Winners and losers of climate warming: Declining growth in Fagus and Tilia vs. stable growth in three Quercus species in the natural beech–oak forest ecotone (western Romania)

Global warming and increasing drought severity are exposing temperate forests to increasing stress, challenging silvicultural decision making. Growth analyses in marginal tree populations at drought-induced range limits may provide valuable information on tree species’ adaptive potentials and species-specific climate turning points. We studied the climate sensitivity and resilience to drought of radial growth, and long-term growth trends of mesic and rear-edge populations of Fagus sylvatica in comparison to three oak species (Quercus petraea, Q. frainetto, Q. cerris) and Tilia tomentosa in natural ecotones from mesic beech to xeric oak forests along three elevation transects in western Romania. Radial growth of all species was positively influenced by summer precipitation and low drought intensity, and negatively by high summer temperatures. The basal area increment (BAI) of F. sylvatica and T. tomentosa has declined in the last 10–20 years with warming and a deterioration of the summer water balance, while the three Quercus species maintained stable growth rates, though at lower BAI levels, suggesting a negative relationship between mean BAI and drought resistance among the five species. Growth reductions during three severe drought events (2000, 2003, 2012) were stronger, and growth resilience lower, in F. sylvatica and T. tomentosa than in the Quercus species, pointing at a thermal limit of beech at June–August temperatures of 20–21 °C. As the climate of the studied ecotones is similar to the predicted climate at colline/submontane elevation in Central Europe in about 50 years, a decline in beech growth and vitality is likely also in drought-affected regions in the distribution centre with future warming. Our results demonstrate that choosing stress-tolerant Q. petraea (as well as Q. frainetto and Q. cerris) instead of more productive timber species is a relatively safe option for Central European forestry in a warmer climate.