Theories of Leaf Longevity

Often, plants in resource-limited habitats retain leaves for longer to compensate for low photosynthetic rates (Small, 1972; Moore, 1980; Kikuzawa, 1995b). Hence, there is a tradeoff between high photosynthetic rate and long leaf longevity (Reich et al., 1992). Williams et al. (1989) considered that the ratio of daily carbon gain to leaf construction costs must be related to leaf longevity. This idea was supported by the comparison of leaf longevities of several tree species (Sobrado, 1991). Moreover, the photosynthetic rate of a leaf decreases with increase in time by nutrient reallocation or by leaf aging. Thus, the decline in leaf photosynthetic rate with increase in time must be incorporated into the model. Carbon gain by a leaf is given as:

where p(t) is the photosynthetic rate and is a decreasing function of leaf age t. The optimum leaf longevity (t*) to maximize gain per unit time (G/t) is obtained by the following equation (Kikuzawa, 1991).

where, a is the maximum photosynthetic rate and is measured as the actual photosynthetic rate (carbon ■ leaf area-1 time-1) at the time of the highest rate. The rate a/b is the decreasing rate of photosynthesis with time (carbon ■ leaf area-1 time-2), b being the intercept of the x axis or potential leaf longevity (time). C is the construction cost of a leaf (glucose ■ leaf area-1 ).

From equation (2), it is supposed that higher photosynthetic rate (a), lower construction cost (C) and shorter potential leaf longevity (b) will shorten leaf longevity, while the reverse will promote leaf longevity. Increased nutrient supply will promote increased photosynthetic rate and thus decrease leaf longevity. Experimental addition of nutrients or water shortened leaf longevity (Shaver, 1981; Lajtha and Whitford, 1989; Cordel et al., 2001). Greater leaf mass per unit leaf area (LMA; the inverse of SLA) is closely related to parameter C (Kikuzawa, 1995b) and positively correlated with leaf longevity (Wright and Westoby, 2002). Actual leaf longevity correlated to potential leaf longevity (Kitajima et al., 1997; Ackerly, 1999). Reich et al. (1991, 1992) have summarized some empirical relationships for some traits in trees or in leaves with leaf longevity.

Two patterns of shoot elongation of trees were found in a tropical rain forest: evergrowing and intermittent (Koriba, 1948, 1958). Similarly, two leaf emergence patterns were known in deciduous broad-leaved trees—leaves appearing successively, one after another, or leaves appearing simultaneously, all at once (Kikuzawa, 1983, 1995a)—as well as in tropical forests (Lowman, 1992). In temperate forests, successive leafing trees are usually found in open habitats such as flood plains and large gaps where light, water and nutrients are available (Kikuzawa, 1988, 1995a), while simultaneous flush trees are

usually found in forest canopies and the forest understorey. Linkages were found among high photosynthetic rate, short leaf longevity and successive leafing of pioneer trees and among low photosynthetic rate, long leaf longevity and simultaneous leafing (Kikuzawa, 1988, 1995a; Koike, 1988). The autumnal leaf coloring pattern is also linked with the leafing pattern. Successive leafing trees first turned color in the lower branches, while simultaneous-leafing trees started in the peripheral zone of a crown (Koike, 1990).

Plants with several leaf cohorts, which appear at different times, can differently utilize the light resource, which fluctuates during the growing season. The winter green shrub, Daphne kamtchatica var. jezoensis, in a deciduous broad-leaved forest leafs out in autumn as well as in spring (Kikuzawa, 1984; Lei and Koike, 1998), both are timed to appear when the canopy trees are leafless (Seiwa and Kikuzawa, 1996). Similarly, an evergreen herbaceous species, Heuchera americana, produces leaves twice in a year; in spring and in autumn (Skillman et al., 1996). The cotton sedge Eriophorum vaginatum can make use of opportunities for early- and late-season photosynthesis by successive leafing (Defoliart etal., 1988). This species produced leaves successively, and nutrients are redistributed from older leaves to actively growing leaves. This efficient use of resources may be important in enabling this species to dominate nutrient-poor sites (Jonasson and Chapin, 1985).

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