ABSTRACT

1  In water-limited environments, the availability of soil water and nutrients to plants depends on environmental conditions, the sizes and shapes of their root systems, and on root competition.  The goal of this study was to predict the sizes and shapes of root systems for plants of different growth forms using data on aboveground plant sizes, climate, and soil texture.
2  A new dataset of >1300 records of root system sizes for individual plants was collected from the literature for deserts, scrublands, grasslands, and shrub- and tree-savannas with £1000 mm mean annual precipitation (MAP).  Root system sizes and shapes for individual plants (i) were characterized by maximum rooting depths (D _i), maximum lateral root spreads (L_i ), and the resultant ratio (L_i/D_i) for seven plant growth forms.
3  Root system sizes differed among plant growth forms and increased in the order predicted from aboveground size: annuals < perennial forbs = grasses < semi-shrubs < shrubs < trees.  Stem succulents were as shallowly rooted as annuals and had lateral root spreads similar to shrubs.
4  Absolute rooting depths increased with MAP in all growth forms except shrubs and trees, but were not strongly related to potential evapotranspiration (PET). Root systems tended to be shallower and wider in dry and hot climates and deeper and narrower in cold and wet climates.  Shrubs were more shallowly rooted under climates with summer than winter rainfall regimes, but grasses and forbs were not.
5  Allometric belowground/aboveground size ratios (D_i : canopy volume; L_i : canopy volume) decreased with increasing MAP in forbs and grasses but not in woody plants.  Plants of a given aboveground size had larger root systems in climates with low rather than high PET.
6  Rooting depths in an independent dataset of 20 test locations were predicted from MAP using regression models for annuals, herbaceous perennials, and woody plants.  The models succeeded in explaining 62% of the observed variance in median maximum rooting depths.
7  Our results suggest that Walter's two-layer model of soil depth partitioning between woody and herbaceous plants is most appropriate in drier regimes (<500 mm precipitation) and in systems with substantial winter precipitation.  The results also highlight many climate change interactions, including the distribution of summer and winter precipitation in monsoonal systems that, when altered, may affect the rooting depth of woody but not herbaceous plants.
 
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