11
O.W. Van Auken and J.K. Bush, Invasion of Woody Legumes, SpringerBriefs in Ecology 4,
DOI 10.1007/978-1-4614-7199-8_3, © Springer Science+Business Media New York 2013
There are estimations suggesting that approximately 60 million ha of grasslands in
the American southwest have been encroached by various woody species and converted to savannas, shrublands, or woodlands (Humphrey 1958 ; Grover and Musick
1990 ). Interestingly, this is more than the total estimated area of these grasslands
(Laurenroth 1979 ). Thus, the composition, cover, density, productivity, and structure
of most if not all of these grasslands has changed. In addition, it has been suggested
that 220–330 million ha of all North American grasslands have been encroached by
woody species (Knapp et al. 2008 ). Not all of this woody plant encroachment is by
woody legumes, but a considerable amount is (Van Auken 2009 ). Prosopis,
Senegalia, and Vachellia are genera of woody legumes with major encroaching species in southwestern North American grasslands, but they are not the only species.
Encroachment of grasslands by woody plants from adjacent communities seems
to be the start of a process called succession. Succession or ecological succession is
the “non-seasonal, directional and continuous pattern of colonization and extinction
on a site by a species populations” (Begon et al. 2006 ; Smith and Smith 2012 ).
There are really two types of terrestrial succession recognized by ecologists.
Primary succession is found or reported on newly exposed areas or landforms. The
area in question has not been previously occupied by biotic species or communities
of any type, and there is no evidence of any soil development. Secondary succession
on the other hand occurs on a site when a community has been removed partially or
completely, and for the most part, only the soil remains. The causes of the community and species removal are various factors. It could be fi res, hurricanes, tornados,
high winds, and disease of some type or anthropogenic activities including tree
harvest, agriculture, or grazing. The start of recovery of the community in terms of
secondary succession begins after the disturbance but seems to be called encroachment in many cases. This is when the woody plants start to establish in the disturbed
grassland community. The return to the previous pre-disturbance community can
take a short time or a very long time depending on the type of community being
replaced and the environmental conditions of the area together with or especially
climatic conditions.
Chapter 3
Encroachment and Secondary Succession
O.W. Van Auken and J.K. Bush, Invasion of Woody Legumes, SpringerBriefs in Ecology 4,
DOI 10.1007/978-1-4614-7199-8_3, © Springer Science+Business Media New York 2013
There are estimations suggesting that approximately 60 million ha of grasslands in
the American southwest have been encroached by various woody species and converted to savannas, shrublands, or woodlands (Humphrey 1958 ; Grover and Musick
1990 ). Interestingly, this is more than the total estimated area of these grasslands
(Laurenroth 1979 ). Thus, the composition, cover, density, productivity, and structure
of most if not all of these grasslands has changed. In addition, it has been suggested
that 220–330 million ha of all North American grasslands have been encroached by
woody species (Knapp et al. 2008 ). Not all of this woody plant encroachment is by
woody legumes, but a considerable amount is (Van Auken 2009 ). Prosopis,
Senegalia, and Vachellia are genera of woody legumes with major encroaching species in southwestern North American grasslands, but they are not the only species.
Encroachment of grasslands by woody plants from adjacent communities seems
to be the start of a process called succession. Succession or ecological succession is
the “non-seasonal, directional and continuous pattern of colonization and extinction
on a site by a species populations” (Begon et al. 2006 ; Smith and Smith 2012 ).
There are really two types of terrestrial succession recognized by ecologists.
Primary succession is found or reported on newly exposed areas or landforms. The
area in question has not been previously occupied by biotic species or communities
of any type, and there is no evidence of any soil development. Secondary succession
on the other hand occurs on a site when a community has been removed partially or
completely, and for the most part, only the soil remains. The causes of the community and species removal are various factors. It could be fi res, hurricanes, tornados,
high winds, and disease of some type or anthropogenic activities including tree
harvest, agriculture, or grazing. The start of recovery of the community in terms of
secondary succession begins after the disturbance but seems to be called encroachment in many cases. This is when the woody plants start to establish in the disturbed
grassland community. The return to the previous pre-disturbance community can
take a short time or a very long time depending on the type of community being
replaced and the environmental conditions of the area together with or especially
climatic conditions.
Chapter 3
Encroachment and Secondary Succession