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Harvard Forest Symposium Abstract 2013

• Title: Root phenology in red oak and eastern hemlock- dominated stands at Harvard Forest
• Primary Author: Rose Abramoff (Boston University)
• Additional Authors: Adrien Finzi (Boston University)
• Abstract:

  Terrestrial ecosystems sequester 1-2 Pg of C from the atmosphere every year, and forests comprise the largest proportion of that C flux. Belowground C allocation accounts for ~25-60% of gross primary production, and is largely influenced by root phenology. However, we know relatively little about root phenology compared to aboveground plant organs. Ecosystem C models have long assumed that above- and belowground phenologies are synchronous, however, the limited research that exists has shown wide variability. For example, root production peaks are often offset from leaf out and shoot elongation. The seasonal pattern of belowground processes related to root production such as root respiration, nonstructural carbohydrate accumulation, and exudation is also unknown. Due to uncertainties in the seasonal pattern and magnitude of allocation to roots, we have collected measurements of root phenology for two contrasting temperate tree species at Harvard Forest: eastern hemlock (Tsuga canadensis) and red oak (Quercus rubra). Bi-weekly to monthly measurements of root production, root respiration, and root nonstructural carbohydrate content are used to determine when roots are receiving C from aboveground and patterns of C use.
  
  
  
  
  
  Minirhizotron and soil core data suggest that fine root biomass does not accumulate in a unimodal peak. In T. canadensis stands, fine root production remains low throughout the year, with significant pulses of root production in August and October. In Q. rubra stands, fine root production begins in late May, about 3 weeks after leaf out and continues throughout the season in oscillating peaks. Standing root biomass for all stands fluctuates seasonally but with high spatial variability, with live fine root biomass averaging 554 ± 241 gC m-2 in Q. rubra and 449 ± 172 gC m-2 in T. canadensis. Root respiration for all stands increases with temperature from early May to late-August, with an average seasonal Q10 of 3.0. Nonstructural carbohydrates in Q. rubra roots experience a drawdown from March to mid-July, coinciding with increases in greenness, root production and respiration. Nonstructural carbohydrates in all stands accumulate after July. The lack of synchronicity of these processes suggests that trade-offs in C allocation, in addition to abiotic factors such as temperature and soil water content, govern the timing of belowground processes.
  
  
  

• Research Category: Forest-Atmosphere Exchange
  Soil Carbon and Nitrogen Dynamics