Longitudinal stem cracks in larch: what makes trees vulnerable?

Abstract

Stem cracks are becoming a major problem affecting timber quality and its economic value; their frequency may increase with increasing frequency and severity of drought stress due to climate change. Several major conifer species are affected, but such damage was mentioned for the first time in France for hybrid larch (Larix x eurolepis) in 1993. Even though a link was made with severe drought some years ago, it was believed, at that time, that the rapid growth of hybrids might be the cause. Our study is intended to disentangle the role of growth versus drought in the appearance of cracks, based on observations in two field genetic trials. Summer drought was clearly identified as the main trigger of longitudinal cracks. Roughly 5–15 years of age seems to be the critical period during which the occurrence of most of the cracks was observed. Wood formed during this period corresponds to juvenile wood, well-known for its weaknesses. Nevertheless, some trees escaped cracking damage: compared to trees with cracks, these looked slender (higher HD ratio), had wider rings and greater grain angle in the early years but similar wood density. Fast growth has often been cited as enhancing crack occurrence. This conclusion is corroborated by our observations that, within a given variety or family, trees with cracks were consistently larger than trees without cracks. Nevertheless, this conclusion should be qualified by the additional observation that, across varieties/families, the faster growing material were not necessarily the most prone to cracks, highlighting the crucial role of the genetic background of the material planted.