44
During preceding glacial and interglacial periods, plant communities moved as
the climate warmed or cooled without the infl uence of man or his animals. Today
anthropogenic forces, man and his animals, play a major role in the development
and distribution of plants and their communities. However, tying together causes
and effects of these changes is much harder to do. Changes in distributions of former plant communities have been noted, and locations have been estimated. Many
of these modifi cations in distributions of populations of plants and animals in the
American southwest and other places have been estimated from pollen records and
fossil packrat middens (Betancourt et al. 1990 ; Miller and Wigand 1994 ; Van
Devender 1995 ; Martin 1999 ). In addition, the use of radiocarbon dating and oxygen isotope data from deep sea cores has been interpreted and used to develop a
single time series to represent global changes in this and other areas throughout the
world (McDowell et al. 1995 ).
Widespread shifts in plant community distributions are predicted in response to
future altered rainfall and temperature regimes associated with the changing global
climate (Adams et al. 2009 ; Kane et al. 2011 ). Projected higher temperatures are
expected to compound the effects of severe droughts and elicit shifts in woody plant
mortality and thus plant communities. The most dramatic shifts are expected to be
caused by hurricanes, tornados, freezes, fi res, and extreme droughts ( Breshears et al.
2005 ). These predicted changes in woody plant mortality could change a plant community dramatically. Forests or woodlands could be converted to grasslands or savannas. This could be considered the reverse or a reset of community succession or a
restart of the successional process (Begon et al. 2006 ). If the disturbance was a severe
drought and caused the death of the majority of the dominant woody plants as occur
recently in the American southwest, the resulting community could be converted to
grassland. This could be the start of another successional process or event. The grassland could be encroached by various woody plants including woody legumes. With
the encroachment, establishment, survival, and growth of the woody legumes and
possibly other woody plants, a savanna would ensue, followed by community succession leading to the area climax community. The climax community would be determined by the interaction of all of the environmental conditions that created the
pre-disturbance community. All of these factors have to be taken into consideration
or examined for the long term, not the short term. Short-term interpretation of longterm events lead to mistakes, misinterpretations, and misunderstandings.
Prior to the current warming cycle, pine parkland and juniper woodland or
savanna was common in what are now southwestern North American semiarid
grasslands or desert grasslands (Betancourt et al. 1990 ; Miller and Wigand 1994 ;
Van Devender 1995 ; Martin 1999 ). These communities have appeared and disappeared in these same areas over the millennia as atmospheric CO 2 levels and global
temperatures have changed. The semiarid and desert grasslands of the American
southwest were lower in elevation and more southward in the past. Desert shrublands were lower in elevation and more southward as well. In spite of what is known
of the locations of these communities, it has been diffi cult to link climatic changes
over the past 160 years to recent vegetation changes because of the relatively short
time, considerable variability, and various interacting factors.
7 Global Climate Change
During preceding glacial and interglacial periods, plant communities moved as
the climate warmed or cooled without the infl uence of man or his animals. Today
anthropogenic forces, man and his animals, play a major role in the development
and distribution of plants and their communities. However, tying together causes
and effects of these changes is much harder to do. Changes in distributions of former plant communities have been noted, and locations have been estimated. Many
of these modifi cations in distributions of populations of plants and animals in the
American southwest and other places have been estimated from pollen records and
fossil packrat middens (Betancourt et al. 1990 ; Miller and Wigand 1994 ; Van
Devender 1995 ; Martin 1999 ). In addition, the use of radiocarbon dating and oxygen isotope data from deep sea cores has been interpreted and used to develop a
single time series to represent global changes in this and other areas throughout the
world (McDowell et al. 1995 ).
Widespread shifts in plant community distributions are predicted in response to
future altered rainfall and temperature regimes associated with the changing global
climate (Adams et al. 2009 ; Kane et al. 2011 ). Projected higher temperatures are
expected to compound the effects of severe droughts and elicit shifts in woody plant
mortality and thus plant communities. The most dramatic shifts are expected to be
caused by hurricanes, tornados, freezes, fi res, and extreme droughts ( Breshears et al.
2005 ). These predicted changes in woody plant mortality could change a plant community dramatically. Forests or woodlands could be converted to grasslands or savannas. This could be considered the reverse or a reset of community succession or a
restart of the successional process (Begon et al. 2006 ). If the disturbance was a severe
drought and caused the death of the majority of the dominant woody plants as occur
recently in the American southwest, the resulting community could be converted to
grassland. This could be the start of another successional process or event. The grassland could be encroached by various woody plants including woody legumes. With
the encroachment, establishment, survival, and growth of the woody legumes and
possibly other woody plants, a savanna would ensue, followed by community succession leading to the area climax community. The climax community would be determined by the interaction of all of the environmental conditions that created the
pre-disturbance community. All of these factors have to be taken into consideration
or examined for the long term, not the short term. Short-term interpretation of longterm events lead to mistakes, misinterpretations, and misunderstandings.
Prior to the current warming cycle, pine parkland and juniper woodland or
savanna was common in what are now southwestern North American semiarid
grasslands or desert grasslands (Betancourt et al. 1990 ; Miller and Wigand 1994 ;
Van Devender 1995 ; Martin 1999 ). These communities have appeared and disappeared in these same areas over the millennia as atmospheric CO 2 levels and global
temperatures have changed. The semiarid and desert grasslands of the American
southwest were lower in elevation and more southward in the past. Desert shrublands were lower in elevation and more southward as well. In spite of what is known
of the locations of these communities, it has been diffi cult to link climatic changes
over the past 160 years to recent vegetation changes because of the relatively short
time, considerable variability, and various interacting factors.
7 Global Climate Change