Fernández R J, Nuñez A H, Soriano A
Departmento de Ecología-IFEVA, Facultad de Agronomía, Universidad de Buenos Aires, 1417, Buenos Aires, Argentina.
Oecologia. 1992 Aug;91(1):39-46. doi: 10.1007/BF00317238.
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/m) than from a difference in their niche overlap with palatable species.
阿根廷巴塔哥尼亚西部的两种优势灌木物种，在绵羊放牧和人为控制资源水平的影响下表现出不同的种群动态行为。丝状千里光在放牧压力增加时盖度增加；这种增加是由个体数量增多而非植株变大导致的；当不放牧时，这种趋势则相反。多刺木莲在放牧压力增加时盖度和密度略有下降，植株大小没有变化。在一项田间试验中，添加水分（初夏时添加30毫米）并去除优势禾本科植物，这两个因素都显著增加了千里光一年生幼苗的数量；木莲唯一明显的反应是由于禾本科植物的去除，幼苗出土量增加。在最后一次处理六年后，千里光的密度增加了十倍，其种群结构在较年轻的等级中出现峰值；木莲则未受影响。在围栏内及其周边进行的幼苗调查中，在研究的两年里，千里光在这些区域的出土情况相似，而木莲在其中一年围栏内的出土量更高。在较湿润年份，千里光的出土量是较干旱年份的3倍。放牧对任何物种的一年生存活率均无影响；在这种情况下以及在操纵实验中，幼苗死亡的主要原因是夏季干旱期间的干燥。我们得出结论，这两种物种对放牧的不同反应主要是由于千里光从资源增加中获益的能力更强，包括那些未被放牧植物利用的资源。这种差异也可能是由于绵羊对木莲果实的偏好导致其种子可获得性降低。这两种灌木利用放牧释放资源的不同能力更多地源于它们不同的种群动态潜力（即快速增加生物量/米的能力），而非它们与适口物种的生态位重叠差异。
