Seasonal patterns of gas exchange and water relations in juveniles of two Neotropical savanna tree species differing in leaf phenology

Abstract

Ecophysiological studies on Neotropical savanna trees have shown that there is a resistance gradient of seasonal water stress in deciduous and evergreen species. Some species perform like drought avoiders, characterized by high transpiration rates during the wet season that can be sustained by evergreen deep rooted species during the dry season. Nevertheless, water deficit in superficial soil layers, fire damages and nutritional requirements could differentially affect juveniles of both phenological types determining their spatial patterns of distribution (evergreen species sparse in open grasslands and deciduous species gathered in groves). This study examined seasonal variations in water relations and photosynthesis in juveniles of two tree species differing in leaf phenology: Curatella americana (evergreen) and Cochlospermum vitifolium (deciduous). We wanted to test the hypothesis that the improved ability of evergreen deep rooted species like C. americana to colonize open savannas is related to drought tolerance and a conservative gas exchange pattern in juveniles that photosynthesize even during the dry season, while one might expect a water spender and avoider character in propagules of deciduous species like C. vitifolium that are active mostly during the rainy season. The studied species showed high transpiration rates during the rainy period, indicating little water availability limitations on surface soil layers. Even so, both species exhibited a distinctive decrease in stomatal conductance towards the end of the rainy season, an indication of water availability restrictions and/or a response to higher air evaporative demands in the shallow rooted juveniles. At the peak of the dry season, C. americana presented comparable transpiration rates with wet season measurements, indicating that the studied juveniles had reached more permanent (or stable) water sources at soil depths of 30 cm (approximate maximum root lengths measured). Yet, C. americana showed a significant decrease in water potentials from wet to dry seasons, and osmotic adjustment as a drought tolerance mechanism. Overall, the evergreen functional type (juvenile plants of C. americana) showed tolerance to water deficit, maintaining functional leaves in the dry season. On the other hand, in absence of a water deficit, juveniles of C. vitifolium showed higher photosynthetic rates that could be translated into high growth rates in wet habitats. However, the low water deficit tolerance of the juveniles of C. vitifolium seems to be the main handicap for its colonization potential of the open savanna environment. In conclusion, as predicted, the greater tolerance to water deficit of the evergreen studied species suggests that resistance to water stress determines the observed distribution pattern of tree species in the study area, where only the evergreen ones colonize the open savanna, acting in its adult stages as a focus for the recruitment of woody species.