Trade-offs between phenology, relative growth rate, life form and seed mass among 22 Mediterranean woody species

Abstract

Mediterranean woody plants exhibit a wide phenological diversity which cannot be explained just on the basis of climatic constraints. We assessed the role of relative growth rate (RGR), life form, seed and fruit mass as potential constraints of plant phenology. In a comparison of traits of 22 Mediterranean woody plant species, the duration of the primary shoot growing period decreased from climbers to shrubs and to trees. A hypothesised negative association between RGR and primary shoot growth duration did not emerge in our species set. The mechanism underlying phenological differences between plant life forms might relate to differences in the proportion of respiring to photosynthetic tissues, which decreases from climbers to trees. It is suggested that the degree of shoot preformation within the bud correlates with primary shoot growth duration, but not with RGR. Development of big fruits and seeds competes for carbon with vegetative growth. Indeed, species with bigger seeds and fruits exhibited shorter primary shoot growing periods, which tended to overlap with flower bud formation and flowering periods. We suggest that duration of primary shoot growth allow to short out the species between two extreme growth strategies: The conservative one would be characterised by a concentration of the primary shoot growth into a short period, free of frosts and droughts, and by a diversion of part of the current resources to assure next year's growth. The opportunistic strategy, on the other extreme, would be defined by the allocation of resources to current growth whenever they are available, achieving longer growing periods at the expense of higher risk of tissue damage. These strategies should have been selected for in environments of predictable and unpredictable resource availability, respectively.