3
These woody legumes are important components of many worldwide arid and
semiarid ecosystems (Whittaker 1975 ; Crisp and Lange 1976 ; Simpson 1977 ;
Benson 1979 ; Felker 1981 ; Barbour and Billings 1988 ). Today there are reports of
large expanses of Prosopis, Senegalia, and Vachellia shrub lands in southwestern
North America including large areas in the South Texas Plains and the plains of
northern Mexico (Gould 1969 ; Correll and Johnston 1979). In the past this region or
at least large parts of it were described as grassland or savanna (Johnston 1963 ;
Gould 1969 ; Correll and Johnston 1979; Anderson 2006 ). Other studies including
interpretations from the earliest travelers (late 1600s) suggest that this area was a
mosaic of vegetation (see Inglis 1962 ) with intermittent thickets or motts, probably
consisting of legumes like Prosopis or various Senegalia and Vachellia species,
probably on deeper soils along some of the creeks or drainages or on rocky outcrops
protected from fi re. Some of the earliest reports of the vegetation were from travelers
in expeditions through northeastern Mexico through the now thorn woodlands
(Tamaulipan woodlands and south Texas Plains) north through the San Antonio area
of Texas. The larger and permanent streams and rivers probably had riparian or gallery forests as they do today (Bush and Van Auken 1984 ). Much of the uplands were
most likely savannas with grassland and shrub land phases, but the grasslands have
apparently been shrinking with increased invasion or encroachment of woody plants
as domestic grazing increased and fi re frequency has been reduced. This phenomenon of decreasing grasslands with a concomitant increase of woodlands, savannas,
or shrub lands is occurring or has occurred worldwide (Van Auken 2000 , 2009 ;
Knapp et al. 2008 ; Scholes 2009 ; Wigley et al. 2010 ; Sharp et al. 2012 ).
Invasion is really the establishment, growth, and survival of a species in a new
area or community with the species coming from afar, probably another continent
(Mooney and Drake 1991 ). Encroachment is the increase in density and spread of
native species into closely associated communities, usually grasslands. These species have been present in these same or adjacent communities for thousands of years
(Inglis 1962 ). Encroachment is the phenomenon occurring in the American southwest and in other grasslands all over the world (Van Auken 2000 , 2009 ). The process of encroachment is usually considered as the entry of woody plants, in many
cases woody legumes, into grasslands. There have been many studies of the changes
wrought by these woody plants as they enter grasslands, especially as they affect
grass forage and domestic animal production (Browning and Archer 2011 ; Allred
et al. 2012 ). However, the process appears to be very similar or the same as early
secondary succession (Begon et al. 2006 ) which will be presented later.
Most of the former grasslands of this area of southwestern North America
today are covered with various woody species including Prosopis glandulosa
(mesquite or honey mesquite and possibly other species of Prosopis ), Celtis pallida (desert hackberry), Leucophyllum frutescens (Texas sage), Aloysia gratissima
(white brush), Baccharis neglecta (baccharis), and other woody species as well as
various woody legumes including Vachellia farnesiana (= Acacia farnesiana=A.
smallii, huisache), V. rigidula (= A. rigidula, blackbrush), V. tortuosa (= A . tortuosa, huisachillo), V. neovernicosa (= A. neovernicosa , viscid acacia), V. constricta
(= A. constricta , mescat acacia), Senegalia berlandieri (= A. berlandieri, guajillo,
1 Introduction
These woody legumes are important components of many worldwide arid and
semiarid ecosystems (Whittaker 1975 ; Crisp and Lange 1976 ; Simpson 1977 ;
Benson 1979 ; Felker 1981 ; Barbour and Billings 1988 ). Today there are reports of
large expanses of Prosopis, Senegalia, and Vachellia shrub lands in southwestern
North America including large areas in the South Texas Plains and the plains of
northern Mexico (Gould 1969 ; Correll and Johnston 1979). In the past this region or
at least large parts of it were described as grassland or savanna (Johnston 1963 ;
Gould 1969 ; Correll and Johnston 1979; Anderson 2006 ). Other studies including
interpretations from the earliest travelers (late 1600s) suggest that this area was a
mosaic of vegetation (see Inglis 1962 ) with intermittent thickets or motts, probably
consisting of legumes like Prosopis or various Senegalia and Vachellia species,
probably on deeper soils along some of the creeks or drainages or on rocky outcrops
protected from fi re. Some of the earliest reports of the vegetation were from travelers
in expeditions through northeastern Mexico through the now thorn woodlands
(Tamaulipan woodlands and south Texas Plains) north through the San Antonio area
of Texas. The larger and permanent streams and rivers probably had riparian or gallery forests as they do today (Bush and Van Auken 1984 ). Much of the uplands were
most likely savannas with grassland and shrub land phases, but the grasslands have
apparently been shrinking with increased invasion or encroachment of woody plants
as domestic grazing increased and fi re frequency has been reduced. This phenomenon of decreasing grasslands with a concomitant increase of woodlands, savannas,
or shrub lands is occurring or has occurred worldwide (Van Auken 2000 , 2009 ;
Knapp et al. 2008 ; Scholes 2009 ; Wigley et al. 2010 ; Sharp et al. 2012 ).
Invasion is really the establishment, growth, and survival of a species in a new
area or community with the species coming from afar, probably another continent
(Mooney and Drake 1991 ). Encroachment is the increase in density and spread of
native species into closely associated communities, usually grasslands. These species have been present in these same or adjacent communities for thousands of years
(Inglis 1962 ). Encroachment is the phenomenon occurring in the American southwest and in other grasslands all over the world (Van Auken 2000 , 2009 ). The process of encroachment is usually considered as the entry of woody plants, in many
cases woody legumes, into grasslands. There have been many studies of the changes
wrought by these woody plants as they enter grasslands, especially as they affect
grass forage and domestic animal production (Browning and Archer 2011 ; Allred
et al. 2012 ). However, the process appears to be very similar or the same as early
secondary succession (Begon et al. 2006 ) which will be presented later.
Most of the former grasslands of this area of southwestern North America
today are covered with various woody species including Prosopis glandulosa
(mesquite or honey mesquite and possibly other species of Prosopis ), Celtis pallida (desert hackberry), Leucophyllum frutescens (Texas sage), Aloysia gratissima
(white brush), Baccharis neglecta (baccharis), and other woody species as well as
various woody legumes including Vachellia farnesiana (= Acacia farnesiana=A.
smallii, huisache), V. rigidula (= A. rigidula, blackbrush), V. tortuosa (= A . tortuosa, huisachillo), V. neovernicosa (= A. neovernicosa , viscid acacia), V. constricta
(= A. constricta , mescat acacia), Senegalia berlandieri (= A. berlandieri, guajillo,
1 Introduction