Oak Ridge National Laboratory Oak Ridge, TN, United States
Above- and belowground phenology determine patterns of plant productivity and resource acquisition. Research focused on aboveground phenology has revealed striking differences among species, both in the average timing of annual events (e.g. leaf out and senescence), as well as in the variability of these events in response to environmental change. Little research has tracked patterns of belowground phenology; yet, these processes mediate uptake of temporally variable resources such as water and nutrients and affect patterns of carbon loss from ecosystems via respiration and priming mechanisms. As such, basic variability of belowground phenology among species, within species across years, and their trade-offs with aboveground phenology are poorly understood. We established a platform for long-term monitoring of leaf, stem, and fine-root phenology in mature trees across 18 species representing a wide range of phylogenetic and functional trait diversity. Following three years of observation (2019-2021) we consider multiple questions including, 1) how do patterns of fine-root phenology vary among species and according to plant phylogenetic groups or functional types? 2) Are there trade-offs in patterns of resource allocation to roots, leaves, and stems? 3) How variable are fine-root phenology and temporal trade-offs with leaves and stems across years?
1) Distinct similarities and differences in root production were observed among species. Some species expressed relatively short periods of concentrated growth with single, sharp pulses of production (e.g. Robinia pseudoacacia, Aesculus glabra) while other species frequently produced multiple discernable pulses of growth across the growing season (e.g. Pinus strobus, Juniperus chinensis). Though gymnosperms sometimes initiated root growth earlier in the season, many angiosperms reached peak root production earlier. 2) Production of different plant tissues were asynchronous with peak production usually occurring first in leaves, followed by stems then roots. 3) Many species exhibited high variability in the timing of peak root production among years shifting by as much as 125 days (e.g. Picea abies) while other species were relatively consistent across all years (e.g. Quercus bicolor). Root production was generally later in 2019 and 2020 compared to 2021, likely due to excessive spring soil moisture in these early years while drier conditions facilitated early production in 2021. These shifts in root production were largely independent of relatively smaller shifts in leaf production. Our first three years of observation highlight high variability among species and across years, but also suggest possible simplifications facilitating tractable representation into modelling applications.