The nutrient limitation hypothesis has often been used to explain the late stage decline in productivity of tree plantations. However, few studies have considered the role of fine roots (≤ 2 mm) may play in the process, and even fewer have considered the impact of understory vegetation. Here we assessed how fine root morphological traits, such as specific root length (SRL) and root tissue density (RTD), as well as fine root biomass (FRB) and root length density (RLD), varied in Chinese fir () and associated understory plants with stand development. Results showed that, for Chinese fir fine roots, the SRL significantly decreased, while the RTD significantly increased, with stand development. There was no significant increase in the relative belowground biomass allocation (indicated by the ratio of total FRB or total RLD to stand basal area) of Chinese fir. This may reflect decreased nutrient foraging by Chinese fir fine roots during the later stage of stand development. Interestingly, understory vegetation had both significantly higher SRL and lower RTD than Chinese fir in every plantation. The weighted mean depth of FRB or RLD of understory vegetation was significantly lower than that of Chinese fir. In the later stage of stand development, understory vegetation tended to dominate the total RLD of the stand in the topsoil, reflecting intense belowground competition of understory plants on Chinese fir. Our study emphasizes that, besides the declined nutrient foraging ability of Chinese fir itself, the intense nutrient competition from understory vegetation could exacerbate nutrient limitation of Chinese fir, which may be an overlooked factor contributing to the decline in plantation productivity during the later stage of stand development.

中文翻译：

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杉木人工林乔木及林下植被细根生物量、形态及垂直分布的变化

养分限制假说经常被用来解释人工林生产力的后期下降。然而，很少有研究考虑细根（≤2毫米）在此过程中可能发挥的作用，更很少有研究考虑林下植被的影响。在这里，我们评估了杉木细根形态特征的变化，例如比根长度（SRL）和根组织密度（RTD）以及细根生物量（FRB）和根长密度（RLD）。具有林分发育的林下植物。结果表明，随着林分的发育，杉木细根的SRL显着降低，RTD显着升高。杉木地下相对生物量分配（以总FRB或总RLD与林分断面积之比表示）没有显着增加。这可能反映了林分发育后期杉木细根采食养分的减少。有趣的是，在每个人工林中，林下植被的 SRL 和 RTD 均显着高于杉木。林下植被的FRB或RLD加权平均深度显着低于杉木。在林分发育后期，林下植被往往占林分表土总RLD的主导地位，反映出杉木林下植物的地下竞争激烈。我们的研究强调，除了杉木本身养分觅食能力下降外，林下植被的激烈养分竞争也会加剧杉木的养分限制，这可能是导致林分后期人工林生产力下降的一个被忽视的因素。发展。