Establishment, Mortality and Growth of Recruits in an Experimentally Disturbed Piñon-Juniper Woodland

Piñon-juniper woodlands across the American Southwest are experiencing widespread overstory mortality due to episodes of prolonged extreme drought. There is a question whether forests can fully recover from these events to their previous state, or whether they are succeeded by more arid woodlands, such as juniper savanna. To better understand the process of vegetation recovery from disturbance, we imposed an experimental disturbance regime by girdling all mature juniper or piñon trees at a plot scale with replication (N=3). Together with three undisturbed control plots, I monitored over three years sapling survival and growth, newly emerged seedling numbers, and their development and survival by tagging nearly 4000 individuals. My hypothesis was that tree death should enhance survival and growth in wet years but potentially reduce growth and survival in dry years. In addition, I hypothesized that the positive effects of intraspecific girdling would be stronger than those of interspecific girdling, indicating niche dynamics, and setting a strong signal for recovery to the mixed-forest state. The girdling treatment increased soil moisture, which was most clearly observed at a depth of 40 – 60 cm. Sapling survival was generally high and depended on sapling size but was independent of treatment. Sapling growth was enhanced in girdle treatments relative to the control but was not observed until the second year after girdling. Though there were subtle signs of niche dynamics overall, the death of juniper and piñon trees had similar effects on saplings growth. The number of new piñon seedlings decreased sharply after girdling piñon trees, but this effect was not significant for juniper. These findings underscore the importance of advance-regeneration through saplings that passed their most vulnerable life stages. Contrary to expectations, sapling growth benefitted from overstory tree death of either species, including in dry years. However, the two species exhibited distinct recruitment dynamics with lower rates of juniper establishment, but higher survival, and in the juniper-girdle treatment signs of recruiting from a stable seedbank. By contrast, piñon seedlings emerged in higher numbers but had higher first-year mortality with little buffering from a persistent seedbank. Larger and presumably older saplings were more often found under the cover of heterospecific trees, compared to smaller saplings, which were more often associated with conspecific trees. Indirectly, this indicates that proximity to heterospecific overstory trees makes for a relatively safer site. Though I did not find direct evidence of stabilizing niche dynamics in the immediate aftermath of disturbance, sapling distribution patterns appear to indicate reduced levels of negative density dependence between heterospecific trees, relative to conspecific trees.

Name thesis:degree_name

Master of Science

Discipline thesis:degree_discipline

Population and Conservation Biology

Grantor

Texas State University

Year dc:date.issued

2026

Author dc:creator

• Sengsirirak, Aarin

Advisor dc:contributor.advisor

• Schwinning, Susan

Committee members dc:contributor.committeemember

• Martin, Noland
• Litvak, Marcy

• dryland ecology
• drought
• population dynamics
• disturbance ecology
• forest recovery
• advanced regeneration
• facilitation
• competition

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/10877/24717

OAI identifier oai:identifier

oai:digital.library.txst.edu:10877/24717