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New Mexico State University (2021)

Weather and Plant-Soil Feedbacks Determine Seed Germination and Seedling Demographic Bottlenecks in Chihuahuan Desert Shrubs

Toth, Caroline R

Titre : Weather and Plant-Soil Feedbacks Determine Seed Germination and Seedling Demographic Bottlenecks in Chihuahuan Desert Shrubs

Auteur : Toth, Caroline R

Université de soutenance : New Mexico State University

Grade : Master of Science (MS) 2021

Résumé
Shrub encroachment is threatening the productivity of southwestern rangelands by diminishing herbaceous productivity, with potential to negatively impact the sustainability of grazing-supported economies. During the past century, for example, native shrubs have expanded in former grasslands to become the dominant species of the Chihuahuan Desert and limit herbaceous growth and reestablishment through direct competition with grasses and indirect positive feedbacks that suppress grass establishment and persistence. In this study, we sought to identify the ontogenetic processes relating to soil type, microbial interactions, and precipitation (including demographic release and bottleneck mechanisms) that have led to distinct spatial patterns of Larrea tridentata (DC.) Coville (creosotebush), Prosopis glandulosa Torr. (honey mesquite), and Flourensia cernua DC. (tarbush) dominance across the Jornada Basin of the Chihuahuan Desert in southern New Mexico. We created a four-stage experiment measuring the germination, survival, and growth responses of these three species during the critical and most vulnerable early life stages, from germination to initial establishment and growth (<3 months post germination). Bottlenecks to establishment of the three primary shrubs that have proliferated on Jornada Basin grasslands differ by species. Under current conditions, F. cernua establishment is limited by extremely low seed viability (<5%) associated with self-incompatibility during pollination. More research is needed to establish under what conditions F. cernua produces viable seed sufficient to explain its historical expansion. P. glandulosa and L. tridentata require distinct seed scarification treatments, correlated with dispersal mechanisms (herbivory and wind, respectively), to break dormancy. We found little evidence for intraspecific or interspecific allelopathy associated with soils or microbial communities associated with P. glandulosa and L. tridentata. However, we did find evidence that germination of L. tridentata is suppressed when exposed to soil dominated by F. cernua. Early seedling establishment ( 24 days post-germination) of both P. glandulosa and L. tridentata is favored by wetter than average conditions. However, in older seedlings (25-74 days), P. glandulosa survival is enhanced by wetter than average conditions, whereas L. tridentata survival is optimal under average conditions but reduced under wetter than average conditions. Disentangling the varying causes of demographic release and demographic bottlenecks is critical for understanding historical shrub encroachment/expansion processes and predicting likely future trends.

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Page publiée le 5 décembre 2021