Study site

1. Responding to temporal resource pulses in a spatially subsidized community: surprising strategies of desert annuals D. Alexander Wait1 and Wendy B. Anderson2 1Dept. of Biology, Missouri State University and 2Dept. of Biology, Drury University, Springfield, Missouri • Conclusions • Species richness on subsidized islands is lower than on unsubsidized islands only in dry years (Fig. 2), but the species found on subsidized islands are not commonly found on either the mainland or unsubsidized islands in any year. Therefore, subsidies impart a very different selection pressure – and limitthe species pool from which to draw on. • In dry environments natural selection should maximize water-use efficiency (WUE) (Ehleringer 1993) or operate to maximize carbon assimilation (Gibson 1998). In this system the regionally common (i.e., unsubsidized species) annual species have both relatively high photosynthetic rates and low WUE (Figs. 4-6). • Interestingly, even though subsidies increase soil moisture (Fig. 3), the dominant species in subsidized areas are even more WUE than the dominant species in unsubsidized areas (Figs. 4 and 6), with no difference in assimilation rates (Fig. 4). Therefore, selection apparently operates to maximize WUE instead of photosynthesis (see Casper et al. 2005). This phenotypic selection is also indicated in Atriplex (Figs 7-8) • Differences in species composition are maintained by periodic wet years because water pulses strengthen selection in subsidized areas for species that are considered more “weedy” (e.g., Amaranthus and Chenopodium)– but in this case these species have more conservative water use strategies and greater plasticity to respond in wet years. • Greater productivity in subsidized areas than in unsubsidized areas (Figs 1-2) is apparently a function of subsidized species being able to keep their stomates open longer into the day (see Fig. 5), and by having longer growing seasons (i.e., they have slower growth rates – but grow for longer periods of time). • Without pulsed years, subsidized islands would probably not look very different from unsubsidized islands with respect to plant species richness and productivity. Results A) Water pulses increase soil organic matter more in subsidized than unsubsidized areas which results in greater soil moisture in subsidized areas even during interpulse years (Fig. 3). Differences in soil moisture help explain differences in cover (Fig. 1) and productivity (Fig. 2) between unsubsidized and subsidized areas. Introduction The physiological responses of plants to pulse events are well studied (e.g., Oecologia vol. 141 #2), while responses to pulse events in spatially subsidized environments have received little attention. Here we examine the physiological characteristics of herbaceous plants growing in a hyper arid region that experiences periodic (every 2-7 years) pulses of rain and contains areas (small islands) that are highly subsidized with nutrient resources via seabird guano (see Fig. 1). We ask how water pulses affect selection for physiological traits in subsidized vs. unsubsidized areas. The results suggest how spatial subsidies in arid environments alter and magnify the impact of temporal variation in rainfall on primary productivity and species richness (see Fig. 2) via selection for conservative water use traits. D) Carbon isotopic discrimination, an integrated measure of water-use efficiency (WUE), indicates that subsidized species are more WUE than unsubsidized species. These data are consistent with instantaneous measures of WUE (Fig. 4). As expected WUE is greater in a dry than a wet year. Fig. 3. Mean (±SE) gravimetric soil moisture at a depth of 8-12 cm on unsubsidized (n=6) and subsidized (n=6) islands in two dry and two wet years. Fig. 6. Mean (±SE) carbon isotope discrimination calculated from carbon isotope ratios of leaf material of dominate C3 species (Cr, Pl, Ch) collected at the end of the growing season in a dry and wet year. Lower discrimination values indicate higher water-use efficiency. B) Species growing in subsidized areas do not have higher instantaneous net photosynthetic rates than species growing in unsubsidized areas (Fig. 4), and therefore must grow over a longer period of time to attain greater biomass (see Fig. 2). In addition, species in subsidized areas are significantly more water-use efficient than species growing in unsubsidized areas (Fig. 4), even though soil moisture availability is greater in the subsidized areas (Fig. 3). E) Atriplex (a species common to both unsubsidized and subsidized areas) net assimilation rates were not significantly different between area types, while instantaneous WUE was greater in Atriplex individuals growing in subsidized areas (Fig. 7). These results mimic those found for the species common to each area type (Fig. 4), and suggest that phenotypic selection for physiological traits is occurring in Atriplex due to both subsidies and water pulses. Fig. 1. Subsidized islands have up to 80% herbaceous cover in a wet year (top), while unsubsidized islands have less then 30% cover in a wet year (bottom). • Study site • Highly water pulsed: Mean (± SD) annual precipitation 1991-2004 was 53± 72 mm; range was 0-300 mm. • 1999- 2002 were dry years, 2003 and 2005 were wet years. Pulse (“wet”) years have on average over 150 mm rain, while interpulse (“dry”) years average less than 20 mm. All data were collected in March-May 2001, 2002, 2003 and 2005. • Six subsidized and six unsubsidized islands in midriff region of Gulf of California; subsidies are primarily in the form of seabird guano deposition. • Subsidized islands have up to 18x higher soil N and P, higher soil moisture in both interpulsed and pulsed years (see Fig. 3), and lower soil pH (Wait et al. 2005). • Productivity is higher, but herbaceous species richness is lower on subsidized than unsubsidized islands (Fig. 2). • There is virtually no overlap in herbaceous species between unsubsidized and subsidized islands. Cryptantha (Cr) and Plantago (Pl) are dominant on unsubsidized islands, while Chenopodium (Ch) and Amaranthus (Am) are dominant on subsidized islands; Atriplex (At) is the only species that is common across all areas; Perytile (Pr) is dominant on subsidized islands only in pulse years. • Island size, physical disturbance, and geology do not explain island wide differences in productivity or species richness (Anderson and Wait 2001, Anderson and Polis 2004). Literature cited Anderson, W. B. and G.A. Polis. 2004. Allochthonous nutrient and food inputs: consequences for temporal stability. In Polis, G. A., M. E. Power, and G. R. Huxel (eds), Food webs at the landscape scale. University of Chicago Press. Anderson, W.B., and D.A. Wait. 2001. Subsidized island biogeography hypothesis: another new twist on an old theory. Ecology Letters. 4:289-291. Casper, B.B., Forseth, I.N. and D.A. Wait. 2005. Variation in carbon isotope discrimination in relation to plant performance in a natural population of Cryptantha flava. Oecologia. (Online First). Ehleringer, J.R. 1993. Carbon and water relations in desert plants: an isotopic perspective. In Ehleringer et al. (eds) Stable isotopes and plant carbon-water relations. Academic, San Diego, pp 155-172. Gibson, A.C. 1998. Photosynthetic organs of desert plants. Bioscience 48:911-920. Wait, D.A., Aubrey, D.P., and W.B. Anderson. 2005. Seabird guano influences on desert islands: soil chemistry and herbaceous species richness and productivity. Journal of Arid Environments. 60:681-695. Fig. 4. Mean (±SE) mid-day instantaneous net assimilation and water-use efficiency for dominant species growing in unsubsidized (Cr and Pl; n=26) and subsidized (Ch and Am; n=26) areas across four different islands in a wet year. Fig. 7. Mean (±SE) mid-day instantaneous net assimilation and water-use efficiency for Atriplex growing in unsubsidized and subsidized (n=6) areas across two different islands in a wet year. C) Species that dominate subsidized areas have lower water contents and are more water stressed in dry years than species that dominate in unsubsidized areas (Fig. 5), indicating that these species keep their stomates open longer into the growing season and into the day. However, in a wet year plant water potential is much higher in species growing in subsidized than unsubsidized areas (Fig. 5), indicating greater plasticity in responding to a water pulse. F) Atriplex individuals in subsidized areas in dry years are more water stressed than Atriplex individuals in unsubsidized areas (Fig. 8). Therefore, physiological traits associated with Atriplex production patterns (which follow overall production patterns; Fig. 2) apparently mimic those found in species only growing in unsubsidized or subsidized areas (Figs. 4-6). D) Leaf Carbon Isotope Acknowledgments We thank the Mexican Department of the Environment for a permit to work on the islands. Funding for this project was provided by the Andrew W. Mellon Foundation. We thank Doug Aubrey and Kate Heckman for help in the field For further information Please contact daw385f@smsu.edu or alexanderwait@missouristate.edu. More information on this (and a link to the poster) can be obtained at: http://courses.smsu.edu/daw385f/homepage.htm#Dr Fig. 5. Mean (±SE) plant water content and mid-day water potential for dominant species growing on unsubsidized (Cr and Pl;n=12) and subsidized (Ch and Am;n=12) areas towards the end of the growing season across six islands in two dry and two wet years. Fig. 8. Mean (±SE) mid-day water potential for Atriplex growing in unsubsidized and subsidized (n=8)areas towards the end of the growing season across six islands in two dry and two wet years. Fig. 2. Mean (±SE) productivity and apparent species richness on unsubsidized (n=6) and subsidized (n=6) islands in two dry and two wet years.