A. Tiktak

Review of sixteen forest-soil-atmosphere models

Abstract The principles and characteristics of sixteen forest-soil-atmosphere models, describing the flows of water, carbon and nutrients in a forest ecosystem were compared. This comparison addresses technical aspects of model implementation and system discretisation, as well as the nature of the included processes. A large variety of integrated models exists, but only a few of them are ‘well-balanced’, describing all aspects of the forest ecosystem with a comparable level of detail. As a result, only a few models can be used to assess effects of air pollution and acid deposition on forest growth. There is considerable agreement with respect to the description of the water cycle and geochemical processes; differences result mainly from different levels of process aggregation. There is no general agreement on the description of the nutrient cycle. Models differ most with respect to nutrient uptake and nutrient (re)allocation in the plant. It is impossible to develop simple lumped models for regional, policy-oriented applications, without a well-based strategy for simplification. Therefore, simple models should always be backed up by more detailed mechanistic models, which can be tested against field observations. Only a few models are well-documented and available to others. The lack of concise and adequate model documentation hampers model exchange and may lead to unnecessary development of new models. Therefore, model documentation must be given high priority.

The Solling dataset. Site characteristics, monitoring data and deposition scenarios

Abstract A data set originating from a forest ecosystem study at Solling (Germany) between 1969 and 1990 is described. The data set was distributed among the participants of the workshop ‘Comparison of Forest-Soil-Atmosphere Models’, which was held in 1993. The purpose was to evaluate the performance of these models on a common data set. The data set is rather complete, although it is focused on soil chemistry and nutrient cycling.

The Solling Norway Spruce site

Abstract The Solling F1 site is a typical Norway Spruce (Picea abies) plantation forest on acid soil, with a well-developed mor humus layer, low soil biological activity and sparse ground vegetation. Inputs, outputs and internal transfers of chemical constituents have been measured continuously for more than twenty years, and were complemented by plant physiological, hydrological, micrometeorological and soil biological measuring programmes in that time. A comprehensive dataset was compiled from many sources to apply a variety of models to the Solling spruce site dataset in the framework of the workshop “Comparison of Forest-Soil-Atmosphere Models” at Leusden in the Netherlands. This paper gives background information on the site, such as land use history, climate, soil and deposition regime, and on the dataset (important trends and limits of accuracy).

Workshop on Comparison of Forest-Soil-Atmosphere Models: Preface

Abstract Between 10 and 14 May 1993, a workshop was organized in Leusden, the Netherlands, in which the concepts and performance of 18 models were discussed. These models were generally aimed at the analysis of effects of acid atmospheric deposition on forest and forest soils. Model concepts were analysed by means of a questionnaire. Model performance was analysed by application of the models to a common dataset for the F1 Norway spruce site, at Solling, for the period 1970–1990. The nature of the participating models varied widely, but all models were process-oriented and dynamic. Included were hydrological models, soil chemistry models as well as integrated models for calculation of forest growth. Overall, average annual hydrology, soil solution chemistry and forest growth of the spruce site between 1970–1990 could be reproduced. Differences between models were attributable more to different parameterizations than to differences of model concepts. Short-term temporal dynamics of soil solution chemistry were not reproduced very well, particularly for nitrate. Models also gave different predictions of the response of the Norway spruce to maintained or reduced fluxes of atmospheric deposition, however, confirming a key role for Mg and N. Complex models did not give better results than more simple models. Integrated models were also judged unbalanced, with respect to description of the hydrologic, soil chemical and forest subsystems. Therefore, it was recommended to improve this balance and to work towards less complex models for prediction of the long-term behaviour of forest stands. Our knowledge about nutrient uptake by forests in nutrient-limited acidified soil systems, and also about the interaction of various forest stresses is still inadequate. Future research should therefore emphasize a better integration of model development and experimental research, e.g., as presently is the case for experimental field manipulations.

Modelling cadmium accumulation at a regional scale in the Netherlands

The cadmium content in soils in the rural environment in the Netherlands may increase towards an exceedance of quality standards due to atmospheric deposition and the use of fertilizers and animal manure. To evaluate this problem, a simple dynamic, process-oriented model SOACAS has been developed which is aimed at predicting the accumulation of heavy metals in the topsoil in a regional context. SOACAS describes the fate of a metal in one completely mixed soil compartment using a Freundlich isotherm and analytical equations to solve the mass balance. We tested if it was possible to reconstruct the soil’s present cadmium contents, using independent estimates of historical cadmium loads (‘hind-cast simulation’). About 2500 recent point observations of cadmium contents in rural areas were available. Before comparison, a map was created to translate the point information obtained from the field data to areal average information (resolution 500 × 500 m2), required for comparison with results of SOACAS. A regression model in combination with a locally-weighted smoother within the framework of Generalized Additive Modelling (GAM) was used for this purpose. A realistic geographical pattern could be obtained with very few a priori assumptions. Comparison of the map obtained by the GAM and the map obtained by hind-cast simulation showed that, despite the large uncertainties about historical cadmium loadings, the current cadmium contents were only slightly underestimated by SOACAS. Moreover, the geographical pattern for the observed and simulated contents compared reasonably well. On this basis of this exercise we believe that the model can be used to predict trends of future metal contents as a function of emission/immission scenario’s. Simulations showed that cadmium contents currently decrease in highly polluted areas around industrial plants in the South-Eastern part of the country, and still increase in arable land.

Towards a landscape scale management of pesticides: ERA using changes in modelled occupancy and abundance to assess long-term population impacts of pesticides.

Pesticides are regulated in Europe and this process includes an environmental risk assessment (ERA) for non-target arthropods (NTA). Traditionally a non-spatial or field trial assessment is used. In this study we exemplify the introduction of a spatial context to the ERA as well as suggest a way in which the results of complex models, necessary for proper inclusion of spatial aspects in the ERA, can be presented and evaluated easily using abundance and occupancy ratios (AOR). We used an agent-based simulation system and an existing model for a widespread carabid beetle (Bembidion lampros), to evaluate the impact of a fictitious highly-toxic pesticide on population density and the distribution of beetles in time and space. Landscape structure and field margin management were evaluated by comparing scenario-based ERAs for the beetle. Source-sink dynamics led to an off-crop impact even when no pesticide was present off-crop. In addition, the impacts increased with multi-year application of the pesticide whereas current ERA considers only maximally one year. These results further indicated a complex interaction between landscape structure and pesticide effect in time, both in-crop and off-crop, indicating the need for NTA ERA to be conducted at landscape- and multi-season temporal-scales. Use of AOR indices to compare ERA outputs facilitated easy comparison of scenarios, allowing simultaneous evaluation of impacts and planning of mitigation measures. The landscape and population ERA approach also demonstrates that there is a potential to change from regulation of a pesticide in isolation, towards the consideration of pesticide management at landscape scales and provision of biodiversity benefits via inclusion and testing of mitigation measures in authorisation procedures.

Application of three forest-soil-atmosphere models to the speuld experimental forest

De invloed op bossen van de depositie van zwavel en stikstof, ozon en van op grote schaal onderzocht. Hiertoe zijn een aantal intensieve monitoring studies opgezet en werden modellen van de kringloop van water, nutrienten en assimilaten ontwikkeld. Dit rapport beschrijft de toepassing van het bodemverzuringsmodel NuCSAM, en de geintegreerde modellen SoilVeg en ForGro op het Speulderbos, een Douglas-opstand op een holtpodzol. In dit bos werd van 1987 t/m 1991 een uitgebreide meetcampagne uitgevoerd. De gesimuleerde bodemwatergehalten, concentraties van stoffen in het bodemwater, naaldmassas, stam-aanwas en nutrientenstatus kwamen redelijk goed overeen met de metingen. De modellen vertoonden echter aanzienlijke onderlinge verschillen op het gebied van grootheden welke niet gemeten konden worden, zoals bosverdamping, drainage, nutrientenopname en mineralisatie. Het gedrag van de geintegreerde modellen werd geverifieerd door toepassing van deze modellen op een irrigatie- en fertigatie experiment op een nabij gelegen Douglas opstand. De modellen konden in het algemeen de effecten van irrigatie en fertigatie op de stam-aanwas redelijk goed voorspellen, maar er waren grote verschillen wat betreft de voorspelde nutrienten status en de stikstof-mineralisatie. De modellen werden vervolgens gebruikt voor scenario analyses voor de periode 1994-2050. Ook hier werden grote verschillen tussen de modellen gevonden voor met name de stikstofkringloop en de nutrienten status (met name het N-gehalte in bladeren). Alle modellen voorspelden dat de concentraties van sulfaat en aluminium in de bodemoplossing snel omlaag gaan na een afname van de verzurende depositie, en dat de concentratie van nitraat een aantal jaren hoog blijft na een afname in de stikstofdepositie. Dit laatste wordt veroorzaakt door opslag van een overmaat aan stikstof in de biomassa en het strooisel. Uit de resultaten van de geintegreerde modellen blijkt verder dat de directe effecten van verhoogde SOx en ozon concentraties in de atmosfeer, alsmede de indirecte effecten van een lage pH en hoge aluminium concentratie een minder groot probleem opleveren dan de effecten van droogte en de overmaat aan stikstof. Onze kennis van de effecten van luchtverontreining en zure depositie op bossen is in het algemeen gebaseerd op laboratoriumstudies en korte monitoring studies. Tot dusverre is het bijna onmogelijk om effecten die in het laboratorium gevonden werden te vertalen naar de veldsituatie. Zolang dit het geval is, blijft elke voorspelling en extrapolatie die met geintegreerde modellen gedaan wordt onzeker, zeker als het gaat om de voorspelling van effecten op een landelijke schaal.

Simulation of movement of pesticides towards drains with a preferential flow version of PEARL.

BACKGROUND As part of the Dutch authorisation procedure for pesticides, an assessment of the effects on aquatic organisms in surface waters adjacent to agricultural fields is required. The peak concentration is considered to be the most important exposure endpoint for the ecotoxicological effect assessment. Macropore flow is an important driver for the peak concentration, so the leaching model PEARL was extended with a macropore module. The new model has two macropore domains: a bypass domain and an internal catchment domain. The model was tested against data from a field leaching study on a cracking clay soil in the Netherlands. RESULTS Most parameters of the model could be obtained from site-specific measurements, pedotransfer functions and general soil structural knowledge; only three macropore-flow-related parameters needed calibration. The flow-related macropore parameters could not be calibrated without using the concentration in drain water. Sequential calibration strategies, in which firstly the water flow model and then the pesticide fate model are calibrated, may therefore be less suitable for preferential flow models. CONCLUSION After calibration, PEARL could simulate well the observed rapid movement towards drains of two pesticides with contrasting sorption and degradation rate properties. The calibrated value for the fraction of the internal catchment domain was high (90%). This means that a large fraction of water entering the macropores infiltrates into the soil matrix, thus reducing the fraction of rapid flow.

Propagation of Uncertainties in Soil and Pesticide Properties to Pesticide Leaching

In the new Dutch decision tree for the evaluation of pesticide leaching to groundwater, spatially distributed soil data are used by the GeoPEARL model to calculate the 90th percentile of the spatial cumulative distribution function of the leaching concentration in the area of potential usage (SP90). Until now it was not known to what extent uncertainties in soil and pesticide properties propagate to spatially aggregated parameters like the SP90. A study was performed to quantify the uncertainties in soil and pesticide properties and to analyze their contribution to the uncertainty in SP90. First, uncertainties in the soil and pesticide properties were quantified. Next, a regular grid sample of points covering the whole of the agricultural area in the Netherlands was randomly selected. At the grid nodes, realizations from the probability distributions of the uncertain inputs were generated and used as input to a Monte Carlo uncertainty propagation analysis. The analysis showed that the uncertainty concerning the SP90 is 10 times smaller than the uncertainty about the leaching concentration at individual point locations. The parameters that contribute most to the uncertainty about the SP90 are, however, the same as the parameters that contribute most to uncertainty about the leaching concentration at individual point locations (e.g., the transformation half-life in soil and the coefficient of sorption on organic matter). Taking uncertainties in soil and pesticide properties into account further leads to a systematic increase of the predicted SP90. The important implication for pesticide regulation is that the leaching concentration is systematically underestimated when these uncertainties are ignored.

Simulation of forest stress for the Solling spruce site with soilveg

Abstract The soilveg model was applied to Solling to evaluate the effects of various stress factors on the Norway spruce stand (F1). First, hydrology, soil solution chemistry, tree growth and nutrient status for a Norway spruce were simulated for 1976–1989. soilveg could reproduce mean concentration levels for all major solutes. However year-to-year variations at greater depths, particularly for nitrate, often could not be explained. soilveg could also reproduce stem growth and nutrient concentrations in foliage. The model shows that in spite of large precipitation surpluses and waterholding capacities, drought is a relevant stress factor for Solling. Magnesium is the most limiting nutrient for growth of the F1 stand. In dry years, drought is the most important factor impeding uptake of Mg, in wet years pH is more important. However, it can not be concluded that a pH decrease due to acid deposition is an important stress factor for the F1 stand as the extent of the pH decrease is unknown. Simulations with optimal irrigation and fertilization, combined with elimination of air pollutants doubled the current stem growth of F1. The absolute effect of individual stress factors can not be quantified because combined manipulations give a larger growth increase than may be expected from single manipulation. The importance of the stress factors for F1 can be ranked: fertilization > irrigation > air pollution. Simulations further indicate that elimination of S and N in throughfall initially (20 years) will decrease stem growth.