Leaf demography and photosynthesis as affected by the environment in the drought semi-deciduous Mediterranean shrub Phlomis fruticosa L.

Abstract

Leaf demography and water relations of the drought semi-deciduous East Mediterranean shrub Phlomis fruticosa L. were seasonally monitored during a 28 month period. Naturally growing individuals at low (c.a. 40 m) and high (c.a. 600 m) elevation, exposed to full sunlight or shaded by the canopy of evergreen sclerophylls were used. At the exposed, low elevation site, photosynthetic rates were also monitored, both daily and annually. It was found that o a) Based on the time of their burst, two kinds of leaves can be distinguished; long lived, spring leaves, with arrested growth and small size during the dry summer period, but resumed growth after the autumn rains, attaining a life-span of about 13 months; and winter leaves, bursting in late autumn/early winter, whose life-span depended on the extent of summer drought. Severely dehydrated, exposed shrubs shed their winter leaves in early summer. Shaded shrubs, however, maintained satisfactory water contents and kept their winter leaves up to late summer. b) Leaf sizes and the percentage contribution of the two leaf types on total leaf population and area depended on season and experimental site. c) Analysis of the above demographic information in relation to the climatic data at the experimental sites and the leaf water content, indicates that water shortage during the summer and low temperature during the winter determine the phenology and limit the growth of leaves in P. fruticosa. d) Daily measurements of net photosynthetic rates taken at nearly monthly intervals were used to assess the relative contribution of the two leaf types on total yearly CO 2 assimilation by the shrubs in the exposed, low elevation site. Although photosynthetic rates of winter leaves were higher compared to spring leaves during their winter and spring co-occurrence on the same shrubs, their yearly based relative contribution was greatly exceeded by that of spring leaves, when the corresponding leaf duration was considered. These results cast doubt on the general conviction that the small spring leaves contribute negligibly to the productivity of drought semi-deciduous Mediterranean shrubs.