Estimation of the growing season start (SGS) for two sites from the CarboEurope network, Sodankylä, Finland (top) and Loobos, Netherlands (bottom). The upper curve illustrates CO₂ fluxes observed above a Scots pine forest, while the lower one shows calculated stomatal conductances from the DO₃SE model, when driven by temperature, irradiance and humidity only (i.e. no explicit phenological factor) (both in arbitrary units). The vertical lines illustrate the estimated SGS from four methods: (1) day when the mean 24-h temperature (T₂₄) remains “permanently” (before midsummer) above 5 °C; (2) first day when T₂₄ exceeds 5 °C for 5 consequtive days; (3) the default EMEP latitude model, SGS = 105 + 1.5(ϕ − 60), where ϕ is latitude; (4) the latitude-height function of Morén and Perttu (1994).