How do trees know when it's spring?

How do trees know when it's spring?

when to unfurl their leaves from their tight-clasped buds?

i'm sure scientists have isolated the hormones certain heat-sensitive cells secrete

mapped their structures and named them

just as they've scrutinised the heat receptors in my skin

and traced the nerves of my limbic system

but that doesn't capture

the pleasure of warmth on the skin, on the bark

it isn't why

we open our softness

to the sun.