The timing of fine-root production and its response to abiotic conditions represents an important strategy for species adaptation that is often left out of functional trait frameworks. However, little is known about basic patterns of inter- and intraspecific variation in belowground phenology and the responses of fine-root production to induced stress. It is therefore not surprising that most terrestrial biosphere models use highly simplistic algorithms to determine the timing of fine-root production in terrestrial ecosystems. Using a combination of field observations, waterlogging trials in potted plants, and modeling exercises we explore the role that root production and root phenology play in shaping plant strategies and ecosystem processes. Our field observations were carried out in 2019 across 13 temperate tree species at The Morton Arboretum (Lisle, IL, USA) using minirhizotrons to observe patterns of root growth in conjunction with aboveground measures of stem growth and leaf phenology. In our waterlogging trial, potted saplings of Magnolia and Sweetgum were subjected to a 3-week period of soil saturation and root production responses were observed via acetate windows. Finally, we assess the ecosystem-level implications of altered fine-root phenology in evergreen forests using updated phenology routines in the E3SM Land Model (ELM).
Results/Conclusions
Both empirical studies highlighted distinct species differences in strategies for fine-root production. Our field observations indicate that fine roots are produced anytime throughout the growing season and highlighted notable differences among species. Peak root production in shagbark hickory occurred in July, shortly after stem wood growth concluded. Despite similar timing of stem wood growth to hickory, fine-root growth in white cedar did not peak until September, likely due to midsummer allocation to leaf production. This highlights different temporal tradeoffs among species with respect to resource allocation and fine-root growth; though more work is needed to determine whether these patterns scale to broader plant functional types. In our waterlogging trial, anoxic stress caused significant mortality of Magnolia roots. Magnolia and Sweetgum both displayed strong pulses of late season root production (August to October) relative to control plants suggesting significant shifts in within-plant allocation priorities during post-stress recovery. Finally, our model simulations observed up to four-fold differences is total vegetation carbon based on differences in the timing and distribution of fine-root production. These combined results emphasize the need to better understand and model inter- and intra-specific differences in fine-root production phenology.