Comparing Black and White Sage in CSUSB’s Preserve: Why Leaf Traits and Temperatures Matter

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Salvia mellifera and Salvia apiana are related shrubs common on the CSUSB reserve, where they are subject to severe summer heat. S. mellifera has visibly smaller and darker leaves than S. apiana, which has larger, light-colored leaves. Because leaf size and color can affect leaf energy budgeting, we examined morphological, optical, and metabolic properties of both species to better understand the role of leaf temperature regulation in these co-occurring species. Crown studies indicate both species are similar in overall canopy size, but S. apiana has greater crown leaf density, and larger leaves. Nevertheless, the crowns of both species absorb a similar proportion of available sunlight. This implies S. apiana orients its leaves at a steeper angle than S. mellifera. Optical reflectance studies indicate S. apiana leaves absorb less direct perpendicular light than S. mellifera. Adaxial leaf trichomes of S. apiana were longer and denser than S. mellifera, explaining differences in leaf reflectance. Differences in light absorption may explain the higher photosynthetic capacity of S. mellifera. Summer leaf temperature data indicate both species operate at approximately the same temperature. Foliar temperature convergence in these two species is likely accomplished through distinct leaf and canopy traits.The smaller, darker, horizontally oriented S. mellifera leaves with minimal trichome coverage presumably shed heat readily through convection and transpiration. Conversely, the large, trichome-dense S. apiana leaves have strong boundary layer resistances to convective and transpirational cooling, though their semi-vertical leaves and high leaf reflectance makes them less subject to radiative heating.