45
Evidence demonstrating links in precipitation patterns or temperature to recent
shrub or woody plant encroachment in southwestern North America since the 1870s
are tenuous. However, a recent prolonged drought has caused considerable Pinion
and Juniperus mortality in Pinion-Juniper woodlands in this area (Breshears et al.
2005 ; Kane et al. 2011 ). Similar mortalities in other communities have not been
documented. The severity and frequency of similar droughts are expected to increase
as the climate continues to be modifi ed. Nevertheless, the irregular nature of
encroachment of woody legumes like Vachellia, Senegalia , Prosopis, and others,
with striking differences in density and basal area in contiguous, fenced, edaphically similar areas, would seem to rule out large-scale climatic infl uences as the
major cause of the encroachment.
Increased levels of atmospheric CO 2 have been proposed as the cause of shrub
and woody legume encroachment into grasslands throughout North America including the semiarid grasslands in the southwest (Mayeux et al. 1991 ; Idso 1992 ; Polley
et al. 1992 ; Johnson et al. 1993 ). There is some evidence to link greater woody plant
density to greater growth at higher atmospheric CO 2 levels (Morgan et al. 2007 ).
This hypothesis is very attractive and could account for the synchronous, widespread encroachment of woody plants into semiarid grasslands and savannas
throughout the world; however, proof is equivocal. Dramatic density differences in
adjacent, fenced, edaphically similar areas would again seem to rule out large-scale
climatic infl uences as the major cause of increased woody plant density and biomass (Archer 1994 , 1995 ). The causes still seem to be more local and management
related (Bahre and Shelton 1993 ; Van Auken 2009 ).
Regardless, the elevated CO 2 hypothesis is based on observations that the woody
legumes and most other woody plants have the C 3 photosynthetic pathway and the
grasses that are being replaced in southwestern grasslands have the C 4 photosynthetic pathway (Begon et al. 2006 ). The C 3 photosynthetic pathway is advantageous
at higher levels of CO 2 . However, there are diffi culties with this hypothesis (Archer
et al. 1995 ). Many C 3 and C 4 species have similar photosynthetic characteristics.
There are a number of C 4 grasses that are more responsive to augmented levels of
CO 2 than previously thought. Substitution or replacement of various C 3 grasses in
the northern cold deserts or cool temperate grasslands by encroachment of C 3 woody
shrubs is not explained by the elevated CO 2 hypothesis. In southwestern North
American, C 3 woody plants like Vachellia, Senegalia , and Prosopis are replacing
the C 4 grasses, but C 3 grasses are not replacing C 4 grasses. Once more, in areas with
similar soils, fences preventing the movement of domestic herbivores reduce the
encroachment of these C 3 woody shrubs and small trees.
Finally, if one examines the timing when most of the encroachment occurred, it
does not match the time of the highest levels of atmospheric CO 2 . There is a temporal disparity between these two factors. The highest levels of atmospheric CO 2 followed the greatest extent of woody plant encroachment. Populations of woody
plants, especially the woody legumes and grasses, have shifted during the Holocene,
but the shifts do not seem to be explained by elevated levels of atmospheric CO 2 .
Furthermore, not all elevated CO 2 studies have demonstrated a fertilizer or stimulatory effect, suggesting other limitations or constraints on the grass plants that are
7 Global Climate Change
Evidence demonstrating links in precipitation patterns or temperature to recent
shrub or woody plant encroachment in southwestern North America since the 1870s
are tenuous. However, a recent prolonged drought has caused considerable Pinion
and Juniperus mortality in Pinion-Juniper woodlands in this area (Breshears et al.
2005 ; Kane et al. 2011 ). Similar mortalities in other communities have not been
documented. The severity and frequency of similar droughts are expected to increase
as the climate continues to be modifi ed. Nevertheless, the irregular nature of
encroachment of woody legumes like Vachellia, Senegalia , Prosopis, and others,
with striking differences in density and basal area in contiguous, fenced, edaphically similar areas, would seem to rule out large-scale climatic infl uences as the
major cause of the encroachment.
Increased levels of atmospheric CO 2 have been proposed as the cause of shrub
and woody legume encroachment into grasslands throughout North America including the semiarid grasslands in the southwest (Mayeux et al. 1991 ; Idso 1992 ; Polley
et al. 1992 ; Johnson et al. 1993 ). There is some evidence to link greater woody plant
density to greater growth at higher atmospheric CO 2 levels (Morgan et al. 2007 ).
This hypothesis is very attractive and could account for the synchronous, widespread encroachment of woody plants into semiarid grasslands and savannas
throughout the world; however, proof is equivocal. Dramatic density differences in
adjacent, fenced, edaphically similar areas would again seem to rule out large-scale
climatic infl uences as the major cause of increased woody plant density and biomass (Archer 1994 , 1995 ). The causes still seem to be more local and management
related (Bahre and Shelton 1993 ; Van Auken 2009 ).
Regardless, the elevated CO 2 hypothesis is based on observations that the woody
legumes and most other woody plants have the C 3 photosynthetic pathway and the
grasses that are being replaced in southwestern grasslands have the C 4 photosynthetic pathway (Begon et al. 2006 ). The C 3 photosynthetic pathway is advantageous
at higher levels of CO 2 . However, there are diffi culties with this hypothesis (Archer
et al. 1995 ). Many C 3 and C 4 species have similar photosynthetic characteristics.
There are a number of C 4 grasses that are more responsive to augmented levels of
CO 2 than previously thought. Substitution or replacement of various C 3 grasses in
the northern cold deserts or cool temperate grasslands by encroachment of C 3 woody
shrubs is not explained by the elevated CO 2 hypothesis. In southwestern North
American, C 3 woody plants like Vachellia, Senegalia , and Prosopis are replacing
the C 4 grasses, but C 3 grasses are not replacing C 4 grasses. Once more, in areas with
similar soils, fences preventing the movement of domestic herbivores reduce the
encroachment of these C 3 woody shrubs and small trees.
Finally, if one examines the timing when most of the encroachment occurred, it
does not match the time of the highest levels of atmospheric CO 2 . There is a temporal disparity between these two factors. The highest levels of atmospheric CO 2 followed the greatest extent of woody plant encroachment. Populations of woody
plants, especially the woody legumes and grasses, have shifted during the Holocene,
but the shifts do not seem to be explained by elevated levels of atmospheric CO 2 .
Furthermore, not all elevated CO 2 studies have demonstrated a fertilizer or stimulatory effect, suggesting other limitations or constraints on the grass plants that are
7 Global Climate Change