Diversity modulates above-ground productivity in response to disturbances: The case of Iberian forests

Disturbances play a key role in forest dynamics globally, with various effects on productivity. Tree diversity is expected to attenuate the negative impact of disturbance on forest productivity, but its overall effect has been little explored and might depend on the functional composition. Here, we analysed whether forest productivity responses to disturbances were modulated by structural and functional diversity, as well as by functional dominance, and whether these responses vary among functional groups. For this, we used 12,075 permanent plots from the Spanish Forest National Inventory spanning the 1986–2019 period and modelled forest above-ground productivity as a function of disturbance occurrence (i.e. harvesting, fire or biotic damage observed in trees) and its pairwise interaction with (i) structural and functional diversity and (ii) functional dominance. We considered the following main functional groups: (i) needle-leaved (50.2% of the data), broad-leaved deciduous (25.8%) and broad-leaved evergreen (24.0%). Harvesting and fire occurrence significantly reduced above-ground productivity of all functional groups, 94.5% and 143.9% on average, respectively. Structural diversity mitigated forest productivity declines associated with harvesting and fire, whereas functional diversity only modulated responses related to harvesting. In general, structural diversity had a more positive effect on forest productivity in disturbed plots than in undisturbed plots. The only exception concerned the impact of fire occurrence on broad-leaved species, which remained similar. In harvested plots, functional diversity increased forest productivity, except for broad-leaved species. Finally, forest productivity decreased in harvested plots dominated by broad-leaved deciduous tree species but increased in fire-affected plots. Synthesis. Forest diversity can mitigate the negative effects of disturbances on productivity. Higher structural diversity enhanced productivity in both harvested and fire-affected stands, while greater functional diversity maintained productivity in harvested forests. These results highlight the importance of promoting structural and functional diversity as a key strategy to sustain forest productivity and reinforce the role of forests as carbon sinks under increasing disturbance regimes.