Coniferous tree species in the southwestern United States (SWUS) are sensitive to climate variability and are being influenced by contemporary climate change in major but poorly understood ways. The local persistence of many species may be determined by natural regeneration, which is declining due to high vulnerability of juvenile trees to moisture limitation, increasing aridity driven by rising temperature and competition with other plant species. Juveniles must therefore partition above and belowground growth to maximize their chance of survival, but these characteristics are largely unknown for coniferous tree species. To address this knowledge gap and enhance understanding of juvenile tree survival and mortality, we evaluated the above and belowground growth characteristics of seven coniferous tree species (Pinus ponderosa, Pinus edulis, Pinus monophyla, Juniperus osteosperma, Juniperus monosperma, Abies concolor, and Pseudotsuga menziesii) across seven diverse SWUS locations. Our objective was to determine if there were common growth characteristics over species in this region, and to establish the tree ages and site conditions where these characteristics diverged.
Results/Conclusions
Aboveground growth characteristics were similar across one- to three-year-old juveniles of all species, with heights averaging 250-350 mm and diameters 8-12 mm. Belowground rooting depths were also similar at these ages, and typically limited to the upper 300 mm of soil. Across-species divergence in growth characteristics became more pronounced for juveniles over age four. We found that microenvironments (sheltered versus unsheltered) had a greater influence on where juveniles were located in our lower elevation sites. This suggests that patterns of tree regeneration in locations with high environmental stress provide insight on future regeneration for current low-stress areas that will become more stressed due to increased aridity expected for the southwest region in the 21st century. The juvenile stage of tree demography is relatively understudied, and our results enhance understanding of the critical process of tree regeneration in the SWUS.