stand): many of the tropical dry forest species were found in the old stands as well
as tropical dry forests from close-by dunes (Castillo-Campos and Medina-Abreo
2005; Castillo-Campos and Travieso-Bello 2006). However, Casuarina stands on
foredunes were species-poor, although the sizes of these trees were similar to the
mature and old Casuarina stands on hydromorphic soils. Foredunes were between
2.5 and 3.5 m tall and soils were not hydromorphic.
Our results show that only under conditions of high moisture in the soil such as
those occurring in the hydromorphic sand of the Port of Veracruz, can native
species colonize the understory of Casuarina stands. These species are also found
in nearby tropical dry forests growing on sand dunes. Thus, under these circumstances, removal of the exotic trees would probably accelerate the natural recovery
of native species since they were already growing beneath the trees. It is also
important to bear in mind that although recovery of natural vegetation is possible
in the circumstances mentioned above, native species should preferably be promoted as the species for plantations because the risk of an aggressive exotic is
always latent and potentially very damaging to biodiversity and the natural
functioning of ecosystems. Native species can probably only grow beneath
Casuarina trees under specific circumstances (hydromorphic soils).
Our studies concur with those of Parrotta (1995), who confirmed that in a
degraded tropical coastal dune in Puerto Rico, up to 19 native forest species were
established beneath a canopy of three exotic species: C. equisetifolia, Eucalyptus
robusta, and Leucaena leucocephala. In our case, however, it was interesting that
in the Casuarina stands growing on relatively drier foredunes and nonhydromorphic soils, we did not find any tropical dry forest species growing beneath the
canopy of Casuarina. In fact, we observed significant differences between
foredune and hydromorphic stands, both in growth forms and species richness and
diversity.
We also found species that are common to coastal wetlands (Hydrocotyle bonariensis, S. diphyllum, Syngonium podophyllum). This provides further evidence
of a humid substrate owing to the closeness of the water table, which is also shown
by the hydromorphism of the soils analyzed. In the older Casuarina stands, the
improved organic matter conditions of the substrate were enhanced by the
hydromorphic attributes of the soil that tend to reduce mineralization rates of the
organic residues.
Seed dispersal syndromes also changed in the different stands. In all of them we
found species dispersed by wind and animals, although the number of species in
each one differed. The relative importance of zoochorous species during colonization by native species in Casuarina plantations in degraded coastal dune vegetation was also found by Parrotta (1995) in Costa Rica. Here, and in coincidence
with our results, the plantations that were introduced are probably acting as bird
perching sites and seeds are constantly being introduced into the plots.
In summary, we observed that tropical dry forest species may colonize the
understory beneath Casuarina stands under particular circumstances:
230
P. Moreno-Casasola et al.
as tropical dry forests from close-by dunes (Castillo-Campos and Medina-Abreo
2005; Castillo-Campos and Travieso-Bello 2006). However, Casuarina stands on
foredunes were species-poor, although the sizes of these trees were similar to the
mature and old Casuarina stands on hydromorphic soils. Foredunes were between
2.5 and 3.5 m tall and soils were not hydromorphic.
Our results show that only under conditions of high moisture in the soil such as
those occurring in the hydromorphic sand of the Port of Veracruz, can native
species colonize the understory of Casuarina stands. These species are also found
in nearby tropical dry forests growing on sand dunes. Thus, under these circumstances, removal of the exotic trees would probably accelerate the natural recovery
of native species since they were already growing beneath the trees. It is also
important to bear in mind that although recovery of natural vegetation is possible
in the circumstances mentioned above, native species should preferably be promoted as the species for plantations because the risk of an aggressive exotic is
always latent and potentially very damaging to biodiversity and the natural
functioning of ecosystems. Native species can probably only grow beneath
Casuarina trees under specific circumstances (hydromorphic soils).
Our studies concur with those of Parrotta (1995), who confirmed that in a
degraded tropical coastal dune in Puerto Rico, up to 19 native forest species were
established beneath a canopy of three exotic species: C. equisetifolia, Eucalyptus
robusta, and Leucaena leucocephala. In our case, however, it was interesting that
in the Casuarina stands growing on relatively drier foredunes and nonhydromorphic soils, we did not find any tropical dry forest species growing beneath the
canopy of Casuarina. In fact, we observed significant differences between
foredune and hydromorphic stands, both in growth forms and species richness and
diversity.
We also found species that are common to coastal wetlands (Hydrocotyle bonariensis, S. diphyllum, Syngonium podophyllum). This provides further evidence
of a humid substrate owing to the closeness of the water table, which is also shown
by the hydromorphism of the soils analyzed. In the older Casuarina stands, the
improved organic matter conditions of the substrate were enhanced by the
hydromorphic attributes of the soil that tend to reduce mineralization rates of the
organic residues.
Seed dispersal syndromes also changed in the different stands. In all of them we
found species dispersed by wind and animals, although the number of species in
each one differed. The relative importance of zoochorous species during colonization by native species in Casuarina plantations in degraded coastal dune vegetation was also found by Parrotta (1995) in Costa Rica. Here, and in coincidence
with our results, the plantations that were introduced are probably acting as bird
perching sites and seeds are constantly being introduced into the plots.
In summary, we observed that tropical dry forest species may colonize the
understory beneath Casuarina stands under particular circumstances:
230
P. Moreno-Casasola et al.