McCord et al., 2014
RESEARCH BRIEF 16
Reducing canopy closure by 30-40% in mature closed-canopy forests increased soft mast production and improved cover preferred by wild turkey hens.
Recurring prescribed fire following canopy reduction maintained desirable cover and food resources for wild turkey poults.
Recurring low-intensity prescribed fire without canopy reduction did not reduce basal area, did not influence understory composition or structure, and did not improve conditions for wild turkeys.
Broadcast application of the herbicide triclopyr in stands following canopy reduction removed the midstory, but tree seedlings quickly recolonized the site and herbaceous groundcover was not increased.
In this study, researchers examined the impact of various silvicultural treatments on habitat for wild turkey broods in upland hardwood forests. They reduced the forest canopy through cutting and disturbed the understory with prescribed fire and/or herbicide application. Following treatments, researchers measured the structure and composition of understory vegetation and the availability of food resources for turkey poults.
Most young turkey deaths occur in the first two weeks of life, when poults are still flightless. A wild turkey hen typically selects areas with a well-developed understory that will conceal poults, yet with an open midstory, so the hen can see above the cover. Poults are most likely to survive in groundcover up to 50 cm tall that provides overhead protection and access to seeds, invertebrates, and soft mast.
Closed canopy upland hardwoods offer wild turkeys hard mast in autumn and winter, but often lack understory structure that is suitable for rearing turkey broods, especially during these critical flightless early weeks.
The study took place between 2000 and 2009 in eastern Tennessee’s 9,825 hectare Chuck Swan State Forest and Wildlife Chuck Swan State Forest and Wildlife Management Area. In this mixed hardwood forest, researchers employed either shelterwood harvests (initial cut only) or retention cuts (removing tree species of low value to wildlife) to reduce the canopy to a 60% target canopy closure for both treatments. Subsequent treatments with herbicide or prescribed fire were applied to influence groundcover and regeneration.
Seven different treatments were applied to the 12 experimental units in each of four stands. Two units per stand received no treatment, and two were treated with prescribed fire but the canopy was not reduced. Shelterwood harvests were conducted on four units, two of which were burned once and two that were not burned. The remaining four units were treated with retention cutting. Of these, two were burned four times and two were not burned. The herbicide triclopyr was broadcast within the under-story and midstory of one burned unit and one unburned unit. Prescribed fires were conducted in April and early May.
Shelterwood harvests reduced basal area to approximately 20 m2/ha. In retention cut units, maples and yellow poplar trees were removed, but trees that provide food for turkeys were retained, such as oaks, black gum, black cherry, and persimmon.
Following treatments, researchers measured vegetation structure and composition, sunlight infiltration to the understory, and food resources suitable for turkey poults. Of particular interest was the level of visual obstruction by plants at heights that would provide optimum cover for poults.
Looking at effects on cover resulting from the various treatments, researchers found that understory conditions for turkey broods generally improved in study units where both thinning and burning took place. The three treatment types that provided the least visual obstruction at ground level, and hence the least protection for young turkeys, were the untreated control units, the units that were burned without canopy reduction, and the retention-cut units treated with herbicide. Much better visual protection was offered in the shelterwood cuts, with and without fire, and in the retention cuts with fire but not herbicide.
The canopy reduction achieved in both the shelterwood cuts and the retention cuts allowed enough sunlight to reach the forest floor to stimulate sufficient ground flora to enhance wild turkey brood protection. However, this benefit was short-lived because regenerating woody plants reached the midstory and shaded out the protective groundcover unless controlled within seven years by periodic prescribed fire. Study results indicated that low-intensity prescribed fire after the canopy was thinned by either method was the best treatment to produce conditions that hens prefer for brood rearing.
Herbicide application did not enhance cover for wild turkey poults and was costly, at $650 to $700 per hectare as compared to $37 for prescribed fire at the time of the study. Although herbicide killed more than 87% of the midstory, this treatment also reduced visual obstruction at ground level, leaving poults vulnerable. Two growing seasons later, herbaceous groundcover had only reached roughly half of the pretreatment level.
In units not treated with herbicide, differences between the various treatment types in terms of herbaceous ground cover growth were not detected. Overall, response by herbaceous flora was lacking (generally <30%); instead, woody regeneration dominated groundcover, which did provide cover for turkey poults.
Soft mast food plants preferred by turkeys found in the study area include blackberry, huckleberry, blueberry, sumac, American pokeweed, greenbriar, and viburnum. Researchers found that the effects on soft mast quantity varied among treatment sites, which may be related to seedbank differences. Of the various treatments, the retention cut followed by prescribed fire produced the most soft mast, primarily blackberries, but only on the second year after the fire, because blackberries develop on mature floricanes. Blackberries offered poults escape cover and protection from avian predators. Blackberry production dominated all site types except those where herbicide had been applied. On these sites, blackberry quantities were greatly reduced or eliminated, and pokeweed and blueberries produced the most soft mast.
The study also looked at effects on invertebrate availability, which is important because invertebrates help turkey poults meet their dietary requirement of 28% crude protein. The study team collected and measured the abundance of ground-dwelling invertebrates that were potentially within reach of wild turkey poults.
Researchers had predicted that invertebrates would increase as herbaceous groundcover responded to treatments, but instead their data showed no difference in invertebrate biomass by year or among treatments. Within all treatments they found ample invertebrates within a 29 hectare area to meet the needs of a 10 poult brood.
Study authors concluded that forest canopy reductions coupled with prescribed fire yielded improved understory conditions that would likely allow poults to find sufficient invertebrates with less vulnerability and exposure as compared to the control stands.
When managing for wild turkeys and other wildlife species that require similar structure and food resources, the authors recommend low-intensity prescribed fire every three to five years. This return interval allows soft mast production and maintains a desirable structure before it gets too tall, reducing visibility in the stand and shading out groundcover.
In subsequent correspondence, the study authors do not recommend burning during the nesting season when managing properties for wild turkeys. In their experience, the effect on vegetation composition is no different than when using dormant-season fire. Woody stems continue to resprout. Although they recognize that no study has documented a population decrease following early growing-season fire, they did document turkey nests consumed while burning in April and May, and they note no advantage to burning at this time.
Instead, they recommend burning during the dormant season to maintain current vegetation composition or during the latter portion of the growing season (August – October) to possibly better control undesirable woody stems and stimulate increased forb coverage.
John M. McCord, Craig A. Harper, Cathryn H. Greenberg (2014) Wildlife Society Bulletin 38: 265-272
FOR FURTHER READING
Kilburg, E., C.E. Moorman, C. DePerno, D. Cobb, and C.A. Harper. 2014. Wild turkeynest survival and nest-site selection in the presence of growing-season prescribed fire.Journal of Wildlife Management 78(6):1033-1039.
The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, land owners and the public about the use, application, and effects of fire in the region. www.oakfirescience.com
This research brief was funded by The Joint Fire Science Program. www.firescience.gov