Seed release without fire in Pinus halepensis, a Mediterranean serotinous wind-dispersed tree

Abstract

1 Although serotiny is frequently considered to have evolved under the selective pressure of fires, the drying conditions that induce cone opening are not necessarily associated with fire. We hypothesized that in Pinus halepensis, a Mediterranean serotinous wind-dispersed tree, xeriscence (seed release induced primarily by drying conditions not generated by fire) bears intrinsic adaptive values, independent of those of pyriscence (fire-induced seed release). 2 We used seed-traps to quantify seed release in two scrubland pine stands in Israel. Contemporary meteorological data were used to seek correlations with climatic factors affecting seed release. 3 Substantial seed release, estimated to be about 60% of the annual crop, was observed in the absence of fire. Seed release was distinctly seasonal, with high rates in spring and autumn, and was strongly correlated with short, infrequent but temporally predictable Sharav events (dry and hot weather). In the most extensive Sharav-induced seed release, seed density reached 117 seeds m-2 and 15% of the annual crop was released within 6 days. Stepwise multiple regression revealed that mean relative humidity (in both stands) and maximum temperature (in one stand) were significant predictors of seed release. 4 Vertical (upwards) wind velocity was significantly positively correlated with dry and hot weather. Seed counts in distant traps (> 20 m from the nearest tree) were significantly greater in periods in which Sharav events occurred than in other periods. Xeriscence may therefore have an adaptive value in promoting dispersal distance by wind. 5 Both xeriscence and pyriscence appear to be involved in determining serotiny in P. halepensis and provide means of exploiting establishment opportunities generated either by fire or by other factors.