Nonstructural Carbohydrates in Red Maple at Harvard Forest and Bartlett Forest 2011-2012

Nonstructural carbohydrates (NSC) are the primary products of photosynthesis, composed mostly of sugars and starch. Recent studies show that NSC pools in mature trees can be quite large and on average a decade old. Thus, NSC pools integrate years of carbon assimilation and represent significant ecological memory at the whole plant and ecosystem level. However, we know very little about how older stored NSC versus newly assimilated NSC are used to support growth and metabolism, or how available older NSC are to trees during stress or following disturbance. To better understand these potential lags in NSC allocation, we studied mature red maple (Acer rubrum) trees in two New England temperate forests. We determined stemwood concentrations of stored sugars and starch of five trees at each site. Applying the radiocarbon (14C) “bomb spike” approach, we estimated the age of carbon in stemwood NSC, ring cellulose, and bole respiration. We also collected stump sprouts regrowing from a separate set of recently harvested red maple trees at each site, and determined the radiocarbon age of this tissue. Our data show that younger NSC is preferentially used for growth and day-to-day metabolic demands. More recently stored NSC contributes to annual ring growth and metabolism in the dormant season. Older reserves are available to the tree after disturbance (e.g. harvesting).非结构性碳水化合物（Nonstructural carbohydrates, NSC）是光合作用的主要产物，其组成以糖类和淀粉为主。已有研究表明，成熟树木体内的非结构性碳水化合物库规模可观，且其平均驻留时长可达十年之久。因此，非结构性碳水化合物库整合了多年的碳同化过程产物，在整株植物乃至生态系统层面体现出显著的生态记忆效应。然而，目前学界对储积已久的非结构性碳水化合物与新同化的非结构性碳水化合物如何分别支撑植物生长与代谢过程，以及树木在逆境或受干扰后可利用的老旧储积非结构性碳水化合物的情况仍知之甚少。为了更深入地理解非结构性碳水化合物分配中存在的这类潜在滞后效应，本研究选取新英格兰地区两处温带森林中的成熟红枫（Acer rubrum）作为研究对象。我们对每个样地内的5棵红枫进行了取样，测定了其木质部中储存的糖类与淀粉含量。本研究采用放射性碳（14C）“炸弹峰值”分析法，估算了红枫木质部非结构性碳水化合物、年轮纤维素以及树干呼吸碳的驻留年龄。此外，我们还在每个样地中采集了另一组近期被采伐的红枫的伐桩萌条，并测定了该组织的放射性碳驻留年龄。研究结果显示，树木优先利用年轻的非结构性碳水化合物满足生长与日常代谢需求；近期储积的非结构性碳水化合物则用于年轮生长与休眠期的代谢活动；而老旧储积的非结构性碳水化合物储备则会在树木受干扰（如采伐）后被调用。