Comparative analysis of albedo and surface energy balance of a grassland site and an adjacent Scots pine forest

Abstract

The climate conditions in the atmospheric boundary layer are influenced by the underlying land-use type because of its impacts on surface energy balance. Furthermore, the performance of atmospheric models depends crucially on a realistic representation of surface processes, e.g. the partitioning of available energy into individual energy-balance components. The present study investigates the similarities and differences in the energy balance of grassland and forest. Data from the period March 1992 to September 1996 were gathered at 2 adjacent sites in the southern Upper Rhine Plain in southwest Germany: a Scots pine Pinus sylvestris L. forest site at Hartheim, and an adjacent grassland site at Bremgarten. The experimental sites constituted a segment of the REKLIP (Regio-Klima-Projekt) network of energy-balance stations. Synoptic conditions were very similar for both land-use types owing to the short distance between the 2 sites. Differences in available energy (AE) between both land-use types were mainly caused by their different albedo. On average, AE of the Scots pine forest was higher than that for grassland by 24%. Normalised by AE, the mean sensible heat flux H accounted for 38% of the AE for grassland and 47% of the AE for the Scots pine forest, while the mean latent heat flux LE reached 62% for grassland and only 53% for the Scots pine forest. Compared to the mean Bowen ratio β of 0.61 for grassland, the relatively high mean β value of 0.89 for the Scots pine forest resulted from the strong stomatal control of the pine trees due to the warm and dry site conditions. In the case of a sufficient water supply, forest AE was converted in equal proportions to LE and H, while for grassland, 60 and 40% of AE were converted to LE and H, respectively. With a low water supply, more AE was converted into H for both land-use types.

Partitioning of turbulent flux reveals contrasting cooling potential for woody vegetation and grassland during heat waves

“…The foregoing reasoning contradicts several recent studies that reported larger λE for grasslands than from forest vegetation covers, mostly at high (>35 • ) latitudes (Baldocchi et al, 2004;Rost and Mayer, 2006;Teuling et al, 2010). An analysis of historical data coupled with more recent measurements in northern Europe by Teuling et al (2010) found forests to have smaller λE, but larger H, than the grasslands despite the former having larger and deeper root systems.…”

“…• 37 E) in Germany, where λE was always larger, while H was smaller, for grassland than for a nearby pine forest (Rost and Mayer, 2006). Our site was at lower latitude (33…”

“…The authors argued that a strong stomatal control of transpiration in trees during unusually hot periods make forests conservative water-users. Rost and Mayer (2006) earlier reported λE to be 10% lower from Scots pine (Pinus sylvestris) compared with nearby grassland. Both the woody and grassy vegetation in these studies had similar accessibility to a soil water supply such that the difference in the soil water content was less than 10% between forests/woodlands and the grasses in the studies of Roberts et al (2005) in the United Kingdom and that of Baldocchi et al (2004) in the United States.…”

Water‐storage capacity controls energy partitioning and water use in karst ecosystems on the Edwards Plateau, Texas

“…AE was higher at the woodland, but a greater fraction was partitioned into H , offsetting differences in AE . Woodlands typically generate proportionally more H because of higher turbulence (Rost and Mayer, ). In two of the years, 2005 and 2008, ET at both sites exceeded annual rainfall.…”

“…In arid and semiarid ecosystems, ET is almost always limited by precipitation, but in regions where annual precipitation is closer to potential ET, temporal dynamics of ET are jointly controlled by available energy ( AE ) and available water, which together govern the partitioning between latent and sensible heat (Milly, ). Woody ecosystems typically have more AE for ET, in large part because albedos are reduced because of multiple reflection and scattering of solar radiation by the canopy (Kessler and Jaeger, ; Baldocchi et al ., ; Rost and Mayer, ). The amount of AE that is partitioned into latent heat is linked to plant‐available water in the root zone.…”