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"Silvoarable agroforestry systems in temperate regions: impact of tree rows on crops, soil and biodiversity"

The 12th of December, paul [dot] pardon [at] ilvo [dot] vlaanderen [dot] be (Paul Pardon) defended his PhD entitled “Silvoarable agroforestry systems in temperate regions: impact of tree rows on crops, soil and biodiversity”. Promotors of the PhD are Prof. dr. ir. Kris Verheyen, Prof. Dirk Reheul, Prof. Dr. ir. Jan Mertens (Ghent University) and Dr. ir. Bert Reubens (Flanders Research Institute for Agriculture, Fisheries and Food).

In temperate regions, interest in agroforestry has been growing for 20 years and several recent reviews have indicated the potential beneficial effects of agroforestry such as carbon sequestration, protection of (ground)water quality through reduction of nitrogen leaching, mitigation of soil erosion, enhanced biomass production and biodiversity conservation. Implementation of agroforestry remains, however, rather limited in large parts of temperate Europe. Besides uncertainties on the legislative and economic level, this is assumed to result from a lack of actual quantification of the potential effects on ecosystem services and biodiversity.

Therefore, the main goal of this PhD research was to analyze and quantify the effect of tree presence on a subset of provisioning, regulating and supporting ecosystem services and biodiversity aspects in a set of silvoarable fields in Belgium. Thereby, the focus was on alley cropping systems because this type of agroforestry system can efficiently be combined with the use of modern farming techniques and machinery for the cultivation of agricultural crops in the intercropping zone between the tree strips. The effect of tree row presence on SOC, pH and soil nutrient conditions (total N, P, K, Mg, Na, Ca) was assessed in the plough layer of the experimental fields.

Although no effects were observed near the young tree rows, significantly increased SOC and soil nutrient concentrations were observed near the boundary planted tree rows. The noted increase of these soil variables was strongly related to the distance from the tree row, resulting in considerable spatial gradients. In addition, the magnitude of the observed effects differed according to tree species and the increase in SOC, total N, K, and Na near the poplar rows was related to the size of the trees, indicating a continuous evolution in SOC and soil nutrient status of the agroforestry system as trees mature.

The main causal factor was assumed to be the input of carbon and nutrients in the top soil layer through tree litter, in particular tree leaves, and to a lesser extent via nutrient enriched throughfall water and a potentially reduced nutrient uptake of the arable crops. Next, the effect of tree row presence on the activity-density and diversity of two types of macro-detrivorous arthropods (woodlice and millipedes) and two types of predatory arthropods (carabids and rove beetles) was assessed. Strongly increased activity-density values and diversity of the macro-detritivoreous arthropods were detected in silvoarable fields. Near the (larger) boundary planted trees, these effects extended into the neighboring arable zone.

Contrasting gradients in abundance were observed for carabids and rove beetles, with increased activity-density values in the arable zone. Finally, the influence of tree rows on yield and quality of key western European arable crops was quantified. Tree size, crop type and distance to the trees were main determinants of the crop yield and quality in the silvoarable fields. While effects on crop yield were limited for all crops near young tree rows, substantial yield reductions were observed near mature trees, in particular for maize and potato. Effects on crop quality were limited for all crops under study, with substantial effects only arising near the oldest tree rows.

To optimize the provisioning service of agroforestry systems, the cultivation of winter cereals may be advisable over maize and potato towards the end of the lifecycle of the tree component. In addition, poplar trees should be harvested when they reach their target diameter for industrial processing. If tree rows are preserved for the delivery of other ecosystem services, however, substantial impacts on crop yield and quality should be taken into account.

This PhD-research was conducted within the framework of the project “Agroforestry in Vlaanderen” and funded by the Flemish Agency for Innovation & Entrepreneurship (Vlaio).