Interplay between topography, fog and vegetation in the central South Arabian mountains revealed using a novel Landsat fog detection technique

Abstract

In the central South Arabian mountains of Yemen and Oman, monsoon fog interception by the endemic cloud forest is essential for ecosystem functions and services. Yet, we know little about the local factors affecting fog distributions and their cumulative effects on vegetation. To examine these relationships, we developed a novel method of high-resolution fog detection using Landsat data, and validated the results using occurrence records of eight moisturesensitive plant species. Regression tree analysis was then used to examine the topographic factors influencing fog distributions and the topoclimatic factors influencing satellite-derived vegetation greenness. We find that the interplay between the complex mountain topography and the incoming fog results in heterogeneous fog densities. Specifically, fog accumulates against steep windward slopes and landforms, resulting in hotspots of fog interception, while lower fog densities occur in leeward locations. We also find that fog distributions correlate with patterns of vegetation greenness, and overall, that greenness increases with fog density. The layer of fog density describes patterns of vegetation greenness more accurately than topographic variables alone, and thus, we propose that regional vegetation patterns more closely follow a fog gradient, than an altitudinal gradient as previously supposed. The layer of fog density will enable an improved understanding of how species and communities, many of which are endemic, range-restricted and in decline, respond to local variability in topoclimatic conditions.

Vegetation and cloud cover shape semi‐arid carbonate landform development

“…The eastern zone shows 10% decrease in cloud observations compared with the central and western zones. The percentage of time with cloud cover computed here is highly correlated ( R 2 = 0.84, Figure S2) to the fog density computed by Ball and Tzanopoulos (2020) from Landsat images, therefore the two parameters can be considered equivalent.…”

“…Relatively verdant vegetation is present in the wet monsoon months, which is in striking contrast to the nearly unvegetated surrounding terrain. The vegetated areas are covered by a drought deciduous cloud forest community known as the Hybantho durae‐Anogeissetum dhofaricae association, and a grassland with perennial shrubs that survive after the monsoon season (Ball & Tzanopoulos, 2020; Friesen et al, 2018; Hildebrandt & Eltahir, 2006). The fog intercepted by the trees is more than 250% of the rainfall (Ball & Tzanopoulos, 2020; Friesen et al, 2018).…”

“…The vegetated areas are covered by a drought deciduous cloud forest community known as the Hybantho durae‐Anogeissetum dhofaricae association, and a grassland with perennial shrubs that survive after the monsoon season (Ball & Tzanopoulos, 2020; Friesen et al, 2018; Hildebrandt & Eltahir, 2006). The fog intercepted by the trees is more than 250% of the rainfall (Ball & Tzanopoulos, 2020; Friesen et al, 2018). While shrubs intercept only 30–45% of the available water (Hildebrandt et al, 2007a, 2007b).…”

“…In this area, water is present as rainfall and fog—also known ‘horizontal precipitation’—during the monsoon season (Friesen et al, 2018), but only in the southern part of the range facing the Arabian Sea (Figure 1). In arid environments, growth of dense vegetation is limited by water availability, and the vegetation responds to fog in a dynamic way (Ball & Tzanopoulos, 2020; Hildebrandt et al, 2007a, 2007b). In situ measurements show that fog distribution can affect vegetation patterns (Ball & Tzanopoulos, 2020; Hildebrandt & Eltahir, 2006; Scholte & De Geest, 2010).…”

Comparison of Machine Learning Methods for Mapping the Stand Characteristics of Temperate Forests Using Multi-Spectral Sentinel-2 Data

“…The topographic factors such as elevation, slope, and topographic solar-radiation index (TRASP) were computed based on the 12.5 m DEM ( Table 3). The TRASP was derived from the elevation based on the aspect map and the values of 0 and 1 were used to indicate the cool, north-facing slope and the hot and dry, south-facing slope, respectively [45,46]. All topographic variables resampled to 10m based on Sentinel-2A data.…”