Diversity and production of Ethiopian dry woodlands explained by climate- and soil-stress gradients

Dry woodlands cover about 14% of the total African land surface and represent about 25% of the natural vegetation. They are characterized by a seasonal climate, with a dry season of 4–7 months. Large parts of these ecosystems are degrading due to grazing, fire or exploitation by people. We studied species richness and productivity patterns of dry woodlands in Ethiopia. For such ecosystems, classic productivity and diversity hypotheses predict that species richness and productivity increase as the wet season length increases, and decrease when soil conditions create water stress. We inventoried and measured trees in 18 2-ha plots distributed in two sites, one higher altitude site with a shorter wet season than the lower altitude site. We found that the stand volume per hectare was lower in the site with a shorter wet season. Across all 18 plots we observed that stand volume decreased with soil water stress (estimated from texture and depth). This was in line with the prediction. The species richness was lower in the short-wet-season woodlands, but was unaffected by variation in soil conditions. This suggests that climate driven constraints (wet season length) set the limits to species richness, and not soil conditions. As far as we know, this study is one of the first studies that evaluated these productivity and diversity hypotheses for dry African woodlands.

Research highlights
► In this study, we investigated eighteen 2-ha plots of Ethiopian dry woodlands, co-dominated by the Frankincense tree species Boswellia papyrifera.
► Soil properties of Ethiopian dry woodlands were associated with species composition, but not with species richness or diversity.
► Soil properties that improve soil water availability (higher clay content and greater soil depth) increased the stand productivity of Ethiopian dry woodlands.
► Species richness decreased with higher altitude, but was independent of soil factors.
► The co-dominant species B. papyrifera lacks juveniles in these woodlands, and will probably be replaced by other species in the near future.