Ellison, A. M. and E. J. Farnsworth. 1996. Spatial and temporal variability in growth of Rhizophora mangle saplings on coral cays: links with variation in insolation, herbivory, and local sedimentation rate. Journal of Ecology 84: 717-731.
1 We used demographic growth analysis to quantify seasonal
and annual patterns of shoot and root module production by Rhizophora
mangle saplings growing on three coral cays in Belize, Central
America. We investigated scaling relationships among root and
shoot modules, leaf life-span, effects of herbivores on module
and whole plant growth, and differences in growth under different
sedimentation regimes.
2 Production of new shoots and aerial roots occurred seasonally.
Annual peaks in solar insolation occurred in May; relative rates
of change in numbers of shoot meristems and leaves, and stem length
peaked one month following. Relative rate of change in numbers
of aerial roots peaked one month following this shoot flush, and
roots elongated primarily during the dry season.
3 Increased water depth was positively correlated with the ratio
of root length to shoot length in saplings. Mean shoot growth
rate was significantly lower at cays exhibiting relatively low
sedimentation rates, as well as at similar locales within cays.
4 Average leaf life-span was 9 months. During an outbreak of
the mangrove skipper Phocides pigmalion, insect herbivores shortened
leaf life-span by increasing leaf abscission rate. Insect folivores
reduced above-ground net primary production available for export
to adjacent marine ecosystems by 5-20%. Up to seven-fold increases
in percentage of roots bored by isopods occurred concomitantly
with annual peaks in new root production. Relative elongation
rate of roots decreased five-fold following isopod attack. However,
whilst both insects and isopods tracked production of and consumed
new modules, neither consumer contributed significantly to variance
in whole-plant growth.
5 Demographic growth analysis is a powerful tool with which to
predict dynamic responses of module production and whole-plant
growth in response to local environmental conditions. Our analyses
illustrate that growth of mangroves are sensitive to seasonal
patterns of insolation, to decreasing sedimentation and to increasing
water depth. Given that growth of mangrove saplings on coral
cays declines significantly with sedimentation rate, persistence
of these forests is unlikely if sea level in the Caribbean increases
as predicted.