Spatial distribution of root length density and soil water of linear agroforestry systems in sub-humid Kenya: implications for agroforestry models

Abstract In simultaneous agroforestry systems trees can compete with crops for water, especially in semi-arid areas. However, in the (sub)humid tropics, on P-fixing Oxisols/Ferralsols small decreases in soil water content caused a decrease in P-transport to roots and therewith a soil-drying induced P-deficiency. The aim of this study was to assess the spatial distribution of soil water content in crop fields bordering tree lines and its relation with root length density distribution of the trees throughout the soil profile. To achieve this, soil water content and tree root length densities throughout the soil profile were measured over a period of 2 years in an experiment with lines of four tree species in the middle of maize fields in sub-humid western Kenya. Soil water content was significantly reduced (2–7 vol.%) near two of the three fast-growing tree species, Eucalyptus grandis and Grevillea robusta , but not near Cedrella serrata and the slower growing Markhamia lutea . These differences were related to differences in water use. Eucalyptus and Grevillea showed high water use and Cedrella and Markhamia low water use. However, soil water content distribution was not related to root length density distribution. Root length densities hardly decreased with distance to Grevillea and clearly decreased with distance to Cedrella . Most water-uptake models, including those of agroforestry models, assume that root length density distribution throughout the profile is proportional to water extraction throughout the profile. The absence of a clear relation between root length density and water extraction near Grevillea tree lines opposed this view. It can be explained by a decrease in water-potential gradient between root and soil at increasing distance from the tree base. If the change in root length density is similar or smaller than the change in water-potential gradient between root and soil, the decrease in water-potential gradient between root and soil is of similar or larger importance for determining tree-water extraction distribution throughout the profile than root length density. Thus, modeling of spatial agroforestry systems cannot assume a direct relation between tree-water extraction and root length density, but needs to include decreasing water-potential gradient between root and soil along roots with increasing distance to the stem base, especially over the horizontal dimension.

[1]  H. Talpaz,et al.  A MACROSCOPIC‐SCALE MODEL OF WATER UPTAKE BY A NONUNIFORM ROOT SYSTEM AND OF WATER AND SALT MOVEMENT IN THE SOIL PROFILE , 1976 .

[2]  P. E. Weatherley THE HYDRAULIC RESISTANCE OF THE SOIL-ROOT INTERFACE — A CAUSE OF WATER STRESS IN PLANTS , 1979 .

[3]  N. Breda,et al.  Soil water dynamics in an oak stand , 1995, Plant and Soil.

[4]  Gail W. T. Wilson,et al.  Architectural analysis of plant root systems 1. Architectural correlates of exploitation efficiency , 1991 .

[5]  G. Campbell Simulation of water uptake by plant roots. , 1991 .

[6]  J. Wallace,et al.  Soil evaporation measurements in an agroforestry system in Kenya , 1999 .

[7]  R. Lascano,et al.  Root water uptake and soil water distribution: test of an availability concept , 1984 .

[8]  D. A. Stone,et al.  Simulation of the water distribution in soil , 1978, Plant and Soil.

[9]  I. Alexander,et al.  Plant Root Systems. Their Function and Interaction with the Soil. , 1978 .

[10]  L. Aylmore,et al.  Non-uniform soil water extaction by plant roots , 2005, Plant and Soil.

[11]  D. C. Mobbs,et al.  Complementarity of light and water use in tropical agroforests: I. Theoretical model outline, performance and sensitivity , 1998 .

[12]  J. Reid,et al.  Soil and plant resistances to water uptake byVicia faba L. , 1986, Plant and Soil.

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

[14]  J. Wallace,et al.  Forests, Water and People in the Humid Tropics: The potential of agroforestry for sustainable land and water management , 2005 .

[15]  B. Theng Soils with Variable Change , 1982 .

[16]  J. Feyen,et al.  AN ATTEMPT AT EXPERIMENTAL VALIDATION OF MACROSCOPIC‐SCALE MODELS OF SOIL MOISTURE EXTRACTION BY ROOTS , 1979 .

[17]  Meine van Noordwijk,et al.  Proximal root diameter as predictor of total root size for fractal branching models , 1994, Plant and Soil.

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

[19]  H. M. Taylor,et al.  Root hydraulic resistance: Implications in modelling nutrient and water uptake , 1992 .

[20]  J. Broadhead Ecophysiology of indigenous trees in agroforestry systems in the semi-arid tropics , 2000 .

[21]  Brent Clothier,et al.  The root zone dynamics of water uptake by a mature apple tree , 1999, Plant and Soil.

[22]  R. Russell,et al.  The Soil-Root Interface , 1979 .

[23]  J. Wallace,et al.  Modelling soil evaporation in an agroforestry system in Kenya , 1999 .

[24]  L. Stroosnijder,et al.  A model of tree-crop competition for windbreak systems in the Sahel: description and evaluation , 2004, Agroforestry Systems.

[25]  J. Roberts,et al.  Root distributions in a Grevillea robusta-maize agroforestry system in semi-arid Kenya , 1999, Plant and Soil.

[26]  Loïc Pagès,et al.  Water uptake by two contrasting root systems (maize, peach tree): results from a model of hydraulic architecture , 1999 .

[27]  G. Schroth,et al.  Tree root characteristics as criteria for species selection and systems design in agroforestry , 1995, Agroforestry Systems.