Transpiration
Desert plants and conifers have adaptations which reduce water loss.  Examples are: thick cuticles, reduced leaf areas, sunken stomata and hairs. All these reduce transpiration and conserve water. Many cacti do photosynthesis in succulent stems, rather than leaves. The surface area of even a fat stem is far less that the total surface of leaves in a tree. Also, stomata of desert plants are usually closed during the day and open at night, when transpiration is lower.