Significance of soil water availability for growth and vitality of recently planted urban street trees

Abstract

Growth conditions at urban street-tree sites can be unfavorable and in northern Europe temperate climates tree vitality will be increasingly threatened by water scarcity due to a changing climate. Nursery grown and recently planted trees in urban areas have a barely developed root system. Therefore, particularly in the first years after planting, young trees along streets might suffer from limited soil water availability and poor site conditions. As soil water is mainly determining tree growth, vitality and mortality, knowledge about root zone soil-water dynamics, hydrological properties of artificial urban planting soils and water stress responses of different urban tree species is crucial, but still lacking. Therefore, this thesis aims to answer to what extent recently planted street-trees were exposed to critical soil water availability and which external parameters are most important for soil drought to occur. Further, investigations were conducted to test if technically mixed structural soil and artificial urban soil were suitable as urban planting media in terms of soil hydrological properties and growth responses of different urban tree species. Finally, research addressed on how and to which extent recently planted street trees respond physiologically to limited soil water availability and if species specific water stress can be explained by growth and vitality parameters. To answer the research questions, three different study approaches were applied and structured in three individual manuscripts. The studies field data were thereby collected on basis of two independent experimental approaches that differ strongly in terms of extent, location and study design: (i) A long-term in situ soil water monitoring in the urban environment and (ii) a large-scale experimental field study in a complete block design with different soil substrates, representative for young urban street tree sites, and commonly used urban street tree species. First, a soil water monitoring network containing 17 young urban street-tree sites in the city of Hamburg, Germany was established. Over four years (2016–2019) the soil water potential and the soil temperature were measured under in situ conditions. Based on a plausible soil water potential (SWP) threshold derived from literature, beyond which tree growth and vitality is likely to be critically affected (SWP; < -1200 hPa), the extent of SWP below was quantified for the soil compartments root ball, planting pit, and surrounding urban soil. During 2018 and 2019, average critical soil water availability in the root ball and planting pit occurred between three to five months per year. Thus, the trees were exposed prolonged periods of potential water stress for two consecutive years. To assess the tree site sensitivity towards meteorological variables, tree- and site characteristics, a data driven randomForest model was adapted and trained by the quantified critical soil water availability data. After planting, critical soil water availability increasingly shifted year wise from the root ball into the entire planting pit as a consequence of root development and increasing water demand of the trees. Considering less usable water within the surrounding sandy soils, indicated by lower water stress intensity, soil water in the planting pit may be depleted earlier and more rapidly with increasing tree age. This finding can likewise be interpreted from the randomForest modelling result that identified tree age as an important predictor. However, long-term (10-day) precipitation was the most important variable predicting the occurrence of critical soil water availability, suggesting a further extension of periods with critical soil water availability as rainy summer days are projected to decrease with climate change. Additionally, soil temperature was identified being a more important predictor than air temperature as it reflects site specific characteristics affecting water- an energy balance. Water infiltration capacity and soil sealing were of medium importance for the occurrence of critical soil water availability in planting pits and urban soils. However, the influence of soil properties was not pronounced in this model approach, likely due to the separated consideration of the soil compartments, their low variability in composition and the thus represented very narrow range of data. The plausible conclusions drawn from the analysis demonstrated the capability of a data-driven method to predict the occurrence of critical soil water availability with high accuracy. In the second study approach, a large-scale experimental field study was established to investigate the response of nine tree species (135 individuals in total) to two common urban planting soils (‘Sand’ and ‘FLL’) and a ‘Loamy Silt’ reference over a three-year period. Therefore, soil hydrological parameters were determined and tree growth and -vitality were measured and monitored regularly. The results revealed for the sandy-textured urban planting soils low plant available water capacities of 6% v/v in the ‘Sand’ and 10% v/v in the ‘FLL’ and also a low unsaturated hydraulic conductivity, that likely limits water resupply to the roots during phases of high demand. Tree growth was restricted in both urban planting soils, but evidently stronger in the ’Sand’ than in ‘FLL’ compared to the reference soil. However, for single tree species different patterns have been identified: e.g. Ostrya carpinifolia with least growth restrictions in ‘FLL’, Liquidambar styraciflua with restricted growth but simultaneously high vitality in ‘Sand’, Amelanchier lamarkii that was weakest in ‘Sand’ and vigorous in ‘FLL’ and Koelreuteria paniculata with no differences in growth between the substrates and however, overall poor vitality. On average, tree growth was most limited in ‘Sand’ (-64% compared to ‘Loamy Silt’) with strongest effect for Quercus palustris and Amelanchier lamarkii. The results suggest that commonly used urban street tree species react differently to water limitations and thus have both, different genetic fixed and actually developed strategies to cope with soil drought. I hypothesize, that tree species which invest resources in fine root growth and exploration to extract water from dry soils with low conductivities might be more successful in urban roadside settings than trees that are only able to lower their root water potential. Thus, as a consequence of low water availability, the use of unfavorable planting soils can cause severe, species-specific growth deficits reflecting limited above-ground carbon uptake or a redistribution of assimilates to plant organs of higher demand (i.e. roots). In the third study approach I intended to investigate the impact of decreased soil water availability on stomatal limitation of assimilation to quantitatively estimate the stress incidences for recently planted urban tree species. Therefore, SWP data of 14 of the 17 in situ soil water monitoring sites in the City of Hamburg, Germany were used over a four years record (2018 – 2021). Thereby, stomatal conductivity (gS) was measured over a two years period. Recorded maximum gS was strongly different in specific species and site combinations. Decreasing SWP significantly decreased gS of most species with differently shaped correlation functions. The individual species x site combination gS response on SWP was highly sensitive to the soil compartment. Adapting a widely used photosynthesis model considering measured stomatal and literature based enzyme kinetic parameters, revealed their high relevance for assimilation. Furthermore, the stomatal limited assimilation depends again strongly on the interplay of the temperature dependent assimilation and the modelled range of stomatal conductivity. Species with lower stomatal regulative ranges and medium ranged values of enzyme kinetic parameters were limited in carbon assimilation due to stomatal regulation earlier and already at low temperatures. Species with higher stomatal regulative ranges were less limited in assimilation and lead to a lower stress incidence during the four years record compared to the species of lower stomatal regulative ranges. Therefore, and as the results correlate significantly with parameters such as isotopic signature (δ13C) and relative growth, the generic assumption that broadly ranged stomatal regulation be coupled with higher incidences of stomatal limitation of assimilation can be questioned. Particularly δ13C data, comparatively easy to gather, has proven to approximate these stress incidences and thus might be an applicable parameter for future investigations at urban tree sites. Based on a set of seven tree species this study approach showed complex interactions of soil water availability and physiological responses, however only valid for the respective sites. To assess future favored plant selection, follow-up studies need to further clarify the benefits of identified patterns (e.g., stomatal regulatory width, temperature sensitivity of assimilation) for urban cooling or death avoidance under increasing climate extremes and at sites with varying soil conditions. Taken together, in addition to protecting and preserving mature, established trees , the results of this study clearly indicate the need of adequate soil conditions at yet to be planted tree sites to increase their water supply and thus the trees resilience and longevity towards climate extremes in an even denser urban environment than their predecessors faced.