364
J. R. Ehleringer
Table 18.1. Percentage abundance of shrub species on slope and in wash microhabitats
along an elevational gradient in the Sonoran Desert near Needles, California
Elevation (m)
180
365
615
715
975
Slope
Encelia farinosa
89
93
0
0
Ambrosia dumosa
6
4
88
0
Krameria parviflora
6
2
12
55
Encelia virginensis
0
0
0
29
Viguiera multiflora
0
0
0
16
Wash
Encelia frutescens
80
0
0
0
0
Hymenoclea salsola
20
96
78
37
3
Bebbia juncea
0
4
14
58
14
Chrysothamnus paniculatus
0
0
6
5
0
Encelia virginensis
0
0
0
0
41
Viguiera multiflora
0
0
0
0
24
Krameria parviflora
0
0
0
0
14
key to understanding aspects of plant carbon gain such as whether or not
annual versus perennial life forms are favored, whether or not acclimation
to drought or drought avoidance will lead to higher rates of carbon gain, and
whether or not conditions are sufficiently predictable to favor the emergence
of one particular set of photosynthetic characteristics over another in a
particular habitat.
18.3.1 Predictability of Precipitation
Rainfall is the life's blood of deserts, yet precipitation amounts are usually
sufficiently low that primary productivity and precipitation are linearly related (Noy-Meir 1973; Evanari et al. 1976; Ehleringer and Mooney 1983; Le
Houerou 1984). Plants of arid regions not only face low annual amounts of
precipitation, but the interannual variation in that precipitation is high,
resulting in a variability of primary production that exceeds the variability in
precipitation input (Le Houerou et al. 1988). The distribution of mean annual
precipitation in deserts can be approximated by a negative binomial or
gamma distribution (Thorn 1958; Hershfield 1962; Hastings 1965; Nicholson
1980). What this indicates is that the coefficient of variation increases as
the mean precipitation decreases (Fig. 18.2). As a consequence, the interannual variation in precipitation becomes progressively greater at drier sites.
Total precipitation in most years then becomes progressively lower than the
arithmetic mean precipitation values.
J. R. Ehleringer
Table 18.1. Percentage abundance of shrub species on slope and in wash microhabitats
along an elevational gradient in the Sonoran Desert near Needles, California
Elevation (m)
180
365
615
715
975
Slope
Encelia farinosa
89
93
0
0
Ambrosia dumosa
6
4
88
0
Krameria parviflora
6
2
12
55
Encelia virginensis
0
0
0
29
Viguiera multiflora
0
0
0
16
Wash
Encelia frutescens
80
0
0
0
0
Hymenoclea salsola
20
96
78
37
3
Bebbia juncea
0
4
14
58
14
Chrysothamnus paniculatus
0
0
6
5
0
Encelia virginensis
0
0
0
0
41
Viguiera multiflora
0
0
0
0
24
Krameria parviflora
0
0
0
0
14
key to understanding aspects of plant carbon gain such as whether or not
annual versus perennial life forms are favored, whether or not acclimation
to drought or drought avoidance will lead to higher rates of carbon gain, and
whether or not conditions are sufficiently predictable to favor the emergence
of one particular set of photosynthetic characteristics over another in a
particular habitat.
18.3.1 Predictability of Precipitation
Rainfall is the life's blood of deserts, yet precipitation amounts are usually
sufficiently low that primary productivity and precipitation are linearly related (Noy-Meir 1973; Evanari et al. 1976; Ehleringer and Mooney 1983; Le
Houerou 1984). Plants of arid regions not only face low annual amounts of
precipitation, but the interannual variation in that precipitation is high,
resulting in a variability of primary production that exceeds the variability in
precipitation input (Le Houerou et al. 1988). The distribution of mean annual
precipitation in deserts can be approximated by a negative binomial or
gamma distribution (Thorn 1958; Hershfield 1962; Hastings 1965; Nicholson
1980). What this indicates is that the coefficient of variation increases as
the mean precipitation decreases (Fig. 18.2). As a consequence, the interannual variation in precipitation becomes progressively greater at drier sites.
Total precipitation in most years then becomes progressively lower than the
arithmetic mean precipitation values.