Marginal holly (Ilex aquifolium L.) populations in Mediterranean central Spain are constrained by a low-seedling recruitment

Abstract

Seedling density and survival was studied in holly woodland located in the mountains of the Sistema Central, Madrid (Spain) in 1998. Ten permanent quadrats of 50×50 cm were randomly placed in each of the following habitats: under closed holly canopy, in canopy gaps, at the edge of the holly woodland, in the adjacent open grassland, and under fleshy- and dry-fruited shrubs located in this grassland. Emerged seedlings were periodically monitored and their survival was checked during the following 2 years. Additionally, a set of 20 seedlings were planted in each of the cited habitats and their survival was followed in parallel with the naturally emerged seedlings. The influence of light on holly survival was tested by experimental planting of holly seedlings under six different controlled light intensities. Seedling emergence was strongly habitat dependent, with the highest seedling densities in the closed holly woodland and in canopy gaps, and a complete absence in the grassland. Survival to the first year was nil. No naturally emerged seedling reached 1 year of age, independent of the habitat studied. Only one of the planted seedlings in the field survived to the first year. Despite this, mean seedling lifespan was different between habitats, with canopy gaps, woodland edge, and under fleshy-fruited shrubs being the habitats with highest average lifespan. Seedling response to the distinct light intensities is largely similar to that seen in the different field habitats, but other factors in addition to light, mainly summer drought and disturbances are proposed to explain the 100% mortality observed. Seedling incorporation is the recruitment dynamic bottleneck of this holly population, being more severe here than in other places studied further north. This may explain the increasing rarity of the species in the southern half of the Iberian Peninsula, where Ilex is close to its southern limit of its distribution.