Pierson Elizabeth A, Mack Richard N
Department of Botany, Washington State University, 99164, Pullman, WA, USA.
Oecologia. 1990 Oct;84(4):526-533. doi: 10.1007/BF00328170.
The effect of tree canopy, understory, herbivores, and litter depth on seedling establishment, survival, and reproduction of the alien grass, Bromus tectorum (cheatgrass), was examined in a series of experiments in four forest habitat types in western North America. Higher recruitment, survival, and reproduction on clearcuts, which would be expected if the overstory alone is limiting the distribution of cheatgrass in forests, were not observed. Removing the understory in an otherwise undisturbed Pinus ponderosa forest did, however, increase the emergence of B. tectorum, but plants in these experimentally-created openings were more vulnerable to grazing by small mammals. In contrast, removing the sparse understory in an Abies forest neither enhanced recruitment nor increased the incidence of grazing of B. tectorum seedlings. Regardless of the forest habitat, most grazed plants died before maturity; even fewer grazed plants produced seeds. Litter depth influenced both recruitment and biomass production: both the rate of germination and the size of resultant seedlings were lower on thick litter (6 cm) compared to results on thin litter (1.5 cm). In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.
在北美西部的四种森林栖息地类型中，通过一系列实验研究了树冠层、林下植被、食草动物和凋落物深度对外来草种——野燕麦（Bromus tectorum，又称 cheatgrass）幼苗建立、存活和繁殖的影响。在皆伐地上并未观察到更高的幼苗补充、存活和繁殖情况，而如果仅上层林冠限制了 cheatgrass 在森林中的分布，这种情况是可以预期的。然而，在原本未受干扰的黄松林中去除林下植被，确实增加了野燕麦的出苗率，但在这些人为创造的空地上的植株更容易受到小型哺乳动物的啃食。相比之下，在冷杉林中去除稀疏的林下植被，既没有提高幼苗补充率，也没有增加野燕麦幼苗被啃食的发生率。无论森林栖息地如何，大多数被啃食的植株在成熟前就死亡了；能产生种子的被啃食植株更少。凋落物深度影响了幼苗补充和生物量生产：与薄凋落物（1.5 厘米）相比，厚凋落物（6 厘米）上的发芽率和由此产生的幼苗大小都较低。在更为开阔的黄松林和花旗松林中，cheatgrass 可能经常仅在林下的空地上定殖。然而，在更为阴暗的巨冷杉林和西部铁杉林中，除非开口延伸穿过林下植被和上层林冠，否则不太可能定殖。因此，除非干扰的规模和发生率大幅增加，否则 cheatgrass 在这些森林中的任何一种中都不太可能增加。
