1. Understanding the processes that control alpine treelines, the elevational limits of tree growth forms, has been a central question in ecology and is growing in importance with concern over climate change. Cool summer air temperatures are currently thought to be the ultimate limiter of upright tree growth at alpine treelines globally. However, winter damage has long been recognized as a shaping force near alpine treelines. Low‐growing krummholz growth forms provide an opportunity to test hypotheses about the controls of upright growth in environments above current treelines.
2. To distinguish between effects of growing season temperature, winter damage and their interaction on preventing upright growth in krummholz, we conducted a field experiment on krummholz growth forms of Pinus albicaulis over the summer and winter of 2015–2016 at 10 mountain top sites in the Tobacco Root Mountains, Montana, USA. We experimentally manipulated four factors using a fully crossed design: shoot position (natural low position in the krummholz mat vs. propped up above the krummholz mat), summer warming (warming chamber vs. ambient), winter exposure (shelter cage vs. exposed), and elevation position (local high vs. low krummholz limits). We also conducted an observational study of the climatic conditions associated with recent natural emergent stem establishment from krummholz.
3. Experimentally propped shoots that were exposed in winter experienced the highest mortality (10%–50%), while propped shoots in shelter cages and shoots located within the krummholz mat, whether caged or not, had low mortality (0%–10%). Summer warming had little influence on shoot mortality. Surviving mat shoots had marginally higher growth rates than surviving propped shoots during the early growing season after treatments were established. Natural emergent stem establishment was associated with warmer than average summer temperatures, but also warmer winter temperatures, lower winter wind speeds, and lower snowpack.
4. Synthesis. Our results suggest winter damage plays a more important role than does growing season temperature in maintaining the krummholz growth form. While warming may increase opportunities for emergent shoot establishment above krummholz mats, establishment of upright trees in the krummholz zone will also require climatic change that reduces wind and snow transport which cause winter damage.

中文翻译：

---

对于保持高寒林线上方的克鲁姆霍兹生长形式而言，冬季损害比夏季温度更为重要

1. 理解控制高山林线的过程，树木生长形式的海拔极限，一直是生态学的中心问题，并且随着对气候变化的关注而变得越来越重要。当前，夏季凉爽的气温被认为是全球高山林线直立树生长的最终限制因素。但是，长期以来，人们一直认为冬季破坏是高山林线附近的塑造力量。生长缓慢的克鲁姆霍兹生长形式提供了一个机会，可以检验有关在当前林木线以上环境中直立生长控制的假设。
2. 生长期温度，冬季损害及其对防止直立生长在krummholz互动的效果区别，我们进行了对krummholz生长形态田间试验松albicaulis在烟草10个山顶站点通过的2015 - 2016年的夏季和冬季根山，蒙大拿州，美国。我们使用完全交叉的设计在实验上操纵了四个因素：拍摄位置（在Krummholz垫中自然处于低位，而在Krummholz垫上方支撑），夏季变暖（保温室与环境），冬季暴露（避难所笼与暴露） ，以及海拔位置（本地krummholz上限与下限值）。我们还对与克鲁姆霍兹州最近自然出没的茎建立相关的气候条件进行了观测研究。
3. 冬季暴露在实验中的支撑芽死亡率最高（10％–50％），而掩蔽笼和位于克鲁姆霍兹垫子内的芽（无论是否有笼）的死亡率均较低（0％–10％）。夏季变暖对苗死亡率没有影响。在建立处理后的早期生长季节，幸存的芽苗的生长速率略高于幸存的芽苗。天然出芽茎的建立与夏季温度高于平均温度有关，但也与冬季温度较高，冬季风速较低和积雪较低有关。
4. 综合。我们的结果表明，冬季损害在保持克鲁姆霍兹生长形式方面比生长季节温度更重要。虽然变暖可能会增加在克鲁姆霍兹垫上发芽的机会，但在克鲁姆霍兹地区建立直立的树木还需要气候变化，以减少风和雪的运输，从而造成冬季破坏。