Nitrogen addition does not mitigate drought-induced growth reduction in Pinus koraiensis: insights from hydraulics, gas exchange and carbon reserves

The composition of non-structural carbohydrates affects the respiration and morphology of tree fine roots in Japan Alps

Nonstructural carbohydrates (NSCs) mediate plant survival when the plant's carbon (C) balance is negative, suggesting that NSCs could predict plant survival under C stress. To examine this possibility, we exposed saplings of six temperate tree species to diverse levels of C stress created by the combination of two light conditions (full light availability and deep shade) and two defoliation levels (severe defoliation and nondefoliation). We then measured survival, biomass and total NSCs and soluble sugars (SSs) concentrations in different organs of both dead and live saplings. We estimated mean NSCs and SSs contents and concentrations per sapling and fitted logistic generalized mixed‐effects models to determine if NSCs and SSs predict survival. Using inverse prediction modelling, we also determined whether there is a common NSCs and SSs threshold across species at the time of sapling's death. Defoliation and shade reduced the mean sapling's NSCs and SSs contents, indicating C stress. Mean sapling NSCs and SSs contents and concentrations predicted survival and the robustness of the models improved with the inclusion of species. At death, saplings of the exotic deciduous tree species Acer pseudoplatanus exhibited significantly lower mean NSCs and SSs contents than saplings of the evergreen conifer species Podocarpus nubigenus and lower stem NSCs and SSs concentrations than the broadleaf evergreen species Drimys winteri. The energetic role that NSCs and SSs play in plants under C stress was evidenced by the capacity of these compounds to predict sapling survival under C stress. No common threshold of NSCs and SSs contents or concentrations for sapling survival amongst species was found, indicating that the level of these compounds may not be good proxies for interspecific comparisons of tolerance to C stress. Presumably, there are species‐specific limits for the mobilization and use of NSCs and SSs in metabolism. Our results anticipate that the inclusion of NSCs and SSs in modelling will improve predictions regarding tree responses to ongoing climate change. Nonetheless, a better understanding of the many roles that carbohydrates play in plant survival under C stress is required to scale predictions up to the community level. Read the free Plain Language Summary for this article on the Journal blog.

We examined the seasonal variations of growth and nutrient accumulation of two-year-old Korean pine (Pinus koraiensis) seedlings in the primary broadleaved Korean pine forest (primary forest) and poplar birch forest (secondary forest) in Changbai Mountain. The seasonal changes of photosynthetically active radiation (PAR), seedling biomass, nonstructural carbohydrate (NSC), total nitrogen (N), and total phosphorus (P) were measured in both forests. The effects of understory light and its seasonal variations in both forests on the growth and nutrient accumulation of understory Korean pine seedlings were also explored. The results showed that the seasonal variations of monthly cumulative PAR under both forests showed a double-peak pattern. In summer, light conditions under the two forests were poor because of the closed canopy. In spring and autumn, light conditions were better due to the deciduous characteristics of canopy broadleaved trees, with light conditions under the secondary forest being obviously better than that of the primary forest. The seasonal variations of biomass, NSC, total N and total P concentrations of Korean pine seedlings in the primary and secondary forests were basically consistent with that of understory light. All those variables increased significantly in spring and autumn, and decreased in summer. In spring, starch concentration increased. In summer, starch and soluble sugar concentrations decreased gradually, and touched bottom in August. In autumn, soluble sugar concentration increased significantly. The biomass and NSC concentration of seedlings under secondary forest were significantly higher than those under primary forest in spring and autumn, but without difference in summer. Therefore, the diffe-rence of understory light conditions in spring and autumn between the two forests might be the key factor driving nutrient accumulation, growth and regeneration of Korean pine seedlings.

Depth of root placement, root size and carbon reserves determine reproduction success of aspen root fragments

Species of the tree genus Pine (Pinus L.) exist all over the world and no other group contains so many attractive forms (Curtis & Bausor, 1943) . Scots pine (Pinus sylvestris L.) is currently the most widely distributed pine and occurs throughout all of Eurasia. In the central alpine valleys, Scots pine is growing at the dry border of its distribution range, which involves overcoming periods with extreme low water availability. Although the species is known for its ability to grow on dry and nutrient poor soils, several extreme droughts during the last two decades have caused a 50% dieback of Scots pine in the dry valleys of the Central Alps in Switzerland. The ability of trees to survive drought is determined by their initial health and their resilience to drought, as well as on the characteristics of a drought event – i.e. timing, duration and intensity. The mechanisms underlying drought-induced mortality are still unclear, as well as the recovery process after soil rewetting. Furthermore, possible mitigation or aggravation of drought effects by elevated nutrient availability in the soil has not been studied before. The carbon (C) balance in trees is used as an indicator for C assimilation, growth, defense and storage processes. When trees are exposed to drought, to changes in soil nutrition or sudden defoliation, the C balance may change. In this thesis, the main objective was thus to combine effects of drought and fertilization to study the C and nitrogen (N) dynamics in Scots pine trees.
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\nIn the first chapter, I give an overview of the state-of-the-art in research on drought-affected C and N dynamics in trees. The aim of the second chapter was to assess the effects of long-term drought release on growth and non-structural carbohydrate (NSC) concentrations of adult P. sylvestris trees. A long-term (13 years) irrigation experiment was conducted in the Pfynwald, a Scots pine dominated forest located at the dry distribution margin of the species in southern Switzerland. I measured growth, NSC, N and phosphorus (P) concentrations, as well as the natural abundance of 13C isotopes on trees with different leaf area in control and irrigation plots. Irrigation resulted in higher growth rates and carbon isotope discrimination, but did not alter NSC levels. Growth and NSC decreased with lower leaf area in both control and irrigated trees, but NSC did not correlate with leaf-level gas exchange indices such as foliar δ13C, which is an indicator for water use efficiency, N or P, which are both stimulants of photosynthesis. Trees with initially low leaf area had limited ability to respond to the long-term irrigation, indicating a legacy effect of previously low crown condition. The NSC constancy across treatments suggests that carbohydrate storage may stay constant when changes in climate are slow enough to allow acclimation. Moreover, total leaf area, rather than leaf gas exchange per unit leaf area, drives variation in whole-tree carbohydrate dynamics in this system.
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\nThe main focus of the third chapter was the mitigation or aggravation of drought effects by nutrient availability in the soil. Three year-old P. sylvestris saplings were exposed to drought during two subsequent years, using four different water and two soil nutrient regimes, and drought was released thereafter. In addition, partial and full needle removal was performed in order to assess effects of changes in source:sink ratio. Biomass, leaf gas exchange and tissue NSC were measured during and after the first and second growing season. Extreme drought reduced stomatal conductance, photosynthesis, biomass and NSC, whereas intermediate drought only slightly affected biomass and NSC. Defoliation stimulated photosynthesis and fertilization increased growth and root biomass fraction, but mainly in the two intermediate drought levels. Only extreme drought pushed P. sylvestris trees to mortality. The third chapter concludes that tree mortality under severe drought periods will not be mitigated, but that the effects of low intensity drought stress could be compensated by increased nutrient availability and decreased source:sink ratio. 
\nThe aim of the fourth chapter was to assess the C and N allocation underlying the biomass changes that were found in chapter 3. I hypothesized that, during drought, increased soil nutrient availability stimulates root metabolism and carbon allocation to belowground tissues under drought stress. I therefore conducted a 15N and 13C labelling experiment in July and August 2016 respectively, on the saplings described above. 15N labelling was conducted with fertilized saplings from all water regimes, while 13C labelling was only conducted with saplings (both nutrient regimes) from two out of four water regimes (well-watered and mild drought). I assessed the abundance of 15N and 13C in the roots, stem and needles after the first growing season and during the second year. C uptake was slightly lower in drought stressed trees, and extreme drought inhibited largely the N uptake and transport. Carbon allocation to belowground tissues was decreased under drought, but not in combination with fertilization. The results indicate a potential positive feedback loop, where fertilization improved the metabolism and functioning of the roots, stimulating source activity and hence C allocation to belowground tissues. We can thus conclude that soil nutrients might play an important role in mitigating drought stress of trees.
\nOverall this thesis shows that the impairment of tree functioning and mortality can be explained with thresholds: long-term drought causes a reduction in tree vigor and leaf area, and if a threshold of approximately 60 – 70% loss of leaf area is reached, trees may follow a trajectory towards mortality, even if drought is released in the soil. In the controlled experiment, soil moisture thresholds were visualized. The impairment of C allocation belowground under mild drought, the reduction of NSC in and impairment of 15N uptake by the roots under extreme drought indicate that roots might be the first tissue to lose function and eventually die off during drought stress. Additional nutrient supply can sustain root functioning under drought, indicating that soil moisture tipping points are not fixed, but can be modified. In general, trees have a strongly coordinated supply – demand regulation for C and N, enabling homeostatic C balances as long as changes in climate are slow or mild enough for trees to acclimate. 
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Halophytic succulence is a driver of the leaf non-structural carbohydrate contents in plants in the arid and hyper-arid deserts of northwestern China.

Taiga-steppe ecotone is a natural grassland habitat with isolated forest islands and low precipitation (c. 100-250 mm yr-1), and is an important part of the boreal biome. The southernmost distribution limit of Siberian taiga occurs in the mountainous areas of Northern and Western Mongolia. In these areas, larch (Larix sibirica) forest patches are normally restricted to the upper part of humid north-facing slopes, whereas steppe covers south-facing slopes and most valley bottoms. Temperatures have been rising faster here than the the global average for decades, but precipitation has not increased concomitantly. Induced by the increasing aridity, a decades-long decline in the growth and regeneration of the larch forests, and differentiation in the growth performance of L. sibirica between within-stand microhabitats (forest edge vs. forest interior) was evidenced by earlier work. However, the physiological mechanisms underlying these ecological responses to drought stress and climate warming are not fully understood. As a contribution to answer these questions, bio- and necromass and morphological properties of fine roots, xylem hydraulic conductivity and wood anatomical properties along the flow pass (root, stem, and branch) together with tree productivity in monospecific L. sibirica stands were studied in this thesis. The study was conducted in two dry sites (the Altai and the Khangai Mountains) in the Mongolian forest-steppe ecotone and a moist site in Central Europe (the Ore Mts. in Germany). The dry sites contain natural L. sibirica stands while the moist site is a L. sibirica plantation outside its natural range. Field work was carried out during three continuous growing seasons (July/August 2010, 2011 and 2012) which started with two dry years (2010–2011) and a subsequent wet year (2012). Variations in fine root mass and morphological properties between the dry and moist years and between the dry and moist site were examined. Spatial distribution of fine roots was also investigated in the Mongolian Altai. The hydraulic architecture of L. sibirica between the dry and moist sites was compared in relation to macroclimate (precipitation, temperature) and tree productivity. We also studied how branch and coarse root hydraulic architecture and xylem conductivity, fine root biomass and necromass, and fine root morphology of L. sibirica respond to different microhabitat conditions in terms of water availability. Fluctuations in fine root biomass (FRB) between years and among study areas revealed high plasticity in fine root system of L. sibirica to different precipitation regimes. In general, increasing FRB was associated with increasing precipitation. Comparatively deep fine root distribution was found in the larch stand in the Altai Mt., which might be an adaptation to top soil drying. However, no obvious plastic change in fine root morphology was observed. Mean annual precipitation was directly related to wood anatomical and hydraulic traits across the three sites, and a higher sapwood area-specific hydraulic conductivity in both roots and branches is correlated with higher productivity in terms of annual radial stem increment. We found a significant reduction of branch hydraulic conductivity in the putatively more drought-affected forest interior in the driest Mongolian Altai site, while no branch xylem modification occurred in the moister Khangai Mountain site. Hydraulic conductivity was several times larger in roots than in branches, but root hydraulics was not influenced by stand density or mean annual precipitation. Very low fine root biomass : necromass ratios at all sites, and in the forest interior in particular, suggest that L. sibirica maintains a relatively high root conductivity by producing large conduits, which results in high root mortality due to embolism during drought. In summary, our results suggest that L. sibirica is adapted to the semi-arid climate at its southernmost distribution limit by considerable plasticity of the branch hydraulic system and a small but apparently dynamic fine root system.

Reserves of non-structural carbohydrates (NSC) stored in living cells are essential for drought tolerance of trees. However, little is known about the phenotypic plasticity of living storage compartments (SC) and their interactions with NSC reserves under changing water availability. Here, we examined adjustments of SC and NSC reserves in stems and roots of seedlings of two temperate tree species, Acer negundo L. and Betula pendula Roth., cultivated under different substrate water availability. We found that relative contents of soluble NSC, starch and total NSC increased with decreasing water availability in stems of both species, and similar tendencies were also observed in roots of A. negundo. In the roots of B. pendula, soluble NSC contents decreased along with the decreasing water availability, possibly due to phloem decoupling or NSC translocation to shoots. Despite the contrast in organ responses, NSC contents (namely starch) positively correlated with proportions of total organ SC. Individual types of SC showed markedly distinct plasticity upon decreasing water availability, suggesting that water availability changes the partitioning of organ storage capacity. We found an increasing contribution of parenchyma-rich bark to the total organ NSC storage capacity under decreasing water availability. However, xylem SC showed substantially greater plasticity than those in bark. Axial storage cells, namely living fibers in A. negundo, responded more sensitively to decreasing water availability than radial parenchyma. Our results demonstrate that drought-induced changes in carbon balance affect the organ storage capacity provided by living cells, whose proportions are sensitively coordinated along with changing NSC reserves.

Wood porosity types (non-porous, diffuse-porous, and ring-porous) reflect evolutionary gradients cha-racteristics of xylem anatomy of temperate tree species. The mechanisms linking porosity type to non-structural carbohydrate (NSC) storage strategy in stem tissues remain unclear. We conducted an experiment with 77 warm-tempe-rate tree species in the Baotianman National Nature Reserve, Henan Province. Among the examined species, there were 3 non-porous wood species, 45 diffuse-porous wood species, and 29 ring-porous wood species (including semi-ring-porous wood). We measured soluble sugars, starch, total NSC concentrations and sugars/starch ratio in bark, sapwood, and heartwood at breast height to explore the influence of wood porosity on NSC storage strategy across stem tissues (bark, sapwood and heartwood). The results showed that the types of trunk tissue and wood porosity had significant effects on the concentration of NSC and its components (soluble sugars, and starch). As for the trunk cross-section, NSC and its components exhibited an inward decline trend, with total NSC concentration in bark (6.4%) being notably higher than that in sapwood (3.2%) and heartwood (2.5%). The total NSC in various trunk tissues was dominant in the form of soluble sugars. Across the three wood porosity types, the concentrations of soluble sugars, total NSC, and sugars/starch ratio in tree bark displayed a counter-evolutionary sequence pattern (non-porous > diffuse-porous > ring-porous), while that in sapwood and heartwood increased along the evolutionary gradient. The concentrations of NSC and its components in sapwood and that in heartwood were significantly correlated for both diffuse-porous and ring-porous tree species. Soluble sugars and starch exhibited significant positive correlations in bark of non-porous and diffuse-porous trees and the three trunk tissues of ring-porous tree species. Along the evolutionary gradient of wood porosity, warm-temperate trees tended to optimize resource allocation by reducing the NSC concentration from bark to sapwood and enhancing the functional differentiation between sapwood and heartwood, reflecting the coordination between xylem anatomical structure and storage function.

ABSTRACTEucalypt saplings in north Australian savannas commonly die back, sometimes to ground level, during the 5 months of the long dry season. Water potentials are lower in saplings than large trees during the dry season, and we hypothesized that low water potentials may lead to high levels of xylem embolism and consequent death of branches and whole shoots. As the dry season progressed, hydraulic conductance of terminal branches decreased by 50% in Eucalyptus tetrodonta but not in Eucalyptus miniata saplings. Hydraulic conductance per leaf area decreased seasonally by 34% in E. tetrodonta branches. These decreases may be associated with the loss of leaves recorded from E. tetrodonta but not E. miniata branches. We modelled the effect of sequential loss of parallel resistors, representing petioles on a branch. This showed there is a non‐linear decrease in flow as basal resistors are lost, which can lead to a decrease in mean flow per resistor due to increased mean path‐length. Thus the observed loss of basal leaves, together with xylem embolism, probably contributed to the seasonal loss of hydraulic conductance in E. tetrodonta saplings. Loss of hydraulic conductance due to xylem embolism was generally low (< 15%) in both species, and the seasonal increase in embolism could not fully account for the decline in hydraulic conductance of E. tetrodonta branches. There was little evidence that branch and shoot death was caused by these levels of embolism. Developing an embolism vulnerability curve for species with long vessels is problematic and this issue is discussed.

Plant hydraulic conductivity (K) refers to the rate of water flow (kg s−1) per unit pressure drop (MPa), which drives flow through the plant organ system. It is an important eco-physiology index for measuring plant water absorption and transport capacity. A field study was conducted in the arid region of the Heihe River Basin in northwestern China, plant hydraulic conductivity was measured by high-pressure flowmeter (HPFM) to investigate the characteristics of hydraulic conductivity of typical dominant desert plants (Reaumuria soongarica M., Nitraria sphaerocarpa M., and Sympegma regelii B.) and their relationship with functional traits of leaves, stems, and roots, and explaining their adaptation strategies to desert environment from the perspective of plant organs hydraulic conductivity. The results showed that the hydraulic conductivity of the leaves and stems of R. soongarica and N. sphaerocarpa (KLA, leaf hydraulic conductivity per unit leaf area; KLW, leaf hydraulic conductivity per unit leaf weight; KSLA, stem hydraulic conductivity per unit leaf area; KSLW, stem hydraulic conductivity per unit leaf weight) were significantly lower than those of S. regelii, while their fine root (KRL, root hydraulic conductivity per unit leaf length; KRSA, root hydraulic conductivity per unit root surface area) and whole root (KTRW, whole root hydraulic conductivity per unit root weight) of hydraulic conductivity were significantly higher than those of S. regelii. In addition, KLA and KLW, KSLA and KSLW, and KRL and KRSA in three desert plants all exhibited consistent trends. Correlation analysis illustrated that the hydraulic conductivity of leaves and stems had a significantly positive correlation, but they had no significant negative correlation with the specific leaf weight (SLW, specific leaf weight). The hydraulic conductivity of fine root weight (KRW, root hydraulic conductivity per unit root weight) and specific root surface area (SRSA, specific root surface area) showed significantly positive correlation (r = 0.727, P < 0.05). The results demonstrated that the R. soongarica and N. sphaerocarpa preserved their water content through the strong leaf absorption capacity of soil water and the low water dispersion rates of leaves to adapt to the harsher arid habitat, which is more drought tolerant than S. regelii.

Grange, R. 1. 1985. Carbon partitioning in mature leaves of pepper: effects of daylength.—J. exp. Bot. 36: 1749–1759. The partitioning of recently fixed carbon has been examined in mature pepper leaves grown in 6, 10 or 14 h photoperiods at different irradiances chosen to give similar radiation integrals and in a 6 h photoperiod at the lowest of these irradiances. The partitioning of carbon into export, starch, sugars and respiration was followed over the photopenod and the subsequent night in a mature leaf. The maximum export rate during the day (approximately 18 μg C cm−2 leaf h−1) was not significantly different among the treatments. Net photosynthesis rate was directly related to irradiance; the proportion of net photosynthesis exported during the day was 33% in 6-h days and 57% in 14-h days. Leaf starch accumulation (as a proportion of net photosynthesis rate) increased slightly when plants were grown in 6-h days. The remobilization of starch and sugars at night allowed export rates to remain similar over 24 h when plants were grown in 10-h or 14-h photoperiods. Leaves grown in 6-h days showed no significant changes in export rate during the first few hours of night but exhausted their starch reserves during the night and export rates declined. Sucrose and hexose levels decreased at the onset of darkness, but did not fall below 40 μg cm−2 in plants grown in 10-h or 14-h photoperiods; when this level was reached after 3–4 h of darkness, starch breakdown began. In leaves grown in both 6-h treatments, sucrose levels fell below 40 μg cm−2 when starch reserves were depleted during the night and the export rate decreased concurrently. The results are discussed in relation to the control of export and starch metabolism in the leaf.

Hydraulics and gas exchange recover more rapidly from severe drought stress in small pot-grown grapevines than in field-grown plants

Aims Carbon and nutrient physiology of trees at their upper limits have been extensively studied, but those of shrubs at their upper limits have received much less attention. The aim of this study is to examine the general patterns of nonstructural carbohydrates (NSCs), nitrogen (N) and phosphorous (P) in shrubs at the upper limits, and to assess whether such patterns are similar to those in trees at the upper limits. Methods Across Eurasia, we measured the concentrations of soluble sugars, starch, total NSCs, N and P in leaves, branches and fine roots (<0.5 cm in diameter) of five shrub species growing at both the upper limits and lower elevations in both summer (peak growing season) and winter (dormancy season). Important Findings Neither elevation nor season had significant effects on tissue N and P concentrations, except for lower P concentrations in fine roots in winter than in summer. Total NSCs and soluble sugars in branches were significantly higher in winter than in summer. There were significant interactive effects between elevation and season for total NSCs, starch, soluble sugars and the ratio of soluble sugar to starch in fine roots, showing lower soluble sugars and starch in fine roots at the upper limits than at the lower elevations in winter but not in summer. These results suggest that the carbon physiology of roots in winter may play an important role in determining the upward distribution of shrubs, like that in the alpine tree-line trees.

In a sequence of one‐step outflow–inflow experiments, this study examines the effects of salinity on soil hydraulic properties with the goal of developing a better understanding of how to predict salinity's impacts on soils. The results of this study are compared with estimates obtained using the Hydrus‐1D model for soil water and solute transport. The model assumes a decline in hydraulic conductivity ( K ), caused by solution chemistry, is constant over the entire hydraulic conductivity function [ K (θ)] and equal to a constant reduction factor established at saturation. In contrast with the model assumptions, the results of this study show an increase in the solution chemistry's effect on K with decreasing soil moisture (θ). These findings demonstrate that current models that exclude solution chemistry, or models that assume a constant K reduction for the entire K (θ) function, may be overestimating drainage, recharge, or bare soil evaporation. This research used undisturbed soil cores and simulated irrigation water with decreasing electrical conductivity (EC) from 20 to 0.25 dS m –1 and sodium adsorption ratio (SAR) held constant. Changes in K (θ) were observed only after dropping below an EC of 1.5 dS m –1 , which is consistent with past studies. At reduced soil water contents, however, K (θ) decreased by up to five orders of magnitude relative to native soils. A second experiment was conducted that increased the SAR from 4 to ≥20, held EC constant at 0.65 dS m –1 , and displayed only moderate reductions in K (θ), although K was reduced proportionally more at reduced soil water contents. Therefore, based on the observed results, an integrated K (EC,θ) and K (SAR,θ) model is needed to better predict salinity effects on soil hydraulic properties and is increasingly important as freshwater resources decline.