Wednesday, August 6, 2008 - 10:10 AMOOS 13-7: Water use of two savanna tree species in relation to leaf phenology
Richard J.T. Verweij1, Edmund C. February1, Steven I. Higgins2, and William J. Bond1. (1) University of Cape Town, (2) Universität Frankfurt A.M.
Background/Question/Methods
Deciduous tree species in African savannas have adapted to seasonal drought by rapid leaf flushing, hence achieving full leaf expansion before or immediately after the onset of the rainy season and/or by gradual leaf abscission as available soil water declines after the last rains, thereby extending their growing season and optimizing their photosynthetic potential. Phenology of different life-forms (trees versus grasses) being out-of-phase may be a mechanism to avoid competition. Phenological niche separation has been proposed as a mechanism that can potentially contribute to equilibrium forms of tree-grass coexistence but is seemingly overlooked in field studies and never investigated in detail.
We assessed water use and levels of water stress of two tree species (Sclerocarya birrea and Terminalia sericea) in relation to their respective leaf phenology patterns. Sap flow velocities were continuously measured during the 2006-2007 growing season using the TDP-method (Granier-type). Phenology and stem circumference increment were monitored bi-weekly from July 2006 – June 2007. The study was done near the Pretoriuskop rest camp in the Kruger National Park, South Africa.
Results/Conclusions
The considered species displayed different phenological patterns. Leaf abscission in S. birrea occured soon after the last rains of the season, while still well hydrated and not water stressed. T. sericea kept a portion of its leaves until very late into the dry season. It then formed new leafs at the onset of the next growing season. This in contrast to S. birrea: leaf flush occurred up to several weeks before the first rains of the new growing season. Data from the dendrometer bands show shrinking of the stem in this period, supporting the hypothesis that water stored in the stem of S. birrea may enable this pre-rain leaf flush. The sap flow velocity measurements indicate that S. birrea is the more conservative water user.
Deciduous tree species in African savannas have adapted to seasonal drought by rapid leaf flushing, hence achieving full leaf expansion before or immediately after the onset of the rainy season and/or by gradual leaf abscission as available soil water declines after the last rains, thereby extending their growing season and optimizing their photosynthetic potential. Phenology of different life-forms (trees versus grasses) being out-of-phase may be a mechanism to avoid competition. Phenological niche separation has been proposed as a mechanism that can potentially contribute to equilibrium forms of tree-grass coexistence but is seemingly overlooked in field studies and never investigated in detail.
We assessed water use and levels of water stress of two tree species (Sclerocarya birrea and Terminalia sericea) in relation to their respective leaf phenology patterns. Sap flow velocities were continuously measured during the 2006-2007 growing season using the TDP-method (Granier-type). Phenology and stem circumference increment were monitored bi-weekly from July 2006 – June 2007. The study was done near the Pretoriuskop rest camp in the Kruger National Park, South Africa.
Results/Conclusions
The considered species displayed different phenological patterns. Leaf abscission in S. birrea occured soon after the last rains of the season, while still well hydrated and not water stressed. T. sericea kept a portion of its leaves until very late into the dry season. It then formed new leafs at the onset of the next growing season. This in contrast to S. birrea: leaf flush occurred up to several weeks before the first rains of the new growing season. Data from the dendrometer bands show shrinking of the stem in this period, supporting the hypothesis that water stored in the stem of S. birrea may enable this pre-rain leaf flush. The sap flow velocity measurements indicate that S. birrea is the more conservative water user.
The early start of S. birrea possibly leads to a competitive advantage over grasses by having exclusive access to resources in the early growing season. On the other hand, T. sericea can continue to photosynthesize longer into the dry season, possibly facilitated by the occasional winter rain, well after grasses have senesced.