Intra-annual density fluctuations in tree rings are proxies of air temperature across Europe

Abstract

Intra-Annual Density Fluctuations (IADFs) are an important wood functional trait that determine trees' ability to adapt to climatic changes. Here, we use a large tree-ring database of 11 species from 89 sites across eight European countries, covering a climatic gradient from the Mediterranean to northern Europe, to analyze how climate variations drive IADF formation. We found that IADF occurrence increases nonlinearly with ring width in both gymnosperms and angiosperms and decreases with altitude and age. Recently recorded higher mean annual temperatures facilitate the formation of IADFs in almost all the studied species. Precipitation plays a significant role in inducing IADFs in species that exhibit drought tolerance capability, and a growth pattern known as bimodal growth. Our findings suggest that species with bimodal growth patterns growing in western and southern Europe will form IADFs more frequently, as an adaptation to increasing temperatures and droughts.

Figure 1

Distribution of study sites, per country and species. (a) Percentage of each species per country. (b) Absolute number of sampled trees of each species per each country. (c) Boxplot showing the distribution of the frequency of IADFs recorded in the different species. ARUN = A. unedo; ERAR = E. arborea; LADE = L. decidua; PIAB = P. abies; PIHA = P. halepensis; PINI = Pinus nigra; PIPI = P. pinaster; PIPN = P. pinea; PISY = P. sylvestris; QUIL = Q. ilex; QURO = Q. robur.

Figure 2

Variation in the frequency of IADFs as a function of tree-ring width (TRW) in gymnosperm and angiosperm tree species. (a) IADFs frequency as a function of tree-ring width in gymnosperm species (y = − 0.0012x² + 0.6306x − 59.101; p < 0.05). (b) IADF frequency as a function of tree-ring width in angiosperm species (y = − 0.0008x² + 0.287x + 0.1009; p < 0.05).

Figure 3

Cumulative IADF frequency for all species in the network in relation to cambial age. IADFs frequency for all species in relation to cambial age. Species abbreviations as in Fig. 1.

Figure 4

Species distribution in relation to mean annual temperature and mean total precipitation. Species distribution. The points represent the average mean annual temperature and the average total annual precipitation. Error bars identify ± 1 standard deviation from the average, providing an indication of the broadness of the considered climatic conditions. Abbreviations as in Fig. 1.

Figure 5

Geographic distribution of study sites and climatic clustering. (a) Geographical distribution of the study sites based on the cluster they belong to. (b) Represented species and number of trees sampled at each site. c Mean air temperature for each climatic cluster. (d) Mean precipitation for each climatic cluster. Cluster 1 includes PINI, PIHA, PIPI; cluster 2 includes PIHA, PIPN, PIPI; cluster 3 includes ARUN, PIPN, ERAR, PIPI, PIHA, QUIL; cluster 4 includes PIAB, PISY, QURO, LADE, PINI. Species abbreviations as in Fig. 1. Maps a, b were created with R package version 5.0.0 (https://CRAN.R-project.org/package=prevR) by Jerzy Piotr Kabala.

Figure 6

Plots of the modeled effects on IADFs frequency. (a) Ring width. (b) Latitude. (c) Climatic cluster. (d) Altitude. The site altitude is included as a linear term, while ring width and latitude are modelled with p-splines, thus accounting for potential non-linearity in their effects; climatic cluster is included as a fixed effect.