Chair of Silviculture, Faculty of Environment and Natural Resources, Freiburg University, D-79085 Freiburg i. Br., Germany.
Tree Physiol. 2014 Feb;34(2):184-93. doi: 10.1093/treephys/tpt124. Epub 2014 Feb 14.
Northern red oak (Quercus rubra L.), a moderately shade-tolerant tree species, is failing to regenerate throughout its native North American range, while successful recruitment in Central Europe has been observed since its introduction. To examine whether comparative photosynthetic performance could explain the regeneration success of this non-native species in Central Europe, we compared the physiological and morphological seedling traits of red oak with three co-occurring tree species under three canopy types in southwestern Germany. Native species included a moderately shade-tolerant native oak (Quercus robur L.) and two shade-tolerant species (Acer pseudoplatanus L. and Carpinus betulus L.). The photosynthetic traits of non-native red oak seedlings were similar to those reported for this species in the native range, where shade-tolerant competitors readily outperform red oak under low light conditions. However, compared with native shade-tolerant species in Europe, red oak seedlings photosynthesized efficiently, especially under closed canopies and in small canopy gaps, exhibiting high photosynthetic capacity, low leaf dark respiration and leaf-level light compensation points that were similar to the more shade-tolerant species. The superior net carbon gain of red oak seedlings at low and moderate light levels was likely facilitated by high leaf areas and reflected by seedling dry masses that were greater than the observed native European species. A competitive advantage for red oak was not evident because relative height growth was inferior to seedlings of co-occurring species. In North America, the inability of seedlings to compete with shade-tolerant tree species in deeply shaded understories is central to the problem of poor oak recruitment. Our study suggests that the ability of non-native red oak to perform equally well to native shade-tolerant species under a variety of light conditions could contribute to the consistent success of red oak regeneration in Europe.
北方红栎（Quercus rubra L.）是一种中等耐荫树种，在其北美原生范围内无法再生，而在引入欧洲中部后，已观察到其成功繁殖。为了研究比较光合作用性能是否可以解释该非本地物种在欧洲中部再生成功的原因，我们比较了德国西南部三种树冠类型下，红栎与三种伴生树种的幼苗生理和形态特征。本地树种包括一种中等耐荫的本地栎（Quercus robur L.）和两种耐荫树种（Acer pseudoplatanus L.和 Carpinus betulus L.）。非本地红栎幼苗的光合作用特征与该物种在原生范围内的报道相似，在低光照条件下，耐荫竞争者很容易胜过红栎。然而，与欧洲本地耐荫物种相比，红栎幼苗光合作用效率高，尤其是在封闭树冠和小树冠空隙下，表现出高光合能力、低叶片暗呼吸和与更耐荫物种相似的叶片水平光补偿点。红栎幼苗在低光和中光水平下的净碳增益优势可能是由高叶片面积促成的，反映在大于观察到的本地欧洲物种的幼苗干质量上。由于相对高度生长劣于伴生物种的幼苗，因此红栎幼苗没有明显的竞争优势。在北美，由于幼苗无法与深阴影下耐荫树种竞争，这是橡树繁殖不良的核心问题。我们的研究表明，非本地红栎在各种光照条件下与本地耐荫物种表现相当的能力可能是其在欧洲持续成功繁殖的原因之一。
