连作杨树人工林细根寿命的代际差异及其影响因素

细根寿命是调控森林生产力形成的关键。通过在连作Ⅰ、Ⅱ代杨树人工林固定样地内埋设微根管,对杨树不同根序细根年度生长动态开展连续观测并进行生存分析。结果表明,杨树不同根序细根累积生存率存在显著差异,高级根(3—5级)寿命较长,其累积生存率显著高于1级和2级细根。杨树细根寿命存在显著的代际差异,连作Ⅱ代人工林活根量、死根量和细根总量均高于Ⅰ代林。连作Ⅱ代人工林细根中位值寿命为(90±16)d,显著低于Ⅰ代人工林((102±22)d)。连作Ⅱ代林各根序细根数量、分布比例均高于Ⅰ代林,低级细根累积生存率低于Ⅰ代林而高级细根累积生存率显著高于Ⅰ代林。连作杨树人工林细根寿命显著受制于土壤环境,1级细根寿命与土壤速效氮相关性极显著(r=-0.861),2级细根寿命与土壤物理性状相关性较强且与土壤酚酸含量呈现极显著相关(r=0.870),高级根序细根寿命与土壤物理性质和养分状况等也具有一定相关性。连作杨树人工林土壤酚酸累积和养分有效性下降影响了细根寿命和周转,并进而造成净初级生产力损耗,相关结论为连作杨树人工林生产力衰退机理模型的建立提供了科学依据。

The difference in fine root lifespan between successive rotations of poplar plantation and the dominant causal factors

The lifespan of fine roots plays an important role in forest productivity. On the basis of minirhizotron technology, this study focused on the annual dynamics of different order fine roots in successive rotation poplar plantations and analyzed their survival. The results showed that the cumulative survival rates of poplar fine roots were significantly different among root orders. The lifespan of higher order roots (3rd-5th order) was longer than that of lower order roots (1st-2nd order), and their cumulative survival rate was also higher. Furthermore, there was a significant difference in the lifespan of poplar fine roots between two plantations (the first and second generations). There were more live roots, dead roots, and total roots in the second rotation forest than in the first. The median lifespan of fine roots was 90±16 d in the second rotation poplar plantation, whereas it was 102±22 d in the first rotation plantation. The root number and the proportion of roots of each order in the second rotation poplar plantation were significantly higher than those in the first rotation plantation. However, the cumulative survival rate of lower order roots was significantly higher, whereas that of higher order roots was significantly lower, in the first rotation poplar plantation. The lifespan of poplar fine roots was responsive to soil environments, with the lifespan of the first order roots being significantly correlated with soil available nitrogen (r=-0.861) and that of the second order roots being significantly correlated with phenolic acid content (r=0.870). The lifespan of the higher order roots also showed some correlation with soil physical properties and nutrient conditions. The findings of this study indicate that the accumulation of phenolic acids and a decreased soil nutrient availability might influence fine root lifespan and turnover of poplar trees and lead to a loss of net primary productivity in poplar plantations, thereby providing an important insight into the mechanism of declining poplar plantation productivity.