Hydraulic redistribution of soil water by neotropical savanna trees

The magnitude and direction of water transport by the roots of eight dominant Brazilian savanna (Cerrado) woody species were determined with a heat pulse system that allowed bidirectional measurements of sap flow. The patterns of sap flow observed during the dry season in species with dimorphic root systems were consistent with the occurrence of hydraulic redistribution of soil water, the movement of water from moist to drier regions of the soil profile via plant roots. In these species, shallow roots exhibited positive sap flow (from the soil into the plant) during the day and negative sap flow (from the plant into the soil) during the night. Sap flow in the taproots was positive throughout the 24-h period. Diel fluctuations in soil water potential, with maximum values occurring at night, provided evidence for partial rewetting of upper soil layers by water released from shallow roots. In other species, shallow roots exhibited negative sap flow during both the day and night, indicating that hydraulic redistribution was occurring continuously. A third sap flow pattern was observed at the end of the dry season after a heavy rainfall event when sap flow became negative in the taproot, and positive in the small roots, indicating movement of water from upper soil layers into shallow roots, and then into taproots and deeper soil layers. Experimental manipulations employed to evaluate the response of hydraulic redistribution to changes in plant and environmental conditions included watering the soil surface above shallow roots, decreasing transpiration by covering the plant and cutting roots where probes were inserted. Natural and manipulated patterns of sap flow in roots and stems were consistent with passive movement of water toward competing sinks in the soil and plant. Because dry shallow soil layers were often a stronger sink than the shoot, we suggest that the presence of a dimorphic root system in deciduous species may play a role in facilitating leaf expansion near the end of the dry season when the soil surrounding shallow lateral roots is still dry.

[1]  C. Springer-Verlag.  Hydraulic lift: water efflux from upper roots improves effectiveness of water uptake by deep roots , 1989 .

[2]  G. Goldstein,et al.  Partitioning of soil water among tree species in a Brazilian Cerrado ecosystem. , 1999, Tree physiology.

[3]  J. Roberts,et al.  Reverse flow of sap in tree roots and downward siphoning of water by Grevillea robusta , 1999 .

[4]  M. Caldwell,et al.  Hydraulic lift: consequences of water efflux from the roots of plants , 1998, Oecologia.

[5]  G. Goldstein,et al.  ADAPTIVE STRATEGIES OF WOODY SPECIES IN NEOTROPICAL SAVANNAS , 1985 .

[6]  B. Walker,et al.  Interactions of woody and herbaceous vegetation in a southern African Savanna , 1985 .

[7]  J. A. Ratter,et al.  Analysis of the floristic composition of the Brazilian cerrado vegetation II: Comparison of the woody vegetation of 98 areas , 1996 .

[8]  M. Caldwell,et al.  Hydraulic lift: Substantial nocturnal water transport between soil layers by Artemisia tridentata roots , 1987, Oecologia.

[9]  G. Goldstein,et al.  Patrones diarios y estacionales en las relaciones hídricas de árboles siempreverdes de la sabana tropical , 1986 .

[10]  M. Caldwell,et al.  Hydraulic lift: water efflux from upper roots improves effectiveness of water uptake by deep roots , 1989, Oecologia.

[11]  P. Nobel,et al.  Rectifier-like Activities of Roots of Two Desert Succulents , 1984 .

[12]  Damian Barrett,et al.  Evaluation of the heat pulse velocity technique for measurement of sap flow in rainforest and eucalypt forest species of south‐eastern Australia , 1995 .

[13]  G. Goldstein,et al.  Atmospheric and hydraulic limitations on transpiration in Brazilian cerrado woody species , 1999 .

[14]  B. Huntley,et al.  Ecology of Tropical Savannas , 1987, Ecological Studies.

[15]  G Goldstein,et al.  Regulation of water flux through tropical forest canopy trees: do universal rules apply? , 2001, Tree physiology.

[16]  D. Marshall.  Measurement of Sap Flow in Conifers by Heat Transport. , 1958, Plant physiology.

[17]  M. Adams,et al.  An improved heat pulse method to measure low and reverse rates of sap flow in woody plants. , 2001, Tree physiology.

[18]  A. G. Marshall,et al.  The Ecology of Neotropical Savannas. , 1984 .

[19]  M. Adams,et al.  Seasonal water acquisition and redistribution in the Australian woody phreatophyte, Banksia prionotes , 2000 .

[20]  P. Nobel,et al.  Radial Hydraulic Conductivity of Individual Root Tissues of Opuntia ficus-indica (L.) Miller as Soil Moisture Varies , 1996 .

[21]  M. Adams,et al.  The redistribution of soil water by tree root systems , 1998, Oecologia.

[22]  A. Franco.  Seasonal patterns of gas exchange, water relations and growth of it Roupala montana, an evergreen savanna species , 1998, Plant Ecology.

[23]  C. Bledsoe,et al.  Seasonal and diurnal patterns of soil water potential in the rhizosphere of blue oaks: evidence for hydraulic lift , 2000, Oecologia.

[24]  T. Dawson.  Hydraulic lift and water use by plants: implications for water balance, performance and plant-plant interactions , 1993, Oecologia.

[25]  F. Rawitscher.  The Water Economy of the Vegetation of the `Campos Cerrados' in Southern Brazil , 1948 .

[26]  J. Felfili,et al.  A comparative study of cerrado (sensu stricto) vegetation in Central Brazil , 1993, Journal of Tropical Ecology.

[27]  B. Walker,et al.  Aspects of the Stability and Resilience of Savanna Ecosystems , 1982 .

[28]  T. S. Filgueiras,et al.  Projeto biogeografia do bioma cerrado : vegetacao & solos , 1994 .