Energy And Water Exchanges From A Temperate Pine Plantation Forest

This paper summarizes the energy and water vapour fluxes for the first 14 months at a newly initiated flux tower site at a 65-year-old temperate conifer forest (Pinus strobus L.) in southern Ontario, Canada. These fluxes have been measured continuously since February 2002 using the eddy covariance technique as a part of long-term tower flux measurement programme over a chronosequence of white pine stands. The site is located 12 km south of Simcoe, near Turkey Point in St Williams Forest Tract, owned and managed by the Ontario Ministry Natural Resources. The site was originally sandy, abandoned land, which was planted with conifer and deciduous forests. The trees are about 22 m in height. The mean trunk diameter of the trees is 33 cm and the stand stem density is 750 ha⁻¹. Leaf area index is about 3·2 to 4·0. The soil is loamy fine sand and very fine sandy loam with low soil-moisture holding capacity. The closure of the daily mean energy balance (H + LE)/(R_n − G − S) was 0·90. Typical values of midday Bowen ratio (H/LE) in the growing season were approximately 1·4, whereas the midday Bowen ratio values for the winter season were typically 14 to 25. Surface conductance followed variations in solar radiation and vapour pressure deficit. In the growing season, surface conductance values were typically 5–12 mm s⁻¹. Transpiration mainly responded to vapour pressure deficit, rather than surface conductance. However, on warm summer days, when soil moisture was low, vapour pressure deficit exerted strong control on canopy conductance, thereby reducing photosynthetic uptake and water loss. Priestley–Taylor α values were low during most of the growing season, indicating dry soil conditions and conservative water loss. In the growing season, maximum daily evaporation values were 3·4 mm day⁻¹. In winter, maximum observed evaporation was 0·05–0·2 mm day⁻¹, mostly as a result of snow sublimation. Total calculated evapotranspiration for the 14 month measurement period was 465 ± 93 mm, and the total measured precipitation was 996 mm, indicating approximately 47% of water loss in the form of evapotranspiration. Copyright © 2005 John Wiley & Sons, Ltd.