Lack of recruitment in Lavandula stoechas subsp. pedunculata: a case of safe-site limitation

Lavandula stoechas subsp. pedunculata regeneration depends exclusively on the establishment of new individuals. Seed availability and seedling emergence and survival are therefore critical life stages and processes for species regeneration. In this study, seedling emergence and survival was monitored for two years in the scrub, both in clearings and adjacent to adult plants, and the surrounding perennial grassland, at 1, 3 and 5 m from the scrub. Soil seed bank spatial distribution was also studied for one year in the same two habitats, using the same sampling design. Soil seed availability in the scrub is high regardless of the distance from the adult individuals. On the contrary, the adjacent grassland shows a drastic fall in seed density, and almost no seedlings were observed there. In the scrub, seedling density was negatively related to distance from the three nearest adult plants in the clearings, and positively related to adult plant size beneath the adult Lavandula plants. There was also a negative relationship between seedling density and the percentage of bare soil. Only one seedling survived the first drought period, with no detection of effects of either position with respect to adult individuals or seedling density. We hypothesized that the study populations suffer a lack of appropriate safe sites within the scrubland while in the adjacent perennial grassland, observed low seed availability was added to safe-site limitation. That results in a lack of successful seedling establishments and a poor expansion potential of Lavandula scrublands, whose edges remain static in the short and medium term. As found in other Mediterranean scrubland, recruitment may only occur in years with particularly favourable weather, under disturbance regimes that increase seedling survival probability or when external dispersal agents increased seed availability in adequate places for Lavandula establishment.