Progress 09/01/00 to 02/28/05
Outputs
Grasslands throughout the southwestern United States have experienced a substantial increase in the abundance of woody plants within the last century. Invasions by woody plants into southwestern rangelands are likely dependent upon both biological and non-biological factors, including presence and identity of grasses, climatic conditions such as rainfall regimes, and soil characteristics. We hypothesized that establishment of seedlings of woody plants was essentially dependent on the availability of water in the soil, which in turn was controlled by the identity of neighboring grasses, soil texture (or particle size) properties, and amount of rainfall during the summer growing season. We conducted a large-scale experiment under field conditions at a representative site (on the Santa Rita Experimental Range) in southeastern Arizona. Our experimental infrastructure centered on six 167 m2 permanent precipitation shelters, constructed on clay-rich and sandy soils. The
experimental plots were planted with Eragrostis lehmanniana (a non-native grass) or Heteropogon contortus (a native grass), or were left bare. Experimental plots were irrigated by hand to receive 50% more or 50% less than average amounts of summer rainfall. We studied the dynamics of mesquite (Prosopis) seedlings (that emerged from planted seed) over a 2-year period. We monitored availability of soil moisture (soil water potential, SWP) at several soil depths during the course of the project; SWP through time clearly reflected differences between soil type, irrigation treatment, and grass presence and identity. For example, SWP of shallow soil was lower on sandy than clayey soil, particularly in dry plots. Plots with non-native grass had lower SWP than plots with native grass, and both grasses reduced soil moisture relative to bare plots. Emergence of seedlings averaged ~40%, and did not differ between sandy and clayey soils; seedling emergence was at least two times greater in wet
than dry plots. In wet plots, emergence was ~50% greater in grass than bare plots, which suggested that the grass canopy might facilitate emergence of seedlings through modifications of the seedlings environment. However, survival of mesquite seedlings was inversely correlated with the density of grass, irregardless of grass identity: two years after seedlings emerged, seedling survival averaged <4% except on bare plots on clay soils (where it averaged ~25%). Ironically, clay soils exhibit relatively low densities of mature mesquite; this suggests that bare soil may be an uncommon feature of clay soils, although this is not borne out anecdotally. Investigations of water use by seedling and mature mesquite suggest that mesquite are more responsive (in terms of water uptake and physiological response) to pulsed summer precipitation events on the sandy soils than on the clayey soils. Thus, the distribution of mesquite on the landscape may depend on a series of wet summers to facilitate
establishment of seedlings on sandy and clayey soils, coupled with relatively rapid and deep infiltration of rainfall into sandy soils where it can be readily accessed by older, more deeply rooted individuals.
Impacts
Results from this project will facilitate predictions about the consequences of climate change - particularly changes in the amount and temporal distribution of summer rainfall - and grass invasions on proliferation of woody plants on strongly contrasting soil types characteristic of the southwestern United States. In sum, native- and non-native grasses differ in their impacts on soil moisture, although grass identity becomes less important to mesquite establishment as mesquite seedlings mature. In particular, although both native and non-native grasses may facilitate early emergence of mesquite, and intermediate-term (e.g., six months to one year) patterns of survivorship suggest that non-native grasses may actually facilitate seedling recruitment, survival of seedlings after two years is low and uniform in plots where cover of grasses is high, regardless of grass identity. Where grasses are absent, seedling survivorship depends on the underlying soil characteristics
and the amount and timing of seasonal precipitation. Thus, management activities directed at control of woody plants in this region should consider both the geomorphic surface under consideration (i.e., as it relates to soil particle size distribution) as well as contemporary or projected patterns of seasonal precipitation. Four graduate and eight undergraduate students have participated in the project, which has generated a number of publications (as well as several manuscripts in preparation).
Publications
- Williams, D.G. and Z. Baruch. 2000. African grass invasion in the Americas: ecosystem consequences and the role of ecophysiology. Biological Invasions. 2:123-140.
- Fravolini, A., D.G. Williams, and T.L. Thompson. 2002. Carbon isotope discrimination and bundle sheath leakiness in three C4 subtypes grown under variable nitrogen, water and atmospheric CO2 supply. Journal of Experimental Botany 53:2261-2269.
- Weltzin, J.F., M.E. Loik, S. Schwinning, D.G. Williams, P. Fay, B. Haddad, J. Harte, T.E. Huxman, A.K. Knapp, G. Lin, W.T. Pockman, M.R. Shaw, E. Small, M.D. Smith, S.D. Smith, D.T. Tissue, and J.C. Zak. 2003. Assessing the response of terrestrial ecosystems to potential changes in precipitation. BioScience 53:941-952.
- Fravolini, A., K.R. Hultine, D.F. Koepke, and D.G. Williams. 2003. The role of soil texture on mesquite water relations and response to summer precipitation. In: Santa Rita Experimental Range: One Hundred Years (1903 to 2003) of accomplishments and contributions; conference proceedings; 2003 October 30 - November 1, Tucson, AZ. Proc. RMRS-P-00. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.
- English, N.B., D.G. Williams, and J.F. Weltzin. 2003. Dynamics of soil temperature and moisture following experimental irrigation on the Santa Rita Experimental Range. In: Santa Rita Experimental Range: One Hundred Years (1903 to 2003) of accomplishments and contributions; conference proceedings; 2003 October 30 - November 1, Tucson, AZ. Proc. RMRS-P-00. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.
- Huxman, T.E., J.M. Cable, D.D. Ignace, J.A. Eilts, N.B. English, J. Weltzin, and D.G. Williams. 2004. Response of net ecosystem gas exchange to a simulated precipitation pulse in a semi-arid grassland: the role of native and non-native grasses and soil texture. Oecologia 141:295-305.
- Fravolini, A., K. Hultine, E. Brugnoli, R. Gazal, N. English and D.G. Williams. 2005. Precipitation pulse use by an invasive woody legume: the role of soil texture and pulse size. Oecologia (accepted with revision).
- Huxman1, T.E., M.D. Smith1, P.A. Fay, A.K. Knapp, M.R. Shaw, M.E. Loik, S.D. Smith, D.T. Tissue, J.C. Zak, J.F. Weltzin, W.T. Pockman, O.E. Sala, B. Haddad, J. Harte, G.W. Koch, S. Schwinning, E.E. Small, and D.G. Williams. 1Authors contributed equally. 2004. Convergence across biomes to a common rain-use efficiency. Nature 429:651-654.
- Chesson, P., R.L.E. Gebauer, S. Schwinning, N. Huntly, K. Wiegand, M.S.K. Ernest, A. Sher, A. Novoplansky, and J.F. Weltzin. 2004. Resource pulses, species interactions, and diversity maintenance in arid and semi-arid environments. Oecologia 141:236-253.
- English, N.B, J.F. Weltzin, A. Fravolini, L. Thomas, D.G. Williams. 2005. Design and performance of large-scale precipitation shelters in semi-desert grassland, Santa Rita Experimental Range, Arizona. Journal of Arid Environments (accepted with revision).
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Progress 10/01/02 to 09/30/03
Outputs
Our experimental infrastructure includes six 167 m2 permanent precipitation shelters, constructed on a late-Pleistocene (clay-rich) and a mid-Holocene (sandy loam) surface, as well as 72 2.7 m2 plots planted with monospecific stands of Eragrostis lehmanniana (a non-native grass) or Heteropogon contortus (a native grass), or left bare. Experimental plots receive 50% more or 50% less than average summer precipitation. We initiated the precipitation treatments later than expected, in June 2002, because of unavoidable delays in construction of the plots. We delayed an explicit investigation of winter precipitation because of the delays in treatment initiation and the relatively low number of replicate plots vis-a-vis interplot variability. We use time domain reflectometry to measure soil water potential (SWP) at 15 cm, 35 cm, and 55 cm; SWP through time clearly reflect differences between depth, soil type, summer watering treatment, grass presence and identity, and
irrigation periodicity. For example, SWP at 15 cm is generally lower on Holocene than Pleistocene soils, particularly in summer dry plots. SWP on Pleistocene soils are consistently lower on plots with Eragrostis than Heteropogon, and both are lower than bare plots. In contrast, on Holocene soils, Heteropogon reduces SWP more than Eragrostis. In August 2002 and 2003, 36 Prosopis seeds were planted into plots and thereafter monitored for patterns of seedling emergence and survivorship. Results to date indicate strong interactive effects of grass species and soil properties on Prosopis emergence and survivorship. For example, emergence rates for the 2002 cohort averaged 40% and did not differ between Holocene and Pleistocene soils, but were 2x - 5x greater in wet than dry plots. In wet plots, emergence was 50% greater in grass than bare plots, which suggests that the grass canopy may facilitate emergence through amelioration of high surface temperatures. On the Holocene site in the dry
plots, survivorship of seedlings was <1% in the presence of grass. In June 2002, we applied a 39-mm pulse of irrigation to each plot, and followed patterns of carbon dioxide and water vapor exchange for various components of the ecosystem for 16 days. Prior to the pulse, both Eragrostis and Heteropogon had less negative PWP, greater leaf photosynthetic rate and stomatal conductance, and greater values for net ecosystem exchange of carbon on the Pleistocene surface than the Holocene surface. After irrigation, stands of Eragrostis had greater initial rates of evapotranspiration (ET) than Heteropogon, whereas maximum instantaneous net ecosystem exchange increased for both surfaces and species at about the same rate. However, differential patterns of ET through time resulted in earlier declines in net ecosystem exchange for Eragrostis than Heteropogon.
Impacts
Results to date indicate that native- and non-native grasses differ in their impacts on soil moisture and subsequent recruitment of Prosopis seedlings. In particular, although both native and non-native grasses may facilitate early emergence of Prosopis, subsequent patterns of survivorship suggest that native grasses may actually suppress seedling recruitment through limitations of light or intensive use of moisture from relatively shallow soil layers. However, this relationship depends on the underlying soil texture and the amount and timing of seasonal precipitation. Thus, management activities directed at control of woody plants or invasive, non-native grasses should take into consideration not only the identity of the grass neighborhood, but should also consider the geomorphic surface under consideration as well as contemporary or projected patterns of seasonal precipitation. To date, four graduate and six undergraduate students have participated in the project,
which has generated four publications (as well as several manuscripts in preparation).
Publications
- English, N.B. et al. 2004. Dynamics of soil temperature and moisture following experimental irrigation on the Santa Rita Experimental Range, Arizona: implications for the establishment of mesquite. Proceedings, Santa Rita Experimental Range One-Hundred Years (1903-2003) of Accomplishments and Contributions. USDA Forest Service Rocky Mountain Research Station. In press.
- Huxman, T.E. et al. 2004. Response of net ecosystem gas exchange to a simulated precipitation pulse in a semi-arid grassland: the role of native versus non-native grasses and soil texture. Oecologia. In press.
- Weltzin, J.F. et al. 2003. Assessing the response of terrestrial ecosystems to potential changes in precipitation. BioScience 53:941-952.
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Progress 10/01/00 to 09/30/01
Outputs
In the first year of this project, we built our pool of personnel for the project and initiated our field research plan. The following timeline provides more information on our progress. Fall 2000 - Site reconnaissance, collected seed from field sites and other sources, arranged for greenhouse space and started seed germination trials. Initiated search for and obtained quotes for precipitation shelters, project truck and water tanks. Designed precipitation shelters and plot design and layout. Constructed project website (http://www.srnr.arizona.edu/uappel/srer). Winter 2000-2001 - Coordinated with Santa Rita Experimental Range managers on potential sites, and initiated request for site use through SRER site administrators. Established accounts with local and regional vendors, and started acquiring project supplies, including water tanks. Salvaged water tanks from previous project. Coordinated with consultant regarding soil water monitoring, and investigated techniques
for soil water monitoring, including Trime probe and TDR probes. Started design and construction of TDR probes. Continued project design and site selection process, including investigation of options for greenhouse supplies for field site. Initiated funds reallocation request for purchase of truck from travel funds. Initial soil sampling and site assessment. Purchased TDR100 and materials for plot boundaries; delivered materials to field site. Spring 2001 - Purchased truck and cell phones for business and emergencies. Ordered materials for TDR probes, began TDR production. Performed Endangered Species/Pima Pineapple Cactus survey on site. Archeological/Historical survey performed by Art McWilliams. Finalized Range Services Agreement with Santa Rita Experimental Range administration. Started plot boundary construction and plot construction, built livestock exclosures, and initiated trenching on 1st site. Started grass seedlings in greenhouse, and hired work study students. Continued
construction of TDR probes, and calibration of soil water monitoring system. Precipitation shelters ordered by U. Tennessee. Summer 2001 (through September) - Completed trenching all plots on both sites, installed TDR probes in all plots, installed plot boundaries, and established experimental plant communities in all plots. Precipitation shelters arrive on-site; construct 6 large precipitation shelters in field, including framework, tie-downs, and covers. Installed rain gauges and rodent exclosures at each precipitation shelter. Completed experimental watering protocols, and started watering experimental plots (using protocols designed for plant community establishment). Established, planted, and began ancillary experiment to investigate role of grass density on seedling establishment. Project truck totaled, began search for new truck. Collected data on plant community, light availability and interception by plant canopy, and initial monitoring and analysis of soil water content.
Minor modifications to plant communities made to homogenize plot characteristics. Initiate experimental water protocol.
Impacts
Encroachment of woody plants into grasslands is a world-wide problem with significant social, cultural, and economic implications. This research will help us understand why invasions by woody plants are patchy on the landscape scale, and how these invasions may be attenuated or exacerbated under future climatic regimes, particularly shifts in the amount or seasonality of precipitation. Proactive management may be required to minimize major increases in woody plant establishment under some, but not all, future precipitation regimes. This research will help elucidate the bounds of potential response in this particular (but widespread) grassland system.
Publications
- No publications reported this period
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