Lecturer in Environmental Science University of Dundee Dundee, United Kingdom
Abstract: Human modification of forest cover for agriculture, particularly in tropical countries, has a significant impact on carbon storage and net primary productivity (NPP) potential at the landscape-scale. However, in the case of perennial crops, like Coffea arabica, carbon storage and NPP may be more similar to undisturbed forest, particularly under high shade conditions. One metric that has been used to estimate this impact is human appropriated NPP (HANPP) and this study explored the utility of HANPP for assessing productivity along an elevational gradient for four coffee farms in Ethiopia. This sample was a subset of 54 farms where coffee yields were monitored and 15 farms that were monitored for carbon storage and decomposition rates. Net primary productivity (NPP) and carbon stock data were collected from 2015-2017 across 4 coffee farms and 2 forest plots located in the Ilubabur Region of Oromia, Ethiopia, adjacent to the Yayu Coffee Forest Biosphere Reserve. These measurements included litterfall, root productivity, and stem growth for coffee shrubs and shade trees as well as berry production of coffee shrubs. Annual NPP values were estimated using Global Environmental Monitoring (GEM) methods. The monitoring period covered two years of El Niño conditions that saw a dramatic collapse in coffee yields. Across the elevational gradient, there was a clear seasonal pattern in NPP over the study time period, with the two forest plots showing the greatest stability and the coffee farms exhibiting decreasing levels of NPP. There was a large variation in observed coffee yields representing varying proportions of farm-level NPP and HANPP. We have shown in cocoa farms, the limited applicability of the HANPP metric for observing sustainable harvesting of farms (Morel et al, 2019); however, there are efforts to produce wall-to-wall maps of HANPP for the continent of Africa. Hence, the results of this study have direct application and relevance for the ecological and agro-ecological community.
References
Morel, A.C., M. Adu Sasu, S. Adu-Bredu, M. Quaye, C. Moore, R. Ashley Asare, J. Mason, M. Hirons, C.L. McDermott, E.J.Z. Robinson, E. Boyd, K. Norris and Y. Malhi (2019) Carbon dynamics, net primary productivity (NPP) and human appropriated NPP (HANPP) across a forest-cocoa farm landscape in West Africa. Global Change Biology. https://doi.org/10.1111/gcb.14661
