Response of Herbaceous Plant Community Diversity and Composition to Overstorey Harvest Within Riparian Management Zones in Northern Hardwoods

Forestry An International Journal of Forest Research Forestry 2013; 86, 111–117, doi:10.1093/forestry/cps060 Advance Access publication 13 September 2012 Response of herbaceous plant community diversity and composition to overstorey harvest within riparian management zones in Northern Hardwoods Eric K. Zenner1*, Michelle A. Martin2,4, Brian J. Palik3, Jerilynn E. Peck1 and Charles R. Blinn2 1School of Forest Resources, Pennsylvania State University, University Park, PA 16803, USA; 2Department of Forest Resources, University of Minnesota, St. Paul, MN 55108, USA; 3USDA FS Northern Research Station, 1831 Hwy 169 E., Grand Rapids, MN 06514, USA; 4Present address: Division of Forestry, MN Department of Natural Resources, 1200 Warner Road, St. Paul, MN 55106, USA *Corresponding author Telephone: +18148654574; Fax: +18148653725; [email protected] Received 25 January 2012 Partial timber harvest within riparian management zones (RMZs) may permit active management of riparian forests while protecting stream ecosystems, but impacts on herbaceous communities are poorly understood. We compared herbaceous plant community abundance, diversity and composition in RMZs along small streams in northern Minnesota, USA, among four treatments before harvest and 1 year and 9 years following treatment. Treatments included a no-harvesting control and three different treatments of the RMZs where the adjacent upland forest was clearcut: (1) an RMZ control, with no harvesting in the RMZ; (2) RMZ TL, in which the RMZ was partially harvested (60 per cent removal of basal area) using tree-length harvesting and (3) RMZ cut- to-length (CTL), in which the RMZ was partially cut (also 60 per cent removal) using CTL harvesting. Herbaceous cover, richness, diversity and most synecological coordinate scores (reflecting tolerances for light, heat, mois- ture and nutrients) varied over time but not among riparian treatments, whereas composition varied over time and by treatment but not differentially among treatments over time. These results indicate a lack of herbaceous plant community response to partial timber harvesting within these RMZs, which is consistent with pre- vious work suggesting that understorey communities may be resistant to change below thresholds of disturbance intensity. Introduction particular are known to be quite sensitive to forest management in upland ecosystems,22 – 24 where timber harvesting can Forested riparian areas are vital to protecting terrestrial and change community composition and abundance.25 – 27 Herb- 1 – 3 aquatic ecosystems. Primarily because of the critical role of aceous communities in riparian areas may be similarly sensi- streamside overstorey structure and composition in maintaining tive, but there have been few studies to quantify this, stream temperature, bank stabilization, filtering of nutrients and particularly in boreal and near boreal forests. Managed riparian 3 –5 organic matter input into the stream timber harvest is often forests have been found to have significantly higher average 6 restricted within riparian buffers or riparian management light levels compared with their unmanaged counterparts, 7 zones (RMZs) to limit changes to sedimentation, soil nutrients resulting in increases in shade-intolerant species in the under- 8,9 and aquatic biota. storey, particularly near the edge of the stream28. The Uncut riparian buffers ameliorate impacts of timber harvest- biomass of understorey vegetation has also been found to be 10 – 12 ing on abiotic and biotic stream conditions. Although suc- greater in partially harvested RMZs than in uncut zones in the cessful in providing stream protection, uncut buffer strips year following treatment.29 The effectiveness of RMZs for main- impede the restoration of, and management for, diverse riparian taining riparian herbaceous communities, therefore, depends on 13,14 areas on the landscape, necessitate forgoing active timber the response of these communities to varying harvest levels. 15 management opportunities and prevent financial gain from Because herbaceous plants are drivers of ecosystem functions 16,17 timber sales. In contrast, active management in riparian such as nutrient cycling,30 their response may further reflect areas (i.e. partial timber harvesting that retains residual vegeta- the degree of alteration in riparian functions that result from tion after harvest) has also been found to lessen harvesting harvesting. effects on stream conditions while providing adequate protection If riparian herbaceous communities are highly sensitive to 18 – 20 of water quality. overstorey management, timber production in the RMZ may Riparian areas can also be especially important repositories come at the cost of herbaceous biodiversity. If, however, these 1,21 of plant biodiversity. Herbaceous plant communities in communities are either resistant to harvest, showing little # Institute of Chartered Foresters, 2012. All rights reserved. For permissions, please e-mail: [email protected] 111 Forestry change following harvest, or resilient, eventually resuming pre- extended from the upland forest on the eastern-facing slope above a harvest compositional conditions, it may be possible to actively stream through the riparian forest along the stream, across the manage riparian forests for a variety of objectives. The objective stream, through the opposite riparian forest, and up toward the upland of the current study, therefore, was to evaluate the impacts of forest on the western-facing slope (Figure 1). In each unit, the RMZ was three levels of management activity (no harvest and partial designated as the area extending 30.5 m from the centre of the stream cut-to-length (CTL) harvest and whole-tree harvest) on herb- into the forest on both sides of the stream. A slight gradient of topo- graphic features (fluvial landforms) extended from the stream through aceous plant community diversity and composition within the RMZ and into the upland forest. RMZs. This examination is important in that it addressed the sus- The completely randomized design involved applying four harvesting tainability of native plant community diversity in managed ripar- treatments randomly assigned to each of three replicate stands in the ian areas, but also because such changes, or the lack thereof, late summer to early fall of 1997.33 The four treatments included (1) may be reflective of the impacts of management on riparian full control (FC, with no harvesting in either the RMZs or upland ecosystem functions. forests); (2) RMZ control (RMZC, no harvesting in the RMZs and clearcutting of the upland forests); (3) RMZ TL (TL, traditional tree-length harvesting in the RMZs and clearcutting of the upland forests) and (4) RMZ Methods cut-to-length (CTL, harvesting in the RMZs and clearcutting of the upland forests). Both RMZ TL and RMZ CTL treatments were partially har- Site description vested (cut trees were ≥10 cm diameter at breast (1.37 m) height (DBH)) to a residual basal area of 13 m2 ha –1 (60 per cent removal). There was The study area included four first-order streams draining into Pokegama ′ ′′ ′ ′′ a gradient of harvesting intensity in the RMZ portion of the stands from Lake (47811 8 N, 93834 29 W) in an area of Minnesota with annual pre- low- to high-intensity removal of overstorey trees from stream to clearcut cipitation of 61–69 cm, 40 per cent of which occurs during the summer edge, with no trees harvested within 5 m of the stream. In addition, growing season. Average temperature is 210.68C in winter and 16.78C 31 blowdown occurred over the 9-year post-treatment period, further redu- in summer. Well-drained to somewhat poorly drained calcareous cing overstorey density by 30 and 50 per cent of the first-year post- soils on an end-moraine support Northern Rich Mesic Hardwood 32 treatment density in the RMZC and the partially harvested treatments, Forest. The study was conducted in 70- to 120-year-old second- respectively.34 growth forest; dominant tree species at the start of the study were trembling aspen (Populus tremuloides Michx., 13–24 per cent relative basal area (RBA)), paper birch (Betula papyrifera Marsh., 10–19 per cent RBA), sugar maple (Acer saccharum Marsh.; 8–22 per cent RBA) and basswood (Tilia Americana L.; 1–15 per cent RBA). Northern white- Sampling cedar (Thuja occidentalis L.) and black ash (Fraxinus nigra Marsh.) were In each unit, five to eight equal-length transects were established per- abundant along the stream edges. Nomenclature follows the USDA pendicular to the stream (Figure 1) and, on each transect, three to six PLANTS database. permanent plots were placed in each of the fluvial landforms (floodplain, slope or terrace) on both sides of the stream. Transects initiated at different distances from the upland edge and crossed the stream, creating a Experimental design ‘short side’ of the transect with one to two plots on one side of the stream and a ‘long side’ with two to four plots on the opposite side. The 12 experimental units were established in 1996 across north–south Short and long sides usually alternated along the stream reach moving oriented stream reaches. Units were 135–200 m long and at least 100 m from north to south. Because plots were placed in different fluvial land- apart when on the same stream such that each ca. 4.8 ha unit forms, distances between plots varied according to inherent variation in topography. Herbaceous vegetation was sampled in 1997 (pre-harvest), 1998 (1-year post-harvest) and 2006 (9 years post-harvest) within 0.5-m2 square subplots that were offset (to the north or south) by 1 m from the permanent plot centre following the methods of Goebel et al.18 Individuals were identified to species (or occasionally to genus or family when identification was uncertain) and per cent cover was estimated using cover classes (1 ¼ trace–1 per cent; 2 ¼ 1–5 per cent; 3 ¼ 5–15 per cent; 4 ¼ 15–30 per cent; 5 ¼ 30–60 per cent and 6 ¼ 60–100 per cent). Cover classes were later converted to per cent cover using the midpoint of the class range.

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