Thermal optima for seed germination of 30 tree species from Central Amazonia: a comparison of approaches and a new proposal

This study compared the seed germination of 30 tree species from Central Amazonia using a wide temperature range (5‑40°C with 12 hours of light daily). Seeds were submitted to pre-germination treatments, whenever necessary, sown on vermiculite or germitest paper, and normal seedling development was assessed until stabilisation. With the same data set, seeds’ optimal germination temperature (T opt ) was determined by comparing the following approaches: (i) 2-step, the temperature with the highest germination and the shortest time to seed population reach 50% of germination was chosen; (ii) the highest Germination Speed Index (GSI); (iii) using the formula developed by Olff and collaborators (Olff’s approach); (iv) three species were evaluated with Covell’s approach; and (v) new formulas for T opt calculation are proposed, based on germination success and five different variables of germination rate (Cipriani’s approach). Comparing the results, T opt differed between 0.6-10.0°C. GSI and 2-step indicated generally higher values than Olff’s formula. The minimum and maximum T opt differences between Cipriani’s approaches were between 0.1-2.2°C. Our study showed that T opt should be determined for each species as, even the 30 Amazonian tree species of the same habitat, T opt ranged between 22.5 and 32.5°C.