||Our goal is to continue investigating the effects of disturbance and climate variables on processes controlling carbon and water processes at AmeriFlux cluster sites in semi-arid and mesic forests in Oregon to address a new set of hypotheses. The observations will be made at three existing and productive AmeriFlux research sites that represent climate and disturbance gradients as a natural experiment of the influence of climatic and hydrologic variability on carbon sequestration and resulting atmospheric CO2 feedback that includes anomalies during the warm/ dry phase of the Pacific Decadal Oscillation. Our objectives are to (1) Combine tower and biological observations at the semi-arid mature and young pine sites to investigate climatologic and hydrologic influences on NPP, NEP and component processes in different aged forests over multiple years that include anomalies in precipitation and temperature (disturbance history/age gradient); (2) Compare climatologic and hydrologic control on measured carbon and water fluxes in forests of the same functional type and similar age (mature pine and Douglas-fir sites), but in different ecoregions over multiple years that include variability in precipitation phase and timing (climate gradient). Our hypotheses are: (1) The controls over the climate feedback through sequestration of CO2 by the two forest types are fundamentally different. (a) Based on climate projections of decreased summer stratus clouds with higher temperatures and evaporative demand in the coastal Douglas-fir region, drought stress will increase and GPP will decline more than soil respiration, leading to reduced net carbon uptake at the MF site in years with these climate conditions; (b) Changes in length and timing of the growing season caused by earlier warming will affect the ponderosa pine sites more dramatically than the Douglas-fir site because of the associated increase in frequency of rain on snow events in winter and spring, and earlier snow melt on which these forests rely during the summer. Such an increase in water demand earlier in the season will deplete soil moisture reserves more rapidly, increase stomatal resistance, and thus reduce CO2 sequestration in summer and autumn; (2) Inherent water-use efficiency (IWUE = GEP*VPD/LE, where LE/VPD is a hydrological measure that approximates ecosystem surface conductance), a proxy for intrinsic leaf-level WUE, will be highest at the young pine site, intermediate at the mature pine site and lowest at the mature fir site due to differences in buffering capacity relative to available soil water.; (3) Over the disturbance gradient, net carbon uptake is lower at the naturally regenerated young pine than the previous young pine plantation and mature pine sites; both young pine sites have not reached productive capacity of the sites, and are more vulnerable to drought.
Relevance to the BER LTM: Our full suite of ecosystem data combined with NACP and global modeling activities will provide a solid foundation for carbon dynamics under conditions of changing climate and disturbance in Pacific Northwest coniferous forests and drought-affected regions. Potential Impact to DOE: The research products from observations and experiments in this study and integration with modeling done outside of this proposal will contribute to the long-term performance measure (LTM) by providing a) New data on carbon cycle mechanisms controlling CO2 exchange with the atmosphere, b) Quantification of terrestrial carbon sources and sinks and how they change in relation to climatologic and hydrologic influences across a climatic gradient, c) Quantification and explanation of trajectories of change in carbon storage and NEP of forests following disturbance.