We studied 24 years of carbon exchange in a beech forest to understand why its productivity rises, peaks and declines each at different times of the year. We mapped climate effects onto short biological windows and found that stored carbon supports early growth, a one‑week bud formation period can set next year’s peak and bursts of dry air drive mid‑summer drops. Hot summers even raised productivity when water was available. These insights help anticipate forest responses to heat and drought.