Combining tree-ring analyses on stems and coarse roots to study the growth dynamics of forest trees: a case study on Norway spruce (Picea abies [L.] H. Karst)

We show the potential of a new method combining tree-ring analyses on stems and on coarse roots of individual trees in order to advance the understanding of growth dynamics in forest trees. To this end, we studied the root–shoot allometry of trees and its dependence on site conditions. Along a gradient in water supply in Southern Germany from dry to moist sites we selected 43 Norway spruce trees (Picea abies [L.] H. Karst.) aged 65–100 years. Increment cores were taken from stem and main roots revealing aboveground and belowground growth course over the last 34 years. Annual growth rates in roots and stems and their allometric relationships were applied as surrogate variables for tree resource allocation to aboveground and belowground organs. The mean sensitivities of both stem and root chronologies were found to be site-specific, and increased from the moist through the dry sites. No temporal offset between aboveground and belowground growth reactions to climate conditions was found in Norway spruce at any of the sites. These results suggest that the root–shoot allometry depends on the specific site conditions only at the driest site, following the optimal biomass partitioning theory (the more restricted the water supply, the more organic matter allocation into the belowground organs).