Albedo, the proportion of solar radiation that is reflected from a surface back to the atmosphere, strongly influences surface temperatures. Understanding how albedo changes with surface type, such as the shift from summer to winter crops in agricultural ecosystems, is important for ecosystem modelling and climate projections. Previous modeling studies simulate winter crop albedo using summer plant traits, which may bias predictions. Here, we quantified the differences between summer and winter crop traits and examined the resulting changes in albedo. Specifically, we measured the albedo of two summer crops, Gossypium hirsutum L. and Sorghum bicolor, and three winter crops, Secale cereale, Triticum aestivum, and xTriticosecale [Secale × Triticum], on farms in the Lubbock, TX area, alongside a variety of leaf traits, including foliar chlorophyll content, leaf density, and leaf area index. We hypothesized that leaf area index would be negatively correlated with albedo because a decreasing leaf area index results in an increasing percentage of exposed soil. We hypothesized that winter crops would have a higher albedo than summer crops because lower winter water and light availability would decrease foliar chlorophyll, which determines reflectance in the visible spectrum, and increase leaf density, which is a possible indicator of a change in near-infrared reflectance.
Results/Conclusions
We found that plot albedo (including both plants and bare soil) is significantly higher in the winter than the summer, while soil albedo is significantly lower in the winter, indicating that plant albedo is the cause of the higher winter plot albedo. Indeed, leaf area index was a strong predictor of albedo. Chlorophyll was lower in winter crops than summer crops and was significantly correlated with albedo, suggesting that winter plants are reflecting more visible light due to lower levels of leaf chlorophyll. Leaf density was higher in winter crops than summer, and while not as strong a predictor of albedo as chlorophyll, was also significantly correlated with albedo, indicating a seasonal change in near-infrared reflectance. These results provide some of the first indications of the mechanisms that drive differences in winter and summer crop albedo. Specifically, the results indicate that the differences are due to changes in the leaf traits that control plant albedo. As cover cropping is becoming a more prevalent practice and is being included in earth system models, our results provide an avenue for reliably predicting seasonal changes in albedo in cover cropping systems.