The physiological basis for the contrasting distribution patterns of Acer saccharum and Thuja occidentalis at cliff edges

(1) Photosynthesis of intact Thuja occidentalis and Acer saccharum seedlings was measured to establish if changes of sensitivity to radiant intensity during early ontogeny could explain their mutually exclusive distribution patterns at cliff edges of the Niagara Escarpment, in southern Ontario, Canada. (2) Measurements of photosynthesis were made following growth at five intensities of photosynthetically active radiation (PAR) and two moisture pre-treatment regimes chosen to simulate conditions found in the field. (3) Two-week-old seedlings (cotyledon stage), six-week-old plants (first adult leaves in Acer and juvenile foliage in Thuja), and two-year-old plants (in Thuja only) were used. (4) Thuja cotyledons had higher net photosynthetic rates and lower specific leaf weights than Acer cotyledons. Thuja may thus rely more on immediate carbon fixation during establishment than Acer, which may rely more on stored metabolites at this stage of development. (5) Thuja seedlings gained more carbon than Acer at lower PAR levels. More Thuja than Acer seedlings survived the first six weeks of growth at the two lowest PAR levels, and were then able to survive an imposed drought for a mean of 5.5 days. (6) Six-week-old Acer seedlings showed low rates of net photosynthesis, biomass production and survival at low PAR and moisture levels, but a greater capacity to respond to favourable conditions. (7) The low net photosynthetic rate of Acer under simulated cliff-edge conditions suggests that it is excluded from the cliff-edge because it is intolerant of hostile conditions that occur there. Unlike Thuja, Acer can benefit from the productive environment in the deciduous forest. Thuja productivity and survival under simulated cliff edge and deciduous conditions were similar. This suggests that Thuja is the more tolerant of cliff-edge conditions but excluded from the deciduous forest zone by other processes.