Photosynthetic performance, height growth, and dominance of naturally regenerated sessile oak (Quercus petraea [Mattuschka] Liebl.) seedlings in small-scale canopy openings of varying sizes

Abstract

Small-scale harvesting methods as practised in close-to-nature forestry may disadvantage the regeneration of more light-demanding tree species such as most oaks and thus cause regeneration failure. Conducted in south-western Germany, this study examined photosynthetic performance and height growth of naturally regenerated 7-year-old sessile oak (Quercus petraea [Mattuschka] Liebl.) seedlings growing in artificially established and fenced canopy openings varying from 0.05 to 0.2 ha in size. We quantified the influence of solar radiation and competing vegetation within gaps on total height, height increment, and dominance of oak seedlings. Measurements were taken on plots systematically established along a north–south transect through gaps. Plot-level solar radiation levels within canopy openings quantified using the total site factor (TSF) were between 20% at southerly positions within small openings and up to 75% of open-field conditions at the centre of larger gaps. Photosynthetic performance, total height, and shoot length of the studied oak seedlings increased with increasing solar radiation. However, height increment and total height did not improve substantially when radiation levels increased from 20 to 50% TSF. Yet, highest levels of oak dominance, where oaks were the tallest individual at a plot, were found around 50% of TSF. Under the conditions at our research site, canopy openings of at least 0.2 ha in size appear necessary to successfully establish natural oak regeneration. Irrespective of gap size, the competition to oaks by woody species needs to be controlled to reduce the risk of regeneration failure.