Background
In describing the context of their study, Hanninen et al. note that "according to the hypothesis presented by Cannell (1985), climatic warming may paradoxically increase the risk of frost damage in boreal and temperate regions, because the trees will deharden and even start to grow during intermittent mild periods in winter and get damaged during subsequent periods of frost." Likewise, they say that "Kellomaki et al. (1995) predicted that frost damage caused by a premature dehardening would severely restrict growth of Scots pine under a climatic warming scenario in central Finland." These are interesting conjectures, to say the least. But are they true?

What was done
Using whole-tree chambers (WTCs) surrounding groups of 40-year-old Norway spruce (Picea abies (L.) Karst.) trees growing under natural conditions in northern Sweden, the authors studied the effects of atmospheric CO2 enrichment (to 700 ppm above the ambient value of 365 ppm) and global warming (ranging from 2.8°C above ambient in summer to 5.6°C above ambient in winter) on the timing of spring bud burst in the trees.

What was learned
The researchers report that the timing of bud burst was unaffected by elevated CO2, but that "the trees growing at an elevated temperature hardened later and dehardened earlier than the control trees." However, they note that the difference was "much smaller" than that implied by Cannell (1985) and predicted by Kellomaki et al. (1995).

What it means
In discussing their results, Hanninen et al. conclude that "regardless of the warming taking place during winter, boreal coniferous trees are able to retain their dormancy and frost hardiness until around the spring equinox," explaining that "high temperatures during bud dormancy induction increase the chilling requirement of rest completion, or in some other way delay bud burst during spring," citing the work of Heide (2003) and Junttila et al. (2003). These observations thus suggest, in their words, that "boreal trees may be able to prevent premature dehardening and growth onset under climatic warming," thereby thwarting the "doom and gloom" scenarios of deleterious frost damage in the face of global warming.

In light of these positive findings, Hanninen et al. additionally conclude that "experiments with WTCs are crucial [our italics] for studying the effects of climatic warming on boreal coniferous trees," because, as they continue, "without the results obtained from such studies, the catastrophic scenarios obtained in modeling studies would probably still prevail today." Their work thus highlights another instance where real-world observations of biological systems reveal something far different than what is suggested by theoretical models that fail to incorporate significant related phenomena (often because the phenomena are simply unknown).

References
Cannell, M.G.R. 1985. Analysis of risks of frost damage to forest trees in Britain. In: Tigerstedt, P.M.A., Puttonen, P. and Koski, V. (Eds.). Crop Physiology of Forest Trees, Helsinki University Press, Helsinki, Finland, pp. 153-165.

Heide, O.M. 2003. High autumn temperature delays spring bud burst in boreal trees, counterbalancing the effect of climatic warming. Tree Physiology 23: 931-936.

Junttila, O., Nilsen, J. and Igeland, B. 2003. Effect of temperature on the induction of bud dormancy in ecotypes of Betula pubescens and Betula pendula. Scandinavian Journal of Forest Research 18: 208-217.

Kellomaki, S., Hanninen, H. and Kolstrom, M. 1995. Computations on frost damage to Scots pine under climatic warming in boreal conditions. Ecological Applications 5: 42-52.