Fine-root dynamics change during stand development and in response to thinning in balsam fir (Abies balsamea L. Mill.) forests

Fine roots play an important role in forest productivity and carbon cycling, however, little is known about their dynamics during stand development or their response to forestry practices. We used the minirhizotron method to measure fine-root production (NPPfr), mortality (Mfr), and year-end standing crop (SCfr) for 5 years at balsam fir (Abies balsamea (L.) Mill.) stands representing a chronosequence after clearcutting that also incorporated thinning of younger and older stands. We used seemingly unrelated regression to maintain mass balance among components of fine-root dynamics in our investigation of relationships with stand development and responses to thinning. The final statistical model explained 58% of the variation in fine-root dynamics, did not use stand age to explain variations in NPPfr, and used stand age to explain variations in Mfr. In the years immediately after clearcutting, SCfr increased rapidly, reaching a maximum in approximately 10 years, and declining slowly thereafter as a stand approached maturity. In the early years as SCfr increased, NPPfr increased and remained approximately constant thereafter. We ascribed the rapid early increase in SCfr to the existence of vigorous advanced growth and high relative fine-root production rates, and the slow decline after reaching a maximum to increasing relative mortality rates. In a pole-stage stand, SCfr recovered approximately 5 years after thinning due to increased relative fine-root production rate and lower mortality rates. In contrast, SCfr did not recover after thinning in a semi-mature stand because relative fine-root production did not increase. Elevated mortality after thinning due to death of root systems of cut trees was observed for several years in thinned stands. The lack of response of fine roots in older thinned stands may limit the response of aboveground components to thinning.

► We studied age-related changes in fine root dynamics and responses to thinning. ► Fine root NPP did not decline with stand age after increasing in early stage. ► Mortality increased with stand age or stage of development. ► Relative NPP increased after thinning a young stand but not an older stand. ► The role of stand age in controlling fine root dynamics is complex.