Tree Rings, Englund Ecotone Scientific and Natural Area, 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 35-36 and 38-43 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）抵消部分此类胁迫压力。气候变化带来的负面影响与CO₂对树木生长的正向影响之间的净平衡，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及分布范围变迁。本研究针对美国中西部北部的稀树草原-森林交界带，量化了气候、林分结构以及CO₂浓度上升对树木年轮年生长量及WUE的影响。本研究采用贝叶斯层级建模法，发现20世纪期间植物WUE提升了约13%~25%，但从平均水平来看，树木年轮年生长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现象一致，我们观测到树木生长对年降水量的敏感性有所下降，这使得当前干旱条件下的树木生长量较过去同龄同规格树木高出25%~65%。但夏季最高温与年降水量之间新出现的交互效应，会削弱高温干旱条件下的节水收益。降水敏感性下降以及温降水交互效应在开阔林冠微气候中表现最为显著，这表明林分结构可能会调节生态系统对未来气候变化的响应。总体而言，尽管在常规干旱条件下，WUE提升可为树木生长带来一定的节水收益，但未来短期内气温升高叠加干旱事件，可能导致树木生长量下降超过50%。本研究成果已用于Heilman等人2020年的手稿《水分利用效率提升降低树木生长对降水的敏感性，但高温会抵消该效应》，目前处于投稿评审阶段。本数据包中的树木年轮数据，与msb-paleon数据包35-36、38-43中的年轮数据，均与msb-paleon数据包34中的同位素数据相匹配。本材料基于以下资助项目完成：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）/拜耳博士预科奖学金。