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  • 1
    Electronic Resource
    Electronic Resource
    Potential growth and drought tolerance of eight desert grasses: lack of a trade-off? (2000)
    Springer
    Oecologia 123 (2000), S. 90-98 
    Details
    ISSN: 1432-1939
    Keywords: Key words Chihuahuan Desert ; C-4 photosynthesis ; Water stress ; Relative growth rate ; Xerophytism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract  Eight perennial C-4 grasses from the Jornada del Muerto Basin in southern New Mexico show five-fold differences in relative growth rates under well- watered conditions (RGRmax). In a controlled environment, we tested the hypothesis that there is an inverse relationship (trade-off) between RGRmax and the capacity of these species to tolerate drought. We examined both physiological (gas exchange) and morphological (biomass allocation, leaf properties) determinants of growth for these eight species under three steady-state drought treatments (none=control, moderate, and severe). When well watered, the grasses exhibited a large interspecific variation in growth, which was reflected in order-of-magnitude biomass differences after 5 weeks. The species had similar gas-exchange characteristics, but differed in all the measured allocation and morphological characteristics, namely tiller mass and number, root:shoot ratio, dry-matter content, and specific leaf area (SLA). Drought affected tillering, morphology, and allocation, and reduced growth by 50 and 68% (moderate and severe drought, respectively) compared to the well-watered controls. With the exception of SLA, none of these variables showed a significant species-by-treatment interaction. We calculated three indices of drought tolerance, defined as the ratio in final biomass between all the possible ”dry”/”wet” treatment pairs: severe/moderate, moderate/control, and severe/control. We found no significant correlation between these drought tolerance indices, on the one hand, and three indices of growth potential (greenhouse RGRmax, final biomass in the control treatment, and final:initial biomass ratio in controls), on the other. Based on these controlled-environment results, we hypothesize that the commonly reported correlation between plant growth potential and drought tolerance in the field may in some cases be explained by differential effects of plants on soil-water content rather than by differences in species responses to drought.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004420050993
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  • 2
    Electronic Resource
    Electronic Resource
    Contrasting demography of two Patagonian shrubs under different conditions of sheep grazing and resource supply (1992)
    Springer
    Oecologia 91 (1992), S. 39-46 
    Details
    ISSN: 1432-1939
    Keywords: Grazing ; Life histories ; Patagonia ; Resource levels ; Shrubs
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Two shrub species, dominant in western Patagonia, Argentina, exhibited different demographic behaviors under the effect of sheep grazing and of manipulated resource levels. Senecio filaginoides showed an increase in cover under increasing grazing pressures; the rise was explained by an increasing number of individuals and not by larger plants; this trend was reversed when no grazing took place. Mulinum spinosum showed a slight decrease in cover and density at increasing grazing pressures, and no change in plant size. In a field experiment in which water was added (30 mm in early summer) and the dominant grasses removed, both factors substantially augmented the number of 1-year-old seedlings of Senecio recruited; the only noticeable response of Mulinum was an increase in seedling emergence because of grass removal. Six years after this last treatment, Senecio exhibited a ten fold increase in density, and its population structure showed a peak in younger classes; Mulinum remained unaffected. In seedling surveys conducted within an exclsure and in its surroundings, the emergence of Senecio was similar in these areas in the two years of the study, whereas the emergence of Mulinum was higher inside the exclosure for one of the years. The emergence of Senecio in the wetter year was 3 times larger than in the drier one. Grazing did not affect first-year survival of any species; the major source of seedlign death in this case, as well as in the manipulative experiment, was desiccation during summer drought. We conclude that the contrasting responses of the two species to grazing are mostly due to the greater ability of Senecio to benefit from any increase in resources, including those not used by grazed plants. The difference might also be due to a reduction in the seed availability of Mulinum caused by sheep preference for its fruits. The dissimilar abilities of the two shrubs to employ the resources freed by grazing would derive more from their contrasting demographic potential (i.e. the ability to rapidly increase biomass/m2) than from a difference in their niche overlap with palatable species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00317238
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    Paper (German National Licenses)
    Fulltext
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