Cladoptosis in Redwood (Sequoia sempervirens)

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

Rare Plant Density by County in California

A total of 2260 native plants are listed as rare in California.  On a density basis, the mean density is 0.33 plants/km² (based on October 2011 CNPS Inventory records) for the 406,388 km².  The inequality on a density basis is evident in the map:  the counties with the highest density statewide are in the San Francisco Bay region, generally.    Most of the northern counties of the state have a density that is less than the mean density, while the larger area counties of southern California have a density that is <1/4 of the mean (that is, below 0.16).  Another hot-spot are the southern three coastal counties. 

Of course absolute numbers for many low density counties are high: Inyo County supports 275 rare plants and San Bernardino County 374, yet on a density basis these jurisdictions are very low.  Kings County supports only 22 plants and is also low density.