Unveiling the hidden reserves: allocation strategies associated with underground organs of Cerrado legumes in fire-prone savannas.

The synthesis and differential allocation of reserve compounds is an important adaptive mechanism that enables species to resprout in fire-prone ecosystems. The analysis of compound allocation dynamics (differential accumulation of compounds between plant organs) provides insights into plant responses to disturbances. The aim was to quantify reserves in eight legume species from Cerrado open savannas with high fire frequency in order to investigate the patterns of allocation and distribution of compounds between leaves and underground organs, drawing ecophysiological inferences. The species were collected in 'campo sujo' areas of the Cerrado. Leaves and underground organs (xylopodium, taproot tubers) were subjected to physiological analyses. Overall, underground organs were characterised by greater deposits of carbohydrates, mainly soluble sugars, and also with the accumulation of proteins and amino acids. This suggests that nitrogen reserves, as well as carbohydrates, may have an ecophysiological function in response to fire, being allocated to the underground organs. Phenols were mainly evident in leaves, but a morphophysiological pattern was identified, where the two species with taproot tubers tended to concentrate more phenols in the underground portion compared to species with xylopodium, possibly due to functional differences between these organs. Such data allow inferring relevant ecophysiological dynamics in legumes from open savannas.

Fire and legume germination in a tropical savanna: ecological and historical factors.

BACKGROUND AND AIMS: In many flammable ecosystems, physically dormant seeds show dormancy-break patterns tied to fire, but the link between heat shock and germination in the tropical savannas of Africa and South America remains controversial. Seed heat tolerance is important, preventing seed mortality during fire passage, and is usually predicted by seed traits. This study investigated the role of fire frequency (ecological effects) and seed traits through phylogenetic comparison (historical effects), in determining post-fire germination and seed mortality in legume species of the Cerrado, a tropical savanna-forest mosaic. METHODS: Seeds of 46 legume species were collected from three vegetation types (grassy savannas, woody savannas and forests) with different fire frequencies. Heat shock experiments (100 °C for 1 min; 100 °C for 3 min; 200 °C for 1 min) were then performed, followed by germination and seed viability tests. Principal component analysis, generalized linear mixed models and phylogenetic comparisons were used in data analyses. KEY RESULTS: Heat shocks had little effect on germination, but seed mortality was variable across treatments and species. Seed mortality was lowest under the 100 °C 1 min treatment, and significantly higher under 100 °C 3 min and 200 °C 1 min; larger seed mass decreased seed mortality, especially at 200 °C. Tree species in Detarioideae had the largest seeds and were unaffected by heat. Small-seeded species (mostly shrubs from grassy savannas) were relatively sensitive to the hottest treatment. Nevertheless, the presence of physical dormancy helped to avoid seed mortality in small-seeded species under the hottest treatment. CONCLUSIONS: Physical dormancy-break is not tied to fire in the Cerrado mosaic. Heat tolerance appears in both forest and savanna species and is predicted by seed traits (seed mass and physical dormancy), which might have helped forest lineages to colonize the savannas. The results show seed fire responses are better explained by historical than ecological factors in the Cerrado, contrasting with different fire-prone ecosystems throughout the world.

Seed germination of a South American forest tree described by linear thermal time models.

Thermal time models may describe and compare seed germination, providing information useful to explain species distribution. However, the capacity of such threshold models to describe germination of tropical native species has been less studied. We evaluated seed germination of a legume tree species (Peltophorum dubium), typical from South American seasonal forests, as described by two linear thermal time models: probit model and graphic model. Seeds were provided from four different provenances in a latitudinal gradient in Brazil, and their physical dormancy mechanically released before the trials. Graphic model and probit regression were used to determine thermal parameters (cardinal temperatures and thermal time requirement) on sub- and supra-optimal ranges for the different seed provenances. Germination rate mainly followed linear relationship with temperature, and regression lines of different germination fractions converged base temperature in the x-axis. Therefore, probit model assumed a single-value of base temperature in the sub-optimal range and a normal distribution of thermal time. Base temperature tended to be higher in the Northern provenance, Porto Velho, showing slower germination under cooler temperatures. Supra-optimal temperatures have shown similar thermal time requirements and different values for ceiling temperature, according to model predictions. No clear patterns have been found between seed provenances and thermal time requirement. Both probit and graphic models have provided reasonable predictions of germination times (t10 and t50), except under the coolest temperatures. Probit regression always described at least 70% of seed germination. Thermal time assumptions, linear models applicability and their limitations are discussed.