Climate change affects temperate forests particularly by changes in water availability as a result of rising temperatures and changing precipitation dynamics. While the annual mean will remain roughly constant, it is the intensity pattern that will change: light precipitation events decrease and heavy precipitation events increase. Droughts and heat waves are assumed to become more frequent, longer and more intense, also as a feedback mechanism of reduced soil moisture affecting evapotranspiration. In addition to meteorological droughts, edaphic droughts are anticipated to increase in the future. These developments impact the soil hydrological functions with altered infiltration conditions, increased surface runoff and an increasing proportion of preferential flow affecting a more complex and heterogeneous water distribution in the subsurface. Yet the link between tree mortality and the reduced and more heterogenous soil water distribution is still not fully understoodThe majority of approaches analysing soil moisture dynamics are based on point measurements, which do not account for the high spatial variability of soil water. Here, we close this knowledge gap by fusing established point-measurements with geophysical methods to assess the spatio-temporal dynamics of water fluxes in the near-surface subsoil from slope to the root zone scale. The questions we ask focus on how infiltration, subsurface water flow, soil moisture distribution and persistence are affected by (i) the subsurface architecture including textural variations as well as preferential flow paths (macro pores, root tracks) and (ii) hydrological extremes (droughts, rain events).Our study site is located in a beech forest near Ebergötzen (central Germany). The Triassic sandstones are overlain by periglacial slope deposits with varying amounts of loess. The Ebergötzen test site is equipped with numerous sensors for analysing water and element fluxes. In addition to meteorological parameters, we collect 15 min times series of throughfall, stemflow, soil water content, water tension and sap flow. This set-up is ideally suited to quantify water fluxes on a point-by-point basis with high temporal resolution, and to validate complementary, beyond-point approaches. To account for the small-scale variability of processes, geophysical methods with a focus on high-resolution electrical resistivity tomography (Dipole-Dipole, 48 electrodes, 15 cm spacing) were used. Measurements were carried out as a combination of a long-term approach (fortnightly/monthly) and event-based measurements (thunderstorm, round the clock).Our data indicate a relatively uniform decrease in soil moisture during prolonged dry periods, with root-water uptake locally causing higher dynamics. In contrast, subsurface moisture penetration after precipitation events is spatially highly variable, confirming the importance of preferential flow for infiltration and distribution of water in the subsurface and thus show the high demand for spatially high-resolution measurements of soil moisture dynamics.
Pyrogenic carbon (PyC) is produced by the incomplete combustion of biomass. It is chemically inert and nutrient-deficient, making it relatively stable in soils. PyC can thus form an important pool of total soil organic carbon (TOC) for C preservation in soils. Despite its significance, data on the nature, level, and relative contribution of PyC to TOC in tropical urban agroecosystems is largely non-existent. In this study, we aim to determine the content and chemical composition of PyC in urban arable soils of Kumasi, a rapidly expanding city in Ghana, West Africa. PyC is likely enriched in these soils, mainly due to soot deposition from traffic, combined with widespread burning of household waste and use of charcoal for cooking.We sampled topsoils (0–10 cm) from arable fields under four levels of urbanisation intensity (UI), from low to high UI. We employed a range of analytical techniques including visual, chemothermal, thermogravimetric, and biomarker analysis, as well as fourier transformed infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. Visual assessment indicated that ≥80% of all bulk soil samples contained charred macro particles, pointing to PyC enrichment in the urban arable soils. Separating TOC into particulate organic C (POC, ≥63 µm particle size) and mineral-associated organic C (MAOC, <63 µm), chemothermal assessment revealed that PyC contributed less than 0.1% to each fraction under all urban intensity conditions. These PyC levels increased notably along with increasing UI in both TOC fractions. Thereby, median PyC levels in the MAOC fraction (7.8–20.4 mg kg-1) were markedly higher compared to those of the POC fraction (0.1–0.3 mg kg-1). This finding highlight a noticeable PyC contribution to TOC preservations in Kumasi’s tropical urban arable soils, although overall contribution is low. Ongoing thermogravimetric, FTIR spectroscopy, NMR spectroscopy, and biomarker analysis will further detail the amount and chemical composition of PyC in these soils. For instance, we will integrate diagnostic ratios of polycyclic aromatic hydrocarbons with masoccharide anhydrides in order to decouple the relative amount of PyC from biomass and that of fossil fuel.By characterising the chemical nature of PyC with this wide range of analytical techniques, insights into the source and transformation of PyC in a tropical urban agricultural context can be provided. This will lead to better understanding of the role of PyC in the urban soil carbon cycle and its implications for urban sustainability and global C sequestration efforts.
Anthropogenic climate change increases the risk of forest fire following drought periods in temperate forests of Central Europe. Areas with an increased proportion of standing deadwood are often considered to be at risk. Especially in national parks, deadwood is not removed, forming an essential part of the local ecosystem. In the Harz National Park, we aim at a comprehensive impact assessment following a fire in a spruce forest that was already disturbed after a massive bark beetle infection to understand deadwood breakdown, vegetation succession, surface erosion and changes in soil properties. The Quesenbank fire of August 2022 burned an area of approx. 13 ha within four days. We scanned 10 ha of burned compared to unburned areas using unoccupied aerial vehicles (UAVs) equipped with multispectral, thermal, high-resolution RGB and light-detection and ranging (LiDAR) sensors. Derived orthoimages, 3D point clouds and canopy height models (CHM) are employed to estimate standing deadwood, fractional cover and succession indicators, thermal ground regime alterations and small-scale morphological changes. To capture the gradual breakdown of deadwood, we collected ground truth on vegetation biophysical parameters, such as fractional cover, plant area index (PAI) and fraction of absorbed photosynthetically active radiation (FAPAR) from upward-directed digital hemispherical photos. The surveys were conducted 2, 9, 11 and 12 months post-fire together with the UAV campaigns in diffuse or near-dusk light conditions. The analysis of the digital CHM and ground models reveal a decline in the detection rate of tree crowns (tree height ≥ 2 m) by 15 %, crown area by 74 %, and a corresponding loss of surface material affecting at least 0.9 ha between October 2022 and October 2023, respectively. The ground reference data confirmed considerably lower fractional cover on burned areas. PAI and FAPAR in burned standing deadwood was lower in unburned stands, altering light, soil moisture and temperature regimes. This is reflected in the occurrence of typical post-fire and light-demanding species such as Epilobium spec. on burned areas, though in lower coverage compared to an unburned, logged site. As the variation in reference data was relatively low over the observation period, we suggest that the main dynamics of the breakdown of standing deadwood had already happened several weeks after the fire. Interestingly, we found a very heterogeneous microtopography due to granite boulders, with subsurface tunneling and unstable ground, influencing post-fire recovery. Upcoming analysis will include analyses of fire-influenced soil properties, morphodynamics and biogeochemical cycling in a region that still shows traces of past land use associated with the mining history of the Harz. We acknowledge the collaboration with the Harz National Park Authority. A preliminary data set from two months after the fire can be accessed via Zenodo: Jackisch, R., Putzenlechner, B., & Dietze, E. (2023). UAV data of post fire dynamics, Quesenbank, Harz, 2022 (orthomosaics, topography, point clouds) (1.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.7554598
Abstract. Spatiotemporal modeling is an innovative way of predicting soil moisture and has promising applications that support sustainable forest operations. One such application is the prediction of rutting, since rutting can cause severe damage to forest soils and ecological functions. In this work, we used ERA5-Land soil moisture retrievals and several topographic indices to model variations in the in situ soil water content by means of a random forest model. We then correlated the predicted soil moisture with rut depth from different trials. Our spatiotemporal modeling approach successfully predicted soil moisture with Kendall's rank correlation coefficient of 0.62 (R2 of 64 %). The final model included the spatial depth-to-water index, topographic wetness index, stream power index, as well as temporal components such as month and season, and ERA5-Land soil moisture retrievals. These retrievals were shown to be the most important predictor in the model, indicating a large temporal variation. The prediction of rut depth was also successful, resulting in Kendall's correlation coefficient of 0.61. Our results demonstrate that by using data from several sources, we can accurately predict soil moisture and use this information to predict rut depth. This has practical applications in reducing the impact of heavy machinery on forest soils and avoiding wet areas during forest operations.
Little is known about how soil properties control tree growth at its upper limit. This paper reviews the state of knowledge and discusses the results specifically related to ecozones, to the scale-dependent importance of single factors, and to new findings from a near-natural treeline ecotone in Rolwaling Himal, Nepal. This paper identifies gaps in literature and shows where new research is needed, both conceptual and geographical. The review shows that at a global scale and throughout diverse ecozones, growing season soil temperature is considered a key factor for tree growth. Soil temperatures differ greatly at a local scale, and are mainly determined by local climatic, edaphic, and topographic conditions. Our result of 7.6 ± 0.6°C for growing season mean soil temperature at treeline in Rolwaling is 1.2 K higher compared to the postulated 6.4 ± 0.7°C for alpine treelines. We suggest a broadening of the ±0.7°C error term to cover the wide range at a local scale. The role of major soil nutrients and soil moisture for treeline shift has been underestimated by far. In Rolwaling, significantly decreasing nutrient availability (N, K, Mg) in soils and foliage with elevation might explain why treeline shift and global warming are decoupled. Further, soil moisture deficits early in the year impede seedling and sapling establishment, which could be an important mechanism that controls treeline position. These findings question previous results which argue that alpine treelines are unaffected by soil nutrient availability and soil moisture. We assume that specific combinations of soil properties as well as single soil properties limit tree growth even below climatic treelines.
