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sapling were shed earlier than sunlit leaves in the upper canopy. Takenaka (2000)
observed individual Cinnamomum japonicum growing at more than 10%, 5% to
10%, and less than 5% full sunlight in the understory of evergreen broad-leaved
forest. Each tree had some shoots in each of the three insolation classes. Takenaka
(2000) compared leaf longevity on shoots in more-shaded positions of better inso-
lated individuals, and vice versa. He found that the better insolated were individu-
als, the stronger was the contrast in shoot growth and leaf turnover between their
well- and poorly insolated shoots. Leaves on poorly insolated shoots were shed
more rapidly than on more-sunlit shoots. This situation in which faster-growing
shoots inhibit slower-growing ones is a form of apical control referred to as correla-
tive inhibition (Cline 1997; Umeki and Seino 2003). If this more rapid shedding of
shaded leaves within individual plants is simply the direct consequence of the shad-
ing rather than apical control (Cline 1997), there should be a correlation between
leaf longevity and plant size in a dense plant population. That is not the case. There
is no significant correlation between mean leaf longevity and individual plant size
and hence shading in dense plantings of Xanthium canadense ; mean leaf longevity
ranged from 20 to 50 days irrespective of plant size (Hikosaka and Hirose 2001).
In summary, individual plants shorten leaf longevity on poorly insolated shoots
when only part of the plant is shaded, but not when the entire plant is shaded.
There is evidence, however, that in more mature trees the relationship between
leaf longevity and insolation reverts to the norm. Mizobuchi (1989) reported that in
large, open-grown Cinnamomum camphora growing on a university campus in
central Japan, leaves on the better insolated southern side of the canopy had a half-
life of about 1 year compared to almost 2 years on the north side. Osada et al. (2001)
studied leaf longevity over more than 3 years at different heights in Dipterocarpus
sublamellatus , Elateriospermum tapos , and Xanthophyllum stipitatum - trees all
more than 30 m tall growing in a mature tropical rainforest. They found that leaf
longevity consistently is shortest in the sunlit upper canopy of individual trees.
Similar results were obtained for 15 tree species in a tropical forest that differ in
maximum height (Meinzer 2003), suggesting that tree maturity rather than just tree
height determines the pattern of leaf longevity with the tree canopy. Miyaji et al.
(1997) studied leaf longevity in 3-m-tall cacao trees ( Theobroma cacao ) growing
under shelter trees in a tropical plantation. Leaf longevity changed depending on the
timing of leaf emergence and level in the canopy (Fig. 7.5 ). Longevity of upper
leaves ranged from 120 to 200 days, the middle layer from 180 to 250 days, and the
lower layer from 280 to 370 days; leaf longevity of bearing-age cacao trees was
longer in the more-shaded, lower canopy. There may be a size-dependent shift in the
degree of branch autonomy such that in the transition from saplings to trees a greater
degree of branch autonomy ensues as apical control shifts from the sapling apex to
individual branches in the tree crown. In this vein, we can rephrase our overall sum-
mary of the relationship between insolation and leaf longevity. When the autono-
mous unit organizing shoot growth is wholly shaded (an individual plant or major
branch), then leaf longevity becomes longer; conversely, when a shoot is only a
poorly insolated part of a larger autonomous unit, then its leaf longevity is shortened
relative to the sunlit part of the autonomous system controlling shoot growth.
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