Stand structural diversity and elevation rather than functional diversity drive aboveground biomass in historically disturbed semiarid oak forests

Abstract

Forests provide biodiversity reservoirs and multiples ecosystem services such as global aboveground biomass dynamics and stock. Ecosystem functions varied according to biotic and abiotic factors, however, the effects of abiotic factors (e.g. elevation) on this relationship was rarely investigated, especially in areas impacted by human activities. We explored the relationships between aboveground biomass (AGB), a key proxy of ecosystem functioning, and plant species diversity in semiarid forests of western Iran submitted to a long history of past disturbances. Forty forest patches were sampled according to an elevation gradient (1700–2100 m) and to two types of forest cover: open cover indicating an intense past use or closed cover for more preserved forests. For each patch, the composition of woody species was recorded and woody species diversity indices (richness, Simpson, Shannon and Evenness) were calculated. We also estimated AGB and computed a wide set of community-weighted mean or functional divergence of ecological traits for woody species. We found that AGB sharply decreased with elevation (from 105.68 ± 10.29 ton/ha at 1700 m to 36.18 ± 6.43ton/ha at 2100 m) for the closed cover but variations were much less pronounced for the open cover (16.32 ± 2.97 ton/ha to 30.83 ± 1.96 ton/ha, respectively). Diversity indices (richness, Simpson, Shannon) increased with elevation for the closed cover and the other traits exhibited strong variations with elevation (either positive or negative) whereas no clear trend was visible in the open cover. Similar results were found when computing the linear relationships between AGB and the different diversity indices which were highly significant in the closed cover only. Surprisingly, the relationships were negative between AGB and most diversity indices (e.g., richness, Shannon, Simpson, Evenness). The dominant Persian oak was in fact the main contributor to AGB which was reduced in stands with a higher tree mixture, a result in line with the mass ratio hypothesis. Our results also illustrate the long-lasting effects of past conservation management on the forest cover and in shaping the relationships between forest ecosystem functions (AGB production) and biodiversity in semiarid areas.