Seed Banks in a Degraded Desert Shrubland: Influence of Soil Surface Condition and Harvester Ant Activity on Seed Abundance
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Journal of Arid Environments 73 (2009) 885–893 Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv Seed banks in a degraded desert shrubland: Influence of soil surface condition and harvester ant activity on seed abundance L.A. DeFalco*, T.C. Esque, J.M. Kane 1, M.B. Nicklas 2 US Geological Survey, Western Ecological Research Center, Las Vegas Field Station, 160 North Stephanie Street, Henderson, NV 89074, USA article info abstract Article history: We compared seed banks between two contrasting anthropogenic surface disturbances (compacted, Received 19 December 2008 trenched) and adjacent undisturbed controls to determine whether site condition influences viable seed Received in revised form densities of perennial and annual Mojave Desert species. Viable seeds of perennials were rare in 16 April 2009 undisturbed areas (3–4 seeds/m2) and declined to <1 seed/m2 within disturbed sites. Annual seed Accepted 17 April 2009 densities were an order of magnitude greater than those of perennials, were one-third the undisturbed Available online 22 May 2009 seed densities on compacted sites, but doubled on trenched sites relative to controls. On trenched sites, greater litter cover comprising the infructescences of the dominant spring annuals, and low gravel Keywords: Granivores content, enhanced seed densities of both annuals and perennials. Litter cover and surface ruggedness Mojave Desert were the best explanations for viable perennial seed densities on compacted sites, but litter cover and the Restoration presence of a common harvester ant explained annual seed densities better than any other surface Reseeding characteristics that were examined. Surface disturbances can have a varied impact on the condition of Seed banks the soil surface in arid lands. Nevertheless, the consistently positive relationship between ground cover of litter and viable seed density emphasizes the importance of litter as an indicator of site degradation and recovery potential in arid lands. Published by Elsevier Ltd. 1. Introduction through trampling or vehicle activity, as well as those that mix topsoil and subsurface soils, can alter the physical, chemical and Viable seed banks of many arid and semi-arid environments are biological properties that influence viable seed densities. However, vulnerable to surface disturbances because they occur in the the relationships between soil surface properties and seed densities uppermost layers of soils (Reichman, 1984; Gutterman, 1993; Guo are poorly understood because soil properties associated with et al., 1998; Marone et al., 1998). Land uses, including heavy or disturbances are rarely characterized at the same time seed banks prolonged grazing, have reduced soil seed densities in the Negev are quantified. Desert (Sternberg et al., 2003), southern Ethiopia (Solomon et al., Soil seed densities vary across a range of soil surfaces in undis- 2006), China’s Horqin steppe and Inner Mongolia (Zhao et al., 2001; turbed arid lands (Reichman, 1984). Understanding how viable seed Zhan et al., 2007), central Australia (Kinloch and Friedel, 2005a,b) densities are related to the surface properties that result after and eastern Nama Karoo in South Africa (Jones and Esler, 2004). disturbance can help to develop indicators for determining the Heavy recreational use in the Sonoran Desert (Walters, 2004) and severity of site degradation, whether management action is required, trampling of coastal sand dunes in Israel (Sternberg et al., 2004) and determine the potential for vegetation recovery. A reduction in reduced seed densities and plant abundance. In contrast, seed organic debris that contains ripening infructescences may diminish densities rebounded after grazing pressure was reduced in grass- inputs into the seed bank and decrease the capture of wind-blown lands of Israel (Osem et al., 2006) and northwestern Patagonia seeds as they disperse across disturbed surfaces. Disturbances often (Ghermandi, 1997). Disturbances that increase soil bulk density remove shrub canopies where seeds are concentrated after seed rain and where seeds of annual and perennial grasses and forbs accu- mulate as they disperse from shrub interspaces (Nelson and Chew, 1977; Price and Reichman,1987; Guo et al.,1998; Marone et al.,1998; * Corresponding author. Tel.: þ1 702 564 4507; fax: þ1 702 564 4600. Caballero et al., 2003, 2005, 2008; Li, 2008). Large or small soil E-mail address: [email protected] (L.A. DeFalco). particle sizes entrap like-sized seeds (Price and Reichman, 1987; 1 Present address: Northern Arizona University, School of Forestry, Flagstaff, AZ 86001, USA. Chambers et al., 1991; Chambers, 1995), so disturbance-induced 2 Permanent address: 244 South Pitkin Road, Craftsbury Common, VT 05827 USA. shifts in particle sizes will likely change the abundance and species 0140-1963/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.jaridenv.2009.04.017 886 L.A. DeFalco et al. / Journal of Arid Environments 73 (2009) 885–893 composition of seeds and the resulting establishment of seedlings. 1949–2004) is 98 Æ 45 mm (Daggett FAA Airport, California; http:// Compaction reduces the number of available microsites that capture www.wrcc.dri.edu). Rainfall during the hydrologic year prior to drifting seeds and produces a mechanical barrier for root growth of seed bank collection was 119 mm, which was above the long-term developing seedlings (Sheldon,1974). By enhancing microslope – the mean. Creosote bush (Larrea tridentata) and bur-sage (Ambrosia slope associated with a seed bank over a topographic area less than dumosa) are the co-dominant species in undisturbed areas with an several meters – disturbances can deplete seed densities through understory of native and non-native species. The study was con- greater water run-off during extreme rainfall events (Caballero et al., ducted at three compacted and three trenched sites distributed 2003). Soil micro-relief, or the collective small crevices and peaks from the northwest to the southeast across the NTC (Fig. 1). due to physical texture or biological crusts, captures seeds and Trenches were constructed one to two years prior to the study, and influences the small-scale redistribution of seeds (Boudell et al., compacted areas received concentrated use during training exer- 2002; Serpe et al., 2006). Thus, disturbances that reduce soil micro- cises for several decades (R. Sparks, NTC, Integrated Training Area relief, such as heavy trampling or vehicle use, can reduce seed bank Management, personnel communication). Compacted sites were abundance whereas others that roughen the soil surface can increase broad areas with recurring and concentrated vehicle traffic. These seed banks. sites were flat with minimal slope and low surface roughness. In addition to the physical properties of soils, seed harvesting Litter was minimal, and only scattered individuals of the peren- activities of granivores influence the regeneration of plant nials Eriogonum fasciculatum, L. tridentata, Croton californica, Ste- communities in arid lands. In the desert southwest of North phanomeria pauciflora and Hymenoclea salsola were present. These America, harvester ants alter plant species abundance and diversity plants were likely adult plants that endured repeated vehicular through seed predation (Brown et al., 1979; MacMahon et al., impacts and survived by crown-sprouting (Gibson et al., 2004). 2000). While seeds from nearby undisturbed areas move onto Annual species observed during the study included Ambrosia disturbed areas via wind or water transport, ants and small acanthicarpa, Cryptantha angustifolia, Pectocarya heterocarpa, mammals can be important dispersal agents for plant species with Amsinckia tessellata, Astragalus acutirostris, and Lupinus humistratus large seeds that are not easily dispersed (Brown et al., 1979; Vander and the non-native species Schismus barbatus, Salsola tragus, Ero- Wall et al., 2006). Seed banks are likely a function of soil properties dium cicutarium, and Bromus madritensis ssp. rubens. and granivore activities that are characteristic of the disturbances In contrast to compacted sites, trenched sites are narrow, linear themselves, yet the influences of numerous physical and biological features where subsoil layers have been mixed with topsoil to soil properties have not been simultaneously evaluated for a depth of w2 m (R. Sparks, NTC, Integrated Training Area degraded arid lands. Little is known about seed bank dynamics Management, personnel communication). Trenched sites have both among contrasting disturbances, or whether there are common slightly higher litter cover and surfaces with higher within-plot surface properties that explain seed bank abundance and species micro-relief than compacted sites due to the wide tire treads of the composition of disturbed surfaces in general. mechanized equipment used to replace the soil. Annual and This study compares the viable seed densities for two different perennial species composition on trenched sites is similar to surface soil disturbances and illustrates how the properties of these compacted sites: perennial cover is low (<1%) and both native and disturbances can interact to influence seed banks in an arid shrub- non-native annual plant species co-occur. land community. The study was conducted at a military training facility in the Mojave Desert of southern California, USA where severely degraded environments provide excellent opportunities 2.2. Sampling design for