Long-Term Changes in Forest Productivity: A Consistent Assessment in Even-Aged Stands
The objective of the study was to provide a stand-scale assessment of long-term productivity changes in even-aged stands and to depict their chronology over the 20th century. We focused on dominant height growth as a proxy for productivity, reconstructed from stem analyses in temporary plots. Height increments from two generations of stands were compared. Stands were associated in pairs to ensure accurate control of intrinsic site fertility conditions. The historical evolution of the growth rate was estimated using a statistical modeling approach based on a mixed-effects model, with a control of both site and developmental stage effects. We placed emphasis on a model formulation that leads to a meaningful interpretation of growth rate evolution. We applied the methodology to a sample of 14 stand pairs and 84 stem analyses of common beech in northeastern France. An accelerated increase in growth rate was identified, reaching +50% over the 20th century. The trend also displayed short-term growth anomalies.
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