I hiked the southern edge of the park, the hills. the trees were almost exclusively on hillsides with south south eastern exposure, plenty of shade, always under a juniper, soil was juniper litter and limestone scree. I think it grows in the juniper litter because it is still more ericaceous than people realize. as for the fruits. probably.just a matter of well timed rains. also it takes a lot of energy to flower and fruit, they may have off years. pinyon pines are like that I've gone out there and none of those trees have cones. other years all are covered in them

It really is an interesting critter. the fact that central texas trees get that leaf blight some years, and that they grow almost exclusively in that juniper mulch makes me think they really prefer their west texas climates where the air is dry and the soils are igneous (and therfore acidic). and almost as interesting as where it is located is where it isn't. if it likes these high relatively cool places on the eastern side of the Edwards plateau then I'd expect to find it in pinyon juniper stands on the southwestern portion too, where it is a little higher , drier, and the limestone is hard and gravelly. there are a lot of goat ranches out there maybe the goats ate all the madrones.

I have heard that they like a specific strata of limestone and the micro stew that is specific to that strata. Possibly the Walnut Limestone. IF I remember right.I am not that up on my geology. The limestone around me is very soft with layers of marl and nodules of flint.

The Madrone up by my shop was shin high when we built the shop. I now walk under overhanging branches. It has been amazing to watch grow.

The book, How To Grow Native Plants of Texas and the Southwest, by Jill Nokes says madrone is "widespread but nowhere abundant on woody, rocky limestone, or igneous soil in canyons or occasionally open plains in the Edwards Plateau and mountains of the Trans-Pecos, also southwestern New Mexico and Mexico south to Guatemala." She says it grows in slightly acid (5.7) to alkaline (8.0) soils with rainfall ranging from 15 to 30 inches per year.

Clearly, Arbutus xalapensis is adaptable to pH. In the south of its range the tree is found in the mountains up to 10,000 ft, and in its northernmost range is below 1,800 ft. It grows taller (80+ ft) where it gets more rain, but is less than 20 ft in west Texas. Here is a map showing Texas Madrone in light green and the Arizona Madrone in dark green.

By the way, since I'm being so pedantic. The name of the river and park is spelled Pedernales. Local vernacular transposes the r to the first syllable.

I don't think it is as adaptable to ph as people think. in this location for example it it only on the best sites growing in a few inches of organic matter, juniper litter mostly. further west soils are volcanic and acidic. in the mountains of mexico soils are acidic. I don't put too much credence in the beneficial microorganism theory. SoIL fauna just make locked up nutrients available in exchange for sugars that leech from the plant. but this plant fails in cultivated settings, presumably when fertilized with water soluble nutrients. problems with cultivation have to be 1) drainage (very common in arbutus and related manzanita) 2) soil ph it is in the Heather family, which are almost all ericaceous and 3) humidity aND related fungal disease. in america at least these are all native to arid west.

The Madrones that I saw at the base of Guadalupe Peak was also on limestone, but different limestone. Very interesting differences in the limestone. We are Cretaceous and they are Permian. I loved the view from up there. One can really feel the ancient Sea level. The mountains feel like barrier islands that have been drained. Oops , I strayed off topic.

The Guadalupes are a little different geologically than the Edwards plateau. the Edwards plateau is pure limestone sea floor that was uplifted. the Guadalupes are a more diverse mineral wise, sedimntary, metamorphic and igneous. the chisos and Davis mountains are even more igneous, acidic.

Maybe local populations have adapted to where they are at but are not flexible to changes in PH if moved from one place to another.

Where I saw the Madrone, it was limestone. It was not igneious.

Could be. arbutus unedo grows in alkaline soil... and clay