Kirtland's warbler occupancy and plantation density

Early studies into the habitat preferences of the Kirtland’s warbler (KW; Setophaga kirtlandii) suggested that these birds exhibited a preference for areas with high jack pine stem densities; therefore, jack pine plantations established as part of the KW recovery and conservation programs have been planted using a 1.5 x 1.8 m spacing (3,588 trees ha-1). In contrast, traditional pine plantations in the Lake States established for roundwood production are typically planted on a 2.1 x 2.4 m spacing that equates to 1,922 trees ha-1. Over more than 40 years, tree spacing in KW habitat plantations has gone largely unchanged, and until very recently there has never been an attempt to verify that these tighter spacings actually provide better KW habitat than would a traditional forestry spacing. We used a retrospective approach to assess the impacts of tree density on KW occupancy and observed an unexpected negative relationship between realized plantation density and maximum occupancy by KW singing males. This finding should be interpreted with caution due to the limited nature of this study; however, a lack of a positive relationship is entirely plausible given the narrow range of densities encountered in plantations, as well as the fact that uniform spacing of plantations should allow for the achievement of optimal jack pine cover at lower densities than would be required in a natural-origin stand.

Methods
We took advantage of unintended variability in plantation spacing to conduct a retrospective assessment of KW usage of plantations across a range of realized plantation densities. Our approach was to use MDNR regeneration survey data to determine realized plantation density and then to relate this to KW occupancy using KW census data. We used digital 3-year post-planting surveys from 2004 - 2010. The maximum density of KW (males ha-1) was used as the metric for analyses and was calculated for each stand by taking the number of singing males from the peak year of occupancy and dividing that by the size (ha) of the stand. In addition to jack pine stem density, we attempted to account for other factors known to influence local KW density, including stand age, stand size, geographic location, and the amount of occupiable habitat in the surrounding landscape. Therefore, for each stand in our analysis we also considered the following as covariates: stand size (ha), functional unit size (ha), stand year of origin, KW Management Area (KWMA), and a measure of habitat connectivity. We calculated a simple metric of habitat connectivity to include in our analysis by summing the total area of potentially occupiable habitat within a defined radius of each stand. For each stand, at the year of peak occupancy, we summed the area (ha) of all jack pine between 5-23 years of age (potentially occupiable habitat) within a 5-km, or 10-km, radius of the centroid of that stand.