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Experimental manipulations of the water and nutrient cycles in forest ecosystems (patch-scale roof experiments: EXMAN)
AGRICULTURAL AND FOREST METEOROLOGY ELSEVIER
Agricultural and Forest Meteorology 73 (1995) 307-320
Experimental manipulations of the water and nutrient cycles in forest ecosystems (patch-scale roof experiments: EXMAN) Michael Bredemeier Research Centre for Forest Ecosystems, University of Goettingen, Goettingen, Germany Received 3 October 1993; revision accepted 11 February 1994
Abstract E X M A N is a joint project funded by the Commission of the European Communities (CEC) Directorate XII, Science and Research (DG XII), with research partners in five European countries. The aim of E X M A N is experimental manipulations of the water and nutrient cycles in forests. These experiments are aimed at testing hypotheses on ecosystem reactions to altered inputs and boundary conditions. E X M A N developed along with the 'roof method' as a means for the manipulation of water and solute fluxes into the forest soil on an ecosystem scale. Large roofs are built underneath the forest canopy to collect the throughfall. This amount of water is then, by means of technical equipment, altered in its chemical composition and/or its quantity and/or its temporal flux distribution and reapplied by sprinkler systems to the soil. The reactions of the soil chemical and hydrological parameters, as well as the physiological reactions of the forest stand and the soil fauna and microfiora, are investigated in an integrated manner. Such patch-scale roof studies are running in Denmark, the Netherlands, Ireland and Germany ( E X M A N sites), as well as in Norway and Sweden. Concerted planning and evaluation of results is secured by periodic contacts between the groups. Results from the manipulation studies may contribute considerably to the progress of the projects GCTE (Global Change and Terrestrial Ecosystems) and BAHC (Biological Aspects of the Hydrological Cycle, e.g. the soil-vegetation-atmosphere transfer (SVAT) models). Relevant BAHC Foci are Focus I, Activity 1, Tasks 1.1.2, 1.1.3 and 1.1.5.
I. Introduction E c o s y s t e m research is in m a n y cases w o r k i n g on the p a t c h (i.e. single e c o s y s t e m ) scale o r even s m a l l e r spatial extensions. This is a c o n s e q u e n c e o f the fact t h a t i n s t r u m e n t s for intensive process studies, such as.gas e x c h a n g e cuvettes, p o r o m e t e r s 0168-1923/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0168-1923(94)05081-3
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or soil tensiometers, are relatively small and only cover areas in the squarecentimetre to square-metre range. To integrate the wealth of knowledge on ecosystem processes into models and assessments of global change, the intensive patch-scale information has to be regionalized. Data from plot study sites should be integrated regionally, as should be the method and the sampling and analysis protocols. The CEC (Commission of the European Communities) research policy fosters such integration and harmonization in ecological research. CEC funding is only available for international joint projects with two or more European partners. The EXMAN project developed within the Environmental Research Programme of the CEC. It includes study sites from Ireland up to Denmark and down to the South of Germany (see Fig. 1), and thus covers considerable gradients of climatic and air pollution regimes. Ecosystem processes are studied using harmonized sampling strategies and analytical methods. The experimental manipulation of the water and nutrient cycles of forests on the ecosystem scale is a specific feature of the EXMAN project (and also of the related NITREX and CLIMEX projects). Reactions of mature forests to such manipulations can be tested under otherwise unchanged on-site conditions. Results from such experiments should be much more realistic and relevant than any laboratory manipulations. EXMAN is related to some further ecosystem manipulation projects which are performed in European cooperation: the NITREX (Nitrogen saturation experiments) and the CLIMEX (_climate change experiment) project. These projects are also funded by CEC-DGXII, and they also employ--at least partly--roof
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M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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manipulations on the ecosystem scale. More detailed information on NITREX can be found in Dise and Wright (1992), on CLIMEX in Jenkins et al. (1993). In this paper, the general idea of the roof manipulation method and the set-up of the EXMAN project are explained. Some example results of reactions to the manipulations are shown. The relevance of the datasets for the project BAHC (Biological Aspects of the Hydrological Cycle) is considered, in the hope of stimulating co-operation between large-scale/fuzzy dataset and patch-scale/intensive dataset approaches.
2. The roof manipulation method
Roofs underneath a forest canopy permit an effective manipulation of the hydrological and chemical input to the soil, whereas other boundary conditions of the forest system remain essentially unchanged. The canopy is exposed to ambient air temperature, humidity and chemistry. Highly transparent roof materials allow for the transmission of 95-98% of the visible light intensity and thus permit continued growth of ground vegetation. If the roofs are built high enough over the ground and have open sides with free ventilation, then temperature and humidity differences compared with the surrounding forest stand are minor. The input vector of water and chemical elements transported with the water is therefore the one significant object of manipulation in a roof study. Fig. 2 shows the principle of roof manipulations and the ecosystem reactions under study. If there are technical facilities for chemical treatment (deionization and reconditioning) and temporary storage (water tanks) of throughfall water, then there are three basic options for input manipulation: (a) change total water flux to the plot; (b) change temporal distribution of water flux (experimental drought periods or optimum irrigation with respect to transpirational demand); (c) change chemical input flux. Manipulations (a) and (b) are particularly interesting in the context of climate change research, as scenarios of global change effects such as increasing summer drought can be simulated. Such scenarios are especially relevant with respect to the hydrological cycle, which is the scope of BAHC. Manipulation option (c) refers more to a change in chemical climate, i.e. a change in air pollution input to forests. Scenarios of reduced air pollution as a result of environmental legislation (e.g. reduced acid inputs) can be simulated by experimental 'clean rain' application. If the roof manipulation facilities are technically constructed in an adequately flexible way, a combination of manipulation options is also feasible, to examine the combined effects of changes in hydrological and chemical input. The manipulation facilities in the EXMAN consortium in general, and particularly the Solling roof site, are set up technically in such a way that manipulation experiments of various kinds and intentions can be run for a long period of time in the future. In Fig. 2(a), the hydrological fluxes in the undisturbed forest system are shown. Throughfall, i.e. the rain and stemflow water in the forest, is formed from the interaction of wet and dry deposition and a canopy exchange component (Fowler,
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a) no roef
preciloitation
dry + occult :lepos.
seepage~uptake
b) roef
storage manipulation reaction ~ ~/&reaction Fig. 2. Schematicsketch of the roof manipulation method in a forest and the ecosystem'sreactions under observation. 1984; Bredemeier, 1988). Throughfall (plus stemflow) reaching the forest floor i s - roughly speaking--subdivided into a seepage and a root uptake/transpiration component. To observe the reaction of these output signals to a changed hydrological input signal is the most interesting point in a hydrological manipulation study (Fig. 2(b)). Also, secondary physiological reponses of the forest to the manipulated water regime are followed, e.g. changes in photosynthesis and gas exchange rates, changes in root growth and vitality (Murach et al., 1993), and changes in soil fauna and soil microflora activities. Ecosystem reactions to a manipulation can be very complex and work on different levels of the system. It is therefore a general strategy in the kind of experiments described here to integrate and concentrate specialist working groups of all
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ecosystem compartments in the roof manipulation studies. More detailed information on which particular investigations are carried out at which roof sites can be found in the recently published E X M A N report (Rasmussen et al., 1993).
3. EXMAN sites and experiments The geographical distribution of the E X M A N sites is shown in Fig. 1. Tables 1 and 2 add information on the regional climatical characteristics. Four of the study sites are lowland forest ecosystems, whereas the two German sites are in the montane elevation zone at about 500 m above sea-level. These two sites also have more continental climate characteristics. The Solling site exhibits particularly high mean annual precipitation, but all E X M A N sites can be classified as humid, with average rainfall well above 750 mm year -I . It is in this climatical zone that the frequency and intensity of drought periods will increase under climatic change, according to the predictions of various models (Houghton et al., 1990; Rind et al., 1990; G o r d o n et al., 1992). The r o o f experiments with drought treatments can test the effects and ecological feedbacks of such model scenarios directly on the ecosystem scale. Further site information regarding the soils and the forest stands is given in Tables 3 and 4, respectively. Soil texture (Table 3), and consequently the water retention characteristics, are different between the experimental regions in E X M A N . The Danish (Klosterhede) and Dutch sites (Kootwijk and Harderwijk) have soils which developed on sandy deposits with little to no silt and clay content. The German (Solling and Hrglwald) and Irish (Ballyhooly) sites, by contrast, have typical loamy soils with moderate clay and high silt and sand contents. This spectrum of soil hydrological characteristics in E X M A N should make the datasets suitable for regional extrapolation exercises within BAHC. The forest stands are all conifer monoculture plantations (Table 4). This forest t y p e - - a l t h o u g h ecologically not very beneficial--is actually the most widespread and economically relevant in Central and Northern Europe. The forest stands represent different age stages and thus different actual biomasses (expressed here by the parameters tree height, diameter and density (trees ha-l)), different growth rates and different evapotranspiration characteristics. The experimental manipulation treatments which are currently conducted at the Table 1 Geographical locations and climate types of the EXMAN sites (from Rasmussen et al., 1993) Site
Longitude
Latitude
Climatetype
Klosterhede Kootwijk Harderwijk H6glwald Solling Ballyhooly
8°24'E 5°50'E 5°40'E ll°10'E 9°34~E 8°25rW
56°29~N 52°10'N 52°20'N 48°30'N 51°38~N 52°09~N
Temperateboreal oceanic Temperatesub-oceanic Temperatesub-oceanic Sub-montane,semi-continental Montane,semi-continental Temperatesub-oceanic
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Table 2 Elevations and local climatic characteristics of the EXMAN sites (from Rasmussen et al., 1993) Site
Klosterhede Kootwijk Harderwijk H6glwald Soiling Ballyhooly
Elevation
Precipitation
(m)
(mm)
30 26 10 540 500 69
860 853 763 800 1088 1000
Mean temperatures (°C) Annual
January
July
9.0 8.9 8.9 7.3 6.4 9.5
0.3 2.0 2.0 -2.0 -2.2 5.5
16.0 16.6 16.6 16.5 14.7 15.5
E X M A N sites are listed in Table 5. At every location in the research network the experiments are run versus an unmanipulated control plot. The Irish site Ballyhooly serves as an overall reference control, as it represents one of the rare locations in Europe for which background air pollution levels--uninfluenced by human activities--can be assumed. (This selection can be explained by the background of the E X M A N project; E X M A N was started as an experiment within the context of air pollution/acid rain research; the relevance for issues related to climate change has emerged only recently). Table 5 shows that the various experimental manipulations are generally aiming at changing the acid-base status of the system, changing the nutrient status, changing the hydrologic input (by means of additional irrigation or drought), or combinations of the above manipulation options. The experiments involving hydrological manipulations are of particular significance for BAHC; such experiments are currently run at all sites of the E X M A N network, as can be seen from Table 5. Example results will be reported in Section 4; more detailed information on the experimental set-up and first results in E X M A N can be found in the recently published CEC report (Rasmussen et al., 1993). Fig. 3 shows a view of the roof experiments in the Solling area, Germany, in more detail, as additional background information for the example results reported below. The drawing in the upper part of the figure (Fig. 3(a)) shows a picture of the site with the triangular arrangement of three roofs of 300 m 2 area each. In the middle lies a central cabin which holds water manipulation and data storage equipment. A 360 ° Table 3 Soil characteristics of the EXMAN sites (from Rasmussen et al., 1993) Site
Soil type
Parent material
Texture (10 cm)
Klosterhede Kootwijk Harderwijk H6glwald Solling Ballyhooly
Typic Haplorthod Plaggic Dystrochrept Typic Udipsamment Typic Hapludult Dystrochrept Typic Haplorthod
Sandy fluvio-glacial deposit Aeolian sand Cover sand Tertiary silty loam with loess Loess derived from standstone Sandstone till-colluvium
Sand Sand Sand Loam Loam Sandy loam
M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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Table 4 Forest characterization of the EXMAN sites (from Rasmussen et al., 1993) Site
Species l
Age (years)
Height (m)
Basal area (m 2 ha -l)
Stocking (ha -a)
Mean diameter (cm)
Klosterhede Kootwijk Harderwijk H6glwald Soiling Ballyhooly
NS DF SP NS NS NS
72 39 39 84 58 52
20 18 10 37 20 24
29 30 29 77 62 44
858 992 1800 603 950 632
22 20 14 40 29 29
INS, Norway spruce; DF, Douglas fir; SP, Scots pine.
sluable crane is located in the centre by the cabin. F r o m its gondola every position in the canopy within the area of the r o o f plots and the control can be reached for physiological measurements. By the side of R o o f 3, the one in the background, the water storage tanks can be seen. They can hold the equivalent of approximately Table 5 Experimental manipulation treatments in EXMAN (adapted from Rasmussen et al., 1993) Site
Plot
Treatment
Roof
Klosterhede, DK
1: Control 2: Drought (summer) 3: Irrigation 4: Fertigation a
Periodic drought Deacidified water ('clean rain') Optimal nutrition
No Yes Yes Yes
1: Control 2: Irrigation 3: Fertigationa 4: Fertigation a
Throughfall + demineralized water Throughfall + nutrition Optimal nutrition
No No No Yes
1: Control 2: Irrigation 3: Fertigation a 4: Fertilization
Throughfall + demineralized water Throughfall + nutrition Throughfall + (NH4)2SO 4
No No No No
1: Control 2: Irrigation 3: Irrigation 4: Liming 5: Irrigation + liming 6: Irrigation + liming 7: Drought
Throughfall + acidic water Throughfall + deacidified water Throughfall + lime Throughfall + lime + acidic water Throughfall + lime + deacidified water Temporary roof/drought periods
No No No No No No Yes
1: Control 2: Roof control 3: Drought (summer) 4: Irrigation
Throughfall Periodic drought Deacidified water ('clean rain')
No Yes Yes Yes
I: Control 2: Drought
Temporary roof/drought periods
No Yes
Kootwijk, NL
Harderwijk, NL
H6glwald, DE
Soiling, DE
Ballyhooly, IE
a Fertigation; a combination of optimum nutrition (fertilization) and optimum irrigation.
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M Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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140mm of throughfall during experimental drought phases for later rewetting. The lower part of the figure (Fig. 3(b)) shows a schematical sketch of the arrangement of experimental installations on the Soiling roof plots. In each of the plots, there is a central patch equipped with lysimeters for soil water sampling and tensiometers for monitoring of soil water potentials. The tensiometers have electric pressure transducers and are read by the computer in the central cabin every 15 min. The soil water potentials and the signals of irrigation events in soil hydrology are thus followed with a very high temporal resolution. The matrix potential detection is so sensitive that even the daily maxima of transpiration of the forest are reflected in the readings. Pressure heads lower than -90 kPa cannot be measured by watercolumn-based tensiometers, however. It is therefore planned to install the time-domain-reflectometry (TDR) technique in the Soiling roof plots in 1994, to obtain information on soil water contents and associated pressure heads in periods of severe drought. Sap flow is measured on several trees in the roof plots and the control as a direct measure of actual transpiration. The method used is the tissue heat balance according to Cermak (1973). There is also a root observation pit with video-rhizoscopes installed in every plot. Root growth, morphology and vitality are continuously observed, and the images are recorded for further digital processing. Thus, together with the physiological measurements in the canopy and the intensive monitoring of soil hydrology and chemistry, a rather complete picture of ecosystem reaction to the manipulative treatments is accomplished. These observations are complemented by studies of the soil fauna and microflora and the ground vegetation in sections of the plots without any destructive sampling activities (see Fig. 3(b)). Three roof plots and an uncovered control plot are available for experimentation at Soiling. The experimental treatments currently pursued are (Fig. 3(b)): Plot D - - n o roof, control; Plot D l - - r o o f , application of 'clean' (preindustrial) rain; Plot D 2 - roof, control for roof effects alone without any further manipulation; Plot D3--roof, experimental drought with subsequent intensive rewetting. In the following section, some results from EXMAN in general and the Soiling drought experiment (D3) in particular are presented.
4. Example results Results can be reported only very briefly within the limited extent of this paper. They are explained in much more detail in the recently published EXMAN report (Rasmussen et al., 1993). The effects of the irrigation treatments in EXMAN on tree growth are illustrated in Fig. 4. Average basal area increment of the trees was higher in all irrigation treatments, although the differences were not statistically significant for the Hrglwald site. Actual transpiration and net photosynthesis vary accordingly between the irrigated and control treatments. It has to be noted that the 'irrigation' treatments did not in every case mean a higher total water input to the soil. Water amounts in the irrigation treatments were 90%, 162%, 143% a n d 141% of the
316
M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
KIosterhede, DK
Kootwijk, NL
Harderwijk, NL
Hbglwald, DE
0
0.5
r 1.5
1
•
i 2
2.5
Basal area increment [m2/(ha*a)] •
Control •
Irrigated
Fig. 4. Basal area increment of the forest stands in four irrigation treatments vs. the respective controls in E X M A N (from Rasmussen et al., 1993).
controls in Klosterhede, Kootwijk, Harderwijk and H6glwald, respectively (over the time period 1988-1990). Thus, in Klosterhede the irrigated plot received less total water than the control, but in an optimal temporal distribution relative to the transpirational demand. This 'time-optimized irrigation' allowed potential transpiration of the Klosterhede forest almost entirely throughout the vegetation period(s) and had
Klosterhede, DK
Kootwijk, NL
Harderwijk, NL
H6glwald, DE
0
50
100
150
200
Growth increment (% of control) Fig. 5. Relative growth enhancement (control represents 100%) in the E X M A N irrigation treatments (from Rasmussen et al., 1993).
M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
317
the strongest effect on growth increase relative to the controls among the four sites (Fig. 5). Not only the amount but also the chemistry of the applied irrigation water has effects on the tree growth. In the Hrglwald manipulation experiments, acid irrigation vs. de-acidified irrigation was sprayed upon the soil (see Rasmussen et al. (1993), Table 5, for details). This resulted in a significant difference in growth response, as shown in Fig. 6. Such interactions of physical climate and ecochemical changes should be duly considered in predicting scenarios of future changes in terrestrial ecosystems. In the Solling drought manipulation the physiological reactions of the trees are investigated with high temporal and spatial resolution (see description of Soiling roof experiment above). A technical innovation in this project is the development ofmicrodendrometers, which are capable of recording variations of tree diameters in the range of 10-100 #m (Dohrenbusch und Grote 1993). The micro-dendrometer is installed by screws on the bark of the tree, and the sensor follows the shrinking and swelling of the stem caused by the daily (and long-term) time-course of transpiration. The daily shrinking maxima are fairly well correlated with the daily xylem water flux rates, as determined by the Cermak apparatus (Fig. 7, adapted from Dohrenbusch und Grote (1993); r 2 = 0.50, P < 0.001). Micro-dendrometers are, however, much more inexpensive and easier to use for long-term measurements. A large number of trees can be equipped and continuously measured by microdendrometers at relatively low cost. In this way, entire forest stands can be observed and transpiration rates be inferred on the stand level, as a prerequisite for regional extrapolation. This monitoring can be done with very high time resolution, and thus gives the chance to follow the effects of single and rapid hydrological events on actual transpiration rate. An example ofmicro-dendrometer observations is given in Fig. 8. It shows the daily diameter fluctuations during the drought experiment in summer 1992 for the control
Relative growth increment 1.4
~
1.2 1 0.8 0.6 o.4
'
i
0.2
I
Normal irr./corCe'ol
Acid irr. / control
Fig. 6. Growth response to clean vs. acid irrigation in the Hrglwaid experiment(control represents 100%) (from Rasmussen et al., 1993).
M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307 320
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(upper curve) and the drought roof plot (lower curve). The experimental drought period lasted for 16 weeks, and it can be seen that in the last week of the manipulation the diameter fluctuations--and hence actual transpiration rate--were much lower in the drought plot than in the control. Immediately upon onset of the rewetting the diameter fluctuations reacted and increased, exhibiting particularly high daily amplitudes during their rise in the rewetting period. After about 2 weeks they had recovered to control levels.
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M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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5. Conclusions 5.1. Relevance f o r B A H C
The facilities within EXMAN to manipulate the hydrologic input vector to forests on the ecosystem scale are unique in Europe and probably in the world. The experiments as such are plot-scale investigations, but the widespread network of research sites together with harmonized sampling and analysis methods give a good opportunity to extrapolate to the regional or possibly to the European continental scale. Besides changes in physical climate, we are probably facing some change in chemical climate and biological structure and feedbacks of terrestrial ecosystems in the future. The interactions of such diverse trends should be examined as closely as possible in predicting environmental scenarios. The EXMAN datasets encompass the effects of changes in chemical budgets, the interactions with climate change, and the biological feedbacks of change in forest ecosystems. This feature should make them suitable to contribute to the progress of BAHC (and of the project Global Change and Terrestrial Ecosystems (GCTE) as well). 5.2. Data availability
The researchers in the EXMAN project will be happy to co-operate and to contribute to joint evaluations and integration exercises in the International Geosphere Biosphere Project (IGBP) research. Published data are available in the report on the first project phase, 1988-1991 (Rasmussen et al., 1993). Furthermore, access to original data can be given, if commonsense copyright and authorship regulations are respected. As EXMAN is in itself already an integrated, multidisciplinary project, we will be happy to integrate the results of our work into a broader scientific exercise. For more information and data from the Soiling roof experiments, interested colleagues may directly contact the author of this paper, For the EXMAN project in general, they should contact the co-ordinator, Dr. Lennart Rasmussen, Forest and Landscape Research Centre, Skovbrynet 16, DK-2800 Lyngby, Denmark.
Acknowledgements EXMAN was and is funded by the CEC (DG XII) within the STEP and ENVIRONMENT programmes. The Solling roof project is also funded by the German Ministery of Research and Technology under Grant OEF 2019-3.
References Bredemeier,M. 1988.Forestcanopytransformationof atmosphericdeposition.WaterAir SoilPollut.,40: 121-138.
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Cermak, J., Deml, M. and Penka, M , 1973. A new method of sap flow rate determination in trees. Biol. Plant. (Praha), 15(3): 171-178. Dise, N. and R.F. Wright, 1992. The NITREX project (Nitrogen saturation experiments). CEC Ecosystem Research Report, 2, pp. 4-14. Dohrenbusch, A. and Grote, R., 1993. Okophysiologische Untersuchungen. In: Jahreszwischenbericht 1992 (interim report), Universit~it G6ttingen, Gottingen, BMFT Project 2019-3, pp. 588-603. Fowler, D., 1984. Transfer to terrestrial surfaces. In: The ecological effects of deposited sulfur and nitrogen compounds. Philos. Trans. R. Soc. London, Ser. B, 10: 259-279. Gordon, H.B., Wetton, P.H., Pittock, A.B., Fowler, A.M. and Haylock, M.R., 1992. Simulated changes in daily rainfall intensity due to the enhanced greenhouse effect. Climate Dyn., 8" 83-102. Houghton, R.A., Jenkins, G.C. and Ephrams, J.J., 1990. Climate Change: the IPCC Scientific Assessment. Cambridge University Press, New York. Jenkins, A,, Wright, R.F., Berendse, F., van Breemen, N., Brussaard, L., Schulze, E.D. and Woodward, F.I., 1993. The CLIMEX project--climate change experiment. In: L. Rasmussen, T. Brydges and P. Mathy (Eds.), Experimental Manipulations of Biota and Biogeochemical Cycling in Ecosystems. Commission of the European Community, Brussels, CEC Ecosystems Research Project, 4, pp. 71-77. Murach, D., Wiedemann, H. and Klaproth, F., 1993. Feinwurzeluntersuchungen auf den Versuchsfl~chen des Solling-Dachprojekts. In: Jahreszwischenbericht 1992 (interim report), BMFT Project 2019-3, pp. 604-616. Rasmussen, L,, Beier, C., Bredemeier, M., Collins, J.F., Cummins, T., de Visser, P.H.B., Farrell, E.P., Kreutzer, K., Pr6bstle, Raben, G.H., Schierl, R., Steinberg, N. and Zuleger, M , 1993. E X M A N - experimental manipulation of forest ecosystems in Europe; project period 1988-1991. Commission of the European Community, DG XII, Brussels, CEC DG XII Ecosystems Research Report 7, 124 pp. Rind, D., Goldberg, R., Hansen, J., Rosenzweig, C. and Ruedy, R., 1990. Potential evapotranspiration and the likelihood of future drought. J. Geophys. Res., 95(7): 9983-10004.
Agricultural and Forest Meteorology 73 (1995) 307-320
Experimental manipulations of the water and nutrient cycles in forest ecosystems (patch-scale roof experiments: EXMAN) Michael Bredemeier Research Centre for Forest Ecosystems, University of Goettingen, Goettingen, Germany Received 3 October 1993; revision accepted 11 February 1994
Abstract E X M A N is a joint project funded by the Commission of the European Communities (CEC) Directorate XII, Science and Research (DG XII), with research partners in five European countries. The aim of E X M A N is experimental manipulations of the water and nutrient cycles in forests. These experiments are aimed at testing hypotheses on ecosystem reactions to altered inputs and boundary conditions. E X M A N developed along with the 'roof method' as a means for the manipulation of water and solute fluxes into the forest soil on an ecosystem scale. Large roofs are built underneath the forest canopy to collect the throughfall. This amount of water is then, by means of technical equipment, altered in its chemical composition and/or its quantity and/or its temporal flux distribution and reapplied by sprinkler systems to the soil. The reactions of the soil chemical and hydrological parameters, as well as the physiological reactions of the forest stand and the soil fauna and microfiora, are investigated in an integrated manner. Such patch-scale roof studies are running in Denmark, the Netherlands, Ireland and Germany ( E X M A N sites), as well as in Norway and Sweden. Concerted planning and evaluation of results is secured by periodic contacts between the groups. Results from the manipulation studies may contribute considerably to the progress of the projects GCTE (Global Change and Terrestrial Ecosystems) and BAHC (Biological Aspects of the Hydrological Cycle, e.g. the soil-vegetation-atmosphere transfer (SVAT) models). Relevant BAHC Foci are Focus I, Activity 1, Tasks 1.1.2, 1.1.3 and 1.1.5.
I. Introduction E c o s y s t e m research is in m a n y cases w o r k i n g on the p a t c h (i.e. single e c o s y s t e m ) scale o r even s m a l l e r spatial extensions. This is a c o n s e q u e n c e o f the fact t h a t i n s t r u m e n t s for intensive process studies, such as.gas e x c h a n g e cuvettes, p o r o m e t e r s 0168-1923/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0168-1923(94)05081-3
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or soil tensiometers, are relatively small and only cover areas in the squarecentimetre to square-metre range. To integrate the wealth of knowledge on ecosystem processes into models and assessments of global change, the intensive patch-scale information has to be regionalized. Data from plot study sites should be integrated regionally, as should be the method and the sampling and analysis protocols. The CEC (Commission of the European Communities) research policy fosters such integration and harmonization in ecological research. CEC funding is only available for international joint projects with two or more European partners. The EXMAN project developed within the Environmental Research Programme of the CEC. It includes study sites from Ireland up to Denmark and down to the South of Germany (see Fig. 1), and thus covers considerable gradients of climatic and air pollution regimes. Ecosystem processes are studied using harmonized sampling strategies and analytical methods. The experimental manipulation of the water and nutrient cycles of forests on the ecosystem scale is a specific feature of the EXMAN project (and also of the related NITREX and CLIMEX projects). Reactions of mature forests to such manipulations can be tested under otherwise unchanged on-site conditions. Results from such experiments should be much more realistic and relevant than any laboratory manipulations. EXMAN is related to some further ecosystem manipulation projects which are performed in European cooperation: the NITREX (Nitrogen saturation experiments) and the CLIMEX (_climate change experiment) project. These projects are also funded by CEC-DGXII, and they also employ--at least partly--roof
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M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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manipulations on the ecosystem scale. More detailed information on NITREX can be found in Dise and Wright (1992), on CLIMEX in Jenkins et al. (1993). In this paper, the general idea of the roof manipulation method and the set-up of the EXMAN project are explained. Some example results of reactions to the manipulations are shown. The relevance of the datasets for the project BAHC (Biological Aspects of the Hydrological Cycle) is considered, in the hope of stimulating co-operation between large-scale/fuzzy dataset and patch-scale/intensive dataset approaches.
2. The roof manipulation method
Roofs underneath a forest canopy permit an effective manipulation of the hydrological and chemical input to the soil, whereas other boundary conditions of the forest system remain essentially unchanged. The canopy is exposed to ambient air temperature, humidity and chemistry. Highly transparent roof materials allow for the transmission of 95-98% of the visible light intensity and thus permit continued growth of ground vegetation. If the roofs are built high enough over the ground and have open sides with free ventilation, then temperature and humidity differences compared with the surrounding forest stand are minor. The input vector of water and chemical elements transported with the water is therefore the one significant object of manipulation in a roof study. Fig. 2 shows the principle of roof manipulations and the ecosystem reactions under study. If there are technical facilities for chemical treatment (deionization and reconditioning) and temporary storage (water tanks) of throughfall water, then there are three basic options for input manipulation: (a) change total water flux to the plot; (b) change temporal distribution of water flux (experimental drought periods or optimum irrigation with respect to transpirational demand); (c) change chemical input flux. Manipulations (a) and (b) are particularly interesting in the context of climate change research, as scenarios of global change effects such as increasing summer drought can be simulated. Such scenarios are especially relevant with respect to the hydrological cycle, which is the scope of BAHC. Manipulation option (c) refers more to a change in chemical climate, i.e. a change in air pollution input to forests. Scenarios of reduced air pollution as a result of environmental legislation (e.g. reduced acid inputs) can be simulated by experimental 'clean rain' application. If the roof manipulation facilities are technically constructed in an adequately flexible way, a combination of manipulation options is also feasible, to examine the combined effects of changes in hydrological and chemical input. The manipulation facilities in the EXMAN consortium in general, and particularly the Solling roof site, are set up technically in such a way that manipulation experiments of various kinds and intentions can be run for a long period of time in the future. In Fig. 2(a), the hydrological fluxes in the undisturbed forest system are shown. Throughfall, i.e. the rain and stemflow water in the forest, is formed from the interaction of wet and dry deposition and a canopy exchange component (Fowler,
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a) no roef
preciloitation
dry + occult :lepos.
seepage~uptake
b) roef
storage manipulation reaction ~ ~/&reaction Fig. 2. Schematicsketch of the roof manipulation method in a forest and the ecosystem'sreactions under observation. 1984; Bredemeier, 1988). Throughfall (plus stemflow) reaching the forest floor i s - roughly speaking--subdivided into a seepage and a root uptake/transpiration component. To observe the reaction of these output signals to a changed hydrological input signal is the most interesting point in a hydrological manipulation study (Fig. 2(b)). Also, secondary physiological reponses of the forest to the manipulated water regime are followed, e.g. changes in photosynthesis and gas exchange rates, changes in root growth and vitality (Murach et al., 1993), and changes in soil fauna and soil microflora activities. Ecosystem reactions to a manipulation can be very complex and work on different levels of the system. It is therefore a general strategy in the kind of experiments described here to integrate and concentrate specialist working groups of all
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ecosystem compartments in the roof manipulation studies. More detailed information on which particular investigations are carried out at which roof sites can be found in the recently published E X M A N report (Rasmussen et al., 1993).
3. EXMAN sites and experiments The geographical distribution of the E X M A N sites is shown in Fig. 1. Tables 1 and 2 add information on the regional climatical characteristics. Four of the study sites are lowland forest ecosystems, whereas the two German sites are in the montane elevation zone at about 500 m above sea-level. These two sites also have more continental climate characteristics. The Solling site exhibits particularly high mean annual precipitation, but all E X M A N sites can be classified as humid, with average rainfall well above 750 mm year -I . It is in this climatical zone that the frequency and intensity of drought periods will increase under climatic change, according to the predictions of various models (Houghton et al., 1990; Rind et al., 1990; G o r d o n et al., 1992). The r o o f experiments with drought treatments can test the effects and ecological feedbacks of such model scenarios directly on the ecosystem scale. Further site information regarding the soils and the forest stands is given in Tables 3 and 4, respectively. Soil texture (Table 3), and consequently the water retention characteristics, are different between the experimental regions in E X M A N . The Danish (Klosterhede) and Dutch sites (Kootwijk and Harderwijk) have soils which developed on sandy deposits with little to no silt and clay content. The German (Solling and Hrglwald) and Irish (Ballyhooly) sites, by contrast, have typical loamy soils with moderate clay and high silt and sand contents. This spectrum of soil hydrological characteristics in E X M A N should make the datasets suitable for regional extrapolation exercises within BAHC. The forest stands are all conifer monoculture plantations (Table 4). This forest t y p e - - a l t h o u g h ecologically not very beneficial--is actually the most widespread and economically relevant in Central and Northern Europe. The forest stands represent different age stages and thus different actual biomasses (expressed here by the parameters tree height, diameter and density (trees ha-l)), different growth rates and different evapotranspiration characteristics. The experimental manipulation treatments which are currently conducted at the Table 1 Geographical locations and climate types of the EXMAN sites (from Rasmussen et al., 1993) Site
Longitude
Latitude
Climatetype
Klosterhede Kootwijk Harderwijk H6glwald Solling Ballyhooly
8°24'E 5°50'E 5°40'E ll°10'E 9°34~E 8°25rW
56°29~N 52°10'N 52°20'N 48°30'N 51°38~N 52°09~N
Temperateboreal oceanic Temperatesub-oceanic Temperatesub-oceanic Sub-montane,semi-continental Montane,semi-continental Temperatesub-oceanic
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Table 2 Elevations and local climatic characteristics of the EXMAN sites (from Rasmussen et al., 1993) Site
Klosterhede Kootwijk Harderwijk H6glwald Soiling Ballyhooly
Elevation
Precipitation
(m)
(mm)
30 26 10 540 500 69
860 853 763 800 1088 1000
Mean temperatures (°C) Annual
January
July
9.0 8.9 8.9 7.3 6.4 9.5
0.3 2.0 2.0 -2.0 -2.2 5.5
16.0 16.6 16.6 16.5 14.7 15.5
E X M A N sites are listed in Table 5. At every location in the research network the experiments are run versus an unmanipulated control plot. The Irish site Ballyhooly serves as an overall reference control, as it represents one of the rare locations in Europe for which background air pollution levels--uninfluenced by human activities--can be assumed. (This selection can be explained by the background of the E X M A N project; E X M A N was started as an experiment within the context of air pollution/acid rain research; the relevance for issues related to climate change has emerged only recently). Table 5 shows that the various experimental manipulations are generally aiming at changing the acid-base status of the system, changing the nutrient status, changing the hydrologic input (by means of additional irrigation or drought), or combinations of the above manipulation options. The experiments involving hydrological manipulations are of particular significance for BAHC; such experiments are currently run at all sites of the E X M A N network, as can be seen from Table 5. Example results will be reported in Section 4; more detailed information on the experimental set-up and first results in E X M A N can be found in the recently published CEC report (Rasmussen et al., 1993). Fig. 3 shows a view of the roof experiments in the Solling area, Germany, in more detail, as additional background information for the example results reported below. The drawing in the upper part of the figure (Fig. 3(a)) shows a picture of the site with the triangular arrangement of three roofs of 300 m 2 area each. In the middle lies a central cabin which holds water manipulation and data storage equipment. A 360 ° Table 3 Soil characteristics of the EXMAN sites (from Rasmussen et al., 1993) Site
Soil type
Parent material
Texture (10 cm)
Klosterhede Kootwijk Harderwijk H6glwald Solling Ballyhooly
Typic Haplorthod Plaggic Dystrochrept Typic Udipsamment Typic Hapludult Dystrochrept Typic Haplorthod
Sandy fluvio-glacial deposit Aeolian sand Cover sand Tertiary silty loam with loess Loess derived from standstone Sandstone till-colluvium
Sand Sand Sand Loam Loam Sandy loam
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Table 4 Forest characterization of the EXMAN sites (from Rasmussen et al., 1993) Site
Species l
Age (years)
Height (m)
Basal area (m 2 ha -l)
Stocking (ha -a)
Mean diameter (cm)
Klosterhede Kootwijk Harderwijk H6glwald Soiling Ballyhooly
NS DF SP NS NS NS
72 39 39 84 58 52
20 18 10 37 20 24
29 30 29 77 62 44
858 992 1800 603 950 632
22 20 14 40 29 29
INS, Norway spruce; DF, Douglas fir; SP, Scots pine.
sluable crane is located in the centre by the cabin. F r o m its gondola every position in the canopy within the area of the r o o f plots and the control can be reached for physiological measurements. By the side of R o o f 3, the one in the background, the water storage tanks can be seen. They can hold the equivalent of approximately Table 5 Experimental manipulation treatments in EXMAN (adapted from Rasmussen et al., 1993) Site
Plot
Treatment
Roof
Klosterhede, DK
1: Control 2: Drought (summer) 3: Irrigation 4: Fertigation a
Periodic drought Deacidified water ('clean rain') Optimal nutrition
No Yes Yes Yes
1: Control 2: Irrigation 3: Fertigationa 4: Fertigation a
Throughfall + demineralized water Throughfall + nutrition Optimal nutrition
No No No Yes
1: Control 2: Irrigation 3: Fertigation a 4: Fertilization
Throughfall + demineralized water Throughfall + nutrition Throughfall + (NH4)2SO 4
No No No No
1: Control 2: Irrigation 3: Irrigation 4: Liming 5: Irrigation + liming 6: Irrigation + liming 7: Drought
Throughfall + acidic water Throughfall + deacidified water Throughfall + lime Throughfall + lime + acidic water Throughfall + lime + deacidified water Temporary roof/drought periods
No No No No No No Yes
1: Control 2: Roof control 3: Drought (summer) 4: Irrigation
Throughfall Periodic drought Deacidified water ('clean rain')
No Yes Yes Yes
I: Control 2: Drought
Temporary roof/drought periods
No Yes
Kootwijk, NL
Harderwijk, NL
H6glwald, DE
Soiling, DE
Ballyhooly, IE
a Fertigation; a combination of optimum nutrition (fertilization) and optimum irrigation.
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M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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140mm of throughfall during experimental drought phases for later rewetting. The lower part of the figure (Fig. 3(b)) shows a schematical sketch of the arrangement of experimental installations on the Soiling roof plots. In each of the plots, there is a central patch equipped with lysimeters for soil water sampling and tensiometers for monitoring of soil water potentials. The tensiometers have electric pressure transducers and are read by the computer in the central cabin every 15 min. The soil water potentials and the signals of irrigation events in soil hydrology are thus followed with a very high temporal resolution. The matrix potential detection is so sensitive that even the daily maxima of transpiration of the forest are reflected in the readings. Pressure heads lower than -90 kPa cannot be measured by watercolumn-based tensiometers, however. It is therefore planned to install the time-domain-reflectometry (TDR) technique in the Soiling roof plots in 1994, to obtain information on soil water contents and associated pressure heads in periods of severe drought. Sap flow is measured on several trees in the roof plots and the control as a direct measure of actual transpiration. The method used is the tissue heat balance according to Cermak (1973). There is also a root observation pit with video-rhizoscopes installed in every plot. Root growth, morphology and vitality are continuously observed, and the images are recorded for further digital processing. Thus, together with the physiological measurements in the canopy and the intensive monitoring of soil hydrology and chemistry, a rather complete picture of ecosystem reaction to the manipulative treatments is accomplished. These observations are complemented by studies of the soil fauna and microflora and the ground vegetation in sections of the plots without any destructive sampling activities (see Fig. 3(b)). Three roof plots and an uncovered control plot are available for experimentation at Soiling. The experimental treatments currently pursued are (Fig. 3(b)): Plot D - - n o roof, control; Plot D l - - r o o f , application of 'clean' (preindustrial) rain; Plot D 2 - roof, control for roof effects alone without any further manipulation; Plot D3--roof, experimental drought with subsequent intensive rewetting. In the following section, some results from EXMAN in general and the Soiling drought experiment (D3) in particular are presented.
4. Example results Results can be reported only very briefly within the limited extent of this paper. They are explained in much more detail in the recently published EXMAN report (Rasmussen et al., 1993). The effects of the irrigation treatments in EXMAN on tree growth are illustrated in Fig. 4. Average basal area increment of the trees was higher in all irrigation treatments, although the differences were not statistically significant for the Hrglwald site. Actual transpiration and net photosynthesis vary accordingly between the irrigated and control treatments. It has to be noted that the 'irrigation' treatments did not in every case mean a higher total water input to the soil. Water amounts in the irrigation treatments were 90%, 162%, 143% a n d 141% of the
316
M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
KIosterhede, DK
Kootwijk, NL
Harderwijk, NL
Hbglwald, DE
0
0.5
r 1.5
1
•
i 2
2.5
Basal area increment [m2/(ha*a)] •
Control •
Irrigated
Fig. 4. Basal area increment of the forest stands in four irrigation treatments vs. the respective controls in E X M A N (from Rasmussen et al., 1993).
controls in Klosterhede, Kootwijk, Harderwijk and H6glwald, respectively (over the time period 1988-1990). Thus, in Klosterhede the irrigated plot received less total water than the control, but in an optimal temporal distribution relative to the transpirational demand. This 'time-optimized irrigation' allowed potential transpiration of the Klosterhede forest almost entirely throughout the vegetation period(s) and had
Klosterhede, DK
Kootwijk, NL
Harderwijk, NL
H6glwald, DE
0
50
100
150
200
Growth increment (% of control) Fig. 5. Relative growth enhancement (control represents 100%) in the E X M A N irrigation treatments (from Rasmussen et al., 1993).
M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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the strongest effect on growth increase relative to the controls among the four sites (Fig. 5). Not only the amount but also the chemistry of the applied irrigation water has effects on the tree growth. In the Hrglwald manipulation experiments, acid irrigation vs. de-acidified irrigation was sprayed upon the soil (see Rasmussen et al. (1993), Table 5, for details). This resulted in a significant difference in growth response, as shown in Fig. 6. Such interactions of physical climate and ecochemical changes should be duly considered in predicting scenarios of future changes in terrestrial ecosystems. In the Solling drought manipulation the physiological reactions of the trees are investigated with high temporal and spatial resolution (see description of Soiling roof experiment above). A technical innovation in this project is the development ofmicrodendrometers, which are capable of recording variations of tree diameters in the range of 10-100 #m (Dohrenbusch und Grote 1993). The micro-dendrometer is installed by screws on the bark of the tree, and the sensor follows the shrinking and swelling of the stem caused by the daily (and long-term) time-course of transpiration. The daily shrinking maxima are fairly well correlated with the daily xylem water flux rates, as determined by the Cermak apparatus (Fig. 7, adapted from Dohrenbusch und Grote (1993); r 2 = 0.50, P < 0.001). Micro-dendrometers are, however, much more inexpensive and easier to use for long-term measurements. A large number of trees can be equipped and continuously measured by microdendrometers at relatively low cost. In this way, entire forest stands can be observed and transpiration rates be inferred on the stand level, as a prerequisite for regional extrapolation. This monitoring can be done with very high time resolution, and thus gives the chance to follow the effects of single and rapid hydrological events on actual transpiration rate. An example ofmicro-dendrometer observations is given in Fig. 8. It shows the daily diameter fluctuations during the drought experiment in summer 1992 for the control
Relative growth increment 1.4
~
1.2 1 0.8 0.6 o.4
'
i
0.2
I
Normal irr./corCe'ol
Acid irr. / control
Fig. 6. Growth response to clean vs. acid irrigation in the Hrglwaid experiment(control represents 100%) (from Rasmussen et al., 1993).
M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307 320
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(upper curve) and the drought roof plot (lower curve). The experimental drought period lasted for 16 weeks, and it can be seen that in the last week of the manipulation the diameter fluctuations--and hence actual transpiration rate--were much lower in the drought plot than in the control. Immediately upon onset of the rewetting the diameter fluctuations reacted and increased, exhibiting particularly high daily amplitudes during their rise in the rewetting period. After about 2 weeks they had recovered to control levels.
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M. Bredemeier / Agricultural and Forest Meteorology 73 (1995) 307-320
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5. Conclusions 5.1. Relevance f o r B A H C
The facilities within EXMAN to manipulate the hydrologic input vector to forests on the ecosystem scale are unique in Europe and probably in the world. The experiments as such are plot-scale investigations, but the widespread network of research sites together with harmonized sampling and analysis methods give a good opportunity to extrapolate to the regional or possibly to the European continental scale. Besides changes in physical climate, we are probably facing some change in chemical climate and biological structure and feedbacks of terrestrial ecosystems in the future. The interactions of such diverse trends should be examined as closely as possible in predicting environmental scenarios. The EXMAN datasets encompass the effects of changes in chemical budgets, the interactions with climate change, and the biological feedbacks of change in forest ecosystems. This feature should make them suitable to contribute to the progress of BAHC (and of the project Global Change and Terrestrial Ecosystems (GCTE) as well). 5.2. Data availability
The researchers in the EXMAN project will be happy to co-operate and to contribute to joint evaluations and integration exercises in the International Geosphere Biosphere Project (IGBP) research. Published data are available in the report on the first project phase, 1988-1991 (Rasmussen et al., 1993). Furthermore, access to original data can be given, if commonsense copyright and authorship regulations are respected. As EXMAN is in itself already an integrated, multidisciplinary project, we will be happy to integrate the results of our work into a broader scientific exercise. For more information and data from the Soiling roof experiments, interested colleagues may directly contact the author of this paper, For the EXMAN project in general, they should contact the co-ordinator, Dr. Lennart Rasmussen, Forest and Landscape Research Centre, Skovbrynet 16, DK-2800 Lyngby, Denmark.
Acknowledgements EXMAN was and is funded by the CEC (DG XII) within the STEP and ENVIRONMENT programmes. The Solling roof project is also funded by the German Ministery of Research and Technology under Grant OEF 2019-3.
References Bredemeier,M. 1988.Forestcanopytransformationof atmosphericdeposition.WaterAir SoilPollut.,40: 121-138.
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Cermak, J., Deml, M. and Penka, M , 1973. A new method of sap flow rate determination in trees. Biol. Plant. (Praha), 15(3): 171-178. Dise, N. and R.F. Wright, 1992. The NITREX project (Nitrogen saturation experiments). CEC Ecosystem Research Report, 2, pp. 4-14. Dohrenbusch, A. and Grote, R., 1993. Okophysiologische Untersuchungen. In: Jahreszwischenbericht 1992 (interim report), Universit~it G6ttingen, Gottingen, BMFT Project 2019-3, pp. 588-603. Fowler, D., 1984. Transfer to terrestrial surfaces. In: The ecological effects of deposited sulfur and nitrogen compounds. Philos. Trans. R. Soc. London, Ser. B, 10: 259-279. Gordon, H.B., Wetton, P.H., Pittock, A.B., Fowler, A.M. and Haylock, M.R., 1992. Simulated changes in daily rainfall intensity due to the enhanced greenhouse effect. Climate Dyn., 8" 83-102. Houghton, R.A., Jenkins, G.C. and Ephrams, J.J., 1990. Climate Change: the IPCC Scientific Assessment. Cambridge University Press, New York. Jenkins, A,, Wright, R.F., Berendse, F., van Breemen, N., Brussaard, L., Schulze, E.D. and Woodward, F.I., 1993. The CLIMEX project--climate change experiment. In: L. Rasmussen, T. Brydges and P. Mathy (Eds.), Experimental Manipulations of Biota and Biogeochemical Cycling in Ecosystems. Commission of the European Community, Brussels, CEC Ecosystems Research Project, 4, pp. 71-77. Murach, D., Wiedemann, H. and Klaproth, F., 1993. Feinwurzeluntersuchungen auf den Versuchsfl~chen des Solling-Dachprojekts. In: Jahreszwischenbericht 1992 (interim report), BMFT Project 2019-3, pp. 604-616. Rasmussen, L,, Beier, C., Bredemeier, M., Collins, J.F., Cummins, T., de Visser, P.H.B., Farrell, E.P., Kreutzer, K., Pr6bstle, Raben, G.H., Schierl, R., Steinberg, N. and Zuleger, M , 1993. E X M A N - experimental manipulation of forest ecosystems in Europe; project period 1988-1991. Commission of the European Community, DG XII, Brussels, CEC DG XII Ecosystems Research Report 7, 124 pp. Rind, D., Goldberg, R., Hansen, J., Rosenzweig, C. and Ruedy, R., 1990. Potential evapotranspiration and the likelihood of future drought. J. Geophys. Res., 95(7): 9983-10004.