ABSTRACT
The extent and consequences of global
land-cover and land-use change are increasingly apparent. One
consequence not
so apparent is the altered structure of plants belowground. This paper
examines such belowground changes, emphasizing the interaction of
altered root distributions with other factors, and their treatment in
models. Shifts of woody and
herbaceous vegetation with deforestation, afforestation, and woody
plant encroachment
typically alter the depth and distribution of plant roots, influencing
soil
nutrients, the water balance, and NPP. For example, our analysis
of
global soil datasets shows that the major plant nutrients C, N, P, and
K
are more shallowly distributed than are Ca, Mg, and Na, but patterns
for each
element vary with the dominant vegetation type. After controlling for
climate,
soil C and N are distributed more deeply in arid shrublands than in
arid
grasslands, and sub-humid forests have shallower nutrient distributions
than
do sub-humid grasslands. Consequently changes in vegetation may
influence
the distribution of soil carbon and nutrients over time (perhaps
decades
to centuries). Shifts in the water balance are typically much
more
rapid. Catchment studies indicate that the water yield decreases
25-40
mm for each 10% increase in tree cover, and increases in transpiration
of
water taken up by deep roots may account for as much as 50% of observed
responses.
Because models are increasingly important for predicting the
consequences
of vegetation change, we discuss the treatment of belowground processes
and
how different treatments affect model outputs. Whether models are
parameterized
by biome or plant functional type (or neither), use single or multiple
soil
layers, or include N and water limitation, all affect predicted
outcomes.
Acknowledging and understanding such differences should help constrain
predictions
of vegetation change.