Báo cáo khoa học: "Evaporation and surface conductance of three temperate forests in the Netherlands"

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Original article Evaporation and surface conductance of three temperate forests in the Netherlands Eduardus J. Moors, Jan A. Elbers, A. Johannes Dolman Wim Snijders the Netherlands PO Box 125, DLO Winand Staring Centre, Wageningen, 12 March 1997; 17 September 1997) (Received accepted Abstract - This paper shows the behaviour of evaporation and surface conductance for three dif- ferent forests in the Netherlands: a pine, larch and poplar forest. Maximum evaporation rates of the forests are similar and approach the equilibrium evaporation rates for large extended sur- faces. There is a tight relationship between available energy and evaporation for poplars, less so for pine and larch. Average evaporation declines in the order: poplar, larch, pine forest. Observed maximum conductances follow this trend with the poplar having the highest conductance of 55 mm s the larch intermediate with 31 mm s and pine the lowest 28 mm s Stomatal control . -1 -1 , -1 was most strong in the pine forest and less strong in the poplar forest. The conductance of all three forests follows a strong near-linear decrease with humidity deficit until 8-10 g kg with a , -1 slowly reducing conductance afterwards. For pine and larch the surface conductance reaches the 50 % reduction value already at solar radiation levels of 150 W m while poplar shows a much , -2 less rapid increase. The maximum conductance found here for pine corresponds well with pre- viously published values for the same species. The value for the larch and poplar stand are high compared to other published results. This may be due to the relatively long sampling period of the present study, which increases the likelihood of obtaining rare high values. The results also sug- gest that at the local to regional scale large differences may be found in forest water use. For pre- dicting water yield of forests at this scale, the local variation in water use and stomatal control will have to be taken into account. (© Inra/Elsevier, Paris.) surface conductance / stomatal conductance / evaporation / forest stand / scaling Résumé - Évapotranspiration et conductance de couvert de trois forêts tempérées aux Pays-Bas. Cet article analyse l’évapotranspiration et la conductance du couvert pour la vapeur d’eau de trois peuplements forestiers aux Pays-Bas : pin, mélèze et peuplier. Les taux maximaux d’éva- poration sont du même ordre de grandeur et étaient proches de l’évaporation d’équilibre pour des surfaces importantes. Il existe une relation étroite entre l’énergie disponible et l’évapotranspira- tion pour le peuplier, et moins forte pour le pin ou le mélèze. L’évapotranspiration moyenne des peuplements est la plus élevée pour le peuplier et la plus faible pour les pins. Les conductances maximales de couvert sont rangées dans le même ordre : celle du peuplier montre la plus forte valeur, 55 mm scelle du mélèze une valeur intermédiaire, 31 mm set celle du pin est la plus , -1 , -1 faible, 28 mm sLe contrôle stomatique est le plus fort chez le pin et le plus faible chez le . -1 * Correspondence and reprints
peuplier. La conductance des trois peuplements montre une forte décroissance linéaire avec le défi- cit de saturation de l’air jusqu’à environ 8 à 10 g kg puis une décroissance plus lente au-delà. , -1 Pour le pin et le mélèze la conductance stomatique atteint 50 % de son maximum pour un rayon- nement global de 150 W m alors que le peuplier montre une augmentation moins rapide. Les , -2 conductances maximales chez le pin trouvées ici correspondent bien aux valeurs publiées. Celles du mélèze et du peuplier sont élevées par rapport aux données de la littérature. Cela est peut-être dû à la longue durée de la période de mesure de cette étude, ce qui augmente la probabilité d’observer des valeurs exceptionnellement fortes. Les résultats montrent aussi que des diffé- rences importantes de consommation en eau par les forêts peuvent être mises en évidence, aussi bien à l’échelle locale que régionale. Pour la prévision du bilan d’eau des forêts, il est nécessaire de prendre en compte les variations locales de consommation en eau et de conductance stomatique. (© Inra/Elsevier, Paris.) conductance de couvert / conductance stomatique / evaporation / échelle 1. INTRODUCTION could be tested and also provide the mates basis obtain parameter values for future to modelling [7]. Despite considerable advances in our understanding of forest hydrological pro- can be described by gra- Evaporation cesses [26], a number of practical forest dient-diffusion theory with two conduc- hydrological problems do continue to exist tances indicating the major controls of in the areas of water and land management. water from the vegetation to the atmo- For instance, since the publication of a sphere. The physiologically based canopy, series of model simulations of water use of surface conductance, describes trans- or typical (model) forest stands for the Nether- port from the saturated leaf stomatal sur- lands [8], forests on the high sandy soils in face to the air just outside the leaf. The the Netherlands have been seen as the prime aerodynamic conductance describes trans- culprits of the increasing water consumption port from the air outside the leaf to the air in these areas. This in turn, has led to plans at a certain reference height above the to replace areas with dark coniferous forests canopy. For forest the main control of (Douglas fir) with species consuming less evaporation is through the surface con- water such as oak and Scots pine. ductance rather than through the aerody- namic conductance, which is generally an At the same time, technological order of magnitude larger. For vegetation progress in fast response sonic anemome- with lower height and aerodynamic rough- try, humidity and trace gas measurement ness, the conductances are of similar mag- (e.g. [23]) has made it possible to rou- nitude or the surface conductance is the tinely measure evaporative fluxes of larger of the two. forests and other vegetation types over prolonged periods of time. This has led to The behaviour of surface conductance an increase in studies analysing the major in evaporation models can be described vegetational controls on land surface atmo- by expressing the actual conductance as sphere interaction at canopy scale [3]. To a maximum conductance limited by a provide additional information to water number of environmental factors, such as resource and land managers in the Nether- temperature, solar radiation (or photo- lands, an extensive project was started, synthetically active radiation), atmospheric aimed at quantifying the water use of humidity deficit and leaf water potential or forests by experimental methods. This soil moisture [14, 31].Although, the exact should provide the observational basis mathematical formulations of the func- against which the initial modelling esti- tions differ among authors, the general
lands, a larch site on a loamy soil in the shape of these functions appears to be broadly similar for various forests [16, North, and a poplar site in one of the pold- 30]. In the observations this maximum ers on a heavy clay soil (figure 1). The value is never obtained, as generally, characteristics of the sites are given in always some form of environmental stress table I. The data quality and methods are is present. In this paper the maximum con- described in Elbers et al. [9] and are only ductance always refers to an observed briefly summarized here. Fluxes of latent value. and sensible heat and momentum were obtained by the eddy correlation method Several reviews have appeared recently from scaffolding towers since early 1995. addressing the surprising lack of variation Only data from 1995 are shown in the cur- of maximum surface conductance analysis. rent amongst the major vegetation types of the world [16, 17, 28]. Similarly, at the leaf The system used consisted of a 3-D level, Körner [18] found small variation sonic anemometer (Solent 1012 R2) and a amongst stomatal conductance of vegeta- Krypton hygrometer (Campbell, KH20) tion types. The fact that at the local or linked to a palm top computer (HP- regional scale large differences in water use of forest may exist, and that at the 200LX) which calculated on-line vari- global scale often all the temperate forests ances and co-variances at half hourly inter- may be described by a few parameters, vals using an moving average filter with a points to an interesting scale problem, viz. time constant of 200 s. An automatic is it possible to use the global compila- weather station took measurements of tions of data, averaged for particular veg- incoming and reflected solar (Kipp and etation types, to make predictions at the Zonen CM21) and long wave (CG1) radi- local or regional scale. For practical water ation, soil heat flux (TNO-WS 31 and management, it is likely that the variation Hukseflux SH1), windspeed (Vector in water use will still be the single most A 101 ML), wind direction (W200P) and important factor on which management temperature and relative humidity (Vaisala decisions will be based. HMP35A). Soil moisture was calculated from measurements of the dielectric con- The current paper aims to analyse the stant of the soil using frequency domain differences and similarities in evaporation sensors at 20 Mhz (IMAG-DLO, and surface conductance of three temper- MCM101).Rainfall was measured above ate forests in the Netherlands. Evapora- the canopy and in the open field with auto- tion rates and surface conductances of the mated tipping bucket rain gauges. Power forests will be compared at both seasonal was supplied by a 12 V battery, connected and diurnal time scales and functional dependencies sought. It is the purpose of to a solar panel and a wind generator. At this paper to seek for generalities on which all sites throughfall was measured by a a useful qualitative comparison can be continuously measuring throughfall gauge based, the modelling approach is the sub- and a system of 40 rainfall gauges under ject of another paper. the canopy, read weekly. Surface conductance was obtained by 2. SITE DESCRIPTION inverting the Penman-Monteith equation AND MEASUREMENTS [equation (1)] using a observed r cor- an rected for the difference in momentum and heat transport [33]. The Penman-Mon- The sites are a site of Scots pine on a high sandy soil in the centre of the Nether- teith equation reads:
3. RESULTS 3.1. Measurements and data quality Overall daily energy balance closure is where λE is the latent heat flux, R the net n good [9] and is summarized in table II. radiative flux, G the soil heat flux, g the a The recovery ratios, defined as the average aerodynamic and g the surface conduc- s energy balance closure for daylight hours, tance, Δ the slope of the saturated specific i.e. the ratio of the measured turbulent humidity temperature curve, c the spe- p fluxes over the sum of net radiation and cific heat of air, p the density of air, y the soil heat flux, are close to unity. Table II psychometric constant and δq the specific also shows the difference in energy par- humidity deficit. titioning between the forest with the poplar stand converting most of its available The use of this equation assumes that energy into evaporation. The reverse is true for the needle carrying forests which the source and sink height of temperature convert most of their available energy into and humidity are located at the same sensible heat. The half hourly data used height; in the case of an understorey the in this paper were selected for dry days upper canopy and under canopy are thus only (minimum 2 d after the last rain), and lumped together in a single isothermal only those 30 min values were used for layer. The surface conductance is in the which energy balance closure was better case of a homogeneous canopy approxi- than 25 %. The first criterion was used to mately equal to the parallel sum of the remove the possibility of contamination stomatal conductances [29]. In practice of the transpiration flux by soil evapora- environmental control on canopy con- tion. Although some soil evaporation may ductance is regulated by the behaviour of still occur after 2 d, this is unlikely to be the guard cells in the stomata. At the substantial. Data suspicious of dew or wet canopy level these controls are lumped canopy after rain were also removed from together and appear more smooth than the analysis. This data screening resulted when observed at the leaf level. This in a data set which thus contained only explains the success of canopy conduc- dry canopy evaporation with minimum or tance models in single leaf evaporation no contamination by soil or wet canopy evaporation. Note that the word evapora- models.
3.2. Seasonal evaporation tion is used to denote both transpiration (i.e. dry canopy evaporation) and soil and surface conductance evaporation, although in practice the terms transpiration and soil evaporation will be In figure 2 the average and maximum used throughout most of the paper. This half hourly transpiration of the three forests usage of evaporation is physically more is shown. Throughout most of this paper precise and avoids using the more impre- both the average and the maximum values cise term evapo-transpiration. of variables are shown. This gives an indi- cation of the statistical variation in the data, The last selection criterion was used and allows a qualitative assessment of the minimize potential advective or heat to main functional relationships between con- storage effects and does not effect, but ductance and environmental variables. It removes a number of uncertain data values is clear from this figure that the poplar from the analysis. Elbers et al. [9] also stand in the polders has the highest average perform a source area analysis which sug- transpiration, followed by the larch. Figure gested that generally during day light con- 2 indicates that the poplar stand transpires ditions fetch requirements were adequate. close to its maximum rate as the differ- For the larch forest only those data were ence between the average and maximum selected with sufficiently long fetch, as at this site, a bog covered by Molinia bor- values is generally small. The conductance ders the forest in a western direction [9]. of forests declines rather smoothly (lin-
early) after an early morning maximum qualitative observations of leaf area devel- during the course of the day [30], with no opment were available. In general it may substantial midday closure effects. This be expected that evergreen needle leaf suggest that for the two other forests, where forests are able to start transpiring earlier the average half hourly transpiration rate is in the season, as they do not first need to roughly two thirds of the daily maximum, grow new needles. This would explain the significant stomatal control is present. difference in early spring transpiration between the stands. The relatively high The maximum transpiration rates for evaporation rates of the poplar stand in the three forest are of similar magnitude the spring are caused by undergrowth of (0.7 mm h This rate corresponds to the ). -1 nettles and shrubs which experienced a equilibrium evaporation rate with a Priest- rapid growth before the leaves started to ley Taylor coefficient of unity [21]. grow on the trees. This results in the high- Although generally a value larger than est total stand evaporation for the poplar unity would be expected [6], the suggestion stand. The higher values of poplar tran- from these results is that the maximum spiration around day 250 originate only evaporation rate from vegetated surfaces from the forest canopy, as the undergrowth is controlled by the physics of the bound- has died down. ary layer and less so by plant physiological control mechanisms. Care must thus be All three forests show a decline in evap- exercised in linking maximum evapora- oration during the dry period from day tion rates to physiological parameters. 210 to 240. This is most likely due to increasing soil moisture stress and or tem- During the winter, after day 300, mea- (see below). stress perature sured evaporation rates are occasionally still of the order of 0.1mm h Although . -1 In figure3 evaporation is plotted the data were selected to minimize effects against the available energy. The pine for- of soil and wet canopy evaporation, this est, on average uses 40 % of the available evaporation must be attributed to stem, energy for evaporation, remarkably con- understorey or soil evaporation. Certainly sistent with values quoted for a Boreal in the poplar stand some of this evapora- Jack pine stand in Canada [2]. In contrast, tion is caused by the soil and dead under- the poplar stand uses 66 % of the avail- storey (litter) as by that time leaves had able energy for evaporation, consistent already fallen off the canopy. This evap- with the estimates for a broad leaved tem- oration gives a quantification of the resid- perate forest [2]. This difference reflects ual, or background evaporation for other primarily the behaviour of the surface con- periods of the year. ductance of both forests, as the roughness All forests show a steep increase in length, and consequently the aerodynamic transpiration in the spring, although the conductance, of the forests are almost sim- timing is slightly different for each forest. ilar. The larch forest is intermediate with The pine forests start to transpire the ear- 46 %. Hinckley et al. [12] note a low liest, around the beginning of April. atmospheric coupling for a poplar stand Leaves started to grow in the poplar stand in the US. Their result fundamentally from the end of April until mid-June and agrees with ours, as low coupling to atmo- fell after early September, a process which spheric vapour pressure deficit as found in their study, would indicate a tight rela- was fully completed only around mid- October. The larch stand started to grow tionship between net available energy and new needles from mid-April till the end evaporation, with no substantial sensitiv- of May and needle fall took place during ity of transpiration to changes in vapour November. Unfortunately in 1995, only pressure deficit.
age. This limits the approach to showing a Figure 4 shows the seasonal behaviour general seasonal trend over 1995. Note, of the conductance of the three forests. that as before, the data were selected to The surface conductance is shown as a exclude periods after strong rainfall to daylight average with a corresponding minimize the inclusion of points when the standard error and as a maximum value. soil surface, understorey or indeed the for- There is not always an equal number of est canopy was still wet. points used in the calculation of the aver-
towards the end of May, and drops after The surface conductance of the poplar stand is generally much higher than that day 200-225, at the end of August, to of the Scots pine and larch stand in accor- increase again after day 240. In the case of dance with the differences in evaporation. the poplar stand this is probably caused The maximum conductance for poplar was by temperature stress rather than soil mois- 55 mm sfor larch 32 mm sand for , -1 , -1 ture limitation as the ground water level the Scots pine 29 mm s The average . -1 at the site remains close to the surface at values are much smaller (18, 10 and 7 mm 1.75 m. Roots still have access to this , -1 s respectively). The forest stands con- reservoir. During this period abnormal tinue to evaporate, even during the win- high temperatures above 30 °C were reg- ter season, with an average diurnal resid- ularly observed and plotting conductance ual conductance of the stand of about against temperature for the poplar (not 2-3 mm sIt is possible that this evapo- . -1 shown) indicated a sharp decrease in con- ration consists of some residual transpi- ductance after 25 °C. In the case of the ration, but it is more likely to be caused Scots pine forest soil moisture stress is by evaporation from the litter or soil layer. more likely to have caused the decline in conductance and evaporation. This is In all forests the average diurnal con- shown more clearly in figure 5, where ductance increases around day 150,
pine. The average conductance of the larch evaporation and conductance are seen to be dropping off at moisture deficits above shows relatively little diurnal variation. 70-80 mm. This level corresponds to about 50 % of the maximum available The difference between maximum and water content of the profile. average conductance can be used as an indication of the amount of stomatal con- trol the trees are able to exert on the tran- spiration rate. A big difference indicates a 3.3. Diurnal evaporation large amount of stomatal control. Total and surface conductance absence of diurnal variation in stomatal control would be shown by similar values of the average and maximum conduc- The surface conductance of forests tances. The Scots pine exerts most con- shows a marked diurnal variation, caused trol on the conductance as the average con- to a large extent by its (bulk) dependence ductance is generally a factor of two lower on solar radiation and atmospheric humid- than the maximum. The larch stand fol- ity deficit [14, 31]. Figure 6 shows the lows this, but the scatter in the maximum diurnal behaviour for the three forests of conductances is larger, which makes it this study. Conductance peaks a few hours impossible to draw firm conclusions. The after sunrise and after that steadily difference between maximum and aver- declines. This is particularly clear in the age conductance for the poplar stand is case of the Scots pine forest, where the smaller, of the order 30-40 %, indicating maximum conductances are reached at 9 to still substantial stomatal control. The diur- 10 hours GMT. The larch and poplar stand nal pattern in conductance and radiation show a clear maximum in conductance gives rise to marked diurnal trend in evap- and a less steep decline than the Scots
pattern of a relatively strong linear oration rates with a well-defined maxi- a decrease until, say 8-10 g kgwith a -1 mum at solar noon. This is also shown in slowly reducing residual conductance figure 6. afterwards (e.g. [30]). This appears to be a The diurnal trend in conductance is to a general feature of the humidity deficit- large part controlled by its response to conductance relationship of forests. radiation and specific humidity deficit. In Also shown is the response to solar figure 7 the response of the conductance of radiation. The pine forest shows a rapid three forests to specific humidity deficit increase with radiation, the 50 % value and solar radiation is shown. Figure 7 is reached at 150 Wm the 50 % value , -2 shows that the conductance of pine forests for larch being almost the same. For the responds most strongly to humidity deficit, poplar stand a much less rapid increase in with almost complete shut down at 16 g kgThe larch forest shows an . -1 conductance with increasing radiation is almost observed. It is important to note that the similar but somewhat more gradual radiation and humidity deficit responses response (e.g [1]). The average conduc- cancel to some extent, as high radiation tances follow this pattern with less ampli- levels are generally associated with high tude. The poplar stand also shows a strong atmospheric humidity deficits. This fall of conductance in the first part of the explains why the maximum values of all curve to a residual conductance of about 5-10 mm sNote, however, that at 8 . -1 three forest tend to decline again with high radiation (> 600 Wm Both needle leaf ). -2 g kg the poplar stand still has an residual -1 conductance of 20 mm swhereas the , -1 forests show a similar response as the forests analyzed by Shuttleworth [30]. The two needle leaf forests are at considerable poplar stand is different from these two, as lower values. All forests appear to follow
steep decline in conductance is observed average conductance for coniferous forests a is 18.7 mm s (± 1.2), which compares -1 with humidity deficit, but a somewhat slower response to radiation. Also the well with the result obtained by Schulze et decline in conductance with increasing al. [28]using a slightly different set of high radiation is less strong than in the forests. They cite an average conductance of 20 mm sThis average number, how- . -1 other two forests. It is tempting to specu- late that this response serves the poplar ever, hides large differences both between species well, because it enables it to keep and within species. For instance the max- imum conductance of larch obtained in on transpiring, and respiring at higher this study is 31.5 mm swhereas a larch , -1 humidity deficits than other species (e.g. stand on arguably a much poorer soil in figure 5). In the rich clay soils on which it is planted, with large amounts of water Siberia reaches a maximum conductance of only 9 mm sThe Pinus results show . -1 available, virtually throughout the year, this behaviour may, although opportunis- more coherence with an average of 24.1 mm s The value for this study is within . -1 tic, give the poplar the ability for increased the range of these other observed values. gas exchange and consequent rapid growth and wood production. It is unknown how the relatively low values for Picea abies of Tenhunen et al. [32] can be explained. Perhaps limited 4. DISCUSSION temporal sampling in this particular study may contribute to these low values. The values obtained in this study are at the The similarity in maximum evapora- tion rates between forests was recently higher end of the observed values: this noted in a review by Kelliher et al. [16]. may be due to the long sampling period obtained by operating continuous mea- They also concluded that maximum evap- surements. This will increase the likeli- oration rates were likely to be determined hood of obtaining rare high values under by large scale boundary layer phenomena specific environmental conditions. It is which tend to reduce the sensitivity of for- less likely that they are caused by con- est evaporation to surface conductance. tamination of the canopy conductance by The results obtained in this study support the soil or understorey. Nevertheless when that hypothesis. comparing conductances, the availability The values of maximum conductance of long-term measurements would appear agree with previously published values, to be a prime requirement. which are listed in tables III and IV. Most values are for coniferous forests and gen- The value obtained for the maximum erally range from low values for Picea conductance of the poplar stand is high species to higher values for Pinus species. compared to the other values published There is, however, considerable variation for deciduous forest (table IV). Excluding in these values, which may partly be the current value for poplar, an average of 21 mm s is obtained. Including our -1 explained by the fact that the maximum values do not always refer to the maxi- yields current measurements an average . -1 sThe high conductance for of 26.7 mum obtained over a complete growing mm season, but refer to a few special days for poplar is however consistent with its high which measurements were available. water use and quick growth rate (e.g. [ 12]). Perhaps more important is the relatively There appears to be no clear relation between leaf area index and maximum strong coupling of transpiration to net conductance; additional leaf area thus does available energy (figure 2) and its stomatal not lead to increased conductance. The control (figure 7).
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