Principles of terrestrial ecosystem ecology.pdf

Leaf area (m 2 )
500
400
300
200
100
0 0
Douglas fir
Western white pine
Ponderosa pine
1000 2000 3000
Sapwood area (cm 2 )
Figure 4.13. Leaf area versus sapwood crosssectional
area for three forest trees. Ponderosa pine,
which typically occupies dry sites, has smaller vessels
and therefore supports less leaf area per unit
sapwood than does Douglas fir from moist sites.
(Redrawn with permission from Canadian Journal of
Forest Research; Monserud and Marshall 1999.)
conducting tissue (the sapwood). There is a
strong linear relationship between the crosssectional
area of sapwood and the leaf area supported
by a tree (Fig. 4.13). However, the slope
of this relationship varies strikingly among
species and environments. Drought-resistant
species generally have less leaf area per unit of
sapwood than do drought-sensitive species
because of the small vessel diameter of
drought-resistant species. The ratio of leaf area
to sapwood area, for example, is generally more
Transpiration, (mL m -2 h -1 )
30
20
10
0
0
Transpiration
Water Movements Within Ecosystems 87
than twice as great in trees from mesic environments
as in trees from dry environments,
due to the smaller-diameter conducting elements
in dry environments (Margolis et al.
1995). Any factor that enhances the productivity
of a tree increases its ratio of leaf area to
sapwood area.This ratio increases, for example,
with improvements in nutrient or moisture
status and is greater in dominant than subdominant
individuals of a stand.
Water storage in stems buffers the plant from
imbalances in water supply and demand. The
water content of trunks of trees generally
decreases during the day, causing water uptake
by roots to lag behind transpirational water loss
by about 2h (Fig. 4.14). The quantity of water
stored in sapwood is substantial, equivalent to
as much as 5 to 10 days of transpiration. This
sapwood water, however, exchanges relatively
slowly, so stores of water in sapwood seldom
account for more than 10% of transpiration.
In dry tropical forests, where trees lose their
leaves during the dry season, this stored water
is critical to support flowering during the dry
season. Trees in these forests that have lowdensity
wood and large stem water storage can
flower during the dry season, whereas trees
with high-density wood and low stem water
storage can flower only during the wet season
(Borchert 1994). Water storage in desert succulents
may allow transpiration to continue for
6 10 12
Time (h)
14 16 18 20 22 24 0
2 4 8
Figure 4.14. Diurnal time course of water uptake
and water loss by Siberian larch. During morning,
transpiration is supported by water loss from stems,
creating a lower water potential in stems and roots,
Water uptake
10
5
Water uptake (L h -1 )
which generates the water potential gradient to
absorb water from the soil.The water stored in stems
is replenished at night. (Redrawn with permission
from BioScience; Schulze et al. 1987.)