Variable retention harvesting to enhance stand complexity: Songbird community response in a red pine (Pinus resinosa) forest

Background/Question/Methods

Whereas forests nearly always respond to natural disturbance (e.g., fire, windthrow) by increasing in complexity, forests tend to become more simplified following human disturbance (e.g., timber harvesting), posing challenges for maintaining biodiversity in perpetuity.  Ecologically-based forestry is being developed as a means to restore and maintain forest complexity; however, long-term, stand-scale studies are needed to better understand the specific mechanisms and patterns by which biodiversity is maintained.  Our study tests for stand-scale responses of breeding songbirds to different spatial harvesting patterns in mature red pine forests situated on the Chippewa National Forest of northern Minnesota, U.S.A.  Treatments consist of three overstory manipulations (large gap, small gap, and dispersed retention), one understory manipulation (brush removal), and controls.

Results/Conclusions

Results of the first breeding season following harvest in 2003 showed no difference in bird community composition between treatments and control sites; lack of difference was attributed to high site fidelity.  The third breeding season following manipulation in 2005, treatments showed greater overall avian abundance   (P = 0.05), richness (P < 0.01), and diversity (P < 0.01), but certain less common species were only found in control sites (i.e., Golden-crowned Kinglet, Scarlet Tanager, and Ruffed Grouse).  This trend continued through 2009.  Preliminary results from the 2011 survey showed similar patterns with greater abundance (P < 0.01) and richness (P < 0.02) within treatment sites compared to the control, but no difference in Shannon’s diversity (P = 0.21).  In particular, the American Redstart, Chestnut-sided Warbler, Nashville Warbler, Yellow-rumped Warbler, Red Crossbill, Evening Grosbeak, and White-throated Sparrow appear to respond positively to overstory treatment.  To date, no discernible differences in avian community composition have been found among overstory treatments with or without brush removal.  Results from this study thus far suggest that overstory retention methods in red pine forests have similar intermediate-term (near decadal) impacts on avian community composition; however, divergence is expected as the forest understory further develops over time.