Why be deciduous in a tropical savanna climate? Insights from leaf deployment, stem growth and sap flow dynamics

Savannas are ecosystems with hot and wet summers and warm and dry winters. How and why savanna trees partition their physiological activity through this seasonal cycle is unknown; yet it is important to understand this temporal partitioning in order to anticipate how anthropogenic climatic change will impact the functioning of savannas. In a South African savanna ecosystem, we examined the temporal pattern of leaf deployment, sap flow and stem increment phenology over three growing seasons for six species. We found that although the phenologies varied between years, there was a high degree of temporal overlap in leaf, sap flow and stem increment phenology between the six study species. Further, we found that a surprisingly large proportion of days on which the measured soil water potential was sufficient for physiological activity remained unused. Although our data are consistent with convergent selection for deciduousness, the unused soil water niche we detected suggests that climate should select for evergreen strategies. Possible explanations for the dominance of deciduousness in these savannas relate to the costs of leaf construction coupled with the risk of leaf destruction due to fire and/or foliovory.