Tree vampire

The trees were seeping from their severed limbs yesterday.  I noticed a puddle on the sidewalk, but everywhere else around was dry, and then I realized the tree above me had been trimmed around power lines.  It's been years since I've seen this happen.  This time, I gave in and put my hand under the stream and then tasted it. 

It wasn't sweet sap.  At first I thought it tasted like nothing, just pure water.  Then I realized that the salt I would normally taste on my skin was totally neutralized.

Today, I saw from the window that the trees were still weeping.  I didn't go out to check if maybe it tasted different today.  It's something I can only do on impulse, apparently, and not deliberately.

Maybe it's spring cleaning for the trees: out with the old water so the new can start circulating.  Or maybe it's more like a tourniquet being removed?

What makes them start and what makes them stop before they bleed out?  I'm sure science has the answer for these questions, but I'd rather just wonder.