Between-year variation in seed weights across altitudes in the high-alpine plant <Emphasis Type="Italic">Eritrichium nanum</Emphasis>

Seed weight is a prominent life history trait of plants affecting dispersal, establishment, and survival. In alpine environments, the few studies investigating the effect of elevation on seed weight within species have mainly detected a decrease in seed weight with increasing elevation. This relationship is generally attributed to the adverse climate at high elevations. In order to test this hypothesis, we analyzed seed weight variation across altitudes (2,435–3,055 m a.s.l.) in two consecutive years that differed in weather conditions in the high-alpine cushion plant Eritrichium nanum. We found a significant reduction in seed weight with increasing elevation in both years, but in the growing season with more adverse weather conditions, the reduction was more substantial than in the more favorable year. We conclude that alpine plants may be able to produce well-developed seeds at low elevations in almost all years, independent of weather conditions, whereas reproduction through seeds is potentially limited to years of favorable weather at high elevation.     

Sensitivity of French temperate coniferous forests to climate variability and extreme events (Abies alba,Picea abies and Pinus sylvestris)

Questions: (1) How do extreme climatic events and climate variability influence radial growth of conifers (silver fir, Norway spruce, Scots pine)? (2) How do elevation and soil water capacity (SWC) modulate sensitivity to climate? Location: The sampled conifer stands are in France, in western lowland and mountain forests, at elevations from 400 to 1700 m, and an SWC from 50 to 190 mm. Methods: We established stand chronologies for total ring width, earlywood and latewood width for the 33 studied stands (985 trees in total). Responses to climate were analysed using pointer years and bootstrapped response functions. Principal component analysis was applied to pointer years and response function coefficients in order to elucidate the ecological structure of the studied stands. Results: Extreme winter frosts are responsible for greater growth reductions in silver fir than in Norway spruce, especially at the upper elevation, while Scots pine was the least sensitive species. Exceptional spring droughts caused a notable growth decrease, especially when local conditions were dry (altitude<1000 m and SWC<100 mm for silver fir, western lowlands for Scots pine). Earlywood of silver fir depended on previous September and November and current‐year February temperature, after which current June and July water supply influenced latewood. Earlywood of Norway spruce was influenced by previous September temperature, after which current spring and summer droughts influenced both ring components. In Scots pine, earlywood and latewood depended on the current summer water balance. Local conditions mainly modulated latewood formation. Conclusions: If the climate becomes drier, low‐elevation dry stands or trees growing in western lowlands may face problems, as their growth is highly dependent on soil moisture availability.     

Canopy status modulates formation of wood rays in scots pine under hemiboreal conditions

Non-structural carbohydrates (NSC) reserves are crucial for trees to cope with weather extremes, thus to ensure their survival and ecological plasticity. The NSC reserves can depend on social status, suggesting uneven plasticity of trees at the stand level. In stemwood of Scots pine (Pinus sylvestris L.), which is a widespread and important species, NSC reserves are stored in parenchyma in wood rays (WR). The quantity of WRs is adjusted intra-annually, allowing retrospective analysis of factors affecting their formation. Accordingly, the differences in WR quantity in stemwood of dominant and intermediate (canopy trees with reduced and narrow crowns) maturing Scots pine were assessed by quantitative wood anatomy. Tangential cuts from the outermost 30 tree-rings were analysed. The relative ray area was intermediate, i.e. covering about 5% of the tangential cut, yet expressed high individuality among the trees. The size and amount of WR mainly differed between the earlywood and latewood; WRs in latewood were higher although narrower in comparison to earlywood, yet their total amount was higher in earlywood. Canopy status had only a slight effect, as quantity and height of WR tended to be higher for the intermediate trees, particularly in earlywood. The size and quantity of WR expressed inter-annual variation, which was mainly related to the meteorological conditions prior to the formation of the tree-ring (previous summer and autumn) indicating legacy effects of climatic factors on NSC and susceptibility of trees to cumulative effects of weather extremes. However, the climatic signals in the inter-annual variation of WR were weaker than observed before, likely due to location of the studied stand in the mid-part of the species range. Nevertheless, the observed differences in mean values and inter-annual variation of WR suggested a within-species diversity of carbon allocation patterns, supporting adaptability of the species.     

Quercus robur survival at the rear edge in steppe: Dendrochronological evidence

Climate change is altering forest ecosystems worldwide, particularly in steppe landscapes, where the rare tree communities are challenged with steadily increasing droughts. In the steppe of Eastern Europe, amid dry conditions, Quercus robur occupies mostly riverine habitats and ravines. Here we study the climate sensitivity and drought vulnerability of a Q. robur population located at the rear edge of the species range, in the steppe of Ukraine. The population occupies two adjacent but clearly contrasting in their microclimatic conditions sites: a river floodplain and a steep-sloping river bank. We develop tree earlywood, latewood, and total ring width site-level chronologies and evaluate their relationship with regional climate variables and the local river's water level using response function analysis. We find that trees growing in the floodplain and at the steep river bank have exhibited slightly different growth patterns. The trees at the flooded site have benefited from water proximity, which facilitated their earlywood growth. These trees have responded positively to the current May and previous September precipitation and previous and current May temperatures. At the non-flooded site, the trees have experienced warm temperatures and the lack of precipitation in June. The extreme drought episodes have triggered a decrease in the latewood and total ring width in trees from both sites. We established that oak growth in the floodplain had been depressed by an unknown stressor around 1900, therefore limiting our ability to identify the more beneficial steppe habitat for Q. robur conclusively. Nevertheless, our results provide a dendrochronological evidence of Q. robur survival in a dry steppe environment and lend new insights into local microclimatic factors contributing to it.     

Climate control on ring width and intra-annual density fluctuations in <Emphasis Type="Italic">Pinus kesiya</Emphasis> growing in a sub-tropical forest of Manipur,Northeast India

Key message

Growth ring study of Pinus kesiya (khasi pine) growing in sub-tropical forest in Manipur, northeast India was performed to understand climate signatures in ring widths and intra-annual density fluctuations.

Abstract

The growth rings in khasi pine (Pinus kesiya Royle ex Gordon) growing in sub-tropical Reserve Forest in Imphal, Manipur, northeast India were analysed to understand environmental signals present in ring-width series and intra-annual density fluctuations (IADFs). For this the growth ring sequences in increment core samples collected from 28 trees were precisely dated and a ring-width chronology spanning AD 1958–2014 developed. The correlation analyses between ring-width chronology and weather data of Imphal revealed that a cool April–May–June favour tree growth. The wood anatomical features of growth rings revealed the occurrence of IADFs in early- and latewoods. The IADFs in earlywood were found to be associated with reduced precipitation in months from April to July. However, the wetter conditions in late growing season, especially August/September triggered the formation of IADFs in latewood. Our findings endorse that the IADF chronologies of khasi pine could emerge as an important proxy of summer monsoon rainfall in long-term perspective in data scarce region of northeast India.

     

13CO2 pulse-labelling of photoassimilates reveals carbon allocation within and between tree rings

Post-photosynthetic fractionation processes during translocation, storage and remobilization of photoassimilate are closely related to intra-annual sigma13C of tree rings, and understanding how these processes affect tree-ring sigma13C is therefore indispensable for improving the quality of climate reconstruction. Our first objective was to study the relationship between translocation path and phloem grain. We pulse-labelled a branch of Larix gmelinii (Rupr.) Rupr. and later analysed the sigma13C distribution in the stem. A 13C spiral translocation path closely related to the spiral grain was observed. Our second objective was to study the use of remobilized storage material for earlywood formation in spring, which is a suspected cause of the autocorrelation (correlation of ring parameters to the climate in the previous year) observed in (isotope) dendroclimatology. We pulse-labelled whole trees to study how spring, summer and autumn photoassimilate is later used for both earlywood and latewood formation. Analysis of intra-annual sigma13C of the tree rings formed after the labelling revealed that earlywood contained photoassimilate from the previous summer and autumn as well as from the current spring. Latewood was mainly composed of photoassimilate from the current year's summer/autumn, although it also relied on stored material in some cases. These results emphasize the need for separating earlywood and latewood for climate reconstruction work with narrow boreal tree rings.     

Effect of Leaf Aging on Water Deficit—Water Potential Relationships of Dogwood Leaves Growing in Two Environments

Water deficit-water potential relationships were determined at approximately monthly intervals from May to August on leaves collected from dogwood trees growing in two environments. The relationships were not the same for leaves of different ages or for leaves of the same age, but growing in the different environments. With aging of the leaves, the relationships shifted to progressively lower water potentials for a given water deficit. Increased leaf dry weight, decreased cell wall elasticity, and decreased osmotic potentials accompanied leaf aging. These changes and the shifts in the relationships were greatest for leaves growing under high light intensity and dry environmental conditions. The lack of constancy in the relationships reduces the usefulness of water deficit or relative turgidity as an estimator of water potential. For the purposes of some workers, however, the relationships may be sufficiently constant for mature leaves of a given species growing in a relatively constant environment.     

Radial-growth and wood-anatomical changes in overaged Quercus pyrenaica coppice stands: functional responses in a new Mediterranean landscape

Recent land-use changes in intensively managed forests such as Mediterranean coppice stands might profoundly alter their structure and function. We assessed how the abandonment of traditional management practices in coppice stands, which consisted of short cutting-cycles (10–15 years), has caused overaging (stems are usually much older than when they were coppiced) and altered their wood anatomy and hydraulic architecture. We studied the recent changes of wood anatomy, radial growth, and hydraulic architecture in two stands of Quercus pyrenaica, a transitional Mediterranean oak with ring-porous wood forming coppice stands in W–NW Spain. We selected a xeric and a mesic site because of their contrasting climates and disturbance histories. The xeric site experienced an intense defoliation after the severe 1993–1994 summer drought. The mesic site was thinned in late 1994. We studied the temporal variability in width, vessel number and diameter, and predicted the hydraulic conductivities (K _h) of earlywood and latewood. In the mesic site, we estimated the vulnerability to xylem cavitation of earlywood vessels. Overaging caused a steep decline in latewood production at a cambial age of 14 years., which was close to the customary cutting cycle of Q. pyrenaica. The diameter distribution of vessels was bimodal, and latewood vessels only accounted for 4% of the K _h. Overaging, acting as a predisposing factor in the decline episode, was observed at the xeric site, where most trees did not produce latewood in 1993–1995. At the mesic site, thinned trees formed wider tree-rings, more latewood and multiseriate tree-rings than overaged trees. The growth enhancement remained 8 years after thinning. Most of the hydraulic conductivity in earlywood was lost in a narrow range of potentials, between −2.5 and −3.5 MPa. We have shown how hydraulic conductivity and radial growth are closely related in Q. pyrenaica and how aging modulates this relationship.     

Dendrochronology and dendroclimatology of Ceiba speciosa (A. St.-Hil.) Ravenna (Malvaceae) exposed to urban pollution in Rio de Janeiro city,Brazil

Nowadays, the biological monitoring through the growth rings has received increasing attention from ecologists and toxicologists. Structural analysis of these rings allows the incorporation of a time component in the study of plant responses to environmental variation. This allows also to evaluate long time series from the woody plants. In this paper, we assessed the dendrochronological characteristics of Ceiba speciosa growing in forest environment and under urbanization impact. Stem samples were obtained with Pressler probe into trees growing the campus of the Oswaldo Cruz Foundation, adjacent to one of the main urban thoroughfares of the city of Rio de Janeiro (Avenida Brasil), and at Tinguá Biological Reserve, an important remnant of Atlantic Forest. The samples were processed and analyzed following usual dendrochronological methods, with COFECHA and ARSTAN softwares. A negative exponential curve was used for standardization of the series. The residual chronologies were correlated with precipitation and temperature indexes obtained from NOAA weather database. Growth rings are distinct and annual, marked by bands of marginal parenchyma, thick-walled and radially flattened fibres in latewood and distended rays in earlywood. In both sites, the intercorrelation between the trees was above 0.40. Ages ranged from 11 to 41 years in the urban site and from 27 to 64 years in the forest site. In urban area, mean annual increment and cumulative average growth rates were 6 mm/year and 142.62 mm, respectively. At the forest site, these rates were 4 mm/year and 173.07 mm, respectively. The comparison between cumulative radial increment of the two sites revealed that trees of the urban site had higher increment rates beginning at the start of their development and consequently, they showed similar diameters despite lower ages. Correlation analysis between the chronologies and climatic factors revealed a positive association between growth and hot and rainy periods for both study sites. However, there is an immediate response of urban trees in relation to the rains and, a late response of forest trees to the same factor. The dry and hot climate, typical of urban environments, and the absence of natural water reserves in urban soil, may explain this more immediate response of urban tree growth to rainfall and temperature indexes. Our results revealed that Ceiba speciosa is a plastic and stress-tolerant species that is able to survive and adapt to polluted urban conditions. These features, along with its wide natural distribution and frequent planting for city landscaping, make this species an important biomarker for environmental monitoring studies.     

Tree-ring growth of Gmelin larch under contrasting local conditions in the north of Central Siberia

While the forest-tundra zone in Siberia, Russia has been dendroclimatologically well-studied in recent decades, much less emphasis has been given to a wide belt of northern taiga larch forests located to the south. In this study, climate and local site conditions are explored to trace their influence on radial growth of Gmelin larch (Larix gmelinii (Rupr.) Rupr.) trees developed on permafrost soils in the northern taiga. Three dendrochronological sites characterized by great differences in thermo-hydrological regime of soils were established along a short (ca. 100 m long) transect: on a river bank (RB), at riparian zone of a stream (RZ) and on a terrace (TER). Comparative analysis of the rate and year-to-year dynamics of tree radial growth among sites revealed considerable difference in both raw and standardized tree-ring width (TRW) chronologies obtained for the RZ site, characterized by shallow soil active layer depth and saturated soils. Results of dendroclimatic analysis indicated that tree-ring growth at all the sites is mostly defined by climatic conditions of a previous year and precipitation has stronger effect on TRW chronologies in comparison to the air temperatures. Remarkably, a great difference in the climatic response of TRW chronologies has been obtained for trees growing within a very short distance from each other. The positive relation of tree-ring growth with precipitation, and negative to temperature was observed in the dry site RB. In contrary, precipitation negatively and temperature positively influenced tree radial growth of larch at the water saturated RZ. Thus, a complicate response of northern Siberian larch forest productivity to the possible climate changes is expected due to great mosaic of site conditions and variability of environmental factors controlling tree-ring growth at different sites. Our study demonstrates the new possibilities for the future dendroclimatic research in the region, as various climatic parameters can be reconstructed from tree-ring chronologies obtained for different sites.     

X-ray microdensitometry applied to subfossil tree-rings: growth characteristics of ancient pines from the southern boreal forest zone in Finland at intra-annual to centennial time-scales

A collection of subfossil wood of Pinus sylvestris (Scots pine) was exposed to X-ray densitometry. The collection of 64 samples from the southern boreal forest zone was dendrochronologically cross-dated to a.d. 673-1788. Growth characteristics were determined by performing density profiles including the following parameters: minimum density, earlywood and latewood boundary density, maximum density, earlywood width, earlywood density, latewood width, latewood density, annual ring width and annual ring density. Seven out of the nine parameters were found to contain non-climatic growth trends and six were found to be heteroscedastic in their variance. Tree-specific records were indexed, to remove the non-climatic growth trends and stabilize the variance, and combined into nine parameter-specific tree-ring chronologies. Growth characteristics of the pines changed in parallel with the generally agreed climatic cooling from the Medieval Warm Period to the Little Ice Age: pine tree-rings showed decreasing maximum densities from the period a.d. 975-1150 to a.d. 1450–1625. A concomitant change in the intra-annual growth characteristics was detected between these periods. The findings indicate that not only the trees growing near the species’ distributional limits are sensitive to large-scale climatic variations but also the trees growing in habitats remote from the timberline have noticeably responded to past climate changes.     

Influence of weather conditions on pallid swift Apus pallidus breeding success

The reproductive ecology of three colonies of pallid swift in a warm continental climate (Piedmont, NW-Italy) was studied over a ten years period About 60% of the clutches were laid in late May and June, but continued until late August and September, when some females laid a second clutch Clutch size and the mean number of fledged young decreased progressively from spring to autumn
Laying dates were influenced by rain and temperature unfavourable weather conditions during May induced most females to postpone egg laying until June Clutch size was not related to the rain or temperature parameters considered
These observations differ from the findings about the common swift studied in cool temperate areas, where a very short stay in the breeding colonies does not allow a delay in laying, and spring weather conditions therefore have a strong influence on clutch size There are also differences about the effect of weather on chick rearing In warmer climates, typical of our colonies, fledging success is hindered both by too dry or too rainy seasons, even if these situations very rarely occur and the three-egg clutch is always the most productive In colder climates the breeding success of the common swift depends primarily on the climatic conditions of May. and in bad weather a two-egg clutch is more productive than a clutch of three In summary, the annual breeding success of the pallid swift appears little influenced by weather changes, due to a lesser impact of adverse conditions in Southern climates and the possibility, for this species, of shifting the laying period in response to a temporary worsening of climatic conditions     

Effect of temperature and precipitation on the annual diameter growth of Scots pine on drained peatlands and adjacent mineral soil sites in Finland

The climate conditions of the current and previous growing seasons have been shown to influence growth of coniferous trees in mineral soils sites. These dependencies may be different in peatlands where growth is generally more dependent on variations in soil water conditions. In the Nordic and Baltic countries, millions of hectares of peatlands and wetlands have been drained in order to enhance forest production. These drainage networks do not guarantee stable soil water conditions for the whole stand rotation. It is thus likely that precipitation in particular may have a different influence on annual growth in peatland to that in mineral soil sites. We studied the effect of precipitation and temperature on the inter-annual diameter growth of Scots pine (Pinus sylvestris L.) in Finland in drained peatland forests. The diameter growth data were limited to periods when growth response to drainage had levelled out. For comparison, growth data were also collected from adjacent mineral soil trees. The climate variables were monthly mean temperature and precipitation in a given location estimated from observations at the nearest weather stations by means of spatial smoothing. We used mixed linear models in describing the annual diameter growth of individual trees as a function of tree size and stand properties and expressed the residual variation as a function of climate parameters. The peatland and mineral soil growth variations showed different dependence on climate parameters. Peatland trees within 5 m of a ditch showed different climate responses compared to those located further away. Precipitation in July was negatively correlated with the diameter growth of peatland trees but there was no correlation with temperature. Growth of trees in mineral soils was positively correlated with March and April mean temperatures and May and June mean precipitation. The residual growth indices showed largely similar patterns in peatlands and mineral soil sites.     

Low soil water and nutrient availability below New Zealand kauri (<Emphasis Type="Italic">Agathis australis</Emphasis> (D. Don) Lindl.) trees increase the relative fitness of kauri seedlings

Tree species can affect the soil they are growing on and this might influence their fitness. The New Zealand gymnosperm tree species kauri (Agathis australis (D. Don) Lindl.) which grows in mixed angiosperm–gymnosperm forests has a substantial effect upon the soil. We studied the hypotheses that: (1) low soil moisture availability below mature kauri trees hampers growth of kauri seedlings and angiosperm seedlings, (2) low nutrient availability below kauri trees hampers only angiosperm seedlings, and (3) angiosperm seedlings are hampered more than kauri seedlings by the conditions below kauri trees. We tested these hypotheses by planting seedlings of kauri and mapau (Myrsine australis (A. Rich) Allan) under kauri trees and applying the following treatments: removal of herbs, removal of litter, removal of nutrient limitation, and elimination of root competition of mature kauri trees. The results indicate that low soil moisture availability, or the combination of low soil moisture availability and low nutrient fertility, hampers the growth of kauri as well as mapau seedlings below kauri trees. The mapau seedlings are hampered relatively more than the kauri seedlings which might result in an increased relative fitness of the latter.     

Application of Picea wilsonii roots to determine erosion rates in eastern Qilian Mountains, Northwest China

As a consequence of global change and human activities, processes of soil erosion are expected to increase in forested areas, resulting in exposed roots. Dendrogeomorphic research was conducted by analyzing exposed roots of Picea wilsonii subjected to continuous denudation along a main road in Tulugou National Forest Reserve, eastern Qilian Mountains, to reconstruct the local soil erosion dynamics. We determined the start of the exposure by examining the shifts in the ring-growth patterns from concentric to eccentric and by analyzing the detailed changes of wood anatomical features of exposed roots. We also find that the width of growth ring, the percentage of latewood and the average cell size of earlywood tracheids are all remarkable signs for soil lowering. According to the analysis of ANOVA (Fisher’s least significant difference method), the reduction of the cell size of earlywood tracheids is verified to be the key indicator for dating the first year of exposure and occurs prior to the other two indicators. Using 40 roots from 23 trees spread along the road, it has been found that erosion rates vary between 3.3 and 13.5 mm/year with an average value about 5.3 ± 2.1 mm/year. The intensity and occurrence of soil erosion may be influenced by the increase of human activities.     

Tracheid anatomical responses to climate in a forest-steppe in Southern Siberia

Tree-rings are precious natural archives to assess ecosystem variability over time. Xylem anatomy in woody tissue is a promising source of information in tree-rings since it is closely linked to tree hydraulics and carbon fixation. However, despite the rising interest for cell anatomy in dendrochronology, still little is known about the interpretation of the variability of cell anatomical responses observed across different environments and species.Here we analyze cell anatomical responses to increasing summer drought on 18 trees from 3 conifer species (Picea obovata, Pinus sylvestris, and Larix sibirica) growing in the transition zone between forest and steppe in the Republic of Khakassia (Russia). Analyses include the comparison of tracheid size distributions along climatic gradients and contrasting micro-topography from 1986 to 2008.Results indicate an overall decrease of earlywood tracheid lumen and cell wall thickness to high temperature and drought regardless of species and site conditions. In particular an increase of one degree Celsius during the summer caused up to 5% reduction of earlywood cell lumen and wall thickness. These anatomical shifts suggest that a downscaling of hydraulic efficiency is not paralleled by increased hydraulic safety, presumably due to carbon limitation.Based on the results of this case study, we suggest that increasing drought stress might hamper the formation of a functional xylem structure, thus being a possible trigger for a miss-acclimation causing long-term decline and higher exposure to hydraulic failures. Despite the promising study approach, more studies including more data (trees, years) and broader climatic gradients would be needed to further improve our mechanistic understanding.     

Tree-ring growth and intra-annual density fluctuations of Pinus pinaster responses to climate: does size matter?

Dendroclimatology generally assumes that climate–growth relationships are age and size independent. However, there is evidence that climate response can be unstable across different age/size classes. In addition, the occurrence of some anatomical features, such as intra-annual density fluctuations (IADFs), is age dependent. The present study investigates whether the climate–growth responses and the occurrence of IADFs in an even-aged stand of Pinus pinaster Ait., growing under Mediterranean climate, are also size-dependent. We randomly selected 60 P. pinaster trees falling within two stem diameter classes: small (<27 cm) and large (>35 cm). Tree rings were crossdated, measured and IADFs identified according to their position within the ring. The residual chronologies of both size classes were strongly correlated, suggesting a common signal. In fact, similar growth–climate relationships were observed in large and small trees. The frequency of IADFs was higher in large than in small trees, suggesting that IADFs were more likely to occur in wider rings of fast-growing trees. In both size classes, most of the IADFs were found in latewood. Latewood IADFs were triggered by the combination of dry June, wet September, and warm December, whereas IADFs located at the end of earlywood were triggered by previous winter precipitation and favorable conditions before summer (high precipitation for large trees and lower temperature for small trees). Our results suggest that IADFs can be a mechanism used at the individual level for adaptation to drought in P. pinaster. The climatic signal of IADFs between earlywood and latewood was mediated by stem size suggesting that future tree-ring studies should include trees stratified by size to better estimate the sensitivity of IADFs to climate.     

Do centennial tree-ring and stable isotope trends of Larix gmelinii (Rupr.) Rupr. indicate increasing water shortage in the Siberian north?

Tree-ring width of Larix gmelinii (Rupr.) Rupr., ratios of stable isotopes of C (δ¹³C) and O (δ¹⁸O) of whole wood and cellulose chronologies were obtained for the northern part of central Siberia (Tura, Russia) for the period 1864–2006. A strong decrease in the isotope ratios of O and C (after atmospheric δ¹³C corrections) and tree-ring width was observed for the period 1967–2005, while weather station data show a decrease in July precipitation, along with increasing July air temperature and vapor pressure deficit (VPD). Temperature at the end of May and the whole month of June mainly determines tree radial growth and marks the beginning of the vegetation period in this region. A positive correlation between tree-ring width and July precipitation was found for the calibration period 1929–2005. Positive significant correlations between C isotope chronologies and temperatures of June and July were found for whole wood and cellulose and negative relationships with July precipitation. These relationships are strengthened when the likely physiological response of trees to increased CO₂ is taken into account (by applying a recently developed δ¹³C correction). For the O isotope ratios, positive relationships with annual temperature, VPD of July and a negative correlation with annual precipitation were observed. The δ¹⁸O in tree rings may reflect annual rather than summer temperatures, due to the late melting of the winter snow and its contribution to the tree water supply in summer. We observed a clear change in the isotope and climate trends after the 1960s, resulting in a drastic change in the relationship between C and O isotope ratios from a negative to a positive correlation. According to isotope fractionation models, this indicates reduced stomatal conductance at a relatively constant photosynthetic rate, as a response of trees to water deficit for the last half century in this permafrost region.     

树木年轮宽度与气候变化关系研究进展

 树木的生长和立地环境密切相关并受多种气候因子的影响。树木年轮宽度的增加与温度、降水、太阳辐射、CO2浓度等气候因子有着复杂的相关关系。在干旱或半干旱地区,温度是限制树木生长的重要气候因子。生长季开始时最低温度的升高有利于延长生长季,与年轮宽度正相关;但是当生长季温度过高时,即使降水非常充裕,当年也只能形成窄年轮。生长季的温度过高则会加快土壤蒸发失水量并提高蒸汽压差,使土壤水分不足而不利于树木生长,因而生长季的高温多表现为与年轮宽度的负相关。生长期内降水量与树木的径向生长也成正相关,但当生长季的降水量充足或过多时,降水对树木径向生长不相关或负相关。受温度和降水共同调控的土壤湿度是树木径向生长的主要限制因子,良好的水分状况对树木生长起决定性作用。某一地区的太阳辐射能量高常会导致高温少雨,故高强度的太阳辐射使表土的湿度降低而不利于树木的径向生长。而在受季风影响的地区,树木年轮宽度的增加与当年雨季的气候变化关系不大。当年季风到来之前的气候（温度和降水）是树木生长的主要限制因子。有关CO2浓度的升高对树木生长的影响,研究的结果很不一致。一些温室实验及田间控制实验证明,CO2浓度的升高能对短命的一年生草本植物和植物幼苗产生“施肥效应”,并有利于其生长;还有些研究证明CO2浓度的升高能使高海拔地带的树木年轮宽度增加;但也有些研究认为CO2浓度的升高对生长在自然条件下的自然植被影响不大。近年来,有关树木径向生长和气候变化的研究越来越引起人们的关注,相关研究也取得了较大的进展。这些研究在帮助人们了解和研究古气候变化对森林植被的影响,以及预测未来全球变化对陆地生态系统的影响等方面有重要的理论和现实意义。综述了气候变化对树木年轮宽度影响的研究进展和应用,并概述了研究方法和发展前景,希望能加快和拓宽这一领域的发展。     

The effect of deficit irrigation on seasonal variations of plant water use in Olea europaea L.

A field experiment on olive trees (Olea europaea L.) was designed with the objective to search for an optimum irrigation scheduling by analyzing the possible effects of deficit irrigation. Treatments were: a non-irrigated control (rainfed) and three treatments that received seasonal water amount equivalent to 33 and 66% of crop evapotranspiration (ET_C) in the period August–September (respectively 33II and 66II), and 66% of (ET_C) from late May to early October (66I-II). Atmospheric evaporative demand and soil moisture conditions were regularly monitored. Irrigation effects on plant water relations were characterized throughout a growing season. Whole-plant water use, in deficit irrigated (66I-II) and rainfed olive trees, was determined using a xylem sap flow method (compensation heat-pulse technique). The magnitude of variations in water use and the seasonal dynamic of water relations varied among treatments, suggesting that olive trees were strongly responsive to both irrigation amount and time. Physiological parameters responded to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. All measurements of tree water status were highly correlated with one another. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the water balance technique. Deficit irrigation during the whole summer (66I-II) resulted in improved plant water relations with respect to other watering regimes; while, severe regulated deficit irrigation differentiated only slightly 33II treatment from rainfed plants. Nevertheless, regulated deficit irrigation of olive trees after pit hardening (66II) could be recommended, at least in soil, cultivar and environmental conditions of this study.