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Harvard Forest Symposium Abstract 2014

  • Title: Seasonality of C allocation to fine roots in three stands
  • Primary Author: Rose Abramoff (Boston University)
  • Additional Authors: Adrien Finzi (Boston University)
  • Abstract:

    Annual forest productivity and carbon storage are affected by the amount and timing of carbon allocated belowground. Though much work has focused on seasonal C partitioning to aboveground plant organs (e.g. leaf expansion, shoot elongation, aboveground woody increment, aboveground respiration), no study yet has measured belowground C partitioning to root production, respiration and exudation together as it changes over the growing season. Here we present a seasonally resolved estimate of C allocated to roots and their partitioning across the growing season for three stands dominated by common northeastern tree species, Quercus rubra, Tsuga canadensis, and Fraxinus americana, at Harvard Forest in Petersham, MA. Beginning in April 2012 through October 2013, 1) fine root production, 2) respiration, 3) nonstructural carbohydrate content and 4) exudation were measured. Data were captured on in situ or excavated fine roots using 1) minirhizotron camera, 2) open path infrared gas analyzer, 3) colorimetry, and 4) cuvette incubation respectively.

    Overall, fine root growth in these stands was characterized by multiple production peaks throughout the growing season that were not explained by soil temperature or indices of precipitation. Q. rubra and F. americana root growth peaked in early and mid-summer, while T. canadensis root growth peaked later in the growing season. Mortality was highest in early spring and during periods of active root growth. T. canadensis experienced a 70% decrease in NPP driven primarily by mortality in 2013 compared to 2012, a trend coincident with the infestation of hemlock wooly adelgid. Fine root respiration peaked mid-summer for all species. The temperature sensitivity of respiration, estimated using the activation energy, Ea, is lowest in mid-summer and highest in early spring and late fall, reflecting seasonal differences in the apparent temperature sensitivity of respiration. We estimate a growing season C efflux of 310, 389, and 491 gCm-2growing season -1, assuming a six month growing season, for Q. rubra, T. canadensis, and F. americana fine roots respectively. Fine root nonstructural carbohydrate content was drawn down in midsummer, coinciding with increasing root respiration and deciduous fine root production. Root exudation rate tended to increase over the growing season, summing to 63, 92, and 110 gCm-2growing season -1 for Q. rubra, T. canadensis, and F. americana. In all stands, C allocation to roots consumed 35-45% of GPP , with the majority (20-30%) allocated to fine root respiration. The remaining 10-15% is allocated to gross fine root production and exudation. Q. rubra allocates relatively more C to gross fine root production than exudation, while F. americana allocates C evenly to each process. T. canadensis allocates only 3% of GPP to gross fine root production, but this may be due in part to adelgid infestation.

    These data demonstrate that some processes such as root growth and accumulation of nonstructural carbohydrates are more responsive to factors other than temperature trends. In contrast, respiration has a strong relationship with temperature. Other processes, such as exudation, have clear differences between species, but their seasonal pattern is not obvious given interannual variability between sample years. Other factors may include seasonally cued trade-offs in C between plant organs (e.g. aboveground use of photoassimilate vs. belowground growth, nonstructural C vs. growth respiration, or nonstructural C vs. exudation), differences in plant type (e.g. vessel size), or nutrient availability. Understanding the potential drivers and magnitude of GPP allocated to each process is fundamental to generating predictions about how warming temperatures may affect belowground C balance.

  • Research Category: Forest-Atmosphere Exchange, Invasive Plants, Pests & Pathogens, Soil Carbon and Nitrogen Dynamics