Plant Ecol DOI 10.1007/s11258-011-9992-1 Established native perennial grasses out-compete an invasive annual grass regardless of soil water and nutrient availability Christopher M. McGlone • Carolyn Hull Sieg • Thomas E. Kolb • Ty Nietupsky Received: 21 December 2010 / Accepted: 6 October 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Competition and resource availability influ- grasses were highly competitive against B. tectorum, ence invasions into native perennial grasslands by non- regardless of water, N, or P availability. Bromus native annual grasses such as Bromus tectorum.Intwo tectorum reproductive potential (viable seed produc- greenhouse experiments we examined the influence of tion) was not significantly influenced by any experi- competition, water availability, and elevated nitrogen mental manipulation, except for competition with P. (N) and phosphorus (P) availability on growth and smithii. In all cases, B. tectorum per-plant production of reproduction of the invasive annual grass B. tectorum viable seeds exceeded parental replacement. Our results and two native perennial grasses (Elymus elymoides, show that established plants of Elymus elymoides Pascopyrum smithii). Bromus tectorum aboveground and Pascopyrum smithii compete successfully against biomass and seed production were significantly reduced B. tectorum over a wide range of soil resource when grown with one or more established native availability. perennial grasses. Conversely, average seed weight and germination were significantly lower in the B. Keywords Bromus tectorum Á Competition Á tectorum monoculture than in competition native Greenhouse Á Nitrogen Á Phosphorus Á perennial grasses. Intraspecific competition reduced Water availability per-plant production of both established native grasses, whereas interspecific competition from B. tectorum increased production. Established native perennial Introduction Resource competition is an important driver of non- C. M. McGlone (&) Á T. Nietupsky native plant invasions. Establishment and spread of non- Ecological Restoration Institute, Northern Arizona native plants in new areas often depends on acquiring University, P.O. Box 15017, Flagstaff, AZ 86011, USA resources faster than, and often at the expense of, native e-mail: [email protected] plants (Rees et al. 2001;Levineetal.2003). Competitive C. M. McGlone Á T. E. Kolb Á T. Nietupsky differences between native and non-native species School of Forestry, Northern Arizona University, depend on the taxa involved and the environmental P.O. Box 15018, Flagstaff, AZ 86011, USA context because of differences in species’ growth and reproductive responses to resource availability (Rees C. H. Sieg USDA Forest Service, Rocky Mountain Research Station, et al. 2001). Understanding competitive interactions at 2500 S Pine Knoll Dr, Flagstaff, AZ 86001, USA the time of invasion can help guide prevention and 123 Plant Ecol post-invasion restoration efforts (Davis et al. 2000; availability, and N and P availability on B. tectorum D’Antonio and Meyerson 2002). and native perennial grass productivity and B. tecto- Bromus tectorum L. (cheatgrass) is a highly inva- rum reproductive potential, we conducted a replace- sive non-native species (Knapp 1996). This Eurasian ment series greenhouse experiment with B. tectorum annual grass is the dominant species on *20 million seedlings and established plants of two perennial grass hectares in the western U.S. (Bradley and Mustard species native to Arizona ponderosa pine forests: 2005). Areas where B. tectorum dominates often have Elymus elymoides (Raf.) Swezey (bottlebrush squir- more frequent fire (Brooks et al. 2004) and altered reltail) and Pascopyrum smithii (Rydb.) A. Lo¨ve biodiversity (Bolton et al. 1993; Brandt and Rickard (western wheatgrass). The plants were grown at both 1994; Belnap and Phillips 2001). Bromus tectorum high and low water availability and with and without N invasions are often driven by disturbance (Bradford and P amendments. To allow for continuity with and Lauenroth 2006), but undisturbed communities previous greenhouse research on B. tectorum—native can also be invaded (Belnap and Phillips 2001). After perennial grass competition, we have elected to use a invasion, B. tectorum can dominate an ecosystem for similar replacement series design to Lowe et al. (2003) decades (Brandt and Rickard 1994). allowing us to determine the relative strength of inter- Competition with perennial grasses can restrict the and intraspecific competition (Jolliffe 2000). We spread of B. tectorum (Yoder and Caldwell 2002; Booth hypothesized that: (1) B. tectorum and native perennial et al. 2003; Chambers et al. 2007). The competitiveness grass growth would be negatively affected by inter- of perennial grasses against B. tectorum is dependent on specific competition, as would B. tectorum reproduc- the life stage of the perennial grasses. Greenhouse and tive potential; (2) competitive relationships between field experiments indicate B. tectorum will generally B. tectorum and native perennial grasses would be out-compete perennial grass seedlings (Lowe et al. altered by increased water availability favoring 2003; Humphrey and Schupp 2004). Yet, field studies B. tectorum; and (3) competitive relationships between suggest that established perennial grasses, particularly B. tectorum and native perennial grasses would be Elymus L. spp. (squirreltail) and Agropyron Gaertn. altered by nutrient amendments favoring B. tectorum. spp. (wheatgrass), can inhibit B. tectorum establish- ment and growth (Yoder and Caldwell 2002; Booth et al. 2003; Chambers et al. 2007). Materials and methods As an annual, B. tectorum is more dependent on the immediate availability of resources than perennial Experimental design grasses (Marschner 1995). Arid and semi-arid regions where B. tectorum has successfully invaded are limited We conducted this study at the Rocky Mountain by water availability. Furthermore, nitrogen (N) avail- Research Station (RMRS) Greenhouse in Flagstaff, ability can alter B. tectorum germination, growth, and AZ (35.1°N 111.69°W). We established two replace- competitive ability (Blank et al. 1994; Lowe et al. ment series experiments (deWit 1960), each contain- 2003; Beckstead and Augspurger 2004). Availability ing a native perennial grass in competition with of phosphorus (P) has also been positively related to B. tectorum in a 6 9 3 9 2 factorial randomized B. tectorum performance (Miller et al. 2006). complete block design with 10 blocks. The first Past research has shown that B. tectorum has poor experiment tested competition between B. tectorum survivorship in Pinus ponderosa C. Lawson (ponder- and E. elymoides. The second experiment tested osa pine) forests in the northwestern United competition between B. tectorum and P. smithii. Both States (Pierson and Mack 1990). Recently, however, experiments tested effects on plant production of six B. tectorum has established persistent populations in levels of interspecific competition, three levels of montane P. ponderosa forests of northern Arizona nutrient availability, and two levels of water avail- (Laughlin and Fule´ 2008; McGlone et al. 2009). Field ability. All plants were grown in 3 l, (16 cm-diameter, research suggests that established perennial grasses and 18.5 cm-tall) plastic pots in a medium of 75% soil and plant-available N and P may influence B. tectorum and 25% perlite. We collected soil 10 km south of invasion in Arizona forests (McGlone et al. 2011). Flagstaff in a P. ponderosa forest with basalt-derived To evaluate the influence of competition, water Typic Argiustolls. All species used in the greenhouse 123 Plant Ecol experiment grow near the soil collection area. We fertilization. Phosphorus was applied at a rate of mixed the soil, collected three samples and measured 5gPm-2 year-1 in a single dry application on April total N, total P, and PO4 at the RMRS Laboratory in 1, 2008. This level significantly increased above- Flagstaff, AZ. Each block contained one replicate of ground growth of native perennial grasses in field each treatment combination for a total of 360 pots per studies in northern Arizona (G. Newman, School of experiment. Each treatment replicate was assigned a Forestry, Northern Arizona University, unpublished random location within each block. The blocks were data). established along a moisture and temperature gradient To test the competitive relationships between based on proximity to the cooling fan at one end of the established native perennials and B. tectorum seed- greenhouse. lings, we planted the native perennial grass seeds Species competitive ability (competition) was esti- several months before planting B. tectorum seeds. In mated by comparing plant growth in species mixtures May 2007, we planted E. elymoides and P. smithii to growth in monocultures. The mixtures were: seeds at three times the desired density. After germi- 5/0, 4/1, 3/2, 2/3, 1/4, 0/5 native/B. tectorum plants. nation the seedlings were thinned to the target density. Elymus elymoides (Sand Hollow cultivar) and In August 2007, commensurate with the timing of field P. smithii (Arriba cultivar) seeds were purchased from germination of B. tectorum, we planted B. tectorum Granite Seed Company in Lehi, Utah. Bromus tecto- seeds at three times the target density and then thinned rum seed was collected in 2007 from P. ponderosa after germination. Locations of B. tectorum and native forests at Flagstaff and Mt. Trumbull, Arizona. Mean seeds within each pot were randomly assigned at annual