Increasing vegetation light use efficiency in the high latitudes of the northern hemisphere. By Rebecca Thomas

In the high latitudes of the northern hemisphere (north of 45N), we have observed an increase in the amplitude of the seasonal cycle of CO2 of 5.0±0.8ppm over the last 50 years. In other words, vegetation exchanged 57±9.8% more CO2 with the atmosphere in 2009-10 compared to 1958-611. In previous posts, I have shown that current terrestrial biosphere models (from the MsTMIP simulations) underestimate the increase in seasonal cycle amplitude. So what has caused the increase in seasonal cycle amplitude, and why are models doing a poor job?

If we decompose the seasonal cycle of CO2, we can begin to answer this question. In the northern hemisphere, the seasonal cycle of CO2 can almost entirely be attributed to the terrestrial biosphere. Therefore, to a good approximation, the seasonal cycle of net ecosystem production (NEP) is proportional to the seasonal cycle of atmospheric CO2. NEP is the difference between Net Ecosystem Production and Heterotrophic Respiration (NPP-Rh).  Changes in NPP or Rh will then result in changes in NEP, and thus the seasonal cycle of CO2. Since we know that the land biosphere is a net sink for CO2, it is unlikely that changes in Rh are driving the change in NEP. Thus, it is the change in NPP that is driving the change in NEP.

Rebecca Thomas blog

Figure 1: A simple schematic of the terrestrial carbon cycle. Changes to any of these fluxes or pools can alter NEP. (from Bonan et al. 2008).

NPP is sensitive to atmospheric CO2 concentrations and temperature, both of which have increased over the last 50 years. These changes have relaxed some of the limitations that had previously prevented vegetation growth, particularly in the high northern latitudes. For example, the warmer temperatures have reduced the amount of snow cover and duration, allowing vegetation to start growing sooner and continue to grow for longer. This increase in growing season length has resulted in an increase in peak greenness, and vegetation greening of 0.25-0.5%/yr has been observed in high northern latitudes since 1982 (first satellite observations)2. The MsTMIP models are able to reproduce this trend, and agree that it is mostly driven by changes in climate. Greening will therefore be partly causing the increase in seasonal cycle amplitude, but this is not the whole story.

NPP can also increase without leading to greening, as vegetation can become more efficient. In particular, an increase in vegetation light use efficiency (LUE) could explain the observed changes (Where LUE is the NPP per unit area of absorbed photosynthetically active radiation (NPP/aPAR)3). I propose three mechanisms, not adequately represented in current models, which can increase LUE and may be responsible for the increase in seasonal cycle amplitude:

  1. CO2 fertilisation:- Increases in ambient CO2 increases leaf internal CO2 and therefore can increase the biochemical rate of photosynthesis. Additionally, a reduction in stomatal conductance at higher ambient CO2 increases vegetation water use efficiency4.
  2. Shift to above ground carbon allocation:- Less carbon needs to be allocated to roots at higher temperatures because nutrient cycling is faster. Also, depending on the species and environment, more carbon can be allocated to wood (increasing NPP without leading to greening) or leaves (increasing NPP and vegetation greenness)5.
  3. Acclimation of autotrophic respiration to sustained higher temperatures:- Most models calculate NPP as the difference between gross primary production and autotrophic respiration (NPP=GPP-Ra). Ra is sensitive to temperature, but it acclimatises to higher temperatures5. While this may have occurred in the real world, models do not include this acclimatisation, and therefore underestimate NPP at higher temperatures.

 

Improving how these mechanisms are represented in models should increase NPP and therefore increase the seasonal cycle amplitude of NEP and atmospheric CO2.

 

References:

  1. Graven, H. D., R. F. Keeling, S. C. Piper, P. K. Patra, B. B. Stephens, and S. C. a. Wofsy (2013), Enhanced seasonal exchange of co2 by northern ecosys- tems since 1960, Science, 341(6150), 1085–1089.
  2. Murray-Tortarolo, G., A. Anav, P. Friedlingstein, S. Sitch, S. Piao, and Z. a. Zhu (2013), Evaluation of land surface models in reproducing satellite-derived lai over the high-latitude northern hemisphere. part i: Uncoupled dgvms, Remote Sensing, 5(10), 4819–4838.
  3. Medlyn, B. E. (1998), Physiological basis of the light use efficiency model, Tree Physiology, 18(3), 167–176.
  4. Franks, P. J., M. A. Adams, J. S. Amthor, M. M. Barbour, J. A. Berry, and D. S. a. Ellsworth (2013), Sensitivity of plants to changing atmospheric co2 concentration: from the geological past to the next century, New Phytologist, 197(4), 1077–1094.
  5. Way, D. A., and R. Oren (2010), Differential responses to changes in growth temperature between trees from different functional groups and biomes: a review and synthesis of data, Tree Physiology, 30(6), 669–688.NPP is sensitive to atmospheric CO2 concentrations and temperature, both of which have increased over the last 50 years. These changes have relaxed some of the limitations that had previously prevented vegetation growth, particularly in the high northern latitudes. For example, the warmer temperatures have reduced the amount of snow cover and duration, allowing vegetation to start growing sooner and continue to grow for longer. This increase in growing season length has resulted in an increase in peak greenness, and vegetation greening of 0.25-0.5%/yr has been observed in high northern latitudes since 1982 (first satellite observations)2. The MsTMIP models are able to reproduce this trend, and agree that it is mostly driven by changes in climate. Greening will therefore be partly causing the increase in seasonal cycle amplitude, but this is not the whole story.

     

    NPP can also increase without leading to greening, as vegetation can become more efficient. In particular, an increase in vegetation light use efficiency (LUE) could explain the observed changes (Where LUE is the NPP per unit area of absorbed photosynthetically active radiation (NPP/aPAR)3). I propose three mechanisms, not adequately represented in current models, which can increase LUE and may be responsible for the increase in seasonal cycle amplitude:

     

    1. CO2 fertilisation:- Increases in ambient CO2 increases leaf internal CO2 and therefore can increase the biochemical rate of photosynthesis. Additionally, a reduction in stomatal conductance at higher ambient CO2 increases vegetation water use efficiency4.
    2. Shift to above ground carbon allocation:- Less carbon needs to be allocated to roots at higher temperatures because nutrient cycling is faster. Also, depending on the species and environment, more carbon can be allocated to wood (increasing NPP without leading to greening) or leaves (increasing NPP and vegetation greenness)5.
    3. Acclimation of autotrophic respiration to sustained higher temperatures:- Most models calculate NPP as the difference between gross primary production and autotrophic respiration (NPP=GPP-Ra). Ra is sensitive to temperature, but it acclimatises to higher temperatures5. While this may have occurred in the real world, models do not include this acclimatisation, and therefore underestimate NPP at higher temperatures.

     

    Improving how these mechanisms are represented in models should increase NPP and therefore increase the seasonal cycle amplitude of NEP and atmospheric CO2.

     

    References:

    1. Graven, H. D., R. F. Keeling, S. C. Piper, P. K. Patra, B. B. Stephens, and S. C. a. Wofsy (2013), Enhanced seasonal exchange of co2 by northern ecosys- tems since 1960, Science, 341(6150), 1085–1089.
    2. Murray-Tortarolo, G., A. Anav, P. Friedlingstein, S. Sitch, S. Piao, and Z. a. Zhu (2013), Evaluation of land surface models in reproducing satellite-derived lai over the high-latitude northern hemisphere. part i: Uncoupled dgvms, Remote Sensing, 5(10), 4819–4838.
    3. Medlyn, B. E. (1998), Physiological basis of the light use efficiency model, Tree Physiology, 18(3), 167–176.
    4. Franks, P. J., M. A. Adams, J. S. Amthor, M. M. Barbour, J. A. Berry, and D. S. a. Ellsworth (2013), Sensitivity of plants to changing atmospheric co2 concentration: from the geological past to the next century, New Phytologist, 197(4), 1077–1094.
    5. Way, D. A., and R. Oren (2010), Differential responses to changes in growth temperature between trees from different functional groups and biomes: a review and synthesis of data, Tree Physiology, 30(6), 669–688.

 

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