Detecting invisible growth rings of trees in seasonally dry forests in Thailand: isotopic and wood anatomical approaches

We measured radial variation of carbon isotope composition and vessel traits in tree species in seasonally dry forests of Northeast Thailand to explore a more reliable and amenable method of tropical dendrochronology for trees that lack visually detectable and consistent growth rings. Six Dipterocarpaceae species (3 Shorea, 2 Dipterocarpus, and 1 Hopea species) with indistinct or irregular growth rings and teak (Tectona grandis), a species which forms distinct growth rings, were examined. The δ¹³C value variations in all species showed annual cyclicity. Dipterocarpaceae species usually marked the lowest values of δ¹³C in the middle of the growing season, whereas teak had the lowest values at nearly the end of the growing season. Since the growing season of the species examined almost corresponds to the rainy season in the study area, the δ¹³C variation was likely caused by the change in moisture availability. The different variation pattern of teak was attributable to its stronger dependence on ¹³C-enriched reserved material early in the growing season. Changes in tree vessel traits for all species examined also showed annual cyclicity. Dipterocarpaceae species showed significant correlation between δ¹³C values and vessel measurements. Vessel lumen (mean area, tangential and radial diameter, and proportion of total area) had a negative correlation, whereas vessel frequency showed a positive correlation. The correlations indicated that changes in vessel traits were caused by the seasonal variation of moisture available to the trees. Thus, we concluded that methods using wood anatomy, as well as δ¹³C, have great potential for use as tools in tropical dendrochronology within the context of seasonal climate.     

Dating of non-oak species in the United Kingdom historical buildings archive using stable oxygen isotopes

Stable oxygen isotope dendrochronology is an effective precision-dating method for fast grown, invariant (complacent) tree-rings and for trees growing in moist, temperate climatic regions where growth may not be strongly controlled by climate. The method works because trees preserve a strong common isotopic signal, from summer precipitation, and therefore do not need to be physiologically stressed to record a dating signal. This study explores the working hypothesis that whilst tree species may differ in their eco-physiology, leaf morphology and wood anatomy they will record an isotopic signal in their growth rings that is sufficiently similar to enable their precise dating against isotopic reference chronologies developed using dated oak tree rings from the same region. Modern and historical samples from six species (sweet chestnut, English elm, ash, alder, European beech and black poplar) were analysed and their oxygen isotopic variability was compared against an oak master chronology previously developed for central southern England. Whilst differences in the relative strength of the agreement between the different species and the master chronology are apparent, the potential for interspecies dating is demonstrated convincingly. The ability to date non-oak species using stable oxygen isotopes opens-up new opportunities for science-based archaeology and will improve understanding of a largely-unexplored, but significant part of the European historical buildings archive.     

A century of tree line changes in sub‐Arctic Sweden shows local and regional variability and only a minor influence of 20th century climate warming

Aim Models project that climate warming will cause the tree line to move to higher elevations in alpine areas and more northerly latitudes in Arctic environments. We aimed to document changes or stability of the tree line in a sub‐Arctic model area at different temporal and spatial scales, and particularly to clarify the ambiguity that currently exists about tree line dynamics and their causes. Location The study was conducted in the Torneträsk area in northern Sweden where climate warmed by 2.5 °C between 1913 and 2006. Mountain birch (Betula pubescens ssp. czerepanovii) sets the alpine tree line. Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics. Results Since 1912, only four out of eight tree line sites had advanced: on average the tree line had shifted 24 m upslope (+0.2 m year^?1 assuming linear shifts). Maximum tree line advance was +145 m (+1.5 m year^?1 in elevation and +2.7 m year^?1 in actual distance), whereas maximum retreat was 120 m downslope. Counter‐intuitively, tree line advance was most pronounced during the cooler late 1960s and 1970s. Tree establishment and tree line advance were significantly correlated with periods of low reindeer (Rangifer tarandus) population numbers. A decreased anthropozoogenic impact since the early 20th century was found to be the main factor shaping the current tree line ecotone and its dynamics. In addition, episodic disturbances by moth outbreaks and geomorphological processes resulted in descent and long‐term stability of the tree line position, respectively. Main conclusions In contrast to what is generally stated in the literature, this study shows that in a period of climate warming, disturbance may not only determine when tree line advance will occur but if tree line advance will occur at all. In the case of non‐climatic climax tree lines, such as those in our study area, both climate‐driven model projections of future tree line positions and the use of the tree line position for bioclimatic monitoring should be used with caution.     

Resizing a River: A Downscaled,Seasonal Flow Regime Promotes Riparian Restoration

While riverine organisms are adapted to the natural flow regime, it is impractical to fully restore natural flows along most regulated rivers. We propose an alternative with the delivery of downscaled flow regimes that provide the seasonal patterns that are essential for aquatic and riparian ecosystems. The Bridge River in British Columbia provided a novel case study as a downscaled flow regime commenced in 2000 along a reach that had generally experienced no flow for the prior half‐century. The experimental flow delivered a mean discharge of about 3 m³/s, versus the pre‐dam mean of 100 m³/s, with a seasonal pattern that mimicked the natural snowmelt‐dominated pattern. To assess the environmental response, we investigated black cottonwoods, Populus trichocarpa, the dominant riparian trees, in the pre‐flow versus post‐flow intervals, using tree ring interpretation for growth analyses and age determination. Sparse mature trees established prior to the 1948 damming did not show significant growth changes in the pre‐ versus post‐flow intervals. In contrast, younger trees that established closer to the river in the decade prior to 2000 displayed significant growth increases by 2002, and juveniles established after 2000 demonstrated faster initial growth than juveniles established before 2000. Further, bands of cottonwood saplings resulted from seedling recruitment along the new river fringe, particularly in 2002, 2003, and 2004, years with gradual flow recession. These responses demonstrate that a downscaled, seasonal flow regime provided environmental benefit, thereby restoring some river function and resulting in a resized river flanked by narrow and reproducing cottonwood bands.     

ARSTAN程序和R语言dplR扩展包进行树轮年表分析的比较研究

在树轮年代学领域,ARSTAN是去趋势处理和建立年表方面应用最为广泛的程序,而新兴的R语言dplR扩展包实现了ARSTAN的主要功能,且具有源代码公开、扩展性强等优点,是传统程序的良好补充。使用贺兰山青海云杉(Picea Crassifolia)树轮宽度数据,分析了ARSTAN和dplR进行树轮年代学分析所得结果的差异。结果显示,两种程序计算平均敏感度和一阶自相关系数的平均误差为0.005—0.008,但具有确定的转换关系;两种程序如果使用同种方法去趋势,拟合曲线的参数相近,建立标准年表的平均误差为0.002;拟合自回归模型时差异较大,其中时域上表现为差值年表起始30a内差异显著,在频域上表现为dplR的差值年表保留的低频信息较少;年表统计量计算和公共区间分析中,不同程序计算样本总体代表性和信噪比的差异较大。分析表明,两程序在拟合生长趋势和自回归模型时存在算法上的较大差异,同时年表统计量和公共区间各指标的算法也不尽一致,但存在较为确定的转换关系。对开展不同来源数据的整合分析提出了建议,应明确不同研究中树轮数据的处理过程,在条件允许时使用同一程序或算法重新处理数据,确保结果的可比性。     

Long‐term growth‐increment chronologies reveal diverse influences of climate forcing on freshwater and forest biota in the Pacific Northwest

Analyses of how organisms are likely to respond to a changing climate have focused largely on the direct effects of warming temperatures, though changes in other variables may also be important, particularly the amount and timing of precipitation. Here, we develop a network of eight growth‐increment width chronologies for freshwater mussel species in the Pacific Northwest, United States and integrate them with tree‐ring data to evaluate how terrestrial and aquatic indicators respond to hydroclimatic variability, including river discharge and precipitation. Annual discharge averaged across water years (October 1–September 30) was highly synchronous among river systems and imparted a coherent pattern among mussel chronologies. The leading principal component of the five longest mussel chronologies (1982–2003; PC1_mussel) accounted for 47% of the dataset variability and negatively correlated with the leading principal component of river discharge (PC1_discharge; r = ?0.88; P < 0.0001). PC1_mussel and PC1_discharge were closely linked to regional wintertime precipitation patterns across the Pacific Northwest, the season in which the vast majority of annual precipitation arrives. Mussel growth was also indirectly related to tree radial growth, though the nature of the relationships varied across the landscape. Negative correlations occurred in forests where tree growth tends to be limited by drought while positive correlations occurred in forests where tree growth tends to be limited by deep or lingering snowpack. Overall, this diverse assemblage of chronologies illustrates the importance of winter precipitation to terrestrial and freshwater ecosystems and suggests that a complexity of climate responses must be considered when estimating the biological impacts of climate variability and change.     

Detecting long‐term growth trends using tree rings: a critical evaluation of methods

Tree‐ring analysis is often used to assess long‐term trends in tree growth. A variety of growth‐trend detection methods (GDMs) exist to disentangle age/size trends in growth from long‐term growth changes. However, these detrending methods strongly differ in approach, with possible implications for their output. Here, we critically evaluate the consistency, sensitivity, reliability and accuracy of four most widely used GDMs: conservative detrending (CD) applies mathematical functions to correct for decreasing ring widths with age; basal area correction (BAC) transforms diameter into basal area growth; regional curve standardization (RCS) detrends individual tree‐ring series using average age/size trends; and size class isolation (SCI) calculates growth trends within separate size classes. First, we evaluated whether these GDMs produce consistent results applied to an empirical tree‐ring data set of Melia azedarach, a tropical tree species from Thailand. Three GDMs yielded similar results – a growth decline over time – but the widely used CD method did not detect any change. Second, we assessed the sensitivity (probability of correct growth‐trend detection), reliability (100% minus probability of detecting false trends) and accuracy (whether the strength of imposed trends is correctly detected) of these GDMs, by applying them to simulated growth trajectories with different imposed trends: no trend, strong trends (?6% and +6% change per decade) and weak trends (?2%, +2%). All methods except CD, showed high sensitivity, reliability and accuracy to detect strong imposed trends. However, these were considerably lower in the weak or no‐trend scenarios. BAC showed good sensitivity and accuracy, but low reliability, indicating uncertainty of trend detection using this method. Our study reveals that the choice of GDM influences results of growth‐trend studies. We recommend applying multiple methods when analysing trends and encourage performing sensitivity and reliability analysis. Finally, we recommend SCI and RCS, as these methods showed highest reliability to detect long‐term growth trends.     

Periodicity of growth rings in Juniperus procera from Ethiopia inferred from crossdating and radiocarbon dating

African pencil cedar (Juniperus procera Hochst. ex Endlicher 1847) is a tropical, irregularly growing species that can produce annual growth rings in response to an annual cycle of wet and dry seasons. In this paper, we assess the periodicity of growth-ring formation for 13 stem discs from a site in Central-Northern Ethiopia by crossdating and radiocarbon dating. The crossdating process is described more transparently than usual to allow open discussion of the methodology employed. Although the ring-width series could be tentatively matched, radiocarbon dating revealed that the growth rings of the junipers from the studied site are neither annual nor represent a common periodicity. It was found that the trees are exceptionally sensitive and respond individually to the complex local climate. For future research, it is recommended to select more mesic sites with an unambiguously unimodal rainfall regime and to gain external evidence to support assumptions about the periodicity of growth-ring formation in Juniperus procera.     

Relating annual increments of the endangered Blanding's turtle plastron growth to climate

This research is the first published study to report a relationship between climate variables and plastron growth increments of turtles, in this case the endangered Nova Scotia Blanding's turtle (Emydoidea blandingii). We used techniques and software common to the discipline of dendrochronology to successfully cross‐date our growth increment data series, to detrend and average our series of 80 immature Blanding's turtles into one common chronology, and to seek correlations between the chronology and environmental temperature and precipitation variables. Our cross‐dated chronology had a series intercorrelation of 0.441 (above 99% confidence interval), an average mean sensitivity of 0.293, and an average unfiltered autocorrelation of 0.377. Our master chronology represented increments from 1975 to 2007 (33 years), with index values ranging from a low of 0.688 in 2006 to a high of 1.303 in 1977. Univariate climate response function analysis on mean monthly air temperature and precipitation values revealed a positive correlation with the previous year's May temperature and current year's August temperature; a negative correlation with the previous year's October temperature; and no significant correlation with precipitation. These techniques for determining growth increment response to environmental variables should be applicable to other turtle species and merit further exploration.