川西亚高山不同年龄紫果云杉径向生长对气候因子的响应

运用树木年轮气候学的基本方法,建立王朗自然保护区紫果云杉在集中分布上限区域的年轮宽度年表,选取差值年表分析不同年龄云杉的径向生长同逐月气候因子的相关及响应关系,结果显示:幼龄组云杉年表的敏感度高于中龄组和老龄组云杉,幼龄组云杉对生长季前及生长季的气温状况显著正相关;中龄组云杉年表仅与当年4月份和7月份的月平均最低气温显著正相关;老龄组云杉的年轮宽度指数同上年生长季(上年8月份)的月平均气温和月平均最低温显著负相关,上年生长季高温的"滞后效应"在老龄组云杉体现的更为突出;幼龄组与中龄组云杉对当年6月份降水持续增加显示出明显的负相关关系,上年12月份的降水会对幼龄组和老龄组云杉径向生长不利。研究表明幼龄组云杉包含的气候信息要优于中龄组和老龄组云杉,在该区域进行相关研究时应根据研究需要选取不同年龄跨度的云杉年表。     

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against ¹³C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced ¹³C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.     

Exploring the growth response of Norway spruce (Picea abies) along a small-scale gradient of soil water supply

The climate-growth response of specific sites and species is one of the main research subjects in classic tree ring studies. Traditional sampling approaches therefore aim at maximizing the climate signal of the analyzed tree ring series, which is typically achieved by focusing on dominant trees or on sites located in particularly temperature or moisture limited environments. However, there is increasing evidence that these selective sampling strategies cannot yield chronologies that are representative for entire populations. One promising approach to gain a deeper understanding of forest dynamics and climate-growth responsiveness is the analysis of climate signal ranges among trees. This individualistic approach requires random sampling and the integration of information on small-scale heterogeneities in site and tree characteristics. Here, we analyze the climate-growth response of 144 Norway spruce trees (Picea abies Karst.) on difference levels of data aggregation. The aim of our study is to investigate the relevance of small-scale heterogeneities in site conditions, particularly in soil water supply, for the detected climate-growth signal. We identify soil water supply and site characteristics, which indirectly modify the water availability for trees, as dominating growth factors across scales. The driest sites show the strongest climate-growth reaction, while the growth response of wetter sites is weak or even insignificant. Therefore, we conclude that integrating small-scale information on site characteristics, particularly on soil water supply, can help to gain a deeper understanding of species specific growth limitations.     

Rapid warming induces the contrasting growth of Yezo spruce (Picea jezoensis var. microsperma) at two elevation gradient sites of northeast China

Accurately assessing the impact of climate changes on tree growth or forest productivity is vital to better understand global carbon cycles. Here, we carried out dendroclimatological research on Yezo spruce (Picea jezoensis var. microsperma) along an elevation gradient in two sites to investigate the effect of rapid warming on spruce growth in northeast China. Results indicated that trees at two low-elevation sites had significantly wider ring widths and higher basal area increment (BAI) compared with high-elevation sites. Ring widths and BAI of Yezo spruce at low elevations showed a clear growth increase during the 1940s–1970s followed by a significant decline after 1980. However, trees at high elevations showed a relatively stable growth during the 1940s–1970s followed by a significant increase after 1980. Rapid warming after 1980 increased the radial growth of Yezo spruce at high-elevation sites, but reduced tree growth at low-elevation sites. Winter precipitation and growing season temperature were positively correlated with radial growth of Yezo spruce at high elevations, but negatively correlated with tree growth at low elevations. A clear pattern of growth and growth-climate relationship changed in 1980. The temperature threshold for determining the impact of climate on Yezo spruce could change with latitude or site. Difference in drought caused by warming may be the main reason for the opposite response of tree growing at different altitudes in northeast China. The mechanism of rapid warming driving contrasting growth at different elevations should also be investigated in other tree species in NE Asia. In the context of future climate warming, our findings are of great significance for tree growth in assessing forest dynamics and carbon cycling.     

No systematic effects of sampling direction on climate-growth relationships in a large-scale,multi-species tree-ring data set

Ring-width series are important for diverse fields of research such as the study of past climate, forest ecology, forest genetics, and the determination of origin (dendro-provenancing) or dating of archaeological objects. Recent research suggests diverging climate-growth relationships in tree-rings due to the cardinal direction of extracting the tree cores (i.e. direction-specific effect). This presents an understudied source of bias that potentially affects many data sets in tree-ring research.In this study, we investigated possible direction-specific growth variability based on an international (10 countries), multi-species (8 species) tree-ring width network encompassing 22 sites. To estimate the effect of direction-specific growth variability on climate-growth relationships, we applied a combination of three methods: An analysis of signal strength differences, a Principal Component Gradient Analysis and a test on the direction-specific differences in correlations between indexed ring-widths series and climate variables.We found no evidence for systematic direction-specific effects on tree radial growth variability in high-pass filtered ring-width series. In addition, direction-specific growth showed only marginal effects on climate-growth correlations. These findings therefore indicate that there is no consistent bias caused by coring direction in data sets used for diverse dendrochronological applications on relatively mesic sites within forests in flat terrain, as were studied here. However, in extremely dry, warm or cold environments, or on steep slopes, and for different life-forms such as shrubs, further research is advisable.     

Climate-tree-growth relationships of European beech (Fagus sylvatica L.) in the French Permanent Plot Network (RENECOFOR)

The influence of climate on the radial growth of Fagus sylvatica was investigated using 15 chronologies developed from mature stands of the French Permanent Plot Network (RENECOFOR) growing under different climatic and soil conditions. The relationships between climate and ring widths were analyzed using extreme growth years, simple correlations and response functions analysis. Monthly climatic regressors were derived by a physiological water balance model that used daily climatic data and stand parameters to estimate soil water deficits. The three most frequent negative pointer years (1959, 1989, 1976) result from a particularly intense and durable drought, whereas positive years (1977, 1958) coincide with wet conditions. The total ring chronology variance attributable to climate averages 34.1% (15.8% –57%). Current early-summer soil water deficit enters in 10 models and the deficit in June explains alone a large part of the radial growth variability (mean value: 26.6%). Temperature or soil water deficit for the other months and weather conditions during the previous season were of little consistency across stands. The response pattern of earlywood is very similar and the percentage of variance explained is higher (16.2% –57.8%). Latewood widths present a different response pattern. High minimum temperature in August and/or September often favour wide latewood widths and monthly water deficits play a secondary role. The percentage of variance explained ranges from 8.8% to 67.4%. Soil water capacity strongly modulates ring characteristics and climate-growth relationships. Mean sensitivity, expressed population signal, signal-to-noise ratio and the strength of growth-climate correlations increase with decreasing soil water capacity.     

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.     

Tree growth and climate sensitivity in open and closed forests of the southeastern Tibetan Plateau

The creation of forest openings is a frequently observed phenomenon in many types of forests. On the southeastern Tibetan Plateau, where the average elevation is greater than 4000 m above sea level, differences in tree growth between forest stands with openings and completely closed stands are poorly characterized. Here, we presented a dendrochronological study of Tibetan juniper (Juniperus tibetica Kom.) and Sikkim spruce (Picea spinulosa (Griff.) Beissn.) in an open and a closed stand, near Qamdo of eastern Tibet. We found that the growth of juniper responded to climate in a similar way in the open and closed stands, and was positively correlated with temperature from October to January and with the Palmer Drought Severity Index (PDSI) from September to June. In contrast, the growth of spruce responded to climate differently in the open and closed stands: growth was positively correlated with the PDSI from September to May in the open stand, whereas it was positively correlated to November and December temperatures (of the prior year) and current June temperature in the closed stand. Interannual variation in, and standard deviations among, juniper tree ring widths were similar in both stands for the past four centuries, whereas they differed in spruce over the past two centuries, particularly in the 1900s. These results suggest that juniper tree ring growth is less sensitive to stand structure than that of spruce, thus providing more reliable climate signals. The data obtained from our study will help forest managers understand the ecology of juniper and spruce in open and closed stands and are therefore useful for management planning.     

Climatic signal in annual growth variation in damaged and healthy stands of Norway spruce [Picea abies (L.) Karst.] in southern Finland

Annual radial increment variation and its dependence on temperature and precipitation were studied in 13 severely damaged and 12 healthy stands of Norway spruce [Picea abies (L.) Karst.] on mineral soil and peatlands in southern Finland. An intervention analysis revealed that the presently dead trees had experienced severe growth reductions since the late 1980s. Even though annual growth variation between the stands was fairly similar, differences in the relationship between radial growth and weather variation were found between sites. High temperature in May increased radial growth in the healthy stands. This was not observed in the damaged stands, where summer temperature was negatively correlated with growth. In addition, high temperature during the previous summer decreased tree growth in next summer. Tree-ring indices showed a strong positive correlation with June precipitation in the damaged stands. Correlation of June precipitation and growth was much weaker in the healthy stands. This result suggests that the damage is connected to drought and is likely to occur at drought-sensitive sites. The finding also fits in well with the fact that many of the damaged sites were rocky or stony.     

不同去趋势方法对树轮气候信号识别的影响

树木生长受到气候因子、随年龄增长的内在生长趋势、环境干扰和其他扰动信号的影响。目前存在不同的去趋势方法对树木年轮进行去趋势以识别树木生长中的气候信号。以往的研究多基于单个方法识别树轮气候信号，而不同去趋势方法识别的树轮气候信号可能会有一定的差别。为了对比不同去趋势方法对树轮气候信号识别的影响，我们基于国际年轮数据库网站获取中国西部地区68个点的树轮宽度数据，采用最常用的"signal-free"方法（SsfCrn）、线性和负指数函数法（std）、67%样条函数法（spline）、firedman方法、以及基于经验模式分解去趋势方法（EEMD）5种去趋势方法分别建立树轮年表，并对比分析同一地点的不同年表对气候响应的异同。结果表明：不同去趋势方法得到的年表对温度、降水以及相对湿度等气候因素的响应具有明显差异。其中，SsfCrn去趋势方法建立的年表对温度（月平均温、月最低温、月平均最低温）响应中相关最高的样点在所有样点中占比最高；EEMD去趋势方法建立的年表对降水量、相对湿度和月最高温响应中相关最高的样点在所有样点中占比最高；firedman去趋势方法建立的年表对月平均最高温响应中相关最高的样点在所有样点中占比最高。研究结果表明SsfCrn，EEMD和firedman方法在识别树轮气候信号方法具有一定的优势。在不同研究区域中，不同去趋势方法建立的年表对不同气候条件响应有差异，因此选择不同的去趋势方法识别树木生长趋势，分析哪种方法可以更好的反应气候变化对树木生长的影响显得尤为重要。     

Determinants of mountain birch growth in situ: effects of temperature and herbivory

Variation in in situ growth performance of the mountain birch as indicated by the widths of annual rings was analysed and related mainly to temperature and herbivory using ring width series from five heath forest sites in the Lake Torneträsk area, northern Sweden. Climate explained 48–64% of the variation in age-corrected mean ring width series. In general, the effect of current year July followed by June temperature was most important at all sites. A warm May resulted in wider rings due to an earlier budburst. Short-term (inter-annual) responses to increased temperature were in most cases not reflected into long-term responses (decades). A large proportion of the variation in stem mean ring width was due to variation among stems within trees (81%) in these polycormic trees, while variation among sites was marginal (0.4%). Within trees, main stems grew faster and were more responsive to climate variation than subordinate stems. No effect of insect herbivory on ring width was found at low defoliation levels (≤12%). At a defoliation level of ca 84% a one-year reduction in stem growth was observed while the growth reduction (ca 50% reduction in ring width) lasted for 4 yr after ca 93% defoliation. After outbreaks resulting in complete defoliation and some stem mortality, ring widths of surviving stems mainly responded with increased growth. Basal sprouts, emerging just after a severe insect outbreak with a high mortality of old stems, grew faster than sprouts occurring during other periods. It is concluded that the mountain birch is well adapted to recover from Epirrita outbreaks; the ability to produce basal sprouts, that can benefit from an existing root system for fast initial growth, is one important mechanism for this.     

Dendroclimatic evaluation of climate-growth relationships of Meyer spruce (Picea meyeri) on a sandy substrate in semi-arid grassland, north China

Dendroclimatic techniques were used to assess the climate-growth relationships of refugial Meyer spruce (Picea meyeri Rehd. et Wils.) on a sandy substrate in semi-arid grassland of north China. Statistical analysis of the tree-ring data showed a mean series intercorrelation of 0.47, a signal-to-noise ratio of 14.44, and a mean sensitivity of 0.18, indicating suitability for climatic analysis. Radial growth was positively correlated with precipitation in February and May of the current year, and in September of the preceding year. However, radial growth of Meyer spruce also correlated negatively with mean monthly temperature in current May, of which mean maximum temperature explained most of the observed variation. In addition, radial growth negatively correlated with solar radiation over most of the year. Rainfall appeared to be the dominant growth-limiting factor in this semi-arid grassland, with temperature and solar radiation being of lesser importance. This study suggests that Meyer spruce in this stand is promising for dendroclimatic and ecological studies because of good cross-dating characteristics and high sensitivity to climate.     

不同生境和去趋势方法下的祁连圆柏径向生长对气候的响应

利用青海不同生境祁连圆柏树木年轮样本,采用3种不同去趋势方法建立树轮年表,结合青海30个气象站的气象资料,分析不同生境和去趋势方法下祁连圆柏径向生长对气候的响应差异。结果表明,祁连山区,生长季前期的平均气温是祁连圆柏树木径向生长的主要限制性因子,NEP树轮标准化宽度年表与生长季前期冬季平均气温相关最好;在柴达木盆地,生长季降水量是该地区树木径向生长的限制性因子,SPL树轮年表对生长季降水量相关较好;在青南高原,祁连圆柏径向生长对春季温度响应最为敏感,而SPL年表与春季温度呈现明显的负相关关系,相关系数达-0.606;而在青海东部地区,祁连圆柏树木径向生长对气候的响应总体不显著。位于青海西部和北部的柴达木盆地和祁连山区祁连圆柏径向生长受西风气候的影响显著,尤其是柴达木盆地,其气候受西风主导;而青南高原受西南季风影响更为显著,该地区祁连圆柏径向生长同时受西南季风气候和海拔高度两方面影响;在青海东部,祁连圆柏径向生长受东亚季风影响更为显著。     

Effect of climate on tree growth in the Pampa biome of Southeastern South America: First tree-ring chronologies from Uruguay

Tree-ring research in the highland tropics and subtropics represents a major frontier for understanding climate-growth relationships. Nonetheless, there are many lowland regions – including the South American Pampa biome – with scarce tree ring data. We present the first two tree-ring chronologies for Scutia buxifolia in subtropical Southeastern South America (SESA), using 54 series from 29 trees in two sites in northern and southern Uruguay. We cross-dated annual rings and compared tree growth from 1950 to 2012 with regional climate variability, including rainfall, temperature and the Palmer Drought Severity Index – PDSI, the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Overall, ring width variability was highly responsive to climate signals linked to water availability. For example, tree growth was positively correlated with accumulated rainfall in the summer-fall prior to ring formation for both chronologies. Summer climate conditions were key for tree growth, as shown by a negative effect of hot summer temperatures and a positive correlation with PDSI in late austral summer. The El Niño phase in late spring/early summer favored an increase in rainfall and annual tree growth, while the La Niña phase was associated with less rainfall and reduced tree growth. Extratropical climate factors such as SAM had an equally relevant effect on tree growth, whereby the positive phase of SAM had a negative effect over radial growth. These findings demonstrate the potential for dendroclimatic research and climate reconstruction in a region with scarce tree-ring data. They also improve the understanding of how climate variability may affect woody growth in native forests – an extremely limited ecosystem in the Pampa biome.     

Preliminary dendrochronological studies on Populus euphratica in the lower reaches of the Heihe River basin in northwest China

To date, preliminary dendrochronological studies of Euphrates poplar trees have not been reported, even though some dendrohydrological studies have been performed. Based on 11 sites in the Ejin Oasis, located in the lower reaches of the Heihe River basin in northwest China, our studies of Euphrates poplar trees concentrates on the characteristics of the tree-ring structure and the stem radial growth, which is the foundation of accurate crossdating and detrending. The results indicate that the Euphrates poplar trees studied have identifiable ring boundaries; the narrow rings and the same ring width variation pattern make the crossdating achievable. The local habitat, the stem radial growth trend of the poplar trees and the completeness of cores should be valued when selecting a detrending method. The complex ring width variation pattern of the poplar trees in the area suggest that comprehensive sampling and preliminary dendrochronological studies are required to gain insights into the spatiotemporal growth patterns of the poplar trees and their relationship with the changing water environment.     

夏干萨特树轮年表中降水信息的探讨与326年降水重建

夏干萨特位于新疆天山中段乌鲁木齐河山区,在此干旱,半干旱地区的森林下限所采集的天山云杉（Picca schrenkinna var.tianschanica)树轮样本建立的树轮宽度年表具有最大的平均敏感度和最大的标准关蓄意助记词件表可能含有最多的降水信息,利用相关分析的方法计算了年轮宽度指数和气候要素间的相关关系,发现5月份的降水与年表的相关最好,进一步计算了旬降水量与年表的相关关系,发现5月下旬的降水与年表的单相关系灵敏最高,5月下旬的降水是森林下限云杉生长的主要限制因子,这一结果具有明确的树木生理学意义,云杉直径生长主要在5-8月份,其中5-6月份形成大约一半的年轮,5月下旬位于年轮形成关键时段的中部,为轮宽形成的最活跃期,同时5月下旬降水的大变率与夏干萨特年表的高敏感度相一致,从另一角度说明了5月下旬降水作为森林下限天山云杉生长关键限制因子的适当性,重建了大西沟气象站5月下旬326a的降水并分析了其长期变化特征,发现320多年来的5月下旬降水分为8个完整的干湿变化阶段,其中最干期出现在1701-1722年,最湿期出现在1961-1981年,降水序列具有显著的64,32,22,14,5和11a变化周期,其中的22a及11a周期与太阳黑子的活动周期相一致,利用方差分析方法进行了未来干湿变化趋势的预测,发现2001-2007年以降水偏少为主,而2008-2015年以偏多为主,这对于北疆重要粮食作物冬小麦的长期产量预报及种植规划具有重要意义。     

Growth and response patterns of Picea crassifolia and Pinus tabuliformis to climate factors in the Qilian Mountains,northwest China

Different tree species exhibit different phenological and physiological characteristics, leading to complexity in inter-species comparison of stem radial growth response to climate change. This study explored the climate-growth responses of Qinghai spruce (Picea crassifolia) and Chinese pine (Pinus tabulaeformis) in the Qilian Mountains, Northwest China. Meanwhile, Vaganov-Shashkin model (VS-oscilloscope) was used to simulate the relationships between radial growth rates and phenology. The results showed that 1) in their radial growth patterns, Qinghai spruce showed a significant increasing trend, while Chinese pine showed a decreasing trend, and Qinghai spruce has a longer growing season than Chinese pine. 2) For the radial growth-climate dynamic response, Qinghai spruce was influenced in an unstable manner by the mean temperature in the mid-growing season of the current year and the late growing season of the previous year and by the mean minimum temperature in the mid-growing season of the current year, while Chinese pine was influenced in a stable manner by the mean temperature and mean maximum temperature during the growing season of the current year. 3) The radial growth rates of the two conifer species were limited by temperature at the initiation and cessation of growth and by soil moisture at the peak of growth. But Chinese pine was more severely affected by soil moisture than Qinghai spruce in the middle of growth. Therefore, different management and restoration measures should be taken based on the differences in ecological responses and physical and physiological properties of the two conifer species to climate change in the subalpine forest ecosystems in the semiarid and arid regions of Northwest China.     

European beech in its northeasternmost stands in Europe: Varying climate-growth relationships among generations and diameter classes

Age, genetics and social status of trees affect their sensitivity to environmental factors, and information about such effects is needed for comprehensive assessment of growth potential. Climatic sensitivity of radial increment (i.e., tree-ring width) of introduced European beech (Fagus sylvatica L.) of different generations and social status, growing in its northeasternmost stands in Europe, was studied by dendroclimatological methods. At present, the studied stands occur outside of the natural distribution area of the species, providing opportunity to study adaptability and potential growth of beech in novel environments under changing climate. The sensitivity of radial growth to climatic factors was modulated by the generation and social status (size) of trees. The first generation trees, which were propagated from the material transferred from the northern Germany, were highly sensitive to climatic factors and showed wide spectrum of responses. The dominant trees were particularly sensitive to June precipitation, indicating sensitivity to water deficit in summer. The suppressed trees were mainly sensitive to temperature in the dormant period. Tree-ring width of the second generation trees, which were propagated from the first generation stands, was mainly affected by water deficit in summer, yet the local factors, modulated the mechanisms of response. In one stand, tree-ring width was affected by conditions during the formation of tree-ring, indicating direct influence of weather conditions on xylogenesis. In the other stand, tree-ring width was correlated to weather conditions in the preceding year, suggesting influence via carbohydrate reserves. The effect of social status on climatic sensitivity in the second generation stands was considerably weaker, likely due to the natural and anthropogenic selection of the material best adapted for local conditions. The effect of climatic factors on radial growth of beech has shifted during the 20th century. The effect of autumn temperature has weakened, likely due to warming; the effect of factors related to water deficit in summer has intensified that could be related to both, changes in climate and ageing. The observed climate-growth relationships suggested that conditions in winter have become suitable for beech, yet careful selection of sites/regions with appropriate hydrological conditions appear necessary to counteract the increasing effect of water deficit, hence to ensure productivity of future beech sites in Latvia.     

The decline of Algerian Cedrus atlantica forests is driven by a climate shift towards drier conditions

Some north-African Atlas cedar (Cedrus atlantica) forests are in decline, following decades of anthropogenic pressure and repeated drought events. We investigated if the recent decline episodes of these forests are linked to precipitation and temperature shifts, leading to a reduction in tree radial growth and climate-growth uncouplings. Tree-ring width chronologies of Atlas cedar in north-western Algeria allow the identification of climate and growth shifts in these vulnerable Mediterranean forests. Such chronologies, built for six sites, showed common patterns of year-to-year variability during the period 1910-2006. The growth at north-facing sites declined from the 1980s until 2006, whereas the growth at mid-elevation sites declined from the early 20th century until the 1940s, remained stable until the 1980s and then declined until 2006. Cool and wet spring conditions enhanced cedar growth. Sites with fast-growing trees, where growth was strongly reduced by dry summer conditions, showed the most-pronounced negative trends. However, a clear climate-growth uncoupling was observed after the 1970s, when the climate rapidly warmed. We also detected a negative growth shift in the 1980s, when mortality increased. This coincided with changes in early-warning signals of the growth series, such as an increase in the first-order autocorrelation of tree-ring width. All these lines of evidence indicate that the 1980s climate shift towards warmer and drier conditions triggered a shift in cedar growth. The use of radial-growth series as early-warning signals should be further investigated in this species and in other drought-sensitive conifers, given the aridification trends expected for the Mediterranean Basin.     

Earlywood vessel size of oak as a potential proxy for spring precipitation in mesic sites

Aim In this study, we evaluate the importance of the mean earlywood vessel size of oaks as a potential proxy for climate in mesic areas. Location The study was conducted in Switzerland at three forest sites dominated by oak (Quercus petraea and Q. pubescens). The three sites were in different climatic zones, varying mainly in terms of precipitation regime. Methods Three 50‐year‐long site chronologies of mean earlywood vessel size and tree‐ring widths were obtained at each site and related to monthly meteorological records in order to identify the main variables controlling growth. The responses of mean vessel size to climate were compared with those of the width variables to evaluate the potential climatic information recorded by the earlywood vessels. Results The results show that the mean vessel size has a different and stronger response to climate than ring‐width variables, although its common signal and year‐to‐year variability are lower. This response is better in particular at mesic sites, where it is linked to precipitation during spring, i.e. at the time of vessel formation, and is probably related to the occurrence of only a few processes controlling vessel growth, whereas radial increment is controlled by multiple and varying factors. Main conclusions The mean earlywood vessel size of oak appears to be a promising proxy for future climate reconstructions of mesic sites, where radial growth is not controlled by a single limiting factor.