Dry forests are among the most threatened ecosystems and have been extensively converted into grasslands, secondary forest, savanna or agricultural land. Knowledge of seed germination and seedling establishment is required for the success of efforts on restoration of these forests. This review focuses 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 variations within the tropical dry forest biome in soil moisture, light, temperature, nutrients and intensity of predation, significantly affect the seed and seedling traits of component species. The majority of dry tropical species possess orthodox seeds which are characterized by dormancy, while a few have recalcitrant seeds which possess little or no dormancy. Seed coat dormancy, which can be overcome by mechanical or acid scarification or sometimes by transit through animal guts, is most prevalent in the dry tropical forest species. Persistent species dominating the undisturbed portions of the forest have bigger seeds compared to those that mostly occur in disturbed regions and require shade for the survival of their seedlings. Shade demand is associated with drought endurance, and may be absolute in species such as Guettarda parviflora and Coccoloba microstachya, or facultative as in Plumeria alba and Bursera simaruba. The fluctuation in temperature significantly affects seed germination in several species of dry Afromontane forest trees of Ethiopia. Seedling mortality is primarily a function of moisture stress during the dry period. Adaptive responses of seedlings to drought stress include increased chlorophyll content, for example in Acacia catechu, and root biomass, as in several dry forest species (for example Drypetes parvifolia, Teclia verdoornia) of Ghana. Mulching, application of fertilizers, interplanting of leguminous species and mycorrhizal inoculation are useful tools for promoting seedling establishment in nutrient-poor dry tropical soils. Periodic forest fires, and predation affect recruitment and seedling development according to their intensity. Many species experiencing frequent fires have evolved thick seed coats, produce fire-hardy seedlings, or escape the effect by temporal separation of seed dispersal and fire events. Predation may result in abortion of fruits or may enhance germination and recruitment by scarification and dispersal, as in most species of the Guanacaste dry forest. Exposure to elevated CO 2 has increased relative growth rate, total leaf area and water use efficiency in most of the dry tropical seedlings 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 higher germination percentage, greater seedling survival and increased growth. Seeds originating from different provenances exhibit differences in germination and seedling growth (for example Prosopis cineraria, Albizia...

The impact of seed size on germination and seedling growth, as affected by water stress, was studied for five tree species from tropical dry forest of India, viz. Albizia procera, Acacia nilotica, Phyllanthus emblica, Terminalia arjuna and Terminalia chebula. Germination tests were conducted under five osmotic potential levels. Seedlings from large (LS) and small (SS) seeds were grown at four soil moisture levels. Observations were made on height, leaf area, biomass and other growth traits such as relative growth rate (RGR), net assimilation rate (NAR), specific leaf area (SLA), and root:shoot (R:S) ratio. Seeds of pioneer species and large seeds, within species, germinated earlier, and with increasing water stress, per cent germination and germination velocity declined. RGR was inversely related with drought tolerance. R:S ratio increased, RGR and SLA declined, but NAR increased with water stress. Notwithstanding successional status, the slow-growing species registered minimum reduction in biomass due to water stress. The response of LS and SS seedlings also differed for some of the growth variables. Increase in NAR could be a compensatory response to water stress, and the marked allocational plasticity could help maximize capture of the limited resource. Seedlings from smaller seeds, particularly of fast-growing species, would be able to cope with mild drought by morphogenetic and physiological plastic response in a better way than those from large seeds. However, seedlings from large seeds had greater survival than those from smaller seeds under intense water stress.

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We examined whether the responses of dry tropical tree seedlings to elevated nitrogen (N) inputs were associated with functional types, and whether the growth traits of seedlings emerging from seeds of different size within a species were differentially affected by increased N inputs. The study comprised five dry tropical tree species: Albizia procera (Roxb.) Benth, Acacia nilotica (L.) Delile, Phyllanthus emblica L., Terminalia arjuna (Roxb.) Beddome, and Terminalia chebula Retz. Of these, Albizia procera, Acacia nilotica, and P. emblica are pioneer species. The former two are N-fixing legumes. Terminalia arjuna and T. chebula are nonpioneer, nonleguminous species. Albizia procera, P. emblica, and T. arjuna are fast growing, while the remaining two are slow-growing species. Seedlings of these species from large and small seeds were grown at four N input levels (0, 30, 60, and 120 kg N·ha–1). Height and leaf area were measured periodically. At the end of the experiment (after 4 months) biomass and other growth traits, namely relative growth rate, net assimilation rate, specific leaf area, and root/shoot ratio, were determined. Foliar N and net CO2 assimilation rates were also determined. The species responded differentially (66%–282% increase in biomass) to elevated N supply, but the response was not associated with between-species seed size variation. However, within species, small-seed seedlings exhibited a greater response. The elevated N input resulted in a greater enhancement in relative growth rate of the slow-growing species. The species response did not follow functional types such as pioneer versus nonpioneer, legumes versus nonlegumes, and deciduous versus evergreen, but rather was individualistic.Key words: Albizia procera, Acacia nilotica, Phyllanthus emblica, Terminalia arjuna, Terminalia chebula, elevated N input.

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