Developing Sustainable Harvest Strategies for Cellulose-Based Biofuels: The Effect of Intensity of Harvest on Migratory Grassland Songbirds

If wildlife resources are to be protected in conjunction with the inevitable large-scale conversion of lands to biofuels production, it is imperative that appropriate harvest strategies be determined. The tall grass prairies of the upper Midwest hold some of the greatest potential for biofuel production, but also provide critical breeding habitat for more than half of the total number of grassland bird species in North America (Knopf 1996). However, harvesting perennial grasses for biofuels may change vegetation structure and bird species composition. My research objectives were to (1) Evaluate the effect of harvest intensity on the density of migratory grassland songbirds and (2) Determine the appropriate harvest strategies for cellulosic biofuels that will minimize the impacts to migratory grassland songbirds. I used a completely randomized block design to determine the effects of treatment application over four blocks in southeastern South Dakota. I assessed harvest intensity at three levels: (1) high intensity (10-cm stubble), (2) low intensity (30-cm stubble), and (3) a non-harvested control. Both harvests were replicated twice within each block. All treatments were applied in the fall of 2008 and 2009. Avian surveys were conducted twice in 2009 and three times in 2010 using spot mapping censuses to determine density (# of territorial males per 100ha) of 4 focal species: savannah sparrow (Passerculus sandwichensi), grasshopper sparrow (Ammodramus savannarum), dickcissel (Spiza americana), and bobolink (Dolichonyx oryzivorus). All 16-ha (40-acre) treatment plots were traversed along transects that were 100m apart while maintaining a 50-m buffer along edges and wetlands within the plot. All males heard and/or seen within 50m of each side of transect were recorded by mapping their location and/or flight path/s on an aerial photograph (Bibby et al. 2000). Thirty vegetative survey points were systematically placed along avian survey transects at set intervals. Vegetative structure was assessed within one week of avian surveys and measured visual obstruction, percentage of ground coverage by growth form, vegetation height, and litter depth. Data were analyzed using a mixed model analysis of variance with vegetative parameters as independent covariates to determine the variation among treatments, blocks, and years. Tests were conducted to look at plots not harvested in either year (control analysis), plots not harvested the first year and then harvested the next year (pre- and postharvest analysis) and plots harvested the second year (2010 treatment analysis). I found visual obstruction, litter depth, live and dead vegetation height, and percent coverage of litter and woody habits to be significantly altered post-harvest. Focal species’ densities varied across harvest treatments. But, in particular, if the percentage of forbs was held constant across treatments, densities of savannah sparrow males were significantly higher on low intensity harvests (mean=33.01; P=0.0529) and high intensity harvests (mean=32.66; P=0.0417) in 2010 than on non-harvested sites in 2009 (mean=14.17). Similarly, if the litter depth was held constant across treatments, densities of dickcissel males were significantly higher on low intensity harvests (mean=69.26; P=0.0393) and high intensity harvests (mean=101.68; P=0.0475) in 2010 than on non-harvested sites in 2009 (mean=-112.22). There were no significant differences in the average maximum density of territorial males among treatments for the grasshopper sparrow or the bobolink. Therefore, a single harvest strategy cannot be recommended. The most important factor regarding biomass stands and their utilization by grassland songbirds is structural diversity. Ultimately, the key to more diverse vegetative structure and bird species composition is to vary harvest intensities throughout the landscape