Ecology of seed and seedling growth for conservation and restoration of tropical dry forest : a review

Dry forests are among the most threatened ecosystems and have been extensiv ely converted into grasslands, secondary forest, savanna or agricultural la nd. Knowledge of seed germination and seedling establishment is required fo r the success of efforts on restoration of these forests. This review focus es on the ecological requirements at seed and seedling stages, and collates the current knowledge of seed viability, dormancy, germination pattern and seedling behaviour of dry tropical tree species. The spatio-temporal varia tions within the tropical dry forest biome in soil moisture, light, tempera ture, nutrients and intensity of predation, significantly affect the seed a nd seedling traits of component species. The majority of dry tropical speci es possess orthodox seeds which are characterized by dormancy, while a few have recalcitrant seeds which possess little or no dormancy. Seed coat dorm ancy, which can be overcome by mechanical or acid scarification or sometime s by transit through animal guts, is most prevalent in the dry tropical for est species. Persistent species dominating the undisturbed portions of the forest have bigger seeds compared to those that mostly occur in disturbed r egions and require shade for the survival of their seedlings. Shade demand is associated with drought endurance, and may be absolute in species such a s Guettarda parviflora and Coccoloba microstachya, or facultative as in Plu meria alba and Bursera simaruba. The fluctuation in temperature significant ly affects seed germination in several species of dry Afromontane forest tr ees of Ethiopia. Seedling mortality is primarily a function of moisture str ess during the dry period. Adaptive responses of seedlings to drought stres s include increased chlorophyll content, for example in Acacia catechu, and root biomass, as in several dry forest species (for example Drypetes parvi folia, Teclia verdoornia) of Ghana. Mulching, application of fertilizers, i nterplanting of leguminous species and mycorrhizal inoculation are useful t ools for promoting seedling establishment in nutrient-poor dry tropical soi ls. Periodic forest fires, and predation affect recruitment and seedling de velopment according to their intensity. Many species experiencing frequent fires have evolved thick seed coats, produce fire-hardy seedling, or escape the effect by temporal separation of seed dispersal and fire events. Preda tion may result in abortion of fruits or may enhance germination and recrui tment by scarification and dispersal, as in most species of the Guanacaste dry forest. Exposure to elevated CO2 has increased relative growth rate, to tal leaf area and water use efficiency in most of the dry tropical seedling tested, but the magnitude of the effect has varied markedly among species. Due to the availability of a large source of energy, large seeds show high er germination percentage, greatet seddling survival and increased growth. Seeds originating from different provenances exhibit differences in germina tion and seedling growth (for example Prosopis cineraria, Albizia lebbeck, Eucalyptus camaldulensis and Acacia mangium), efficiency of nodulation (for example Acacia nilotica, A. auriculiformis), and stress resistance (for ex ample Populus deltoides, Dalbergia sissoo). The review points out the need for coordinated, long-term, field-based studies for identification of multi ple cues and niches for germination, on seed and seedling dynamics in respo nse to fire, and on within-species genetic variability for selection of sui table provenances. Field-based studies at species and community levels are also needed to perm it manipulations of biotic components to augment the recruitment of desired species and to suppress that of undesirable species.