Woody encroachment is a general phenomenon that occurs worldwide, but it involves species with widely different functional characteristics, suggesting that circumstances of woody encroachment differ among regions. In Texas, honey mesquite (Prosopis glandulosa) is the dominant encroacher on deep prairie soils from west to central Texas while Ashe juniper (Juniperus ashei) dominates on the shallow and rocky soils of the Edwards Plateau, a limestone karst. We hypothesized that mesquite trees are more negatively affected by limitations of soil depth than juniper.  To test this, we followed the water status (i.e. water potential and rates of gas exchange) of the two species in three size classes across two sites differing in soil depth during a year that ended in a severe drought (2008).

Results/Conclusions

On the deep soil site, where small juniper trees were absent, large and medium mesquite trees had substantially higher water potentials and gas exchange rates than large and medium juniper trees. These differences were maximized in the wettest part of the year. On the shallow soil site, where large mesquite trees were absent, water potentials and gas exchange rates were more similar between species, and overall lower compared to those measured on deep soils. Differences between drought and normal conditions were also lower on the shallow soil site than on the deep soil site. Thus, even though both species were negatively affected by the shallow soil, the negative effect on mesquite was much greater, supporting our original hypothesis. We speculate that juniper expansion on deep soils is primarily limited by seedling establishment, since no small individuals were found at the site, but mature trees, once established, did quite well. By contrast, mesquite expansion on shallow soils appears to be limited by seed supply since mature trees did poorly compared to trees on deep soils and large seed trees were altogether absent. Our results suggest that woody encroachment may be limited by different life history stages in different regions.  This understanding has important management implications. For example, woody species primarily limited by growth potential due to site conditions may never become a problem because such limitations are not easily modified by changes in land management. On the other hand, species primarily limited by seedling establishment may become a problem, since changes in management often do affect seedling survivorship.