Growth response of Abies georgei to climate increases with elevation in the central Hengduan Mountains,southwestern China

Climatic harshness is expected to increase at higher elevations; however, elevational trends of tree radial growth response of high-elevation forests to climate change need to be investigated at different locations because of existing local variability in site-specific climatic conditions. We developed tree-ring width chronologies of Yunnan fir (Abies georgei) along elevation gradients at two sites in the central Hengduan Mountains (HM). High-elevation forests of A. georgei showed growth synchronicity and common growth signals along elevation gradients, indicating a common climatic forcing, although tree radial growth rates decreased with increasing elevation. Radial growth of Yunnan fir showed positive correlations with summer temperatures and February precipitation and moisture availability, but were negatively correlated with spring temperatures. The strongest positive relationship indicated summer (July) mean and minimum temperatures are the most important growth determining climatic factors for tree radial growth in the cold environment of HM, and this relationship revealed a clear elevational trend with stronger correlations at higher altitudes. In contrast, tree radial growth was negatively correlated with June precipitation and moisture availability. The whole study period 1954–2015 was split in two sub-periods of equal length. Comparing the early sub-period (1954–1984) to the later sub-period (1985–2015), tree growth response to the summer temperatures strongly increased, while it became weaker to June precipitation and moisture availability. High-elevation Yunnan fir forests in the HM currently benefit from elevated growing season temperatures under humid summer conditions. However, increasing temperatures may induce drought stress on tree radial growth if the observed decreasing trend in humidity and precipitation continues.     

不同去趋势方法的新疆东天山高低海拔雪岭云杉树轮宽度年表对气候的响应

利用新疆东天山高低海拔雪岭云杉(Picea schrenkiana Fisch.et Mey.)年轮样本,采用样条函数法、负指数函数法和区域曲线法3种去趋势方法研制树轮宽度标准化年表,通过分析不同去趋势方法的新疆东天山高低海拔云杉树轮宽度标准化年表特征、年表与气候响应的关系和年表间在不同频域互相关,及其与其它资料对比发现:(1)3种去趋势法对东天山年表质量影响较小,低海拔树轮年表含有更多可靠的气候变化信息。(2)高海拔的雪岭云杉树木径向生长与6—9月平均气温均呈显著正相关,其中标准化树轮年表与7月平均气温相关系数达0.553(P0.01,n=58),夏季温度可能是东天山高海拔雪岭云杉径向生长的主要限制因子;低海拔雪岭云杉树木径向生长与春季降水显著正相关,同时与春季平均气温显著负相关,春季高温和缺水共同作用的春旱可能是影响低海拔雪岭云杉径向生长的主导因子,且负指数函数年表对气候响应更敏感。(3)区域曲线法能够保留树轮标准化年表中更多低频信息,但优势不明显。高低海拔标准化年表在低频域相关系数较大,并和历史干旱事件有很好的对应。在东天山高低海拔雪岭云杉的去趋势方法研究中,负指数函数法比样条函数法和区域曲线法更适合。     

Long‐term physiological and growth responses of Himalayan fir to environmental change are mediated by mean climate

High‐elevation forests are experiencing high rates of warming, in combination with CO₂ rise and (sometimes) drying trends. In these montane systems, the effects of environmental changes on tree growth are also modified by elevation itself, thus complicating our ability to predict effects of future climate change. Tree‐ring analysis along an elevation gradient allows quantifying effects of gradual and annual environmental changes. Here, we study long‐term physiological (ratio of internal to ambient CO₂, i.e., C_i/C_a and intrinsic water‐use efficiency, iWUE) and growth responses (tree‐ring width) of Himalayan fir (Abies spectabilis) trees in response to warming, drying, and CO₂ rise. Our study was conducted along elevational gradients in a dry and a wet region in the central Himalaya. We combined dendrochronology and stable carbon isotopes (δ¹³C) to quantify long‐term trends in C_i/C_a ratio and iWUE (δ¹³C‐derived), growth (mixed‐effects models), and evaluate climate sensitivity (correlations). We found that iWUE increased over time at all elevations, with stronger increase in the dry region. Climate–growth relations showed growth‐limiting effects of spring moisture (dry region) and summer temperature (wet region), and negative effects of temperature (dry region). We found negative growth trends at lower elevations (dry and wet regions), suggesting that continental‐scale warming and regional drying reduced tree growth. This interpretation is supported by δ¹³C‐derived long‐term physiological responses, which are consistent with responses to reduced moisture and increased vapor pressure deficit. At high elevations (wet region), we found positive growth trends, suggesting that warming has favored tree growth in regions where temperature most strongly limits growth. At lower elevations (dry and wet regions), the positive effects of CO₂ rise did not mitigate the negative effects of warming and drying on tree growth. Our results raise concerns on the productivity of Himalayan fir forests at low and middle (<3,300 m) elevations as climate change progresses.     

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.     

Different trends and divergent responses to climate factors in the radial growth of Abies georgei along elevations in the central Hengduan Mountains

Climate change has been unprecedented in the last half-century. Tree growth dynamics and responses to climate warming at different elevations vary by study area due to regional diversity in site-specific climatic conditions in the central Hengduan Mountains. A. georgei is the dominant species in high-elevation montane forests in the central Hengduan Mountains. To study the response of A. georgei radial growth to climate and identify tree growth trends at different elevations, tree-ring width chronologies at four elevations across the subalpine A. georgei forest belt were built and growth-climate relationships were analyzed. The primary findings of this study were as follows: (1) radial growth rates of A. georgei decreased with elevation; (2) warming alleviated the limitation of low temperatures and abundant precipitation on tree radial growth at the highest sampling site; and (3) unlike at other elevations, the trend of trees basal area increment (BAI) at the lowest sampling site showed a significant decline over the past 20 years. This suggests the presence of an elevational inflection point, likely between 3800 m and 4000 m, where tree growth trends diverge. These results confirmed that A. georgei at higher elevation in the central Hengduan Mountains currently benefits from higher temperatures. However, the effects of drought on A. georgei at lower elevations would cause radial growth to decrease with climate warming. Therefore, it is critical to establish effective management strategies based on how A. georgei responds to climate change at various elevations.     

Asynchronous recruitment history of Abies spectabilis along an altitudinal gradient in the Mt. Everest region

Aims Forest growth and recruitment and their relationships to climate are complex. The aims of our study are (i) to examine the patterns in tree radial growth and recruitment along an altitudinal gradient in Mt. Everest region and (ii) to identify the climatic factors that are responsible to the observed patterns in tree growth and recruitment.Methods Four plots, each 30 × 60 m in size, were established from the lower to upper limits of the eastern Himalayan fir forest in Dingjie County of the Mt. Everest Nature Reserve, China. Dendrochronological techniques were applied to obtain information about the radial growth and age of the trees in the plots. Correlation analysis was used to identify the relationships between radial growth and recruitment of trees and climatic variables, i.e. monthly mean temperature and Palmer Drought Severity Index (PDSI). The population age structure was analyzed to investigate the recruitment history.Important findings The timberline plot was characterized by significant tree recruitments in the recent three decades and sporadic recruitments in earlier periods. The other three plots showed recruitment pulses during 1880–1910 in Plot 3?700 m, during 1870–80 and 1920–30 in Plot 3?520 m and during 1900–40 in Plot 3?410 m. The recruitment of fir trees in the timberline was sensitive to summer (June–September) temperatures, but it was mainly controlled by episodic disturbances in lower altitudes. Fir radial growth in the upper two plots was positively correlated with previous winter and current August temperature. Fir radial growth at the two lower plots was positively correlated with PDSI from previous September to current September.     

长白山不同海拔树木生长对气候变化的响应差异

以长白落叶松和红松为例,探讨了长白山地区不同海拔树木生长对气候变化的响应。利用长白山北坡不同海拔4个长白落叶松样点和6个红松样点的树轮宽度资料建立差值年表,通过聚类分析、相关分析和响应分析等方法,研究树木生长特征及其气候响应。结果表明:两个树种年表的平均敏感度、树轮宽度指数的年际变率、信噪比等特征值较高,反映年表含有较强的环境信息。随海拔升高,长白落叶松年表特征值呈先下降后增加的趋势,红松年表特征值则呈先增加后下降的趋势。聚类分析将长白落叶松年表分成高、低海拔两类,红松年表分成高、中、低海拔三类。树木生长对气候响应存在海拔差异。高海拔长白落叶松生长受当年气温影响;低海拔长白落叶松生长对气候存在"滞后响应"。高海拔红松不仅受降水限制,且对气温有"滞后响应";中海拔红松不仅受气温限制,且对降水有"滞后响应";低海拔红松生长主要受气温限制。     

滇西北玉龙雪山不同海拔云南松(Pinus yunnanensis)径向生长对气候因子的响应

云南松(Pinus yunnanensis)是重要的造林树种,在我国西南地区广泛分布。研究不同海拔云南松径向生长对气候变化的响应,有助于了解气候变化背景下云南松的敏感性和适应性。在滇西北丽江玉龙雪山不同海拔采集了云南松树木年轮样品,采用传统的树木年轮方法制作了不同海拔云南松树轮宽度标准化年表,并分析了不同海拔云南松径向生长与气候因子的相关性。结果表明:1)低海拔样点云南松具有较快的年平均生长速率。2)不同海拔云南松对气候因子的响应模式一致,树轮宽度与当年5—6月的降水量、帕尔默干旱指数(PDSI)和相对湿度呈正相关,与同期温度呈负相关。3)不同海拔的云南松径向生长对气象因子的响应程度不一样,即低海拔样点云南松树轮宽度与当年5月份的干旱指数、相对湿度、降水量相关系数较高;而高海拔样点的云南松树轮宽度与5—6月的降水、相对湿度、干旱指数的相关系数较低。研究表明春末夏初的水分条件是玉龙雪山云南松径向生长的主要限制因子,且低海拔地区云南松生长受水分限制更为严重,区域气候变暖和干旱化趋势可能对低海拔地区云南松的生长产生持续的负面效应。研究结果可为探讨气候变化下云南松的适宜分布区、以及云南松人工林的经营和可持续管理提供参考。     

芦芽山阳坡不同海拔华北落叶松径向生长对气候变化的响应

为研究树木生长对气候变化的响应状况,选取芦芽山阳坡的3个海拔高度建立了华北落叶松(Larix principis-rupprechtii)的树轮宽度年表。年表的统计参数表明,3条年表均为研究气候信息的可靠资料。结果表明,芦芽山阳坡华北落叶松的径向生长和生长与气候的关系均具有海拔差异,中海拔(2440 m)和高海拔(2540 m)的华北落叶松具有相似年际生长变化,而二者均与低海拔(2330 m)华北落叶松的年际生长不同。低海拔华北落叶松的生长与4月平均气温和上一年11月降水量显著负相关,而中海拔和高海拔的生长均与上一年10月平均气温和6月降水量显著负相关。通过年表与气候因子之间的滑动相关分析发现,3个海拔高度华北落叶松生长与气候因子的关系均不稳定,生长与气温条件之间的显著相关关系是随着气温升高而出现的。气温的升高引起了华北落叶松生长与气温因子关系的海拔差异,以及径向生长的海拔差异。这一结果对于气候变化对植被垂直梯度影响的研究具有一定参考价值。     

Ranging of <Emphasis Type="Italic">Rhinopithecus bieti</Emphasis> in the Samage Forest,China. II. Use of Land Cover Types and Altitudes

We investigated composition and structure of a temperate montane forest called Samage at Baimaxueshan National Nature Reserve in Yunnan, one of the last refuges for the highly endangered black-and-white snub-nosed monkeys (Rhinopithecus bieti). There is a patchwork of vegetation types at Samage, and we distinguished 6 major land cover types within the home range of the focal group. We tracked the semihabituated Gehuaqing band for a full annual cycle to study their habitat utilization and altitudinal ranging. We analyzed the group’s selective use of particular habitat types via selection ratios. We calculated habitat availability from a GIS database. We found that they used mixed deciduous broadleaf/conifer forest disproportionately to its availability in all months. Subjects completely avoided meadows. Pine and evergreen broadleaf forests acted as corridors between patches of mixed forest and monkeys visited them occasionally, but at low frequencies and mostly in transit. The focal band stayed at elevations ranging from ca. 2600 m to 4000 m, and the mean elevation used is 3200 m. We found evidence for seasonal variation in use of elevational zones. The band stayed at significantly higher elevations in summer than in spring. The descent in spring was likely related to a flush of immature leaves at low-lying elevations. Availability of preferred fruits also had a highly positive influence on altitudinal ranging, i.e., during months with high fruit availability (late summer, early fall), the band stayed at medium elevations where preferred fruits were most abundant. Higher concentrations of lichens and the snub-nosed monkeys’ search for not yet depleted fruits probably caused them to remain at mid-elevations in winter. There is no significant correlation between climate parameters and elevation used. One of the main inferences of this investigation is that, contrary to previous accounts, Rhinopithecus bieti is not universally associated with high-elevation dark fir forest, but at Samage exhibits an overwhelming preference for mixed forest. Moreover, our analyses support the hypothesis that elevational migration, in this temperate-subtropical forest, is influenced by the temporal fruiting of major food trees and that climate has only a negligible effect on elevation use. Dayong Li and Cyril C. Grueter contributed equally to the paper.     

Silver fir and Douglas fir are more tolerant to extreme droughts than Norway spruce in south‐western Germany

Improving our understanding of the potential of forest adaptation is an urgent task in the light of predicted climate change. Long‐term alternatives for susceptible yet economically important tree species such as Norway spruce (Picea abies) are required, if the frequency and intensity of summer droughts will continue to increase. Although Silver fir (Abies alba) and Douglas fir (Pseudotsuga menziesii) have both been described as drought‐tolerant species, our understanding of their growth responses to drought extremes is still limited. Here, we use a dendroecological approach to assess the resistance, resilience, and recovery of these important central Europe to conifer species the exceptional droughts in 1976 and 2003. A total of 270 trees per species were sampled in 18 managed mixed‐species stands along an altitudinal gradient (400–1200 m a.s.l.) at the western slopes of the southern and central Black Forest in southwest Germany. While radial growth in all species responded similarly to the 1976 drought, Norway spruce was least resistant and resilient to the 2003 summer drought. Silver fir showed the overall highest resistance to drought, similarly to Douglas fir, which exhibited the widest growth rings. Silver fir trees from lower elevations were more drought prone than trees at higher elevations. Douglas fir and Norway spruce, however, revealed lower drought resilience at higher altitudes. Although the 1976 and 2003 drought extremes were quite different, Douglas fir maintained consistently the highest radial growth. Although our study did not examine population‐level responses, it clearly indicates that Silver fir and Douglas fir are generally more resistant and resilient to previous drought extremes and are therefore suitable alternatives to Norway spruce; Silver fir more so at higher altitudes. Cultivating these species instead of Norway spruce will contribute to maintaining a high level of productivity across many Central European mountain forests under future climate change.     

Interannual growth response of Norway spruce to climate along an altitudinal gradient in the Tatra Mountains, Poland

Ring-width chronologies of Picea abies (L.) Karst. from ten sites in the Tatra Mountains, Poland, were developed to explore growth/climate responses in stands along an altitudinal gradient ranging from 839 to 1468 m a.s.l. There were positive relationships between current-year radial growth and mean monthly temperatures in March, April, June and July, but with increasing elevation, the strength of this correlation declined for March–April and increased for June–July temperatures. The mean monthly temperature in October of the previous year positively influenced radial growth of trees at all sites. Lower mean temperatures in January negatively affected growth of trees at the high-elevation sites. Trees at the low-elevation sites responded positively to a warm early spring, whereas trees at the high-elevation sites showed positive growth responses to higher summer temperatures. Growth of trees at the high-elevation sites was better explained by the temperature regime than was growth at the lower-elevation sites. Therefore, it is likely that these sites may be particularly sensitive to potential effects of temperature change.     

芦芽山阳坡不同海拔白杄径向生长对气候变暖的响应

使用树轮生态学方法研究了山西芦芽山建群种白杄(Picea meyeri)径向生长对气候变暖的响应状况, 发现随着气温升高, 不同海拔白杄生长与气候因子关系的变化存在差别。研究区气温可以分为1958-1983年的气温降低阶段和1984-2007年的气温升高阶段。由气温降低阶段进入气温升高阶段, 低海拔白杄树轮年表的序列间相关系数和第一主成分解释量均增大, 而高海拔白杄树轮年表的序列间相关系数和第一主成分解释量均减小, 表明气候条件对低海拔白杄生长的影响增强而对高海拔白杄生长的影响减弱。随着气温升高, 不同海拔白杄径向生长与气候因子的关系均出现了变化。1958-1983年, 低海拔(2 060 m)白杄生长与7月降水量显著正相关(p < 0.05), 而在1984-2007年, 这一关系表现为极显著正相关(p < 0.01), 同时与生长季中5-7月平均气温呈现显著负相关(p < 0.05)。海拔2 330 m, 白杄在1958-1983年与7月降水量极显著正相关(p < 0.01), 进入1984-2007年后与气候因子没有显著相关关系。海拔2 440 m, 白杄生长由1958-1983年的与气候因子没有显著相关关系转变为1984-2007年的与上一年10月平均气温显著负相关(p < 0.05)。高海拔(2 540 m)白杄生长在1958-1983年与上一年11月平均气温极显著负相关(p < 0.01), 在1984-2007年与上一年10月、当年1月平均气温和6月降水量均显著负相关(p < 0.05)。滑动相关分析结果表明, 随着气温升高, 低海拔主要气候因子对生长的影响增强, 而高海拔主要气候因子对生长的影响减弱, 这可能成为高海拔白杄生长对气温升高敏感性降低的原因。在气候变暖的驱动下, 海拔引起的白杄生长与气候因子关系的差异发生了变化。     

Growth variation in Abies georgei var. smithii along altitudinal gradients in the Sygera Mountains, southeastern Tibetan Plateau

A network of nine Smith fir (Abies georgei var. smithii) ring-width chronologies was constructed from sites ranging in elevation from 3,550 to 4,390 m above sea level (a.s.l.) in the Sygera Mountains, southeastern Tibetan Plateau. High-elevation trees had lower growth rates than did low-elevation trees. The mean tree-ring series intercorrelation (RBAR) increased with elevation. Principal component analysis identified three elevation zones (around 3,600, 3,800, and >4,200 m a.s.l.) with distinctive tree-ring growth patterns. Five chronologies with elevation >4,200 m a.s.l. were highly correlated. Overall, the initiation of tree-ring growth in Smith fir is controlled by common climatic signals, such as July minimum temperature, across a broad altitudinal range. Precipitation was not a growth-limiting factor across stands. Regardless of differences in stand elevation, topographical aspect, and tree age, the radial growth of Smith fir trees was markedly similar in response to common climatic signals, perhaps as a result of the relatively high-elevation of these forests (above 3,550 m a.s.l.) and the abundant summer monsoon rainfall. In addition, radial tree growth along the altitudinal gradients was indicative of a recent warming trend on the southeastern Tibetan Plateau.     

Anomalous temperature–growth response of Abies faxoniana to sustained freezing stress along elevational gradients in China’s Western Sichuan Province

A network of ten Faxon fir tree-ring width chronologies was constructed from sites ranging in elevation from 3,000 to 3,450 m in the Wolong Natural Reserve in Western Sichuan Province, China. The site chronologies display significant inter-site correlations (mean R = 0.647, p < 0.001) and the first principal component (PC1) accounts for 68.32 % of the total variation of the chronologies, implying a high degree of similarity in growth variation among the elevation gradients. Correlation analysis using monthly climate data indicates that the radial growth response of Faxon fir along the elevation gradients is markedly similar to common climatic signals, such as sunshine duration (positive) and cloud cover (negative), from January to March. Thus, it appears that winter freezing stress, which is caused by low solar radiation and high cloudiness, is the major environmental factor regulating the growth of trees across the elevational gradients. In addition, the site chronologies have no elevation-dependent growth responses to temperature or precipitation. Irrespective of the elevational differences of the sample sites, an anomalous reduction in radial growth occurred consistently since the 1960s, diverging from the instrumental temperature records since the 1990s. The cause of this divergence may be ascribed to the recent accelerated winter freezing stress and its role in controlling radial growth.     

Radial Growth of Two Dominant Montane Conifer Tree Species in Response to Climate Change in North-Central China

North-Central China is a region in which the air temperature has clearly increased for several decades. Picea meyeri and Larix principis-rupprechtii are the most dominant co-occurring tree species within the cold coniferous forest belt ranging vertically from 1800 m to 2800 m a.s.l. in this region. Based on a tree-ring analysis of 292 increment cores sampled from 146 trees at different elevations, this study aimed to examine if the radial growth of the two species in response to climate is similar, whether the responses are consistent along altitudinal gradients and which species might be favored in the future driven by the changing climate. The results indicated the following: (1) The two species grew in different rhythms at low and high elevation respectively; (2) Both species displayed inconsistent relationships between radial growth and climate data along altitudinal gradients. The correlation between radial growth and the monthly mean temperature in the spring or summer changed from negative at low elevation into positive at high elevation, whereas those between the radial growth and the total monthly precipitation displayed a change from positive into negative along the elevation gradient. These indicate the different influences of the horizontal climate and vertical mountainous climate on the radial growth of the two species; (3) The species-dependent different response to climate in radial growth appeared mainly in autumn of the previous year. The radial growth of L. principis-rupprechtii displayed negative responses both to temperature and to precipitation in the previous September, October or November, which was not observed in the radial growth of P. meyeri. (4) The radial growth of both species will tend to be increased at high elevation and limited at low elevation, and L. principis-rupprechtii might be more favored in the future, if the temperature keeps rising.     

Variations in carbon source–sink relationships in subalpine fir across elevational gradients

• Cold‐adapted trees display acclimation in both carbon source and carbon sink capacity to low‐temperature stress at their upper elevational range limits. Hence a balanced carbon source–sink capacity might be required for their persistence and survival at the elevational tree limits.
• The present study examined the spatial dynamics of carbon source–sink relationship in subalpine fir (Abies fargesii) trees along elevational gradients in the northern slope of the temperate region and in the southern slope of the subtropics in terms of climate in the Qinling Mountain range, north‐central China.
• The results showed that non‐structural carbohydrate (NSC) concentrations in both the source and sink tissues increased with the increase in elevation. The ratio of carbon source–sink displayed a consistent decreasing trend with the increase in elevation and during growing season, showing that it was lowest at a ratio of 2.93 in the northern slope and at a ratio of 2.61 in the southern slope at the upper distribution elevations in the late growing season. Such variations of carbon source–sink ratio might be attributable to the balance between carbon source and sink activities, which changed seasonally across the elevational distribution range.
• We concluded that a ratio of carbon source–sink of at least 2.6 might be essential for subalpine fir trees to persist at their upper range limits. Therefore, a sufficient source–sink ratio and a balanced source–sink relationship might be required for subalpine fir trees to survive and develop at their upper elevational distribution limits.

     

Tree growth response to drought and temperature in a mountain landscape in northern Arizona, USA

Aim To understand how tree growth response to regional drought and temperature varies between tree species, elevations and forest types in a mountain landscape. Location Twenty‐one sites on an elevation gradient of 1500 m on the San Francisco Peaks, northern Arizona, USA. Methods Tree‐ring data for the years 1950–2000 for eight tree species (Abies lasiocarpa var. arizonica (Merriam) Lemm., Picea engelmannii Parry ex Engelm., Pinus aristata Engelm., Pinus edulis Engelm., Pinus flexilis James, Pinus ponderosa Dougl. ex Laws., Pseudotsuga menziesii var. glauca (Beissn.) Franco and Quercus gambelii Nutt.) were used to compare sensitivity of radial growth to regional drought and temperature among co‐occurring species at the same site, and between sites that differed in elevation and species composition. Results For Picea engelmannii, Pinus flexilis, Pinus ponderosa and Pseudotsuga menziesii, trees in drier, low‐elevation stands generally had greater sensitivity of radial growth to regional drought than trees of the same species in wetter, high‐elevation stands. Species low in their elevational range had greater drought sensitivity than co‐occurring species high in their elevational range at the pinyon‐juniper/ponderosa pine forest ecotone, ponderosa pine/mixed conifer forest ecotone and high‐elevation invaded meadows, but not at the mixed conifer/subalpine forest ecotone. Sensitivity of radial growth to regional drought was greater at drier, low‐elevation compared with wetter, high‐elevation forests. Yearly growth was positively correlated with measures of regional water availability at all sites, except high‐elevation invaded meadows where growth was weakly correlated with all climatic factors. Yearly growth in high‐elevation forests up to 3300 m a.s.l. was more strongly correlated with water availability than temperature. Main conclusions Severe regional drought reduced growth of all dominant tree species over a gradient of precipitation and temperature represented by a 1500‐m change in elevation, but response to drought varied between species and stands. Growth was reduced the most in drier, low‐elevation forests and in species growing low in their elevational range in ecotones, and the least for trees that had recently invaded high‐elevation meadows. Constraints on tree growth from drought and high temperature are important for high‐elevation subalpine forests located near the southern‐most range of the dominant species.     

Effects of climate on diameter growth of co-occurring Fagus sylvatica and Abies alba along an altitudinal gradient

In high-elevation forests, growth is limited by low temperatures, while in Mediterranean climates drought and high temperatures are the main limiting factors. Consequently, the climate-growth relationships on Mont Ventoux, a mountain in the Mediterranean area, are influenced by both factors. Two co-occurring species were studied: silver fir (Abies alba Mill.) and common beech (Fagus sylvatica L.), whose geographical distribution depends on their low tolerance to summer drought at low altitude/latitude, and low temperatures (late frost and short length of the growing season) at high altitude/latitude. Firs and beeches distributed along an elevational gradient were investigated using dendroecological methods. Silver fir growth was found to be more sensitive to summer water stress than beech. On the other hand, beech growth was more impacted by extreme events such as the 2003 heat wave, and negatively related to earlier budburst, which suggests a higher sensitivity to late frost. These results are confirmed by the different altitudinal effects observed in both species. Beech growth decreases with altitude whereas an optimum of growth potential was observed at intermediate elevations for silver fir. Recent global warming has caused a significant upward shift of these optima. As found for the period 2000–2006, rising temperatures and decreasing rainfall may restrain growth of silver fir. If these trends continue in the future beech might be favored at low altitudes. The species will have a reduced capacity to migrate to higher altitudes due to its sensitivity to late frosts, although an upward shift of silver fir is likely.     

Variations in the timing of different phenological stages of cambial activity in Abies pindrow (Royle) along an elevation gradient in the north-western Himalaya

High mountains around the globe are some of the most vulnerable ecosystems to climate change and of great concern for conservation. The Himalayan Mountains are experiencing a higher warming than average global warming, which can significantly impact their biodiversity, vegetation distribution and ecosystem structure. There is a need to study the process of wood formation in Himalayan conifers to have a better understanding of their growth responses to predicted climate change. Variations in the climatic factors regulating cambial activity would result in changes in the timing of cambial phenology. In this study, the variations in the timing of different stages of cambial phenology (cell enlargement stage, wall-thickening stage and cell maturation stages) in pindrow fir (Abies pindrow) were investigated from anatomical observations of wood microcores collected during 2014-15 along an elevation range of c.2300−3000 m asl in the north-western Himalaya. The onset of all three cambial phenological stages was significantly correlated with elevation, with onset of cambial activity happening more than a week earlier at the lowest elevation than at the highest elevation site. Although the termination of wall-thickening and maturation stage appeared minimally related to elevation, the cell-enlargement stage showed significant correlation with elevation, with tracheid formation ceasing approximately three weeks earlier in trees at the highest elevation. The timing of these phenological stages did not show strong variations between the two study years. Our findings provide new data on the timings of cambial phenophases and help to understand tree growth response to ongoing changing climate in the Himalayan region.