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Invasive Scotch Broom Alters Soil Chemical Properties in Douglas-Fir Forests of the Pacific Northwest, USA

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Invasive Scotch Broom Alters Soil Chemical Properties in Douglas-Fir Forests of the Pacific Northwest, USA Plant Soil (2016) 398:281–289 DOI 10.1007/s11104-015-2662-7 REGULAR ARTICLE Invasive scotch broom alters soil chemical properties in Douglas-fir forests of the Pacific Northwest, USA Robert A. Slesak & Timothy B. Harrington & Anthony W. D’Amato Received: 8 April 2015 /Accepted: 3 September 2015 /Published online: 14 September 2015 # Springer International Publishing Switzerland 2015 Abstract was no difference in labile-P measures between the Backgrounds and aims Scotch broom is an N-fixing presence and absence of Scotch broom at either site, invasive species that has high potential to alter soil but there were notable reductions (25–30 %) in the properties. We compared soil from areas of Scotch intermediately-available P fraction when broom was broom invasion with nearby areas that had no evidence present. Extractable nutrient cations (notably K) were of invasion to assess the influence of broom on soil P lower in the presence of broom at both sites, with the fractions and other chemical properties. effects most pronounced at the fine-textured OR site. Methods The study was conducted at two contrasting Conclusions Lasting effects of Scotch broom invasion Douglas-fir sites in Oregon (OR) and Washington (WA), are likely to be associated with variable changes in soil USA with broom invasion for 10 years. We used the C, N, and decreases in extractable nutrients and avail- Hedley sequential fractionation procedure to assess ef- able P. These changes, and other documented effects of fects of Scotch broom invasion on P pools of varying Scotch broom on soil, are likely to have lasting effects bioavailability, and also measured total C, N and ex- on Douglas-fir growth after Scotch broom removal that tractable nutrient cations. will vary depending soil nutrient status at a given site. Results Total soil C and N were significantly higher with broom present at the fine-textured OR site, but Keywords Cytisus scoparius . Hedley sequential there was no effect at the coarse-textured WA site. There fractionation . Forest soils . Invasive species effects on soils . Soil phosphorus Responsible Editor: Hans Lambers. R. A. Slesak Introduction Department of Forest Resources, University of Minnesota, 115 Green Hall, 1530 Cleveland Avenue N, St. Paul, MN 55108, USA Scotch broom (Cytisus scoparius (L.) Link) is a non- R. A. Slesak (*) native, invasive plant species of major concern in coast Minnesota Forest Resources Council, St. Paul, MN 55108, USA Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. e-mail: [email protected] menziesii) forests of the Pacific Northwest capable of T. B. Harrington ecosystem alteration and degradation. Ballistic dispers- USDA Forest Service, Pacific Northwest Research Station, 3625 al, abundant production, and prolonged viability of its 93rd Avenue Southwest, Olympia, WA 98512-9193, USA seeds enable Scotch broom to invade young forest, A. W. D’Amato woodland, and prairie sites (Bossard 1993; Bossard Rubenstein School of Environment and Natural Resources, and Rejmánek 1994; Smith 2000). Once established, University of Vermont, Burlington, VT 05405, USA Scotch broom has a competitive advantage over many 282 Plant Soil (2016) 398:281–289 native plant species because of its rapid seedling growth found no effect of Scotch broom on grass biomass rate (Grotkopp and Rejmánek 2007), and its ability to when the two species were grown together in a bioas- reduce availability of soil water and light (Watt et al. say study, but the investigators had amended the soils 2003; Wearne and Morgan 2004). Scotch broom is also used in the bioassay with P fertilizer, ameliorating any a symbiotic nitrogen fixing species that can fix as much potential Scotch broom-induced P deficiency if one as 100 kg N ha−1 per year (Watt et al. 2003), which existed. Although not conclusive, these studies sup- provides a competitive advantage in traditionally N- port the notion that changes in soil P following Scotch limited ecosystems (Vitousek and Horwath 1991). broom invasion may lead to lasting effects on soil Scotch broom invasion may alter soil properties and properties. have lasting effects on soil functions even after its We assessed the effect of Scotch broom invasion for a removal (Weidenhamer and Callaway 2010). At two period of 10 years on soil P fractions and other soil nitrogen-deficient sites in Australia, Scotch broom was chemical properties at two sites with contrasting soils more competitive with native shrubs on Scotch broom- and invasion history. We hypothesized that the presence invaded soils than on native woodland soils, suggesting of Scotch broom would reduce readily available soil P that the species altered soil chemical or other properties pools and increase less-available pools, but have no to favor its own regeneration and growth (Fogarty and effect on total P. We also hypothesized that Scotch Facelli 1999). Others have also documented reductions broom would increase soil C and N and decrease nutri- in plant growth using bioassay approaches (Haubensak ent cation pools. Our overall objective was to assess the and Parker 2004; Grove et al. 2012), but the mecha- potential for Scotch broom to have lasting effects on soil nisms leading to these reductions are unclear. Introduc- chemical properties. tion of alleopathic compounds by Scotch broom has been hypothesized as one possible mechanism, because species of the Cytisus genus are known to produce defensive alkaloid compounds that have been shown Methods to detrimentally affect functions of some plants in con- trolled laboratory settings (Wink and Twardowski Site descriptions 1992). Grove et al. (2012) concluded that observed negative effects of Scotch broom on Douglas-fir growth The assessment was conducted at two Douglas-fir sites were largely driven by reductions in ectomycorrhizal affiliated with the North American Long-Term Soil fungi (EMF) colonization, which would limit nutrient Productivity (LTSP) study located in Washington and water acquisition by the native trees. (WA) and Oregon (OR) USA (Harrington and Excess N inputs may also be a driver of altered soil Schoenholtz 2010)(Table1). Soils at the WA site are properties as there is the potential to increase cation loss classified as sandy-skeletal, mixed, mesic, Dystric (Adams et al. 1997; Smethurst et al. 2001) and increase Xerorthents formed in glacial outwash with slopes rang- acquisition of soil P (Caldwell 2006; Shaben and Myers ing from 0 to 3 % (Soil Survey Staff, USDA-NRCS). 2009). Increased P acquisition and availability associat- Soils at the OR site are classified as fine-loamy, isotic, ed with N-fixing species is supported theoretically mesic Andic Dystrudepts formed in basic agglomerate (Houlton et al. 2008) and empirically (Giardina et al. residuum with slopes ranging from 2 to 40 % (Soil 1995; Compton and Cole 1998), but the effect of in- Survey Staff, USDA-NRCS). The regional climate is creased acquisition on the availability of P pools over Mediterranean, characterized by cool, wet winters and time is unclear. Phosphorus availability has large control warm, dry summers with periods of prolonged drought. over biological activity in general (Vitousek et al. 2010), Potential natural vegetation includes the western hem- and any change in soil pools following Scotch broom lock (Tsuga heterophylla (Raf.) Sarg.)/salal (Gaultheria invasion could have lasting effects on soil functions. shallon Pursh) plant association at the WA site This would be especially true if readily-available pools (Henderson et al. 1989) and the western hemlock/ of inorganic P are reduced (Caldwell 2006), accelerating Oregon-grape (Mahonia nervosa (Pursh) Nutt.)/ the transformation of inorganic P into less-available swordfern (Polystichum munitum (Kaulf.) Presl) and organic and occluded forms (Walker and Syers 1976; western hemlock/Oregon grape-salal plant associations Compton and Cole 1998). Shaben and Myers (2009) at the OR site (Halverson et al. 1986). Plant Soil (2016) 398:281–289 283 Table 1 Site characteristics and selected pre-treatment soil properties to a depth of 60 cm for study sites in Washington and Oregon (from Devine et al. 2011) Characteristic or property WA site OR site Location (latitude, longitude) 47.206 °N, 123.442 °W 45.196 °N, 122.285 °W Elevation (m) 35 549 Mean annual temperature (°C) 10.7 11.2 Mean annual precipitation (cm)1 240 170 Soil particle size distribution (% sand/silt/clay)2 65 / 14 / 21 37 / 34 / 29 Bulk density (Mg m−3) 1.45 (0.05)3 0.98 (0.02) Soil coarse fragments by mass (%) 67.6 (1.3) 37.7 (2.2) Soil water holding capacity (mm)4 55 142 Total soil C (0–60 cm; Mg ha−1) 92.4 (5.8) 169.5 (12.0) Total soil N (0–60 cm; Mg ha−1) 3.3 (0.15) 7.2 (0.41) Precipitation was estimated for the period, 1950–2005 (PRISM Climate Group 2012) 2 Determined with the hydrometer method 3 Standard error in parentheses, n=8 for bulk density at OR site, n=16 for all others 4 Estimated from water release curves constructed with pressure plate analyses on intact soil cores Experimental Design Soil samples were collected in the fall of 2013 at both sites using bucket augers. At each treatment replication, Each of the sites was clear cut harvested in spring 2003. three samples were randomly selected from a nine point Following cutting, Scotch broom began to proliferate grid that encompassed the central 2-×2 m area of the across each of the sites from seed that was likely present plot. Given the plot arrangement described above, this in the seed bank prior to harvest (WA site) or brought in sampling scheme resulted in samples being collected on the machinery during forest harvesting (OR site). In from areas that were either in close proximity to broom the fall of 2013, 3-m radius circular plots were replicated (in broom-present plots) or in areas with limited over- with Scotch broom either: i) present since harvest or ii) story cover (in broom-absent plots).
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