Linking Seed Size and Thermal Tolerance in Seed Germination of Hymenaea stigonocarpa, a Fire-Prone Neotropical Savanna Tree

The Neotropical Savanna (Cerrado) is a fire-prone biome characterized by seasonal climate, nutrient-poor soils, and variable fire regimes. While fire-induced germination responses are well documented in Cerrado plants, the role of seed size in mediating thermal tolerance remains poorly understood. Here, we investigate how seed size and fire-related heat treatments influence germination in Hymenaea stigonocarpa, a keystone Cerrado tree species. Specifically, we test the predictions that (i) low to moderate fire temperatures (<270 °C) do not impair seed germination and (ii) larger seeds exhibit greater heat tolerance than smaller seeds. We exposed 360 seeds from 30 individual trees to five heat-shock treatments (27, 100, 150, 200, and 270 °C) simulating fire intensities typically experienced in the Cerrado. Our results show that H. stigonocarpa produces relatively large seeds with an average germination rate of approximately 42%. The average time required for germination was 12.18 ± 0.43 (average ± standard error) days. The time required for seed germination varied significantly as a function of heat-shock treatment and seed mass, with seeds exposed to the highest temperature (270 °C) taking longer to germinate. Moreover, seed mass had a positive effect on the time required for seed germination. The germination percentage remains stable across heat treatments and seed sizes, indicating that H. stigonocarpa seeds exhibit characteristics typical of heat-tolerant species rather than those of heat-stimulated species. Our study showed that H. stigonocarpa trees produce large seeds that germinate quickly and are tolerant to moderate temperatures. These seed traits play a crucial role in the reproductive success of individual plants in fire-prone, nutrient-poor, and water-limited ecosystems. Furthermore, our results offer important guidance by emphasizing the role of seed size in effective restoration initiatives.