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The current landscapes in north-western Europe are the result of natural processes and human-induced land use and land use changes. Whereas during past centuries, the natural ancient forest cover disappeared and was -sometimes abruptly- converted in intensively used cultural landscapes, an opposed trend has become apparent recently. This trend reversal is at least partly related to the decision of the European Union and several of its member states to actively promote afforestation of -mainly- farmland, in order to respond to surplus agricultural production and to contribute to the international efforts to reduce the nitrate pollution of water bodies and the emission of greenhouse gases. However, many questions pertain regarding the implementation and evaluation of environmental effectiveness and efficiency of this afforestation policy.

The ultimate goal of this book is to assist stakeholders, such as forest and landscape planners and policy makers, in scheming and planning new forests in an environmentally sound way with as many positive and as few negative effects on the environment as possible. The book focuses on the influence of afforestation on carbon sequestration, nitrogen deposition, nitrate leaching, water recharge and potential biodiversity. It deals specifically with the ex-ante evaluation of afforestation measures based on i) field analyses in chronosequences of forest stands planted on former arable land, ii) modelling of these systems using mechanistic models describing the water, C- and N-cycles as well as the understory, iii) simplification of the process-based models into a metamodel (METAFORE) and iv) integration of data and models in a computerized spatial decision support system (the AFFOREST-sDSS).

There is limited knowledge of the contribution of afforested arable land to mitigation of greenhouse effects. In the AFFOREST project we evaluated the rate and magnitude of carbon (C) sequestration in biomass and soils following afforestation of cropland. Two oak () and four Norway spruce () afforestation chronosequences (age range 1 to 90 years) were studied with respect to C sequestration in Denmark, Sweden and the Netherlands.

Biomass C sequestration ranged between 2.7 and 4.6 Mg C ha yr for stands younger than 45 years with no clear influence of different site characteristics. Such effects were probably masked by the soil enrichment, which is a legacy of former agriculture. Biomass C sequestration differed more between sites after 40-50 years owing to different management, tree species-specific growth patterns and less influence of former fertilization.

For the total soil compartment studied, i.e. forest floor and mineral soil 0-25 cm, afforestation of cropland as a minimum resulted in unchanged soil C contents and in most cases led to net C sequestration. Rates of soil C sequestration ranged from being negligible in two of the Danish chronosequences to 1.3 Mg C ha yr for the Dutch chronosequence. The allocation of sequestered soil C was also quite different among chronosequences. While forest floor development consistently led to C sequestration, there was no general pattern in mineral soil C sequestration. In the short term (30 years), tree species had little influence on total soil C sequestration. Afforestation of nutrient-poor sandy soils seemed to result in larger C sequestration in forest floors and the whole soil than afforestation of nutrient-rich, clayey soils.

For the afforested ecosystem as a whole, the general contribution of soils to C sequestration (i.e. to a net gain in C stock) was about one third of the total C sequestration. The contribution of soil varied among the chronosequences from none to 31%, which is not far from reported contributions of soil in similar studies. In the short term (30-40 years), total C sequestration was higher in Norway spruce than in oak whereas soil type did not clearly influence the rate of C sequestration.

The work in AFFOREST has improved the knowledge of C sequestration in afforested cropland. The new results may help to bridge the gap between existing knowledge and policy demands.

The long-term effects of afforestation on hydrological fluxes were investigated using a series of forests of different age planted on comparable soils (chronosequences) in Sweden, Denmark and the Netherlands. Rainfall, throughfall, soil moisture contents and groundwater dynamics were monitored at two oak chronosequences and four spruce chronosequences during a period of one to two years. At all chronosequences, the hydrological fluxes were simulated using a hydrological simulation model. The model was validated on measured throughfall data, soil water contents and Cl fluxes. Afforestation has a clear influence on the water recharge of the considered sites. Water recharge is generally lower under spruce compared to oak. In the spruce stands 5–30% of the incoming precipitation leads to water recharge to ground and surface water, whereas water recharge in the oak stands ranges between 20–35% of the precipitation. In general, water recharge declined with an increase of the stand age. At the oak stands leaching decreased from 35 to 20% of the precipitation in the first 30 years. In the spruce stands the water recharge varied considerable between the four investigated chronosequences but in general, the decline in water recharge was approximately 100-150 mm (10-20%). In both oak and spruce stands, losses by soil evaporation slightly declined. Transpiration slightly increased in the oak stands and transpiration remained fairly stable in the spruce stands. It can be concluded that afforestation leads to a reduction in water recharge compared to agricultural use. This reduction is mainly due to an increase in interception evaporation. The strongest reduction is found when sites are afforested with dense spruce forests. The smallest impact is found in open deciduous forest, which has lower interception evaporation.

Knowledge on the impact of afforestation of arable land on N deposition and leaching of nitrate to groundwater and surface waters is limited. In the AFFOREST project we evaluated nitrogen (N) deposition and nitrate leaching following afforestation of cropland. Two oak () and four Norway spruce () afforestation chronosequences (age range 1 to 90 years) were studied with respect to deposition and nitrate leaching in Denmark, Sweden and the Netherlands. This paper presents a synthesis of these six chronosequence experiments. Three to six forest stands of Norway spruce and/or common oak were monitored in each chronosequence for a period of two years. In each stand, throughfall and soil solutions beneath the root zone were sampled and nitrate leaching was calculated. For all sites and tree species, the throughfall deposition of N increased with stand height (and age). The forests varied substantially in their ability to retain N in the ecosystem. No consistent pattern was apparent in the three countries. However, in some chronosequences nitrate leaching was low or negligible in the early phase of afforestation and increased after canopy closure (> 15-20 years). In general, nutrient-rich clayey soils leached more nitrate than nutrient-poor sandy soils. In the first approximate 35 years after afforestation, nitrate leaching below the root zone was generally higher below oak than below Norway spruce. The presented results are compared to available studies and discussed.

Eutrophication largely results from deposition of atmospheric N. The emission of N mainly originates from agriculture (NH), traffic, power plants and industry (NO). The most important ammonia source is emission from animal manure. The extent of this emission depends on manure composition and meteorological conditions. After N is emitted (either as NH or as NO) it will be transported over short or long distances. Deposition rates vary with the structure of the earth surface, and it is often assumed that the interceptive properties of vegetation, expressed as roughness length, are constant and the same for all wind speeds and all transferable quantities. Careful evaluation of individual components of the overall transfer resistance is crucial to an understanding of how and where N will be distributed within a particular forest canopy. In the context of local scale ammonia, like encountered in the AFFOREST project, NH will be mostly dry deposited to the forest. The Eutrend model was used for the calculation of N deposition, and it calculates concentrations and depositions as a function of surface characteristics. The model is able to describe both short and long-distance transport. The land use changes occurring in an afforestation process have different effects. The first effect is related to the removal of especially NH3 emission, while the second is related to changing deposition characteristics when turning agricultural land into forest. On top of that, after afforestation the deposition of N will change due to the effect of growing forest. Growing forest will have an effect on the roughness length, which is an important factor determining the deposition velocity. Emission from a specific location is not only deposited at that same location but is also transported and deposited in surrounding areas. Because of such transport through the air, a spatial interaction between the emission source and the deposition receptor is introduced as a complicating factor. Further investigation was needed to evaluate the relevancy of spatial interaction for the AFFOREST-sDDS. After corrections, the AFFOREST-sDSS is realistically well able to describe the deposition situation after a certain amount of years after afforestation.

In many European countries agricultural areas are currently being converted to forest. Both spatial arrangement of new forests and habitat quality play a role in the development of the field layer in newly planted forests on former agricultural land. One important goal of afforestation is the development of a natural ecosystem with a valuable field layer.

In the course of succession, open-canopy species are replaced by climax species with a denser canopy, and the light availability on the forest floor is expected to decrease. The differences in the environmental conditions between ancient forests and afforested arable land have not been studied in a successional context. Differences between ancient forests and recent forests on former arable land can be caused by many different factors. Several studies have addressed the vegetational change in the field layer under the influence of nitrogen (N) deposition. The plant ecological approach followed in this study focused on the characteristics of individual species that determine their ecological behavior in relation to light and N availability. A greenhouse experiment was carried out in order to study the interactive effect of light and N on the performance of species of ancient and recent forests.

In addition, an field layer model has been developed of which the main growth processes are based on two compartments: a shoot compartment responsible for the acquisition of carbon (C) through photosynthesis and a root compartment involved in the uptake of N. The relative growth rate of the shoot compared to the growth rate of the root depends on the ratio between the N and C concentration in the pool.

This study showed the importance of the interactive effect of light and N on plant performance. Reduction of the light availability in the young forests will reduce the growth of fast-growing and competitive species while the growth of forest species, given that they already occur in the forest, will hardly be affected. This result stresses the importance of design (e.g. choice of tree species and density of the trees) and management of the tree layer, in controlling the development of the field layer vegetation.

A forest/tree model has been developed of which the main growth processes are based on the CENW model. The model links the flows of carbon (C)), energy, nutrients and water in trees and soil organic matter. Modelled tree growth depends on physiological plant factors, the size of plant pools, such as foliage mass, environmental factors, such as temperature and rainfall, and the total amount and turn-over rates of soil organic matter, which drives mineraliZation of soil organic nitrogen (N). The forest/tree model has been developed as a generic model for coniferous trees. In addition, the model has been extended generically for deciduous trees by shedding of leaves in autumn and including growth buds in which C can be stored during winter time, and from which re-growth is initiated in spring. In spring the initial C gives the leaf activity an augmentation for photosynthesis. For the purpose of the available input parameters in the AFFOREST project it was needed to change the daily time step of the original CENW-model into a fortnightly time step. In relation to this, the original simple hump functions for N, water and temperature dependencies are replaced by smooth minimum/optimum/maximum functions. The model was validated against data obtained for Quercus robur (Oak) from the AFFOREST chronosequence in Vestskoven in Denmark. The different tree stands covered a chronosequence over a period of 35 years. The forest/tree model was successfully able to simulate all aspects of tree growth, which included water, N and C dynamics reflected in biomass production. An associated structure variable, i.e. mean tree height was also simulated successfully, which is an important input parameter for the amount of atmospheric N deposition in forests. The model includes essentially all relevant pools and processes that determine forest growth under a range of natural conditions.

Existing complex mechanistic models require too many input data, which are generally unavailable for application at a regional scale. For the development of a simplified soil model, however, we used existing complex models to start with. This soil model includes all relevant processes in order to simulate the temporal trajectory of carbon (C) sequestrations, nitrate leaching and water recharge. The soil model was derived from the existing models NUCSAM, SMART2 and SMB. For the water balance the existing model WATBAL was used. The principal question was whether the derived simplified soil model is acceptable for use within the AFFOREST project. To test this, the soil model was applied to two afforested oak chronosequences and three spruce chronosequences in Sweden, Denmark and the Netherlands. Validation of the soil and water model was performed by:

• applying the model to the AFFOREST chronosequences and comparing the output on: (i) C sequestration in the soil, (ii) water recharge and (iii) nitrate concentration and nitrate leaching with (a) results from the detailed soil model NUCSAM and (b) measured values in chronosequences

• testing the plausibility of the model behaviour through the evaluation of scenarios on nitrogen (N) deposition levels and temperature for various combinations of soil type and tree species.

Carbon sequestration in the soil is modelled satisfactory but has a tendency to be underestimated. Modelled water recharge fluxes and nitrate leaching fluxes are also in accordance with experimental results reported in literature. In general, we conclude that the soil and water part of the metamodel is an acceptable and even a necessarily alternative for a more complex process oriented model and that it fulfils the necessary requirements for use within the AFFOREST project.

This chapter describes the development of the METAFORE metamodel for the AFFOREST project, focusing on aspects that are important in defining the role of the metamodel in the entire system. Two modes are distinguished: one in which the METAFORE metamodel operates in a batch mode for generating the AFFOREST-sDSS tables for decision support, and another mode of operating with an extended user interface and extended possibilities for evaluating detailed results. The various detailed process-based models are from different sources and each of the institutes had experts on the processes that were modelled. From these models, the individual partners developed meta-descriptions of their parts of the system. The task of the METAFORE metamodel was to combine all this knowledge into a single model executable, and assure a correct calculation of the values needed for the AFFOREST database and the AFFOREST-sDSS. The design of the METAFORE distinguishes a metamodel framework and the model components or submodels. The metamodel framework focuses on the interface and communication between the different submodels and it is responsible for the communication between the submodels. In this design, the submodels are merely servers, waiting to be initialized or called to perform one step of the simulation (i.e. one month or one year of the simulation). To do this, each submodel has only a limited set of exposed methods. Although the detailed process models, as part of their scientific development process, have been extensively validated and calibrated, this does not automatically assure a proper simulation of the processes by the metamodel. During the entire process of the development of METAFORE, the simulation behaviour of the METAFORE modules were constantly tested against the detailed process models. In the end, METAFORE has been developed as a simplification of the detailed models with a lesser demand on data. This means that the detailed behaviour in the process models can at best result in similar but aggregated behaviour in the METAFORE metamodel. It is concluded that the results are satisfactory.

A spatial decision support system (AFFOREST-sDSS) has been built to address ‘Where’, ‘How’, ‘How long’ and ‘What if’ questions related to the environmental performance (EP) of afforestation of agricultural land in north-western Europe at two spatial resolutions. EP is defined in terms of carbon (C) sequestration in biomass and soil, nitrate losses through leaching and groundwater recharge, as determined by 36 possible afforestation management practices. Management practices are a combination of tree species choice, site preparation and thinning regime. Time series of EP are precomputed by means of the metamodel METAFORE and stored in a spatial database in a GIS-environment. The GIS has been upgraded to a AFFOREST-sDSS by incorporation of a Goal Programming tool which allows for optimisation based on the three components of EP. A user-interface was developed to allow non-specialist users making use of the AFFOREST-sDSS. The output of the AFFOREST-sDSS is complementary to and should be used together with other sources of information like empirical evidence, expertise and scientific literature. The AFFOREST-sDSS shows a promising road to the integrated valorisation of knowledge and technical capabilities to better integrate environmental concerns in the planning and management of human interventions in the rural environment.