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J.R. Ehleringer
these two parameters, but that the relationships for both summer and winter
storms are the same.
18.3.2 Drought Duration
Of importance in trying to understand the constraints imposed on plant
performance by drought is length or duration of the drought period. One
way of examining this character is to examine how long it takes, after the
"winter season" or "summer season" begins, before significant precipitation
occurs. A 10-mm event is often considered a minimal trigger for plant
growth activities in the desert (Beatley, 1974a,b). Winter precipitation is
driven by frontal storms coming off the Pacific Ocean. If we examine the
date of occurrence of the first lO-mm precipitation for the winter season at
sites differing in the mean annual precipitation, we see that this date is
highly variable (Fig. 18.5). The "beginning" of the winter season may be
essentially any date between October 1 and May 15 in the Sonoran Desert;
the frequency distribution is flat enough that there is little tendency for the
winter growing period to begin during any specific window. Moreover, at
the two driest sites (Indio and Needles), there was not a single storm large
enough to trigger growth according to the criteria of Beatley (1974b) in
almost one-third of the years. As such, we would obviously expect that
perennial plants at the driest sites would require features to insure persistence
throughout an entire year without precipitation.
The entry of moist tropical air into the Sonoran Desert is more reliable.
In the more southerly sites, the dates of the first summer monsoonal rains
are predictable (Fig. 18.6). The date of the first summer rains is, however,
very much dependent on the average amount of summer rains received. In
the regions receiving less summer precipitation, the date of these first rains
is less predictable, such that at the drier sites this summer moisture (if any)
could come at virtually any time during the summer.
Once the first rains have arrived, just how good is early season moisture
as a predictor of the overall quality of the growing season? If the first rains
events portend a generally wet and favorable season, then plants should
respond (break dormancy, germinate) shortly after these rains events to
capitalize on the available soil moisture and to be positioned to effectively
use later moisture inputs. However, if the first rains of the season do not
provide qualitative information about the remainder of the season, then
perhaps responding to those early rains is a chance event. For 10 of the 12
stations listed in Fig. 18.2, there is a highly significant, positive correlation
(P < 0.01) between the amount of winter precipitation falling in the first 30
days of the growing season and the total amount for that growing season.
For 7 of 12 stations, there was an equivalently positive and significant
correlation for summer rains as well. Thus, early season precipitation is
statistically a reliable indicator of future moisture inputs. Pianka (1967) ex-
J.R. Ehleringer
these two parameters, but that the relationships for both summer and winter
storms are the same.
18.3.2 Drought Duration
Of importance in trying to understand the constraints imposed on plant
performance by drought is length or duration of the drought period. One
way of examining this character is to examine how long it takes, after the
"winter season" or "summer season" begins, before significant precipitation
occurs. A 10-mm event is often considered a minimal trigger for plant
growth activities in the desert (Beatley, 1974a,b). Winter precipitation is
driven by frontal storms coming off the Pacific Ocean. If we examine the
date of occurrence of the first lO-mm precipitation for the winter season at
sites differing in the mean annual precipitation, we see that this date is
highly variable (Fig. 18.5). The "beginning" of the winter season may be
essentially any date between October 1 and May 15 in the Sonoran Desert;
the frequency distribution is flat enough that there is little tendency for the
winter growing period to begin during any specific window. Moreover, at
the two driest sites (Indio and Needles), there was not a single storm large
enough to trigger growth according to the criteria of Beatley (1974b) in
almost one-third of the years. As such, we would obviously expect that
perennial plants at the driest sites would require features to insure persistence
throughout an entire year without precipitation.
The entry of moist tropical air into the Sonoran Desert is more reliable.
In the more southerly sites, the dates of the first summer monsoonal rains
are predictable (Fig. 18.6). The date of the first summer rains is, however,
very much dependent on the average amount of summer rains received. In
the regions receiving less summer precipitation, the date of these first rains
is less predictable, such that at the drier sites this summer moisture (if any)
could come at virtually any time during the summer.
Once the first rains have arrived, just how good is early season moisture
as a predictor of the overall quality of the growing season? If the first rains
events portend a generally wet and favorable season, then plants should
respond (break dormancy, germinate) shortly after these rains events to
capitalize on the available soil moisture and to be positioned to effectively
use later moisture inputs. However, if the first rains of the season do not
provide qualitative information about the remainder of the season, then
perhaps responding to those early rains is a chance event. For 10 of the 12
stations listed in Fig. 18.2, there is a highly significant, positive correlation
(P < 0.01) between the amount of winter precipitation falling in the first 30
days of the growing season and the total amount for that growing season.
For 7 of 12 stations, there was an equivalently positive and significant
correlation for summer rains as well. Thus, early season precipitation is
statistically a reliable indicator of future moisture inputs. Pianka (1967) ex-