Biol Invasions (2019) 21:1055–1073 https://doi.org/10.1007/s10530-018-1885-y (0123456789().,-volV)(0123456789().,-volV) ORIGINAL PAPER Scotch broom (Cytisus scoparius) modifies microenvironment to promote nonnative plant communities David R. Carter . Robert A. Slesak . Timothy B. Harrington . David H. Peter . Anthony W. D’Amato Received: 9 February 2018 / Accepted: 21 November 2018 / Published online: 24 November 2018 Ó Springer Nature Switzerland AG 2018 Abstract Scotch broom [Cytisus scoparius (L.) vegetation plots. On a percent cover basis, Scotch Link] is a globally important nitrogen (N)-fixing broom had a substantially greater negative influence invasive plant species that has potential to alter soil on SWC than did background vegetation. Surpris- water dynamics, soil chemistry, and plant communi- ingly, Scotch broom was not consistently associated ties. We evaluated the effects of Scotch broom on soil with increases in total soil N, but there was evidence of moisture, soil chemistry, soil temperature, photosyn- increasing soil water N when Scotch broom was thetically active radiation (PAR), and vegetation present. Scotch broom-only plots had greater concen- communities over 4 years at a site recently harvested trations of soil water magnesium (Mg2?) and calcium for timber. Treatments of Scotch broom (either present (Ca2?) than other treatments. On a percent cover basis, via planting or absent) and background vegetation Scotch broom had a uniquely high demand for (either present or absent via herbicide treatments) potassium (K?) relative to the background vegetation. were applied to 4 m2 plots. Background vegetation Average soil temperature was slightly greater, and soil was associated with the greatest decrease of soil water surface PAR lower, with Scotch broom present. content (SWC) among treatments. During the driest Scotch broom-absent plots increased in species diver- year, Scotch broom showed some evidence of sity and richness over time, while Scotch broom- increased early-and late-season soil water usage, present plots remained unchanged. Scotch broom and, briefly, a high usage relative to background presence was associated with an increase in cover of nonnative sweet vernalgrass (Anthoxanthum odora- tum L.). Scotch broom generated positive feedbacks Electronic supplementary material The online version of with resource conditions that favored its dominance this article (https://doi.org/10.1007/s10530-018-1885-y) con- tains supplementary material, which is available to authorized and the establishment of nonnative grass. users. D. R. Carter (&) T. B. Harrington D. H. Peter Department of Forest Resources and Environmental Á USDA Forest Service, Pacific Northwest Research Conservation, Virginia Polytechnic Institute and State Station, 3625 93rd Avenue Southwest, Olympia, University, 228G Cheatham Hall, Blacksburg, VA 24060, WA 98512, USA USA e-mail: [email protected] A. W. D’Amato Rubenstein School of Environment and Natural R. A. Slesak Resources, University of Vermont, Burlington, VT 05405, Department of Forest Resources, University of Minnesota, USA 1530 Cleveland Ave N, Saint Paul, MN 55108, USA 123 1056 D. R. Carter et al. Keywords Soil properties Á Pacific Northwest Á Generally the traits that enable a plant species to Extended growing season Á Soil water become invasive—high growth rate and leaf nutrient concentrations—are traits that also increase decom- position rates and nutrient cycling (Ehrenfeld et al. 2001; Ehrenfeld 2003; Allison and Vitousek 2004). Introduction Invasive N-fixing plant species generally have a greater impact on patterns of N-cycling (Vila` et al. Native to the Mediterranean (Tutin et al. 1968), Scotch 2011; Broadbent et al. 2017) than non-N-fixing plants. broom (Cytisus scoparius (L.) Link) is an invasive Compounding this, edaphically young sites, like those species of major ecological concern (Bossard and commonly found in recently glaciated regions of the Rejmanek 1994). Now found on six continents (Potter western PNW, generally possess an N:P stoichiometry et al. 2009), Scotch broom is capable of dominating favorable for biological N-fixation (Williams 1981). sites (Bossard and Rejmanek 1994; Richardson et al. Similarly, greater soil phosphorus is often correlated 2002) and altering ecosystem function (Haubensak with plant species invasion (Weidenhamer and Call- and Parker 2004; Slesak et al. 2016). Scotch broom is away 2010). an aggressive invader in early-successional coast Several studies have investigated the impact of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco Scotch broom invasion on soil properties and nutrient var. menziesii) forests of the Pacific Northwest cycling with varying results. Increases in soil C, N, and (PNW), where this study was conducted. Scotch P have been documented following invasion in several broom is well-adapted to the climate and summer studies (Wheeler et al. 1987; Fogarty and Facelli 1999; droughts characteristic of the region. High seed Haubensak and Parker 2004; Caldwell 2006; Grove production coupled with a ballistic seed dispersal et al. 2015), whereas others have reported no change in strategy and decades-long seed viability, allows this N and a decrease in P (Shaben and Myers 2009). species to rapidly dominate and persist on a site for Caldwell (2006) found that soils under Scotch broom many years (Bossard and Rejmanek 1994). had higher activities of two soil enzymes involved in The dramatic annual fluctuation of precipitation the acquisition of phosphorus. It is still not known characteristic of Mediterranean climates, like that of whether greater P depletion in the presence of legumes the PNW, results in soil water being a primary limiting is due to plant-or Bradyrhizobium-(bacterial sym- resource during the growing season (Brubaker 1980). bionts of Scotch broom) demands (Hardy and Havelka Scotch broom has been found to be a strong competitor 1976). N-fixation is considered a P-demanding pro- for soil water in similar systems (Richardson et al. cess, yet, other studies have demonstrated no increase 2002; Watt et al. 2003) and to have a high evapotran- in Bradyrhizobium fixation rates but an increase in spiration (ET) rate, owing to its large size (Boldrin host biomass (Crews 1993) under P fertilization. et al. 2017). Conversely, Scotch broom possesses Dassonwille et al. (2008) found a general tendency several traits that make it tolerant and avoidant of for invaders to increase soil nutrient pools on low- drought conditions: high root length density, low leaf fertility sites and decrease soil nutrient pools on high area to root mass ratio, low specific leaf area, quality sites. Comparisons made by Slesak et al. photosynthetic stems, and a drought-deciduous phe- (2016) between two sites of contrasting soil textures nology (Bannister 1986; Bossard and Rejmanek 1994; and nutrient availabilities showed changes in soil Matı´as et al. 2012; Boldrin et al. 2017). As a result, chemical properties in the presence of Scotch broom Scotch broom is considered highly adapted to drought- may be site-dependent. Any change in nutrient cycling prone, Mediterranean climates, likely facilitating its caused by an invasive species has the potential to alter dominance of sites in those regions around the globe. the distribution and availability of nutrients, thereby Given these potentially conflicting attributes of Scotch influencing plant communities (Prober and Lunt broom—as a plant with the ability to deplete soil water 2009). due to its high ET and as a plant that avoids excessive Invasive species can possess traits that facilitate soil water usage during peak drought conditions— competitive success over native plant species. Such questions remain about Scotch broom’s influence on traits include: plastic phenologies, rapid growth, and soil water. extensive seedbank accrual (Knapp and Seastedt 1986; 123 Scotch broom (Cytisus scoparius) modifies microenvironment 1057 Beatty and Sholes 1988; Wearne and Morgan 2006; in isolation; and (v) increases in nonnative plant Zhou et al. 2009; Matı´as et al. 2012; Boldrin et al. species cover and decreases in native plant cover in the 2017). Soil nutrient enrichment, particularly N, asso- presence of Scotch broom. Lastly, we expected (vi) ciated with plant invasions has been hypothesized to Scotch broom to have greater decreasing effects on facilitate the establishment and success of other soil water content and soil nutrient variables relative to nonnative invaders (Weidenhamer and Callaway native plants on a per unit of cover basis. 2010). Invasions can be so severe as to lead to drastic modification or loss of native plant communities (Ruesink et al. 1995). Similarly, Scotch broom has Methods caused changes in plant community composition and richness (Parker et al. 1997) through several mecha- Study site nisms. For example, the rapid growth rate of Scotch broom facilitates its dominance over competing The study was located near Matlock, WA, USA species (Fogarty and Facelli 1999). The resultant (47.215°N, 123.417°W) at a recently harvested forest shade and litter accumulation under large Scotch site. The soils are classified as a sandy-skeletal, mixed, broom affect soil temperature and, correspondingly, mesic, Dystric Xerorthents formed in glacial outwash conditions affecting germination for other species (Soil Survey Staff, USDA-NRCS; Table 1). The local (Waterhouse 1988; Wearne and Morgan 2004). Grove climate is Mediterranean with a winter rainy season et al. (2012, 2015) partially attributed reductions in and seasonal