Báo cáo khoa học: "Tree canopy and herb layer transpiration in three Scots pine stands with different stand structures"

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Original article Tree canopy and herb layer transpiration in three Scots pine stands with different stand structures Lüttschwager Steffen Rust Monika Wulf b Dietmar Jacqueline Forkert Reinhard F. Hüttl a for Center and Land use Research, Eberswalder Straße 84, 15374 Müncheberg, Germany Agricultural Landscape b University of Brandenburg, 03013 Cottbus, Germany Technical (Received 30 March 1998; accepted 25 January 1999) Abstract - To evaluate the impact of herb layer structure on the transpiration of Scots pine ecosystems in north-eastern Germany, we measured tree canopy and herb layer transpiration in three stands. Parameters of tree hydraulic architecture were measured and their drought stress monitored. Despite striking differences in ecosystem structure, combined tree and herb layer transpiration was equal for all three sites. Transpiration rate per needle area and tree canopy transpiration were least at the site dominated by the tall grass species Calanzagrostis epigeios. Pine pre-dawn water potential in the Calamagrostin-Cultopinetum sylvestris was never lower than in the Myrtillo-Cultopinetum sylvestris, indicating that severity of competition of ground vegetation was not much different. Huber val- ues, xylem hydraulic conductance and leaf-specific conductance of pine were least in the Calamagrostio-Cultopinetum sylvestris. Thus, pine transpiration rate might have been adjusted to lower tree hydraulic conductance and the herbaceous species used the water left by the trees. (© Inra/Elsevier, Paris.) canopy / herb layer / transpiration / hydraulic conductance / Scots pine Résumé - Transpiration des arbres et de la strate herbacée dans trois peuplements de pins sylvestres de différentes struc- tures. Dans le but d’évaluer les effets de la strate herbacée sur la transpiration d’écosystèmes de pins sylvestres en Allemagne du nord-est, la transpiration des houppiers et de la strate herbacée a été mesurée dans trois peuplements. Les paramètres de l’architecture hydraulique et le niveau de contrainte hydrique ont été mesurés. Malgré des différences importantes dans la structure de chacun de ces trois peuplements, leur transpiration totale (arbres plus herbe) était identique. Le taux de transpiration par unité de surface foliai- re, ainsi que la transpiration par arbre étaient les plus faibles dans le site à dominante de Calamagrostis epigeios. Le potentiel hydrique de base dans le site à Calamagrostio-Cultopinetum sylvestris n’a jamais été inférieur à celui mesuré dans le site à Myrtillo- Cultopinetum sylvestris, ce qui permet de conclure à un niveau de compétition entre les arbres et l’étage herbacé peu différent. Les valeurs de Huber, la conductance hydraulique du xylème, ainsi que la conductance hydraulique spécifique foliaire des pins étaient les plus faibles dans le Calamagrostio-Cultopinetum sylvestris. Ainsi, le taux de transpiration des pins semble s’ajuster pour réduire la conductance hydraulique, la ressource hydrique laissée par les arbres étant consommée par la strate herbacée. (© Inra/Elsevier, Paris.) couvert / strate herbacée / transpiration / conductance hydraulique / pin sylvestre * Correspondence and reprints dluettschwager@zalf.de
1. Introduction The aim of this paper is to describe the drought stress. different rates of the tree and herb layer of transpiration pine ecosystems with various structures. In particular, Scots pine is the dominant tree species in more than we want to estimate the contribution of the herb layer to thirds of the forests in north-eastern Germany. Site two the stand transpiration rate. Furthermore, we want to factors, especially soil pH, nutrient and soil water avail- investigate whether a pine stand with a denser cover of ability cause important differences in the structure and grasses used more water and, as a result of competition species composition of these pine forests [6]. The differ- between trees and herbs, whether the trees were more ent types of stands are characterised by the dominance of likely to suffer drought stress. various herb species. For example, mature pine stands on podsolic soils poor in nutrients have only a sparse cover of grass species on the forest floor, whereas stands richer 2. Materials and methods in nutrients have a dense cover of grasses, e.g. Brachypodium sylvaticum and Calamagrostis epigeios 2.1. Site description [2, 11, 15]. The various forest ecosystem types have markedly different rates of biomass production. Sites were selected to represent major pine ecosystem Calamagrostio-Cultopineti, i.e. stands with dominance types of northern Germany. The stands are 45 (Taura)- to of Calamagrostis epigeios, produce 4-5 t biomass (Neuglobsow and Rösa)-year-old Scots pine (Pinus 65 -1 -1 ha a in the herb layer. Stands dominated by sylvestris) forests, located in the former GDR. Edaphic Deschampsia flexuosa, so-called Avenello-Cultopineti, factors and climate are very similar (table I and [30]). reach only 0.8thaa[11]. -1-1 Data of precipitation and tensions in the upper soil dur- ing the period of measurements are shown in figure 1. In many earlier ecosystem studies total stand transpi- ration could not be partitioned into the contribution of The site Rösa suffered from heavy air pollution for at the tree canopy and the herb layer. However, this is very least 20 years until the re-unification of Germany in important in order to understand the impact of stand 1989. In that year, needle loss was estimated at 45 % structures on the water balance of pine ecosystems. [12]. Since then, trees have partially recovered [7]. In Some authors (e.g. [11]) assume that pine stands domi- Neuglobsow, needle loss was always low (8 % in 1989 nated by Calamagrostis epigeios consumed significantly [12]). According to the forest administration the site Rösa received approximately 1 000 kg ha of nitrogen -1 more water than those dominated by Deschampsia flexu- osa, and therefore the pine trees were more prone to as urea in the years 1970-1985 (unpublished).
2.2. Tree biomass and leaf area index 2.4. Tree water status Five trees per stand were sampled as a stratified ran- From 1993 to 1995, the water status of the stands was dom sample for needle mass in September 1995. All assessed by periodical measurements of pre-dawn water branch diameters and the needle mass of one branch per potential. Two twigs per tree from the upper crown of whorl were measured. Using the close correlation of ten trees per stand were collected with a shotgun and the branch diameter and needle mass [14, 17], data were balancing pressure of two fascicles per twig was imme- scaled to tree level. Specific needle area (projected) was diately measured with a pressure chamber. estimated with an image analysis system (CUE-3 Image analyser, Olympus) samples being stratified for crown location, age and length. A regression of the projected needle area on sapwood area was used to scale to stand 2.5. Tree canopy transpiration level [1, 31]. Tree canopy transpiration was estimated by sap flow 2.3. Tree hydraulic conductivity in 15 representative trees per stand using measurements a constant heating method [8]. Two gauges were installed at breast height in each tree ranging from 0 to small (basal diameter 0.5 cm) and two In 1995, ten 2.1 cm and 2.2 to 4.4 cm from the cambium, respective- diameter 2.5 cm) branches per tree were larger (basal ly. Automatic readings were taken every 30 s and aver- collected from the top of the crown of five trees per aged over 30-min periods. Data were collected between stand and immediately re-cut under water. On the small branches hydraulic conductivity K(kg s m MPa -1 ) -1 August 1993 and November 1995. h and vulnerability to embolism were measured in 2-year- old segments 5 mm in diameter (including bark) and sapwood area was measured in all 45 Conductive 40 mm in length using a conductivity apparatus as by computer-tomography [5, 10, 19] in col- sample trees described by Sperry et al. [23]. Branches were bench-top laboration with the Centre for Radiology of the Phillips- dried. Hydraulic conductance K(kg sMPa and K -1-1 ) University Marburg. From inventories of the study plots T h of the larger branches were measured in the field with a and the data on sapwood area in the sample trees, stand sapwood area was calculated. Stand sap flow was calcu- high-pressure flowmeter [27, 33]. We used de-ionised, de-gased, filtered (0.2 μm) 0.01 N HCl and, for the seg- lated as the product of average sap flow density and ments, a pressure of 6 kPa. stand sapwood area.
2.6. Ground vegetation: species, biomass and LAI 3. Results 3.1. Ground vegetation At each site, five to eight plots of 9 m were estab- 2 lished in the summer of 1994. The plots were divided vegetation of Neuglobsow was dominated by The into four quadrants to estimate cover degree of all plant Deschampsia flexuosa (about 15-23 % from April to species to the nearest percent. Because transpiration was July) and Vaccinium myrtillus (about 8-13 % from April not measured for mosses, their cover was estimated to July) indicating a site without major deposition. Rösa, without differentiating for species. All plots were pooled however, was dominated by Calamagrostis epigeios to calculate monthly averages of cover. We followed the (about 12-29 % from April to July) and Brachypodium nomenclature of Schmeil and Fitschen [21]. sylvaticum (ranged from about 4-18 %), showing the influence of recent N-fertilisation and Ca-deposition. plots (0.25 m per site all living herbaceous ) 2 In three The species in Taura were a mix of N-indicators such as collected in height strata of 10 cm, dried at plants were Calamagrostis epigeios and Rubus idaeus and acid-toler- 80 °C and weighed. For each relevant species means of ant species such as Deschampsia flexuosa, and the latter the biomass were scaled to a hectare basis. Specific leaf reached cover degrees of about 44-57 % from April to area for these species was estimated with an image July [32]. analysis system (CUE-3 image analyser, Olympus). Using the specific leaf area and the leaf biomass the leaf Large differences between sites were found for the area index of these species (LAI was calculated. The ) part LAI (table II). Rösa, because of the prevalence of wide- LAI of the herbaceous layer is the sum of the . part LAI leafed species, had two to three times the LAI of Neuglobsow. 2.7. Transpiration of the ground vegetation and leaf area index of the trees 3.2. Needle mass Transpiration was measured monthly for species with Needle mass was highest in Rösa (7.22 ± 0.53 t ha), -1 least 10 % cover within an minimum area of 200 m . 2 at intermediate in Taura (5.89 ± 0.72 t ha and lowest in ) -1 In Rösa, these were Brachypodium sylvaticum, Neuglobsow (5.42 ± 0.51 t ha The higher specific ). -1 Calamagrostis epigelos and Rubus idaeus, in Taura needle area and needle mass of Rösa resulted in the high- Deschampsia flexuosa and in Neuglobsow Deschampsia est LAI (3.71 ± 0.27 compared to Neuglobsow 2.38 ± flexuosa and Vaccinium myrtillus. In the growing season 0.15 and Taura 2.65 ± 0.32). of 1995 diurnal courses were measured during periods of bright days with a climatised porometer (compact CO porometer, Walz, Effeltrich). Five-minute O /H 2 3.3. Hydraulic conductivity averages of exposed leaves of one species were recorded from dawn until dusk. The daily output of transpiration of a species was scaled up to the stand level using its leaf 2-year-old segments with an outer diameter of ca In index (LAI and water potentials close to 0 MPa, the hydraulic 5 ). part area mm conductivity K was significantly higher in Neuglobsow h Wedler [29] expected only low differences in the rela- (P < 0.013). Over much of the tested range of xylem tionship of transpiration rates of patch types in the field water potential, K of segments from Neuglobsow was h layer within a week. According to this fact we assumed highest, but there was no interaction effect of xylem that the relation of the transpiration rates of different water potential and site on K (figure 2). The leaf specif- h herb species to each other were nearly equal within 2 to ic conductance LSC, i.e. the hydraulic conductivity 3 consecutive days. The measured daily transpiration of divided by the projected needle area distal to the mea- sured segment, was 52 % higher in Neuglobsow than in a herb species was related to the canopy transpiration on the same day. Continuously measured canopy transpira- the other stands (significance of difference P < 0.005). tion used reference to calculate the total herb The Huber value (sapwood area/needle area) of segments was as layer transpiration. The ratio of ground vegetation tran- from Rösa was significantly lower than in Neuglobsow. Since the conductivity per cross-sectional area (specific spiration to tree transpiration was interpolated through periods without measurements and used to estimate herb conductivity) was not significantly different (data not layer transpiration from tree transpiration during these shown), this resulted in higher LSC in Neuglobsow over times. the range 2-15 mm xylem diameter.
of drought in 1994 pre-dawn water potential fell from For stems, the leaf area to sapwood area ratio at breast height (1.3 m) for the three stands was highest in Rösa above -0.5 MPa in spring to below -1.0 MPa at the end and lowest in Neuglobsow (table III). of July. In Neuglobsow trees reached the lowest needle water potentials with single trees as low as -2.6 MPa, on 3.4. Water status of the trees average -1.65 ± 0.24 MPa as compared to Rösa with - 1.16 ± 0.21 MPa. In 1995, pre-dawn water potentials Pre-dawn water potentials differed substantially never fell below -1.0 MPa. between 1994 and 1995 (figure 3). During a long period
ditions, the ratio of sap flow densities of inner and 3.5. Tree canopy transpiration outer 0.88 in Neuglobsow, but 0.40 in Rösa. In sapwood was Taura, we found a ratio of 0.63 (all differences signifi- The ratio of sap flow densities of inner and outer sap- cant at P < 0.001). For the entire growing season of wood differed significantly between the stands (table 1994, sapflow densities at the outer sensors in Rösa were IV). In Rösa the mean flow density in the outer sapwood significantly higher than in Neuglobsow, but significant- was higher than at the other sites, but decreased much ly lower at the inner sensors. On average, sap flow per more steeply towards the heartwood than in Taura and tree in Rösa was 90 % of that in Neuglobsow. Neuglobsow. Over 4 weeks of comparable climatic con-
Daily tree canopy transpiration per ground area for 1994 and 1995 is shown in figure 4. On fine days, tran- spiration reached approximately 1 mm d in , -1 Neuglobsow up to 1.5 mm d Because of declining soil . -1 water availability, transpiration in Neuglobsow fell to less than one third from mid July to mid August 1994, in spite of fairly constant climatic conditions. Tree canopy The ratio of sap flow densities of inner and outer sap- transpiration during the growing season of 1994 (April to were not constant, but changed from wood, however, September) was 106 mm in Rösa, 82 mm in Taura and year to year and rose close to unity in periods with low 113 mm in Neuglobsow. In 1995, the values were Rösa flow rates, e.g. at the beginning and the end of the grow- 94, Taura 90 and Neuglobsow 122 mm. ing season.
4. Discussion Transpiration per needle area (stand transpiration per hectare divided by projected needle area per hectare) was lower for the nitrogen-fertilised and polluted stands in Rösa and Taura in all 3 years. For days with non-limiting The cumulated LAI of the herb layer in Taura is simi- soil water availability, i.e. soil water potential above lar to 1.54 reported by Wedler et al. [29] for a 30-year- - 100 hPa, there was a highly significant difference in old pine stand at Hartheim in the upper Rhine valley. transpiration per needle area between these stands The LAI of the herb layer at Rösa was higher because of (figure 5). the dominance of the wide-leafed species Calamagrostis epigeios and Brachypodium sylvaticum. The absence of these species is the reason for the low LAI in 3.6. Contribution of the ground vegetation Neuglobsow, although the leaf area of moss species was to stand transpiration not taken into account. During summer the transpiration During fine summer days, ground vegetation transpi- of the herb layer of up to 50 % of the stand transpiration ration exceeded tree transpiration. In Neuglobsow, where was higher than expected. Granier et al. [9], from sap transpiration rates were highest, ground vegetation tree flow and eddy correlation measurements at Hartheim, transpiration (excluding mosses) reached half the tree estimated a herb layer contribution to total vapour flux transpiration (table V). Comparing the results of tables II of 26 %. A contribution of the herb layer to stand tran- and V, the relative contribution of a species to stand tran- spiration comparable to our results was found by Tan spiration is mainly controlled by leaf area index and spe- and Black [25], Black [3], Roberts et al. [18] and cific transpiration rates. While in July the LAI of Rubus Spittlehouse [24]. Due to the low number of days mea- idaeus did not exceed 6 % of the total herb layer in Rösa, sured at each site and the variable weather conditions this species contributed 12 % to herb transpiration. during field works our data can only be rough estimates. Vaccinium myrtillus, however, transpired less than 18 % However, investigations in a Scots pine ecosystem in the of the herb layer, although its partial LAI was 23 %. upper Rhine valley have shown that the relationships among transpiration rates of different patch types in the Stand transpiration is the sum of field layer transpira- field layer do not change significantly within a week tion and tree canopy transpiration. Since ground vegeta- [29]. Additionally, the counteracting effects of measur- tion transpiration data were only available for some ing exposed, leafy plant parts and excluding plant stems days, stand transpiration estimates have to be rather in the procedure of up-scaling are not known. While the rough. For the growing season of 1995, these are first leads to an overestimation of transpiration, the in in Taura and 184 in Rösa, 173 185 mm mm mm exclusion of plant stems may cause an underestimation. Neuglobsow.
The leaf indices of the tree canopies are signifi- of water potentials that cause xylem embolism. The ment area cantly different drought stress developed during drought in Rösa was not because of the differences in needle bio- higher than in Neuglobsow. This, together with the mass and specific leaf area. While potential evapotran- assumption that tree transpiration rates in Rösa were spiration at the three sites was comparable, soil water more limited by hydraulic architecture than in availability was highest at Rösa with 150 mm as com- Neuglobsow, leads us to the conclusion that there was no pared to 100 mm extractable soil water in the upper severe competition of ground vegetation. Rather, the 50 cm at Neuglobsow [30]. Nevertheless, a lower tran- herbaceous species used the water left by the trees. spiration rate on a needle area basis caused stand canopy Therefore, stand transpiration for all three stands is of transpiration in Rösa to be lower than in Neuglobsow, the same magnitude, although there are large differences despite the higher LAI of pine in Rösa. The largest dif- in species composition and stand structure. ferences between stands occurred during periods of drought. A reason might be the lower leaf specific con- Acknowledgements: This study was funded by the ductivity of the xylem. Our estimates of hydraulic con- German ministry of education and science. We thank ductivity of whole trees, stems, and branches indicate a Mel Tyree for giving Steffen Rust the chance to study lower conductivity of the pine trees in Rösa than in their methods at the Proctor Maple Research Station and Neuglobsow. The leaf area to sapwood area ratio found André Granier for critical comments on this paper. We in Rösa (2 078 cm cm was twice that of Neuglobsow 2 -2) thank our technicians Bodo Grossmann and Lothar and highly compared to other studies. Van Hees and Löwe. Bartelink [28] report 900-1 300 cmcm and 2-2 Mencuccini and Grace [17] found 800-1 700 cm cm 2 -2 for Scots pine. Models [13, 26] and field experiments [4, References 16] show that stomatal regulation can play an important role in controlling the development of xylem embolism. [1]Albrektson A., Sapwood basal area and needle mass of Because of their lower conductance, trees in Rösa would pine (Pinus sylvestris L.) trees in Central Sweden, Scots have to develop a much steeper water potential gradient, Forestry 57 (1984) 36-43. with the risk of xylem dysfunction and decreasing con- ductivity, if they were to sustain a transpiration rate as [2] Bergmann J.H., Das Sandrohr (Calamagrostis epigeios (L.) Roth). Forschungsbericht, Zeneca Agro, Frankfurt/M, high as the trees in Neuglobsow [13, 23, 26, 34]. 1993. However, xylem water potentials of the stands were always in the same range, with Neuglobsow at the lower [3] Black T.A., Tan C.S., Nnyamah J.U., Transpiration rate end. Transpiration rate might be adjusted to tree of Douglas fir trees in thinned and unthinned stands, Can. J. hydraulic conductance in a way that avoids the develop- Soil. Sci. 60 (1980) 625-631.
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