Seasonal variation in energy and water fluxes in a pine forest: an analysis based on eddy covariance and an integrated model

Abstract

We used a combination of eddy flux, stem sap flow and environmental measurements with an integrated resistance/energy model to analyse the seasonality of energy and water fluxes in a 50-year-old boreal Scots pine forest (62°51′N, 30°40′E). The analyses utilised data obtained from three contrasting growing seasons (days 120–270 of 2000–2002). The measured latent heat above the canopy (LE e) accounted for the majority of forest net radiation ( R n) from June to mid-August (37.6–42.7%), while sensible heat ( H e) was the dominant consumer of net radiation (41.3–52.4%) at other times during the growing season. The seasonal sums of LE e were 362, 395 and 418 MJ m −2 for 2000–2002, respectively. The lower LE e in 2000 than in the other years resulted from the low rainfall and high vapour pressure deficit in July–August of that year. Canopy transpiration (LE c) accounted for 82–87% of LE e, and was generally coupled with seasonal patterns of daily mean temperature and net radiation, but was reduced by the low soil water content (<0.25 m 3 m −3) and a high vapour pressure deficit (>1.1 kPa) during the summer. The modelled flux components at the forest floor were significant throughout the growing season, with daily-averaged rates ranging between 2 and 50 W m −2 for net radiation, 5 and 30 W m −2 for latent heat, and 5 and 40 W m −2 for sensible heat. The Bowen ratio ( H e/LE e) was within the range 0.5–3.0 for over 80% of the time during the season, and the ratio between the forest floor sensible and latent heat ( H s/LE s) varied from 1.2 to 4.5. Day-to-day variation in the ratio H s/LE s was greatly controlled by the soil moisture in summer, but by the mean daily temperature in spring and autumn. Although the canopy heat storage and soil heat flux were important on diurnal and seasonal time scales, their seasonal integrations accounted for only a small portion of net radiation.