February 18, 2004
Effects of climate variation and vegetation changes.
4. What is the actual evidence that the introduction of cattle and sheep grazing
changes plant communities? (continued from previous lecture …)

In the Santa Rita Experimental Range near Tucson, repeat photography has been used to
record vegetation changes. This does not strictly record vegetation changes associated
with the introduction of sheep and cattle grazing, because the earliest photographs follow
cattle grazing by at least 50 years, but this technique does illustrate ongoing changes over
the last 100 years. The series of photographs document an ongoing spread of mesquite
into the desert grassland.
5. Scales of vegetation change

Vegetation is constantly in flux. However changes that take place over different time
spans tend to have different characteristics and have different causes.

On the time span of the last 10,000s of years, vegetation changes are associated with the
transition from the last glacial into the current interglacial. Driven by large changes in
average precipitation and temperature, whole geographic regions transition from forest to
desert grassland. Looking at it spatially, the boundaries between ecosystems move along
elevation gradients, leaving only forested “sky islands” where once was continuous
forest.

On the time span of the last 100s of years, the most prominent change was caused by the
introduction of cattle and sheep grazing as discussed above. The takeover of the
American West by settlers of European origin, who brought along their familiar farm
animals, can be looked at as one of the consequences of global human population
explosion (see lecture 1). This brought about the largest vegetation changes in the last
5000 years, and can be described as a community reorganization: some previously
dominant species became rare, some previously rare species became dominant, and
introduced exotic species became widespread.

On the time span of 10s of years, we observe the waxing and waning of particular
species, most likely associated with climate variation. Populations may expand and
contract, as they go through periods of high recruitment or high mortality, often linked to
precipitation patterns.

On the time scale of years, vegetation changes are seen predominantly as variation in
plant productivity, linked to annual precipitation totals and timing.
6. Changes on the scale of 10s of years: climate effects on vegetation dominance

Climate affects the vegetation of arid/semiarid lands predominantly through variation in
precipitation amount and timing. To understand this we must first review the effects that
rain has on soil moisture.

Firstly, larger storm events infiltrate deeper than smaller events. However, multiple rain
events can infiltrate deeper than one event, if soil water accumulates between events.
This happens when precipitation > evapotranspiration, which is more likely to happen in
February 18, 2004
winter and/or if rainfall is intense. In Tucson, soil moisture at or below 50 cm is often
recharged twice: in the summer monsoon system (because of rainfall intensity) and in
winter (because of low evapotranspiration rates). By contrast, on the Colorado Plateau,
which is a more arid cold desert, deeper soil layers are recharged only once, in winter.
Shallow soil water (0 – 20 cm) is recharged often but is depleted rapidly, while deeper
soil layers (>50 cm) are recharged only once or twice a year, but are depleted more
slowly by plant uptake over the course of months.

These differences in the depth and the annual dynamics of soil water creates ecological
niches for plants. Grasses are specialized to take up shallow soil moisture, through a
dense, shallow root system and a physiology that allows them to respond to rainfall fast.
Shrubs and trees are better able to use deeper soil water, with their deeper root systems.
This contrasting pattern of water uptake (representing two niches) is supported by stable
isotope studies, which show greater uptake of water from recent summer rainfall events
in shallow-rooted annuals, grasses and cacti than in woody shrubs and trees. (Note: stable
isotopes are atomic variants of elements with a different number of neutrons in the atomic
nucleus. Unlike unstable radioactive isotopes, stable isotopes do not break apart. Many
elements important to life have relatively common heavier stable isotopes, such as H, O,
N, and C. In the case of the H and O isotopes which make up water, we can use the
fraction of the heavier isotopes to determine the source of water: water from winter
precipitation recharging deeper soil water has a different heavy isotope fraction than
water from summer precipitation sitting in the shallow soil.)

Walter explained the stable coexistence of grasses and woody plants in savannas through
their differential use of shallow and deeper soil water. Grasses specialize on shallow soil
water, which may be all there is in particularly dry regions. In this case, there will only be
grasses. But as it gets wetter, some water will infiltrate below the root zone of the grasses
and become available to deeper-rooted woody plants. At this point, grasses and woody
plants can coexist. Increases in summer precipitation will support a greater density of
shrubs and eventually will support even deeper-rooted trees.

James Brown and collaborators argued that increases in winter precipitation can also
increase the density of shrubs. He explained the expansion in woody vegetation in the
Chihuahua Desert as the consequence of a decade of higher than normal winter rainfall.

However, experiments by Steve Archer and coworkers suggest that precipitation may
only be a minor element in the phenomenon of woody expansion.