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Abiotic constraints on the establishment of Quercus seedlings in grassland

机译：非生物性因素限制草地栎类幼苗的建立

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High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first-year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland-forest ecotone (shade). In the 2000 growing season, precipitation was 35% below the long-term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late-season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present-day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue. [References: 50]
机译：相对于美国东部的落叶林，高蒸发量需求和周期性干旱是中西部草原生长季的特征，并且预测的气候变化表明这些极端气候可能会加剧。尽管建立树苗的环境不是最理想的，但落叶树在上个世纪内已扩展到相邻的草丛大草原，导致土地覆盖率发生了戏剧性的变化。为了确定光照和温度对幼苗建立的作用，我们评估了暴露于以下三种条件之一的一年生栎栎幼苗的碳水关系和地上生长：（1）完整的高草草原群落（对照），（2 ）去除了地上的草本生物质（去除了草），以及（3）遮荫加生物质去除，以将光（PFD）减少到草原-森林过渡带的典型水平（阴影）。在2000年的生长季节，降水量比长期平均水平低35％，这对橡树幼苗在中期的碳吸收有明显的负面影响（光合作用下降到最大速率的10％）。然而，在遮荫处理中净光合作用和气孔导度约为。在此干旱期间，分别比对照处理幼苗高出2.5倍和1.5倍。在此期间，对照幼苗的叶片和空气温度相似，而遮阴处理中的叶片温度仍低于空气温度。季末恢复期与气温下降同时发生，导致所有幼苗的净光合作用增加。与全日照相比，遮荫幼苗的光合速率和水分关系增加，这表明，至少在干旱年份，高光照和高温可能会对橡树幼苗的性能产生负面影响。然而，所有幼苗的高成活率表明，大果藜幼苗能够耐受当前和未来的极端气候。除非恢复历史悠久的火灾制度，否则森林扩张和土地覆盖变化可能会继续。 [参考：50]

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