Old-field succession along a precipitation gradient in the semi-arid coastal region of Tenerife

Secondary succession on abandoned fields was studied in the semi-arid region of Tenerife (Canary Islands). At four different sites along a precipitation gradient four chronosequences were sampled. On the whole, 11 fields, abandoned for eight to 70 years, and adjacent near-to-natural stands were studied, with regard to species richness, species composition, vegetation structure, life form distribution and the importance of endemic/exotic species, using uni- and multivariate statistical methods.In the three drier chronosequences species richness increased significantly during succession whereas in the wettest sequence it peaked very early with a subsequent decline towards the mature stand. Temporal changes in floristic composition were significant and revealed clear directional trends. We could distinguish two pathways of succession: one for the northern coast under more humid conditions and one for the southern coast under arid conditions. Therophytes were generally substituted by nanophanerophytes, while hemicryptophytes and chamaephytes showed low abundances with peaks in late successional stages. At all sites, endemic species replaced exotic species, with regard to both relative number and cover. The relative importance of exotic species in early succession increased with increasing precipitation. Cover of perennials, stand height and stand biomass increased with time since abandonment and water availability. Floristic composition recovered faster than stand structure and, in particular, stand biomass.We have the following conclusions (i) The absence of disturbance and in presence of near-to-natural stands in the surroundings, coastal scrub on Tenerife has the capacity to regenerate completely albeit slowly after traditional agricultural use. (ii) The rate and pathway of succession are influenced by site productivity, i.e. mean annual precipitation. (iii) Life form distribution changed in the second phase of succession in relation with the precipitation gradient. (iv) The initially dominant exotic annual plant species seem to be a temporary problem only since, in the course of succession, they are substituted by native perennials.