Simon1,
I tapped our silver birch a couple of years back, around this time of year i guess, and at the time i did guess! you can't not notice it when it pours from the bark. I don't think the fact that yours is young should affect whether it does or doesn't, maybe how much though. At a wild guess again, i'd say you could be too early. i haven't seen birch for a few weeks, living in the city as i do, so am detached from the wild, but what are the leaves doing? if they are shooting through it should be about the right time, the time at which they grow the fastest and have the most amount of sap supplied to them, when they stop growing so fast then maybe that is the time at which sap flow slows, and if there are not many leaves shooting through then perhaps it has not begun sufficiently. All this is conjecture on my part however.
I would have thought 4cm is much deeper than the radius at which the sap flows. is it not just beneath the bark in the sapwood region? this is a genuine question, i don't know.
When i got the sap, i wanted to do something interesting, instead of just drinking it like i should have! so i boiled it down to a rather woody, burnt sugar, so that was disapointing. any ideas on successful birch sap reducing people? any help much appreciated.
cheers,
-ian