Browsing by Subject "Juniperus"
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Item Biotic and abiotic controls on carbon dynamics in a Central Texas encroaching savanna(2014-12) Thijs, Ann; Hawkes, Christine V.; Litvak, Marcy E.Anthropogenic activities are responsible for increases in atmospheric CO₂ and climate change. These increases are partly counterbalanced by natural processes, such as carbon uptake in land surfaces. These processes are themselves subject to climate change, creating a coupled carbon-climate system. I investigated the carbon sink that woody encroachment represents, using a Central Texas savanna as study site, and studied how climatic factors influence this carbon sink. Woody plant encroachment, a worldwide structural change in grassland and savanna ecosystems, alters many ecosystem properties, but the net effect on the carbon balance is uncertain. Woody encroachment represents one of the key uncertainties in the US carbon balance, and demands a more detailed understanding. To come to a process-based understanding of the encroachment effect on carbon dynamics, I analyzed patterns of carbon exchange using eddy-covariance technology. I expected the imbalance between carbon uptake and release processes associated with the encroaching trees specifically, to be responsible for the carbon sink. I also expected that the sink would vary in time, due to strong links between carbon fluxes and soil water in this semi-arid ecosystem. I further studied the ecophysiology of the dominant species, as well as soil respiration processes under different vegetation types, and scaled these findings in space and time. I found that the ecosystem was a significant carbon sink of 405 g C m⁻² yr⁻¹. The encroaching trees increased photosynthesis by 180% and decreased soil respiration by 14%, compared to the grassland, resulting in a strong carbon sink due to the encroachment process. The encroaching process also altered carbon dynamics in relation to climatic drivers. The evergreen species Ashe juniper effectively lengthened the growing season and widened the temperature range over which the ecosystem acts as a carbon sink. The drought resistance of the encroaching trees reduced the sensitivity of this savanna to drought. I conclude that encroachment in Central Texas savannas increased the carbon sink strength by increasing the carbon inputs into the ecosystem. Woody encroachment also reduced the sensitivity to climatic drivers. These two effects constitute a direct effect, as well as a negative feedback to the coupled carbon-climate system.Item Climate Warming and Drought Effects on Pinus and Juniperus Species: Contrasting Drought Tolerance Traits Limit Function and Growth in Tree Seedlings(2013-04-29) Lenoir, Katherine JudithJunipers and pines exhibit contrasting patterns of growth decline and mortality with climate change-type warming and drought; yet, the underlying physiological mechanisms are not fully understood. Does warming exacerbate the effects of drought on gas exchange physiology and growth? Do the combined effects of drought and warming differ for pines and junipers? To what extent do isohydric vs. anisohydric responses to water limitation in pines and junipers constrain net leaf CO2 exchange and plant growth response to drought and warming? To address these questions, we compared responses of leaf gas exchange and growth in seedlings of juniper (Juniperus scopulorum, J. virginiana) and pine (Pinus edulis, P. taeda) species of contrasting arid and mesic origin in a study of combined warming (ambient, +1.8 ?C) and enhanced summer drought (long-term mean, -40%). Warming and enhanced summer drought each reduced photosynthesis and growth and effects were largely independent, suggesting that warming exacerbates drought effects on growth. Enhanced summer drought and warming had distinct impacts on photosynthetic carbon gain that were differentially revealed depending upon soil water content. Warming reduced light-saturated net photosynthesis (Asat) under low soil water contents, whereas carry-over effects of drought treatment were evident under well-watered conditions. Short-term soil drying led to greater reduction of Asat in pines (-51%) rather than junipers (-30%). Under short-term water-limited conditions, Asat and gs were about two-times higher for junipers compared to pines. Relative growth rate of junipers declined with warming (-28%) and drought (-50%) treatments. In contrast, pine growth and Asat declined more with warming than drought. Only P. edulis exhibited increased mortality in response to warming and drought, reaching 75% in the combined warming and drought treatment. Diminished sensitivity of R to water limitations, coupled with steeper reductions in Asat with decreasing soil water content in isohydric pines compared to anisohydric junipers could account for the greater sensitivity of pines to warming and drought under climate change.