28
canopy, total nitrogen values were approximately 0.06 %. Total soil nitrogen levels
were higher below the Prosopis canopy compared to the surrounding gaps or open
area soils to a depth of 20–25 cm.
Total soil carbon and nitrogen levels approximately doubled over a period of 20
years in Vachellia communities a little farther north and east (Bush 2008 ). Similar
trends were found in other woody legume communities with soil nitrogen below the
canopy of S. berlandieri and V. rigidula being 5 and 13 times higher compared to
gaps or open areas between the plants. There are similar fi ndings for soil nitrogen
levels in other parts of the arid regions of southwestern North America including
areas in both the Chihuahuan and Sonoran deserts (Virginia and Jarrell 1983 ;
Virginia 1986 ; Schlesinger et al. 1996 ).
Lower light levels below the Prosopis canopy have also been demonstrated
(Bush and Van Auken 1987 ). Light levels below the Prosopis glandulosa canopy
above the herbaceous layer were 20–45 % of full light levels in associated gaps.
Below the herbaceous layer, light levels were further reduced to 10–25 % of the full
light levels (Fig. 5.4 ).
There were signifi cant differences in light levels above the surface vegetation
and below the surface vegetation in the open compared to under the canopy (oneway ANOVA for both positions, P < 0.001). Prosopis glandulosa growth is promoted by high levels of visible light suggesting that it is a sun not a shade species
and its growth is suppressed in its own shade below its own canopy or by the shade
of other canopy species (Bush and Van Auken 1987 ). Its dry mass increased linearly
with increased light levels over the range of light levels examined ( r
2 = 0.99,
P ≥ 0.001) (Fig. 5.5 ).
Fig. 5.3 Mean total, aboveground, and belowground dry mass of Prosopis glandulosa with complete nutrient addition ( left ), minus nitrogen, minus phosphorus, and a control with no added
nutrients( right ) (Van Auken and Bush 1989 ). The ANOVA for total, aboveground, and belowground dry mass was signifi cant ( P < 0.0001 for each variable), with the complete media (all
added) being signifi cantly different from the control (no added nutrients), but not different from
the No-N or No-P added nutrients (Scheffé Multiple Range Test, P > 0.05)
5 Factors that Determine Growth Rates
canopy, total nitrogen values were approximately 0.06 %. Total soil nitrogen levels
were higher below the Prosopis canopy compared to the surrounding gaps or open
area soils to a depth of 20–25 cm.
Total soil carbon and nitrogen levels approximately doubled over a period of 20
years in Vachellia communities a little farther north and east (Bush 2008 ). Similar
trends were found in other woody legume communities with soil nitrogen below the
canopy of S. berlandieri and V. rigidula being 5 and 13 times higher compared to
gaps or open areas between the plants. There are similar fi ndings for soil nitrogen
levels in other parts of the arid regions of southwestern North America including
areas in both the Chihuahuan and Sonoran deserts (Virginia and Jarrell 1983 ;
Virginia 1986 ; Schlesinger et al. 1996 ).
Lower light levels below the Prosopis canopy have also been demonstrated
(Bush and Van Auken 1987 ). Light levels below the Prosopis glandulosa canopy
above the herbaceous layer were 20–45 % of full light levels in associated gaps.
Below the herbaceous layer, light levels were further reduced to 10–25 % of the full
light levels (Fig. 5.4 ).
There were signifi cant differences in light levels above the surface vegetation
and below the surface vegetation in the open compared to under the canopy (oneway ANOVA for both positions, P < 0.001). Prosopis glandulosa growth is promoted by high levels of visible light suggesting that it is a sun not a shade species
and its growth is suppressed in its own shade below its own canopy or by the shade
of other canopy species (Bush and Van Auken 1987 ). Its dry mass increased linearly
with increased light levels over the range of light levels examined ( r
2 = 0.99,
P ≥ 0.001) (Fig. 5.5 ).
Fig. 5.3 Mean total, aboveground, and belowground dry mass of Prosopis glandulosa with complete nutrient addition ( left ), minus nitrogen, minus phosphorus, and a control with no added
nutrients( right ) (Van Auken and Bush 1989 ). The ANOVA for total, aboveground, and belowground dry mass was signifi cant ( P < 0.0001 for each variable), with the complete media (all
added) being signifi cantly different from the control (no added nutrients), but not different from
the No-N or No-P added nutrients (Scheffé Multiple Range Test, P > 0.05)
5 Factors that Determine Growth Rates