The integration of trees within agricultural systems, known as agroforestry, provides an effective means of improving water retention and nutrient cycling on farmland. Studying an established agroforestry system (AFS) combining Italian alder (Alnus cordata) as a linear windbreak with a blackberry (Rubus fruticosus) crop, we explore the water use dynamics and soil properties influenced by the tree component. This interdisciplinary investigation, conducted on an active berry farm in South Africa, utilized a range of hydrological, pedological, and dendrological measurements to elucidate the effects of the windbreak on the surrounding microclimate, water balance, and nutrient distribution.
Hydrological Phenomena
Water Cycle Processes
The windbreak played a significant role in shaping the local water dynamics through various pathways. Precipitation interception by the tree canopy reduced the overall water input to the soil by approximately 3.5% during the study period. However, the windbreak’s disruption of the wind field led to a potential 15% increase in precipitation on the leeward side of the trees. These spatially variable effects on precipitation highlight the need to consider the windbreak’s influence on the local microclimate when assessing water fluxes in an AFS.
Soil infiltration was generally favorable, with hydraulic conductivity values exceeding the maximum observed precipitation intensities at both the windbreak and blackberry locations. This indicated that the majority of precipitation was able to percolate into the soil rather than generating substantial surface runoff. Nevertheless, we did observe instances where the soil water storage changes exceeded the precipitation input, suggesting the occurrence of either surface runoff or lateral subsurface flow. The heterogeneous soil surface conditions, with compacted areas between the blackberry rows and flatter, less dense soil within the rows, likely contributed to this spatial variability in water movement processes.
Soil Hydrology
The soil profile exhibited a relatively high porosity and air capacity, allowing a significant portion of the infiltrated water to drain from the topsoil rather than being stored. This was beneficial, as the deeper soil layers were protected from evaporation and could serve as a water source for the plants’ roots. Interestingly, the plant-available water (PAW) was higher in the topsoil near the windbreak compared to the blackberry crop, indicating the trees’ potential to enhance the soil’s water retention capacity.
Continuous monitoring of soil moisture and matric potential revealed that the topsoil was consistently drier than the deeper soil layers, likely due to a combination of evaporation and plant water uptake. The permanent wilting point was frequently reached at the windbreak location, suggesting that the trees utilized more water than the blackberry crop. However, the overall soil water status did not indicate severe water stress, likely due to the supplementary irrigation applied to the system.
Urban Hydrology
The windbreak’s influence on the local microclimate was evident in the shading effects it exerted on the surrounding area. Simulations showed that the windbreak could reduce incoming solar radiation by up to 75% in its immediate vicinity on a sunny day. This shading effect could lead to a decrease in potential evapotranspiration (PET) of up to 54% in the affected areas, potentially reducing the water demand for the blackberry crop.
The combination of reduced wind speeds and decreased solar radiation due to the windbreak’s presence suggests that the AFS experienced a shift from a water-limited to an energy-limited system, with the actual evapotranspiration (AET) approaching the PET. This finding highlights the potential of strategically placed windbreaks to enhance the water use efficiency of agricultural systems in water-scarce regions.
Pedological Factors
Soil Composition
The soil at the study site was classified as a Cambic Mollic Umbrisol, characterized by a loamic texture throughout the profile and a high base saturation in the upper horizon. The topsoil exhibited a moderate bulk density, with organic matter content decreasing with depth.
Spatial analysis of the topsoil samples revealed distinct patterns in carbon and nitrogen content, with the highest values observed within the windbreak rows and decreasing with increasing distance. This suggests that the alder trees contributed to the accumulation of organic matter and nutrients in the adjacent soil, likely through litter fall, root exudation, and the trees’ nitrogen-fixing capacity.
Soil Structure
The soil hydraulic properties varied between the windbreak and blackberry locations. The topsoil near the windbreak had a higher porosity and water content at field capacity, resulting in a greater PAW compared to the blackberry crop area. Conversely, the hydraulic conductivity was nearly three times higher in the blackberry topsoil, indicating a more favorable infiltration capacity in that location.
These differences in soil physical properties may be attributed to the influence of the windbreak, which could have led to the deposition of finer soil particles through erosion processes, resulting in a denser topsoil near the trees. Alternatively, the accumulation of organic matter around the windbreak may have enhanced soil aggregation and water retention capacity.
Soil Biogeochemistry
The distribution of carbon and nitrogen in the topsoil suggested the occurrence of nutrient translocation and enrichment around the windbreak. This could be the result of a combination of surface runoff, which transported fine soil particles and associated nutrients downslope, and the trees’ own contribution of organic matter and nitrogen fixation.
The observed patterns in soil nutrient concentrations highlight the potential of strategically placed windbreaks to enhance the overall soil fertility and ecosystem productivity within an AFS. By acting as a sink for nutrients and organic matter, the windbreak could potentially improve the long-term sustainability of the agricultural system.
Urban Agroforestry Practices
Land Use Patterns
Agroforestry systems, such as the one studied, represent a purposeful integration of trees and crops within the same land management unit. This deliberate arrangement aims to optimize the use of available resources, including water, nutrients, and space, while providing a range of ecosystem services.
The linear windbreak configuration adopted in this AFS is a common practice in many agricultural landscapes, as it can effectively reduce wind speeds and modify the local microclimate. By strategically placing the windbreak within the blackberry crop, the farmers were able to harness the protective and regulative functions of the trees to potentially enhance the productivity and resilience of the overall system.
Crop-Tree Interactions
The interactions between the blackberry crop and the Italian alder windbreak were multifaceted, with both positive and negative effects observed. On the one hand, the windbreak’s influence on the local water balance, nutrient cycling, and microclimate appeared to be beneficial for the crop’s growth and yield. The trees’ capacity to improve soil properties and water use efficiency could contribute to the long-term sustainability of the AFS.
However, the windbreak also had the potential to compete with the crop for resources, such as water and light. The higher water demand of the alder trees, as evidenced by the frequent reaching of the permanent wilting point in the topsoil near the windbreak, highlighted the need for careful management to balance the water requirements of the different components within the AFS.
Ecosystem Services
Beyond the direct impacts on crop productivity, the incorporation of the Italian alder windbreak in this urban AFS provided additional ecosystem services. The trees’ ability to fix atmospheric nitrogen and contribute to the accumulation of organic matter in the soil can enhance the overall fertility and long-term resilience of the system.
Furthermore, the structural diversity introduced by the windbreak is likely to have a positive influence on the local biodiversity, providing habitat and resources for a range of organisms. This increased biodiversity can, in turn, support beneficial ecosystem processes, such as integrated pest management and pollination services, further contributing to the sustainability of the AFS.
Environmental Impacts
Microclimate Regulation
The windbreak’s influence on the local microclimate was a key factor in shaping the water dynamics and energy balance within the AFS. By reducing wind speeds and intercepting solar radiation, the trees moderated the environmental conditions experienced by the blackberry crop, potentially mitigating the effects of drought and extreme temperatures.
The simulated reduction in potential evapotranspiration due to the windbreak’s shading effect highlighted the trees’ ability to optimize the water use efficiency of the system. This finding is particularly relevant in water-scarce regions, where the strategic placement of windbreaks can be a valuable climate change adaptation strategy for agricultural systems.
Nutrient Cycling
The spatial patterns observed in soil carbon and nitrogen concentrations around the windbreak suggested that the trees played a significant role in the translocation and accumulation of nutrients within the AFS. This effect was likely driven by a combination of factors, including the trees’ nitrogen-fixing capacity, the deposition of organic matter through litter fall, and the redistribution of fine soil particles and associated nutrients via surface runoff and erosion processes.
By acting as a nutrient sink and enhancing the overall soil fertility, the windbreak contributed to the long-term productivity and sustainability of the AFS. This nutrient cycling function is particularly valuable in urban and peri-urban agricultural systems, where nutrient depletion and land degradation can be pressing challenges.
Biodiversity Conservation
The structural diversity introduced by the windbreak within the predominantly monoculture blackberry crop likely had a positive impact on the local biodiversity. The trees provided additional habitat and resources for a range of organisms, including pollinators, natural enemies of crop pests, and other beneficial species.
This increase in biodiversity can contribute to the overall resilience of the AFS, as it supports key ecosystem services such as pollination, biological pest control, and nutrient cycling. The conservation of biodiversity within urban and peri-urban agricultural landscapes is crucial for maintaining the long-term sustainability and multifunctionality of these systems.
In conclusion, the investigation of this Italian alder-blackberry AFS in South Africa highlighted the multifaceted effects of incorporating trees within agricultural systems. The windbreak’s influence on the local water cycle, soil properties, nutrient dynamics, and microclimate demonstrated the potential of strategically placed agroforestry components to enhance the sustainability and resilience of urban and peri-urban agricultural landscapes. These insights can inform the design and management of similar agroforestry systems in TriCounty Tree Care’s service area and beyond, contributing to the development of climate-smart and ecologically robust land use practices.