Environmental control of vessel traits in Quercus ilex under Mediterranean climate: relating xylem anatomy to function

Time series of tree-ring anatomical features are important to understand the functional role of xylem plasticity over the life span of trees, and thus to reconstruct past ecological and climatic conditions. Holm oak (Quercus ilex L.) is a drought-tolerant tree widely distributed in the Mediterranean Basin. Chronologies of tree-ring width (TRW), vessel lumen area [maximum (MAX) and mean (MVA)] and vessel density (VD) were developed for the period 1942–2001. Each ring was divided into three sections to compare the intra-annual variation of vessel features with the climate conditions during the growing season. The common variability of tree-growth and vessel features was analyzed using a principal component analysis (PCA). Vessel lumen area (MAX and MVA) and TRW loaded positively on the first axis (PC1), whereas VD from the first and second part of the ring (VD1 and VD2) loaded negatively, suggesting that these variables share a common variance. On the other hand, VD in the last third of the ring (VD3) loaded positively on second axis (PC2). PC1 showed a strong positive correlation with precipitation during the hydrological year (prior October–September) and a negative correlation with temperature in spring (April–May), while PC2 showed a negative correlation with precipitation in June. Our results showed that TRW and vessel lumen area were mainly dependent on moisture conditions along the growing season, while vessel density probably plays an important role in the balance between hydraulic conductivity and safety to embolism to better adjust the hydraulic system to water availability.     

A lonely dot on the map: Exploring the climate signal in tree-ring density and stable isotopes of clanwilliam cedar,South Africa

Clanwilliam cedar (Widdringtonia cedarbergensis; WICE), a long-lived conifer with distinct tree rings in Cape Province, South Africa, has potential to provide a unique high-resolution climate proxy for southern Africa. However, the climate signal in WICE tree-ring width (TRW) is weak and the dendroclimatic potential of other WICE tree-ring parameters therefore needs to be explored. Here, we investigate the climatic signal in various tree-ring parameters, including TRW, Minimum Density (MND), Maximum Latewood Density (MXD), Maximum Latewood Blue Intensity (MXBI), and stable carbon and oxygen isotopes (δ¹⁸O and δ¹³C) measured in WICE samples collected in 1978. MND was negatively influenced by early spring (October-November) precipitation whereas TRW was positively influenced by spring November-December precipitation. MXD was negatively influenced by autumn (April-May) temperature whereas MXBI was not influenced by temperature. Both MXD and MXBI were negatively influenced by January-March and January-May precipitation respectively. We did not find a significant climate signal in either of the stable isotope time series, which were measured on a limited number of samples. WICE can live to be at least 356 years old and the current TRW chronology extends back to 1564 CE. The development of full-length chronologies of alternative tree-ring parameters, particularly MND, would allow for an annually resolved, multi-century spring precipitation reconstruction for this region in southern Africa, where vulnerability to future climate change is high.     

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.     

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.     

Diverse climate sensitivity of Mediterranean tree-ring width and density

Understanding long-term environmental controls on the formation of tree-ring width (TRW) and maximum latewood density (MXD) is fundamental for evaluating parameter-specific growth characteristics and climate reconstruction skills. This is of particular interest for mid-latitudinal environments where future rates of climate change are expected to be most rapid. Here we present a network of 28 TRW and 21 MXD chronologies from living and relict conifers. Data cover an area from the Atlantic Ocean in the west to the Mediterranean Sea in the east and an altitudinal gradient from 1,000 to 2,500 m asl. Age trends, spatial autocorrelation functions, carry-over effects, variance changes, and climate responses were analyzed for the individual sites and two parameter-specific regional means. Variations in warm season (May–September) temperature mainly control MXD formation (r = 0.58 to 0.87 from inter-annual to decadal time-scales), whereas lower TRW sensitivity to temperature remains unstable over space and time.     

Temporal instability in tree-growth/climate response in the Lower Bavarian Forest region: implications for dendroclimatic reconstruction

This paper explores the temporal stability of growth/climate relationships in ring-width chronologies of Norway spruce [ Picea abies (L.) Karst] and silver fir ( Abies alba Mill) in the Lower Bavarian Forest region in southern Germany. These chronologies were compiled, using both historic and living tree-ring data, with the main aim of developing a dendroclimatic reconstruction for the region covering the last 500 years. Moving window correlation analysis shows that prior to the twentieth century, both species co-vary in a similar way (1480–1899 mean r =0.66). There is no significant correlation between the species chronologies since ca. 1930, which partly reflects anomalous growth trends in the fir chronology since ca. 1960. Multiple regression analysis was utilised to assess the ability of both species chronologies to model March–August precipitation. The precipitation signal of the spruce data was found to be both stronger than the fir data (1872–1930 calibration: r ²=0.45 vs 0.25) and more time stable. After ca. 1930, the fir chronology loses it ability to model March–August precipitation until there is no climate signal at all in the fir data in recent decades. The spruce data also express a later weakening in their climate signal in the mid 1970s. We present compelling evidence indicating that the anomalous trends observed in the fir data, since the mid 1960s, appear to be predominantly related to local SO₂ emissions from power plants and refineries. It is also likely that this local anthropogenic forcing is the cause of the weakening of the climate signal in the spruce data since the mid 1970s. The conclusions from this study are: (1) The fir tree-ring data cannot be used for traditional dendroclimatic calibration, although prior to the twentieth century the decadal variability in the fir data is very similar to spruce and so these data could be used to extend potential reconstructions in the future; (2) The recent decline and recovery event in the fir data appears to be unique to the twentieth century and is not part of a natural episodic phenomenon; (3) Traditional dendroclimatic calibration of March–August precipitation will be made using solely the spruce ring-width data. However, due to SO₂ forcing in recent decades, the calibration period will be shortened to the 1871–1978 period.     

Dendroclimatic signals in the pine and spruce chronologies in the Solovetsky Archipelago

Searching for a robust tree-ring parameter useful for paleoclimatic purposes is one of the most demanding topics in the modern paleoscience. Since Blue Intensity has already expressed itself in different geographical locations all over the world as a possible replacement for maximum density, close attention is paid to investigate features of the inferred signal. The Solovki Islands is a unique location in Northern Russia where two important factors that make this territory attractive for developing a long tree-ring chronology have been met: modern long-living trees and building activities using old trees that were started by monks in the middle of the 16th century. The main goal of the research is to develop pine and spruce chronologies based on tree-ring width (TRW) and delta Blue Intensity (dBI) and to assess the ability of these parameters to be used as climate predictors. As a result, 14 conifer chronologies from 7 sites (4 for pine and 3 for spruce) were developed. The composite pine and spruce chronologies span a period of 474 and 378 years each. Cross-correlation of dBI-based chronologies of both conifers is high (r = up to 0.71 while for TRW-based chronologies it is lower on average (−0.18 to 0.63). Intra-species correlation of TRW chronologies in some cases achieved even negative values (r = −0.18. Discrepancies found between TRW chronologies of pine and spruce could be explained by differences in climatic signals. Response function analysis with monthly temperatures revealed that growth of pine depends on the previous August, while spruce has a temporally stable and strong relation to June temperatures. Compared to TRW, dBI-based chronologies have a high correlation with summer temperatures (r = 0.64 and 0.66 for spruce and pine, respectively). Presented research points out the importance of the response function analysis suggesting that depending on goals of the study several tree-ring parameters could be used, e.g., tree-ring width of spruce responses to June temperatures, while dBI to the whole summer.     

Climate signals in the ring widths and stable carbon, hydrogen and oxygen isotopic composition of Larix decidua growing at the forest limit in the southeastern European Alps

The southeastern border of the European Alps is not well resourced with high-resolution climate proxies and experiences a distinct climatic regime from the northern and western Alpine zones. Here, we present new high-resolution climatic proxies (AD 1907–2006) from ring widths and stable carbon (δ¹³C), non-exchangeable hydrogen (δ²H) and oxygen (δ¹⁸O) isotope ratios of cellulose extracted from Larix decidua tree rings, growing at the forest limit in the southeastern European Alps (Slovenia). δ¹³C, δ²H and δ¹⁸O are strongly (p < 0.001) and positively correlated with each other. June temperature has the strongest control on tree ring width (TRW), while later summer conditions (July–August) influence the stable isotope composition. All four proxies are strongly correlated (r > 0.4; p < 0.001) with summer temperature and also sunshine hours, while precipitation has less impact. A combination of TRW and δ¹³C provides the greatest potential for reconstructing past temperatures (June–August) with significant (p < 0.001) correlations with gridded temperatures extending across a very large part of southern and western Europe west of the Carpathian Mountains. The water isotopes (oxygen and hydrogen) record conditions in the Adriatic and Mediterranean, which are the source area for the air masses that bring precipitation to this region giving strong correlations with temperatures in southern Italy and the western part of the Balkan Peninsula. Combining proxies with different spatial and temporal signals allows the strength and spatial footprint of climate signals to be enhanced. These findings open new perspectives for climate reconstruction in the southeastern European Alps and Western Balkans.     

Seasonal and synoptic climatic drivers of tree growth in the Bighorn Mountains,WY, USA (1654–1983 CE)

In the United States’ (US) Northern Rockies, synoptic pressure systems and atmospheric circulation drive interannual variation in seasonal temperature and precipitation. The radial growth of high-elevation trees in this semi-arid region captures this temperature and precipitation variability and provides long time series to contextualize instrumental-era variability in synoptic-scale climate patterns. Such variability in climate patterns can trigger extreme climate events, such as droughts, floods, and forest fires, which have a damaging impact on human and natural systems. We developed 11 tree-ring width (TRW) chronologies from multiple species and sites to investigate the seasonal climatic drivers of tree growth in the Bighorn Mountains, WY. A principal component analysis of the chronologies identified 54% of shared common variance (1894–2014). Tree growth (expressed by PC1) was driven by multiple seasonal climate variables: previous October and current July temperatures, as well as previous December and current April precipitation, had a positive influence on growth, whereas growth was limited by July precipitation. These seasonal growth-climate relationships corresponded to circulation patterns at higher atmospheric levels over the Bighorn Mountains. Tree growth was enhanced when the winter jet stream was in a northward position, which led to warmer winters, and when the spring jet stream was further south, which led to wetter springs. The second principal component, explaining 19% of the variance, clustered sites by elevation and was strongly related to summer temperature. We leverage this summer temperature signal in our TRW chronologies by combining it with an existing maximum latewood density (MXD) chronology in a nested approach. This allowed us to reconstruct Bighorn Mountains summer (June, July, and August) temperature (BMST) back to 1654, thus extending the instrumental temperature record by 250 years. Our BMST reconstruction explains 39–53% of the variance in regional summer temperature variability. The 1830s were the relatively coolest decade and the 1930s were the warmest decade over the reconstructed period (1654–1983 CE) – which excludes the most recent 3 decades. Our results contextualize recent drivers and trends of climate variability in the US Northern Rockies, which contributes to the information that managers of human and natural systems need in order to prepare for potential future variability.     

An unstable tree-growth response to climate in two 500 year chronologies,North Eastern Qinghai-Tibetan Plateau

Two new Juniper tree-ring-width (TRW) chronologies spanning more than 500 years were developed in the Yellow River source area, North Eastern Qinghai-Tibetan Plateau (NE-QTP). For the two studied sites, located approximately 50 km apart, split correlation and coherence analysis reveal unstable tree-growth responses to local moisture availability. While significant correlations are obtained with April–June local precipitation, Palmer Drought Severity Index (PDSI) and river flow from 1948/1954 to 1998 and from 1948/1954 to 1970s, these correlations vanish for the time period 1970s-1998. The local instrumental climate data (precipitation, PDSI and river flow) exhibit opposite correlations with large scale modes of variability (El Niño Southern Oscillation, ENSO, and Pacific Decadal Oscillation, PDO) before and after the 1977 PDO shift. One tree-ring chronology is coherent and anti-phased with instrumental ENSO/PDO indices at 5.2-year frequency. On the longer time span, this TRW chronology is compared with PDO reconstructed from historical Chinese data. This comparison also exhibits unstable multi-decadal relationships, notably in the mid 19th century. Altogether, the comparison between our two chronologies, local instrumental climate records, and ENSO/PDO indices suggest a cautious use of local TRW records for paleoclimate reconstructions. Further studies are needed to explore both the spatial coherency of tree-ring records and the temporal stability of their response to local and large scale climate variability.     

Recent climate hiatus revealed dual control by temperature and drought on the stem growth of Mediterranean Quercus ilex

A better understanding of stem growth phenology and its climate drivers would improve projections of the impact of climate change on forest productivity. Under a Mediterranean climate, tree growth is primarily limited by soil water availability during summer, but cold temperatures in winter also prevent tree growth in evergreen forests. In the widespread Mediterranean evergreen tree species Quercus ilex, the duration of stem growth has been shown to predict annual stem increment, and to be limited by winter temperatures on the one hand, and by the summer drought onset on the other hand. We tested how these climatic controls of Q. ilex growth varied with recent climate change by correlating a 40‐year tree ring record and a 30‐year annual diameter inventory against winter temperature, spring precipitation, and simulated growth duration. Our results showed that growth duration was the best predictor of annual tree growth. We predicted that recent climate changes have resulted in earlier growth onset (?10 days) due to winter warming and earlier growth cessation (?26 days) due to earlier drought onset. These climatic trends partly offset one another, as we observed no significant trend of change in tree growth between 1968 and 2008. A moving‐window correlation analysis revealed that in the past, Q. ilex growth was only correlated with water availability, but that since the 2000s, growth suddenly became correlated with winter temperature in addition to spring drought. This change in the climate–growth correlations matches the start of the recent atmospheric warming pause also known as the ‘climate hiatus’. The duration of growth of Q. ilex is thus shortened because winter warming has stopped compensating for increasing drought in the last decade. Decoupled trends in precipitation and temperature, a neglected aspect of climate change, might reduce forest productivity through phenological constraints and have more consequences than climate warming alone.     

Elevation-dependent climate sensitivity in Eucalyptus pauciflora Sieb. ex Spreng

We examine climate sensitivity in tree-ring chronologies from Eucalyptus pauciflora Sieb. ex Spreng at three elevations—1,350, 1,475 and 1,600 m above the sea level. Consistent with the principle that the sensitivity of tree-ring chronologies increases with proximity to the limits of tree growth, statistics reflecting chronology reliability increased with elevation. Climatological analyses of the three elevation classes revealed that whilst ring width is significantly and positively correlated with maximum air temperature during spring (September–November) in the chronology from the highest elevation class, significant correlations with maximum temperature are not present at low elevations. Similarly, whilst ring width in the chronology from 1,350 m was significantly and positively correlated with precipitation during late summer and early autumn, no significant correlations are evident at higher elevations. Our results illustrate the importance of careful site selection in dendroclimatological studies of eucalypts and demonstrate the potential of E. pauciflora for climatological studies.     

Instability of climate signal in tree-ring width in Mediterranean mountains: a multi-species analysis

Temporal instability of climate signal in tree-ring width of the five dominant species (Pinus nigra, P. sylvestris, P. uncinata, Abies alba, Fagus sylvatica) growing under Mediterranean mountainous climate was studied over the last century (1910–2004). To disentangle the tree–climate–site complex, the effects of both soil water availability (SWA) (dry, mesic and humid sites) and altitude (from 430 to 1,690 m) were investigated on the response patterns. Responses to climate were analysed using bootstrapped correlation coefficients from 17 ring-width chronologies built from 293 trees sampled in 64 stands in South-Eastern France. Temporal analyses were performed considering forty-six 50-years intervals (from 1910–1959 to 1955–2004). May–June drought was the primary limiting factor. For P. sylvestris, summer precipitation also played a key role. F. sylvatica was the less responding species with no clear common pattern. Low SWA led to an increasing correlation with precipitation in May for P. nigra and A. alba. Precipitation from May to August prevailed on the driest conditions for P. sylvestris. Correlation analyses suggested that warm autumn or winter enhanced growth, except for F. sylvatica. For P. nigra, the importance of April temperature increased with increasing altitude. Temporal analyses revealed a stability of sensitivity for the highest contexts (P. uncinata and F. sylvatica). At lower altitudes, the correlation with minimum temperature in April increased while temperature more often exceeded the threshold of 0°C over the last decades. For precipitation, a decrease in the strength of correlation was observed without close relationships with local xericity.     

Long-term changes in tree-ring–climate relationships at Mt. Patscherkofel (Tyrol,Austria) since the mid-1980s

Although growth limitation of trees at Alpine and high-latitude timberlines by prevailing summer temperature is well established, the loss of thermal response of radial tree growth during last decades has repeatedly been addressed. We examined long-term variability of climate–growth relationships in ring width chronologies of Stone pine (Pinus cembra L.) by means of moving response functions (MRF). The study area is situated in the timberline ecotone (ca. 2,000–2,200 m a.s.l.) on Mt. Patscherkofel (Tyrol, Austria). Five site chronologies were developed within the ecotone with constant sample depth (≥19 trees) throughout most of the time period analysed. MRF calculated for the period 1866–1999 and 1901–1999 for ca. 200- and ca. 100-year-old stands, respectively, revealed that mean July temperature is the major and long-term stable driving force of Pinus cembra radial growth within the timberline ecotone. However, since the mid-1980s, radial growth in timberline and tree line chronologies strikingly diverges from the July temperature trend. This is probably a result of extreme climate events (e.g. low winter precipitation, late frost) and/or increasing drought stress on cambial activity. The latter assumption is supported by a <10% increase in annual increments of ca. 50-year-old trees at the timberline and at the tree line in 2003 compared with 2002, when extraordinary hot and dry conditions prevailed during summer. Furthermore, especially during the second half of the twentieth century, influence of climate variables on radial growth show abrupt fluctuations, which might also be a consequence of climate warming on tree physiology.     

Effects of a severe drought on Quercus ilex radial growth and xylem anatomy

We assessed the response of Quercus ilex subsp. ballota to the severe summer drought recorded in 1994 in NE Spain through the study of changes in radial growth and wood anatomy. We selected a coppice stand in the Iberian Peninsula, which is characterized by a Mediterranean climate under continental influence. We measured internode length, tree-ring width, mean and maximum vessel diameter, and vessel density for 1981–1997. The annual predicted hydraulic conductance (K_h) was calculated following Hagen-Poisseuille's law. We compared the tree-ring width, vessel diameter and K_h of Q. ilex subsp. ballota and co-existing ring-porous oaks (Q. faginea, Q. pyrenaica) for a dry summer (1994) and a wet summer (1997). To evaluate the drought-resistance of xylem for Q. ilex subsp. ballota (dominant under continental conditions) and Q. ilex subsp. ilex (dominant in mild areas) we determined vulnerability curves. Dimensionless indices of internode length, tree-ring width, and vessel density were compared with climatic data (monthly total precipitation and mean temperature) using correlation analyses. Internode length, tree-ring width, K_h, and mean and maximum vessel diameter declined in 1994. According to vulnerability curves, Q. ilex subsp. ballota showed a greater drought resistance than Q. ilex subsp. ilex. During the year of growth, we found a positive influence of January and June–August precipitation on the internode length, tree-ring width, and vessel density. The response of Q. ilex subsp. ballota radial-growth to summer drought was comparable to that of Q. faginea latewood. Overall, growth and wood anatomy of Q. ilex subsp. ballota showed a plastic response to drought.     

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。     

Spatial differences in the radial growth responses of black locust (Robinia pseudoacacia Linn.) to climate on the Loess Plateau,China

The black locust (Robinia pseudoacacia Linn.) is the dominant tree species in the “grain for green” project on the Loess Plateau (LP) of China, and brings many ecological benefits to this planted region. However, there are concerns regarding its suitability as a plantation forest species in different regions of the LP. We used a dendroclimatological approach to investigate the radial growth response of black locust to varying climate in two sites on the LP with differing precipitation gradients. We took tree-ring samples from black locust in Yongshou County (in the semi-humid southern LP) and Shenmu County (in the semi-arid northern LP), and developed tree-ring width (TRW) chronologies for each. We performed moving correlation analyses between TRW chronologies and aggregated thermal (maximum temperature (TMX), minimum temperature (TMN), mean temperature (TMP)) and hydroclimatic factors (precipitation, self-calibrated Drought Severity Index (scPDSI), and humidity). The results demonstrated the increased influence of thermal factors during autumn, and the generally decreased influence of hydroclimatic factors on black locust radial growth in Yongshou, compared with the decreasing influence of thermal factors (during all seasons) and increasing influence of hydroclimatic factors (precipitation during summer, scPDSI and humidity during autumn) on black locust radial growth in Shenmu. The results indicated that black locust radial growth might benefit from the current climatic conditions in the southern LP. However, black locust radial growth stressed by water availability in the northern LP, which may reduce its vitality and productivity as climate warms in the future. These results have implications for regional forestry planning and ecological restoration strategies on the LP.     

Spatial and age-dependent tree-ring growth responses of Larix gmelinii to climate in northeastern China

Tree-ring width chronologies from 276 Larix gmelinii cores taken in northeastern China were used to analyze spatial and age-dependent growth–climate response relationships. Tree radial growth from five localities showed similar patterns, while exhibiting different tree-ring growth responses to local climate. The rotated principal component analysis (RPCA) indicated that tree age, growing season moisture conditions, and ambient air temperature variations resulted from location differences (e.g., longitude, latitude, and altitude), which could explain the non-stationary spatial climate–growth relations observed. The study tested the fundamental assumption that the climate–growth of L. gmelinii was age independent after the removal of size trends and disturbance signals. The age-related climate–growth relationship might potentially improve the veracity of past climate reconstructions. Bootstrapped correlation function analyses suggested that the response of L. gmelinii radial growth to climate differed between trees ≥150 years old and <150 years old. Mean sensitivity and standard deviation for trees increased with age in the <150 years old tree class; whereas trees ≥150 years old had no significant relationship with age. These results showed that the assumption of age-independent climate–growth relationship is invalid at these sites. Physiological processes and/or hydraulic constraints dependent on tree age, together with detrending techniques could be the possible causal factors of clear age-dependent responses. These results suggested the importance of incorporating trees of all ages into the chronology to recover a detailed climatic signal in a reconstruction of L. gmelinii for this region.     

Summer temperature dependency of larch budmoth outbreaks revealed by Alpine tree-ring isotope chronologies

Larch budmoth (LBM, Zeiraphera diniana Gn.) outbreaks cause discernable physical alteration of cell growth in tree rings of host subalpine larch (Larix decidua Mill.) in the European Alps. However, it is not clear if these outbreaks also impact isotopic signatures in tree-ring cellulose, thereby masking climatic signals. We compared LBM outbreak events in stable carbon and oxygen isotope chronologies of larch and their corresponding tree-ring widths from two high-elevation sites (1800–2200 m a.s.l.) in the Swiss Alps for the period AD 1900–2004 against isotope data obtained from non-host spruce (Picea abies). At each site, two age classes of tree individuals (150–250 and 450–550 years old) were sampled. Inclusion of the latter age class enabled one chronology to be extended back to AD 1650, and a comparison with long-term monthly resolved temperature data. Within the constraints of this local study, we found that: (1) isotopic ratios in tree rings of larch provide a strong and consistent climatic signal of temperature; (2) at all sites the isotope signatures were not disturbed by LBM outbreaks, as shown, for example, by exceptionally high significant correlations between non-host spruce and host larch chronologies; (3) below-average July to August temperatures and LBM defoliation events have been coupled for more than three centuries. Dampening of Alps-wide LBM cyclicity since the 1980s and the coincidence of recently absent cool summers in the European Alps reinforce the assumption of a strong coherence between summer temperatures and LBM defoliation events. Our results demonstrate that stable isotopes in tree-ring cellulose of larch are an excellent climate proxy enabling the analysis of climate-driven changes of LBM cycles in the long term.