Biogeography of woody encroachment: why is mesquite excluded from shallow soils?

While some studies aim to generalise the attributes of woody encroachers, examining their functional differences across biogeographic regions may also be instructive. Most of Texas is encroached byProsopis glandulosa, but on the eastern Edwards Plateau, a limestone plateau with thin soils,P. glandulosais rare andJuniperus asheiis dominant. We hypothesised thatP. glandulosais excluded from sites where bedrock at a depth of 1 m or less restricts the development of taproots, thus rendering this normally deep‐rooted species too vulnerable to drought. To test this idea, we monitored the physiological status of the two species on a site where both species were encroaching and the soil was 1·5–2·5 m deep, thus relatively deep for the Edwards Plateau region but not for regions whereP. glandulosadominates. Data were collected across three tree size classes from May to November 2006. Stem water potentials were similar across species and water potentials and photosynthetic rates decreased with tree size. Based on isotopic evidence, the effective rooting depth ofP. glandulosaincreased with tree size, but not inJ. ashei. P. glandulosahad a higher rate of leaf gas exchange overall as expected for a drought‐avoider; except for large trees during a summer drought, when species differences in photosynthesis rates diminished and the stomatal conductance forJ.asheiexceeded that ofP. glandulosa. We speculate that restrictions of soil depth may limit the invasiveness ofP. glandulosain part through negative effects on large trees, which may lower fecundity.