Reduced Temperature Sensitivity of Maximum Latewood Density Formation in High-Elevation Corsican Pines under Recent Warming

Physiological meaning of bimodal tree growth-climate response patterns

Correlation coefficients are widely used to identify and quantify climate signals in proxy archives. Significant relationships between tree-ring chronologies and meteorological measurements are typically applied by dendroclimatologists to distinguish between more or less relevant climate variation for ring formation. While insignificant growth-climate correlations are usually found with cold season months, we argue that weak relationships with high summer temperatures not necessarily disprove their importance for xylogenesis. Here, we use maximum latewood density records from ten treeline sites between northern Scandinavia and southern Spain to demonstrate how monthly growth-climate correlations change from narrow unimodal to wide bimodal seasons when vegetation periods become longer and warmer. Statistically meaningful relationships occur when minimum temperatures exceed 'biological zero' at around 5° C. We conclude that the absence of evidence for statistical significance between tree growth and the warmest summer temperatures at Mediterranean sites is no evidence of absence for the physiological importance of high summer temperatures for ring formation. Since correlation should never be confused with causation, statistical values require mechanistic understanding, and different interpretations are needed for insignificant correlations within and outside the growing season.

Revising Alpine summer temperatures since 881 CE

Europe experienced severe heat waves during the last decade, which impacted ecological and societal systems and are likely to increase under projected global warming. A better understanding of pre-industrial warm-season changes is needed to contextualize these recent trends and extremes. Here, we introduce a network of 352 living and relict larch trees (Larix decidua Mill.) from the Matter and Simplon valleys in the Swiss Alps to develop a maximum latewood density (MXD) chronology calibrating at r = 0.8 (p > 0.05, 1901-2017 CE) against May-August temperatures over Western Europe. Machine learning is applied to identify historical wood samples aligning with growth characteristics of sites from elevations above 1900 m asl to extend the modern part of the chronology back to 881 CE. The new Alpine record reveals warmer conditions in the tenth century, followed by an extended cold period during the late Medieval times, a less-pronounced Little Ice Age culminating in the 1810s, and prolonged anthropogenic warming until present. The Samalas eruption likely triggered the coldest reconstructed summer in Western Europe in 1258 CE (-2.32 °C), which is in line with a recently published MXD-based reconstruction from the Spanish Pyrenees. Whereas the new Alpine reconstruction is potentially constrained in the lowest frequency, centennial timescale domain, it overcomes variance biases in existing state-of-the-art reconstructions and sets a new standard in site-control of historical samples and calibration/ verification statistics.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00382-024-07195-1.

Temperature and Turgor "Limitation" and Environmental "Control" in Xylem Biology and Dendrochronology

Trees and other woody plants are immensely ecologically important, making it essential to understand the causes of relationships between tree structure and function. To help these efforts, we highlight persistent traditions in plant biology of appealing to environmental factors "limiting" or "controlling" woody plant features. Examples include the idea that inevitable drops in cell turgor with plant height limit cell expansion and thus leaf size and tree height; that low temperatures prohibit lignification of cells and thus the growth of woody plants at high elevation; and notions from dendrochronology and related fields that climate factors such as rainfall and temperature "control" growth ring features. We show that notions of "control," "limitation," and the like imply that selection would favor a given trait value, but that these would-be favored values are developmentally impossible to produce. Such "limitation" scenarios predict trait frequency distributions that are very narrow and are abruptly curtailed at the upper limit of developmental possibility (the right-hand side of the distribution). Such distributions have, to our knowledge, never been observed, so we see little empirical support for "limitation" hypotheses. We suggest that, as a more productive starting point, plant biologists should examine adaptation hypotheses, in which developmental possibility is wide (congruent with the wide ranges of trait variation that really are observed), but only some of the possible variants are favored. We suggest that (1) the traditional the proximate/ultimate causation distinction, (2) purging scenarios of teleology/anthropomorphism, and (3) stating hypotheses in terms of developmental potential and natural selection are three simple ways of making "limitation" hypotheses clearer with regard to biological process and thus empirically testable.

250 years of flood frequency and discharge in an ungauged Corsican mountain catchment: A dendrogeomorphic reconstruction

The primary goal of paleoflood hydrology is to estimate the frequency and magnitude of past floods. Botanical evidence, and particularly scars on trees, has been used repeatedly as paleostage indicators to reconstruct peak discharges and flood height. Yet, these reconstructions depend on the presence of visible scars on tree stems which tend to be masked as trees grow older. Here, we estimated flood magnitude using an alternative approach based on growth disturbances in tree-ring series, tree positions and the minimal discharge necessary to submerge the root collar of a tree as estimated by hydraulic modeling. We tested the reliability of this newly developed approach by using the traditional scar-based reconstruction as a benchmark. To this end, we sampled 60 trees showing evidence of flood damage on their stems along a 787-m long segment of the Asco river (Corsica, France). Based on 440 growth disturbances dated in tree-ring series, we reconstructed 28 floods between 1759 and 2020 and 18 during the 20th century. Using the two-dimensional Iber hydraulic model and detailed topographic data of the study site obtained from UAV imagery, we estimated that peak discharges of the 28 reconstructed events ranged between 10 and 210 m³s^-1, with 200 m³s^-1 being considered as the threshold for extreme floods. Not only do the scar-based and root collar submersion approaches yield similar results, findings are also clearly in line with the sparse information available from historical archives and short gauge station records on past floods. The unprecedented length and depth of the record presented here opens new avenues for climate change and flood impact research.

Maximum July-August temperatures for the middle of the southern Tien Shan inferred from tree-ring latewood maximum densities

Long-term temperature reconstructions are urgently needed to prolong meteorological climatic data, which are too short to evaluate the anthropogenic effect on climate change since the Industrial Revolution. The maximum latewood chronology (MXD) of Picea schrenkiana in the middle of the southern Tien Shan was established, and it showed a strong correlation with the mean maximum temperature of the current July to August (T_maxJA), with r = 0.773 (p < 0.001, 1959-2016), which implies that a high temperature in the late growing season could increase the cell wall thickness and lead to high latewood density. Then, the T_maxJA of the middle of the southern Tien Shan was reconstructed over the period of 1720-2018. Three MXD chronologies from Kyrgyzstan significantly correlated with our T_maxJA reconstruction at the interannual scale, and they also showed similar variations on decadal scales. None of these MXD series showed a warming trend in the past century, which was also found in several MXD series from different regions of the world. Spatial correlation analysis revealed that our T_maxJA reconstruction showed significant correlations with that in eastern Asia, southern Europe, and north-western Africa, forming a teleconnection called the Silk Road Pattern. However, moving correlation analysis between our T_maxJA reconstruction and Hokkaido temperature series indicated that this teleconnection was unstable in the past 3 centuries. The volcanic eruptions from the mid-high latitudes in the Northern Hemisphere showed a stronger cooling effect than those from the Southern Hemisphere and the low latitudes of the Northern Hemisphere. The summer North Atlantic Oscillation was also shown to affect the temperature in the Tien Shan to a certain extent.