Needle Nitrogen Content Research Articles

Brown bear digging decreases tree growth: Implication for ecological role of top predators in anthropogenic landscapes.

Large carnivores have recently increased in number and recolonized in human-dominated landscapes; however, their ecological roles in these landscapes have not been well studied. In the Shiretoko World Heritage (SWH) site, brown bears have recolonized a previously abandoned mosaic landscape of natural forests and conifer plantations after land abandonment. We previously reported that the bears had recently begun to dig for cicada nymphs in association with the creation of larch plantations. As a result, this digging activity decreased soil nutrients. To deepen the understanding of the novel ecological role of brown bears within human-modified landscapes, we examined the impacts of brown bear digging on the growth of larch trees. We found that brown bear digging decreased fine root biomass of larch, soil water, and nitrogen availability. Brown bear digging negatively affected needle nitrogen content, but not carbon isotope ratios, a water stress index of trees. Tree ring data suggest that digging negatively affected the radial growth of larches. The results imply that digging decreases tree growth due to limited soil nitrogen uptake. Our findings indicate that the ecological roles of large carnivores may differ between natural and anthropogenic landscapes.

Arctic Plant Responses to Summer Climates and Flooding Events: A Study of Carbon and Nitrogen‐Related Larch Growth and Ecosystem Parameters in Northeastern Siberia

AbstractBuilt upon a 5‐year field investigation and a 13‐year satellite data set, this study examines the intricate interrelationships among ecophysiological parameters of Larix gmelinii trees and the prevailing ecosystem, climatic, and environmental factors present in the Indigirka lowlands of northeastern Siberia. It identified spatial‐temporal patterns in July needle nitrogen (N) content as an indicator of N availability from 2009 to 2013. Needle N content (%) revealed distinct yearly values: 2012 (1.31 ± 0.24), 2013 (1.67 ± 0.39), 2009 (1.72 ± 0.15), 2011 (1.84 ± 0.34), and 2010 (2.08 ± 0.25). Positive correlations were found between ecosystem and larch parameters, as well as between September temperature or February/May precipitation and subsequent July ecosystem productivity. Soil moisture (SM) primarily influences N availability across sites, with higher SM levels reducing N availability. However, July air temperature (AT) is the primary driver of interannual N availability changes, with higher temperatures enhancing N availability. Larch photosynthesis is mainly influenced by solar radiation (SR), temperature, N availability, and SM. Annual fluctuations in SR positively impact larch photosynthesis, while high temperatures or wetting events impose limitations on photosynthesis, even if N availability has increased. Consequently, a moderate correlation exists between N availability and photosynthesis across various sites and years (r = 0.422, P = 0.133, n = 14). In summary, this research provides valuable insights into climatic and environmental impacts on larch trees and ecosystems, emphasizing the significance of SM, AT, and SR for predicting future growth patterns of larch.

Large investment of stored nitrogen and phosphorus in female cones is consistent with infrequent reproduction events of Pinus koraiensis, a high value woody oil crop in Northeast Asia.

Pinus koraiensis is famous for its high-quality timber production all the way and is much more famous for its high value health-care nut oil production potential since 1990's, but the less understanding of its reproduction biology seriously hindered its nut productivity increase. Exploring the effects of reproduction on nutrient uptake, allocation and storage help to understand and modify reproduction patterns in masting species and high nut yield cultivar selection and breeding. Here, we compared seasonality in growth and in nitrogen ([N]) and phosphorus ([P]) concentrations in needles, branches and cones of reproductive (cone-bearing) and vegetative branches (having no cones) of P. koraiensis during a masting year. The growth of one- and two-year-old reproductive branches was significantly higher than that of vegetative branches. Needle, phloem and xylem [N] and [P] were lower in reproductive branches than in vegetative branches, although the extent and significance of the differences between branch types varied across dates. [N] and [P] in most tissues were high in spring, decreased during summer, and then recovered by the end of the growing season. Overall, [N] and [P] were highest in needles, lowest in the xylem and intermediate in the phloem. More than half of the N (73.5%) and P (51.6%) content in reproductive branches were allocated to cones. There was a positive correlation between cone number and N and P content in needles (R2 = 0.64, R2 = 0.73) and twigs (R2 = 0.65, R2 = 0.62) of two-year-old reproductive branches. High nutrient sink strength of cones and vegetative tissues of reproductive branches suggested that customized fertilization practices can help improve crop yield in Pinus koraiensis.

Trend analysis of long-lasting air temperature and precipitation time series in a mountainous fir forest in central Greece. Implications for nitrogen uptake by plants

<p>In the present work meteorological data, concerning temperature and precipitation in a mountainous Bulgarian fir (Abies borisii-regis Mattf.) forest, was analyzed in order to find trends for the period 1973-2019. The results showed that temperature values on annual and seasonal base, especially in summers, showed a significant increasing trend. In addition, the number of days with particularly high temperatures increased together with the daily temperature range. With regard to the rain height an increasing trend was found significant at the 0.1 probability level. The nitrogen concentration in current year needles of the Bulgarian fir measured in the period 1997-2019 showed an increasing trend. This fact could be a sign of increased mineralization rates of organic matter as a response to the temperature increase.</p>

Effects of snow manipulation on larch trees in the taiga forest ecosystem in northeastern Siberia

Changes in winter precipitation (snow) may greatly affect vegetation by altering hydrological and biochemical processes. To understand the effects of changing snow cover depth and melt timing on the taiga forest ecosystem, a snow manipulation experiment was conducted in December 2015 at the Spasskaya Pad experimental larch forest in Eastern Siberia, which is characterized by a continental dry climate with extreme cold winters and hot summers. Variables including soil temperature and moisture, oxygen and hydrogen isotope ratios of soil moisture and stem water, foliar nitrogen and carbon contents and their isotopes, phenology, and soil inorganic nitrogen were observed at snow removal (SNOW−), snow addition (SNOW+), and CONTROL plots. After snow manipulation, the soil temperature at the SNOW− plot decreased significantly compared to the CONTROL and SNOW+ plots. At SNOW− plot, snowmelt was earlier and soil temperature was higher than at other plots during spring because of low soil moisture caused by less snowmelt water. Despite the earlier snowmelt and higher soil temperature in the SNOW− plot in the early growing season, needle elongation was delayed. Leaf chemistry also differed between the CONTROL and SNOW− plots. The needle nitrogen content in the SNOW− plot was lower in the middle of July, whereas no difference was observed among the three plots in August. The soil inorganic nitrogen content of each plot corresponded to these results. The amount of soil ammonium was lower in the SNOW− plot than in the other plots at the end of July, however, once production started in August, the amount of soil ammonium in the three plots was comparable. Extremely low soil temperatures in winter and freeze–thaw cycles in spring and dry soil condition in spring and early summer at the SNOW− plot may have influenced the phenology and production of soil inorganic nitrogen.

Recovery of silver fir (Abies alba Mill.) seedlings from ungulate browsing mirrors soil nitrogen availability.

Abies alba (Mill.) has a high potential for mitigating climate change in European mountain forests; yet, its natural regeneration is severely limited by ungulate browsing. Here, we simulated browsing in a common garden experiment to study growth and physiological traits, measured from bulk needles, using a randomized block design with two levels of browsing severity and seedlings originating from 19 populations across Switzerland. Genetic factors explained most variation in growth (on average, 51.5%) and physiological traits (10.2%) under control conditions, while heavy browsing considerably reduced the genetic effects on growth (to 30%), but doubled those on physiological traits related to carbon storage. While browsing reduced seedling height, it also lowered seedling water-use efficiency (decreased $\delta ^{13}$C) and increased their $\delta ^{15}$N. Different populations reacted differently to browsing stress, and for seedling height, starch concentration and $\delta ^{15}$N, population differences appeared to be the result of natural selection. First, we found that populations originating from the warmest regions recovered the fastest from browsing stress, and they did so by mobilizing starch from their needles, which suggests a genetic underpinning for a growth-storage trade-off across populations. Second, we found that seedlings originating from mountain populations growing on steep slopes had a higher $\delta ^{15}$N in the common garden than those originating from flat areas, indicating that they have been selected to grow on N-poor, potentially drained, soils. This finding was corroborated by the fact that nitrogen concentration in adult needles was lower on steep slopes than on flat ground, strongly indicating that steep slopes are the most N-poor environments. These results suggest that adaptation to climate and soil nitrogen availability, as well as ungulate browsing pressure, co-determine the regeneration and range limit of silver fir.

Geography and climate regime affect needle stoichiometry character of Pinus bungeana across China

Leaf stoichiometric pattern and its potential determinants are of great significance in the modeling nutrient cycling, detecting ecosystem stability and predicting the response of ecosystem to global changes. To reveal the stoichiometric characteristic of Pinus bungeana, we analyzed the needle nitrogen (N) and phosphorous (P) concentrations of 76 needle samples from 15 sampling sites throughout the country, and took the local geographical and climatic variables into consideration. The generalized linear mixed model (GLMM) and principal component analysis (PCA) were used to explore the effects of environmental factors on needle stoichiometry. This study revealed that both the concentrations and ratio of N, P content showed significant differences in needles of P. bungeana among different sampling sites. The needle N, P concentrations and N: P ratio showed quadratic curve correlation with climatic factors. The PCA indicated that all variables can be explained by three principal components, namely, temperature, N and P, which were very important factors affecting the geographical distribution of P. bungeana. Among the three components, N is most crucial factor limiting the growth and metabolism of P. bungeana. In summary, our study will help to elucidate the patterns of needle N and P concentrations in needles of P. bungeana, thus revealing the causes and driving forces for its natural region.

Effect of forest stand type on host plant quality and direct and indirect effects on pine sawfly performance

Abstract Observed lower levels of herbivory in mixed compared with monoculture stands have been hypothesized to depend on top‐down forces, through higher predation pressure by natural enemies or through bottom‐up mechanisms through plant quality effects on herbivore performance. In this study, we compared the performance measured as host plant induced mortality, cocoon weight, and predation mortality of the European pine sawfly Neodiprion sertifer (Geoffroy) (Hymenoptera, Diprionidae) in mixed and monoculture forest stands. We did not observe a difference in host plant induced mortality, cocoon weight, or predation mortality between mixed and monoculture forest stands. We did find an effect of local conditions around each experimental tree on pine sawfly performance. For example, the nitrogen content of pine needles is negatively affected by the proportion of pine around the experimental tree, which in turn increases the survival of sawfly larvae. The results suggest that local conditions around individual trees are more important for the performance of the European pine sawfly than stand type, i.e. mixed or monoculture plant stands. We conclude that the ongoing trend for diversification within commercial forestry calls for more research where the effects of both bottom‐up and top‐down effects are studied at several spatial scales.

Heterophyllous Shoots of Japanese Larch Trees: The Seasonal and Yearly Variation in CO2 Assimilation Capacity of the Canopy Top with Changing Environment.

Japanese larch (Larix kaempferi = L. leptolepis) is often characterized by its high growth rate with heterophyllous shoots, but the functional differences of heterophyllous shoots still remain unclear. Recently, abrupt high temperature and drought during spring induced high photosynthetic rate via change in leaf morphology of the deciduous habit. In order to reveal the photosynthetic characteristics of both short and long-shoot needles of sunny canopy of the larch trees using a canopy tower, we calculated the seasonal change of gas exchange characters and leaf mass per area (LMA) and foliar nitrogen content (N) of heterophyllous needles: short and long-shoot needles over 3 years. No marked difference in light-saturated photosynthetic rates (Psat) was observed between short and long shoots after leaf maturation to yellowing, although the difference was obvious in a specific year, which only shows that seasonal change in temperature and soil moisture determines the in situ photosynthetic capacity of needles. The large annual and seasonal variations in Psat in both shoots were found to be mainly determined by climatic variations, while shoot types determined the strategy of their photosynthetic N utilization as well as the stomatal regulation.

Biochar amendment increases tree growth in nutrient-poor, young Scots pine stands in Finland

Biochar is charred material formed by the pyrolysis of organic matter. The addition of biochar to soil may offer a chance to mitigate climate change by increasing soil carbon stocks, improving soil fertility and enhancing plant growth. Vast majority of biochar studies are conducted in agricultural soils, and field experiments studying the effect of biochar on tree growth in boreal forests are lacking. We applied spruce biochar amendments of 5 and 10 Mg ha−1 to the soil surface in young boreal Scots pine (Pinus sylvestris L.) forests in Southern Finland and studied the responses in tree growth and needle nitrogen (N) and 15N concentrations during the first three years after treatment. The biochar amendment of 10 Mg ha−1 increased the diameter growth of dominant trees significantly, on average by 1 mm year−1, which corresponded to 25% increase compared to control during the three years study period. The positive growth responses were less pronounced in height than in diameter growth. The biochar amendment of 5 Mg ha−1 increased the height growth of dominant trees by 0.16 m or 12% compared to the control during three years. Biochar amendments did not affect N and 15N concentrations in needles. The results suggest that wood biochar amendment can be a climate-friendly method to increase tree biomass production in nutrient poor, xeric, young Scots pine forests.

Can needle nitrogen content explain the interspecific difference in ozone sensitivities of photosynthesis between Japanese larch (Larix kaempferi) and Sakhalin fir (Abies sachalinensis)?

Although deciduous and evergreen conifers are expected to utilize needle nitrogen differently, their interspecific differences in the ozone sensitivity remain unclear. We compared the growth and physiological responses to elevated O3 concentrations in deciduous Japanese larch (Larix kaempferi) and evergreen Sakhalin fir (Abies sachalinensis) seedlings. Seedlings of both species were exposed to 62.5 ± 1.95 nmol(O3) mol-1 during the day time in open-top chambers. Elevated O3 had no effect on height or diameter growth in either of the species but suppressed photosynthetic parameters. A decline in photosynthetic nitrogen-use efficiency was observed in both species. However, needle N content in larch was decreased by O3, whereas it was not decreased in fir. Needle N responses to O3 were different between larch and fir seedlings, indicating that the O3 sensitivity of photosynthesis could vary depending on needle N dynamics. We also observed a higher correlation between photosynthesis and needle N even under O3 exposure, particularly in the mass-based relationship with higher accuracy. The result indicated that mass-based needle N could explain difference in O3 sensitivity of photosynthesis between the different leaf habits in larch and fir seedlings.

Changes in Physiological Functioning in Loblolly Pine Trees Undergoing Bark Beetle Simulated Mortality

AbstractThe southeastern United States contains extensive loblolly pine (Pinus taeda L.) plantations at risk from bark beetle damage that can change ecosystem biogeochemical cycling. Functional changes in tree physiology have the potential to occur before visual evidence of mortality making them difficult to incorporate into ecosystem models. Therefore, we girdled loblolly pines to simulate bark beetle damage and measured physiological processes including sapflow rates, photosynthesis, litterfall, and needle nitrogen concentrations to determine the physiological changes occurring in trees undergoing mortality. We found that the girdling treatment took 5 months to significantly reduce sapflow rates but visual crown mortality occurred more than one year after girdling. Girdled pines had approximately 2.5 times lower water use than control pines and exhibited greater susceptibility to atmospheric water stress. Girdled and control pines had similar needle nitrogen concentrations and photosynthetic rates measured during the mortality year. However, more litterfall with higher nitrogen concentrations occurred in the mortality year than in the previous year, resulting in redistribution of carbon and nitrogen in the ecosystem. Overall, these data allow for better quantification of the effects of background disturbance levels and individual tree mortality on water, carbon, and nitrogen cycling within a loblolly pine ecosystem.

A major trade-off between structural and photosynthetic investments operative across plant and needle ages in three Mediterranean pines.

Pine (Pinus) species exhibit extensive variation in needle shape and size between juvenile (primary) and adult (secondary) needles (heteroblasty), but few studies have quantified the changes in needle morphological, anatomical and chemical traits upon juvenile-to-adult transition. Mediterranean pines keep juvenile needles longer than most other pines, implying that juvenile needles play a particularly significant role in seedling and sapling establishment in this environment. We studied needle anatomical, morphological and chemical characteristics in juvenile and different-aged adult needles in Mediterranean pines Pinus halepensis Mill., Pinus pinea L. and Pinus nigra J. F. Arnold subsp. salzmannii (Dunal) Franco hypothesizing that needle anatomical modifications upon juvenile-to-adult transition lead to a trade-off between investments in support and photosynthetic tissues, and that analogous changes occur with needle aging albeit to a lower degree. Compared with adult needles, juvenile needles of all species were narrower with 1.6- to 2.4-fold lower leaf dry mass per unit area, and had ~1.4-fold thinner cell walls, but needle nitrogen content per dry mass was similar among plant ages. Juvenile needles also had ~1.5-fold greater mesophyll volume fraction, ~3-fold greater chloroplast volume fraction and ~1.7-fold greater chloroplast exposed to mesophyll exposed surface area ratio, suggesting overall greater photosynthetic activity. Changes in needle traits were similar in aging adult needles, but the magnitude was generally less than the changes upon juvenile to adult transition. In adult needles, the fraction in support tissues scaled positively with known ranking of species tolerance of drought (P. halepensis > P. pinea > P. nigra). Across all species, and needle and plant ages, a negative correlation between volume fractions of mesophyll and structural tissues was observed, manifesting a trade-off between biomass investments in different needle functions. These results demonstrate that within the broad trade-off, juvenile and adult needle morphophysiotypes are separated by varying investments in support and photosynthetic functions. We suggest that the ecological advantage of the juvenile morphophysiotype is maximization of carbon gain of establishing saplings, while adult needle physiognomy enhances environmental stress tolerance of established plants.

Decomposition of harvest residue needles of different needle ages in a Chinese fir (Cunninghamia lanceolata) plantation

The canopies in evergreen coniferous plantations are often composed of various-aged needles. Plantation management, such as thinning, produced abundant harvest residue, including needles with different needle ages. However, little attention was paid to the effect of needle age on decomposition, although the needle chemical properties varied substantially with leaf ages. A field experiment was conducted for 3 years to investigate the decomposition of harvest residue needles at different needle ages, and determine the main controlling factors in different stages of decomposition in a Chinese fir plantation. We found that the initial decomposition rate varied 5-fold among different needle ages in a Chinese fir plantation. Litter quality controlled the overall litter decomposition rate, especially the initial decomposition rate. Needle nitrogen content was positively correlated to decomposition rate during the early stage of decomposition. However, it was negatively correlated to decomposition rate during the later stage of decomposition. The contents of needle tannins increased the asymptotic mass remaining (A, proportion of mass remaining at which decomposition approaches zero, i.e., the fraction of slowly decomposing litter). We also found that the initial litter decomposition rates in soil fauna presence were significantly higher than those in soil fauna absence across different needle ages. Moreover, the effect of soil fauna on initial litter decomposition is independent of needle quality. Our results suggest that needle age and plant secondary metabolites should be considered to understand the response of litter decomposition and nutrient cycling to management practices, such as thinning, in conifer plantations.

Below‐ and above‐ground effects of deadwood and termites in plantation forests

AbstractDeadwood is an important legacy structure in managed forests, providing continuity in shelter and resource availability for many organisms and acting as a vehicle by which nutrients can be passed from one stand to the next following a harvest. Despite existing at the interface between below‐ and above‐ground systems, however, much remains unknown about the role woody debris plays in linking these zones. Moreover, it remains untested whether the accelerative effects of wood‐feeding insects on wood decomposition influence tree growth or nutritional status in forests. In this study, we added different quantities of pine logs to the bases of saplings in two‐year‐old loblolly pine (Pinus taedaL.) plantations in Mississippi,USA. We included a treatment in which subterranean termites (Blattodea: Rhinotermitidae:Reticulitermes) were excluded from logs to determine how these insects affect the release of nutrients from wood and, in turn, tree growth. After 51 months of decomposition, we quantified below‐ground effects by measuring microbial biomass, plant‐available forms of N, and ectomycorrhizal diversity associated with fine tree roots. Meanwhile, above‐ground measurements focused on the elemental concentrations in decomposing wood either protected or unprotected from termites and tree metrics related to growth and nutrient status. We found additions of wood to significantly increase nitrate and potential net nitrification relative to reference treatments but detected no significant effects on tree growth, needle nitrogen concentrations, or ectomycorrhizal diversity. Soil nitrate and potential net nitrification were higher under protected vs. unprotected logs, and plant‐available forms of N were mostly more abundant short distances away from both protected and unprotected logs than directly under them. The wood of logs protected from termites had significantly lower concentrations of most elements compared to that of unprotected logs, largely due to the large amounts of soil imported into unprotected logs by termites. Termite exclusion had no measurable effect on tree growth, nutritional status, or ectomycorrhizal diversity, however. Our findings indicate that deadwood and termites both contribute to the spatial heterogeneity of soil properties but may have limited short‐term local effects on tree growth. Longer‐term studies and studies on less fertile sites are needed.

Needle age dependence of photosynthesis along a light gradient within an Abies alba crown

Age and shade effects on needle structure and photosynthesis were determined within a lower part of Abies alba crown along a horizontal increasing gradient of branches self-shading. It was hypothesized that a decrease in net CO2 assimilation rate with increasing needles’ age would be related to: (1) structural age-related changes in needles, (2) reduction of stomatal conductance, (3) nitrogen translocation from old to young needles, and (4) decrease in efficiency of photochemical processes. Leaf mass-to-area ratio increased non-linearly with needle age. In a needle cross section, distance between the vascular bundles decreased, and height of palisade parenchyma cells increased with age. The structural changes observed in our study might lead to an increase in internal resistance to CO2 with greater needle age. Total needle nitrogen concentration linearly decreased with age due to dilution and/or translocation to younger needles. When expressed per needle area, nitrogen content was reduced in 6-year-old needles compared with younger ones. Net CO2 assimilation rate per needle area decayed and was accompanied by a decrease in transpiration and water and photosynthetic nitrogen use efficiency. Old needles maintained high photochemical efficiency which compensated to some extent for light deficit in their micro-light environments. Our results have suggested that there is a mechanism controlling the relation between efficiency of light and dark photosynthetic processes along the needle age and shade gradient in A. alba crown.

Impact of Arceuthobium sichuanense infection on needles and current-year shoots of Picea crassifolia and Picea purpurea in Qinghai Province, China

Arceuthobium sichuanense is an aerial parasitic plant (dwarf mistletoe) which causes severe damage to spruce forests on the Qinghai-Tibet Plateau. Picea crassifolia and Picea purpurea, two main host species of A. sichuanense, have different growth characteristics and elevational distributions. The effects of A. sichuanense infection on P. crassifolia and P. purpurea trees were evaluated by examining needle and current-year shoot morphology, needle water use efficiency and needle nitrogen concentration, with 30 needle samples, 30 current-year shoot samples and 10 A. sichuanense aerial shoot samples for each host species. The most apparent effects were significant reductions in both needle size and current-year shoot length. The high degree of correlation in foliar δ 15N values between the dwarf mistletoe and its host trees indicated that nitrogen in the dwarf mistletoe was derived entirely from its host. Percent reductions in needle and current-year shoot length were smaller for P. purpurea than for P. crassifolia, possibly suggesting that P. purpurea exhibits a lesser capacity to accommodate the parasitic effects of mistletoe infection by adjusting the growth of needles and current-year shoots. The reductions in needle nitrogen concentration and δ 13C values were lower in P. purpurea than in P. crassifolia, indicating that P. purpurea suffered more nitrogen and water stresses and exhibited lower vigor compared to P. crassifolia. Our results demonstrated that P. purpurea may suffer more negative effects induced by dwarf mistletoe infections at needle and branch levels than P. crassifolia.

Increased Needle Nitrogen Contents Did Not Improve Shoot Photosynthetic Performance of Mature Nitrogen-Poor Scots Pine Trees.

Numerous studies have shown that temperate and boreal forests are limited by nitrogen (N) availability. However, few studies have provided a detailed account of how carbon (C) acquisition of such forests reacts to increasing N supply. We combined measurements of needle-scale biochemical photosynthetic capacities and continuous observations of shoot-scale photosynthetic performance from several canopy positions with simple mechanistic modeling to evaluate the photosynthetic responses of mature N-poor boreal Pinus sylvestris to N fertilization. The measurements were carried out in August 2013 on 90-year-old pine trees growing at Rosinedalsheden research site in northern Sweden. In spite of a nearly doubling of needle N content in response to the fertilization, no effect on the long-term shoot-scale C uptake was recorded. This lack of N-effect was due to strong light limitation of photosynthesis in all investigated canopy positions. The effect of greater N availability on needle photosynthetic capacities was also constrained by development of foliar phosphorus (P) deficiency following N addition. Thus, P deficiency and accumulation of N in arginine appeared to contribute toward lower shoot-scale nitrogen-use efficiency in the fertilized trees, thereby additionally constraining tree-scale responses to increasing N availability. On the whole our study suggests that the C uptake response of the studied N-poor boreal P. sylvestris stand to enhanced N availability is constrained by the efficiency with which the additional N is utilized. This efficiency, in turn, depends on the ability of the trees to use the greater N availability for additional light capture. For stands that have not reached canopy closure, increase in leaf area following N fertilization would be the most effective way for improving light capture and C uptake while for mature stands an increased leaf area may have a rather limited effect on light capture owing to increased self-shading. This raises the question if N limitation in boreal forests acts primarily by constraining growth of young stands while the commonly recorded increase in stem growth of mature stands following N addition is primarily the result of altered allocation and only to a limited extent the result of increased stand C-capture.

Altitudinal trends in δ13C value, stomatal density and nitrogen content of Pinus tabuliformis needles on the southern slope of the middle Qinling Mountains, China

In this study, a coniferous tree species (Pinus tabuliformis Carr.) was investigated at a moderate-altitude mountainous terrain on the southern slope of the middle Qinling Mountains (QLM) to detect the trends in carbon isotope ratio (d13C), leaf nitrogen content (LNC) and stomatal density (SD) with altitude variation in north-subtropical humid mountain climate zone of China. The results showed that LNC and SD both significantly increased linearly along the altitudinal gradient ranging from 1000 to 2200 m, whereas leaf d13C exhibited a significantly negative correlation with the altitude. Such a correlation pattern differs obviously from that obtained in offshore low-altitude humid environment or inland high-altitude semi-arid environment, suggesting that the pattern of increasing d13C with the altitude cannot be generalized. The negative correlation between d13C and altitude might be attributed mainly to the strengthening of carbon isotope fractionation in plants caused by increasing precipitation with altitude. Furthermore, there was a remarkable negative correlation between leaf d13C and LNC. One possible reason was that increasing precipitation that operates to increase isotopic discrimination with altitude overtook the LNC in determining the sign of leaf d13C. The significant negative correlation between leaf d13C and SD over altitudes was also found in the present study, indicating that increases in SD with altitude would reduce, rather than enhance plant d13C values.

Photosynthetic and growth responses of Pinus koraiensis seedlings to canopy openness: Implications for the restoration of mixed-broadleaved Korean pine forests

The mixed-broadleaved Korean pine (Pinus koraiensis Sieb. et Zucc.) forest (MBKPF) is a zonal climax ranging from Northeast China to the Far East region of Russia. However, since the 1950s, most of the MBKPFs in Northeast China have become the mixed broadleaved secondary forests due to the heavy forest exploitation by timber harvesting. The natural regeneration of Korean pine is generally poor because of the light limitation in Northeast China. This study was conducted to understand how Korean pine seedlings adapt to different canopy openness (CO) and what would be the minimum CO for its establishment and growth in the natural forest stands. In order to test the hypothesis that Korean pine seedlings have acclimated to low light regimes, the responses of photosynthetic, morphological and growth parameters to light environments were examined in Korean pine seedlings that were field-grown in secondary forests. The results can provide the references for the restoration of MBKPFs in Northeast China. Three-year-old Korean pine seedlings were under-planted in secondary forest stands with three levels of CO at 30%, 15% and 7%, which were created by modifying the branches of the secondary forest stands. After one year of acclimation under the secondary forest stands, the variations in the physiological and morphological characteristics of the needles as well as the growth parameters were measured. The maximum net photosynthesis (Amax), dark respiration (Rd), needle chlorophyll (Chl) contents and total needle nitrogen (N) contents all responded positively to the increased CO. The total non-structural carbohydrate (TNC), Amax, Rd, and N contents in the 7% stand were significantly lower than those in the 15% and 30% stands (p<0.05). The capture efficiency of excitation energy by open Photosystem II (PSII) reaction centres (Fv′/Fm′) in the 7% stand was the lowest and was significantly different from those in the 15% and 30% stands (p<0.05), although no significant differences were found between 15% and 30% CO (p>0.05). These results indicated that 7% CO seedlings were experiencing some degree of low light stress. The leaf mass area (LMA) and height growth decreased as the CO decreased from 30% to 15% to 7%, while the lateral shoot growth increased. Therefore, needle morphology, tree height and crown growth of Korean pine seedlings displayed a large plasticity in response to variations in light intensities. These findings suggest that Korean pine seedlings can strongly acclimate in the forest canopy by adjusting physiological traits that are associated with light energy distribution in the PSII and by adjusting the morphological features of their needles. These results suggest that the minimum canopy openness required for the survival of P. koraiensis seedlings (4 years old) grown under a canopy should be more than 7%.