Effects of fire on Phragmites Australis (cav.) trin. ex steudel and associated species at Delta Marsh, Manitoba
Thompson, Donald James.
In Delta Marsh the responses of a Phragmites australis dominated community were monitored after prescribed burning performed in the spring, summer and fall. The aerial standing crop produced by Phragmites australis in 1979 in this stand was similar to that reported for similar habitats in other temperate regions. Much of the within stand variation in P. australis performance was explained with reference to a soil moisture gradient within the stand and to the uneven age structure of the stand at different parts of the gradient. P. australis accounted for 91% of the aerial standing crop of the community, which contained seven substorey species. To a greater degree than that of the dominant the performance of the substorey species was related to the moisture gradient. Two upland species (Cirsium arvense and Urtica dioica) occurred more frequently in the drier part of the stand but labiates such as Lycopus asper, Mentha arvensis and Teucrium occidentale were more frequent at intermediate moisture levels. Several microenvironmental changes occurred after summer burning. Light intensity increased and air soil temperatures fluctuated more. There was a release of mineral nutrients including nitrate, phosphate and potassium. Reed regrowth began approximately a week after burning and mowing, and continued until frost in late October. The aerial standing crop of regrowth was similar after burning and mowing but shoot density was greater following mowing. Shoots on summer burned plots were more phenologically advanced than those produced on mowed plots. There was a decline in rhizome reserves following burning and mowing in comparison with the controls. Seeds of several substorey species germinated after burning and mowing which was related to the microenvironmental changes resulting from litter and canopy removal. Of the substorey species C. arvense and U. dioica sprouted regrowth most vigorously following both treatments. In the 1980 growing season the aerial standing crop of P. australis increased after fall and spring burning but declined following summer burning. Shoot density increased dramatically after all burning treatments due to the production of many smaller vegetative shoots. Flowering shoot density increased after spring burning. Both flowering and vegetative shoots were shorter and of smaller basal diameter following summer burning, which has potential as a method of controlling P. australis growth. Below-ground production was greater following fall and spring burning but after summer burning was similar to the controls. Changes in the performance of P. australis were related to changes in its morphology and seasonal growth cycle which occurred because of the burning treatments. After spring and summer burning, with the onset of vernal growth, the total non-structural carbohydrate (TNC) content of P. australis rhizomes declined to lower levels than in the controls but by the end of the growth season rose to higher levels than in the controls. There were differences in the timing of the drop in rhizome reserves related to differences in the phenological development of the aerial shoots which arose from the rhizomes. The greater decline in reserve levels with the onset of vernal growth after spring burning was related to increased shoot density and in inflorescence production. The decline after summer burning was related to increased shoot density as well as an initially lower TNC level at the begining of the growing season as a result of the production of regrowth in the previous fall. In the 1980 growing season the magnitude of changes in the dynamics of the plant community depended upon the season of burning. The changes were greatest after summer burning and mowing where the competitive ability of P. australis was reduced. In contrast almost no change in substorey composition or production occurred after spring burning, which enhanced P. australis performance. Fall burning had intermediate effects with increased production of P. australis but there were changes in species composition. The species diversity (based on the biomasses of individual substorey species) increased on summer burned and mowed plots but not on fall or spring burned plots. Species diversity increased following summer burning lun to increased production by C. arvense and due to the establishment of Sonchus arvensis and Atriplex patula which produced considerable biomass.