祁连山中部祁连圆柏年内径向生长对气候因子的响应

通过对祁连山中部葫芦沟流域的祁连圆柏连续采集微树芯,对其形成层活动和径向生长动态进行了连续两年的监测研究。结果表明,2012年细胞壁加厚和细胞成熟阶段开始时间分别发生在6月26日和7月24日,比2013年细胞壁加厚（6月22日）和细胞成熟阶段（6月26日）开始时间分别晚5 d和28 d。2012年细胞扩大、细胞壁加厚和细胞成熟阶段结束时间分别为7月16日、8月9日和9月8日,比2013年各阶段结束时间分别晚7、28 d和24 d。2012年最大细胞分裂速率为0.33细胞/d,共形成20.9个细胞,细胞分裂速率和木质部细胞总数均高于2013年。通过与附近气象站记录的气象数据进行对比,发现祁连圆柏生长开始时间在温暖年份显著早于寒冷年份,说明祁连圆柏的径向生长开始时间与温度有关。但2013年春季和夏初的高温导致区域干旱程度加剧,使祁连圆柏生长结束时间显著早于2012年,并导致2013年的木质部细胞总量和生长速率都小于2012年。研究表明,在寒冷干旱地区,尽管升温会使生长季提前,但升温导致的干旱胁迫可能对树木的生长速率和木质部细胞总量产生重要影响。     

Moisture‐driven xylogenesis in Pinus ponderosa from a Mojave Desert mountain reveals high phenological plasticity

Future seasonal dynamics of wood formation in hyperarid environments are still unclear. Although temperature‐driven extension of the growing season and increased forest productivity are expected for boreal and temperate biomes under global warming, a similar trend remains questionable in water‐limited regions. We monitored cambial activity in a montane stand of ponderosa pine (Pinus ponderosa) from the Mojave Desert for 2 consecutive years (2015–2016) showing opposite‐sign anomalies between warm‐ and cold‐season precipitation. After the wet winter/spring of 2016, xylogenesis started 2 months earlier compared to 2015, characterized by abundant monsoonal (July–August) rainfall and hyperarid spring. Tree size did not influence the onset and ending of wood formation, highlighting a predominant climatic control over xylem phenological processes. Moisture conditions in the previous month, in particular soil water content and dew point, were the main drivers of cambial phenology. Latewood formation started roughly at the same time in both years; however, monsoonal precipitation triggered the formation of more false rings and density fluctuations in 2015. Because of uncertainties in future precipitation patterns simulated by global change models for the Southwestern United States, the dependency of P. ponderosa on seasonal moisture implies a greater conservation challenge than for species that respond mostly to temperature conditions.     

Temporal dynamic of wood formation in Pinus cembra along the alpine treeline ecotone and the effect of climate variables

We determined the temporal dynamic of cambial activity and xylem development of stone pine (Pinus cembra L.) throughout the treeline ecotone. Repeated micro-sampling of the developing tree ring was carried out during the growing seasons 2006 and 2007 at the timberline (1,950 m a.s.l.), treeline (2,110 m a.s.l.) and within the krummholz belt (2,180 m a.s.l.) and the influence of climate variables on intra-annual wood formation was determined. At the beginning of both growing seasons, highest numbers of cambial and enlarging cells were observed at the treeline. Soil temperatures at time of initiation of cambial activity were c. 1.5°C higher at treeline (open canopy) compared to timberline (closed canopy), suggesting that a threshold root-zone temperature is involved in triggering onset of above ground stem growth. The rate of xylem cell production determined in two weekly intervals during June through August 2006–2007 was significantly correlated with air temperature (temperature sums expressed as degree-days and mean daily maximum temperature) at the timberline only. Lack of significant relationships between tracheid production and temperature variables at the treeline and within the krummholz belt support past dendroclimatological studies that more extreme environmental conditions (e.g., wind exposure, frost desiccation, late frost) increasingly control tree growth above timberline. Results of this study revealed that spatial and temporal (i.e., year-to-year) variability in timing and dynamic of wood formation of P. cembra is strongly influenced by local site factors within the treeline ecotone and the dynamics of seasonal temperature variation, respectively.     

Adjustment Capacity of Maritime Pine Cambial Activity in Drought-Prone Environments

Intra-annual density fluctuations (IADFs) are anatomical features formed in response to changes in the environmental conditions within the growing season. These anatomical features are commonly observed in Mediterranean pines, being more frequent in younger and wider tree rings. However, the process behind IADF formation is still unknown. Weekly monitoring of cambial activity and wood formation would fill this void. Although studies describing cambial activity and wood formation have become frequent, this knowledge is still fragmentary in the Mediterranean region. Here we present data from the monitoring of cambial activity and wood formation in two diameter classes of maritime pine (Pinus pinaster Ait.), over two years, in order to test: (i) whether the differences in stem diameter in an even-aged stand were due to timings and/or rates of xylogenesis; (ii) if IADFs were more common in large trees; and (iii) if their formation is triggered by cambial resumption after the summer drought. Larger trees showed higher rates of cell production and longer growing seasons, due to an earlier start and later end of xylogenesis. When a drier winter occurs, larger trees were more affected, probably limiting xylogenesis in the summer months. In both diameter classes a latewood IADF was formed in 2012 in response to late-September precipitation, confirming that the timing of the precipitation event after the summer drought is crucial in determining the resumption of cambial activity and whether or not an IADF is formed. It was the first time that the formation of a latewood IADF was monitored at a weekly time scale in maritime pine. The capacity of maritime pine to adjust cambial activity to the current environmental conditions represents a valuable strategy under the future climate change conditions.     

Ciclo di Accrescimento E Differenziazione Delle Gemme in Piante Perenni Nel Territorio di Bari

Summary

Growth cycle and buds differentiation in perennial plants growing in Bari's area. — V. The evolution of the wood ring in Rhammus Alaternus L. from December 1946 to March 1949.

In Rhamnus Alaternus L. the cambium awackens in March with the production of an early wood, characterized by large vessels and loose fibers. It keeps on dividing the following months, producing intermediary wood, whose vessels become narrower and narrower. In June the wood ring is almost complete and the cambial activity slows down. In July late wood is formed, with narrow tracheae and highly lignified fibers. From August to February cambial activity is very scarce and irregular, in the wood produced fibers being tangentially compressed.

In a young branch of R. A. cambium becomes suddenly active in February, that is one month before than in the stem, it goes on during the month of March-June, with an evident diminution in the size of the wood elements. This diminution of size instead of being regular, shows some oscillations, expecially during the month of May. In Sept. the cambium starts again dividing, but rather poorly and irregularly, forming some vessels a little wider than the preceeding ones, so that an incomplete false ring is formed only in some points of the branch. As a rule the autumn wood is a typical late wood. This scarce and irregular cambial activity goes on also during Oct.-Dec. In January is more active and forms rather wide vessels in comparison with those formed in the preceeding month. This new formed wood however does not show the characteristics of the early wood of a new ring.

Concluding, the characteristics of the cambial activity of A. R. are

In the stem: 1) March: early wood. 2) March-June: intermediary wood. 3) July: late wood, resting period. 4) At the end of Autumn a scarce production of late wood is sometimes possible.

In the young branch: 1) February: early wood. 2) March-June: intermediary wood, with positive and negative oscillations in the vessels width. 3) At the end of June the late wood is already differentiated. 4) July-August: resting period. 5) Spt.-Juan.: the cambium starts again dividing very scarcely and producing a transition wood between the two rings. This wood can be regarded neither as a true ring nor a false one, it represents rather the last phase of the late wood formation. 6) In the young branch of the female specimen the cambium starts dividing somewhat later than in the male one. 7) The wood ring width does not vary much in relation to the yearly amount of rainfall. The lack of relation between rainfall and ring width is particularly evident in 1948 (a rainy year). The, «Compleasance» of R. A. could show that this species is, in Puglia, in its own habitat. On the other hand it is rather peculiar that R. A. forms only one wood ring during one year, with production of early wood during the spring, and of late wood in the autumn, instead of producing a second autumn ring following a distint summer rest, as could be expected.     

Notes on the Effect of Cutting Bracken (Pteris aquilina L.)

Background: Undisturbed high elevation treelines follow a common growing season isotherm, irrespective of latitude. Small stature plants thrive at much higher elevations because they grow in a favourable microclimate near the ground, whereas trees are aerodynamically coupled to free atmospheric circulation, hence the uniform treeline elevation in a given region.

Aims: I argue that the treeline results from tree architecture and not from a tree-specific inferior physiology. At tissue level, all cold adapted higher plants (including winter crops) face similar temperature related limitations and are constrained by similar thermal thresholds.

Methods: In order to explore this hypothesis, winter rape and winter oat were grown in the field and the daily rate of leaf expansion was measured during early winter, with temperatures similar to those at treeline at the beginning of the growing season (monthly mean temperature of 5 °C).

Results: Leaves of these winter crops only grew when the degree hours above 5 °C for a given day exceeded zero. Rape and oats showed very similar responses. The data support a common temperature threshold for tissue formation in winter crops and treeline trees.

Conclusions: A literature survey revealed lower temperature thresholds (close to freezing point) in arctic and Antarctic algae. It appears that the formation of complex, partly lignified tissue does not happen below 5 °C, whereas such temperatures exert little constraints on photosynthesis. The study illustrates that tissue growth in winter crops can help understand growth of treeline trees, given that both exhibit similar minimum temperature requirements for meristem functioning.     

Evoluzione Della Cerchia Legnosa in Quercus Aegilops L. a Tricase

Abstract

The author has examined the cambial activity and the formation of the annual ring in Quercus aegilops from February 1954 to February 1955. Material was collected at Tricase (Lecce) the only Italian locality where this oriental species grows although it is doubtful whether it is spontaneous.

Cambial activity starts in the middle of April both in the main stem and in the branches following flowering and bud development. It goes on for about five months until the second half of August forming a complete growth ring. This annual ring shows the normal sequence of early wood with wide vessels formed during the second half of April and the whole of May, intermediate wood with medium and small size vessels, parenchima, and fibres formed in June-July, and late wood with few small size vessels, parenchima, and fibres formed from the end of July to middle August.

There is no reawakening of cambial activity in autumn even if the climatic conditions are favourable. Tylosis are very easily formed in the wood vessels and by this process the plant can withstand the drought of the Apulian summer.     

Temperature thresholds for the onset of xylogenesis in alpine shrubs on the Tibetan Plateau

Key message

The threshold minimum air temperature driving xylem growth of alpine  Rhododendron aganniphum is lower than that commonly observed at the treeline of conifers.

Abstract

Understanding how alpine shrubs grow and which environmental factors drive their biomass gain could help to functionally differentiate trees and shrubs. The cambium is the main meristem responsible for wood formation in trees and shrubs. Thus, a better knowledge of cambium growth dynamics in alpine shrubs would allow explaining why shrubs displace trees above the treeline. Here, we aim to investigate the timings and dynamics of xylogenesis and to identify the thermal thresholds controlling the onset of xylem growth of Rhododendron aganniphum, a tall shrub growing above the alpine treeline on the Tibetan Plateau. Timings of xylogenesis and radial growth rates were assessed from anatomical observations of the developing xylem during three growing seasons (2011, 2012, and 2013). The threshold temperature at which xylogenesis had a 0.5 probability of being active was calculated with logistic regressions. The onset of xylogenesis was observed between mid and late June, whereas the end of xylogenesis lasted from mid to late September. Overall, the duration of xylem growth lasted 88–101 days, and 94 % of the ring was formed from June to August. The threshold for the onset of xylem growth was observed at 2.0 ± 0.6 °C for the minimum air temperature, lower than that commonly observed for treeline conifers (ca. 6 °C). This low thermal threshold allows alpine shrubs to have a growing season long enough to complete xylem production and maturation during the warmest summer months. Our results suggest that the time required to complete xylogenesis is critical to understand why shrubs displace trees above the treeline.

     

Ciclo Di Accrescimento E Differenzazione Delle Gemme in Piante Perenni Nel Territorio Di Bari

Summary

The evolution of cambial activity during one year in Viburnum Tinus L. in Bari has been studied. The research seems to be particularly difficult in this evergreen shrub. The wood is of the porous diffused type with scarse evidence of wood rings. The vessel diameter varies rather irregularly in the wood ring; on the other side the fibers show wide variations and may be assumed as a good index of the wood ring evolution. Both in the branch and in the stem only one wood ring each year is formed.

Cambial activity prosecutes during the whole year, with an irregular step. During the period July-beginning of September the cambium devides very slowly, or possibly stops deviding.

The early wood is produced earlier in the branch than in the stem; namely in February-end of May in the branch and in March-beginning of June in the stem. The stimulating growth stuffs evidently proceeds downwards from the top to the base of the plant. The relations between ring evolution and climatic factors are discussed. The peculiar cambial poussée during the month of June seems to be correlated with the exceptionally aboundant rainfall of May in Puglia in 1947.

The late wood is formed during the other months discontinuosly. The alternation between the two phases of cambium division and wood lignification has been focussed. The wood ring in Viburnym Tinus is annual and the early wood differentiates in spring.     

Evidence of threshold temperatures for xylogenesis in conifers at high altitudes

Temperature is the most important factor affecting growth at high altitudes. As trees use much of the allocated carbon gained from photosynthesis to produce branches and stems, information on the timing and dynamics of secondary wood growth is crucial to assessing temperature thresholds for xylogenesis. We have carried out histological analyses to determine cambial activity and xylem cell differentiation in conifers growing at the treeline on the eastern Alps in two sites during 2002–2004 with the aim of linking the growth process with temperature and, consequently, of defining thresholds for xylogenesis. Cambial activity occurred from May to July–August and cell differentiation from May–June to September–October. The earliest start of radial enlargement was observed in stone pine in mid-May, while Norway spruce was the last species to begin tracheid differentiation. The duration of wood formation varied from 90 to 137 days, depending on year and site, with no difference between species. Longer durations were observed in trees on the south-facing site because of the earlier onset and later ending of cell production and differentiation. The threshold temperatures at which xylogenesis had a 0.5 probability of being active were calculated by logistic regressions. Xylogenesis was active when the mean daily air temperature was 5.6–8.5°C and mean stem temperature was 7.2–9°C. The similar thresholds among all trees suggested the existence of thermal limits in wood formation that correspond with temperatures of 6–8°C that are supposed to limit growth at the treeline. Different soil temperature thresholds between sites indicated that soil temperature may not be the main factor limiting xylogenesis. This study represents the first attempt to define a threshold through comparative assessment of xylem growth and tissue temperatures in stem meristems at high altitudes.     

Seasonal development of secondary xylem and phloem in <Emphasis Type="Italic">Schizolobium parahyba</Emphasis> (Vell.) Blake (Leguminosae: Caesalpinioideae)

The cambial activity and periodicity of secondary xylem and phloem formation have been less studied in tropical tree species than in temperate ones. This paper describes the relationship between seasonal cambial activity, xylem and phloem development, and phenology in Schizolobium parahyba, a fast growing semideciduous seasonal forest tree from southeastern Brazil. From 2002 to 2003, wood samples were collected periodically and phenology and climate were recorded monthly in the same period. S. parahyba forms annual growth increments in wood, delimited by narrow initial parenchyma bands. The reduction of the cambial activity to a minimum correlates to the dry season and leaf fall. The higher cambial activity correlates to the wet season and the presence of mature leaves. In phloem, a larger conductive region was observed in the wet season, when the trees were in full foliage. The secondary phloem did not exhibit any incremental zone marker; however, we found that the axial parenchyma tends to form irregular bands.     

Unravelling cell wall formation in the woody dicot stem

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.     

树木形成层活动及其影响因素研究进展

树木生长是森林生态系统固碳的主要方式,树木生长过程受到气候与非气候因素的共同作用。树木径向生长长期定位监测是明确树木生长对气候变化响应的重要研究手段。本文对运用微树芯法的树木形成层活动及径向生长过程研究进行了总结。首先,综述了气候因素对树木形成层活动的影响: 寒冷湿润区温度决定树木生长开始和停止,干旱半干旱区水分和温度共同决定生长开始,水分决定生长停止;生长速率和持续时间共同决定生长量,最大生长速率出现在夏至前后;短期施氮并不能影响树木径向生长动态。其次,探讨了生物因素对树木径向生长过程的调控: 形成层活动开始时间因树种、树龄、竞争关系而有所差异;非结构性碳水化合物的季节动态与径向生长过程相耦合。最后,阐述了气候因素和生物因素交互作用下树木次生生长的响应机制。针对以上进展,本文提出了目前研究尚存在的问题并展望了未来的发展前景,以期为进一步的科学研究提供参考。     

Nitrogen status of conifer needles at the alpine treeline

Background and Aims: High elevation treelines occur worldwide at similar mean growing season temperatures. Does this result from direct impact of low temperature on growth or carbon metabolism, or does nutrient limitation, induced by low soil temperature, play a role? Similar treeline elevations at contrasting soil fertility argue against the latter, but the actual nutritional status of treeline trees (here addressed as foliage nitrogen concentration) has never been assessed systematically. Although needle nitrogen (N) concentration does not necessarily indicate growth limitation by N, the relative abundance of N would indicate obvious depletions at the treeline.

Methods: A central problem with any foliage nutrient assay is that the units for describing the element concentration are dependent on elevation themselves. Here we separate changes in N per unit tissue from changes in reference units.

Results: Needles of Pinus cembra and Picea abies in the Alps do not show elevational differences in N concentration per dry weight, water content, area or volume, thus, there is no N depletion near the elevational tree limit. Hence, nutrient supply is either unaltered, or growth is adjusted so that nutrient depletion in needle tissue does not occur.

Conclusions: Chronic N shortage at needle level is not an explanation for low tree vigour at the treeline.     

Behavioral responses of leaves and vascular cambium of Prosopis cineraria (L.) Druce to different regimes of coal-smoke pollution

Abstract

The foliar and cambial responses of Prosopis cineraria to five different coal-smoke regimes in Delhi, India, were studied using the monthly collections of leaves and cambial blocks as well as the data on SO₂, NO₂ and particulate concentrations from each of the five study sites. Coal-smoke inhibited pigment concentration, NR activity and sugar content and promoted stomatal index and the nitrate and sulfur contents. Stomatal conductance was low, leading to a drop in the net photosynthetic rate. Cambial activity started quite late and the annual increment of wood was reduced despite a prolonged active phase, possibly due to a distinctly retarded rate of cambial cell division. Vessel proportion in the wood increased but the size of vessel elements and fibers decreased. Such studies may help in identifying trees suitable for cultivation in the pollution-affected areas.     

Tree recruitment at the treeline across the Continental Divide in the Northern Rocky Mountains,USA: the role of spring snow and autumn climate

ABSTRACT

Background: Topoclimate can influence tree establishment within treeline ecotones. Yet much less is known about how regional topography, such as the Continental Divide, Rocky Mountains, mediates the role of climate in governing treeline dynamics.

Aims: To utilise the Continental Divide to test whether contrasts in growing-season moisture regimes to the west (summer-dry) and east (summer-wet) impact the spatio-temporal patterns of tree establishment and rates of treeline advance in the Northern Rocky Mountains.

Methods: We sampled trees at sites on north- and south-facing slopes, west and east of the Continental Divide. We used dendroecological techniques to reconstruct patterns of tree establishment. Age-structure data were quantitatively compared with climate to evaluate possible mechanistic linkages.

Results: Across all sites, 96% of trees established after 1950. There was a treeline advance (range = 39–140 m) accompanied by increases in tree density. Significantly more trees established during wet springs on both sides of the Divide.

Conclusions: Overall, snow duration in spring and autumn temperatures appear to influence patterns of tree recruitment at the treeline. Continued warming will likely amplify the role of autumn climate in regulating tree establishment throughout treeline ecotones in the Northern Rocky Mountains, particularly west of the Divide where summer-dry conditions persist.     

Compensatory mechanisms mitigate the effect of warming and drought on wood formation

Because of global warming, high‐latitude ecosystems are expected to experience increases in temperature and drought events. Wood formation will have to adjust to these new climatic constraints to maintain tree mechanical stability and long‐distance water transport. The aim of this study is to understand the dynamic processes involved in wood formation under warming and drought. Xylogenesis, gas exchange, water relations and wood anatomy of black spruce [Picea mariana (Mill.) B.S.P.] saplings were monitored during a greenhouse experiment where temperature was increased during daytime or night‐time (+6 °C) combined with a drought period. The kinetics of tracheid development expressed as rate and duration of the xylogenesis sub‐processes were quantified using generalized additive models. Drought and warming had a strong influence on cell production, but little effect on wood anatomy. The increase in cell production rate under warmer temperatures, and especially during the night‐time warming at the end of the growing season, resulted in wider tree‐rings. However, the strong compensation between rates and durations of cell differentiation processes mitigates warming and drought effects on tree‐ring structure. Our results allowed quantification of how wood formation kinetics is regulated when water and heat stress increase, allowing trees to adapt to future environmental conditions.     

Spatial patterns of Smith fir alpine treelines on the south-eastern Tibetan Plateau support that contingent local conditions drive recent treeline patterns

Background: Recent work has shown little change in the position of the Smith fir treeline on the south-eastern Tibetan Plateau in response to global warming. However, the relationship between tree distribution patterns within the treeline ecotone and low responsiveness is unknown, and additional constraints than climate might be major drivers of these patterns (e.g. microsite availability for regeneration).

Aims: To characterise the spatial patterns of Smith fir alpine treelines and to infer the underlying processes driving their dynamics.

Methods: We investigated spatial patterns of Smith fir trees across two treeline ecotones in the Sygera Mountains, south-eastern Tibetan Plateau. The O(r)-ring statistic was used to analyse the univariate and bivariate spatial point patterns of three size classes (adults, juveniles and seedlings).

Results: Mature trees presented random spatial patterns. Clusters of juveniles and seedlings colonised areas not occupied by mature trees. Seedlings were clustered and established preferentially near juvenile firs, Rhododendron mats and over moss–lichen and organic matter substrates, indicating the importance of microsite availability for successful Smith fir recruitment.

Conclusions: Local factors such as microsite availability may play a major role in driving recent Smith fir treeline patterns and determine the lack of significant warming-induced upward shifts of these ecotones.     

Evoluzione Annuale della Cerchia Legnosa in Individui Maschili E Femminili di Ginkyo Biloba L.

Abstract

Wood ring evolution in male and female plants of GINKYO BILOBA L. — A morphological examen of the wood rings in Ginkyo biloba L. has been performed on stem sections from specimens of the Botanical Institute of Florence, on wood samples from the collection of Adriano Fiori and on wood cores extracted from the stem of male and female plants growing in the Botanical Garden of Florence.

At the same time the development of the growth ring has been followed on wood samples tacken off the stems of two adult Ginkyo plants growing in the Botanical Garden of Florence.

All the material which has been studied shows the same characters, that is the frequency of uncomplete crescent-shaped wood rings, and consequently a large degree of eccentricity of the stems, depending not only upon the uncomplete wood rings, but also on the changing width of each ring.

Microscopical analysis has pointed out the presence of false rings, limited by discontinuous late wood, particularly in the female plant. This character has been observed only in a fossil wood of Ginkyo bonesii.

It has not been possible to assers how many rings or crescents are formed during one year. From dating records made on wood cores it seems that more than one ring could differentiate during one year, but the cambium shows a very irregular activity in Ginkyo. The factors which control the formation of false and uncomplete wood rings (wood crescents) are also unknown; their formation possibly depending upon environment conditions.

External factors such as water deficiency probably control cambial activity through auxin production. Larson (1964) has shown that draught causes a decrease in tracheids diameter, and the formation of false growth rings; thus the same causes could have been in action in our plants.

Noteworthy are also the data by Gungkel and Thimann (in Larson 1964) showing that auxin content increases downwards in the shoots of Ginkyo and that the apical bud has a very low auxin production, so that it seems possible that in Ginkyo the leaves supply something which the cortical tissue converts into auxin.

Lastly, the almost continuous growth activity of the cambium in Ginkyo must be pointed out, this causes a great difficulty in checking the beginning and the end of this process along the year. It is any way possible to establish the time of the highest cambial activity, which lies in June-July both in the female and in the male plants.     

Temporal variations of mobile carbohydrates in Abies fargesii at the upper tree limits

Low temperatures are associated high‐altitude treelines, but the functional mechanism of treeline formation remains controversial. The relative contributions of carbon limitation (source activity) and growth limitation (sink activity) require more tests across taxa and regions. We examined temporal variations of mobile carbon supply in different tissues of Abies fargesii across treeline ecotones on north‐ and south‐facing slopes of the Qinling Mountains, China. Non‐structural carbohydrate (NSC) concentrations in tissues along the altitudinal gradient on both slopes changed significantly in the early and late growing season, but not in the mid‐growing season, indicating the season‐dependent carbon supply status. Late in the growing season on both slopes, trees at the upper limits had the highest NSC concentrations and total soluble sugars and lowest starch concentrations compared to trees at the lower elevations. NSC concentrations tended to increase in needles and branches throughout the growing season with increasing elevation on both slopes, but declined in roots and stems. NSC concentrations across sampling dates also indicated increases in needles and branches, and decreases in roots and stem with increasing elevation. Overall altitudinal trends of NSC in A. fargesii revealed no depletion of mobile carbon reserves at upper elevation limits, suggesting limitation of sink activity dominates tree life across treeline ecotones in both north‐ and south‐facing slopes. Carbon reserves in storage tissues (especially roots) in the late growing season might also play an important role in winter survival and early growth in spring at upper elevations on both slopes, which define the uppermost limit of A. fargesii.