Regeneration growth and crown architecture of European beech and silver fir depend on gap characteristics and light gradient in the mixed montane old-growth stands

Abstract

Light is a vital factor affecting tree growth, survival and reproduction. In forests, natural and anthropogenic disturbances are the main drivers of abrupt changes in environmental conditions, including solar irradiance. We examined interspecific differences in the responses of European beech and silver fir to canopy gap characteristics and positions within gaps across various life stages (small seedlings, tall seedlings and saplings) in old-growth stands in Świętokrzyski National Park, Central Poland. Additionally, we analyzed sapling architectural responses to light. Regeneration densities were strongly affected by gap characteristics. The largest interspecific differences in responses to gap characteristics were found for small seedlings, which indicated their different preferences not only for light conditions created within gaps, but also toward various microsites distributed on a fine-scale after gap creation. Small beech seedlings responded only to the position within the gap, but small fir seedlings reacted in a more complex way, including such variables as gap size, shape and distance from a gap border. Gap size and the distance from its border had the most noticeable positive effect on tall seedlings and saplings, indicating that successful transition of regeneration to taller life stages may be possible only in larger gaps and in places further from a canopy border. Light conditions did not affect sapling length to diameter ratio. However, both direct and diffuse light had a negative effect on beech and fir crown area and length. Fir had wider but shorter crowns compared with beech in the whole light spectrum. This result indicates that these two species might have contrasting strategies regarding light interception, with fir being better adapted to increase a crown laterally, and with beech either being more tolerant to self-shading than fir or able to keep a favorable spatial leaf arrangement to avoid self-shading. Beech had the highest height growth increments in low levels of direct light, but its height growth was positively affected by diffuse light. Fir height growth rates showed no response to direct light, but were highest in medium levels of diffuse light. No effect of light on beech height to lateral growth rate ratio was found. The effect of direct light on this parameter for fir was negative, and nonlinear in the case of diffuse light, with the highest ratios observed in medium light levels. Our results stress the need of using both direct and diffuse light when analyzing light effects on sapling architecture.