Contrasting responses of seedling development to soil barriers in two woody encroachers

Background/Question/Methods

To seedlings emerging on shallow rocky soils over fractured bedrock, the ability to efficiently navigate around obstructions may be an important component of establishment success. The purpose of this study was to examine if species differences in response to artificial soil barriers under non-limiting soil resource conditions could help explain the distinct geographical distribution of honey mesquite, which proliferates on deep soils, and Ashe juniper, which is strongly associated with the shallow rocky soils of the Edwards Plateau karst in central Texas. We hypothesized that two horizontal barriers at 15 and 30 cm soil depth, each with 3% of its area perforated to allow root passage, have a strong negative effect on root development of mesquite, which is known for the rapid growth of a central tap root, but only minor effects on root development in juniper, which produces a more branched root system that could make it more likely to encounter perforations in the soil barriers.  We tested the hypothesis by comparing plant height, rooting depth, above- and below- ground biomass, R/S ratios and specific root length in barrier and no-barrier pots for juniper and mesquite seedlings at three stages of early seedling development. 

Results/Conclusions

As hypothesized, barriers did not significantly affect juniper seedling growth, rooting depth, biomass allocation, or specific root length, but this was due to a large degree because juniper seedlings remained relatively shallow-rooted. Upon encountering a barrier, juniper roots grew in a straight line above the barrier, growing downward if a passage was encountered.  By contrast, in mesquite, barriers reduced plant height, rooting depth, overall biomass, and increased root/shoot ratios and specific root length. When a mesquite root encountered a barrier, it grew in a widening spiral until a downward passage was encountered. Thus, it appeared that the two species used two recognized but contrasting search strategies upon encountering a barrier (area-restricted search in mesquite and extensive (range) search in juniper, and that juniper’s search strategy involved no significant cost to seedling growth while mesquite’s search strategy did. We discuss these results in the context of search theory and the nature of plant adaptations to shallow soil conditions.