To what extent does the species composition of Macaronesian laurel forests depend on their human disturbance history and environmental drivers?

Abstract

Deforestation and forest fragmentation have serious negative consequences for biodiversity and species distribution, but many studies approach species distributions and speciation processes without taking in account the local history of human disturbances. Nevertheless, the complex recovery process after forest destruction is widely distributed around the world and takes place under a wide mosaic of biotic and abiotic factors that may be influencing the species distribution. In this study aerial pictures from 1951 and 2019 were used to assess forest recovery during about sixty years. We tested if the initial stage (forest completely destroyed or young growth forest in 1951) determines the successional process, evaluating the role of different spatial drivers (distance from remnant forest fragments and unfelled areas around each site) and mesoclimate conditions, by examining 40 disturbed sites and 18 old-growth forest sites distinguished in 1951 aerial photos. The final stage during this successional process was assessed using 2019 aerial photos, covering the same these sites. At each plot, plant species composition and forest structure were studied, which allowed us to obtain a maturity index. Our results reveal that the initial stage is not the only driver explaining forest structure and species composition at the end of the 60-year period analyzed. The role of each forest recovery driver varies depending on each stage. In the early successional stages, the slope aspect (exposure) was the most important factor, which is correlated with mesoclimatic conditions, followed by the distance from young-growth forest in 1951. However, in intermediate successional stages the cover of old-growth forest around each plot in 1951 was the main factor in the recovery. At the end of the study period (2019), the differences in species richness between young-growth and old-growth forest were not significant, although old-growth forests were characterized by high densities of endemics, threatened species and β-diversity, showing the species composition a strong dependence on forest structure. Our results reveal the important need to consider human disturbance history in forest research and how climate conditions and the lack of nearby remnants can negatively affect the forest recovery process, greatly increasing the time needed to return to old-growth laurel forest conditions, which requires specific management recommendations.