The phenology of fine roots in trees may be a key to improving global carbon exchange models. Of particular interest is the timing of fine root production in relation to shoot production and whether allocation is sensitive to climate shifts. Our objective was to determine the periodicity of fine root production in pure stands of spruce (Picea spp.) in comparison to stem growth. Our working hypothesis is that extending the growing season in temperate forests increases root turnover at the expense of above-ground allocation because warming temperatures alone do not compensate for reduced photoperiod. We installed three minirhizotron tubes in each of two spruce stands in east-central NY State in October 2015. Trees in both stands were fitted with high-precision two-point dendrometers and sap flow meters. We also recorded soil temperature, soil moisture, understory PAR, and canopy reflectance using satellite and lower-elevation video imagery during the 2016 and 2017 growing seasons. Root images were analyzed to determine the beginning and end of new growth, root elongation, and root death for comparison with other monitoring data and local weather patterns.
Dendrometer data indicated basal diameter increase starting in early April and ending mid-September. Stem elongation ceased in June as measured by LiDar imagery. Initial leaf growth for Picea spp in 2016 was recorded in mid-May with a corresponding first fine root growth appearing simultaneously in one of the six in situ root tubes. However, no additional tubes showed fine root growth until early June. Root elongation in both spruce stands was detected in some amount after initial appearance until cessation as late as November. Root growth appears to be a function of soil temperature since growth began synchronously with leaf out. Early internal circulation of water, nutrients, and C may be important for initializing leaf and root growth. Root growth is responding differently to seasonal change indicating a phenological pattern with reference to carbon allocation.