O'Connor T G
Döhne Agricultural Development Institute, Private Bag X15, 4930, Stutterheim, South Africa.
Oecologia. 1995 Aug;103(2):214-223. doi: 10.1007/BF00329083.
Acacia karroo Hayne is the most important woody invader of grassland in South Africa, and can greatly reduce the productivity of grassland. A field experiment was conducted to test the hypotheses that emergence, growth and the 1st year's survival of Acacia karroo would be enhanced by (1) defoliation of the grass sward, (2) increased irradiance, (3) increased moisture availability and (4) its germination within cattle dung pats. The study was conducted on one site above and one below the natural altitudinal treeline of this species in grassland of the eastern Cape, South Africa. Not one seedling emerged from dung pats. Neither location nor the other treatments affected the density of emerging seedlings, although only 40.4 seedlings m emerged of the 200 seeds m planted. Shading dramatically increased the density of surviving seedlings. In the open, only 3 and 1.5 seedlings m remained respectively at the end of the growing season or the beginning of the next, compared to 23.3 and 19.5 seedlings m under shading for these respective times. This was attributed to the effect of shade on moisture availability in a season which received only 54% of average rainfall. Seedling survival until the end of the growing season was enhanced (30%) by shade at both sites, but also by supplemental water (24%) and defoliation of the sward (7%) at the site above the treeline. Across sites and treatments, seedling survival was related to moisture availability, with no or poor survival for < 500 mm rainfall, indicating this species can only establish in certain rainfall years. Seedling survival over winter was not influenced by treatment, but was greater for larger seedlings. Treatments affected seedling size, in particular seedlings growing under shade and within a dense grass canopy were etiolated. A. karroo seedlings are capable of establishing and surviving within a dense grass sward for at least a year, tolerant of low irradiance and of interference, which, because most seeds do not persist for much longer than a year, suggests this species forms predominantly a seedling bank. This has implications for the invasion of grassland by woody species.
卡鲁金合欢是南非草原最重要的木本入侵植物，会大幅降低草原生产力。开展了一项田间试验，以检验以下假说：（1）草皮去叶、（2）光照增加、（3）水分供应增加以及（4）在牛粪堆中发芽会促进卡鲁金合欢的出苗、生长及第一年存活。该研究在南非东开普省草原该物种自然海拔树线之上和之下的一个地点进行。没有一粒种子在牛粪堆中出苗。尽管种植的200粒种子每平方米仅出苗40.4株，但地点和其他处理均未影响出苗幼苗的密度。遮荫显著增加了存活幼苗的密度。在开阔地，生长季节结束时或次年年初每平方米分别仅剩下3株和1.5株幼苗，相比之下，在这些相应时间，遮荫条件下每平方米有23.3株和19.5株幼苗。这归因于在一个仅获得平均降雨量54%的季节里，遮荫对水分供应的影响。两个地点遮荫均使生长季节结束时的幼苗存活率提高（30%），但树线以上地点补充水分（24%）和草皮去叶（7%）也有此效果。 across sites and treatments, seedling survival was related to moisture availability, with no or poor survival for < 500 mm rainfall, indicating this species can only establish in certain rainfall years. 越冬幼苗存活率不受处理影响，但较大幼苗的存活率更高。处理影响幼苗大小，特别是在遮荫条件下和茂密草丛冠层内生长的幼苗黄化。卡鲁金合欢幼苗能够在茂密草皮中扎根并存活至少一年，耐受低光照和干扰，由于大多数种子存活时间不超过一年，这表明该物种主要形成幼苗库。这对木本物种入侵草原具有重要意义。
