Below-ground competition between trees and grasses may overwhelm the facilitative effects of hydraulic lift
Ludwig, F. ; Prins, H.H.T. ; Berendse, F. ; Kroon, H. de; Dawson, T.E. - \ 2004
Ecology Letters 7 (2004)8. - ISSN 1461-023X - p. 623 - 631.
african humid savanna - mojave desert - water-uptake - rooting patterns - east-africa - plants - vegetation - shrubs - transport - nutrients
Under large East African Acacia trees, which were known to show hydraulic lift, we experimentally tested whether tree roots facilitate grass production or compete with grasses for below-ground resources. Prevention of tree-grass interactions through root trenching led to increased soil water content indicating that trees took up more water from the topsoil than they exuded via hydraulic lift. Biomass was higher in trenched plots compared to controls probably because of reduced competition for water. Stable isotope analyses of plant and source water showed that grasses which competed with trees used a greater proportion of deep water compared with grasses in trenched plots. Grasses therefore used hydraulically lifted water provided by trees, or took up deep soil water directly by growing deeper roots when competition with trees occurred. We conclude that any facilitative effect of hydraulic lift for neighbouring species may easily be overwhelmed by water competition in (semi-) arid regions.
Hydraulic lift in Acacia tortilis trees on an East African savanna
Ludwig, F. ; Dawson, T.E. ; Kroon, H. de; Berendse, F. ; Prins, H.H.T. - \ 2003
Oecologia 134 (2003)3. - ISSN 0029-8549 - p. 293 - 300.
water-uptake - soil-water - artemisia-tridentata - grass interactions - rooting patterns - humid savanna - plants - woody - kenya - nutrients
Recent studies suggest that savanna trees in semi-arid areas can increase understorey plant production. We hypothesized that one of the mechanisms that explains the facilitation between trees and grasses in East African savannas is hydraulic lift (HL). HL in large Acacia tortilis trees was studied during the first 3 months of the dry season during a relatively wet year (1998) and a very dry year (2000). In 1998, we found distinct diel fluctuation in soil water potential (psi(S)), with increasing values during the night and decreasing again the following day. These fluctuations in psi(S), are consistent with other observations of HL and in A tortilis were found up to 10 in from the tree. In 2000, during a severe drought, fs measurements indicated that HL was largely absent. The finding that HL occurred in wetter years and not in drier years was supported by data obtained on the 5180 values in soil, rain and groundwater. The 6180 of water extracted from the xylem water of grasses indicated that when they grew near trees they had values similar to those of groundwater. This could be because they either (1) use water from deeper soil layers or (2) use hydraulically lifted water provided by the tree; this was not seen in the same grass species growing outside tree canopies. While our data indicate that HL indeed occurs under Acacia trees, it is also true that psi(S) was consistently lower under trees when compared to outside tree canopies. We believe that this is because tree-grass mixtures take up more water from the upper soil layers than is exuded by the tree each night. This limits the beneficial effect of HL for understorey grasses and suggests that in savannas both facilitation via HL and competition are active processes. The importance of each process may depend upon how wet or dry that particular site or year is.