Norwegian Journal of Agricultural Sciences : Birch dieback - caused by prolonged early spring thaws and subsequent frost

Abstract

The dieback of yellow birch (Beula alleghaniensis Britton) and white birch (B. papyrifera Marsh) has from the middle thirties been a mystery in Eastern Canada and Northeast US. The extent was enormous, covering an area of at least 490.000 km2. An idea of early spring climatic damage was set forth in 1957, and thaw- and freezing experiments in Norway since I 989 have reproduced the birch dieback symptoms; small, more or less chlorotic and curled leaves, failure of bud growth and progressive dying back of twigs from the ends. Such symptoms on yellow birch appeared by frost of at least -5C at bud burst stage 3, where green tips of leaves were visible. In March and early April this stage seems to need at least I00 day degrees (base temp. 4C), whereas about 50 day degrees are adequate in late April and May. Between 1936 and 1954 four thaw-frost events in Canada and US exceeded these values of day degrees before the frost, and the areas correspond very well with those hit by birch dieback. Rootlet dying, which was considered the first symptom, turns out to be a secondary one after the frost injuries. The frost damage to the vulnerable crown brings the tree out of physiological balance, the water content increases and movement becomes irregular.