Principles of terrestrial ecosystem ecology.pdf

82 4. Terrestrial Water and Energy Balance
Soil depth (cm)
0
50
100
150
200
Trees
Shrubs
Grasses
0 0.2 0.4 0.6 0.8 1
Cumulative root biomass (fraction of total)
Figure 4.6. The cumulative fraction of roots found
at different soil depths for three plant growth forms
averaged over all biomes. (Redrawn with permission
from Oecologia; Jackson et al. 1996.)
Figure 4.7. Maximum rooting depths of selected
species in the major biome types of the world
showing that species in each biome differ widely.
have dry seasons. This may explain the occurrence
in such forests of deep-rooted trees
that tap water from depths of more than 8m
(Nepstad et al. 1994). Deep-rooted plants may
be more common than generally appreciated.
Deep roots often extend into cracks in bedrock,
where they tap water as it drains through rock
channels to groundwater.
Rooting depth has important ecosystem
consequences because it determines the soil
volume that can be exploited by vegetation (see
Chapter 12). California grassland soils below a
1-m depth, for example, remain moist even at
the end of the summer drought, whereas an
adjacent chaparral shrub community uses water
to a depth of 2m (Fig. 4.8). This greater rooting
depth contributes to the longer growing season
and greater productivity of the chaparral. Even
in the chaparral, species differences in rooting
depth (Box 4.2) lead to differences in moisture
supply and drought stress.
Woody species in dry environments are often deeply
rooted. (Redrawn with permission from Oecologia;
Canadell et al. 1996.)