Leaf metabolic and morphological responses of dwarf willow (Salix herbacea) in the sub-arctic to the past 9000 years of global environmental change

Abstract

Ice-core records of the concentration of atmospheric CO₂ and its stable isotope ratio (δ¹³C_a) have shown that the global C cycle has not remained in steady-state over the past 11000 yr, implying a possible change in vegetation activity over this period. Here we evaluated the ecophysiological responses of the dwarf willow (Salix herbacea) over the past 9000 yr by measuring the stable carbon isotope composition and stomatal characters of a unique, well dated, high-latitude (68 °N) sub-fossil leaf sequence. After correction for corresponding changes in δ¹³C_a, a 9000-yr record of variations in the ratio of intercellular (c_i) to atmospheric (c_a) CO₂ concentration was established. Intercellular[ratio ]atmospheric CO₂ concentration ratios provide a time-integrated indicator of the set-point of leaf gas exchange, and the historical variations revealed in this record have been interpreted as an impact of environmental changes on leaf gas exchange. The sequence shows a progressive fall in c_i/c_a 9000–3000 yr BP as well as the climatic effects of the Medieval Warm Period, the Little Ice Age and the post-industrial CO₂ rise. Leaf stomatal index (proportion of epidermal cells as stomata), but not stomatal density, was significantly (P<0.01) correlated with Holocene atmospheric CO₂ variations. A process-based interpretation of the changes in c_i/c_a was made using two different coupled photosynthesis-stomatal conductance models. Calculated in this way, S. herbacea photosynthetic rates were relatively stable throughout the Holocene whilst stomatal conductance progressively declined. Both, however, showed the marked effects of the Medieval Warm Period and the Little Ice Age. Overall, the results demonstrate that S. herbacea leaf metabolism, like the global C cycle, has not remained in steady state during the Holocene but has responded to changes in atmospheric CO₂ concentration and short-term climatic oscillations.