ABSTRACT
Studies in global plant biogeography have almost exclusively
analyzed
relationships of abiotic and biotic factors with the distribution and
structure
of vegetation aboveground. The goal of this study was to extend
such
analyses to the belowground structure of vegetation by determining the
biotic
and abiotic factors that influence vertical root distributions in the
soil,
including soil, climate, and plant properties. The analysis used
a
database of vertical root profiles from the literature with 475
profiles
from 209 geographic locations. Since most profiles were not
sampled
to the maximum rooting depth, several techniques were used to estimate
the
amount of roots at greater depths, to a maximum of 3 m in some
systems.
The accuracy of extrapolations was tested using a subset of deeply
(>2
m) sampled or completely sampled profiles. Vertical root
distributions
for each profile were characterized by the interpolated 50% and 95%
rooting
depths (the depths above which 50% or 95% of all roots were located).
General linear models incorporating plant life-form dominance, climate,
and soil variables explained as much as 50% of the variance in rooting
depths for various biomes and life forms. Annual potential
evapotranspiration (PET) and precipitation together accounted for the
largest proportion of the variance (12 to 16% globally and 38% in some
systems). Mean 95% rooting depths increased with decreasing
latitude from 80° to 30° but showed no clear trend in the
tropics. Annual PET, annual precipitation, and length of the warm
season were all positively correlated with rooting depths.
Rooting depths in tropical vegetation were only weakly correlated with
climatic variables but were strongly correlated with sampling depths,
suggesting that even after extrapolation, sampling depths there were
often insufficient to characterize root profiles. Globally, more
than 90% of all profiles had at least 50% of all roots in the upper 0.3
m of the
soil profile (incl. organic horizons) and 95% of all roots in the upper
2 m. Deeper rooting depths were mainly found in water-limited
ecosystems. Deeper 95% rooting depths were also found for
shrublands compared to grasslands, in sandy soils versus clay or loam
soils, and in systems with relatively
shallow organic horizons compared with deeper organic horizons.