Tree Rings, Glacial Lakes State Park Site 2, Minnesota

Both increases in temperature and changes in precipitation may limit future tree growth, but rising atmospheric CO2 could offset some of these stressors through increased plant Water Use Efficiency (WUE). The net balance between the negative impacts of climate change and positive effects of CO2 on tree growth will be most important for systems already at plant physiological limits, where increased climate stress could drive mortality and shifts in range distribution. Here, we quantify the effects of climate, stand structure, and rising CO2 on both annual tree-ring growth increment and WUE at a savanna-forest boundary in the Upper Midwest United States. Taking a Bayesian hierarchical modelling approach, we find that plant WUE increased by ~13-25% over the course of the 20th century, but on average, tree-ring growth increments do not significantly increase. Consistent with higher WUE under increased CO2 and recent wetting, we observe a decrease in sensitivity of tree growth to annual precipitation, leading to 25-65% higher growth under dry conditions compared to trees of similar age and size in the past. However, an emerging interaction between summer maximum temperatures and annual precipitation diminishes the water-savings benefit under hot and dry conditions. Both the decrease in precipitation sensitivity, and the interaction between temperature and precipitation are strongest in open canopy microclimates, suggesting that stand structure may modulate response to future changes. Overall, we find that while higher WUE may provide some water savings benefits to growth under normal drought conditions, near-term future temperature increases combined with drought events could drive growth declines of over 50%. These products are used in the manucript, Heilman et al., 2020, Increased water use efficiency leads to decreased precipitation sensitivity of tree growth, but is offset by high temperatures. Submitted for review. The tree rings in this data package and those in msb-paleon packages 35-39 and 41-43 correspond with the isotope data in msb-paleon package 34. This material is based upon work supported by the National Science Foundation Doctoral Dissertation Improvement Grant no. DEB-1701897, the National Science Foundation PalEON MacroSystems Biology under grant no. DEB-1241874, and University of Notre Dame Center for Environmental Science and Technology (CEST)/Bayer Predoctoral Fellowship.气温升高与降水格局变化均可能限制未来树木生长，但大气CO₂浓度上升可通过提升植物水分利用效率（Water Use Efficiency, WUE）抵消部分此类气候胁迫。气候变化的负面影响与CO₂对树木生长的正向效应之间的净平衡，对于已处于植物生理极限的生态系统尤为关键——在此类系统中，加剧的气候胁迫可能引发树木死亡与分布范围转移。本研究针对美国中北部的稀树草原-森林交错带，量化了气候、林分结构与CO₂浓度上升对树木年轮年生长增量及WUE的双重影响。研究采用贝叶斯层次建模方法，发现20世纪期间植物WUE提升了约13%~25%，但平均而言树木年轮生长增量并未出现显著增长。与CO₂升高下WUE提升及近期降水增加的研究结果一致，我们观测到树木生长对年降水量的敏感性有所降低，使得在干旱条件下，相较于历史上同龄同规格的树木，当前树木的生长量提升25%~65%。然而，夏季最高温与年降水量之间新近出现的交互作用，会抵消高温干旱条件下的水分节约收益。降水敏感性降低以及温-降水交互作用在开放冠层微气候中表现最为显著，这表明林分结构可调节生态系统对未来气候变化的响应。总体而言，我们发现尽管在正常干旱条件下更高的WUE可为树木生长提供一定的水分节约收益，但近期气温升高叠加干旱事件可能导致树木生长量下降超50%。 本研究使用的相关成果已发表于稿件Heilman等人2020年的《提升的水分利用效率降低树木生长对降水的敏感性，但高温会抵消该效应》（已提交评审）。本数据包中的年轮数据，与msb-paleon数据包35-39及41-43中的年轮数据，均与msb-paleon数据包34中的同位素数据相匹配。本研究依托的工作得到了美国国家科学基金会博士论文改进基金（编号DEB-1701897）、美国国家科学基金会PalEON宏观系统生物学项目（编号DEB-1241874）以及圣母大学环境科学与技术中心（CEST）/拜耳博士前奖学金的支持。