I wrote a small article about my master’s thesis project in the Tejon Ranch Conservancy Newsletter:
EPIC Saltbush Study on Tejon Part of Master’s Journey
By Mitchell Coleman, M.S., CSU Bakersfield Lecturer, Researcher
Greetings! I am writing this fresh off my master’s thesis defense of November 2017. The focus of my thesis was to elucidate factors affecting saltbush (Atriplex polycarpa (Torr.) S. Watson) seedling recruitment in the San Joaquin Valley of California. I specifically wanted to find out why saltbush seedlings do not seem to recruit in landscapes that are heavily invaded by non-native grasses. In the last 200 years, much of the valley’s native saltbush shrublands have been completely extirpated (approximately 5% of the original range remains today) to make room for ranching, agriculture, and later, urban and petroleum developments. Coinciding with this disturbance, much of the valley was invaded by highly-competitive, ecosystem-transforming annual grasses of Mediterranean origin. These grasses likely compete vigorously for water and nutrients in the soil during the wet season (typically January to March).
Relatively understudied is the role the grasses play in transforming the environment in ways that might affect recruitment of saltbush seedlings. When the grasses die during the dry season, the senesced shoots form residual dry matter (RDM), a structural modification to otherwise native habitat because it shades, and thus cools the soil. Throughout the San Joaquin Valley, it is now common to observe remnant stands of saltbush adjacent to large invasive grasslands. These shrublands do not seem to expand into the grassy areas, suggesting that the grasslands prevent the natural ecological succession of saltbush shrublands. I hypothesized that invasive annual grasses limit saltbush seedling recruitment due to both competition during the wet season and the structural modification of the RDM during the dry season. I predicted that this occurs due to alterations in soil moisture, soil temperature, and light penetration to the ground (saltbush has a high light and temperature-loving C4 photosynthetic pathway).
To assess my predictions, I conducted a number of experiments and observational studies. Overall, I found that the grass RDM limits saltbush germination to a greater degree than grass competition, apparently by reducing surface temperature and light levels of the soil. I also found that if saltbush seedlings successfully recruit amongst thick RDM, they have a greater chance of survival compared to established seedlings in the relatively open saltbush areas. The reason is that small herbivores (such as rabbits) occur in higher densities in native saltbush compared to invasive grasslands (grass RDM often forms thick and impenetrable barriers which impedes small mammal movement). Thus, saltbush seedlings that occur amongst thick RDM are more protected from herbivory (which is often deadly to young seedlings) relative to saltbush seedlings in open areas. This creates a seed-seedling mismatch wherein saltbush germination is significantly hampered in dense RDM areas, but seedlings which do germinate can survive at higher rates compared to seedlings in native saltbush habitat. Thus, combined land management practices which minimize the presence of RDM as an inhibitory factor (to increase saltbush germination) is important for saltbush seedling recruitment. Interestingly, once they are established they should form relatively open stands that are kept open by herbivores that inhabit the shrubs.
A significant component of my thesis research was conducted at native saltbush stands in the Comanche Point region of Tejon Ranch, with the support of the Environmental Educational Partnership Impacting Colleges and Careers (EPIC). EPIC is a partnership developed between CSU Bakersfield and the Tejon Ranch Conservancy. The program provides CSUB students the opportunity to work with the Conservancy on research and/or land management initiatives during the summer in the form of internships.
The EPIC program was an enormous asset and my thesis benefited tremendously because of it. I received logistical support from Drs. Michael White and Ellery Mayence of the Conservancy, and their guidance greatly improved the quality of my research. The EPIC program also provided funds which I used to purchase the necessary research materials. Looking to the future, I look forward to continuing my relationship with the Tejon Ranch Conservancy on future research projects. I am very grateful for the opportunity to have been a part of EPIC!
Full Article: https://spark.adobe.com/page/Sa8vRMftQ7P7l/