Dissertation: Street tree plantings provide ecosystem services (21.11.2016)
In urban forestry, the assessment and valuation of ecosystem services provided by urban trees are increasingly important both for the rationale of planting new trees and for retaining and managing existing tree populations. To support the field of practical urban forestry, research is needed on the net effects of ecosystem services and costs. The aim of this thesis was to analyse the ecosystem service potential of young street tree plantings. To this end, transplanting recovery, tree growth and carbon and water exchange were studied on two case study streets, one planted with Tilia × vulgaris Hayne and the other with Alnus glutinosa (L.) Gaertn. f. pyramidalis Sakari . The relationships between tree growth, tree and soil water and carbon exchange, environmental variables and tree properties were examined.
Transplanting recovery of Tilia trees was delayed due to excess soil water, while Alnus trees recovered within the first few years. Alnus shoot growth responded positively and Tilia negatively to an increase in soil water content. Branch leaf area in relation to branch basal area varied, showing effects of transplanting and subsequent adaptation of the trees to the new growing sites. The studied trees accumulated carbon in their woody biomass during the first decade after transplanting, but the sequestration was small relative to carbon loss from the man-made tree soils. Several additional decades of tree growth were estimated to be needed to attain net carbon sequestration in these street tree plantings if peat originating C and/or renewable C lost from tree soils was counted as C loss. Biomass equations developed in traditional forests predicted total aboveground street tree biomass fairly well, but performed unsatisfactorily in estimating specific aboveground biomass compartments. The biomass distribution and litter production of street trees also require further study to gain insights into the role of tree litter in urban biogeochemical cycles.
The annual variation in tree water use of the studied trees was high, but within one year, a Penman-Monteith-based evapotranspiration model with added stomatal conductance and leaf area dynamics description, together with soil water status, explained the variation in tree transpiration quite well. Using a single parameterization over all four years examined did not produce reliable tree water use estimates however. Scaling tree transpiration to different canopy cover percentages implied that especially the columnar Alnus trees could transpire a considerable proportion of annual rainfall with attainable canopy cover, potentially contributing to stormwater management.