Periodic flooding and grazing are thought to be important in the maintenance of native temperate humid grasslands of the Flooding Pampa in eastern Argentina. Studies were undertaken to assess the effect of the interaction between flooding and grazing upon recruitment of dallisgrass Paspalum dilatatum, a dominant native perennial grass of the Flooding Pampa in this region.
Experiments were undertaken to investigate the effects of microhabitat conditions associated with winter flooding and spring-summer grazing on dallisgrassseed germination and seedling establishment.
Twenty (30 x 40 x 20 cm) soil blocks (‘mesocosms’) with vegetation intact were collected in late summer from grazed natural grassland. Seedling emergence from the seed bank (and also from seeds buried in mesh bags) was studied in a replicated factorial experiment. Three months after collection, treatments applied to the blocks were: unflooded or flooded in winter (i.e. 5 cm depth for 15 days in August), and low or high intensity defoliation (mowing to 2 cm height when vegetation reached 5 cm, or mowing to 10 cm height when it reached 20 cm, respectively). Seedling emergence and survival were monitored regularly until the end of growing season (March).
Additional laboratory experiments were undertaken to investigate germination under different combinations of temperature, light and simulated flooding.
Dallisgrass germination and seedling emergence were promoted by flooding and high intensity defoliation (simulating heavy grazing). Gaps were created by flooding and maintained by the higher defoliation which further led to increased seedling recruitment.
In the summer following treatment, unflooded blocks had about 10 seedlings/m² (regardless of mowing intensity). Flooded blocks with low intensity defoliation had about 55 seedlings/m²; those under high intensity defoliation about 105 seedlings/m².
Flooding broke seed dormancy and higher germination rates were associated with alternating temperature and light.
These results indicate that microhabitat conditions associated with current disturbances, i.e. flooding and grazing, synergistically promote the recruitment of dallisgrass.
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