NSFDEB-NERC:Tropical deadwood carbon fluxes: Improving carbon models by incorporating termites and microbes

Objectives of this project are to measure rates of tropical deadwood (coarse woody debris;
CWD) turnover and identify mechanisms that determine fates of CWD carbon (C) under current
and future climates. Woody plants are the largest aboveground terrestrial biotic C store,
and CWD biomass is likely substantial but poorly estimated. Much of our knowledge of turnover
comes from temperate systems, where CWD is thought to be slow cycling. Less is known about
tropical CWD pools and turnover, despite large living tree biomass. Due to differences in
climate (warmer temperatures and sometimes greater precipitation), wood construction, and
loss pathways (increasing role of termites), it is likely that CWD is dynamic in tropical
systems. This project combines field, molecular, and modeling approaches in tropical Australia,
where termites and fungi are key agents of wood turnover. A continuous belt of forest from
rainforest to savanna with existing long-term data and infrastructure makes it an ideal location.
Three questions will be addressed: Q1. What controls rates of CWD C turnover? Turnover rates
will be determined by termite and fungal activity, which are dependent on climate and wood
construction. Warmer and wetter conditions should increase turnover, but termites should increase
turnover relatively more in dry conditions as they have water conservation strategies. Dense
highly lignified wood should decay more slowly. Q2. What controls fates of C from CWD? C fates
(CO2, CH4, organic residues) will depend on stage of decay and functional composition of termite
and microbial communities. Later stages of decay, increased methanogens, decreased methanotrophs
and changes in termite species should result in greater CH4:CO2. Greater organic residue formation
will occur when C loss is via termites. Q3. How do mechanisms of wood turnover scale up to
affect ecosystem-level C fluxes under climate change? Climate warming in Northern Australia
will increase turnover rates and alter C fates of wood. To date, CWD is poorly parameterized
in Earth system models (e.g., CWD decays only via physical fragmentation). Field data from
Q1 and Q2 will be used to drive new predictive models of wood turnover and greenhouse gas
(GHG) production under climate change. To test Q1 and determine relative saprobic microbe
and termite decay rates in response to precipitation variation, blocks of a novel substrate
(Pinus radiata) known to attract termites will be placed at 6 sites (Rainfall gradient experiment).
Blocks will be enclosed in fine-mesh with or without holes manipulating termite access; blocks
will be harvested at end of wet and dry seasons for 4 years. To determine influence of wood
construction on decay, replicate logs of 10 species/site similarly enclosed will be placed
at rainforest and savannah sites (Common garden experiment). Logs will be harvested at end
of 2 wet and 2 dry seasons. For logs/blocks, initial and final mass, density and chemistry
will be measured. Local weather stations will provide climate data. To test Q2, wood subsamples
and termites in wood will be collected to determine termite community, microbial community
and functional gene composition, and organic residue formation. CO2 and CH4 will be measured
for harvested logs/blocks. As some termites live in mounds, experimental mounds (Termite mesocosm
experiment) will be established and mound diel CO2 and CH4 fluxes will be measured monthly
for 1 yr. To test Q3, field data will be incorporated into woodCLM, an ecosystem model derived
from the Community Land Model. Results from the modified and original models will be compared.
Using woodCLM, wood dynamics and GHG emissions will be simulated under future climate scenarios

The impacts of this proposal are in three areas: education and training, contributions
to the field and contributions to the broader public. The project capitalizes on a US NSF/DEB - UK
NERC Lead Agency Opportunity, which would facilitate international collaborations and sharing of
expertise between US (GWU and UCI) and UK (NHM) institutions. Funding through this opportunity
would have repercussions in all three Broader Impact areas.