J Arid Land (2013) 5(3): 298–309 doi: 10.1007/s40333-013-0172-0 jal.xjegi.com; www.springer.com/40333

Annual-perennial plant relationships and species selection for desert restoration Scott R ABELLA1∗, Stanley D SMITH2 1 2

Department of Environmental and Occupational Health, University of Nevada Las Vegas, Las Vegas, Nevada 89154-3064, USA; School of Life Sciences, University of Nevada Las Vegas, Las Vegas, Nevada 89154-4004, USA

Abstract: Exotic plant invasion is a growing concern in the conservation and management of indigenous arid land ecosystems. By creating areas of ameliorated microclimates and fertile soil below their canopies, perennial plants might influence exotic annual plant invasions. We conducted a quantitative literature review of studies that compared exotic annual plant abundance among native perennial plant species and interspace (open areas) microsites in North America’s Mojave Desert, where exotic plant invasion has corresponded with increasing extent of wildfire and broad-scale ecosystem transformation. Ten studies compared exotic annual plant abundance between interspaces and below a total of 36 native perennial species. These studies revealed that: (1) With few exceptions, most native perennial species supported a greater abundance of exotic annuals than interspaces, indicating overall facilitation of exotic species by native perennials. (2) Exotic species abundance varied by orders of magnitude among native perennial species, with some perennial species harboring amounts of exotics similar to interspaces. (3) Distributions of dominant exotic species varied, where Bromus rubens displayed a greater affinity for below-perennial microsites than did Schismus spp. and Erodium cicutarium that often were most abundant in interspaces. Results suggest that the degree of facilitation of exotic plants warrants consideration when selecting native perennial species for revegetation and restoration projects. Keywords: Bromus rubens; exotic; fertile island; invasive; nurse plant; positive plant interaction; Schismus Citation: Scott R ABELLA, Stanley D SMITH. 2013. Annual-perennial plant relationships and species selection for desert restoration. Journal of Arid Land, 5(3): 298–309.

A major principle of arid land ecology is that locations of perennial plants influence spatial distributions of microclimate, soil properties, and organisms (Tielbörger and Kadmon, 1997; Flores and Jurado, 2003; Allen et al., 2011). Through shading, microclimates below many perennial plant canopies exhibit moderated temperatures, reduced solar insolation, and lower evaporation rates compared to interspaces between perennial plants (Franco and Nobel, 1989). Via stemflow and root macrochannels, perennial plants also can increase water infiltration during precipitation events (Devitt and Smith, 2002; Bhark and Small, 2003). Soil nutrients are often more concentrated below perennial plants, through several processes such

as trapping of dust and litter, root uptake of nutrients from interspaces, and enhanced litter decomposition (Schlesinger et al., 1996; Titus et al., 2002; Yin et al., 2010). These nutrient-enriched soils are termed “fertile islands” and create heterogeneity in soil fertility at spatial scales of ’ symbols above bars indicate that a ratio is greater than 10. Perennial species are abbreviated as: LT, Larrea tridentata; AD, Ambrosia dumosa; HS, Hymenoclea salsola; KE, Krameria erecta; PS, Psorothamnus fremontii; and BJ, Bebbia juncea. Data for 1995 and 1997 were used for Brooks (1999) and Brooks (2000a).

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Table 1 Summary of studies comparing annual plant measures between interspaces and below perennial plant species in the Mojave Desert, USA Study and measure (interspace value)1 Brooks (1999)3

Native perennial species 2

LT AD HS KE KG PS BJ CR EF EV ET ES GS MS PR SM TM LA YS ––––––––––––––––––– Below-perennial : interspace abundance ratio –––––––––––––––––––

1994 exotic richness (1.5/0.1 m2)

0.5

1995 exotic richness (1.6/0.1 m2)

1.3

2

1994 exotic biomass (20 kg/hm )

0.3

1995 exotic biomass (13 kg/hm2)

9.2

1994 Bromus rubens biomass (0.6 kg/hm2)

2.1

1995 Bromus rubens biomass (0.4 kg/hm2)

37.1

1994 Bromus berteroanus biomass (