Although forest and savanna biomes predominate in tropics regions, the factors that control their distribution remain unclear. South American savannas occur in regions that are considered warm and humid enough to support forests, indicating that agents other than climate determine the occurrence of one or the other physiognomy. Herbivory, fire and water deficit have been considered environmental filters that limit the forest species encroachment in savanna physiognomies, but the effects of these filters on the capability of these species to recruit from seeds remain poorly understood. In this study we investigated how stress factors characteristic of savanna environments, such as soil desiccation, heat shocks and high temperatures affect the survival and germination of seeds from savanna and forest tree species. We found that desiccation (to 5%) reduced the germination percentage of forest seeds, but had no effect on the germination of savanna seeds. Forest seeds were less tolerant to heat shocks of 140°C and 200°C, and showed lower germination percentage at temperatures of 35 and 40°C, when compared with savanna seeds. Savanna seeds presented longer germination times and higher germination variance than forest seeds, indicating a risk-spreading germination strategy among savanna species. The low tolerance of forest seeds to desiccation, heat shock and high temperatures may explain the low recruitment of forest trees into savanna physiognomies. Climate change models predict lower soil moisture, higher temperatures and higher fires frequency for South America biomes. Our results suggest that savanna species are likely to be more capable of withstanding the effects of these changes than forest species. METHODS Study areaSeeds were collected at the IBGE (Brazilian Institute for Geography and Statistics) Ecological Reserve (15°55′-15°58′S, 47°52′-47°55′W) and at the adjacent experimental farm Fazenda Água Limpa (FAL) (15°56′-15°59′S, 47°55′-47°58′W), both located near Brasília, DF, Brazil, at 268 L. C. RIBEIRO AND F. BORGHETTI

Fire is considered an important factor in influencing the physiognomy, dynamics and composition of Neotropical savannas. Species of diverse physiognomies exhibit different responses to fire, such as population persistence and seed mortality, according to the fire frequency to which they are submitted. The aim of this study is to investigate the effects of heat shocks on seed germination of Anadenanthera macrocarpa (Benth.) Brenan, Dalbergia miscolobium Benth., Aristolochia galeata Mart. & Zucc., Kielmeyera coriacea (Spreng.) Mart. and Guazuma ulmifolia Lam., which are native species of the Brazilian savanna. The temperatures and exposure times to which the seeds were submitted were established according to data obtained in the field during a prescribed fire: 60 °C (10, 20 and 40 min), 80 °C (5, 10 and 20 min) and 100 °C (2, 5 and 10 min). Untreated seeds were used as controls. Seeds of A. galeata and K. coriacea showed high tolerance to most heat treatments, and seeds of A. macrocarpa showed a significant reduction in germination percentage after treatments of 80 °C and 100 °C. Treatments of 100 °C for 10 min reduced germination percentage for all species except G. ulmifolia, which has dormant seeds. For this species, germination was accelerated by heat treatments. The high temperatures applied did not interfere with the time to 50% germination (T(50) ) of the tolerant seeds. Seeds of the savanna species K. coriacea and A. galeata were more tolerant to heat shocks than seeds of the forest species A. macrocarpa. Guazuma ulmifolia, the forest species with seeds that germinate after heat shock, also occurs in savanna physiognomies. Overall, the high temperatures applied did not affect the germination rate of the tolerant seeds.

Nomenclature Lista de Esp ecies da Flora do BrasilAbstract Questions: Does seed mass influence the tolerance of seeds to the effects of heating in fires? Is the tolerance to heat shocks during fire events dependent mostly on seed mass itself or to other traits linked to the species ecological origin, e.g. non-fire-prone (forest) and fire-prone (savanna) environments?Location: Savanna and forest ecosystems of the Cerrado biome, Brazil.Methods: Heat shocks were applied to seeds of 17 selected common tree species in savanna (ten species) or forest ecosystems (seven species) of the Cerrado biome. After being submitted to a heat shock, seeds were allowed to germinate in chambers set at 30°C. Germination percentage was calculated for each treatment and species. For comparison purposes, species were grouped into two functional groups (forest and savanna) and also into seed size classes (small, medium and large). We used generalized linear model to analyse heat shock effects on seed germination of savanna and forest tree species, and how seed size affected the germination response.Results: Seed mass influenced seed tolerance to heat shock. Larger seeds had higher survival when subjected to heat shock, independent of species' ecological origin. In addition, seeds from species occurring in fire-prone environments (savanna) were more tolerant to high levels of heat shock than seeds from species occurring in non-fire-prone environments (forest). Conclusion:In response to global climatic change, fire regimes in savannas of South America are expected to become more frequent and intense, even expanding into currently non-fire-prone environments. In this sense, our findings indicate that species from savannas (fire-prone environments) might have an adaptive advantage to these consequences of climatic changes, potentially leading to expansion of the savanna ecosystems.

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x nos níveis de alguns estresses ambientais (aumento no déficit hídrico do solo, maiores temperaturas máximas e aumento na frequência de fogo) em diversas regiões onde o bioma Cerrado ocorre, os quais podem influenciar diretamente o processo germinativo. Considerando tais aspectos e de acordo com os resultados descritos para as três espécies deste estudo, mudanças ambientais serão, possivelmente, mais limitantes ao recrutamento de espécies, por sementes, às populações dos cerrados do Brasil Central.