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Oak Woodland Restoration: Understory Response to Removal of Encroaching Conifers Warren D

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Oak Woodland Restoration: Understory Response to Removal of Encroaching Conifers Warren D WOODLAND RESEARCH REPORT Oak Woodland Restoration: Understory Response to Removal of Encroaching Conifers Warren D. Devine, Constance A. Harrington, and David H. Peter ABSTRACT Oregon white oak (or Garry oak, Quercus garryana) woodlands and savannas of the coastal Pacific Northwest are legacies of an anthropogenic fire regime that ended with European settlement in the mid-1800s. Historically, these oak stands had a sparse overstory and an understory dominated by fire-tolerant grasses and forbs. Post-settlement fire suppression resulted in widespread invasion and subsequent overstory dominance by conifers, causing mortality of shade-intolerant oak trees and shifting understory plant communities to shade-tolerant species. In a study on four southwestern Wash- ington sites, our objective was to determine the effects of overstory conifer removal, primarily Douglas-fir (Pseudotsuga menziesii), on microclimate, native and non-native understory cover, and sapling growth. Overstory conifer removal created a warmer, drier understory microclimate during summer months. Conifer removal had little effect on native understory cover during five years post-treatment; however, cover of non-native plants, primarily grasses and woody understory species, increased significantly during the same period. Height growth of Oregon white oak and Douglas-fir saplings exhibited a delayed, but positive, response to overstory conifer removal, although the treatment response of Douglas-fir was 133% greater than that of oak. Increases in non-native understory cover and the rapid growth of young Douglas-fir indicate the importance of pre- and post-treatment understory management to control undesirable plants and promote native species such as Oregon white oak. Keywords: Douglas-fir (Pseudotsuga menziesii ), microclimate, Oregon white oak (Quercus garryana), restoration, savanna, understory Introduction Harrington 2006). Conifers overtop centuries to replace due to their slow and suppress the shade-intolerant oak growth (Stein 1990). Where former he Oregon white oak (or Garry trees, leading to crown die-back and canopy oak trees are already dying as oak; Quercus garryana) wood- T eventual mortality of the oak. a result of suppression, promoting the lands, savannas, and associated prairies Restoration of Oregon white oak growth and survival of oak seedlings of the coastal Pacific Northwest are woodlands and savannas to presettle- and saplings becomes critical. part of an anthropogenic ecosystem, ment conditions requires removal of This encroachment of conifers is historically maintained by frequent, the conifer overstory to “release” the widespread, and substantial losses low-intensity fires set by Native Amer- suppressed oak trees from competition of prairie and oak habitats also have icans (Agee 1993). After European and to restore a more open stand struc- resulted from conversion of land to settlement in the mid-1800s, the lack ture (Harrington and Devine 2006). agricultural and urban uses (Crawford of regular burning allowed fire-intol- Results from a companion study have and Hall 1997). In British Columbia, erant conifers, primarily Douglas-fir shown that large oak trees (diameter less than five percent of pre-settlement (Pseudotsuga menziesii), to regenerate at breast height over 20 cm) respond Oregon white oak habitat remains (Lea in prairies and oak stands where they to release from overtopping Douglas- 2006) (Figure 1). Recognizing the loss were previously excluded (Thysell and fir by increasing their rate of growth, of this ecosystem, private landowners, Carey 2001). Growth and survival of expanding leaf area through formation conservation organizations, and state Oregon white oak trees are substan- of new (i.e., epicormic) branches, and and federal land managers in Oregon’s tially reduced where these conifers increasing acorn production (Devine Willamette Valley, the Puget Sound have invaded (Stein 1990, Devine and and Harrington 2006). Preservation region of Washington, and Vancouver of existing oak trees is a priority when Island, British Columbia, have become Ecological Restoration Vol. 25, No. 4, 2007 ISSN 1522-4740 E-ISSN 1543-4079 restoring these stands because the trees involved in various aspects of restoring ©2007 by the Board of Regents of the provide an important structural com- Oregon white oak communities. Two University of Wisconsin System. ponent and would take decades or non-profit organizations, the Oregon December 2007 ECOLOGICAL RESTORATION 25:4 • 247 colonize these former oak savanna and woodland sites. The Douglas-fir had been lightly thinned two to three times before this study at 10- to 15-year intervals, most recently immediately prior to the study (2000–2001). Stand basal area averaged 34.3 ± 8.2 m2/ha for Douglas-fir and 4.7 ± 2.4 m2/ha for oak. Approximate age range for the suppressed oak was 90 to 150 years, while Douglas-fir averaged 65 to 90 years. A detailed description of these oak trees appears in Devine and Harrington (2006). The understory of the oak woodland and savanna stands in the Puget Sound region was historically dominated by grasses and forbs in a plant commu- nity likely similar to the Oregon white Figure 1. An Oregon white oak (Quercus garryana) woodland (dormant season) with encroaching oak, long-stolon sedge (Carex inops), Douglas-fir Pseudotsuga( menziesii ) in the background. In the foreground is our most problematic invasive shrub, Scotch broom (Cytisus scoparius). Photo by Warren Devin common camas (Camassia quamash) community (Chappell and Craw- ford 1997). A lack of regular fire and Oak Communities Working Group information on understory response increasing conifer dominance results and the Garry Oak Ecosystems Recov- will facilitate selection of appropriate in a more shade-tolerant, shrubby ery Team (British Columbia), have restoration techniques in similar plant understory community such as the focused on preserving and restoring communities. Oregon white oak, Douglas-fir/snow- Oregon white oak ecosystems since the berry (Symphoricarpos albus), sword- late 1990s. When Oregon white oak Study Area fern (Polystichum munitum) commu- is released from overtopping conifers, nity (Chappell and Crawford 1997) the intensity of release ranges from The study took place in four stands of which was present at our study sites. individual-tree treatment (removing Oregon white oak and Douglas-fir on The most common understory spe- only the conifers adjacent to selected the Fort Lewis Military Reservation cies present across all treatments and oak trees) to total removal of conifers in southwestern Washington, east of years are listed in Table 1. Control at the stand level. These practices are the city of Tacoma. The four sites, of the invasive shrub Scotch broom influenced by the fact that on many located 10 to 15 km apart, are on gla- (Cytisus scoparius) is part of nearly all private lands the invading conifers cial outwash terraces and moraines, at oak savanna and prairie restoration represent a source of timber revenue elevations from 85 to 135 m. Soils are efforts in the region; we performed while the oak trees may have little gravelly to very gravelly sandy loams, a one-time removal of Scotch broom monetary value. Thus, depending on and loamy fine sands, and are moder- from the study by cutting (at ground- the merchantability of the conifers, ately to somewhat excessively drained. line to prevent sprouting) or uprooting a release treatment may require an Annual precipitation in Tacoma is 995 all plants within one tree-height radius expenditure, allow the owner to break mm, but total precipitation from 1 of the study trees at the beginning of even, or even yield a profit. May through 30 September averages the study in June 2001. The number The goal of this study was to only 158 mm (WRCC 2005). Mean of Scotch broom plants per study tree evaluate the influence of oak release temperatures in January and July are was variable, averaging 43 ± 80 and treatments on the understory, as 5 and 19°C respectively. ranging from zero to 350. the effects of release are currently The four stands were similar in spe- not well understood. We compared cies composition, with an overstory Study Design and primarily composed of Douglas-fir overtopped and released conditions Sampling to determine how removal of coni- averaging 167 ± 46 trees/ha (mean ± fers affects microclimate, native and one standard deviation) and a mid- At each of four sites, we selected 18 non-native understory cover, and story of suppressed Oregon white oak oak trees for the study that were each growth of Oregon white oak and averaging 115 ± 66 trees/ha. This is overtopped by at least two Douglas-fir Douglas-fir saplings. Our hope is that the first generation of Douglas-fir to trees. Average height of these 72 oak 248 • December 2007 ECOLOGICAL RESTORATION 25:4 Table 1. Shade tolerance and percent coverage in years 1 and 5 by basal area. The full-release treatment treatment for species occurring on at least 5% of microplots. Shade tolerance represents a near-total removal of classification is tolerant (T), intermediate (M), or intolerant (I). The symbol * overstory competition, while the half- indicates a significant change in cover p( < 0.05) between year 1 and year 5. release represents an individual-tree Cover (%) release that has been applied by some Control Full Release landowners to prolong the life of oak Shade Group / Species Yr 1 Yr 5 Yr 1 Yr 5 trees while removing relatively few Tolerance overstory conifers. During thinning Native forbs and treatment, trees were cut with Polystichum munitum (swordfern) T 16.6 20.1 7.2 12.1 chainsaws and moved to the landings Galium aparine (stickywilly) M 7.0 1.6* 6.0 3.4 with skidders. Clinopodium douglasii (yerba buena) T 2.8 2.0 1.1 2.2 We measured air temperature and Nemophila parviflora M 2.4 0.3 2.8 1.2 relative humidity every two hours near (smallflower nemophila) 18 randomly selected full-release and Fragaria vesca (woodland strawberry) M 0.5 1.1 1.1 1.6 control treatment trees using HOBO® Pro dataloggers (Onset Computer Native grasses Carex inops (long-stolon sedge) M 1.9 2.1 2.2 3.6 Corp., Bourne, MA).
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