The regeneration niche of trees at the alpine treeline: competition and facilitation by alpine vegetation during germination and seedling establishment

树木生长的潜在海拔极限可能是由于生长季节缺乏温暖所致。然而，建树的海拔极限可能是由截然不同的气候因素设定的，例如冰冻温度和相互作用的压力因素，如过量的太阳辐射，这可能解释了为什么目前很少有高山树木达到潜在的树木生长极限。建立的限制也与树线对气候变化的反应有重要关系。虽然目前大多数生态生理树线研究集中在成年树的生长限制上，但我们的项目集中在最早的生命阶段、萌发和幼苗表现，这些都是决定树线动态和位置的强大人口瓶颈。我们的目的是阐明生长季温暖、生长季长度、霜冻和辐射胁迫对高山生命带树木形成树种的初步建立的相对重要性和相互作用。我们的初步结果表明，对强烈太阳辐射的保护可以防止光抑制，提高存活率，但会减少生长和碳水化合物的储存。这些影响显示了已建立的植被在更新中的模糊作用：一方面它提供了对严酷高山气候的保护，另一方面它竞争资源。在这个正在进行的项目的一年延期中，我们将解决这个模糊的问题，并完成我们对幼苗建立的小气候限制的实验。这些实验自2011年秋季/2012年春季开始实施(2010年秋季/2011年春季的实验因博士生调换而停止)。在2014年夏季继续监测，将控制天气影响，如2012年夏季的干旱条件，并允许监测苗木的第二年和第三年，包括使用我们新的叶绿素荧光监测设备(自2013年年中开始使用)在所有季节进行详细的业绩监测。两个新的关于植被相互作用的实验，其中一个被整合到今年(2013年)开始的全球树线协作实验中，第三个实验计划在明年开始。通过关注苗木和明确整合和排序多种原因，该项目将是解决当前许多关于树线动态的争议的重要一步。

The potential altitudinal limit for tree growth is probably set by a lack of warmth during the growing season. However, the altitudinal limit for tree establishment is probably set by quite different climatic factors, such as freezing temperatures and interacting stressors such as excess solar radiation, which may explain why few alpine treelines currently reach the potential tree-growth limit. Establishment limitations also have important bearings for treeline responses to climatic changes. While most current ecophysiological treeline research focuses on growth limitations for adult trees, our project focuses on the earliest life stages, germination and seedling performance, which represent strong demographic bottlenecks determining treeline dynamics and positions. Our aim is to clarify the relative importance and the interactions of growing-season warmth, growing-season length, and frost and radiation stress for the initial establishment of treeline-forming tree species in the alpine life zone. Our preliminary results suggest that protection from strong solar radiation prevents photoinhibition and increases survival but decreases growth and carbohydrate storage. These effects show the ambiguous role of established vegetation for regeneration: on the one hand it provides protection from the harsh alpine climate and on the other hand it competes for resources.In this one-year extension of the on-going project we will address this ambiguity and finish our experiments on microclimatic restrictions for seedling establishment. These experiments have been in place since autumn 2011/spring 2012 (the experiments installed autumn 2010/spring 2011 were discontinued due to a change of PhD student). Continuing them to throughout the 2014 summer will provide a control for weather effects, such as the dry conditions in the 2012 summer, and allow a monitoring of seedlings into their second and third year, including a detailed performance monitoring throughout all seasons with our new chlorophyll fluorescence monitoring equipment (in use since middle 2013). Two new experiments on vegetation interactions, one of which integrated in a collaborative global treeline experiment started this year (2013), while a third experiment is planned for next year.By focusing on seedlings and by explicitly integrating and ranking multiple causes, this project will be an important step to resolve many of the current controversies concerning treeline dynamics.