Constraints on tree seedling establishment after fires: passing the germination bottlenecks.

Persistence and colonization by tree species in an environment following a fire depends on the effects on seed germination and seedling development. We used seeds of Kielmeyera coriacea and Qualea parviflora as a model to test the effects of high temperatures on germination and initial development of tree seedlings. We exposed the seeds to heat flow (70, 100, 130, 150 or 170 °C) for 2 or 5 min and compared the germination with that of unheated seeds (control). Seedlings were then harvested after 3, 7 or 15 days to evaluate aerial and root mass, root:shoot ratio, presence of cotyledon opening, true leaves, and secondary roots. We found no effect on germination for seeds exposed to temperatures ≥150 °C. However, germination was significantly reduced for seeds exposed to 100 °C for both 2 and 5 min. The mass of 15-day-old K. coriacea seedlings was smaller when seeds were heated at 70 °C for 5 min or at temperatures higher or equal to 100 °C. Qualea parviflora seedlings did not show any difference in mass, but there were marginal differences in the presence of roots and the opening of cotyledons. Kielmeyera coriacea seedlings allocated biomass faster than Q. parviflora. High temperatures affect both quantity and quality of germinable seeds, as well as biomass allocation during initial seedling development. These factors may explain the decrease in seedlings observed after fire, suggesting a bottleneck effect that influences population dynamics and species persistence in systems with frequent fires.

Frequent fires alter tree architecture and impair reproduction of a common fire-tolerant savanna tree.

Several Cerrado tree species have traits and structures that protect from fires. The effectiveness of a trait depends on the fire regime, especially the frequency. We used Vochysia elliptica, a common Cerrado tree, as a model to test whether different fire frequencies alter crown architecture and flower, fruit and seed production. We analysed the effect of fire on the production of inflorescences, fruits and seeds, as well as seed germination and tree architecture of 20 trees in each of three plots of a long-term ecological experiment managed with different fire regimes: burned every 2 years (B), burned every 4 years (Q) in mid-dry season and an area protected from fire (C). We found a large negative effect of fire frequency on crown architecture and on flower and fruit production. Trees in C and Q had significantly more main branches and a larger crown area than trees in B. At its peak, a tree in C was expected to produce 2.4 times more inflorescences than Q, and 15.5 times more than B, with similar magnitudes for fruits. Sixty per cent of trees in B and 10% in Q produced no fruits. The differences in architecture might explain the reduction in sexual reproduction due to a smaller physical space to produce flowers at the branch apices. Resource limitation due to plant investment to replace burned vegetative parts may also decrease sexual reproduction. Our results indicate potentially severe consequences of high fire frequencies for population dynamics and species persistence in Cerrado communities.

Fire-induced damage to Qualea multiflora Mart. seeds depends on fruit protection and the position in the tree crown.

Although the Cerrado vegetation is adapted to fire, anthropogenic fires may impair the reproduction of some species. This study aimed to test the role of fruits of Qualea multiflora and their position in the tree crown in protecting the seeds during fires. Ten trees were selected in an area protected from fire (NB) and ten in a biannually burned area (B). Fruits were counted before the 2008 fire and weekly thereafter for 5 weeks. Fruit dehiscence, damage and position in the tree crown were recorded. Seed germination was assessed for seeds from both areas. In NB, 5.7% of fruits were located higher than 2 m, while in B 49.5% were located at this height. One week after the fire, the proportion of dehiscent fruits was lower in NB (19.5%) than in B (34.5%). Five weeks after the fire, all fruits in NB had dispersed their seeds, whereas in B only 47% of the fruits had. Immediately before the fire, the germination rate of seeds collected in NB was 80% (0-1 m), 97% (1-2 m) and 92% (≥2 m). Fifteen days after the fire, the germination rate of seeds in the B area was 3% (0-1 m), 13% (1-2 m) and 78% (≥2 m). Protection of Q. multiflora seeds from high temperatures is related to the fruit position in the tree crown rather than to fruit protection. Therefore, the results suggest that fire may alter the recruitment of new individuals.