Agriculture Reference
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Gap
Understory
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Leaf age (months)
Fig. 8.1 Survivorship of Elateriospermum tapos (Euphorbiaceae) leaves on the forest floor and
in canopy gaps. (From Osada et al. 2003)
shade-tolerant tree Elateriospermum tapos was greater in the understory than in
canopy gaps (Osada et al. 2003; Fig. 8.1 ). Kai et al. (1991) reported similar
observations for the semideciduous shrub Ligustrum obtusifolium and then
experimentally confirmed the role of insolation in affecting leaf longevity. They
subjected cloned plants growing in a nursery to 7%, 20%, and 100% full sun; in
100% sunlight, almost all leaves were shed before mid-December, whereas in
the shaded plots some leaves remained until the next autumn. The evergreen
shrub Daphniphyllum macropodum normally retains leaves 4-5 years in the
understory of deciduous broad-leaved forests but only 2 years in canopy gaps; a
similar trend is observed in the low-growing evergreen Pachysandra terminalis
(Kikuzawa 1989). Finally, leaf survivorship in the evergreen shrub Rhododendron
maximum decreased for plants growing in the understory of more-open forests
from 5 years under an evergreen canopy, to 4 years under a deciduous canopy,
and only 3 years in canopy gaps (Nilsen 1986; Fig. 8.2 ). Because canopy gaps
arise suddenly, existing leaves on understory species can be subjected abruptly
to substantially greater insolation; understory species with fairly long-lived
leaves should be more tolerant of high insolation after gap formation than those
with relatively short-lived leaves (Lovelock et al. 1998). Lovelock et al. tested
this expectation by assessing the degree of photoinhibition in 12 tree species
from tropical rainforest, finding that species with long-lived leaves (more than
3.5 years) were more tolerant of abrupt increases in light than species with short-
lived leaves (less than 2 years).
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