Lobaria pulmonaria is an epiphytic lichen that, in south-eastern Canada, inhabits deciduous forests where it must acclimate to large seasonal changes in temperature and in light caused by closing and opening of the leaf canopy. On a seasonal timescale, this acclimation occurs via large shifts in the macromolecular complexes of the photosynthetic system, within a photobiont population that shows no seasonal change in cell numbers. In this study, samples of L. pulmonaria were harvested in February and in May from a natural population near Sackville, New Brunswick, and subjected to two simulated intense seasonal changes: (1) early spring warming, simulated by a shift from high light at 5°C to high light at 16°C (February shift), and (2) late spring canopy closure, simulated by a shift from high light at 16°C to low light at 16°C (May shift). Thallus samples were collected daily throughout each week-long shift. There were no significant changes in photobiont cell population size or in the fraction of cells dividing during either shift. During the first day of the February temperature shift, there were, however, large changes in the pools of chlorophyll, the major light capture molecule in the photobionts, the PsbA (D1) core protein of photosystem II whose turnover is highly responsive to changing light and temperature, and the RbcL major subunit of the carbon-fixing RUBISCO enzyme whose levels correlate strongly with achieved photosynthesis in lichens. A static population of photobionts was therefore able to perform large and rapid macromolecular reallocations to cope with rapid environmental change. No significant changes were seen in the chlorophyll, photosystem II or RUBISCO pools across the May light shift, although seasonal-scale macromolecular reallocation does occur in response to decreased light in the summer.