Root and shoot phenology and root longevity of Norway spruce saplings grown at different soil temperatures

Tree roots comprise a huge carbon pool. Their dynamics are driven by environmental factors and thereby affected by climate change. We studied the effects of soil temperature on root and shoot phenology and their linkages in Norway spruce (Picea abies). Saplings were grown in controlled-environment rooms for three simulated growing seasons (GS1, GS2, GS3). Soil temperature treatments 9, 13, 18 and 21°C were applied during GS2. Root growth was monitored with minirhizotrons. Root growth commenced in all treatments simultaneously. Temporal growth patterns of short and long roots were usually bimodal. Root growth was very low in the coldest treatment during GS2 but increased during GS3 as an after-effect. Short root growth also continued later after colder than warmer treatments during GS3. Reduced sink strength of roots and increased carbohydrate accumulation into needles at 9°C during GS2 probably enabled compensatory root growth under restored temperatures in GS3. Soil temperature did not affect shoot phenology, and root and shoot phenology varied between growing seasons; thus the linkage of root and shoot phenology was inconsistent. Root longevity was shorter and turnover rate higher in warmer than colder soil. This can further affect soil carbon dynamics and ecosystem carbon cycling in boreal forest ecosystems.