Linking conifer root growth and production to soil temperature and carbon supply in temperate forests

TL;DR: A large pool of NSC, principally in the form of starch, existed in fine roots of P. abies, and acted as an energy supply for root production throughout the year, even when photosynthesis was limited.

Abstract: In temperate conifer forests, soil temperature is an important driver of fine root growth and winter root growth can occur during aerial dormancy. We hypothesize that in conifers, stocks of non-structural carbohydrates (NSC) in fine roots are high enough to provide energy for root growth and production throughout the year, even when photosynthesis is reduced. We measured monthly root production (i.e. the number of roots undergoing elongation) and their elongation rate (RER) in mature Picea abies for one year, along a soil temperature gradient (three altitudes of 1400, 1700 and 2000 m). Every two months we harvested needles, branches, stem, large, medium and fine roots, and quantified starch and soluble sugars in each organ using analytical methods and near infra-red spectroscopy (NIRS). Soil water potential was monitored continuously. We analysed RER data with regard to climate variables and NSC levels of the current and preceding month. NIRS was a reliable method for measuring starch and soluble sugars. NSC was high in the crown and roots but very low in the trunk all year round. Soil temperature was positively correlated to RER (of the current month) between 0 and 8 °C, above which RER stabilised and was not explained by NSC levels or soil water potential. However, mean RER of fine roots in the month following the measurement of NSC was significantly and negatively correlated to soluble sugar and positively correlated with starch content. Very fine root starch content was also positively correlated with root production in the month following the starch measurement. Soil temperature was a major driver of fine root elongation, but at low temperatures only. At soil temperatures >8 °C, no particular driver was dominant. NSC levels were negligible in the stem and root-bases, suggesting that wood production is a major sink that depletes carbohydrates. A large pool of NSC, principally in the form of starch, existed in fine roots of P. abies, and acted as an energy supply for root production throughout the year, even when photosynthesis was limited. Soluble sugars were depleted in fine roots during the growing season, but no relationship was found between fine root production and soluble sugars. The physiological mechanism by which NSC accumulation actively or passively occurs in fine roots is not known but could be due to the symplastic pathway of phloem unloading in conifer root tips, which suggests a passive mechanism.