27
A species that takes its place during secondary succession is C. laevigata. It appears
to be a shade or shade-tolerant species, and its dry mass did not increase linearly
with increased light levels ( r = 0.79, p > 0.05), but reached a plateau at low light
levels (Fig. 5.2 ). Vachellia farnesiana seedlings are found in open disturbed areas
and open grassland associated with supposed parent trees (partial shade) early in
succession (Fig. 4.3 ), but they are not present when the canopy closes with community development probably because of low light levels below the canopy. The
reverse is true for C. laevigata (Fig. 4.3 ), and its growth is promoted and not suppressed in low light (Bush and Van Auken 1986 ).
Transplant studies have been done with both V. farnesiana and C. laevigata
(Lohstroh and Van Auken 1987 ; Van Auken and Lohstroh 1990 ). Both species were
germinated from seed and placed in a Frio clay-loam low nutrient soil (Taylor et al.
1966 ). Plants of both species were grown in a greenhouse for seven weeks before
being transplanted into the fi eld site. Plants were grown in 15 cm deep plastic pots
containing 1,400 g of dried and sieved soil. The experiment was a factorial experiment, and plants were placed under the canopy of mature V. farnesiana trees or in
gaps between the trees. Nutrients were added or not added, roots of other species
were present or removed via trenching to a 20 cm depth, and insect herbivores were
present or removed via the addition of insecticide. Mean light levels below the canopy were 515 ± 153 μM/m
2 /s, and in the gaps, they were 2,173 ± 174 μM/m
2 /s on a
clear midday, measured with a Li-cor
® LI-188 integrating quantum sensor. The
experiment was harvested on the fi rst of November which was 12 weeks after transplanting into the fi eld. The only signifi cant differences between treatments were for
canopy position. Vachellia farnesiana aboveground dry mass in the open or gaps
was approximately twice as high as the dry mass below the V. farnesiana canopy
with none of the other treatments showing signifi cant differences. The reverse was
true for C. laevigata, and aboveground dry mass below the V. farnesiana canopy
was approximately four times higher than dry mass in the open or gaps with none of
the other treatments showing signifi cant differences.
The appearance and function of Prosopis glandulosa (Archer et al. 1988 ) in former arid and semiarid C 4 temperate, subtropical, and tropical grassland seems very
similar to what is reported for V. farnesiana . Prosopis species appear to be early
successional species that encroach and establish on low nutrient or low nitrogen
soils or in disturbances in grasslands or savannas with constant, heavy domestic
grazing (Archer et al. 1988 ; Archer 1994 ). Prosopis will grow in low nutrient soils,
but its growth is promoted by nutrient addition to the soil, including N, P, K, and S
(Fig. 5.3 ) (Van Auken and Bush 1989 ). The greatest stimulation was with the addition of all four nutrients.
The importance of high rates of biological nitrogen fi xation in early successional
or disturbed communities and the role of legumes, like Prosopis , in these communities have been known for a considerable time (West and Klemmedson 1978 ). Soil
resource levels, especially nitrogen, below a Prosopis julifl ora canopy in the
Sonoran desert were higher than values in gaps or intervening areas between canopies (West and Klemmedson 1978 ). Total nitrogen levels in the intervening gaps
between Prosopis plants were 0.02–0.04 %, while at the surface below the Prosopis
5 Factors that Determine Growth Rates
A species that takes its place during secondary succession is C. laevigata. It appears
to be a shade or shade-tolerant species, and its dry mass did not increase linearly
with increased light levels ( r = 0.79, p > 0.05), but reached a plateau at low light
levels (Fig. 5.2 ). Vachellia farnesiana seedlings are found in open disturbed areas
and open grassland associated with supposed parent trees (partial shade) early in
succession (Fig. 4.3 ), but they are not present when the canopy closes with community development probably because of low light levels below the canopy. The
reverse is true for C. laevigata (Fig. 4.3 ), and its growth is promoted and not suppressed in low light (Bush and Van Auken 1986 ).
Transplant studies have been done with both V. farnesiana and C. laevigata
(Lohstroh and Van Auken 1987 ; Van Auken and Lohstroh 1990 ). Both species were
germinated from seed and placed in a Frio clay-loam low nutrient soil (Taylor et al.
1966 ). Plants of both species were grown in a greenhouse for seven weeks before
being transplanted into the fi eld site. Plants were grown in 15 cm deep plastic pots
containing 1,400 g of dried and sieved soil. The experiment was a factorial experiment, and plants were placed under the canopy of mature V. farnesiana trees or in
gaps between the trees. Nutrients were added or not added, roots of other species
were present or removed via trenching to a 20 cm depth, and insect herbivores were
present or removed via the addition of insecticide. Mean light levels below the canopy were 515 ± 153 μM/m
2 /s, and in the gaps, they were 2,173 ± 174 μM/m
2 /s on a
clear midday, measured with a Li-cor
® LI-188 integrating quantum sensor. The
experiment was harvested on the fi rst of November which was 12 weeks after transplanting into the fi eld. The only signifi cant differences between treatments were for
canopy position. Vachellia farnesiana aboveground dry mass in the open or gaps
was approximately twice as high as the dry mass below the V. farnesiana canopy
with none of the other treatments showing signifi cant differences. The reverse was
true for C. laevigata, and aboveground dry mass below the V. farnesiana canopy
was approximately four times higher than dry mass in the open or gaps with none of
the other treatments showing signifi cant differences.
The appearance and function of Prosopis glandulosa (Archer et al. 1988 ) in former arid and semiarid C 4 temperate, subtropical, and tropical grassland seems very
similar to what is reported for V. farnesiana . Prosopis species appear to be early
successional species that encroach and establish on low nutrient or low nitrogen
soils or in disturbances in grasslands or savannas with constant, heavy domestic
grazing (Archer et al. 1988 ; Archer 1994 ). Prosopis will grow in low nutrient soils,
but its growth is promoted by nutrient addition to the soil, including N, P, K, and S
(Fig. 5.3 ) (Van Auken and Bush 1989 ). The greatest stimulation was with the addition of all four nutrients.
The importance of high rates of biological nitrogen fi xation in early successional
or disturbed communities and the role of legumes, like Prosopis , in these communities have been known for a considerable time (West and Klemmedson 1978 ). Soil
resource levels, especially nitrogen, below a Prosopis julifl ora canopy in the
Sonoran desert were higher than values in gaps or intervening areas between canopies (West and Klemmedson 1978 ). Total nitrogen levels in the intervening gaps
between Prosopis plants were 0.02–0.04 %, while at the surface below the Prosopis
5 Factors that Determine Growth Rates