Arctic and Alpine Treelines in Alaska: Controls on Performance of White Spruce - Implications for Global Change

9978143 Sveinbjornson北极和高山树线的位置，而部分历史，主要是环境变量限制树的功能，特别是物质积累的条件的反映。为了能够预测树线运动响应全球气候和大气CO2浓度的变化（以及这些运动反过来对全球变化的后果），必须了解目前限制树种在树线上表现的最重要的环境因素和植物过程。 白色云杉是阿拉斯加的主要树种。它的生长，繁殖努力和繁殖成功率在树线上比邻近的森林低。尽管显着减少土壤氮矿化的林线相比，在楚加奇山脉，阿拉斯加中南部的森林，氮肥的应用刺激白色云杉生长只有很小的程度，效果是没有更大的林线比内部的森林。因此，似乎白色云杉在树线上的氮限制并不不同。相反，树线白色云杉似乎是碳限制的基础上，目前的研究结果，更少的增长，更早的针叶死亡率，减少菌根感染水平，增加根呼吸。该项目的PI预测在树线处的根死亡率增加，他们建议通过过早的针叶损失来加剧碳流失。他们预计，增加的风，低土壤温度和更大的土壤运动导致观察和预测的组织周转增加。 在这个项目中，他们将评估减少生长的替代材料限制，即营养限制，可能是楚加奇树线的磷缺乏和其他地方的磷和/或氮缺乏。支持养分限制的趋势，增加针保持沿着海拔梯度内的森林，高养分浓度的树线针和细根，并在斯堪的纳维亚半岛的树线山桦树海拔增加土壤氮的限制。该项目将通过研究地理上不同的北极和高山白色云杉林线以及阿拉斯加邻近的森林地点来检验营养限制假说。选定的拟议地点代表了极端的气候和地理条件。在每个地点，该项目将研究三个树线地点和三个森林地点，测量植被组成、过去的树木生长和累积的针叶损失。在每个地点，将在有和没有遮蔽物以及施肥和不施肥的情况下种植幼苗。将在每个地点监测树木和幼苗生长、针叶和细根周转、树木种子生产、种子发芽和幼苗建立。 还将在每个地点监测气候和土壤养分状况。小气候研究将表明不同气候参数对组织损失和组织生长的相对重要性。 为了评估大气CO2浓度升高是否会使白色云杉幼苗补偿它们在树线处遭受的组织损失，它们将暴露于不同的大气CO2水平（环境和1.5倍）和不同程度的针叶和/或根修剪（完整和30%去除）。 通过将两种营养水平（有或没有肥料的树木土壤）纳入处理，将有可能进一步完善碳与营养的评估。 将测量茎和根的生长以及光合作用和水分的关系。将对来自田间和生长室的树木和幼苗的土壤和组织进行取样，进行化学分析（还对根部的菌根感染水平进行分析），以测试假设的生长减缓机制。总之，现场和实验室研究将允许过早的组织周转和材料限制树性能的重要性进行评估。这些知识，再加上对树种生长与繁殖努力和成功之间关系的理解，将为现实地预测树线对全球变化的反应奠定基础。

9978143SveinbjornssonThe location of arctic and alpine treelines, while partly historic, is primarily a reflection of conditions where environmental variables limit tree function, particularly material accumulation. In order to be able to predict treeline movements in response to global changes in climate and atmospheric CO2 concentrations (and the consequences of these movements in turn on global change), it is essential to understand the most important environmental factors and plant processes presently limiting tree species performance at treelines. White spruce is the dominant treeline tree species in Alaska. Its growth, reproductive effort, and reproductive success is lowered at the treeline as compared to the adjacent forest. In spite of dramatically reduced soil nitrogen mineralization at the treeline as compared to the forest below in the Chugach Mountains, south-central Alaska, nitrogen fertilizer application stimulates white spruce growth only to a small degree, and the effect is no greater at the treeline than inside the forest. Thus it appears that white spruce is not differentially nitrogen limited at the treeline. Rather the treeline white spruce appears to be carbon limited based on current findings of less growth, greater premature needle mortality, reduced mycorrhizal infection levels, and increased root respiration. The PIs for this project predict increased root mortality at the treeline, which they suggest exacerbate the carbon drain by premature needle loss. They expect that increased wind, low soil temperatures, and greater soil movement cause the increase in observed and predicted tissue turnover. In this project they will evaluate an alternate material limitation reducing growth, i.e. nutrient limitation, possibly a phosphorus deficiency at the Chugach treeline and phosphorus and/or a nitrogen deficiency elsewhere. Supporting a nutrient limitation are trends of increased needle retention along an elevational gradient within the forest, high nutrient concentrations in treeline needles and fine roots, and an elevationally increasing soil nitrogen limitation in treeline mountain birch in Scandinavia. This project will test the nutrient limitation hypothesis by studying geographically distinct arctic and alpine white spruce treelines and adjacent forest sites in Alaska. The proposed sites have been selected to represent climatic and geographical extremes. At each location the project will study three treeline sites and three forest sites at which vegetation composition, past tree growth, and accumulated needle loss will be measured. At each site seedlings will be planted with and without shelter and with and without fertilizer application. Tree and seedling growth, needle and fine root turnover, tree seed production, seed germination, and seedling establishment will be monitored at each site. Climatic and soil nutrient conditions will also be monitored at each site. The microclimatic study will indicate the relative importance of different climatic parameters on tissue loss and tissue growth. In order to evaluate whether rising atmospheric CO2 concentration will allow the white spruce seedlings to compensate for tissue loss they suffer at the treeline, they will be exposed to different atmospheric CO2 levels (ambient and 1.5 times) and different degree of needle and/or root pruning (intact and 30% removal). By including two nutrient levels (treeline soil with or without fertilizer) to the treatments, it will be possible to further refine the carbon versus nutrient assessment. Shoot and root growth will be measured as well as photosynthesis and water relations. Soils and tissues of trees and seedlings from the field and the growth chamber will be sampled for chemical analysis (roots for mycorrhizal infection levels also) in order to test the hypothetical growth reduction mechanisms. Together, the field and laboratory studies will allow an evaluation of the importance of premature tissue turnover and material limitation on tree performance. That knowledge, in conjunction with an understanding of the relationship between tree species growth and reproductive effort and success, will lay the foundation for a realistic prediction of treeline response to global change.