This dissertation further investigates predominant issues in the educational pipeline, that is, both the higher education and K-12 context. In particular, I first attempt to answer why post-secondary enrollment at U.S. colleges and universities has increased among all students, but racial/ethnic and gender inequalities persist in the college experience. That is, Latinx and Black students continue to be less likely to graduate from post-secondary institutions and women are underrepresented in science, technology, engineering, and mathematics (STEM) fields. Second, I further contextualize suspensions and the criminalization of students of color in schools today. Today, many schools rely heavily on police and security officers to maintain discipline. Therefore, in part, this dissertation explores this phenomenon and investigates how the type of staff (e.g., support vs. policing) may contribute to the days of lost instruction time due to out-of-school suspensions. Overall, to investigate the issues proposed, my dissertation uses social network methods to examine college-level educational trajectories at the institutional level and, using quantitative methods, I examine discipline disparities using national data.

Soil hypoxia occurs when soil oxygen is used by roots and soil organisms at a faster rate than it is replenished by diffusion through the soil. Once soil oxygen levels fall below the amount necessary to support aerobic cellular respiration, new root production is reduced, water and nutrients uptake decline and energy demand switches from active ion uptake to cellular maintenance. In the Mediterranean climate of California, soil hypoxic conditions happen typically in periods of high rainfall, but they can also occur during the growing season if a high amount of irrigation water is applied. As the diffusion of oxygen through water filled pores is approximately 10,000 times slower than through air filled pores, the concentration of soil oxygen can drop rapidly, especially if root respiration is high. To test the impact of hypoxia on root growth and activity at different phenological stages, we flooded the root systems of young (5th leaf) almond (Prunus dulcis) trees three times for 3 days during the growing season and measured root population dynamics and root/soil activity.

We found that there were no impacts of saturating the rootzone of almond trees for three consecutive days in either late spring, summer, and fall, even though the treatments succeeded in creating hypoxic conditions. We had hypothesized that hypoxia would have the greatest negative impact in the spring when most new fine root production was expected to occur. However, due to very dry conditions and resulting little root production prior flooding, we observed that most root production occurred in the two weeks after each spring and summer saturation event. We concluded that this response is likely due to drought alleviation reducing soil strength and enhancing root production, rather than a response to the short-term hypoxic conditions. Future field studies will need to incorporate higher irrigation and perhaps longer saturation treatments.

California signed the Sustainable Groundwater Management Act (SGMA) into law in 2014. SGMA requires groundwater-dependent regions to halt overdraft and develop plans to reach an annual balance of pumping and recharge. Groundwater aquifers can be recharged by flooding agricultural fields when fallow, but this has not been an option for perennial crops such as fruit and nut trees. While flooding these crops might be possible during the dormant season, it is not known what impact flooding might have on tree-root systems, health and yield. We followed root production, tree water status and yield in two almond orchards in Northern California for 2 years to test the impact of applying captured winter water runoff for groundwater recharge purposes on tree performance. Results showed that more than 90% of the water applied to sandy soil and 80% of the water applied to loamy soil percolated past the root zones, with no measured adverse effects on tree water status, canopy development or yield. Groundwater recharge did not negatively affect new root production and tended to extend root lifespan. Based upon these data, applying additional water in late December and January is not likely to have negative impacts on almond orchards in moderately drained to well-drained soils.