Driebe E M, Whitham T G
Department of Biological Sciences, Box 5640, Northern Arizona University, Flagstaff, AZ 86001, USA e-mail:
Oecologia. 2000 Apr;123(1):99-107. doi: 10.1007/s004420050994.
Cottonwoods are dominant riparian trees of the western United States and are known for their propensity to hybridize. We compared the decomposition of leaf litter from two species (Populus angustifolia and P. fremontii) and their hybrids. Three patterns were found. First, in one terrestrial and two aquatic experiments, decomposition varied twofold among tree types. Second, backcross hybrid leaves decomposed more slowly than those of either parent. Third, the variation in decomposition between F and backcross hybrids was as great as the variation between species. These results show significant differences in decomposition in a low-diversity system, where >80% of the leaf litter comes from just two species and their hybrids. Mechanistically, high concentrations of condensed tannins in leaves appear to inhibit decomposition (r =0.63). The initial condensed tannin concentration was high in narrowleaf leaves, low or undetectable in Fremont leaves, and intermediate in F hybrid leaves (additive inheritance). Backcross hybrids were high in condensed tannins and were not different from narrowleaf (dominant inheritance). Neither nitrogen (N) concentration nor the ratio of ash-free dry weight to N (a surrogate for carbon:nitrogen ratio) were significantly correlated with decomposition. The N content of leaf material at the end of each year's experiment was inversely correlated with rates of litter mass loss and varied 1.6- to 2.1-fold among tree classes. This result suggests that hybrids and their parental species are used differently by the microbial community.
三角叶杨是美国西部主要的河岸树木，以其易于杂交而闻名。我们比较了两种三角叶杨（窄叶杨和弗里蒙特杨）及其杂种的落叶分解情况。发现了三种模式。第一，在一项陆地实验和两项水生实验中，不同树木类型的落叶分解速度相差两倍。第二，回交杂种的叶子比任何一个亲本的叶子分解得都慢。第三，F代杂种和回交杂种之间的分解差异与物种之间的差异一样大。这些结果表明，在一个低多样性系统中，落叶分解存在显著差异，其中80%以上的落叶仅来自两个物种及其杂种。从机制上讲，叶片中高浓度的缩合单宁似乎会抑制分解（r = 0.63）。窄叶杨叶片中的初始缩合单宁浓度较高，弗里蒙特杨叶片中的浓度较低或无法检测到，F代杂种叶片中的浓度处于中间水平（加性遗传）。回交杂种的缩合单宁含量较高，与窄叶杨没有差异（显性遗传）。氮浓度以及无灰干重与氮的比率（碳氮比的替代指标）均与分解没有显著相关性。每年实验结束时叶片材料中的氮含量与凋落物质量损失率呈负相关，不同树木类别之间相差1.6至2.1倍。这一结果表明，微生物群落对杂种及其亲本物种的利用方式不同。
