NOAA/WDS Paleoclimatology - Maxwell - UM Biological Station Flux Site - PPGR - ITRDB MI027

Several important environmental influences of tree growth and carbon sequestration have changed over the past several decades in eastern North America, specifically, more frequent pluvial conditions, increased carbon dioxide (CO2) concentrations, and decreased acidic deposition. These factors could lead to changes in the relationship between tree growth and water availability, and perhaps even decouple the two, having large implications on how future climate change will impact forest productivity and carbon sequestration. Here, we examine the concurrent influence of the climatic water balance (precipitation minus potential evapotranspiration), CO2 concentrations, and sulfate and nitrogen deposition on radial tree growth, carbon isotopes, and intrinsic water-use efficiency (iWUE) for several hardwood tree species in the Midwestern United States. We found that when considering the simultaneous influence of these factors, the climatic water balance is the dominant influence on annual growth. Therefore, the recent pluvial period is the primary cause of the weakening relationship between radial growth and water availability. Even during pluvial periods, water availability is the primary control on growth, with increasing CO2 concentrations and decreased SO4 deposition being secondary factors. Importantly, the weakening in the climate-growth relationship is species specific, with Acer species having stable relationships with the climatic water balance, Liriodendron tulipifera showing a strengthening relationship, and Quercus species and Populus grandidentata exhibiting weakening. Thus, interannual variations in soil moisture unevenly impact tree growth and carbon sequestration. Our findings suggest that, despite recent pluvial conditions, increasing CO2 concentrations and decreasing acidic deposition have not buffered the impact of water availability on tree growth and carbon sequestration.

过去数十年来，北美东部地区影响树木生长与碳固存的若干关键环境因子已发生改变，具体表现为湿润气候条件愈发频繁、二氧化碳（CO₂）浓度升高，以及酸性沉降减少。上述因子可能改变树木生长与水分可利用性之间的关联，甚至使二者解耦，这对未来气候变化如何影响森林生产力与碳固存具有重要意义。在此，我们针对美国中西部地区的多种硬阔树种，探究了气候水分平衡（降水量减去潜在蒸散量）、CO₂浓度、硫酸盐与氮沉降对树木径向生长、碳同位素以及内在水分利用效率（intrinsic water-use efficiency, iWUE）的协同影响。研究发现，当综合考量这些因子的协同作用时，气候水分平衡是主导树木年生长量的核心影响因子。因此，近期的湿润气候阶段是导致径向生长与水分可利用性之间关联减弱的主要原因。即便处于湿润气候阶段，水分可利用性仍是调控树木生长的首要因素，而CO₂浓度升高与硫酸盐沉降减少仅为次要影响因子。值得注意的是，气候-生长关联的减弱存在物种特异性：槭属（Acer）树种与气候水分平衡的关联保持稳定，北美鹅掌楸（Liriodendron tulipifera）的关联呈现增强趋势，而栎属（Quercus）树种与大齿杨（Populus grandidentata）的关联则表现为减弱。由此可见，土壤湿度的年际变化对树木生长与碳固存的影响并不均衡。我们的研究结果表明，尽管近期处于湿润气候阶段，CO₂浓度升高与酸性沉降减少并未缓解水分可利用性对树木生长及碳固存的影响。