Flooding and drought tolerance in seeds and seedlings of two Mora species segregated along a soil hydrological gradient in the tropical rain forest of Guyana.

Mora excelsa and M. gonggrijpii are well segregated along a soil hydrological gradient. M. excelsa is positively associated with soil hydromorphic characteristics such as gley, mottling and groundwater within 1.20 m, whereas M. gonggrijpii is negatively associated with these characteristics. Growth and mortality of artificially installed seedlings were studied in both species in occasionally flooded forest and dryer uphill forest. In a moderate year (1992, no pronounced flooding, no drought), there was no difference between the two species in growth or mortality in the two forest types. M. gonggrijpii was larger in both forest types. Flooding tolerance of seeds and seedlings were studied under controlled water regimes. Seeds of M. gonggrijpii appeared to be very intolerant to flooding, since germination in this species dropped to 50% after only 11 days of flooding. Seeds of M. excelsa floated and 80% of the seeds were viable after as much as 50 days of flooding. Artificially submerged seeds of the latter species had an intermediate survival response. Flooding in seedlings resulted in cessation of growth in both species. Mortality was nil in most treatments, but all M. gonggrijpii individuals died after a treatment of 8 weeks of continuous flooding. Drought tolerance of seedlings of M. excelsa and M. gonggrijpii was studied in a drying-out experiment. Seedlings of both species of approximately equal size differed widely in a number of characteristics: total leaf area, leaf dry weight, leaf thickness, leaf size and leaf area ratio (LAR) were all larger in M. gonggrijpii, while stomatal density and specific leaf area (SLA) were smaller in this species. Seedlings did not differ in stem hydraulic conductivity. M. excelsa showed lower osmotic potential at full hydration. Turgor potential loss points were not nearly approached in the forest during the middle of the dry season in either species. M. gonggrijpii had much lower stomatal conductance than M. excelsa, due to lower stomatal density. Boundary layer conductance was of the same magnitude as stomatal conductance, especially in the morning. In a drying-out experiment, total plant transpiration was higher in M. gonggrijpii, as the lower conductance observed in this species was compensated for by its larger leaf area. M. gonggrijpii was able to extract water from dryer soils than M. excelsa and may be able to utilize its higher leaf water content under moderate drought in the forest understorey.