Robson T Matthew, Rodríguez-Calcerrada Jesús, Sánchez-Gómez David, Aranda Ismael
Instituto Nacional de Investigaciones Agrarias y Tecnologías Agroalimentarias, Centro de Investigación Forestal, Carretera a Coruña Km 7.5, E-28040 Madrid, Spain.
Tree Physiol. 2009 Feb;29(2):249-59. doi: 10.1093/treephys/tpn023. Epub 2008 Dec 5.
Refugia of mixed beech forest persist in the central mountains of the Iberian Peninsula at the south-western limit of European beech (Fagus sylvatica L.) distribution. The lack of beech regeneration is a concern in this region that has experienced reduced rainfall and higher temperatures over the past 30 years. Beech is considered especially susceptible to climate change because of its conservative shade-tolerant growth strategy; hence seedling responses to drought stress in gaps and in the understory are of particular interest. During the summer of 2007, a watering treatment raised the soil water content by up to 5% in gap and understory plots of beech seedlings in a mixed beech forest. Root-collar diameter was increased by our watering treatment in understory seedlings. Neither drought-avoidance through stomatal closure nor physiological drought-tolerance mechanisms were able to mitigate the effects of water stress in the understory seedlings, whereas osmotic adjustment enhanced the ability of the gap seedlings to tolerate water stress. Overall, high photosynthetic rates in the gaps, despite the photoinhibitory effects of high radiation, allowed gap seedlings to survive and grow better than the understory seedlings irrespective of water availability. Our results indicate that further intensification of summer drought, predicted for the Iberian Peninsula, will hinder the establishment of a beech seedling bank in the understory because of the conflicting seedling trait responses to simultaneously withstand water stress and to tolerate shade.
在欧洲山毛榉（Fagus sylvatica L.）分布的西南边界，伊比利亚半岛中部山区留存着混交山毛榉林的避难所。在过去30年里，该地区降雨减少、气温升高，山毛榉缺乏更新是一个令人担忧的问题。由于山毛榉具有保守的耐荫生长策略，它被认为对气候变化特别敏感；因此，研究幼苗在林窗和林下对干旱胁迫的反应尤为重要。2007年夏季，在一片混交山毛榉林的山毛榉幼苗林窗和林下样地进行浇水处理，使土壤含水量提高了5%。我们的浇水处理增加了林下幼苗的根茎直径。林下幼苗既不能通过气孔关闭来避免干旱，也没有生理耐旱机制来减轻水分胁迫的影响，而渗透调节增强了林窗幼苗耐受水分胁迫的能力。总体而言，尽管高辐射有光抑制作用，但林窗内较高的光合速率使林窗幼苗无论水分供应情况如何，都能比林下幼苗更好地存活和生长。我们的研究结果表明，预计伊比利亚半岛夏季干旱将进一步加剧，这将阻碍林下山毛榉幼苗库的建立，因为幼苗在同时抵御水分胁迫和耐荫方面的性状反应相互矛盾。
