| Non-structural carbohydrates (NSC, mainly soluble sugars and starch) are important carbon reserves in trees, which vary seasonally due to changes in the strength of carbon sink or source. Trees living under similar environmental conditions may show different seasonal patterns of NSC storage, but no systematic studies had been conducted in Chinese temperate forest. In addition, we still knew little about the spatial variations in NSC of trees. Using twelve tree species [Korean spruce (Picea koraiensis), Korean pine(Pinus koraiensis), Mongolian pine(Pinus sylvestris var. mongolica), Dahurian larch(Larix gmelinii), Manchurian walnut (Juglans mandshurica), Japanese elm (Ulmus japonica), Machurian ash(Fraxinus mandshurica), Mongolian oak (Quercus mongolica), Amur linden (Tilia amurensis), Ussuri poplar (Populus ussuriensis), Korean aspen(Populus davidiana) and white birch (Betula platyphylla)] with different leaf phenology (evergreen vs. deciduous), life forms (needleleaved vs. broadleaved) and wood types (non-, semi-ring-, ring-, or diffuse-porous wood), the spatiotemporal variations in NSC were systematically studied for major tree species in Chinese temperate forest.The major objectives were to:(1) quantify the spatial variations in NSC concentrations in stems for two diffuse-porous tree species, i.e., Betula platyphylla and Tilia amurensis, after leaf-fall in a temperate forest in northeastern China;(2) Investigate the radial, axial and inter-specific variations in concentrations and contents of NSC in stems for12Chinese temperate tree species;(3) Examine the seasonality of NSC concentrations in stem outer wood (light-colored) and inner wood (dark-colored) for12tree species;(4) Compare the seasonality of NSC concentrations in new twigs and old branches, explore relationships between concentrations of soluble sugars and starch in these tissues, and explore factors controlling the inter-specific variations in NSC concentrations in branches. Here we measured the NSC concentrations in the stem wood and branch at seven phonological stages. For each species, stem bark (periderm and phloem), outer wood (light-colored) and inner wood (dark-colored) were sampled at four heights along the stem (stump, breast height, crown base and mid-crown). New twigs (current year) and old branches (≥1years old, diameter≤3cm) were also sampled in mid-crown. Concentrations of soluble sugars and starch were determined with a modified phenol-sulphuric method, and expressed as a percentage of dry matter (%DM). The main results were bellow:(1) In October, the concentrations of total NSC (TNC, sum of soluble sugars and starch) and soluble sugars in the stem xylem decreased with the depth increasing from the cambium to the pith for both white birch(Betula platyphylla) and Amur linden(Tilia amurensis), whereas those of starch showed little radial variations. There was a substantial amount of NSC existing even in the inner wood close to pith. The concentrations of NSC decreased slightly from the stump to the breast height, and then increased vertically. The maximum concentrations of TNC, sugars and starch occurred at different heights depending on species and TNC components. The ratio of sugar to starch showed contrasting vertical trends for the two species, i.e., increasing from the stump to the top of tree for white birch, but decreasing for Amur linden. The concentration of TNC (1.0%DM) in the stem of white birch (a shade-intolerant species) was significantly lower than that (4.3%DM) of Amur linden (a shade-tolerant species), which might be related to their different life-history strategies. Applying the sampling protocols that take the axial and radial variations in concentrations of NSC would effectively reduce potential uncertainty in estimation of NSC storage at tree or stand levels.(2) Concentrations of TNC, soluble sugars and starch were much higher in bark than those in wood. On average, contents of soluble sugars and starch accounted for48%and52%of the TNC, respectively; and contents of TNC in bark, outer wood, and inner wood accounted for34%,38%, and28%of the stem total, respectively. Bark was the major pool of sugars in the stem (accounting for50%of the stem total on average), while outer wood was the major pool of starch (41%). The concentration of soluble sugars varied axially for all the conifers but did not for the broadleaved species. Mean concentrations of TNC, sugars and starch in stem varied by more than two folds among the species. However, there were no significant differences in these values for the species groups with different leaf phenology or wood types. Ignoring the radial, axial and inter-specific variations in NSC in stem would introduce large bias in estimating NSC storage at tree or ecosystem levels.(3) The TNC concentration in the inner wood showed similar seasonal patterns as that in the outer wood for most species, but these patterns varied with leaf phenology. Two evergreen conifers showed an increase of TNC before leaf-bud break, whereas most deciduous trees had an overall decline of TNC during leaf flushing. The TNC kept at a low level in the summer, and increased in the late summer and autumn. The seasonal course of TNC was generally followed the dynamics of starch. Significant higher seasonal mean TNC concentration was detected in outer wood than in inner wood for seven species. The seasonal amplitudes of the TNC (i.e. relative variations) varied from35%to70%in outer wood and from39%to55%in inner wood. The storage capacities of the TNC per unit biomass (i.e. the seasonal maximum minus the minimum) ranged from0.5%DM to3.9%DM in outer wood and from0.2%DM to0.9%DM in inner wood. Surprisingly, the seasonal mean, amplitude and storage capacity of TNC in outer wood of deciduous trees were comparable with those in evergreen trees. The difference in TNC storage capacity between outer wood and inner wood was significant only for the ring-porous trees. However, the ring-porous and semi-ring-porous species have a higher storage capacity of TNC in outer wood than the other two wood types, but a lower one in inner wood than others. As the high concentrations, dramatic seasonal variations of TNC in inner wood, and synchronicity with those in outer wood, we concluded that inner wood may act as an active pool of carbon reserves in temperate forest trees.(4) The concentrations of NSC in new twigs and old branches showed similar seasonal patterns of NSC concentration for all the tree species except for Korean aspen and Ussuri poplar. The concentrations of NSC in the new twigs were usually higher than those in the old branches. The concentrations of TNC in the old branches of the evergreen species increased significantly before bud-break, declined slightly during bud-break, and kept relatively stable after leaf flushing until autumn when the concentration of NSC rebounced slightly. The concentrations of TNC in the old branches of the deciduous species decreased significantly before or during bud-break, and then progressively increased in both new twigs and old branches along the season. Meanwhile, a conversion of starch to soluble sugars was observed in October when the dormant season started. There was a significant and positive linear relationship between the concentrations of soluble sugars and starch for both new twigs and old branches when the data were pooled across all the species. The seasonal mean concentration, maximum concentration, and storage capacity of TNC in the old branches decreased significantly with the duration of shoot extension increasing for all the species except for Mongolian oak (the species with an intra-annual regrowth of branch), Korean spruce and Japanese elm (no data available). These findings suggested that the shoot growth of these temperate trees relies strongly on NSC reserves, and the duration of shoot length growth is inter-connected with TNC storage in branches. Future researches on forest carbon cycling modeling and global climate change should consider this functional inter-connection.This study quantified the spatiotemporal variability in NSC in branch and stem of twelve tree species in a Chinese temperate forest. This is the first report on spatiotemporal variability in NSC of various species grouped with contrasting properties. This work not only extended the theory of ecological strategy on co-occurring temperate tree species, but also will supply important data for tree ecophysiology, comparative ecology and global change science. Analyzing NSC variability in leaf and root tissues and exploring NSC storage at whole tree and ecosystem scales will offer much more information. |
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