Tell them what you’re going to tell them: Spanish oaks are decreasing in numbers aroundAustin. Why does this matter?
In and around greater Austin, Spanish oaks can be found growing, especially on rocky slopes and in greenbelt areas. These trees grow in their natural niche among the ash junipers (cedars), native Texas ash trees, occasional live oaks and cedar elms.
What’s especially interesting about these greenbelt areas is that they are, as is typical in all forested areas, extremely dense when unbothered. This state is important to the very survival of the Spanish oak’s efforts to grow on poor, often steep, rocky limestone slopes. Due to erosion and high limestone content, the soils in these areas are extremely shallow causing Spanish oak roots to grow in next to no soil.
Central Texas summers are extremely harsh with us experiencing little to no rainfall and often months of temperatures over 100 degrees. As the Spanish oak is fairly drought resistant, the leaves are still quite thin with a fair amount of surface area. The waxy cuticle over the leaf that helps prevent sun scald (sunburn) and water loss is comparatively thin on the Spanish oak leaf compared to other species. Severely drought stressed Spanish oaks get bacterial leaf scorch, the technical term for sunburn, on the leaves, leading to defoliation and death. As we consider heat and drought conditions, it is also important to note how porous the limestone is.
So why does this matter? Well, limestone, unless non-porous, which is fairly rare around here (see perched water tables), generally does not retain water. Instead, water permeates through the stone when it rains and whatever does not flow down slopes and off ridges and spurs, where the Spanish oaks are growing, will permeate right through the soil, providing no water retention. During the hottest summer months, by the day after a rain, the soil will often be bone dry the day after a rain, especially in 100+ degree dry heat. Mulch formed by leaf litter and shedding plant parts, which helps ensure healthy roots and soils formation, is often either partially or completely washed away through erosion, further eliminating the possibility of water retention.
So how do these trees survive? The short answer is symbiosis. Symbiosis occurs when one plant or animal species benefits another. In the harsh rocky slopes of the Austin greenbelts, Spanish oaks are found in the summer on the ‘edge’ or around the outside of the forested areas, where the natural forested areas end due to neighborhoods, developments, and cleared areas that are dying and defoliating. Why? The answer is actually very simple.
In the greenbelts, the trees grow very close together. Although tree and plant roots cross over and compete with each other in very little soil, it is important to note how the densely packed canopies cool the root zones. Evapotranspiration is the technical term for water being pulled through the vascular system and out the holes in the undersides of the leaves (stomates). In simple terms, these stomates open to exchange gasses and to allow a certain amount of water loss to cool the leaves. In very hot conditions, they close to stop this water loss.
When tree/plant roots are no longer in these shady greenbelt areas, they are now exposed to the direct heat of the summer sun on the now exposed roots for months at a time. Because they cannot retain any water, and the surface of the thin Spanish oak leaves are comparatively large, they have to put more water through the stomates to cool the leaves enough to prevent sun-scald (bacterial leaf scorch). Subsequently, they defoliate and sections of the tree start to decline. Eventually, this leads to mortality.