Rob Wallace
Hi Jamie!

I couldn't check-in to WC last night, having to get some grading done for class today (I spent my normal grading time getting ready for a turning demo I did Sunday afternoon, comparing six hollowing systems at our February chapter meeting) - sorry for the delayed response....

The primary trigger for beginning sap flow in deciduous trees in early spring is daily temperature fluctuations, where daytime temperatures come above freezing, but drop back down below freezing at night. A few of these cycles consistently over a period of days or weeks will start the tree's metabolic machinery to start mobilizing water and nutrients in preparation for the energy demanding chore of breaking dormant buds and producing the whole aboveground mass of leaves, which is quite an investment for the tree. The sapwood in most species (although I don't know about walnut specifically) becomes very saturated with water and mobilized (translocated) sugars from breakdown of starches stored over winter in the roots. This process has been studied best in sugar maple, due of course to its economic importance in the maple syrup industry. Trees growing in environmental conditions subject to winter freezing, and springtime daily freeze-thaw cycles are assumed to behave similarly with respect to early season preparation for active growth.

I found a link (see below) that might explain it better.....

I hope this helps! Have a great day!

Rob