Abstract:
Dwarf mangroves on peat substrate growing
in eastern Puerto Rico (Los Machos, Ceiba State
Forest) were analyzed for element concentration, leaf
sap osmolality, and isotopic signatures of C and N in
leaves and substrate. Mangrove communities behind
the fringe presented poor structural development with
maximum height below 1.5 m, lacked a main stem,
and produced horizontal stems from which rhizophores
developed. This growth form departs from
other dwarf mangrove sites in Belize, Panama, and
Florida. The dwarf mangroves were not stressed by
salinity but by the low P availability reflected in low
P concentrations in adult and senescent leaves. Low P
availability was associated with reduced remobilization
of N and accumulation of K in senescent leaves,
contrasting with the behavior of this cation in
terrestrial plants. Remobilization of N and P before
leaf abscission on a weight basis indicated complete
resorption of these nutrients. On an area basis,
resorption was complete for P but not for N. Sulfur
accumulated markedly with leaf age, reaching values
up to 400%, compared with relatively modest accumulation
of Na (40%) in the same leaves. This
suggests a more effective rejection of Na than sulfate
at the root level. Dwarf mangrove leaves had more
positive d13C values, which were not related to
salinity, but possibly to drought during the dry season
due to reduced flooding, and/or reduced hydraulic
conductance under P limitation. Negative leaf d15N
values were associated with low leaf P concentrations.
Comparison with other R. mangle communities
showed that P concentration in adult leaves below
13 mmol kg-1 is associated with negative d15N
values, whereas leaves with P concentrations above
30 mmol kg-1 in non-polluted environments had
positive d15N values.