Kumari, N; Saco, PM; Rodriguez, JF; Johnstone, SA; Srivastava, A; Chun, KP; Yetemen, O (2020). The Grass Is Not Always Greener on the Other Side: Seasonal Reversal of Vegetation Greenness in Aspect-Driven Semiarid Ecosystems. GEOPHYSICAL RESEARCH LETTERS, 47(15), e2020GL088918.

Our current understanding of semiarid ecosystems is that they tend to display higher vegetation greenness on polar-facing slopes (PFS) than on equatorial-facing slopes (EFS). However, recent studies have argued that higher vegetation greenness can occur on EFS during part of the year. To assess whether this seasonal reversal of aspect-driven vegetation is a common occurrence, we conducted a global-scale analysis of vegetation greenness on a monthly time scale over an 18-year period (2000-2017). We examined the influence of climate seasonality on the normalized difference vegetation index (NDVI) values of PFS and EFS at 60 different catchments with aspect-controlled vegetation located across all continents except Antarctica. Our results show that an overwhelming majority of sites (70%) display seasonal reversal, associated with transitions from water-limited to energy-limited conditions during wet winters. These findings highlight the need to consider seasonal variations of aspect-driven vegetation patterns in ecohydrology, geomorphology, and Earth system models. Plain Language Summary Sunny (equatorial-facing) slopes receive more solar radiation than shady (polar-facing) slopes. A common assumption in water-limited semiarid ecosystems is that this difference in solar radiation results in shady slopes being greener than sunny slopes, because they lose less water to the atmosphere due to evapotranspiration. Some studies have suggested seasonal changes to this pattern, but the lack of a global-scale analysis has prevented a clear understanding of the extent of this phenomenon and its causes. Here, we used an 18-year record of remotely sensed monthly data to compare vegetation activity on opposing slopes in 60 semiarid catchments with different climates from all over the world. Our results show three different patterns: (1) always greener shady slopes; (2) greener shady slopes in summer but greener sunny slopes in winter; and (3) no discernible difference between slopes. Contrary to the common belief that shady slopes are always greener in semiarid landscapes, the majority of the studied sites show a seasonal reversal of this patterns in vegetation greenness. We attribute this contrasting behavior to the timing of precipitation and different growth responses of vegetation types on opposing slopes. At sites having wet winters, sunny slopes benefit more from solar radiation; hence, their vegetation grows more rapidly than that of shady slopes. These findings underline the importance of considering the seasonal variations of vegetation pattern on opposing slopes in ecohydrological, geomorphological, and Earth system models.