Effects of Intra-Seasonal Drought on Kinetics of Tracheid Differentiation and Seasonal Growth Dynamics of Norway Spruce along an Elevational Gradient.

Research Highlights: Our results provide novel perspectives on the effectiveness and collapse of compensatory mechanisms of tracheid development of Norway spruce during intra-seasonal drought and the environmental control of intra-annual density fluctuations. Background and Objectives: This study aimed to compare and integrate complementary methods for investigating intra-annual wood formation dynamics to gain a better understanding of the endogenous and environmental control of tree-ring development and the impact of anticipated climatic changes on forest growth and productivity. Materials and Methods: We performed an integrated analysis of xylogenesis observations, quantitative wood anatomy, and point-dendrometer measurements of Norway spruce (Picea abies (L.) Karst.) trees growing along an elevational gradient in South-western Germany during a growing season with an anomalous dry June followed by an extraordinary humid July. Results: Strong endogenous control of tree-ring formation was suggested at the highest elevation where the decreasing rates of tracheid enlargement and wall thickening during drought were effectively compensated by increased cell differentiation duration. A shift to environmental control of tree-ring formation during drought was indicated at the lowest elevation, where we detected absence of compensatory mechanisms, eventually stimulating the formation of an intra-annual density fluctuation. Transient drought stress in June also led to bimodal patterns and decreasing daily rates of stem radial displacement, radial xylem growth, and woody biomass production. Comparing xylogenesis data with dendrometer measurements showed ambivalent results and it appears that, with decreasing daily rates of radial xylem growth, the signal-to-noise ratio in dendrometer time series between growth and fluctuations of tree water status becomes increasingly detrimental. Conclusions: Our study provides new perspectives into the complex interplay between rates and durations of tracheid development during dry-wet cycles, and, thereby, contributes to an improved and mechanistic understanding of the environmental control of wood formation processes, leading to the formation of intra-annual density fluctuations in tree-rings of Norway spruce. [ABSTRACT FROM AUTHOR]