In glacier forelands, seeds readily germinate, however, a high proportion of seedlings die shortly after their appearance. We hypothesized that besides drought, frost and missing safe sites, heat on the ground surface could be one of the major threats for seedlings. The heat strain in different ground strata was assessed from 2007 to 2010. The heat tolerance (LT50) of eleven alpine species from different successional stages was tested considering imbibed (G1) and germinated seeds (G2) as well as seedlings (G3). Additionally, the heat hardening capacity of seedlings was determined in the field. Across all species, LT50 decreased significantly by 9 K from G1 (55 °C) to G3 (46 °C), similarly in all species of the successional stages. Field-grown seedlings had mostly an increased LT50 (2K). Intraspecifically, LT50 of seedlings varied between 40.6 and 52.5 °C. Along the chronosequence, LT50 in G1 was similar, but was higher in G2 and G3 of early successional species. The highest temperatures occurred at 0-0.5 cm in air (mean/absolute maximum: 42.6/54.1 °C) posing a significant heat injury risk for seedlings when under water shortage transpirational cooling is prevented. Below small stones (0-0.5 cm), maxima were 4 K lower, indicating heat safer microsites. Maxima >30 °C occurred at 32.3, >40 °C at 6.2 %. Interannually, 2010 was the hottest year with heat exceeding LT50 at all microsites (0-0.5 cm). Temperature maxima on sandy surfaces were lower than on microsites with gravel (diameter <5-10 mm). The hot summer of 2010 may be a small foretaste of in future more severe and frequent heat waves. Ground surface temperature maxima at the pioneer stage are already now critical for heat survival and may partly explain the high seedling mortality recognized on recently deglaciated terrain.

中文翻译：

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生长室和田间冰川前陆物种早期发育阶段的耐热性。

在冰川前陆中，种子很容易发芽，但是，很大一部分幼苗在出现后不久就会死亡。我们假设除了干旱，霜冻和缺少安全地点外，地表的热量可能是幼苗的主要威胁之一。评估了2007年至2010年不同地层的热应变。测试了11个不同演替阶段的高山物种的耐热性（LT50），其中考虑了吸收的（G1）和发芽的种子（G2）以及幼苗（G3）。另外，在田间确定了幼苗的热硬化能力。在所有物种中，LT50从G1（55°C）到G3（46°C）显着下降了9 K，在演替阶段的所有物种中也是如此。田间生长的幼苗大部分具有增加的LT50（2K）。在种内，幼苗的LT50在40之间变化。6和52.5°C。沿时间序列，G1中的LT50相似，但早期演替物种的G2和G3中的LT50较高。最高温度出现在空气中0-0.5厘米处（平均/绝对最大值：42.6 / 54.1°C），当在缺水状态下防止蒸发蒸腾冷却时，对幼苗造成严重的热伤害风险。在小石头（0-0.5厘米）以下，最大值降低了4 K，表明热场所更安全。在32.3时出现最大值> 30°C，在6.2％时出现> 40°C。每隔一年，2010年是最热的一年，所有微型站点（0-0.5厘米）的热量都超过LT50。沙质表面的最高温度低于含砾石的微场所（直径<5-10毫米）。2010年炎热的夏天可能预示着将来还会出现更严重，更频繁的热浪。