The effect of water-level fluctuations on swamp forest colonization by seedlings of Tabebuia cassinoides DC. (Bignoniaceae)

Establishment of Tabebuia cassinoides seedlings is related to water-level fluctuations in southeastern Brazil swamp forests. Nine years of annual monitoring of 48 individuals established during a drought in November 1997, when the swamp was unflooded, suggested that their establishment depends on this unpredictable event. This conclusion is further sustained by the wide variability of the seedling cohort size structure, and the fascicular root conformation that holds the shoots erect.

Germination and seedling establishment in wetlands are poorly understood [1], and in most cases involve adaptations to survive oxygen deprivation [2,3].Therefore, flooding is the main cause of low frequency of germination [4].Seed buoyancy and long-term dormancy are often listed as important traits that allow seeds to survive and/or reach safe sites [5,6] that are crucial for their development.
Even understory forest plants can serve as safe sites.For instance, the Brazilian swamp forest tree Tabebuia cassinoides DC. (Bignoniaceae) germinates inside the tanks of the understory bromeliad Nidularium procerum Lindm [7].However, not all swamps in southeastern Brazil have understory bromeliads, and young T. cassinoides seedlings are flood-intolerant.Therefore, although asexual reproduction has been reported as an important characteristic of this species [7], other seed-establishment strategies must operate, to account for the high abundance of this species in most Brazilian swamps [8], even if we consider that only a small fraction of the seeds produced will survive to maturity [9].
Although this deciduous tree flowered and set seeds every year (between August and November), no new seedlings became established at this site.This could be due to intolerance of the seeds or seedlings to permanent flooding, or to transport of seeds to other sites by water.Therefore, low water levels may favor species germination, as shown in Chinese wetlands [10].
Height and basal diameter of cohort members showed wide variation from November 2000 to December 2006 (Table 1), and diameter was always closely correlated to height (r 2 > 0.89; Table 1).The cohort density stability indicated no density-dependent or size-dependent mortality patterns.
The fascicular root morphology of the saplings seems to hold the shoots erect, which might be important for fixation or for maintenance of a larger flood-free surface area of the shoots, as is important for internal aeration [11].Therefore, the onset of root fixation in unflooded soil conditions, allowing substrate trapping, may be cru- cial to survival in the subsequent deep flooding, as suggested for Carapa guianensis Aubl. in Amazonian varzea forests [12].In some restored wetlands, low water levels favored the establishment of T. cassinoides [13].
Other local swamp tree species, such as the Clusiaceae Symphonia globulifera L.f. and Calophylum brasiliense Cambess., that also have seeds that float on the water, differently from T. cassinoides, germinate forming pivotal root structures that after fixation keep their shoots static and erect.These root patterns may restrict the ability of these species to colonize permanently and deeply flooded sites.Indeed, C. brasiliense occurs in Picinguaba wetlands, but mainly in shallow swamps (or periodically flooded sites).This pattern was previously observed at the National Reserve of Poço das Antas [7].
Our observations suggest that the seedling colonization pattern of Tabebuia cassinoides is dependent on synchronicity between annual seed production and an apparently stochastic water-level fluctuation.Thus, survival chances in the swamp may also be related to root morphological patterns and to success in reaching a safe site during a drought period.

Table 1 .
Summary of structural parameters of a Tabebuia cassinoides DC. (Bignoniaceae) cohort established in 1997 at Picinguaba, São Paulo, Brazil during a severe drought.