but these patterns varied among ecoregions. More specifically, AFS in tropical zones quickly increased in SOC to peak levels, while AFS in temperate zones increased SOC at a slower rate but notably peaked at a greater overall level of SOC compared to tropical areas. To our surprise, there was no direct linkage between biomass C and changes in total SOC stock among the various AFS when examined with structural equation modeling.

Our findings indicate that AFS such as shelterbelts and alley cropping enterprises (Figure 1) can be a valuable global strategy for mitigating climate change in the long-term by sequestrating C in both tree biomass and soil. Moreover, our work indicates that the planting of multiple tree species will enhance C sequestration in AFS by increasing biomass C stock and jump-starting the process of biomass C accumulation over time. Combined, this process will contribute to the urgent need to sequester C in response to combat ongoing greenhouse gas emissions from fossil fuel combustion. Since a positive impact of AFS establishment on SOC storage was only observed in cropland-based AFS but not in perennial pasture-based AFS, we suggest that establishing AFS within cropped landscapes be a priority. Finally, our study suggests that establishing agroforestry practices in tropical and temperate zones could be a particularly effective strategy for increasing SOC stock in the short and long term, respectively.

 Figure. 1. Results from a recent global meta-analysis show that agroforestry systems, such as shelterbelt systems (top) and alley cropping (bottom), benefit carbon sequestration in the agricultural landscape, particularly if multiple tree species are used and if trees are allowed to grow longer in the system (Ma et al., 2020). (Photo credit: Scott Chang)

References

Albrecht, A., and S. T. Kandji. 2003. Carbon sequestration in tropical agroforestry systems. Agriculture Ecosystems & Environment 99:15-27.

Ma, Z., H. Y. Chen, E. W. Bork, C. N. Carlyle, and S. X. Chang. Carbon accumulation in agroforestry systems is affected by tree species diversity, age and regional climate: A global meta‐analysis. Global Ecology and Biogeography. DOI: 10.1111/geb.13145

Nair, P. K. 2011. Agroforestry systems and environmental quality: introduction. Journal of Environmental Quality 40:784-790.

Schoeneberger, M., G. Bentrup, H. De Gooijer, R. Soolanayakanahally, T. Sauer, J. Brandle, X. Zhou, and D. Current. 2012. Branching out: Agroforestry as a climate change mitigation and adaptation tool for agriculture. Journal of Soil and Water Conservation 67:128A-136A.

Schroth, G., and J. A. McNeely. 2011. Biodiversity conservation, ecosystem services and livelihoods in tropical landscapes: towards a common agenda. Environmental Management 48:229-236.

Shi, L. L., W. T. Feng, J. C. Xu, and Y. Kuzyakov. 2018. Agroforestry systems: Meta-analysis of soil carbon stocks, sequestration processes, and future potentials. Land Degradation and Development 29:3886-3897.