The degree of coupling between aboveground assimilation and transport with below-ground metabolism is an indicator of ecosystem nutrient cycling and energy turnover in the rhizosphere as well as having a large impact on their long-term storage capacity in the soil. Understanding how and when assimilates arrive below-ground for mineralization is necessary to predict how nutrient and energy cycles might be altered by current and future changes in climate, species distribution and land use. Currently, there are two proposed mechanisms that describe the link between assimilation and below-ground respiration via the phloem: 1) the transport of assimilates basipetally according to the Münch theory, and 2) pressure-concentration waves. The transport of assimilates through the phloem by mechanism 1 is often quantified through isotopic labeling studies. Thus, the time between isotopic labeling in the canopy and when the labeled carbon is respired from the rhizoshpere characterizes the degree of coupling between aboveground and below-ground metabolism. The timing between the uptake and below-ground respiration of the labeled carbon is termed the "speed of link". Based on statistical approaches, recent studies have reported a speed of link on the order of one day or less in mature forests, which is too fast for phloem transport by molecular diffusion or classical sink-source dynamics. These studies often cite mechanism 2 to support their conclusions despite the lack of experimental evidence. In this presentation, we report results from experiments designed to observe the mechanisms behind the speed of link of Douglas-fir saplings. We kept the plants for several days (0,1 and 6 days) in the dark to create a large carbon source-sink gradient with the intention of inducing a strong pressure-concentration wave. Following the no light treatment, in a controlled growth chamber, we introduced labelled CO2 prior to exposing the plant to light. Upon exposing the plants to light, the labeled CO2 is carboxylated through photosynthesis and after transport, is respired below-ground. During the experiment we monitored the carbon isotopic composition of CO2 and the CO2 flux from the soil. We observed in our control trees (no darkness treatment), a rapid increase in soil respiration after the exposure to light that occurred on the order of minutes. For the same trees the label arrived three to five hours later. The concentration increase of the rapid CO2 release was on the order of 100- 200ppm above background while the labeled assimilates increased this value by about 50ppm. Two concentrations peaks were clearly observed each displaying unique kinetic responses. We have taken stem samples after the experiment to analyse the anatomical properties as well as leaf and root samples for carbohydrate analyses. Results from the trees placed in the dark are forthcoming. With these experiments we show experimental evidence for both mechanisms, pressure concentration waves and molecular diffusion, that are behind assimilate flow between the plant and the rhizosphere and we characterize their dynamics.