Báo cáo khoa học: "Temporal and spatial variation in transpiration of Norway spruce stands within a forested catchment of the Fichtelgebirge, Germany"

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Original article Temporal and spatial variation in transpiration of Norway spruce stands within a forested catchment of the Fichtelgebirge, Germany Martina Alsheimer Barbara Köstner, Eva Falge, John D. Tenhunen Department of Plant Ecology II, Bayreuth Institute for Terrestrial Ecosystem Research, University of Bayreuth, 95440 Bayreuth, Germany 27 June 15 (Received 1997) January 1997; accepted Abstract - Tree transpiration was observed with sapflow methods in six Norway spruce (Picea abies) stands located in the Lehstenbach catchment, Fichtelgebirge, Germany, differing in age (40 years up to 140 years), structure, exposition and soil characteristics. The seasonal pattern in tree canopy transpiration, with the highest transpiration rates in July, was very similar among the stands. However, young dense stands had higher transpiration compared to older less dense stands. Because of forest management practices, stand density decreases with increasing stand age and provides the best predictor of canopy water use. Measured xylem sapflux density did not dif- fer significantly among stands, e.g. vary in correlation with stand density. Thus, differences in canopy transpiration were related to differences in cumulative sapwood area, which decreases with age and at lower tree density. While both total sapwood area and individual tree sapwood area decrease in older less dense stands, leaf area index of the stands remains high. Thus, transpiration or physiological activity of the average individual needle must decrease. Simulations with a three-dimensional stand model suggest that stand structural changes influence light climate and reduce the activity of the average needle in the stands. Nevertheless, age and nutrition must be con- sidered with respect to additional direct effects on canopy transpiration. (© Inra/Elsevier, Paris.) transpiration / canopy conductance / sapwood area / stand age / stand density / Picea abies Résumé - Variations spatiotemporelles de la transpiration de peuplements d’épicéas dans bassin-versant du Fichtelgebirge (Allemagne). La transpiration des arbres a été évaluée au un moyen de méthodes de mesure du flux de sève dans six peuplements d’épicéas (Picea abies), situés dans le bassin-versant du Lehstenbach, Fichtelgebirge (Allemagne), qui différaient en âge (40 à 140 ans), structure, exposition, et en caractéristiques de sol. L’allure des variations saisonnières * Correspondence and reprints Tel: (49) 921 55 56 20; fax: (49) 921 55 57 99; e-mail: john.tenhunen@bitoek.uni-bayreuth.de
transpiration des arbres, avec notamment un maximum en juillet, était très similaire entre de la peuplements. Néanmoins, les jeunes peuplements denses ont montré une plus forte transpi- ces ration que les peuplements âgés et moins denses. La densité du peuplement s’est avérée être la meilleure variable explicative de la transpiration, car les pratiques sylvicoles réduisent la densité des peuplements en fonction de l’âge. La densité de flux de sève n’a pas montré de différences significatives entre les peuplements. Ainsi, les différences de transpiration étaient seulement dues aux différences de surface de bois d’aubier, qui diminue avec l’âge et la densité. Alors que la surface de bois d’aubier à l’échelle du peuplement comme à celle de l’arbre diminuaient dans les peuplements âgés et peu denses, l’indice foliaire de tous les peuplements étudiés restait élevé. Ainsi, il est probable que la transpiration ou l’activité physiologique des aiguilles diminuent avec l’âge des arbres. Des simulations réalisées au moyen d’un modèle de couvert 3D suggèrent que les modifications de structure des peuplements influencent le microclimat lumineux et rédui- sent l’activité foliaire. Malgré tout, l’âge et la nutrition doivent être pris en compte dans leurs effets sur la transpiration des arbres. (© Inra/Elsevier, Paris.) transpiration, conductance du couvert, surface de bois d’aubier, âge, densité, Picea abies of heterogeneity in 1. INTRODUCTION improved analysis and function of spruce stands structure within landscapes and along chronose- Norway spruce (Picea abies (L.) quences and new analytic capabilities to Karst.), because of its importance in tim- separate the complex effects of multiple ber production, is one of the most widely factors on carbon fluxes, i.e. potentials for studied forest trees of Europe. The empir- comparison of sites as may be achieved ically derived yield tables for Norway with process-oriented simulation models. spruce demonstrate that substantial dif- ferences in stand development and pro- Landscape heterogeneity in transpira- ductivity occur regionally within Germany tion occurs as a result of the presence of [3, 30, 54, 56, 73] and between neighbor- different species, variation in site quality, ing countries (Austria in Marschall [44]; local climate gradients, the spatial mosaic Slovakia in Halaj [26]; Switzerland in in stand age as well as stand density, and Badoux [5]). Observations and recon- silvicultural treatment. Heterogeneity in structions of height growth and wood vol- transpiration potential is accompanied by ume increment for Norway spruce at long- shifts in foliage mass to sapwood area term sites demonstrate 1) a rapid increase ratios [43]. Espinosa-Bancalari et al. [13] in growth and production followed by found that variations in foliage area to sap- growth decline after approximately wood area ratios are strongly correlated 80-100 years [12, 57], 2) a clear differ- with mean annual ring width of the sap- entiation in development due to climate wood, implying that growth potential is and soils [30, 54] and 3) a recent trend for an important component in the dynamic growth stimulation even in older stands maintenance of xylem water supply capac- due, among other factors, to high nitro- ity. Sapwood permeability is directly pro- gen deposition [16, 17, 54]. An evalua- portional to tree growth rate [74]. tion of the relative importance of long- term changes in site climate (temperature, Greater latent heat exchange and CO 2 precipitation and atmospheric CO site fixation in young as compared to old ), 2 quality (also as affected by atmospheric stands of Pinus banksiana were observed in northern Canada [63]. Decreases in nitrogen deposition), and tree physiology canopy transpiration of 35 % with aging on forest growth requires both an
the catchment is covered with Norway spruce of Norway spruce were reported by Schu- [Picea abies [L.] Karst.]. The exposed sub- bert (in [37]) in a comparison of 40- and strates are mainly phyllite and gneiss and the 100-year-old stands. Yoder et al. [75] most common soils are brown earths and pod- found that photosynthetic rates decreased sols. Where ground water is near the surface, in old trees of Pinus ponderosa, suggest- local boggy organic layers form. The mean ing that canopy gas exchange is reduced in annual air temperature is approximately 5.8 °C old stands as growth potential decreases. (at an altitude of 780 m) and mean annual pre- cipitation is 1 000-1 200 mm. There is also a Falge et al. [14] reported in Picea abies, high occurrence of fog (100-200 d per year) that the observed data were compatible and only a short growing season (100-130 d with an unaltered mesophyll photosyn- year). per thetic capacity but a greater stomatal lim- Six spruce stands differing either in age and itation as trees aged. structure, in exposition, or in soil characteris- tics were chosen for study. Three of the stands In the present study, tree canopy tran- were of approximately the same age (40 years). spiration was simultaneously examined The stand Schlöppner Brunnen compared to along a chronosequence of Picea abies the other stands is growing on very wet and stands growing in relatively close prox- boggy soil (subsequently: 40-year boggy imity within a forested catchment of the stand), while the stands Weiden Brunnen (sub- Fichtelgebirge, Germany. Our purpose sequently: 40-year stand) and Schanze are was to determine whether regulation of located on moderately moist to moist soils. The stand Schanze has a north-east exposition the transpiration flux differed, and if so, (subsequently: 40-year NE stand) while all potential causes of this variation, i.e. other stands occur on south-facing (south-east potential differences in microclimate, in to south-west) slopes. In addition to these three canopy structure and light interception, in stands of the same age, the 70-year old stand site quality and tree nutrition, or in water Süßer Schlag (subsequently: 70-year stand), supply capacity as reflected in the foliage the 1 10-year old stand Gemös (subsequently: 110-year-stand) and the 140-year-old stand area to sapwood area ratio. While tree Coulissenhieb (subsequently: 140-year stand) canopy transpiration can be measured or located on drained but moist soils were inves- estimated via micrometerological meth- tigated. Tree density of the stands decreases ods, homogeneous areas lend themselves with age owing to thinning and removal of best to interpretation with these methods wood in forest management. Stand character- and large fetch distances are required. istics are summarized in table I. Measurements of water flux at the leaf or Investigations were carried out primarily shoot level are limited due to problems in the year 1995 from the middle of April to encountered in a direct scaling-up of rates the middle of November (preliminary experi- ments with fewer stands were conducted dur- to the stand level [39]. Thus, xylem ing 1994 as described below). Air tempera- sapflow measurements were used in our ture, relative humidity and net radiation or study and are viewed as the most appro- global radiation were recorded automatically at priate method for obtaining coupled infor- meteorological stations above the canopy at mation about the physiology of individ- the 40-year boggy, the 40-year NE and the ual trees, tree structural development, and 140-year stand as well as for several weeks in site factors as they affect water relations. autumn at the 40-year stand. Vapor pressure deficit (D) was calculated from temperature and relative humidity measurements at the first three sites. The remaining sites were consid- 2. MATERIALS AND METHODS ered most similar to the 140-year stand and transpiration at these sites was related to D at the 140-year stand. Precipitation was measured The experimental sites are located within in an open field near the 140-year stand. At the Lehstenbach catchment, Fichtelgebirge, the 140-year stand, rainfall, throughfall and northeastern Bavaria, Germany at an altitude of approximately 750-800 m. More than 90 % of windspeed as well as soil temperature were
null-balance method of Kucera additionally recorded. Soil matrix potentials steady-state, were measured with self-recording tensiometers [36] Cermák et al. [9] and Schulze et al. et al. [42], which were installed at 35 and 90 cm [60]. With the Granier methods applied in all deep at the 40-year stand, the 40-year boggy stands, cylindrical heating and sensing ele- stand and the 140-year stand, and with manu- inserted into the trunks at breast ments were ally recorded tensiometers at 20 cm deep at above the other ca 15 cm apart, height, one the 40-year NE stand, the 70-year stand and and the upper element was heated with con- the 110-year stand. Predawn water potentials of stant power. The temperature difference sensed small twigs of the trees at the 140-year, 40- between the two elements was influenced by year, 40-year boggy and 40-year NE stand were the sap flux density in the vicinity of the heated measured every 2 weeks from the end of June to the middle of August, using a pressure cham- element. Sap flux density was estimated via ber [58]. calibration factors established by Granier [19]. The steady-state, null-balance instrumentation Sapflow installations were made in mid- was used to compare methods on the same trees April in three stands but were delayed until within the 40-year stand. A constant tempera- middle of May at the 40-year NE stand and ture difference of 3 K was maintained between until beginning of June at the 70-year and 110- a sapwood reference point and a heated stem year stands. Within all stands, transpiration section. The mass flow of water through the was monitored on ten trees except in the case of xylem of the heated area is proportional to the the 140-year-old stand where 12-13 trees were energy required in heating. Additionally, both examined. Two methods for measuring xylem methods were used (on separate trees) to esti- sapflow were used: thermal flowmeters con- mate transpiration in the 140-year stand. structed according to Granier [19, 20] and the
from five trees of the 40-year, the 40-year Total sapflow per tree was obtained by mul- boggy and the 40-year NE stand. Nutrient con- tiplying sap flux density by the cross-sectional tent of the needles of the 140-year stand was area of sapwood at the level of observation. determined in October 1992 and in October Sapwood area of sample trees was estimated 1995. from regressions relating GBH (girth at breast height) to sapwood area determined either with Needle biomass of five individual trees per an increment borer, by computer tomography site, selected over the GBH distribution (girth [25], or from stem disks of harvested trees. breast height), was determined by applying at Since no correlation was found between tree the ’main axis cutting method’ of Chiba [10]. size and sap flux density except at the 40-year Needle area/needle biomass was determined NE stand, stand transpiration (mm d was) -1 for sub-samples taken from the lower-, mid-, estimated (except at the 40-year NE stand) by and upper-third of the canopy with a Delta-T multiplying mean flux density of all sample image analyzer (DIAS). Regression equations trees by total cross-sectional sapwood area of relating total needle surface area for trees to the stand and dividing by stand ground sur- GBH were used to sum leaf area for trees in face. At the 40-year NE stand where flux den- the stand and to estimate LAI. Harvest results sity was correlated with tree size, tree transpi- indicated that trees from 40-year stands were of ration was extrapolated to stand transpiration similar structure and these data were pooled according to the frequency of occurrence of for needle surface area regressions. For the trees in different size classes. For days with older stands, LAI estimates are based on five missing data owing to technical failures as well trees per stand. Cross-sectional sapwood area as for the early season before sensors could be of stands was estimated from regressions relat- installed in some stands, canopy daily transpi- ing GBH to sapwood area determined either ration sums were estimated from correlations with an increment borer, by computer tomog- established between the measured daily tran- raphy [25], or from stem disks of harvested spiration and daily maximum vapor pressure (cf. figure 9). trees deficit (D cf. figure 4). , max From tree canopy hourly transpiration rates and hourly average D measured above the 3. RESULTS canopy, values of total canopy conductance (G were derived. The time courses for mea- ) t sured sap flow were shifted by 0.5-1.5 h until compatability between morning increases in 3.1. Stand climate and water supply photosynthetic photon flux density and esti- mated tree canopy transpiration were achieved. During the intensive measurement Thus, our analysis assumes that a linear shift phase, which was carried out from the compensates for the capacitive delay in flow middle of April to the beginning of detection at breast height as compared to crown November 1995, a pronounced period of level transpiration. Further details regarding the estimate of G as dependent on shifted tree t cloudy and rainy weather occurred in June, canopy transpiration and on D are given by with sunny warm weather in early and mid Köstner et al. [32, 34] and Granier et al. [22]. and cool clear weather in fall. summer, Tree canopy conductance was calculated Monthly changes in climate factors are according to the following formula: given in table II. T and, thus, D were max max consistently lower (ca 15 %) at the 40- year NE stand as compared to the 40-year where g is tree canopy conductance (mm s ), -1 c and 140-year stand which were adjacent E is tree canopy transpiration (kg H m 2 -2 O c on the northern divide of the watershed. ), -1 h D is vapour pressure deficit (hPa), G is v The lowest D (20 % less than 40-year max gas constant (0.462 m kPa kg K T is air 3 -1 -1 k ), stand owing to evaporation from standing temperature (Kelvin). water and mosses in the understory) was measured for Needle nutrient content was found in the 40-year boggy stand. In mid- twig samples collected in July in the sun crown July and in August, moderate drying of of five harvested trees at the 70-year and at the the surface soil layers occurred. However, 110-year stands and at the end of October 1994
the lowest recorded soil matrix potentials and is also high at the 40-year boggy stand (0.83 ± 0.12 mg g , -1 1-year-old at the 110-year-stand (ca -550 hPa at 20 needles), while at the other stands the Mg -con- 2+ cm soil depth) do not indicate that the trees were subjected to water stress. Ten- centration in the needles of this age class ranges between 0.25 ± 0.09 mg g (40--1 siometer values from other stands fluctu- year NE stand) and 0.63 ± 0.39 mg g -1 ated within the same range as observed in the110-year stand. Lowest predawn water (70-year stand). Therefore, these other potentials of the trees measured at the 40- stands show values far below the limit of year stand during the end of June to the adequate mineral nutrient concentration middle of August fluctuated only between for optimal growth according to Bergmann [6]. The Mg of the 40- -concentrations 2+ -0.4 and -0.5 MPa. year boggy stand and the 110-year stand are significantly different (P < 0.05) from the Mg of the 40-year- -concentrations 2+ 3.2. Needle nutrient concentration stand, the 40-year NE stand and the 140- year stand. Needle analysis of twig samples showed that there are differences in needle Differences between stands were also found in the Ca of the -concentration 2+ nutrient concentration among stands. Mg 2+ needles. Lowest Ca in 1- -concentration 2+ - concentration (± standard deviation), for example, is highest at the 110-year stand year-old needles was measured at the 40- , -1 mg g1-year-old needles) year NE stand (1.41 ± 0.32 mg g A ). -1 (1.12 ± 0.21
3.3. Tree canopy transpiration concentration of 2.46 ± 0.78 mg Ca per 2+ was found at the 40-year- g dry weight stand. The 40-year boggy stand, the 70- comparison of the estimated daily A year stand and the 140-year stand had transpired by six trees of the 40- water almost the same relatively high Ca -con- 2+ year stand Weiden Brunnen when mea- centration in the needles (4.28 ±1.21 mg sured with the ’Granier’ and ’Cermák/ g4.28 ± 2.34 mg gand 4.29 ± 1.42 -1 Schulze’ methods is illustrated in figure 1. , -1 On an individual tree basis, there are sys- mg grespectively). Highest Ca -con- 2+ , -1 tematic differences observed in transpira- centration was observed at 110-year stand tion estimates (average sapflux density) (7.38 ± 1.52 mg g). -1 which depend on instrumentation speci- ficities, local variation in wood structure, -concentration + K of the 1- The mean etc. However, with a sufficiently large year-old needles reached higher values in number of installations (estimated require- the 40-year-old stands (5.97 ± 0.52 mg ment of 8-10 [35]), which are carried out 6.59 ± 1.11 mg g and 6.34 ± -1 , -1 g in consistent fashion (in our study ten per 0.93 mg gat the 40-year stand, the 40- -1 stand), flux rates observed with both sys- year boggy stand and the 40-year NE stand, respectively) than in older stands (4.97 ± 0.52 mg g and 5.53 ± 0.45 mg -1 gat the 70-year stand and the 140-year -1 stand). The lowest K -concentration + (3.46 ± 0.480 mg g was measured in 1- ) -1 year-old needles of the 110-year stand, which was significantly different from the -concentration + K of the needles of the other stands. The needle nitrogen concentration is higher in the 40-year-old stands (3-year- old needles; 40-year stand: 15.1 ± 1.5 mg ; -1 g40-year boggy stand: 15.5 ± 1.7 mg g 40-year NE stand: 13.7 ± 0.6 mg g ) -1 ; -1 than in the 70-year stand (3-year-old nee- dles: 12.5 ± 0.8 mg g the 110-year- ), -1 stand (3-year-old needles: 11.8 ± 1.4 mg ) -1 g and the 140-year stand (3-year-old needles: 11.7 ± 1.0 mg g Therefore two ). -1 of the 40-year-old stands (40-year stand and 40-year boggy stand) and the three older stands were, concerning the nitro- gen concentration of the 3-year-old nee- dles, significantly different (P < 0.05) and also the differences between the 40-year NE stand and the 140-year stand were sig- nificant.
well. Studies by Köstner et al. Weiden Brunnen site, we feel confident tems agree that the calibration factors provided by [33] and Granier et al. [22], which have Granier [19] function well in estimating the two methods of sapflow compared tree transpiration of spruce, at least when measurements within the old spruce stand there is no apparent water stress. Thus, Coulissenhieb and in the case of Pinus the ’Granier’ method provides a useful sylvestris, also indicate that similar esti- and appropriate means for comparing tran- mates of transpiration flux are obtained. spiration rates and water use in the six The ’Cermák-Schulze’ system should inte- selected experimental stands. grate over any changes in flux density that may occur with depth in the trunk and pro- The average estimated half-hourly vide a direct measurement of total flow in transpiration of all six stands water use as long as the electrodes span the entire is shown for two clear summer days hav- conducting sapwood. Given the good ing different time course patterns in vapor agreement found for these methods at the pressure deficit (D) in figure 2. The simi-
simultaneously by light and D, but also larity at all locations in the diurnal pattern by endogenous factors related to water of water use is quite striking and the storage, hormonal regulation, and further importance of variation in PPFD is obvi- as yet unexplained variables. ous. On these days, the highest maximum hourly transpiration rates of ca 0.25 mm To obtain an impression of the overall -1 h were observed for the 40-year boggy influence of light and D on regulation of spruce stand, while the lowest hourly rates water loss from the spruce stands, the day- of only 0.11 mm h were found for the -1 time half-hour values of stand conduc- 140-year stand. On 28 June, D increased tance (G in figure 2) over the entire season t continuously and rapidly for a long period were examined for agreement with sev- reached in the after- until 14 hPa ca was eral simple models. We hypothesized that decreased during the late noon, and then D stand conductance should increase with afternoon hours. On 1 August, a similar increasing PPFD incident on the canopy maximum in D was achieved (ca 15 hPa), and then saturate at sufficiently high light but D was already large during the previ- when stomata are open in all canopy lay- ous night owing to warm air temperatures ers. We expected that increasing D would and increases in D occurring during the impose an additional linear restriction on day were very gradual. A close compari- the maximum stomatal conductance son of the estimated time courses of tran- attained in each canopy layer. The data spiration illustrates that the actual rate were separated into classes with differing occurring at 15 hPa D on these two days ranges of D (0-5, 5-10, 10-15, 15-20 and depends on the time course of change in > 25 hPa) and fit with non-linear regres- conditions. Maximum values of G were sion techniques. An example of the general t depressed in August at all sites by ca 40 %, results is shown for the 40-year stand in figure 3. An equation in which conduc- when D remained high during the night. tance saturates with increasing light pro- Thus, canopy conductance is affected
vided a good explanation of observations stands saturated at D values of ca max when D was greater than 10 hPa. At lower 20 hPa (figure 4). Daily maximum G t D, saturation did not occur and G was lin- decreased strongly with increasing D t max early related to incident PPFD. A simple (figure 5). Thus, stomatal regulation with model combining PPFD and D effects respect to D plays an important role in over the entire range of observations, cf. determining stand maximum transpiration Lu et al. [41], resulted in an increasing rate. While linear approximations to the stimulation of conductance with increasing dependencies shown in figure4 may be PPFD at low D and, thus, was not further useful for coarse estimates of water bal- developed as a practical description. Time- ances, the variation in response shown and dependent endogenous effects such as dis- these stomatal regulatory phenomena sug- cussed above, time lags in sap flow gest that models such as Haude [27] should response that we attempted to correct in be applied with appropriate caution. While relation to above canopy conditions, and daily integrated tree canopy transpiration potential measurements errors at low vapor was correlated with daily maximum D, pressure deficit contribute to the derived transpiration rates in late September and description of conductance behavior and October seemed to be influenced by the may cause difficulties in these simple previous night minimum air temperature. empirical models. Maximum rates of daily tree canopy Daily transpiration has been linearly transpiration at our sites increased from 2.4 mm d in May to 2.8 mm d in July -1 -1 related to vapor pressure deficit measured at various times of day in a number of sim- at the the 40-year boggy stand, at which plified hydrological models. In Germany, time the highest water use was measured, and decreased from 2.6 mm d in August -1 the time of observation at standard weather -1 d in October. As would be stations is used as the critical input vari- 1.2 to mm able [1, 27]. Integrated daily tree canopy from the results shown in fig- expected transpiration in our study increased curvi- ures 2 and 4, this seasonal pattern in tree linearly with daily maximum D, and the canopy transpiration was found in all six maximum capacity for transpiration in all investigated stands (figure 6) and system-
(due to evaporation from atic differences between stands occur. the lowest D max Similar magnitudes in water use and dif- standing water and mosses in the under- story), was greater at the 40-year boggy ferences between stands were observed during 1994, when tree transpiration was stand Schloeppner Brunnen (208 mm measured in only three of the stands. The total). Comparative analyses of needles at daily sum of tree canopy transpiration was the sites indicated a significantly higher Mg content at the 40-year boggy stand reduced by approximately 50 % during which may be related either to delivery in periods of overcast skies, and to essen- flowing water or better retention of Mg tially zero when overcast and rain retarded transport away from the occurred. In June 1995, these factors due to tree roots. Further experiments must be reduced the monthly sum of canopy tran- carried out in order to determine whether spiration by approximately 60 % in com- this change in nutrient status is causally parison to July 1995 and by approximately related to the higher level of physiological 50 % in comparison to June 1994 (half as many ’bad weather’ days; figure 6). activity of the 40-year boggy stand. Transpiration of the oldest stand is Seasonal total overstory transpiration in the two 40-year-old stands on drained much lower than in the young stands (fig- soil differed in proportion to stand leaf ure 6; e.g. in 1995 transpiration of the 140- year-stand was only 81% of the 40-year area indices, 134 mm at the 40-year stand Weiden Brunnen and171 mm at the 40- stand and only 52 % of the 40-year boggy year NE stand Schanze. T and, thus, stand), despite having greater equal max or LAI. Seasonal transpiration from the older D are consistently lower at Schanze as max stands (147, 163 and 109 mm in the 70-, compared to Weiden Brunnen. LAI appears to increase in north-exposed 110- and 140-year-old stands on a ground area basis, respectively) was similar after stands, tending to maintain a similar stand water balance as discussed by Miller et standardizing for LAI (figure 7). Current al. [47]. Seasonal canopy transpiration, management practices in the Fichtelge- birge, result in decreases in stand density even after adjusting for LAI and despite
that are correlated with stand aging. As different methods at the 40-year stand Weiden Brunnen. The same results were illustrated in figure 7, stand density was found to be the best predictor of seasonal obtained either by measurement of sap- wood area from stem disks of harvested transpiration, even better than stand age. Differences in transpiration among the 40- trees, from stem cores, or from measure- ments with computer tomography (fig- year-old stands as a group and the older stands as a group could also reflect the ure 9A). The overall comparison of stands (figure 8) is based on coring and stem disk influences of increasing N deposition in analysis. If the data from young (40-year recent decades and early tree development stands) and old stands (> 40 years) are under differing nutrient regimes. compared (figure 9B), then it is quite clear that there is a shift in sapwood area rela- Sapflux density in July for all trees var- ied between 0.017 kg d cm and -1 -2 tionships on an individual tree basis, the 0.147 kg d cm Although large dif- -1 -2 . amount of sapwood area for similar size trees decreasing in older stands. ferences in overstory transpiration occurred, measured xylem sapflux den- While both total sapwood area and indi- sity did not differ significantly among vidual tree sapwood area decreases in stands, i.e. vary in correlation with stand density (figure 8). Thus, differences in older less dense stands, leaf area index of canopy transpiration were related to dif- the stands remains high (table I). Thus, the needle area which must be supported ferences in cumulative sapwood area, by a specific sapwood area increases (fig- which decreases with age and at lower tree density (figure 8). To obtain greater con- ure 10). With the same sapflux density, transpiration or physiological activity of fidence in our estimations of the cumula- the average needle must decrease. This tive sapwood area of the stands, sapwood was found independently for Norway area of individual trees was measured by
the basis of cuvette gas on spruce values (90-300 mm year reported by ) -1 exchange measurements [14]. Given the Cermak [50], who derived these values limited water supply to needles in older from xylem sap flow measurements of stands and the greater stomatal restriction Picea abies at various sites. of needle gas exchange, total transpira- Values between 0.25 and 0.7 mm h -1 tion of older less dense stands is greatly for Picea abies by Lade- reported were reduced. foged ([37]; 2.6-3.8 mm d and ) -1 McNaughton and Jarvis [46] and similar high rates for Pseudotsuga menziesii by 4. DISCUSSION Granier [21].Tajchman [67] and Brech- tel [7] determined water use by Norway The influence of light and vapor pres- spruce at two sites in Germany of 360 and deficit on tree canopy transpiration 280 mm yearrespectively. Heimann , -1 sure was similar among the stands investigated, [28] reported annual transpiration for a which resulted in a similar overall pattern 40-year-old spruce stand located at the in seasonal water use resembling that Harz, Germany, of 292 mm (± 97 mm, reported for Picea abies by Ladefoged standard deviation). Roberts [55] sum- [37] and for Douglas-fir by Granier [21]. marized studies by Calder [8] indicating Relatively low canopy transpiration rates 290, 330 and 340 mm year transpiration -1 in June 1995 (figure 6) were due to high for spruce sites in the United Kingdom. precipitation during this month. As found Explanation of these apparent regional by Graham and Running [ 18] for Pinus flux differences requires a better under- contorta, conductance during warm spring standing of differences in site quality and and summer periods was mainly deter- the relative importance of simultaneous mined by vapor pressure deficit of the air, variations in climate, canopy LAI and the while under cooler conditions (in our case understory contribution to evapotranspi- in October and in their case during spring) ration. In some cases, the transpiration conductance was correlated with previous estimates have been derived from hydro- night minimum air temperature. logical or meteorological measurements, assuming a negligible understory flux. The absolute values of maximum Transpiration from the understory can be hourly transpiration rates in spruce stands of the Fichtelgebirge (from 0.11mm hat-1 large and its relative contribution to total the 140-year stand up to 0.25 mm h at-1 evapotranspiration may be underestimated the 40-year boggy stand; 1.4 mm din -1 [72]. Stand transpiration and conductance July at the 140-year stand up to 2.8 mm of coniferous stands are reduced strongly -1 din July at the 40-year boggy stand) are under conditions of limited soil water availability [11, 31, 66]. Water supply lim- relatively low for coniferous forests [62]. itations may be ruled out in terms of Similar low rates of maximum transpira- mm h were found for a 120- ) -1 tion (0.15 explaining the low rates observed in the Fichtelgebirge. Measurements of soil spruce stand in the Bayerische year-old Wald (700 m NN; [51]) and low annual matrix potentials indicate that trees were transpiration ( 145 mm in a 35- to 55-year- not exposed to water stress during the growing season of 1995. This is supported old and 137 mm in a 100-year-old spruce stand, respectively) was measured by by the relatively high predawn water Gülpen [24] in the Black Forest of Ger- potentials (-0.4 up to -0.5 MPa) measured in twig samples. Gross and Pham-Nguyen many. The seasonal totals for canopy tran- [23] found for spruce trees that moderate spiration found in our studies (109- 208 mm year are within the range of ) -1 restriction in water supply was associated
is small but that differences in cumulative with predawn water potentials in the range of -0.7 to -0.8 MPa, while trees exposed sapwood area are extremely important (fig- to strong water stress exhibited predawn ure 8). While the total sapwood area and individual tree sapwood area decreases in water potentials of -1.2 to -1.4 MPa. area index older, less dense stands, leaf On the other hand, low Mg -concen- 2+ of the stands remain high and the needle tration in needles is typical for this region area which must be supported by a par- where the forest stands grow on acidified ticular sapwood area increases. A similar soils poor in cations [38, 61]. Not only effect of stand age on the leaf area/sap- were Mg very low at the -concentrations 2+ wood area ratio of stands was reported by 40-year- and the 40-year NE stand, but Albrektson [2], while Aussenac and also the Ca was lowest in -concentration 2+ Granier [4] showed that this ratio is influ- these two young stands. The highest nee- enced by tree density and, therefore, by dle yellowing and needle loss (table I, nee- thinning practices. Changes within stands dle loss) was also recorded for the 40-year seem to be related to the response to light stand, along with the lowest transpiration climate [64]. Thinning results in large rates among the 40-year-old stands. changes in tree density at the sites inves- Locally high water tables occur which tigated and on the leaf area/sapwood area seem to result in improved nutrition (suf- ratio (figure 10). This means that the ficient nutrient supply at the 40-year boggy amount of needles supported by a sap- stand, especially magnesium) either due wood element increases as tree density of to supplemental delivery of nutrients in the stands decreases (as described in [65] flowing water or due in some manner to or [29]) and as stand age increases. There- better nutrient retention. Direct measure- fore, with the same sapflux density aver- ments which might determine whether age transpiration of the average needle higher gas exchange capacity is found in must decrease. needles from the 40-year boggy stand must be undertaken. Higher magnesium con- Pothier et al. [52] found that sapwood centrations in the needles were also found permeability increases with increasing age, at the 110-year stand, which was fertil- which is partly due to an increase in tra- ized with magnesium in 1983. It is inter- cheid length. Pothier et al. [53] concluded esting that water use by the 110-year stand that: ’...tracheid length and sapwood rel- was low despite fertilization. Differences ative water content are the two most between the 40-year-old stands as a group important characteristics of sapwood with and the older stands as a group were found which we can explain the variation of sap- in needle nitrogen concentration. Increased wood permeability with stand develop- N-deposition in recent decades, as it pos- ment’. Water conductance is influenced sibly affects the growth patterns, 3-D tree by sapwood permeability, by sapwood structure and stand light climate, as well as area and by the length of the pathway. needle physiology are further factors that Pothier et al. [52] found with jack pine may contribute to the differences observed (Pinus banksiana Lamb.) at good quality in canopy transpiration of different aged sites that sapwood conductance decreases stands. with age. Mattson-Djos [45] also reported decrease with age in conductance for the Site fertility is known to influence the a leaf area/sapwood area ratio, which in turn entire pathway between roots and foliage of P. sylvestris. One reason for this affects growth rates, hydraulic conduc- tivity of trees and stand transpiration [13, decrease in sapwood conductance may be 40]. Our measurements showed that vari- the increased resistance to water flow in ation in sap flux density among the stands minor branches compared to the main stem
([68, 69, 70, 76] all in Pothier [52]) and growth and which affect canopy form and needle clumping may provide an addi- greater biomass distribution to minor means for trees to maintain the bal- tional branches in older trees. At the spruce sites between xylem water supply and investigated in the Fichtelgebirge, shifts ance canopy water demand. in individual tree function apparently occur that allow a degree of equilibration to thin- Correlation was found between stand ning practices and xylem sapflux density age and tree canopy transpiration at sites that remains within a restricted relatively within the Lehstenbach catchment. Age constant range. Since spruce canopies are dependencies of transpiration rates were quite dense, mechanisms involved in reported by Schubert (in Ladefoged [37J)
who found a decrease in transpiration of ca be explained. More work is required to 35 % in 100-year-old spruce trees in com- examine the degree to which model pre- dictions might be improved if physiolog- parison to 40-year-old trees. Yoder et al. ical differences of needles due to needle [75] reported differences in net photosyn- age [38] and tree age [75], due to differing thesis between 45-year-old and 250-year- nutrition, or due to acclimation along light old pine trees (P. ponderosa and P. con- gradients within the canopy [48, 49] and if torta) of approximately 14-30 % with the the distribution of tree structures within interpretation that the changes are not due stands are considered. to changes in mesophyll photosynthetic capacity but are related to decreased hydraulic conductivity in larger trees and decreased stomatal conductance in the old ACKNOWLEDGEMENTS trees. At our sites, tree density decreases We are grateful to Michael Wedler, Yuki- with age and is a better predictor of tran- hiro Chiba, Bernhard Manderscheid, Gunnar spiration than age (figure 7B). The lower Lischeid and Martina Mund for valuable dis- canopy transpiration in old versus young cussions and assistance. We thank Annette stands is clearly related to differences in Suske, Ralf Geyer, Jörg Gerchau, Gerhard average physiological activity of the nee- Müller, Gerhard Küfner, Andreas Kolb, Karin dles. The hypothesis that the observed dif- Wisshak and personnel of the Department of ferences in tree canopy transpiration Plant Ecology I at the University of Bayreuth for their support during tree harvest studies. between stands can be explained by Financial support was provided from the Bun- changes in average structure and spacing desministerium für Bildung, Wissenschaft, of individual trees, was tested with the aid Forschung und Technologie, Germany (BEO of the forest canopy light interception and 51-0339476A). gas exchange model STANDFLUX [15]. Using only a single average tree type and the same physiology for all needles, model REFERENCES estimates of water use are very similar to measured tree canopy transpiration rates Albrecht F., Die Methoden zur Bestimmung [1] der Verdunstung der natürlichen Erdober- (figure 11). When all data are pooled, fläche, Arch. Meteor. Geoph. Biokl., Seric B 80 % of the variation in daily water flux is 2 (1950) 1-38. explained. In some stands (40-year boggy Albrektson A., Sapwood basal area and nee- [2] stand and 40-year NE stand), transpira- dle mass of Scots pine (Pinus sylvestris L.) tion rates were underestimated, suggest- trees in Central Sweden, Forestry 57 (1984) 35-43. ing greater average physiological activity Assmann E. , Franz F., Vorläufige Fichten- [3] at these sites. Thus, this preliminary scal- crtragstafeln für Bayern. Inst. f. Ertragskunde ing-up of cuvette gas exchange measure- d. Forstl. Forschungsanstalt. 2, Auflage 1972 ments with the 3-D model STANDFLUX (1963) Münich, Vorläufige Fichten- Ertragstafeln für Bayern, Forstw Cbl 84 independently suggests that a large por- (1965) 13-43. tion of the observed variation in tree Aussenac G., Granier A., Effects of thinning [4] canopy transpiration is due to changes in on water stress and growth in Douglas-fir, intercepted photon flux. A large mass of Can J. For. Res. 18 (1988) 100-105. needles in the shade crown of older stands Badoux, Tables de production pour l’épicéa [5] may not contribute greatly to photosyn- Siusse, Inst. Fédéral Rech. Forest. à Bir- en mensdorf, 1964. thetic carbon gain. In simulations, the pho- Bergmann W., Farbatlas Ernährungsstörun- [6] tosynthetic rate and water use efficiency of gen bei Kulturptlanren, Fischer, Jena, 1986. the older conifer stands are low compared Brechtel H.M., Influence of species and age [7] to the younger stands. On the other hand, of stand on evapotranspiration and ground substantial variation in response remains to recharge in the Rhine-Main Valley, water
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