Retained structures in managed pine stands
Although forest certification systems ask landowners to consider stand-level structural elements (e.g., snags, downed woody debris, green trees) in managing for biodiversity, little information exists on the amount of retained structures in managed stands, and how these features influence wildlife populations. Developing empirically-based criteria for structural retention ensures that forest landowners are using the best available scientific information when managing for biological diversity.
A recent study conducted by Mississippi State University examined this issue by (1) documenting the amount of vertical structure and downed woody debris remaining on harvested stands after site preparation and (2) evaluating relationships of retained structures with bird communities. Collaborators on this study included NCASI, Hancock Natural Resources Group, Potlatch Corporation, Resource Management Services, and Weyerhaeuser Company. The study documented a wide range of retained structure in recently harvested management units. The study also found that “stringers” along ephemeral drains and streamside management zones (SMZs) had similar structural characteristics; these 2 retained features accounted for nearly 30% of bird species richness in management units. The overall conclusion was that retained structural features support and add to biological diversity in recently harvested pine stands.
The student on the project, Dr. Michael Parrish, recently completed his dissertation. The abstract from his dissertation follows:
In the southern United States, institutional forest owners engaged in forest certification programs often retain unharvested or less-intensively harvested vegetation when clearcut harvesting intensively managed pine (Pinus spp.) forests (“IMPFs”), a practice called ‘green tree retention’. I investigated resultant patterns of land cover and retained structural elements in recently-harvested IMPF management units (“MUs”) and related them to avian biodiversity to provide information to support harvest decisions. First, to provide forest managers baseline data on retention, I screen-digitized land cover on 1187 MUs (totaling 51646 ha) and characterized green tree retention levels and internal land cover attributes (Chapter 2). I found MU land cover was dominated by regenerating clearcuts (mean: 80.5%), streamside management zones (“SMZs”; vegetated buffers surrounding intermittent and perennial streams; 14.0%) and stringers (buffers surrounding ephemeral streams; 3.3%). Next, I surveyed 60 MUs for vegetation stem density and cover (Chapter 3). Concurrently, I surveyed avian community density and richness (Chapter 4). Vegetation and avian metrics were compared and contrasted across the dominant cover types (with emphasis on stringer/SMZ similarity) to understand impacts of retained structural elements on biodiversity outcomes. I found that snag and log density, midstory pine density, understory deciduous cover, and ground cover were not different in stringers and SMZs; however, overstory (pine and deciduous) and midstory (deciduous) tree density was lower in stringers than in SMZs, and understory pine density was greater in SMZs. Species overlap between cover types was high (74% to 84%), but SMZs and stringers provided 27% of MU species richness. Stringers appeared to benefit both shrubland- and forest-associated birds. Finally, I sampled land cover across 4450 sq-km surrounding the 60 MUs, and performed ordination analyses to identify associations between local-scale (MU interiors) and landscape-scale (3-km buffers around MUs) land cover and avian guild diversity (Chapter 5). I found the region to be >90% forested. Cover type data explained 41% of the partial variation in avian density and total species richness. Local-scale MU characteristics appeared more important than landscape-scale characteristics in explaining avian biodiversity responses. My results suggest that retained structural features support and enhance MU biodiversity in harvested IMPFs.
Parrish, M. C. 2018. Effects of green tree retention on birds of southern pine plantations. (Ph.D.), Mississippi State University, Mississippi State, Mississippi, USA.
Parrish, M. C., Demarais, S., Ezell, A. W., Wigley, T. B., Jones, P. D., & Riffell, S. K. 2017. Retained vegetation density of streamside management zones and stringers in southern intensively managed pine forests. Forest Ecology and Management 397(2017): 89-96. http://doi.org/10.1016/j.foreco.2017.04.024.
Parrish, M. C., Demarais, S., Wigley, B. T., Riffell, S. K., Ezell, A. W., & Jones, P. D. 2018. Operational green tree retention and land cover patterns in intensively managed pine forest landscapes of the southeastern United States. Forest Science 64(5): 564-576. http://doi.org/10.1093/forsci/fxy009.
Parrish, M. C., Demarais, S., Wigley, T. B., Jones, P. D., Ezell, A. W., & Riffell, S. K. 2017. Breeding bird communities associated with land cover in intensively managed pine forests of the southeastern U.S. Forest Ecology and Management 406(2017): 112-124. http://doi.org/10.1016/j.foreco.2017.09.063.