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，主要由糖类与淀粉构成。已有研究显示，成熟树木体内的NSC库规模可观，且平均存储时长可达十年之久。因此，NSC库整合了多年的碳同化过程，在整株植物乃至生态系统层面承载了显著的生态记忆。然而，当前学界对老旧存储NSC与新同化NSC如何分别支撑植物生长与代谢过程，以及在逆境或受干扰后树木可利用的老旧NSC储备状况仍知之甚少。为深入解析NSC分配过程中存在的这类潜在滞后性，我们对两处新英格兰温带森林中的成熟红枫（Acer rubrum）开展了研究。我们在每个样地选取5棵红枫，测定其茎干木质部内存储的糖类与淀粉浓度。采用放射性碳（14C）“炸弹峰值”法，我们估算了茎干木质部NSC、年轮纤维素以及树干呼吸中碳的年龄。此外，我们还在每个样地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年龄。研究数据表明，年轻NSC会被优先用于支撑植物生长与日常代谢需求；新近存储的NSC则参与休眠期的年轮生长与代谢活动；而老旧储备会在树木受干扰（如采伐）后被动员利用。