Lammas shoots in Norway spruce - a future challenge? (1)
Recently formed shoots are not frost tolerant, and the risk of frost damages in the autumn increases as growth cessation is delayed. Actually, lammas shoots may also enhance the risk of frost damages during the winter.
Lammas shoots is not a new phenomenon, such shoots have been recorded in progeny and provenance trials already from the 1960s. It used to be a problem associated in productive spruce forests in the lowlands, particularly when established on previous arable land, but now it appears to increase both in abundance and frequency beyond that.
A recent study from the inner Oslofjord showed that 40 to 50% of the trees in high productive areas had lammas shoots. Although there was a substantial variation between sites, there was a clear and positive relationship between productivity and occurrence of such shoots. Data from progeny and provenance trials show that there is also a genetic component in formation of lammas shoots.
The problem of lammas shoots is particularly related to damages that may arise if the terminal bud is killed by frost. Then lateral shoots will take the lead. Thus, there is a clear relationship between lammas shoots and development of two or more terminal shoots, which is very negative for the future quality of the log.
Although we have a lot of data on lammas shoots from different trials, we know very little about the underlying mechanisms. The climate influences the growth cycle of Norway spruce, and a warmer climate will affect both the timing of bud break in spring and growth cessation in the autumn. Based on this, we assume that both a warmer climate and increased availability of nutrients may increase the occurrence of lammas shoots in the future. This is an important challenge, both for the forestry and forest tree breeding in Scandinavia.
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