Abstract
Abstract. The endemic argan tree (Argania spinosa) populations in southern Morocco are highly degraded due to overbrowsing, illegal firewood extraction and the expansion of intensive agriculture. Bare areas between the isolated trees increase due to limited regrowth; however, it is unknown if the trees influence the soil of the intertree areas. Hypothetically, spatial differences in soil parameters of the intertree area should result from the translocation of litter or soil particles (by runoff and erosion or wind drift) from canopy-covered areas to the intertree areas. In total, 385 soil samples were taken around the tree from the trunk along the tree drip line (within and outside the tree area) and the intertree area between two trees in four directions (upslope, downslope and in both directions parallel to the slope) up to 50 m distance from the tree. They were analysed for gravimetric soil water content, pH, electrical conductivity, percolation stability, total nitrogen content (TN), content of soil organic carbon (SOC) and C/N ratio. A total of 74 tension disc infiltrometer experiments were performed near the tree drip line, within and outside the tree area, to measure the unsaturated hydraulic conductivity. We found that the tree influence on its surrounding intertree area is limited, with, e.g., SOC and TN content decreasing significantly from tree trunk (4.4 % SOC and 0.3 % TN) to tree drip line (2.0 % SOC and 0.2 % TN). However, intertree areas near the tree drip line (1.3 % SOC and 0.2 % TN) differed significantly from intertree areas between two trees (1.0 % SOC and 0.1 % TN) yet only with a small effect. Trends for spatial patterns could be found in eastern and downslope directions due to wind drift and slope wash. Soil water content was highest in the north due to shade from the midday sun; the influence extended to the intertree areas. The unsaturated hydraulic conductivity also showed significant differences between areas within and outside the tree area near the tree drip line. This was the case on sites under different land usages (silvopastoral and agricultural), slope gradients or tree densities. Although only limited influence of the tree on its intertree area was found, the spatial pattern around the tree suggests that reforestation measures should be aimed around tree shelters in northern or eastern directions with higher soil water content or TN or SOC content to ensure seedling survival, along with measures to prevent overgrazing.
Highlights
The degradation of dryland forests is a major problem, since trees help prevent erosion and desertification (Dregne, 2002; FAO, 2019; Verón et al, 2006)
The averages show that there is a continuous decline of values from T1 along T2 and IT3 to IT4 for the parameters soil water content, EC, percolation stability (PS), total nitrogen content (TN), soil organic carbon (SOC) and C/N
We found higher average suspended sediment concentrations (4.42 g L−1 compared to 2.18 g L−1 under argan trees) and lower average infiltration rates in the intertree areas (229.56 mm h−1 compared to 452.57 mm h−1 under argan trees; Kirchhoff et al, 2019a)
Summary
The degradation of dryland forests is a major problem, since trees help prevent erosion and desertification (Dregne, 2002; FAO, 2019; Verón et al, 2006). Due to a sparser vegetation cover, lower amounts of organic matter and rare but intense rain events, soils in those regions are generally more vulnerable to erosive processes which result from and cause degradation and desertification (Ravi et al, 2010; Vásquez-Méndez et al, 2011). The higher amount of nutrients, especially the higher input of organic matter in soils underneath trees, leads to a higher resilience against erosion and is responsible for the darker insular appearance (Auerswald, 1995; de Boever et al, 2015; Pérez, 2019). Besides the added fertility to the soil, dryland forests are an important source of fodder for grazing livestock, making silvopastoral systems the most characteristic form of land use in drylands (Solorio et al, 2017; Soni et al, 2016)
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