ADAPTIVE STRATEGIES OF WOODY SPECIES IN NEOTROPICAL SAVANNAS

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Abstract
Summary: 1. In this review we discuss the adaptive strategy of woody species in tropical savannas. The low, evergreen, broadleaved, sclerophyllous tree is considered as the typical woody representative in these ecosystems. The discussion is largely based on data concerning four widespread neotropical species: Curatella americana, Byrsonima crassifolia, Bowdichia virgilioides and Casearia sylvestris, together with more fragmentary information available on other American and African savanna woody species.2. Several types of savanna ecosystems with contrasting ecological features have to be distinguished. Our discussion refers to tree species in one of these types: seasonal savannas, that occur in a tropical wet and dry climate, with constantly high temperature, and on well‐drained soils. Most of these savannas are normally burned once a year, towards the end of the dry season.3. Woody species in seasonal savannas exhibit a quite distinctive morphology. They have low, tortuous trunks, deep and extensive root systems, relatively high R/S and L/S ratios, and large, highly scleromorphic leaves. Their annual phenodynamics appears somewhat puzzling since leaf renewal and expansion, as well as blooming, take place during the dry, apparently less favourable, part of the year.4. Savanna trees maintain high leaf conductance throughout the year. Some species show a moderate midday decrease in leaf conductance suggesting partial stomatal closure, particularly under very high atmospheric water demands, or in young, developing leaves. However, given the steep vapour density gradient, transpiration flux density tends to be high, especially on clear dry‐season days.5. There is no drastic drop in leaf water potential, as might be expected with a high transpiration rate. The most negative values attained in either season only rarely exceed the leaf turgor loss point. This moderate fall in ψ permits leaf expansion in the dry season. Variable hydraulic resistance contributes to maintain high water flow when steep ψ gradients between soil and leaves are produced.6. When all factors are taken into account, it seems that savanna trees maintain a favourable water budget all the year, thanks to their extensive root systems that may extract soil water from deep layers, thus allowing the maintenance of a high water flux through the soil‐plant‐atmosphere system even during the dry season. In this way, these trees have the least seasonal behaviour of all plant components in the seasonal savanna ecosystem.7. Seasonal savannas occur on extremely poor, nutrient‐deficient soils. As an apparent consequence of this nutrient stress, the concentration of nitrogen, phosphorus, potassium, calcium and magnesium in leaves tends to be significantly lower than in forest trees or in drought‐deciduous species.8. Two mechanisms contribute to improve the nutrient economy. One is the reallocation of absorbed nutrients between old and young tissues; the other, the minimization of nutrient losses due to low leaf wettability, low leaf cuticular conductance, and leaf renewal in the rainless season.9. Savanna trees have low photosynthetic capacity. This is probably due to high internal resistance of leaves induced by their low nitrogen concentration. However, under field conditions rates of CO2 uptake may be maintained near their optimum because leaf conductance is high all day, and leaf temperature closely matches air temperature, remaining therefore within the optimal range for photosynthesis.10. All in all, it appears that the physiological behaviour of savanna trees favours a continuously high water flux through the plant that, even if it lowers water‐use efficiency, maintains leaf temperatures near optimum for CO2 uptake, prevents sharp drops in leaf water potential, and induces a high passive uptake of soil nutrients. In this way, the close interaction between water, carbon and nutrient economies leads to the increased fitness of these populations in the seasonal savanna environment.