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Harvard Forest Symposium Abstract 2017

  • Title: Does water stress contribute to reduced woody growth and disrupted nutrient cycles on the Barre Woods Soil Warming Experiment?
  • Primary Author: Michael Bernard (Marine Biological Laboratory)
  • Additional Authors: Francis Bowles (Marine Biological Laboratory); Jerry Melillo (Marine Biological Laboratory); William Werner (Marine Biological Laboratory)
  • Abstract:

    Climate change models project increasing temperatures and altered precipitation such that wet regions may become wetter – albeit, with increasing variability in intensity and frequency of precipitation events. Despite increases in precipitation, instances of drought may become more common as warmer temperatures and variable precipitation lead to earlier snowmelt, longer periods without precipitation, and less water captured by the soil during intense events. Forests store a significant amount of terrestrial carbon and how they respond to the future climate will have a profound impact on the global carbon cycle.

    Over the last 14 years, we have used buried heating cables to warm a 30 m × 30 m forest plot five degrees above ambient soil temperatures measured on an adjacent control plot at Harvard Forest. Over the course of the study, we have observed an average annual increase in net nitrogen mineralization of 75% and wood carbon storage of 42% on the heated plot, which indicates that increased productivity or changes in carbon allocation may partially offset increases in soil carbon efflux with warming.

    During the last seven years, we have observed five dry periods (lasting 1-5 months) when soil moisture approached the wilting point on the heated plot. Only two similar periods were observed in the previous seven years. Wood growth on the heated plot was equivalent to the control plot during four of the last seven seasons and N:P ratios progressively increased in the litter of red oaks, the dominant canopy species. Red maple, a codominant species, has had elevated N:P ratios in its litter since the start of the experiment.

    Fine root biomass has been reduced on the heated plot and the ectomycorrhizal community has been restructured such that it is dominated by morphotypes that lack long hyphal networks. Both belowground changes may reduce access to water and nutrients during drought, which has been shown to decrease availability of soil P more than soil N. We hypothesize that growth may be constrained by water stress – either directly through reductions in evapotranspiration and photosynthesis, or indirectly, through a disproportionate reduction in P uptake relative to N. Further investigation is necessary to determine whether certain water use strategies during periods of hydrologic stress (isohydric vs. anisohyrdic behavior) or mycorrhizal association types (arbuscular mycorrhizae vs. ectomychorrhizae) increase susceptibility to nutrient imbalances and what the implications might be for forest ecosystem productivity in a future climate.

    Disentangling the effects of soil warming and reduced soil moisture on woody growth and plant nutrient acquisition and retention strategies will require a 2 × 2 factorial design consisting of: non-warmed treatments experiencing ambient conditions, non-warmed treatments experiencing reductions in soil moisture (likely by diverting throughfall), warmed treatments experiencing reductions in soil moisture (a consequence of using heating cables), and warmed treatments with soil moisture replenished. Data collection using current techniques will allow for detection of changes in belowground and aboveground processes related to plant-soil-microbe interactions. Such an experiment would be long-term and provide insights into the strength of the land carbon sink under a future climate.

  • Research Category: Soil Carbon and Nitrogen Dynamics