Thermal ecology of an embedded dwarf succulent from southern Africa (Lithops spp: Mesembryanthemaceae)

The 'stone plants' of southern Africa (Lithops spp.) live embedded in the hot surface layers of the soil: a 'window' for admitting light into the leaf is the only part of the plant exposed to the air. Many of the common mechanisms for keeping the leaves of desert plants cool are therefore ineffective for leaf temperature control in these plants. Model simulations and experimental measurements in the field indicate that leaf temperatures of Lithops are governed by three principles: (1) leaf and soil temperatures are strongly coupled; (2) variations of the surface energy budgets of the leaves alone have little effect on leaf temperature; (3) variation of window clarity causes significant variation of leaf temperature. Among the implications of these principles for the thermal ecology of Lithops and other embedded dwarf succulents are: (1) thermally coupling the plant and soil combines the plant's thermal capacity with the soil's and reduces daily variation of leaf temperature; (2) the steep vertical heterogeneity of temperature that is typical in soil keeps the deeper areas of the plant (containing the chlorenchyma) cool relative to the hotter surface regions of the plant during the hottest periods of the day; (3) variation of leaf color has little effect on leaf temperature, so that leaf color may be selected primarily for reasons other than for temperature adaptation; (4) variation of window clarity is probably the sole thermal adaptation to hot conditions that embedded dwarf succulents can employ.