Aboveground biomass allocation patterns within Mediterranean sub-shrubs: A quantitative analysis of seasonal dimorphism

The monthly patterns of aboveground biomass allocation were studied in the branches of six Mediterranean subshrubs with different leaf phenology. Four of them were seasonally dimorphic species, and the remaining two were a winter deciduous and a cushion plant with photosynthetic stems. By the analysis of these species we aimed to identify different aboveground biomass allocation patterns within seasonally dimorphic species and to understand the role of seasonal dimorphism as a strategy to avoid the main stresses of mediterranean climate: summer drought and winter cold. The biomass allocation to the different living and photosynthetic fractions of 3-year-old branches was studied monthly for a minimum of 13 months per species. Leaf area (LA, mm(2)) and leaf mass per area (LMA, mg cm(-2)) measurements were used to characterize the diverse types of leaves of each species. Standing dead and senescent tissues accounted for a great percentage of the branch biomass of seasonally dimorphic species both during summer and winter. Different patterns of photosynthetic biomass allocation were found within the seasonally dimorphic species analysed. These patterns ranged from the moderate photosynthetic biomass oscillation of Salvia lavandulifolia to the almost deciduousness of Lepidium subulatum, and they were achieved by keeping alive, drying out or shedding different types of branches and leaves throughout the year. The formation of stress tolerant leaves and the reduction in the amount of photosynthetic biomass responded both to the occurrence of summer drought and winter cold. These results demonstrate that seasonal dimorphism is a flexible ecological strategy, as it comprises very different leaf phenologies and enables plants to escape both summer drought and winter cold. (c) 2006 Elsevier GmbH. All rights reserved.