中文摘要：

采用控制环境生长室，研究了CO2浓度升高对2个种植密度下红桦幼苗生长和氮（N）、磷（P）含量的影响。试验设置CO2浓度为350和700umol·mol^-1 2个水平，每个CO2浓度水平下又设密度28和84株·m^-2 2个水平。结果表明：CO2浓度升高，红桦株高和叶面积指数（LAI）均增加，净同化率（NAR）值增加，叶质比（LMR）和比叶面积（SLA）均下降，但相对生长率（RGR）提高。CO2浓度增加，红桦幼苗茎枝、叶、根和总生物量提高，氮（N）、磷（P）含量降低，但单株N、P总吸收量均增加。CO2浓度升高，氮磷利用效率（NUE和PUE）提高，氮磷累积速率（NACR和PACR）显著增加。CO2浓度升高，红桦幼苗体内N、P浓度下降是由于生物量迅速增加引起的稀释效应造成的，而NUE和PUE的提高可以有效缓解CO2浓度升高后，亚高山和高山地区森林土壤中养分元素不足对森林生产力的限制。CO2浓度升高导致的植物生长的增加量会随植株密度的增加而降低，不同器官养分吸收量的增加量在低密度条件下比高密度条件下大得多，主要是因为高种植密度显著降低了植株各部位的干质量。

英文摘要：

By using growth chamber, Betula albosineasis seedlings were planted with two densities （28 and 84 plants · m^-2） and treated with 350 and 700 umol CO2 · mol^-1 to study the effects of elevated CO2 on the growth and nutrient contents of the seedlings. The results showed that under elevated CO2 , the plant height, leaf area index, net assimilation rate and relative growth rate of B. albosineasis seedlings with both of the two planting densities increased, while the leaf-mass ratio and specific leaf area decreased. Elevated CO2 increased the dry mass of stem, branch, leaf and root and the uptake of N and P per plant, but decreased the N and P contents of each organ. It also benefited the use efficiency and accumulation rate of N and P. The decrease of N and P contents in each organ was mainly resulted from the dilution effect of the rapid accumulation of biomass, while the increase of N and P use efficiency might be helpful to the alleviation of the impact of soil nutrients deficiency, a limiting factor on the productivity of subalpine and alpine mountain forests owing to the elevated CO2 environment in the future. The increment of plant growth under elevated CO2 decreased with increasing planting density, and the more enhancement of nutrient uptake per plant was observed under low than under high planting density, mainly because the increase of planting density significantly decreased the dry mass of B. albosineasis seedlings.