Modelling nutrient transport from forest ecosystems to surface waters : The model ForSAFE-2D

Sammanfattning: Forests provide multiple products and services which are all are linked to water resources. Trees need water to grow and, at the same time, they change the quality and the quantity of runoff by modifying water and nutrient cycling. The understanding of the interactions between forest and water is fundamental to assess the consequences of natural and anthropogenic pressures, such as climate change and forest management, on the provision of forest products and services.Due to the complexity of ecosystems, models are often used to understand the interactions between different system components under a changing environment. ForSAFE is a dynamic, mechanistic ecosystem model simulating the storage and fluxes of chemical elements in forest ecosystems. It was developed to better understand the effects of environmental changes on the chemistry of forest biomass, soil and soil water at the forest plot level.The first two studies in this thesis are examples of the application of ForSAFE in forest stands in Southern Sweden. The model is used to simulate the effects of anthropogenic and natural disturbances on different ecosystem indicators, including indicators of soil water quality. The studies show that nutrient leaching below the rooting zone is positively related to the nutrient availability at the site, soil disturbances and the amount of organic material left in the forest after tree felling or a storm. Both types of disturbance produce a temporary increase of the acidity of the soil solution, but long-term effects where not predicted by the model. Compared to harvesting, a higher nutrient release in the soil solution can occur after storms due to root lifting causing increased mineralisation, a larger amount of biomass left at the site due to technical and economic constraints and larger canopy openings. In addition, sea-salt episodes can increase the acidity of the soil solution in the first years after the storm. When considering other ecosystem services, trade-offs can exist between the reduction of nutrient loads in the soil solution and the accumulation of carbon in the forest.The conclusions drawn from the application of ForSAFE at the forest plot level are valid for the soil water chemistry in the unsaturated zone. In this thesis, an effort has been made to expand the model simulations from the plot to the hillslope scale to understand how forest ecosystems can affect the chemistry of the streams. A new hydrology concept was integrated in ForSAFE-2D that simulates two-dimensional flows of water and chemical elements from the forest to the stream.ForSAFE-2D allows a better representation of the moisture content by simulating an increasing water saturation level in deeper soil layers and towards the stream. The simulated transport of a tracer along a hillslope shows that the model is capable of capturing the average concentrations of the tracer in the stream. This capability is based on a correct representation of the long-term average runoff and of tracer concentrations in the soil solution.The results also highlight some of the issues that should be addressed by follow-up research studies. The partitioning of water between base flow and peak flows suggests that the simulation of flow paths by ForSAFE-2D should be re-evaluated. A correct representation of flow paths will be crucial when simulating the transport of elements or compounds which change concentration with depth or distance from the stream (e.g. dissolved organic matter). In addition, the effects of saturation on weathering, as well as decomposition, show that the regulation of these processes at increasing moisture contents should be updated. Finally, the process regulating the allocation of carbon and nutrients to foliage should be revised to increase the share of foliage in the tree biomass and thereby correct the simulation of evapotranspiration.