Hi Ciaran, The Phenology Network should have information for you. Try Alison Donnelly - was in Trinity now at Milwaukee. Warm wishes, Eugenie

Thank you Eugenie. I've been in touch with Alison and her colleagues fronting the International Phenological Gardens of Europe network and they have been most helpful providing phenological data for this specific species and other Salix species. However, her informative work to date appears to have focused on modelling the timing of budburst for Betula pubescens. Any other suggestions would be most welcome as I have contacted many growers regarding this question and I have been in touch (via email) with some research insititutes in England also.

You are unlikely to find a 'threshold' as such. I don't think anyone actually understands the true mechanism that determines bud-burst, but if you look at bud-burst models, most tend to have a chilling and warming component. (early models tended to use only a cumulative temperature sum). This means that the 'threshold' you seek is variable depending on the chilling which has occurred. It is highly unlikely that anyone has sufficient data for Salix ie many years of observational data coupled with temperature data. [I've found it limited for even for Oak] . . .As data is limited, you tend to use the same data to parameterise a model!

For data contact the UK Phenology Network (Kate Lewthwaite, Project Manager via [email protected]). The UKPN has over 3 million records of the timing of natural events going back to 1736. I do not know how many records they have for willow species. It would also be worth you contacting Tim Sparks ([email protected]). If anyone has looked at the phenology of willows in the British Isles, Tim is sure to know.

I should have said, if the UKPN do have data on willow species and the analysis has not been done then the phenology data can be coupled with Central England Temperature data and/or the Met Office's 5km gridded climate data. The following articles might inspire you as to what can be done with UKPN data:

• Phillimore, A.B.; Stålhandske, S.; Smithers, R.J. and Bernard, R. (2012) Dissecting the contributions of plasticity and local adaptation to the phenology of a butterfly and its host plants. The American Naturalist, 180:5, 655–670.

• Smithers, R.J. and Sparks, T.H. (2011) No matter how long the winter, spring is sure to follow. British Wildlife 22, 254–257.

• Amano, T.; Smithers, R.J.; Sparks, T.H. and Sutherland, W.J. (2010) A 250-year index of first flowering dates and its response to temperature changes. Proceedings of the Royal Society B, Biological Sciences, 277, 2451–2457.

• Phillimore, A.B.; Hadfield, J.D.; Jones, O.R. and Smithers R.J. (2010) Differences in spawning date between populations of common frog reveal local adaptation. Proceedings of the National Academy of Sciences of the United States of America, 107:18, 8292–8297.

• Smithers, R.J. and Sparks, T.H. (2010) Climate is what we expect, weather is what we get. British Wildlife 21, 236–239.

• Thackeray S.J.; Sparks, T.H.; Frederiksen, M.; Burthe, S.; Bacon, P.; Bell, J.R.; Botham, M.S.; Brereton, T.M.; Bright, P.W.; Carvalho, L.; Clutton-Brock, T.; Dawson, A.; Edwards, M.; Elliott, J.M.; Harrington, R.; Johns, D.; Jones, I.D.; Jones, J.T.; Leech, D.I.; Roy, D.B.; Scott, W.A.; Smith, M.; Smithers, R.J.; Winfield, I.J. and Wanless, S. (2010) Trophic level asynchrony in rates of phenological change for marine, freshwater and terrestrial environments. Global Change Biology. 16:12, 3304–3013.

• Smithers, R.J. (2009) UK Spring Indicator – Spring Index. Defra Biodiversity Research and Development Programme Review. Defra, London.

• Sparks, T.H. and Smithers, R.J. (2009) The blurring of the seasons: a focus on autumn phenology. British Wildlife 20, 247–249.

Lennartsson and Ogren(2004) calculated threshold temperatures and temperature sums required for half budburst for controled and field-grown plants for a variety of clones including an English sourced known as "Bowles" with all clones having a threshold temperature of between 1.0 and 1.2oC so data does exist for different regions (this work was done in Finland) but how applicable this data is to an Irish and English oceanic temperate climate is another question. The alternative is to decide upon a suitable date and temperature to start accumulating thermal days (degree days) and chilling days from, which is what I have been doing. I will investigate the above attached papers however. Thank you Tim and Richard for your helpful inputs.

We did some similar sort of work in northern Ontario on jack pine (Pinus contorta) (see Di Giovanni and others). We looked at strobilus function (pollen release) and found that growing degree days were a trigger, but then more proximate conditions (weather on particular days) were the immediate trigger. The effects of radiative solar heating must also play a role. I have some published work on the temperatures within the catkins of Salix in the Canadian arctic. There are interesting differences between staminate and pistillate catkins, and sun-facing and shaded sides (see Kevan's publications). Those effects also influence the behaviour of insects inhabiting the catkins.

I suspect that to answer your question you will need to approach the problem from a hierarchical point of view. First considering GGD above freezing, possibly including sunshine (radiative warming) and wind (cooling), and then consider the influence of weather around the time of bud-burst for sun, cloud, rain, and wind.

Some old work that you should look into is that by Budel (in German and in beekeeping magazines!) but nicely summarized in Geiger's classical book on micrometeorology (The Climate near the Ground, if memory serves is the title). Budel's work is cited in my own (see above).

I hope this helps, and please don't dismiss the gems in the old literature.

Peter