Temporal variation in floral and vegetative development in Dirca occidentalis, a rare endemic shrub

Background/Question/Methods Dirca occidentalis is a rare, deciduous shrub endemic in patchy populations near the San Francisco Bay.  The species is considered winter-blooming, which raises questions about mechanisms of pollination and other aspects of its reproductive biology.  To provide a baseline of information relevant to our broad goal of understanding how D. occidentalis is adapted to reproduce sexually during winter, we chronicled overt stages of floral development and assessed temporal variation in flowering and subsequent seasonal vegetative growth of 27 individuals in diverse habitats at Jasper Ridge Biological Preserve during the winter of 2007-2008.  Floral development began in late October with the emergence of pistils from buds that became swollen after autumnal rains.  Stigma surfaces appeared receptive both before and after anthers of the same inflorescence released pollen, a finding that is consistent with previously reported observations that suggest D. occidentalis can form seeds after both cross- and self-pollination.

Results/Conclusions On day 8 of winter (December 29), five of the 27 monitored plants were at or beyond anthesis, but bud swell of 12 other plants had not begun.  Such temporal variation was evident both within and among habitats and continued through day 64 of winter (February 23), when eight of the 27 plants remained preanthesis.  Flowering was staggered on plants that began to bloom comparatively early.  Such plants developed new flowers throughout winter, whereas flowering was more synchronous among plants with late incipient flowers.  Vegetative growth had not begun on two of the 27 plants by day 64 of winter, while among other plants, mean elongation of new leaves and stems was as great as 74 mm and 102 mm, respectively.  Our data reveal that the timing of flowering and the initiation of seasonal shoot growth of this endemic shrub are highly variable within and among the diverse habitats in which the species occurs.  Such temporal variation likely is important to the capacity for D. occidentalis to persist despite the likelihood for limited pollinator activity and for episodes of unfavorably low temperatures during portions of long period from late autumn through early spring when plants may be in bloom.