Stand management to reduce fire risk promotes understorey plant diversity and biomass in a semi-arid Pinus halepensis plantation

Questions How do thinning and understorey management practices (ploughing, clearing) to reduce fire risk affect plant species composition, diversity and biomass within pine plantations on previous croplands? Do managed pine plantations promote changes in plant succession compared to unplanted areas (old fields) towards restoration goals including holm oak forest or other alternative land-use types? Location Altiplano del Conejo, Guadix-Baza Basin, SE Iberian Peninsula, Spain. Methods We evaluated plant species composition, diversity, cover and biomass in the understorey of a semi-arid 10-yr-old afforestation area of Pinus halepensis through a manipulative experiment in which four different thinning regimes (T0 treatment: control plots; T100, T75, T60 and T48 treatments: 100%, 75%, 60%, and 48% of mean basal area removed, respectively), and two understorey vegetation treatments (clearing, ploughing) were applied, simulating stand practices to reduce fire risk. Sampling was performed annually for 5 yr (from 2006 to 2010). For comparison, four other land-use types (rain-fed cereal crops, old fields, alpha grass steppes and holm oak forests) were sampled to quantify understorey vegetation community composition and diversity. Results Afforestation plots were compositionally distinct from each other within 5 yr, mainly as a result of canopy thinning and to a lesser extent, vegetation treatments. Total species richness was highest in the low to medium densities of pines, where richness approached that found in native communities. However, afforestation plots were most similar in diversity, life-form type and composition to abandoned agricultural fields, and large differences were found between afforestation plots, native forests and alpha grass steppes. Conclusions This study demonstrates that plant species composition in the understorey of pine plantations on former cropland can respond rapidly to common management practices intended to reduce fire risk. In particular, tree thinning increases both understorey plant diversity and biomass, but does not lead to recruitment of species that are typical of native forests. Rates of native forest succession appear to be slow in this semi-arid Mediterranean ecosystem. Current pine plantation management practices, while useful for increasing understorey biomass and plant diversity will need to be coupled with additional restoration techniques (e.g. seeding and transplants) to recover species composition and structure of native forests.