Microhabitat and diel tissue acidity changes for two sympatric cactus species differing in growth habit

Of two cactus species occurring sympatrically in the north-western Sonoran Desert, Echinocereus engelmannii has short stems occurring in tightly packed clumps, whereas Opuntia acanthocarpa has stems that freely branch, producing a taller and more open canopy; the stems of E. engelmannii are also much more spinose, being shaded 62% by spines compared with only 10% for O. acanthocarpa. Because of differences in height, the mean daily wind speed was 2.4-fold higher and the daytime air temperature 5 degrees C lower at the top of O. acanthocarpa than at the top of E. engelmannii; similar root depths for the two species led to similar soil water potentials, CO2 levels and temperatures in the root zone. Nocturnal increases in stem acidity and osmotic pressure in the field were about twice as high for O. acanthocarpa as for E. engelmannii, as was the nocturnal CO2 uptake in the laboratory, consistent with the higher photosynthetic photon flux density on the stem surface of the former species. Root length and the ground area explored by roots per unit shoot area were about threefold higher for O. acanthocarpa, consistent with a threefold higher daily transpiration per unit shoot area, than for E. engelmannii. Thus, O. acanthocarpa apparently has distinct advantages over E. engelmannii with respect to water acquisition and net CO2 uptake.