Abstract
In subtropical montane semi-moist forest in SW China (SMSF), a large majority of evergreen tree and tall shrub species was found to have only one cohort of old leaves in early spring. In contrast, almost all species of evergreen tree and tall shrub in warm temperate rain forest (WTRF) in Japan and sclerophylls in Mediterranean-climate forest (MSF) of the Mediterranean Basin have two or more cohorts of old leaves in early spring; they drop their oldest cohort during or soon after leaf outgrowth in spring. Japanese WTRF has no dry season and MSF a dry summer. SMSF has a dry winter. On four evergreen Rhododendron species from SW China with only one cohort of old leaves in spring when in cultivation in Scotland, the majority of leaves in the senescing cohort fell by the end of December. We hypothesize that with dry winters, there is an advantage to dropping older leaves in autumn, because there is a low chance of appreciable positive assimilation in winter and a high chance of desiccation, reducing the resorption of dry mass and mineral nutrients from ageing leaves. Our hypothesis may be extended to cover evergreens at high altitude or high latitude that experience cold soils in winter.
Highlights
In the extensive literature on leaf longevity, scant attention has been paid to the question of why evergreen temperate plants and subtropical montane plants drop their old leaves when they do [1]
In contrast to what has been reported for warm temperate rain forest (WTRF) in Japan and Mediterranean-climate sclerophyll forest (MSF) in the Mediterranean Basin, we have found for many of the evergreen trees and shrubs in semi-moist forest in SW China (SMSF) that in early spring (April), there is only one cohort of old leaves present, while or before the new leaves expand, and there is no sign of ageing leaves about to fall
At the SMSF sites, thirteen species in eight families were seen with only one cohort of old leaves in mid-April, and seven of these had new leaves beginning to expand (Table 4)
Summary
In the extensive literature on leaf longevity, scant attention has been paid to the question of why evergreen temperate plants and subtropical montane plants drop their old leaves when they do [1]. The current understanding of the advantage of having evergreen leaves in the temperate zones goes back to the seminal paper of Small [2], which provided a rationale in terms of leaves lasting for two or more growing seasons and did not consider the function of the leaves in the coolest months. He was concerned with plants of nutrient-poor soils and argued, in effect, that the evergreenness of many woody plants of nitrogen-poor soils in the Northern Hemisphere is incidental, and their advantage lies in keeping the leaves for two or more growing seasons. In the last forty years, this interpretation of the value of longer leaf-life has become generally accepted for plants on nutrient-poor soils and in deep shade and, to some extent, those of dry sites [1,3,4,5,6,7]
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