Xylem Phenology And Growth Response Of European Beech, Silver Fir And Scots Pine Along An Elevational Gradient During The Extreme Drought Year 2018

Highlights: European beech (Fagus sylvatica L.) and silver fir (Abies alba Mill.) displayed parabolic elevational trends of the cessation of xylem cell differentiation phases. Xylem phenology and growth rates of Scots pine (Pinus sylvestris L.) appeared to be less influenced by the 2018 drought, whereas beech reduced growth on the lowest elevation and fir seemed negatively affected in general. Background: The year 2018 was characterized by multiple drought periods and heat waves during the growing season. Our aim was to understand species-specific responses of xylem phenology and growth to drought and how this effect was modified along an elevational gradient. Materials and Methods: We sampled microcores and increment cores along an elevational gradient in the southwestern Black Forest (SW Germany) region and analyzed xylem phenology and growth response to drought. Results: Termination of cell enlargement and lignification occurred earliest in beech and latest in pine. Beech had the highest growth rates but shortest growth durations, fir achieved moderate rates and medium durations and pine had lowest growth rates despite long growth durations. In contrast to pine, onsets of cell differentiation phases of fir and beech did not show clear linear relationships with elevation. Cessation of cell production and lignification of beech and fir followed a parabolic elevational trend and occurred earliest on low elevations, whereas pine showed no changes with elevation. Tree-ring width, generally, depended 3-4 times more on the growth rate than on growth duration. Conclusions: The possibly drought-induced early cessation of cell differentiation and considerable growth reduction of beech appeared to be most severe on the lowest elevation. In comparison, growth reductions of fir were larger and seemed independent from elevation. We found evidence, that productivity might be severely affected at lower elevations, whereas at high elevations wood production might not equally benefit during global warming.