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Factors affecting seedling recruitment of the desert shrub Atriplex polycarpa(Torr.) S. Watson (Chenopodiaceae) in the San Joaquin Valley of California
Invasive annual grasses of mediterranean origin dominate large areas in the arid and semi-arid regions of the San Joaquin Valley in California and create numerous management challenges. Much of the invaded habitat would be native saltbush shrub communities (Atriplex spp.); however, the presence of these grasses may inhibit saltbush recruitment. One way this could occur is due to competition for water between the grasses and saltbush seedlings. Another possibility is that when invasive annual grasses die after setting seed, they leave behind a dense cover of residual dry matter (RDM) as dead shoots, which alters the habitat by shading the soil. I tested the hypothesis that grasses limit saltbush seedling recruitment, leading to persistently invaded grasslands and thereby inhibiting saltbush succession. I predicted that competition from grasses during the rainy season reduces soil moisture available to saltbush seedlings at the critical seedling stage of their life cycle. Furthermore, I predicted that the presence of dense RDM blankets formed during the dry season shades the soil and inhibits the natural succession of saltbush shrublands in the San Joaquin Valley at old-field areas. Moreover, I predicted that grass competition and RDM affects saltbush recruitment due to alterations in soil temperature, soil moisture, and light penetration to the ground. To address these questions, I used a combination of manipulative and natural field experiments to assess the factors affecting recruitment of the saltbush Atriplex polycarpa (Torr.) S. Watson. I conducted experiments manipulating the effects of invasive grass competition and RDM on saltbush seedling germination, mortality, and survival. I also conducted a number of comparative studies between six mature saltbush stands at Tejon Ranch, wherein I assessed differences in seedling recruitment, demographic composition, water relations, and soil conditions. The goal of the study was to more precisely elucidate mechanisms governing saltbush germination in the San Joaquin Valley. Overall, I found than invasive grass RDM functions to inhibit saltbush seed germination to a degree that precludes long-term seedling recruitment in the San Joaquin Valley. Competition between saltbush seedlings and the grasses during the wet season also inhibits recruitment to a smaller degree. Saltbush recruitment is affected by alterations to soil moisture, temperature, and light levels created by invasive grasses. I also found that older seedling survival is greater in grassy areas compared to native saltbush habitat, ostensibly due to higher pressures of herbivory in the saltbush habitat. This creates a seed-seedling conflict wherein seed germination is inhibited in grassy areas, but seedling survival is higher in grassy areas. Thus, field conditions with a low amount of RDM over saltbush seeds and no seedling herbivory are ideal for saltbush recruitment in the San Joaquin Valley. My findings are relevant for the conservation and management of important remaining saltbush habitat and the continued expansion of invasive annual grasses in the San Joaquin Valley and throughout the mediterranean-type climate regions of California.
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