Thirteen-year growth response of ponderosa pine to various manipulations of understory vegetation was studied to determine if a threshold of understory cover can be established for plantation productivity and whether nitrogen-fixing Ceanothus species benefit plantation growth compared to non N-fixing Arctostaphylos species, given their ability to improve site fertility. Results showed that completely controlling competing vegetation increased plantation growth, as was universally expected, but the effects of partial shrub control on plantation performance was inconsistent. We did not find a benefit of nitrogen-fixing shrubs on tree growth, as young plantation growth was slightly better in the non N-fixing plots compared to the N-fixing plots due to differences in shrub cover. An understory cover threshold for tree growth was not observed. Due to the overriding effect of shrub competition, controlling herbaceous species had little effect on plantation growth even though it benefited tree seedling survival. A long-term value of N-fixing shrubs at these sites, if there is one, must be balanced by successful plantation establishment and rapid early growth.

研究了黄松对不同林下植被的十三年生长响应，以确定是否可以为林下生产力建立一个林下覆盖的阈值，以及固氮西奥努斯物种与非固氮弧菌相比是否有利于人工林生长物种，因为它们具有提高站点繁殖力的能力。结果表明，完全控制竞争性植被可以提高人工林的生长，这是普遍预期的，但是部分灌木控制对人工林性能的影响不一致。我们没有发现固氮灌木对树木生长的好处，因为由于灌木覆盖率的差异，非固氮地块的幼林生长比固氮地块稍好。未观察到树木生长的林下覆盖阈值。由于灌木竞争的压倒性作用，即使控制草种有利于树木幼苗的存活，但对人工林的生长几乎没有影响。这些地点的固氮灌木的长期价值（如果有的话）必须通过成功的人工林建设和快速的早期生长来平衡。