Phenology

Examples of the necessity for appropriate timing of reproductive processes can be found in marginal areas from the poles to the tropics. The brief growing seasons of polar and alpine regions contain many examples of the necessity for prompt and precise phenological control in relation to flowering. Similarly, marginal areas in the tropics, such as lake and river margins, also provide examples where seedling establishment faces pheno-logical problems due to fluctuating environments.

4.4.1 Reproduction in flood-prone tropical lake and river margins

The annual flood pulses of the central Amazonian rivers inundate marginal forests to a depth of 10 m or more every year (Fig. 4.7). The regularity of this flooding has created a complex relationship between forest plants and the fish that migrate into the flooded forests and consume the seeds which are shed during the peak of the flooding season. Dispersal by floodwaters (hydrochory) and dispersal by fish (ichthyochory) are both possible means of seed dissemination. In a study of seed dispersal by the Amazonian catfish (Auchenipterichthys longimanus) digestion of the seeds was never found to be harmful for germinability and in many cases caused an improvement. Ichthyochory has been related to the striking ability of the vegetation of these flooded Amazonian lakes to recover even after extensive industrial habitat destruction. For example, in Amazonia, Lake Bata has suffered severe silting from the effluent of bauxite mine tailings being deposited in a layer more than 2 m deep over one third of the lake, destroying the lake vegetation and that of the neighbouring forest. However, despite this massive devastation, almost 80% of the former plant species recolonized the area 10 years after the impact of

Fig. 4.7 The understorey of a seasonally flood-tolerant Amazonian forest. These trees are flooded annually to depths of 4-6 metres for up to 6 months. The red arrow shows the flood-line mark on the foreground tree.

silting (Mannheimer et al., 2003). The arrival of species that were not in the surrounding forest suggests that the catfish, which is the most abundant opportunistic fruit eater in the region, may have played a significant role in the rapid re-establishment of the vegetation.

A constant risk for seeds that are shed into flood-waters is the high probability of not reaching a site where they can become established. Carapa guianensis is a hardwood tree from the Brazilian Amazon with large recalcitrant seeds that can germinate both in flood-free (terra firme) and flood-prone forests (varzea). This particular species fruits throughout the year. Consequently, some seeds fall onto water and are dispersed by the currents, while in the dry season others fall on the ground where they may either be dispersed by rodents or float away when the forest eventually floods. For the seeds that land in water, in addition to the risk of not finding a suitable landing site, there is always the probability that even if they germinate on dry ground only a few months later they may have to withstand the dangers of a full submergence for six months or more.

An experimental study of the effects of flotation on seed germination in this species has shown that floating can delay germination in some of the seeds whereas others germinate while they are floating. Seeds that germinate while floating develop both shoots and roots and are able to continue to float and maintain their reserves until they reaching a suitable landing site. The seeds that delayed germination while floating showed a decline in germinability which was particularly abrupt after 2-2.5 months of floating and was probably due to depletion of endosperm reserves. However, all the seeds that germinated did so without showing any injury as a result of a prolonged floating period. This seed polymorphism with regard to dormancy is a feature of this species which under natural conditions gives a degree of ecological plasticity that enables the species to adapt to different seasons and thus contribute to the ecological success and wide geographical range of the species (Scarano et al., 2003).

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