Cladoptosis is the shedding of plant canopy units.
In many conifer 'leaves' – needles – are retained
for a period of years, then are usually shed as units consisting of the
needles, sheathing bracts, and associated short shoot. In Redwood (Sequoia sempervirens), leaf
surface area in the canopy is renewed by shedding of sheathing bracts, long shoots and associated
needles. The location and expression of
an abscission zone is such that often two or perhaps three year old leaves are
often shed, but sometimes abscission occurs after one year. One potential function regulation of cladoptosis
in Redwood could involve the value of the leaf area vs. the propensity for the
leaves to become colonized by lichens.
Lichen covered leaves presumably are less effective photosynthetically,
so their renewal and replacement would be a function of lichen interactions. The dynamics of this process suggests that abscission
might have a regulatory feedback controlled by leaf ‘worth’.
The photo shows two leaf segments of Redwood that were shed as a single
unit, the lower segment 2 yr old, with lichen colonization over about one-third of the two year old portion of the leaf unit, and a few new lichen'ets' beginning on the tip of the one year old portion of the leaf unit
