A phenological hypothesis on the thermophilous distribution of Pistacia lentiscus L.

In this study we present evidences of the importance of the phenological pattern on the distribution limits of the dioecious Pistacia lentiscus L. This species, though displaying a thermophilous distribution, has been proved to resist low freezing temperatures during winter. We try to explain this apparent paradox by studying the effects of an extreme cold event that occurred in December 2001 on a natural population of P. lentiscus in the NE of the Iberian Peninsula. In previous phenological studies conduced over 4 years, no spring branch damage was noticeable among population individuals. Female individuals of this species present a prolonged phenophase development which extends until early winter with the conclusion of fruit set. Therefore, we hypothesize that female plants would be more affected than male ones in terms of the following: (1) vegetative and reproductive organs survival; and (2) next-year vegetative growth and fruit production. To test this hypothesis, we selected 225 adult individuals (113 females and 112 males) in April 2002, and estimated their crown volume, percentage of frozen branches and reproductive buds, and the amount of 2001 fruits frozen. In June 2002 we evaluated, in the same individuals, the percentage of vegetative buds flushed into shoots and of reproductive buds producing infrutescences. Branch mortality was significantly higher in female plants and females with increased frost-damage displayed a higher amount of frozen fruits. The loss of reproductive buds caused a decrease in 2002 fruit production, while 2002 vegetative growth was unaffected by the degree of frost damage. These results verify most of the predictions of our hypothesis. Moreover, they suggest that the limited distribution of P. lentiscus in the cold areas of Mediterranean climate could be more determined by the long extent of the phenological activity of the crown than by its frost tolerance during winter.