Soils play a large role in the cycling of atmospheric trace gases and are an important component of the climate system. The bulk of my thesis was directed at the role of soils in the global molecular hydrogen (H\u2082) cycle. I conducted field measurements of H\u2082 uptake in three Southern California ecosystems, and found that both the diffusion of H\u2082 into soils and the distribution of biological activity with depth controlled uptake rates at the surface. I then moved into the laboratory, where I mapped out the temperature and moisture controls on the biological uptake of H\u2082 in both desert and boreal forest soils. These experiments yielded simple relationships between moisture, temperature, and uptake rate, which I then used to constrain H\u2082 uptake by soils in a mechanistic model. The model is based on the 1D diffusion equation with a sink term, and is driven by a combination of remote sensing products and land surface modeling output. I calculated a mean annual soil H\u2082 sink of 67.3 \u00b1 5.5 Tg. The model was able to reproduce the seasonal cycle at high northern latitudes, and implies that seasonal variability in snow cover is a key process controlling H\u2082 uptake. I found that snow cover and soil moisture control the uptake of H\u2082 globally, which may have important implications for the hydrogen budget in future climate change scenarios.
\r\n\r\nMy second thesis topic involved the development of a remote sensing technique using passive microwave brightness temperatures to identify the freeze-thaw status of soils, which I applied to areas north of 45\u00b0N. I found a significant increase in the growing season length in North America by 3.8 days/decade, driven by both an earlier spring thaw and later fall freeze. The lengthening of the growing season may affect the carbon and hydrogen cycles at high northern latitudes, and is a new metric of global change.
", "doi": "10.7907/BXV8-HH61", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:2399", "collection": "thesis", "collection_id": "2399", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06022006-105702", "primary_object_url": { "basename": "LRW_Thesis_final.pdf", "content": "final", "filesize": 4515539, "license": "other", "mime_type": "application/pdf", "url": "/2399/1/LRW_Thesis_final.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Links Between Carbon and Water Cycles in Northern Ecosystems: Constraints from Stable Isotopes", "author": [ { "family_name": "Welp", "given_name": "Lisa Renee", "orcid": "0000-0001-7125-0478", "clpid": "Welp-Lisa-Renee" } ], "thesis_advisor": [ { "family_name": "Randerson", "given_name": "James T.", "clpid": "Randerson-J-T" } ], "thesis_committee": [ { "family_name": "Adkins", "given_name": "Jess F.", "clpid": "Adkins-J-F" }, { "family_name": "Eiler", "given_name": "John M.", "clpid": "Eiler-J-M" }, { "family_name": "Randerson", "given_name": "James T.", "clpid": "Randerson-J-T" }, { "family_name": "Wennberg", "given_name": "Paul O.", "clpid": "Wennberg-P-O" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "High-latitude climate change will have an impact on the carbon and water cycles in northern ecosystems. Stable isotopes in these systems can serve as indicators of changes and feedbacks. Monitoring the stable isotopic composition of Arctic river discharge provides a means to investigate integrated basin-scale hydrologic changes in remote northern regions. I measured water \u03b4\u00b9\u2078O and \u03b4D from the Kolyma River in Siberia and local precipitation to partition the river flow into 60% snow and 40% rain inputs. Comparing this estimate with seasonal precipitation across the watershed showed a significant portion of snowmelt is retained in the soils of this permafrost dominated region, and contributes to ~40% of the growing season transpiration. The seasonal cycles of atmospheric CO\u2082 and \u03b4\u00b9\u2078O-CO\u2082 at high northern latitudes have the potential to serve as indicators of ecological change. Effective interpretation of atmospheric observations requires an understanding of how different species and ecosystems contribute to biosphere-atmosphere exchange. By combining isotopic signatures of ecosystem water pools with measured CO\u2082 fluxes in three stands of an Alaskan boreal fire chronosequence (recent burn, intermediate-aged deciduous and mature evergreen forests), I compared the relative effects of stand age on the phase and amplitude of the seasonal cycles of CO\u2082 and \u03b4\u00b9\u2078O-CO\u2082. Higher rates of mid-summer net carbon exchange and a shorter growing season at the deciduous stand resulted in the largest seasonal CO\u2082 amplitude and also delayed the drawdown of atmospheric CO\u2082 compared to the evergreen stand. Reduced levels of photosynthesis at the deciduous stand early in the growing season caused atmospheric \u03b4\u00b9\u2078O-CO\u2082 to increase more slowly compared to fluxes from the evergreen stand. The distribution of stand ages in northern boreal forests is likely to determine the response of net ecosystem exchange (NEE) to future climate changes. I used three years of NEE measurements from the Alaskan fire chronosequence to determine that the sensitivity of growing season NEE to spring air temperatures and summer drought was greater at the deciduous forest than the evergreen forest. As forest fire disturbance increases due to climate warming, the shift to younger forests should increase interannual variability in atmospheric CO\u2082 concentrations.
", "doi": "10.7907/KY54-DC47", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" } ]