19
this to happen and the communities would probably continue as woody legume
savannas with higher levels of carbon and nitrogen below the canopy and lower
levels in the intervening patches (Jurena and Van Auken 1998 ). This would probably be similar to what has been reported in other woody legume communities in the
Tamaulipan region of south Texas, northeastern Mexico, and the Chihuahuan Desert
in New Mexico (Archer et al. 1988 ; Schlesinger et al. 1996 ).
In time, the Vachellia or Senegalia species that are the focus of these patches of
woodland in the savannas would die and disappear from these communities (Bush
and Van Auken 1987a ; Jurena and Van Auken 1998 ), similar to what happens to
other woody legume patches or thickets in this region (Archer et al. 1988 ; Archer
1994 ). Vachellia farnesiana would not be able to replace itself in the savanna patches
or woodland patches probably because of lower light levels below its canopy (Bush
and Van Auken 1986a ). Replacement species would be any number of trees or
shrubs capable of starting growth in the higher nitrogen soil and the low light environment of the canopy understory (Bush and Van Auken 1986b , 1987b ). The
replacement species in relatively mesic areas, in parts of south Texas and northeastern Mexico, would probably be Celtis laevigata (hackberry or sugarberry), Ulmus
crassifolia (cedar elm or fall elm), C. pallida (desert hackberry), Bumelia lanuginosa (bumelia), Melia azedarach (chinaberry), Ehretia anacua (anacua), possibly
Quercus fusiformis (live oak), and others (Van Auken and Bush 1985 ). However,
light and soil nitrogen requirements of most of these species are not known.
In drier areas of the American southwest, the proposed woody legume pioneer
species or encroaching species would be different and could include Senegalia
berlandieri (gaujillo), Vachellia rigidula (blackbrush acacia), V. neovernicosa
( A. neovernicosa ), V. constricta ( A. constricta ), and possibly others. Canopies
would be more open with higher light levels below the canopy, but in addition, there
would still be higher levels of soil nitrogen below the canopies (Jurena and Van
Auken 1998 ; Powell 1998 ). Seedlings of the same mature canopy species were
found below the canopy of V. rigidula and S. berlandieri in these communities, but
at low density, and 17–24 other woody or succulent species were found below the
canopies as well. Some species were only found below the woody legume canopies,
but fi ndings were not consistent. The canopies seemed to be important for some
species either because of higher levels of nitrogen in the soil or lower light levels,
but it was not obvious which species would be replacement species. The communities with these species of woody legumes are extensive in area covered in the
American southwest but less than the area covered by Prosopis . These communities
are more open with lower stature than the mature V. farnesiana communities that are
in areas of higher rainfall and soil moisture.
The studies that we have just cited concerning encroachment and secondary succession are not temporal studies but spatial studies or studies where the species
seem to fi t into a successional sequence based on dendrochronological and other
observations, but the communities that we studied were not really a temporally
sequence as indicated above. These studies were used to develop a descriptive
model of woody legume secondary succession in the southwestern North American
4 Woody Legume Community Structure
this to happen and the communities would probably continue as woody legume
savannas with higher levels of carbon and nitrogen below the canopy and lower
levels in the intervening patches (Jurena and Van Auken 1998 ). This would probably be similar to what has been reported in other woody legume communities in the
Tamaulipan region of south Texas, northeastern Mexico, and the Chihuahuan Desert
in New Mexico (Archer et al. 1988 ; Schlesinger et al. 1996 ).
In time, the Vachellia or Senegalia species that are the focus of these patches of
woodland in the savannas would die and disappear from these communities (Bush
and Van Auken 1987a ; Jurena and Van Auken 1998 ), similar to what happens to
other woody legume patches or thickets in this region (Archer et al. 1988 ; Archer
1994 ). Vachellia farnesiana would not be able to replace itself in the savanna patches
or woodland patches probably because of lower light levels below its canopy (Bush
and Van Auken 1986a ). Replacement species would be any number of trees or
shrubs capable of starting growth in the higher nitrogen soil and the low light environment of the canopy understory (Bush and Van Auken 1986b , 1987b ). The
replacement species in relatively mesic areas, in parts of south Texas and northeastern Mexico, would probably be Celtis laevigata (hackberry or sugarberry), Ulmus
crassifolia (cedar elm or fall elm), C. pallida (desert hackberry), Bumelia lanuginosa (bumelia), Melia azedarach (chinaberry), Ehretia anacua (anacua), possibly
Quercus fusiformis (live oak), and others (Van Auken and Bush 1985 ). However,
light and soil nitrogen requirements of most of these species are not known.
In drier areas of the American southwest, the proposed woody legume pioneer
species or encroaching species would be different and could include Senegalia
berlandieri (gaujillo), Vachellia rigidula (blackbrush acacia), V. neovernicosa
( A. neovernicosa ), V. constricta ( A. constricta ), and possibly others. Canopies
would be more open with higher light levels below the canopy, but in addition, there
would still be higher levels of soil nitrogen below the canopies (Jurena and Van
Auken 1998 ; Powell 1998 ). Seedlings of the same mature canopy species were
found below the canopy of V. rigidula and S. berlandieri in these communities, but
at low density, and 17–24 other woody or succulent species were found below the
canopies as well. Some species were only found below the woody legume canopies,
but fi ndings were not consistent. The canopies seemed to be important for some
species either because of higher levels of nitrogen in the soil or lower light levels,
but it was not obvious which species would be replacement species. The communities with these species of woody legumes are extensive in area covered in the
American southwest but less than the area covered by Prosopis . These communities
are more open with lower stature than the mature V. farnesiana communities that are
in areas of higher rainfall and soil moisture.
The studies that we have just cited concerning encroachment and secondary succession are not temporal studies but spatial studies or studies where the species
seem to fi t into a successional sequence based on dendrochronological and other
observations, but the communities that we studied were not really a temporally
sequence as indicated above. These studies were used to develop a descriptive
model of woody legume secondary succession in the southwestern North American
4 Woody Legume Community Structure