Qingmin Han

Fruit production in three masting tree species does not rely on stored carbon reserves

Fruiting is typically considered to massively burden the seasonal carbon budget of trees. The cost of reproduction has therefore been suggested as a proximate factor explaining observed mast-fruiting patterns. Here, we used a large-scale, continuous 13C labeling of mature, deciduous trees in a temperate Swiss forest to investigate to what extent fruit formation in three species with masting reproduction behavior (Carpinus betulus, Fagus sylvatica, Quercus petraea) relies on the import of stored carbon reserves. Using a free-air CO2 enrichment system, we exposed trees to 13C-depleted CO2 during 8 consecutive years. By the end of this experiment, carbon reserve pools had significantly lower δ13C values compared to control trees. δ13C analysis of new biomass during the first season after termination of the CO2 enrichment allowed us to distinguish the sources of built-in carbon (old carbon reserves vs. current assimilates). Flowers and expanding leaves carried a significant 13C label from old carbon stores. In contrast, fruits and vegetative infructescence tissues were exclusively produced from current, unlabeled photoassimilates in all three species, including F. sylvatica, which had a strong masting season. Analyses of δ13C in purified starch from xylem of fruit-bearing shoots revealed a complete turn-over of starch during the season, likely due to its usage for bud break. This study is the first to directly demonstrate that fruiting is independent from old carbon reserves in masting trees, with significant implications for mechanistic models that explain mast seeding.

Leaf traits, shoot growth and seed production in mature Fagus sylvatica trees after 8 years of CO2 enrichment

BACKGROUND AND AIMS Masting, i.e. synchronous but highly variable interannual seed production, is a strong sink for carbon and nutrients. It may, therefore, compete with vegetative growth. It is currently unknown whether increased atmospheric CO(2) concentrations will affect the carbon balance (or that of other nutrients) between reproduction and vegetative growth of forest species. In this study, reproduction and vegetative growth of shoots of mature beech (Fagus sylvatica) trees grown at ambient and elevated atmospheric CO(2) concentrations were quantified. It was hypothesized that within a shoot, fruiting has a negative effect on vegetative growth, and that this effect is ameliorated at increased CO(2) concentrations. METHODS Reproduction and its competition with leaf and shoot production were examined during two masting events (in 2007 and 2009) in F. sylvatica trees that had been exposed to either ambient or elevated CO(2) concentrations (530 µmol mol(-1)) for eight consecutive years, between 2000 and 2008. KEY RESULTS The number of leaves per shoot and the length of terminal shoots was smaller or shorter in the two masting years compared with the one non-masting year (2008) investigated, but they were unaffected by elevated CO(2) concentrations. The dry mass of terminal shoots was approx. 2-fold lower in the masting year (2007) than in the non-masting year in trees growing at ambient CO(2) concentrations, but this decline was not observed in trees exposed to elevated CO(2) concentrations. In both the CO(2) treatments, fruiting significantly decreased nitrogen concentration by 25 % in leaves and xylem tissue of 1- to 3-year-old branches in 2009. CONCLUSIONS Our findings indicate that there is competition for resources between reproduction and shoot growth. Elevated CO(2) concentrations reduced this competition, indicating effects on the balance of resource allocation between reproduction and vegetative growth in shoots with rising atmospheric CO(2) concentrations.

Effect of stem radial growth on seasonal and spatial variations in stem CO2 efflux of Chamaecyparis obtusa

Key messageBesides stem temperature, seasonality and vertical gradient in stem diameter growth strongly affect both seasonal and vertical variations in stem CO2efflux.AbstractStem CO2 efflux (Es) is known to vary seasonally and vertically along tree stems. We measured Es at various heights in a 50-year-old hinoki cypress [Chamaecyparis obtusa (Sieb. et Zucc) Endl.] stand over 2 consecutive years in Central Japan. Effects of stem temperature (Ts), daily stem diameter increment (Di), and difference in vapor pressure deficit between nighttime and daytime (VPDdiff) on seasonal variation in daily Es were examined by a generalized linear model (GLM). Daily Es exhibited a substantial seasonal variation, which was mostly affected by Ts and Di. Any effect of VPDdiff, which was assumed to be an index of daily sap flow rate, was not detected. The GLM prediction of daily Es from the combination of Ts and Di showed good agreement with the observed seasonal trend in daily Es. These results suggest that, in addition to stem temperature as the important environmental factor, stem radial growth is a significant phenological factor influencing seasonal variation in Es. Additionally, the intercept of GLM, which indicates the basal daily efflux that is independent of Ts and Di, was closely related to the annual diameter increment. Furthermore, a strong relationship was found between annual Es and annual diameter increment. These findings suggest that variation in stem diameter growth along stems is considerably responsible for the observed vertical variation in Es. Therefore, stem radial growth can affect both seasonal and spatial variations in stem CO2 efflux.

Photosynthesis and growth of Thujopsis dolabrata var. hondai seedlings in the understory of trees with various phenologies

We have studied the photosynthetic production and growth of Thujopsis dolabrata var. hondai (hiba) seedlings under typical light conditions found in mixed forests, including constant shade, phenological gaps under broad-leaved trees with different lengths of foliation period, and in an open plot. Leaves sampled from the open plot had significantly higher rates of light-saturated gross photosynthesis and dark respiration, and a significantly lower specific leaf area than leaves from the other plots. The relative growth rate of whole plant biomass was significantly higher in plots that received larger amounts of light. The variations in these properties under the various light conditions are considered to reflect the ability of hiba seedlings to adapt to available light. In the late autumn, leaves sampled from the open and from the gap under deciduous trees indicated photoinhibition. However, the estimated value of monthly net photosynthesis of hiba seedlings was greater in these plots, indicating that the benefit of a greater light intensity for photosynthesis seems to be larger than the detriment of photoinhibition. The seasonal pattern of photosynthesis by understory hiba seedlings was affected by the phenology of canopy trees. Light availability under the canopy of deciduous trees associated with phenological gaps helped hiba seedlings to tolerate the relatively dark conditions during the subsequent foliation period. These results for reaction to the light regime and for the phenology of hiba seedlings are practicable for hiba forest management.

Drought-dependent Responses of Photosynthesis,Transpiration and Water Use Efficiency of Japanese Cypress and Japanese Red Pine Seedlings

This study was conducted to investigate the potential for modifying drought tolerance of Japanese cypress (Chamaecyparis obtusa Endl.) and Japanese red pine (Pinus densiflora Sieb. et Zucc.). Three-year-old seedlings were controlled for five-months at three different soil water potentials ({ie73-1}). Japanese cypress exposed to high {ie73-2} was able to maintain higher photosynthesis (Phn), transpiration (Tr) and stomatal conductance to H2O (gH2O) in comparison to low {ie73-3} pretreatments, however, there was no significant difference in Phn for Japanese red pine. Soil water potential at the threshold from the maximum to limited Phn was higher in high {ie73-4} pretreatments than in low {ie73-5} pretreatments. Net photosynthesis, Tr and gH2O decreased more rapidly in high {ie73-6} pretreatments than in low {ie73-7} pretreatments. Transpiration decreased more significantly than Phn, thus, resulted in increased water use efficiency. All these factors are likely to result in significant improvements in the drought tolerance. Japanese red pine seems more drought-tolerant than Japanese cypress. Japanese cypress is suitable to soil of −0.05 MPa water potential, and Japanese red pine is suitable to −0.16 MPa and even dryer soils.

Influence of reproduction on nitrogen uptake and allocation to new organs in Fagus crenata

The contributions of the internal nitrogen (N) cycle and N uptake from soil to growth in mature trees remain poorly understood, especially during reproduction. In order to elucidate how reproduction affects N uptake, allocation and remobilization, we applied pulse 15N labelling to three fruiting (F) and three non-fruiting (NF) Fagus crenata Blume trees after the leaves were fully unfurled. Three-year-old branches were sampled from upper crowns at about 2 week intervals until leaf fall. 15N content per organ dry mass (15Nexcess) and N concentration in all new shoot organs were determined. Fruiting led to greater 15Nexcess uptake from the soil during the first month following application. Cupules absorbed the highest fraction of 15Nexcess initially and nuts contained about half the 15Nexcess at the end of the growing season. Biomass of reproductive organs represented up to 70% of new shoot growth in F trees. This fruit burden led to 34% and 38% reduction in biomass and 15Nexcess, respectively, in mature leaves compared with NF trees. Moreover, the increment of 15Nexcess in new shoots of F relative to NF trees was lower than the increment of biomass between the two. These results indicate that N is a limiting resource during masting in F. crenata. 15Nexcess incorporated into nuts started to increase dramatically once 15Nexcess in leaves, branches and cupules hit seasonal maxima. Similar seasonal biomass growth patterns were also found in these organs, indicating that sink strength drives uptake and allocation of 15Nexcess between new shoot compartments. These results, together with translocation of 15Nexcess from cupules and senescing leaves to nuts (contributing to fruit ripening), suggest that a finely tuned growth phenology alleviated N limitation. Thus, fruiting did not influence the N concentration in leaves or branches. These reproduction-related variations in N uptake and allocation among new shoot compartments have implications for N dynamics in the plant-soil system.

Recent developments in understanding mast seeding in relation to dynamics of carbon and nitrogen resources in temperate trees

Mast seeding, the synchronous intermittent production of large seed crops in populations of perennial plants, is a widespread and widely studied phenomenon. Economy of scale has been demonstrated to provide the ultimate selection factor driving the evolution of masting, for example, in terms of the predator-satiation and pollination-efficiency hypotheses; however, its physiological mechanism is still poorly understood. The resource budget (RB) model assumes that an individual plant requires more resources to flower and fruit than it gains in a year, and therefore only flowers when a specific threshold amount of stored resources is surpassed. Although the RB models have been well explored theoretically, including for resource depletion and pollen coupling, empirical data to support these assumptions are still disputed. Here, we explore the extent to which the RB model applies to masting tree species, focusing on the dynamics of carbon and nitrogen resources in natural temperate forests. There is little empirical evidence that plants use carbohydrates stored over several years to produce fruits; however, nitrogen stores in temperate trees are more commonly depleted after masting. We review the internal nitrogen cycle including resorption during leaf senescence, storage and remobilization, discussing the effect of masting on these processes. Overall, carbohydrates and nitrogen are clearly involved in the proximate mechanisms driving mast seeding, but the determinant resource seems to be species specific.

Photosynthetic Responses to Lightflecks of Fagus crenata Seedlings Grown in a Gap and Understory of a Deciduous Forest

Photosynthetic responses to a series of 1-min lightflecks (1,000µmol m−2 s−1) superimposed on a background with different duration (1, 5, and 10 min) and intensity (25 and 50µmol m−2 s−1) of low background photosynthetic photon flux density (PPFD) were measured in the leaves ofFagus crenata grown in a gap and understory of aFagus crenata forest in the Naeba Mountains. The two background PPFD intensities most frequently occurred in understory and gap sites respectively. The maximum net photosynthetic rate (PNmax) and maximum stomatal conductance (gsmax) were higher in the gap seedlings than in the understory seedlings. However, when the background PPFD was 25µmol m−2s−1, the net photosynthetic rate (P25) and stomatal conductance (gs25) were almost the same between the gap and understory. When the background PPFD duration was 1-min, the net photosynthetic rate (PN) at the end of each lightfleck increased progressively. When the background PPFD duration was 5- and 10-min, the increase inPN at the end of each lightfleck was less. This indicates that background PPFD duration is important to photosynthetic responses to lightflecks. The higher ratios ofP25/PNmax andgs25/gsmax in the understory seedlings indicate that the understory seedlings can maintain relatively lower levels of biochemical and stomatal limitations than the gap seedlings under low light conditions. The ratios ofPN/PNmax at the end of each lightfleck (IS) and light utilization efficiency of single lightflecks (LUEs) that showed the influence of lightflecks on carbon gain were higher in the understory seedlings than in the gap seedlings when the background PPFD was 25µmol m−2 s−1. This means that understory seedling are capable of utilizing fluctuating light more efficiently under low light conditions than the gap seedlings although the net carbon gain of single lightflecks (CGs) in the understory seedlings was not higher than that in the gap seedlings. There were no significant differences inIS andLUEs between understory seedlings at a background PPFD of 25µmol m−2 s−1 and gap seedlings at a background PPFD of 50µmol m−2 s−1. However,CGs in gap seedlings was higher than in understory seedlings. These results provide more evidence thatF. crenata acclimate to a natural light environment in respect to relative induction state at low background PPFD and can capture the fluctuating light at the same efficiency in both the gap and understory seedlings under natural light environments.

The effect of the planting depth of cuttings on biomass of short rotation willow

ABSTRACT In order to investigate how the depth to which a cutting is inserted into the ground influences biomass of short-rotation coppice willows, 1-year-old cuttings from three clones each of Salix pet-susu and Salix sachalinensis were planted and harvested after two-seasons growth under natural conditions. All cuttings were 20 cm long, with either 18 or 10 cm below ground level (referred to as deep planting [DP] and shallow planting [SP], respectively, hereafter). Aboveground dry biomass from DP was 40% greater than from SP, although root biomass did not differ between the two treatments and shoot number per cutting was lower for DP than SP. The roots of the DP cutting were found deeper in the soil than those of SP cutting, suggesting that sustained soil water supply resulted in the higher aboveground biomass.