Questions: Can gender of nurse plants affect regeneration patterns and spatial population structure? Is there a seed-seedling conflict in the regeneration process? What factors are responsible for the clumped spatial population structure observed for adult trees?
Location: Mediterranean cold semi-arid high mountains in Spain.
Methods: The spatial pattern of adult Juniperus thurifera trees was studied by means of Ripley's K-analysis. χ2 analyses were used to test for natural seedling frequency in each of three main microhabitats: (1) under female and (2) male tree canopies and (3) in open interspaces. The observed pattern was explained experimentally by studying seed and seedling survival for two years. Survival probabilities were calculated across life stages for each of three main microhabitats.
Results: Adult J. thurifera trees were aggregated in space. Most seedlings were found underneath female J. thurifera trees. Experimental studies demonstrated that from seed dispersal to seedling survival all life stages showed the same positive or negative trend within a given microhabitat, indicating stage coupling and no seed-seedling conflicts. Attraction of frugivo-rous birds by reproductive female junipers and improvement of environmental conditions beneath tree canopies were the main factors responsible for the variation in seedling density among microhabitats; highest underneath female trees and lowest in open interspaces.
Conclusions: In dioecious species, the gender of nurse plants can significantly determine the spatial population structure. In J. thurifera forests, facilitation beneath female trees occurs among all life stages without any sign of seed-seedling conflict. The most critical factors shaping the spatial population structure were directed seed dispersal and environmental amelioration beneath female conspecific trees.