Efeitos da seca sazonal e disponibilidade de luz na sobrevivência e crescimento de Bowdichia virgilioides em duas fitofisionomias típicas dos cerrados do Brasil Central

Bowdichia virgiliodes Kunth is a common legume tree in the savannas of Central Brazil. This study investigated the effects of shading and of seasonal variations of soil water on the establishment and development of B. virgilioides in the 'campo sujo', a grassland with scattered trees and shrubs, and the 'cerradao', a woodland with a well-developed arboreous canopy. Soil remained wet during most of the rainy season, but short dry spells affected soil water potential of the upper layers. The soil under 'campo sujo' vegetation dried faster in the dry season. A larger percentage of seedlings emerged in the 'campo sujo'. Most of the mortality occurred shortly after germination, still in the wet season. Seasonal drought was not an important mortality factor. The 'cerradao' was not a favourable environment for the development of B. virgiliodes. Few cerradao seedlings survived, and 15-month-old plants reached only 3.8 ± 0.7 cm (mean ± SD) of total stem length, whereas the 'campo sujo' plants reached 6.9 ± 1.3 cm. Canopy shading reduced the photosynthetic photon flux density (PFD) available for understory seedlings and may be a major constraint for seedling growth. Shading by the tree layer in the cerradao had a much higher impact on PFD interception. A horizontal leaf located 5 cm aboveground would potentially fix 30 to 60% less CO2 than a leaf beneath the grass canopy of a 'campo sujo' and 60 to 80% less CO2 than an unshaded leaf. Because the height of the herb layer was less than 50 cm, shading decreased markedly with plant height in the 'campo sujo'.