384
J.R. Ehleringer
0
24
Hymenoclea salsola ecotypes
o-e
A
c
2
0
r = .91
+::
22
eel
C
E
·c
u
0 leaf
en
• twig
:0
20
Q)
0.
0
+-'
0
.~
18
c
0
.0 ....
r2 = .69
eel
()
16
~
80
B
0
,-.... 70
C>
-
.~
60
eel
Q)
+
r2 =.77
-.J
- 50
eel
Q)
~
40
30
10
20
30
40
(0, mbar bar- 1
Fig. 18.12. A Carbon isotope discrimination of leaves and twigs from different ecotypes
of Hymenoclea salsola grown under common garden conditions as a function of (0 the leafto-air evaporative gradient weighted for seasonal precipitation input. B The ratio of leaf
area as a proportion of total photosynthetic area plotted as a function of (0. (Comstock
and Ehleringer 1992)
18.6.2 Utilization of Summer Moisture Inputs
The evolution of greater life-form diversity in desert regions with bimodal
precipitation patterns may relate niche differentiation to the use of moisture
resources. Cannon (1911) and Cody (1986) both noted striking differences in
the root distributions of desert plants, suggesting that plants were using
different moisture resources. From illustrations provided in those studies, it
is evident that there can be substantial mot overlap and thus it is unclear
that plants have nonoverlapping soil moisture sources. The water resource