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Harvard Forest Research Project 2022

  • Title: Disintegrating rhizosphere and enriching phyllosphere microbiomes under urbanization: understanding consequences for tree growth and mortality.
  • Principal investigator: Jennifer Bhatnagar (
  • Institution: Boston University
  • Primary contact: Jennifer Bhatnagar (
  • Team members:
  • Abstract:

    Urbanization is increasing around the world, impacting ecological communities in consistent ways that are not well understood. In the temperate zone, urbanization often increase tree growth, yet the mechanisms by which this occurs is unclear. Urban trees maintain foliar nutrient levels similar to rural trees, suggesting that rapid growth in urban systems is sustained by a parallel increase in nutrient uptake. Nevertheless, urban soils can be extremely nutrient-poor. Plant growth is typically enhanced by colonization of roots by fungal symbionts (mycorrhizal fungi), which extract nutrients (e.g.; nitrogen (N) and phosphorus (P)) from soil and provide it to their host plant in exchange for carbon (C, in the form of photosynthate). However, urbanization consistently decreases abundances of mycorrhizal fungi on tree roots and in soils within the temperate zone. Instead, urbanization increases N and P inputs to tree canopies in throughfall. Elevated foliar nutrients can shape the composition and activity of the phyllosphere microbiome, including nutrient uptake and transfer to plants, as well as proliferation of plant and animal pathogens. Nevertheless, it is unknown whether this occurs in urban ecosystems and what the downstream consequences are for urban C cycling. The goal of this proposed research is to understand the relationship between changing plant-microbial interactions and biogeochemistry in urbanizing forests. Our overarching hypothesis is that urban tree growth and mortality is inversely related to root mycorrhization, because urban environmental conditions prevent mutualistic exchange with mycorrhizal fungi, generating negative feedbacks to C and nutrient loss in urban forests.