Coping with Central European climate – xylem adjustment in seven non-native conifer tree species

The introduction of tree species raises the question of whether they find suitable conditions in their secondary range and whether they can successfully adapt to the new conditions in the context of an unstable climate. Stem secondary growth of trees and the changes in xylem cell structure make it possible to trace the adaptation process to the new climate on an inter- and intra-seasonal scale. The objective of this study was to determine whether the seven non-native conifer species in Central Europe differ in climate sensitivity and to what extent late frost and spring-summer drought can influence xylem changes. In our study, we investigated the growth pattern and xylem cell structure modifications of seven introduced conifer species Pseudotsuga menziesii, Pinus strobus, Tsuga canadensis, Abies grandis, Pinus banksiana, Pinus rigida, Pinus nigra, growing under uniform conditions in Poland. To determine species-specific temporal variations in climate sensitivity, we used stationary and non-stationary correlations of ring width with daily climate variables: mean, minimum, and maximum temperatures, precipitation sum, and SPEI, supported by a pointer-year analysis. Changes in xylem structure (frost rings and intra-annual density fluctuations) were analyzed using light and fluorescence microscopy. Warming of the winter-spring period benefits both southern European and North American introduced conifers, as reflected in the extended responsiveness period. However, as the growing season warms and the risk of late frosts increases, the potential for acclimatization of non-native species decreases, especially with respect to their juvenile growth phase. Negative precipitation and evapotranspiration balance at spring-summer period can lead to reduced growth and anomalies in earlywood xylem structure, commonly referred to as intra annual density fluctuations. P. strobus, the species that loses both frost-induced cambium damage and drought-induced xylem anomalies in summer is the fastest growing non-native species, along with P. menziesii. Although the results presented here show the potential and the limitations in the acclimatization of coniferous species, the obvious limitations related to local environmental conditions and the different ages of the trees make us cautious in generalizing the conclusions. Therefore, we call for the creation of a pan-European dendro-network of sites of non-native coniferous species.