Photosynthesis and stomatal conductance of mature canopy Oak (Quercus robur) and Sycamore (Acer pseudoplatanus) trees throughout the growing season

1. Gas exchange is strongly seasonal in temperate deciduous forests; howeve r, virtually no studies have described measurements made at the leaf scale through the whole growing season. This level of detail is important to unde rstanding processes as substantial differences may occur within the canopy between species and between sun and shade leaves. 2. Gas exchange was measured weekly throughout the growing season in leaves of Oak (Quercus robur) and Sycamore (Acer pseudoplatanus) trees at Wytham Woods, UK. In Q. robur sun, shade and intermediate leaves were sampled on e ach occasion. 3. Acer pseudoplatanus came into leaf before Q. robur and commenced net ass imilation (A) earlier. Maximum irradiance saturated photosynthetic rates (A (max)) in Q. robur were reached approximately 50 days after budburst and we re maintained until approximately 20 days before abscission. Acer pseudopla tanus lost its leaves before Q. robur in the autumn and showed no seasonal trend in A(max). The lag between budburst and attainment of maximum photosy nthetic rate in Q. robur was substantial and represented a quarter of the p otential growing season. 4. A(max) of A. pseudoplatanus (3.5 mu mol m(-2) s(-1)) sun leaves was subs tantially lower than that of Q. robur (10.4 mu mol m(-2) s(-1)) despite A. pseudoplatanus being a fast-growing species. The earlier flushing of A. pse udoplatanus leaves in the spring could not completely compensate for this. It is likely that other variables, such as total leaf area, are responsible for the high growth rate of A. pseudoplatanus, rather than high photosynth etic rates. 5. Shade leaves of Q. robur had lower A(max) and mean A than sun leaves; th eir quantum efficiency was also lower. Consistent with this, sun leaves had a higher nitrogen content per unit area, resulting from a lower specific l eaf area (SLA). 6. Week to week variations in solar radiation influenced A; this could be d escribed by a non-rectangular hyperbola. Stomatal conductance (g(s)) decrea sed in conditions of high vapour pressure deficit (VPD). However, this did not decrease A as high VPD was associated with high solar radiation and the stimulation of photosynthesis caused by high photosynthetic photon flux de nsity (PPFD) more than compensated for the reduction by VPD.