Reproductive output of mesquite and juniper across sites of varying soil depth

Background/Question/Methods   A key characteristic of a woody encroacher is the capacity for high seed production. In Texas, honey mesquite (Prosopis glandulosa) is the dominant encroacher on deep prairie soils of the Blackland Prairie (BP), but not on the shallow rocky soils of the eastern Edwards Plateau (EP), where Ashe juniper (Juniperus ashei) is the dominant encroacher. We hypothesize that shallow soil limits the invasiveness of mesquite on the EP by limiting the development of productive seed trees, while another mechanism must be invoked to explain the limitations of juniper in the BP. In an earlier study, we established that mature mesquite trees on the EP become highly water-stressed during summer, while their counterparts in the BP do not. To link compromised physiological performance of mesquite trees to reproduction, we determined components of reproductive function of each species across three sites differing in soil depth: two EP sites with 0.3 m and 1.5 m soil depth, respectively, and one BP site with a soil depth >10 m. We analyzed reproductive functions in 5 trees in each of 3 size classes (when present), on 3 branches per tree, from February – September 2009.

Results/Conclusions   In mesquite, reproductive output covaried with branch size, and we corrected for differences due to branch size by reporting marginal means obtained in an analysis of covariance. For mesquite, flower numbers per branch increased with tree size and large trees had more flowers per branch on the deepest soil than on the intermediate soil. Pod and seed numbers increased with soil depth. No seeds were produced on the shallowest soil, and seeds that were produced on the soil of intermediate depth were 20% lighter than seeds produced on the deepest soil. For juniper, seed numbers per branch also increased with tree size and with soil depth, although seed numbers of large trees on the shallowest soil were 60% reduced compared to those on the deepest soil. Seeds were significantly lighter on the shallowest soil, but not different among the two deeper soils. We conclude that soil depth affects the reproductive functions of both tree species, but unlike juniper, mesquite suffered total reproductive failure on the site with a soil depth of 0.3 m. This supports our hypotheses, but leaves open for future study, why juniper encroachment is limited in the BP.