Rapid diversity and structure degradation over time through continued coffee cultivation in remnant Ethiopian Afromontane forests

Tropical deforestation and effects on biodiversity and ecosystem services are relatively well studied, but the extent and impact of forest degradation remains much more cryptic. Most of the remaining Ethiopian moist Afromontane forests are currently being degraded due to forest management for coffee production. So far, effects of degradation on biodiversity and forest structure have been estimated by comparing forests with different management regimes in terms of coffee cultivation intensity. Here, we performed an analysis at two time points (2009 and 2017), through the resurveying of 32 plots located in moist Afromontane forest remnants managed for coffee production, to evaluate how rapidly further degradation occurs. In contrast to space-for-time substitutions, our study allows for an accurate assessment of changes in biodiversity and forest structure due to coffee cultivation, without confounding effects of local environmental conditions. We found reductions in woody vegetation richness (-23%) and diversity (-14%), and a further degradation of forest structure, as quantified by tree and shrub density (-31%). Overall tree and shrub height increased over time (+11%). No changes were found in basal area, dominant tree height and woody community composition over an eight-year period. Our findings indicate a continuation in loss of woody vegetation diversity and related ecosystem services due to an intensive low thinning regime where the smallholder coffee farmers removed predominantly intermediate and suppressed trees from the understorey and maintain the tallest trees in the upperstorey. To halt further forest degradation, implementation of REDD+ and coffee certification programs could provide options, however their effectiveness remains to be tested in the field.