Pierson Elizabeth A, Mack Richard N
Department of Botany, Washington State University, 99164, Pullman, WA, USA.
Oecologia. 1990 Oct;84(4):519-525. doi: 10.1007/BF00328169.
With increasing elevation and corresponding changes in the macroclimate, forest zones in the Intermountain Region of western North America are often dominated in turn by Pinus ponderosa, Pseudotsuga menziesii, Abies grandis, an Thuja plicata. Bromus tectorum (cheatgrass), and introduced annual grass now abundant in the Region's steppe, is uncommon in mature stands representative of these forest zones. In order to determine whether B. tectorum is largely excluded from these forests by insufficient seed dispersal or environmental restriction(s), the grass's demography was compared in each of four years among populations experimentally-introduced into mature forests. The number of recruits did not differ among the Pinus, Pseudotsuga, and Abies sites; recruitment was however significantly lower on the coolest site dominated by Thuja. Emergence in both the low elevation Pinus and Pseudotsuga sites was about the same in autumn, winter, and spring. In the cooler, moister Abies and Thuja sites, emergence was limited to autumn and early winter. Survival in these forest sites ranged between 0 and 87%. The percentage of the total population to survive until harvest was highest in the Pseudotsuga site, intermediate in the Pinus and Abies sites, and lowest in the Thuja site. Compared with B. tectorum in the steppe, the surviving plants were small, and few produced seeds. All parents were members of either the autumn or winter cohort, and most parents produced only one seed. No seeds were produced at the Thuja site. Although phenotypic plasticity apparently contributes to the wide ecological amplitude of this grass, its growing season on these sites in most years is too short for it to reproduce. Consequently, these forest zones broadly define the current environmental limits to the distribution of cheatgrass in this portion of its new geographic range.
随着海拔升高以及宏观气候的相应变化，北美西部山间地区的森林带依次常由西黄松、花旗松、巨冷杉和西部铁杉主导。雀麦（ cheatgrass），一种现已在该地区草原大量存在的外来一年生草本植物，在代表这些森林带的成熟林分中并不常见。为了确定雀麦在这些森林中主要是由于种子传播不足还是环境限制而基本被排除在外，在四年中的每一年，对实验性引入成熟森林的种群中的该草本植物的种群统计学进行了比较。在西黄松、花旗松和巨冷杉林地中，新苗数量没有差异；然而，在由西部铁杉主导的最冷林地中，新苗数量显著更低。低海拔的西黄松和花旗松林地在秋季、冬季和春季的出苗情况大致相同。在较凉爽、湿润的巨冷杉和西部铁杉林地，出苗仅限于秋季和初冬。这些林地中的存活率在0%至87%之间。直到收获时存活的总种群百分比在花旗松林地最高，在西黄松和巨冷杉林地居中，在西部铁杉林地最低。与草原上的雀麦相比，存活的植株较小，很少产生种子。所有亲本均属于秋季或冬季种群，且大多数亲本仅产生一粒种子。在西部铁杉林地没有产生种子。尽管表型可塑性显然有助于这种草本植物具有广泛的生态幅度，但在大多数年份，它在这些林地的生长季节太短，无法繁殖。因此，这些森林带大致界定了雀麦在其新地理分布范围内这一部分的当前环境分布界限。
