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A renewed interest in natural gap dynamics has developed from a recent focus on sustainable forestry in Europe. To improve our understanding of the role of a canopy opening on the physical environment in a natural forest, a case study was undertaken in and around a gap in a semi-natural, beech-dominated ( Fagus sylvatica L.) forest. Spatial and temporal variations in photosynthetically active irradiance ( I P ), soil and air temperature ( T S and T A ) and soil water content (SWC) were studied until the third growing season after natural formation of the gap. I P was highest in the northern part of the gap and in the adjacent forest, particularly around mid-summer and on sunny days, when I P was predominantly direct. Total I P in the southern part of the gap and along gap edges declined by about 19.6% from the first to the second year after gap formation, possibly due to growth of canopy and sub-canopy trees. Maximum and mean T S were generally highest in the southern central part of the gap, and in late spring and summer also under the closed canopy east of the gap. The insulating effect of dense regeneration could to some extent explain the spatial variation of T S , though this was not tested directly. SWC in the gap was near field capacity and generally higher than that under the closed canopy during summer. A decrease in SWC from the first to the second year after gap formation may have been attributed to released edge trees, although water use by the dense regeneration could not be ruled out. In general, the effect of this small irregular gap on light, soil and air temperature, and soil moisture was influenced by the considerable microsite variation. Furthermore, growth of regeneration and edge trees, resulting in increased interception, shading and water extraction by roots, may have modified the gap effect seen as early as the second year after gap formation.

Ash (Fraxinus excelsior L.) and beech (Fagus sylvatica L.) seedlings were grown in the field under three levels of natural light: (1) open, (2) gap and (3) shade. Light acclimation of photosynthesis was characterized by means of modulated chlorophyll a fluorescence of intact leaves and growth parameters were measured at the end of the growing season. Measurements of maximum photochemical efficiency (F v/F m) of dark-adapted leaves at intervals through the day showed that ash had a higher F v/F m than beech in open and gap plots but not in shade plots. This indicated a larger build-up of photoinhibition in beech under gap and open conditions. Steady-state light response curves of the operating efficiency of PSII (F′ q/ F′m), the electron transport rate (ETR) and the photochemical efficiency factor (F′q/F′v) showed greater variability across light treatments in ash than in beech. Both species exhibited similar responses of non-photochemical quenching (NPQ) to light. When the data were normalized to the mean maximum irradiance in the growth environment, all photochemical parameters showed a reduction in variation across treatments, indicating that light acclimation in the two species occurred primarily through adjustments in rates of photochemistry. Adjustments in thermal heat dissipation were small in both species. This pattern was stronger in ash, suggesting a greater degree of phenotypic plasticity in photosynthetic capacity in this earlier successional species. Contrary to our expectations, the build-up of photoinhibition in beech did not appear to have a negative effect on total biomass accumulation relative to ash.