Dendroecological analysis of a rare sub-canopy tree: Effects of climate, latitude, habitat conditions and forest history

Dendroecological knowledge about non-dominant sub-canopy trees is scarce but needed to fully understand the tree-ring dynamics in temperate deciduous forests. Here I present a study on the effects of climate, habitat conditions and forest management on the radial growth in the rare tree species Sorbus torminalis. Increment cores were collected in 19 populations along a latitudinal gradient ranging from the regional distribution limit in Switzerland, through Germany, to the absolute northern limit in northeastern Germany and Denmark. The ring width measurements were crossdated and correlated with climate and habitat descriptors. The annual response of the chronologies showed a predominantly negative relation with monthly temperature, and both positive and negative relation with monthly precipitation. In some populations, high temperature together with low precipitation may thus cause water stress and thereby limit tree-ring growth. Mean tree-ring width correlated significantly with mean annual precipitation as well as with indicators for soil nutrient content, soil moisture, temperature and light, and cover of the vegetation layers. A third factor, which proved important for understanding the variation in tree-ring growth, was forest management history. The species responded with abrupt growth increase to clearing of the dominant canopy species. In addition, forest management in Europe has become more extensive during the past 50 years leading to reduced growth of S. torminalis due to competition with the dominant tree species. There was no latitudinal trend in radial growth, but chronologies showed a higher sensitivity and magnitude of growth changes towards the northern distribution limit.