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Wildlife, Animals, and Plants |
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FIRE EFFECTS
IMMEDIATE FIRE EFFECT ON PLANT:Black cottonwood sprouts from the lateral roots, root crown, and bole after top-kill by fire [49,58]. It also establishes from seed [3,24,131,158]. Gom and Rood [49] studied the effects of severe fires along the Oldman River near Lethbridge, Alberta, which occurred in early and mid-April before cottonwoods' spring bud burst. The fires burned during the afternoon; strong Chinook winds increased fire severity. Black cottonwood, narrowleaf cottonwood, plains cottonwood, and hybrids among them dominated these riparian areas. All cottonwoods observed were girdled by the fires. Fire damage to trunks varied from "slight bark charring to complete trunk incineration." This caused 100% top-kill in trees even though not all canopies were severely damaged, demonstrating the susceptibility of these cottonwood species to severe fires. The exact amount of mortality was not determined (because it is difficult to tell from which tree's lateral root system suckers originate), but mortality was definitely less than 25%, as 75% of trunks (including all species) produced coppice sprouts. The most severe fires were in older stands, likely the result of higher fuel accumulation, and there were many pits in the ground on these sites where root systems were burned. Young stands close to the river (without as much fuel loading) experienced less canopy damage but were top-killed via girdling. Hardwood saplings (cottonwood, alder, willow) less than 2 inches (5 cm) in diameter experienced 100% mortality in 168 and 42 BTU/second/foot intensity fires in a white spruce, quaking aspen, and balsam poplar (likely including some black cottonwood and hybrids) stand northeast of Edmonton, Alberta [80]. Hall and Hansen [58] stated that in low and moderate severity fires older black cottonwood trees with thick bark may not be top-killed, but little quantitative data examining the relations between fire severity, tree age, and top-kill or mortality is available. DISCUSSION AND QUALIFICATION OF FIRE EFFECT:No entry PLANT RESPONSE TO FIRE:One year after the fires along the Oldman River in Alberta (described above), approximately 75% of the top-killed cottonwoods (including all narrowleaf and black cottonwoods, and balsam poplar) produced shoots from remnants of trunks. Sprouting was not observed on control transects (on adjacent unburned areas). Of the trunks producing sprouts, 90% were from either narrowleaf or black cottonwood (the 2 species' sprouts were not easily differentiated). The average number of coppice sprouts was 17 per trunk with a range of 1 to 60. Height of the tallest sprout on each tree averaged 16 inches (41 cm) and ranged from 3.5 to 30 inches (9-77 cm). The authors speculated that the high rate of sprouting was in part due to the occurrence of the fires while the cottonwoods were dormant. The degree of canopy or trunk damage did not have much impact on coppice sprouting frequency or vigor. Sprouts were even observed from trunks that had burned to 10 inches (25 cm) below ground. Of trees that experienced "heavy" canopy damage, 60% produced shoots from stumps; 80% of those with "light" damage produced coppice sprouts. The average height of coppice sprouts was not significantly (p>0.05) affected by the degree of trunk damage, and trunk diameter had no significant (p>0.05) effect on either the number of coppice sprouts or the height of sprouts. Over 1,000 root suckers were observed on the study transects, 80% of which were either narrowleaf or black cottonwood. The density of root suckers (including all species) was about 1/ 3 m2). Suckers were more common near trees that had many coppice sprouts. After 1 growing season the average height of root suckers was approximately 3 feet (1 m) [49]. Transects on the Oldman River were surveyed again 5 years after fire. At this time there was an average of 4 coppice sprouts per trunk on the 30% of the cottonwoods that still had living sprouts. Average sprout height was approximately 10 feet (3 m) with a range of 5 to 16 feet (1.5 to 5 m). Of the 34 trunks with surviving coppice sprouts 33 were either narrowleaf or black cottonwood and 1 was plains cottonwood. Survival of root suckers over the 5 years was about 50%. A density of 1 sprout per 7 m2 was observed as 550 sprouts still survived on the transects. The average height of root suckers increased from about 3 feet (1 m) to 8 feet (2.4 m). Seven root suckers were present on unburned control sites; the authors stated that these might have been induced by flooding damage 2 years prior to the survey [49]. Gom and Rood [49] summarized the response of several cottonwood species to fire, showing the high sprouting/suckering ability of balsam poplar and black and narrowleaf cottonwoods relative to Fremont and eastern cottonwoods:
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:No entry FIRE MANAGEMENT CONSIDERATIONS:To increase the longevity of cottonwood stands managers may choose to prescribe fire when the stand is in the "pole" stage, as sprouting is high at this point [58]. Dormant season, particularly fall, fires induce the most sprouting [49]. Gom and Rood [49] suggest that fire- induced sprouting, particularly in old stands, may be used to regenerate stands much the way flooding disturbance allows development of new stands. They also state that regeneration may be more rapid after fire than after flooding because the growth rate of sprouts is about 3 times greater than that of seedlings. If the goal of prescribed fire is to maintain a cottonwood stand, 5 years of grazing exclusion after fire are recommended [58]. Herbivory and fire interactions have been well documented for quaking aspen stands. In the Rocky Mountains (study sites ranged from Jasper National Park, Canada to Rocky Mountain National Park, Colorado), White and others [157] found that prolific quaking aspen sprouting after fire could not overcome the effects of elk grazing, and, with a short fire return interval, quaking aspen stands would decline. Fire can be used to maintain black cottonwood stands, but it may cause negative impacts such as erosion and sedimentation. These impacts both on the site of fire and on sites downstream can be increased by heavy postfire rainfall [132].
Related categories for SPECIES: Populus balsamifera ssp. trichocarpa | Black Cottonwood |
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