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Key message: Transpiration in conifer species was controlled by wind speed, similar to broadleaf species, indicating that caution is required when using the simplified Penman-Monteith equation. Abstract: The significance of aerodynamic controls on tree transpiration ( E) is critical when modeling E using the simplified Penman-Monteith equation. However, few studies have explicitly compared the significance of aerodynamic controls on E between conifer-type needles and broadleaf leaves. Although previous studies assumed that the response of E to wind speed ( w) could be insignificant in conifers, we hypothesized based on the evapotranspiration theory that the significance of E regulation by w is difficult to determine based on apparent such leaf-type differences. We performed sap flux measurements using a wind tunnel on two conifer species, Chamaecyparis obtusa var. formosana and Araucaria cunninghamii, and three broadleaf species, Swietenia mahagoni, Michelia compressa, and Plumeria rubra. Chamaecyparis obtusa and P. rubra showed no distinctive E response to w, while A. cunninghamii, S. mahagoni, and M. compressa had significant increases in E due to w, supporting our hypothesis. Our experimental and model calculation data suggested that the aerodynamic controls on E by w probably result from differences in stomatal conductance rather than leaf type. We emphasize the need to confirm the significance of aerodynamic controls on E using both direct and indirect measurements for coupling conditions between the leaf surface and atmosphere when modeling E, irrespective of leaf type. [ABSTRACT FROM AUTHOR]

To clarify characteristics of carbon (C) allocation in a Bornean tropical rainforest without dry seasons, gross primary production (GPP) and C allocation, i.e., above-ground net primary production (ANPP), aboveground plant respiration (APR), and total below-ground carbon flux (TBCF) for the forest were examined and compared with those from Amazonian tropical rainforests with dry seasons. GPP (30.61 MgC ha year, eddy covariance measurements; 34.40 MgC ha year, biometric measurements) was comparable to those for Amazonian rainforests. ANPP (6.76 MgC ha year) was comparable to, and APR (8.01 MgC ha year) was slightly lower than, their respective values for Amazonian rainforests, even though aboveground biomass was greater at our site. TBCF (19.63 MgC ha year) was higher than those for Amazonian forests. The comparable ANPP and higher TBCF were unexpected, since higher water availability would suggest less fine root competition for water, giving higher ANPP and lower TBCF to GPP. Low nutrient availability may explain the comparable ANPP and higher TBCF. These data show that there are variations in C allocation patterns among mature tropical rainforests, and the variations cannot be explained solely by differences in soil water availability. [ABSTRACT FROM AUTHOR]

Changing trends in runoff and water balance under a warming atmosphere are a major subject of interest in recent climatic and hydrological research. Forest basins represent the most complex systems including critical hydrological processes. In this study, we investigate the relationship between annual total runoff ( Q), precipitation ( P), and mean temperature ( T) using observed data collected from 829 (forest) site years around the world. It is shown that the strong linear relationship between annual P and Q is a function of mean T. By empirically perturbing observed annual Q and P with T, a set of ΔQ-zero lines are derived for different mean T. To evaluate the extent to which the future changes in annual P and T alter Q, the future projections of ΔP and ΔT under a warming scenario (A1B) from five coupled AOGCMs (Atmosphere-Ocean General Circulation Models) are compared with the empirical ΔQ-zero lines derived in this study. It is found that five AOGCMs show different distributions with respect to the ΔQ-zero lines, which can be attributed to the contrasting dominant sensitivities of various influencing factors to water balance partitioning among models. The knowledge gained in this empirical study is helpful to predict water resources changes under changing climate as well as to interpret hydrologic simulations in AOGCM future projections. [ABSTRACT FROM AUTHOR]

Wind speed profiles above a forest canopy relate to scalar exchange between the forest canopy and the atmosphere. Many studies have reported that vertical wind speed profiles above a relatively flat forest can be classified by a stability index developed assuming wind flow above a flat plane. However, can such a stability index be used to classify vertical wind speed profiles observed above a sloping forest at nighttime, where drainage flow often occurs? This paper examines the use of the bulk Richardson number to classify wind speed profiles observed above a sloping forest at nighttime. Wind speed profiles above a sloping forest were classified by the bulk Richardson numberRi B. Use ofRi B distinguished between drainage flow, shear flow, and transitional flow from drainage flow to shear flow. These results suggest thatRi B is useful to interpret nighttime CO2 and energy fluxes above a sloping forest. Through clear observational evidence, we also show that the reference height should be high enough to avoid drainage-flow effects when calculatingRi B. [ABSTRACT FROM AUTHOR]

Using data for one year, we examined the vertical wind speed profiles on a mountain slope covered with forest in northern Thailand under a tropical monsoon climate. We defined two profile patterns: higher wind speeds at greater heights (Pattern 1) and lower wind speeds at greater heights (Pattern 2). We classified 9.4% of the data as Pattern 2; this pattern tended to occur during the night, at low wind speeds, and with high outgoing longwave radiation. In addition, stable stratification and decoupling between the canopy surface air and the overlying layers were observed when Pattern 2 occurred frequently. These facts suggested that Pattern 2 was caused by a nocturnal drainage flow. The occurrence of Pattern 2 showed a clear seasonal trend, indicating that there is a seasonal trend in the occurrence of nocturnal drainage flows. Pattern 2 was observed more frequently between August and February and less frequently between March and July. This corresponded to the seasonal trend in wind speed, but did not correspond to the seasonal trend in the outgoing longwave radiation. [ABSTRACT FROM AUTHOR]