Oecologia (2004) 141: 254–268
DOI 10.1007/s00442-004-1682-4
PULSE EVENTS AND ARID ECOSYSTEMS
Travis E. Huxman . Keirith A. Snyder . David Tissue .
A. Joshua Leffler . Kiona Ogle . William T. Pockman .
Darren R. Sandquist . Daniel L. Potts .
Susan Schwinning
Precipitation pulses and carbon fluxes in semiarid and arid
ecosystems
Received: 13 February 2004 / Accepted: 9 July 2004 / Published online: 27 August 2004
# Springer-Verlag 2004
Abstract In the arid and semiarid regions of North
America, discrete precipitation pulses are important
triggers for biological activity. The timing and magnitude
of these pulses may differentially affect the activity of
plants and microbes, combining to influence the C balance
of desert ecosystems. Here, we evaluate how a “pulse” of
water influences physiological activity in plants, soils and
ecosystems, and how characteristics, such as precipitation
pulse size and frequency are important controllers of
biological and physical processes in arid land ecosystems.
We show that pulse size regulates C balance by
determining the temporal duration of activity for different
components of the biota. Microbial respiration responds to
very small events, but the relationship between pulse size
and duration of activity likely saturates at moderate event
sizes. Photosynthetic activity of vascular plants generally
increases following relatively larger pulses or a series of
small pulses. In this case, the duration of physiological
activity is an increasing function of pulse size up to events
that are infrequent in these hydroclimatological regions.
This differential responsiveness of photosynthesis and
respiration results in arid ecosystems acting as immediate
C sources to the atmosphere following rainfall, with
subsequent periods of C accumulation should pulse size be
sufficient to initiate vascular plant activity. Using the
average pulse size distributions in the North American
deserts, a simple modeling exercise shows that net
ecosystem exchange of CO2 is sensitive to changes in
the event size distribution representative of wet and dry
years. An important regulator of the pulse response is
initial soil and canopy conditions and the physical
structuring of bare soil and beneath canopy patches on
the landscape. Initial condition influences responses to
pulses of varying magnitude, while bare soil/beneath
canopy patches interact to introduce nonlinearity in the
relationship between pulse size and soil water response.
Building on this conceptual framework and developing a
greater understanding of the complexities of these eco-
hydrologic systems may enhance our ability to describe
the ecology of desert ecosystems and their sensitivity to
global change.
Keywords Desert plants . Precipitation . Carbon .
Photosynthesis . Respiration
Introduction
The availability of water, like other resources limiting
biological activity, is spatially and temporally heteroge-
neous on multiple scales (Lambers et al. 1998). Although
water availability changes over short (hourly and daily)
and long (seasonal and yearly) time scales, most studies
have focused on the ecological implications of long-term
T. E. Huxman (*) . D. L. Potts
Ecology and Evolutionary Biology, University of Arizona,
Tucson, AZ 85721-0088, USA
e-mail: huxman@email.arizona.edu
Tel.: +1-520-6218220
K. A. Snyder
USDA—ARS Jornada Experimental Range,
Las Cruces, NM, USA
D. Tissue
Department of Biological Sciences, Texas Technical University,
Lubbock, TX, USA
A. J. Leffler
The Ecology Center, Utah State University,
Logan, UT, USA
K. Ogle
Ecology and Evolutionary Biology, Princeton University,
Princeton, NJ, USA
W. T. Pockman
Department of Biology, University of New Mexico,
Albuquerque, NM, USA
D. R. Sandquist
Department of Biological Science, California State University,
Fullerton, CA, USA
S. Schwinning
Renewable Natural Resources, University of Arizona,
Tucson, AZ, USA

dynamics. Differences in plant functional type abundance
and life history diversity across the four North American
deserts is influenced by seasonal and annual water
availability (Ehleringer 1985; Smith et al. 1997; Smith
and Nobel 1986). Similarly, seasonal and annual precip-
itation inputs explain much of the variation in ecosystem
processes, such as primary production (Webb et al. 1978;
Gutierrez and Whitford 1987; Knapp and Smith 2001;
Huxman et al. 2004a, b) .
Surprisingly, how short-term fluctuations in water
availability influence ecological processes has not been
evaluated to the same extent as other environmental
variables. For example, the importance of light distribution
has been critically evaluated across multiple temporal and
spatial scales from the tropics to the tundra (Pearcy et al.
1985; Pearcy 1990; Smith and Knapp 1990). Similarly,
seasonal, monthly and diurnal variations in temperature
have been cited as important drivers of physiological
processes in many biomes (Mooney and Billings 1961;
Valentini et al. 2000; Huxman et al. 2003; Enquist et al.
2003). Infrequent, discrete, and largely unpredictable
precipitation events (pulses; e.g., Schwinning and Sala
2004, this issue) have been suggested to be an important
driver of arid land ecosystem structure and function (Noy-
Meir 1973; Ehleringer et al. 1999), yet only now is a
mechanistic understanding of their role in ecological
processes emerging (Weltzin and Tissue 2003). The focus
of this paper is to consider how variation in precipitation
characteristics, such as pulse size or frequency, affects
ecosystem C fluxes in semiarid and arid regions, and how
those flux patterns may be influenced by variation in the
edaphic, microbial and vegetation components of these
ecosystems.
While we are beginning to understand how plant
function and productivity are influenced by variation in
episodic precipitation inputs (Osmond et al. 1987; Smith et
al. 1997; Ehleringer et al. 1999; Schwinning and
Ehleringer 2001; Whitford 2002; Huxman et al. 2004a,
b), we still lack information on how the large-scale fluxes
of CO2 in arid lands are controlled by changes in water
status. For example, Reynolds et al. (2004, this issue)
suggest that our understanding of plant function in the
North American deserts would be improved by consider-
ing multiple precipitation pulses (storms) as single,
biologically relevant events. Additionally, Austin et al.
(2004, this issue) point out that even fairly small rain
events influence soil biogeochemical processes. It is the
combination of these plant and microbial processes that
combine to influence ecosystem C pools and fluxes; here
such factors as seasonal rainfall event size distribution
may be critical to ecosystem function. Understanding how
precipitation events differentially influence these ecosys-
tem components may shed light on the ecosystem CO2
exchanges of arid ecosystems, and how these regions may
respond to climate changes, which may include shifts in
the magnitude, seasonal timing and event size pattern of
precipitation pulses (Weltzin et al. 2003).
This paper addresses two fundamental questions about
CO2 exchange dynamics: (1) how does a “pulse” of water
availability influence C metabolism from microbes and
leaves to whole ecosystem and (2) how do pulse
characteristics, such as size and frequency, control C
dynamics in arid lands?
Ecosystem component responses to precipitation
pulses
As with all other biological activities, the ability of
organisms to acquire and utilize C depends on the
presence of sufficient water. Since the organisms facilitat-
ing different components of the C cycle are partially
separated in space, the physical distribution of soil water
following rainfall links ecosystem C exchanges to precip-
itation patterns. The vertical distribution of soil moisture
likely exerts overwhelming control on patterns of ecosys-
tem C exchange. For example, as several contributions in
this issue have pointed out (e.g., Austin et al. 2004;
Schwinning and Sala 2004), microbes located on or just
beneath the soil surface are hydrated most frequently, and
even minute rainfall events may enhance the microbial
contribution to ecosystem activity, while being ineffective
for triggering the autotrophic processes of vascular plants.
Even biological soil crusts require fairly large-sized events
to achieve net C gain (Belnap et al. 2004a, b; Cable and
Huxman 2004, this issue). Often overlooked, the hor-
izontal distribution of soil moisture may be equally
important in determining ecosystem C fluxes. Runoff
and runon patterns redistribute precipitation from the plot
to the landscape level (Loik et al. 2004, this issue) and,
other processes, such as canopy interception may
significantly interfere with ecosystem water use, particu-
larly of small rainfall events. Both the vertical and
horizontal distributions of precipitation-derived water in
the soil are strongly influenced by edaphic factors;
however, our understanding of these complexities are
still quite limited.
Below, we briefly review both the microbial and higher
plant responses to soil moisture pulses. Both phenomena
are covered in depth by other contributions in this special
issue (e.g., Austin et al. 2004; Belnap et al. 2004; Cable
and Huxman 2004; Huxman et al. 2004b; Ogle and
Reynolds 2004; Schwinning and Sala 2004; Snyder et al.
2004). Here, we expand on the question how precipitation
pulse patterns, interacting with physical and edaphic site
factors, impact the balance of respiration and assimilation
in arid/semiarid ecosystems. We also present a simple
model to conceptualize the role of precipitation patterns in
influencing ecosystem C cycling, using the example of the
three North American warm deserts.
Microbial response to precipitation pulse
In arid ecosystems, a precipitation pulse into dry soil
immediately alters the C balance of the system in several
ways. First, high concentrations of CO2, built up from
inorganic C sources and soil microbial activity during the
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