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Mechanisms for Maintenance of Dominance in a Nonclonal Desert Shrub

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Mechanisms for Maintenance of Dominance in a Nonclonal Desert Shrub Mechanisms for maintenance of dominance in a nonclonal desert shrub STANLEY G. KITCHEN, SUSAN E. MEYER, AND STEPHANIE L. CARLSON United States Department of Agriculture, Forest Service, Rocky Mountain Research Station, Shrub Sciences Laboratory, Provo, Utah 84606 USA Citation: Kitchen, S. G., S. E. Meyer, and S. L. Carlson. 2015. Mechanisms for maintenance of dominance in a nonclonal desert shrub. Ecosphere 6(12):252. http://dx.doi.org/10.1890/ES15-00083.1 Abstract. Blackbrush (Coleogyne ramosissima: Rosaceae) is a slow-growing, non-clonal shrub that is regionally dominant on xeric, shallow soils in the North American Mojave Desert-Great Basin transition zone and southern Colorado Plateau. Blackbrush seed production is concentrated in mast years, and most seeds are cached and later consumed by heteromyid rodents. Vegetation histories show that blackbrush stands can persist apparently unchanged for over a century. We used dendrochronological techniques to examine plant age distributions, recruitment patterns and growth rates, to ascertain how blackbrush achieves this long-term population stability. Our study addressed the following questions: (1) What is the role of within-clump recruitment in long-term patterns of clump persistence? Do blackbrush clumps accrue new cohorts through time? (2) How does recruitment vary temporally, specifically in relation to years of mast seed production and climate variability? (3) What impact does intra-specific competition have on plant growth rates? To address these questions, we aged stems from 208 clumps in five Mojave Desert and four Colorado Plateau populations. Individual plant age estimates ranged from 3 to 122 years. Clumps comprised of multiple-aged cohorts were ubiquitous. Within clumps, plant and cohort number increased with clump age, suggesting a steady accumulation of new cohorts over time. Clumps in Colorado Plateau populations accumulated cohorts at a significantly faster rate than clumps in Mojave Desert populations. Recruitment occurred in relatively frequent pulses. It was only partially synchronized with mast years, with some seedling establishment following years of low seed production. Individuals that recruited into established clumps averaged half the radial growth rate of individuals that recruited into openings. Blackbrush recruitment is bimodal, with initial colonization of open spaces from rodent caches but with long-term clump persistence a product of periodic, within-clump recruitment of new plants. This dual recruitment strategy provides a mechanism for continued community dominance in an abiotically stressful environment under low levels of disturbance. Key words: age structure; blackbrush; Coleogyne ramosissima; Colorado Plateau; dendrochronology; growth rate; life- history strategies; masting; Mojave Desert; seedling bank; stress tolerance. Received 13 February 2015; revised 6 July 2015; accepted 7 July 2015; published 8 December 2015. Corresponding Editor: E. Ribbens. Copyright: Ó 2015 Kitchen et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. http://creativecommons.org/licenses/by/3.0/ E-mail: [email protected] INTRODUCTION and that life history strategies in turn are shaped by environmental factors related to abiotic stress It has long been recognized that plant com- and disturbance regimes (Grime 1977). Species munity dynamics are strongly influenced by the long-term success and persistence in a plant life history strategies of the component species community can best be described as full self- v www.esajournals.org 1 December 2015 v Volume 6(12) v Article 252 KITCHEN ET AL. replacement through time, implying the opera- long-lived, forming stable populations that have tion of life-history traits that provide a balance been shown to persist for centuries (Webb et al. between costs and potential benefits of traits 1987, 2011). Its leaves are microphyllous and essential for individual longevity and new facultatively deciduous (Bowns 1973, Ackerman recruitment (Silvertown and Charlesworth and Bamberg 1974, Ackerman et al. 1980), stress- 2001). On the other hand, the capacity to occupy resistance adaptations enabling plant survival and dominate a plant community over time is a through multiple cycles of drought (Smith et al. measure of success that goes beyond basic self- 2004). Unlike many stress-tolerant desert shrubs, replacement. Dominance can be defined in terms however, blackbrush regularly produces large of total biomass or resource capture capacity seed crops and appears to have generally high relative to the community as a whole (Whittaker investment in sexual reproduction, with estimat- 1965). Plant communities in which ecosystem ed seed yields as high as 300 kg haÀ1 in high- processes perpetuate existing patterns of domi- production years (S. Meyer, unpublished data). nance can be considered stable. Conversely, Production of the moderately large seeds (one- processes which alter ecosystems to the extent seeded achenes; mean mass ’17 mg; Pendleton that dominant species become poorly adapted 2008) is limited primarily to mast years, espe- reduce community stability. Thus, knowledge of cially at drier sites (Pendleton et al. 1995; Meyer adaptive strategies that enable species to achieve and Pendleton 2015a). Masting, defined as ‘the and maintain dominance is needed to predict synchronous intermittent production of large plant community stability in response to change. seed crops by perennial plants’ (Kelly and Sork Life-history strategies that enable a species to 2002) has rarely been reported for desert shrubs. first effectively occupy a large proportion of It is commonly associated with the need to satiate available space and second, to hold that space seed predators, in this case the heteromyid once occupied through ongoing self-replacement, rodents that also disperse and scatter-hoard the should facilitate community dominance. seeds (Bowns and West 1976, Auger 2005, Meyer In xeric environments, dominant shrub species and Pendleton 2015b). Successful seedling estab- must be resilient to extremes in the physical lishment takes place primarily from rodent environment and associated abiotic stresses. caches, though emergence under shrub crowns They are expected to exhibit strategies for stress from seeds that have not undergone secondary tolerance that feature slow growth, late maturity, dispersal is also possible (Meyer and Pendleton and long life span (Grime 1977). Life-history 2015b). Thus, masting validates seed predation as traits that result in infrequent recruitment while a selective force without masking the counter- maintaining community dominance are also balancing benefits of predator-assisted seed often associated with long-lived shrubs in warm dispersal on fitness. In addition to selecting for deserts (Ackerman 1979, McAuliffe 1988, Hunter intermittent reproduction, post-dispersal verte- 1989, Cody 2000). Individual plant longevity, and brate seed predation favors reduced seed bank thus stand stability, is often maintained through residence time, which can be detected as an such mechanisms as vegetative regeneration or absence or reduction in the dormancy mecha- clonal expansion (e.g., creosote bush (Larrea nisms necessary for maintenance of persistent tridentata (DC.) Coville; McAuliffe et al. 2007). seed banks (Pendleton and Meyer 2004, Meyer Blackbrush (Coleogyne ramosissima Torrey: Ro- and Pendleton 2005). saceae) is a desert shrub that is often dominant Interactions among seedling, juvenile and where it occurs, frequently forming near-mono- adult conspecifics within plant communities are cultures on shallow soils in areas transitional dynamic and feature a combination of facilitative between cold and warm deserts of western North and competitive effects (Holmgren et al. 1997, America (Bowns and West 1976, West 1983, Montgomery et al. 2010). In stressful environ- Callison and Brotherson 1985, Meyer and Pend- ments, juveniles may benefit from close proxim- leton 2005). Some features of blackbrush life ity to adults through amelioration of the abiotic history fit the paradigm for a stress-tolerant environment (Brooker et al. 2008, Valladares et dominant species while others appear contradic- al. 2008). Benefits to sub-adult plants may tory. Blackbrush is slow-growing and apparently include reductions in direct irradiance, air and v www.esajournals.org 2 December 2015 v Volume 6(12) v Article 252 KITCHEN ET AL. soil temperatures and vapor pressure deficit, and even herbaceous plants if appropriate protocols from increases in soil nutrients. Nurse plants are employed (Schweingruber and Poschlod may also provide protection from herbivory 2005). Interest in shrub dendrochronology has (McAuliffe 1986). The net outcome of competi- increased rapidly in recent years, especially for tive and facilitative interactions may be transito- alpine and arctic ecosystems experiencing shrub ry, switching with ontogenetic development as invasion related to global warming (Myers-Smith maturing plants become less vulnerable to stress et al. 2015). Recent studies on shrubs of arid and (Miriti 2006, Wright et al. 2014). semi-arid ecosystems are less common (e.g., Blackbrush has a long history in the American Milton et al. 1997, Seligman and Zelman 2000, desert southwest. Woodrat midden macrofossil Poore et al. 2009, Srur and Villalba 2009, Olano et assemblages indicate
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