Early response of Pinus sylvestris and Picea abies seedlings to an experimental canopy gap in a boreal spruce forest

We studied the early response of size and morphology of Pinus sylvestris (Scots pine) and Picea abies (Norway spruce) seedlings to an experimental canopy gap and the consequent distribution of photosynthetically active radiation (PAR) in a boreal Norway spruce forest. Because of the asymmetric distribution of radiation in the gap and its surroundings, which was due to solar geometry at high latitudes, the highest amount of radiation was received several metres north of the gap centre. As a result, seedlings growing under the canopy at the northern edge of the gap received amounts of radiation similar to those received by seedlings growing in the southern part of the gap. Seedling size (aboveground dry biomass, height, and projected leaf area) of both species increased with increasing radiation, but shade-intolerant P. sylvestris responded more strongly than shade-tolerant P. abies. P. sylvestris seedlings reached their maximum aboveground dry biomass in the northwestern part and P. abies seedlings in the northern half of the gap. As height difference between species was small, the greater aboveground dry biomass of P. sylvestris seedlings was explained by its larger projected leaf area compared to P. abies. In addition, P. sylvestris responded to radiation with a more flexible morphology than P. abies, allowing P. sylvestris to increase its needle length with increasing radiation. Longer needles contributed to increased aboveground dry biomass, projected leaf area, and specific leaf areap. Consequently, with increasing radiation, P. sylvestris seedlings allocated proportionally more aboveground dry biomass to assimilating parts versus structural parts compared to P. abies. This allowed P. sylvestris to capture more light and compete better for resources than P. abies in gap and gap-edge environments