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Uncertainty in the representation of vegetation in Earth System Models is a major contributor to the intermodel spread in climate projections under global warming. Empirical soil moisture stress parameterizations to model drought effects on photosynthesis have been identified as a major driver of this uncertainty, leading to a call to develop more mechanistic models that leverage the principles of soil and plant hydraulic theory. The goal of this dissertation is to develop and install a simplified plant hydraulics representation within a major Earth System Model, compare its dynamics with a non-hydraulic model, and refine methods to use transient leaf water potential observations to infer vegetation water-use strategy. Chapter 1 presents the full model description of Plant Hydraulic Stress (PHS), which we developed to implement plant hydraulics within the Community Land Model (CLM). PHS has since been adopted as the default representation of vegetation water use in version 5 of the CLM. PHS updates vegetation water stress and root water uptake to better reflect plant hydraulic theory, advancing the physical basis of the modeled vegetation hydrodynamics. Point simulations of a tropical forest site (Caxiuanã, Brazil) under ambient conditions and partial precipitation exclusion highlight the differences between PHS and the previous CLM implementation. Model description and simulation results are contextualized with a list of benefits and limitations of the new model formulation, including hypotheses that were not testable in previous versions of the model. Key results include reductions in transpiration and soil moisture biases relative to a control model under both ambient and exclusion conditions, correcting excessive dry season soil moisture stress in the control model. The new model structure, which bases water stress on leaf water potential, could have significant implications for vegetation-climate feedbacks, including increased sensitivity of photosynthesis to atmospheric vapor pressure deficit. Chapter 2 extends the analysis of PHS to the global scale. Historical simulations with and without plant hydraulics are compared to understand the influence on interannual soil moisture and photosynthesis dynamics. The focus of this chapter is on analyzing model dynamics across the semi-arid tropics. The PHS simulation yields longer soil moisture memory and increases interannual photosynthesis variability as compared to the non-hydraulic model. With an analytical derivation and analyses of soil moisture dynamics, we demonstrate the importance of the root water uptake parameterization for soil moisture memory and carbon cycle variability. Chapter 3 investigates methods to use transient leaf water potential observations to infer vegetation water-use strategy. We use a set of soil-plant-atmosphere models, ranging in complexity, to investigate the underlying meaning of three isohydricity metrics and identify potential classification errors. The model-based approach allows us to derive analytical expressions for the three metrics and to more methodically sample both environmental space and trait space to generate idealized experiments to test the fidelity of the resulting water-use strategy classifications. We consider two previously defined metrics, isohydricity slope and hydroscape area, in comparison to a third metric, relative isohydricity, defined herein. We describe classification challenges resulting from trait coordination and environmental variability, suggest practical recommendations for metric retrieval, and discuss the value and limitations of isohydricity and the broader pursuit of response-based metrics of vegetation traits. Our results indicate that the major limitations of the isohydricity slope and hydroscape area metrics can be corrected with the relative isohydricity methods described here.

Bibliography: pages 162-177. / The seasonal and diurnal patterns of photosynthetic gas exchange and the water relations of seven species of the mediterranean-climate region of South Africa (fynbos) were investigated. The following species, representing the major fynbos elements, were chosen for intensive investigation: Erica plukenetii and Erica hispidula (ericoid element), Thamnochortus lucens and Askidiosperma paniculatum (restioid element), Protea laurifolia and Leucadendron salignum (proteoid element). Metrosideros angustifolia, a shrub of riparian habitats, was also studied.

The growth and recovery of trees from disturbances such as fire and browsing is driven by the intensity of the disturbances and the availability of resources. In savannas, resprouting has become recognized as a key functional trait in plant ecology over the past decade. Although this may indeed be the case, there is still limited information about the physiology and growth strategies of resprouting trees. Available information about the influence of disturbance comes from ecosystems that are in many ways different from tropical and sub-tropical savannas. Therefore it is important to know and understand post-disturbance tree responses and limitations so as to establish sustainable use and management practices. This thesis reports the findings of a study, conducted in the Mpumalanga province of South Africa, aimed at achieving a better understanding of the influences of resource availability (water and nutrients) and disturbances (herbivory and repeated cutting) on the coppicing of a widely distributed savanna tree species that is both ecologically and economically important; Terminalia sericea. To investigate the effects of resource availability, cut trees were exposed to different levels of water and nutrient (nitrogen and phosphorus) supplementation over a period of two years in a factorial experimental design. A number of coppice regrowth variables (e.g. shoot production, resprout shoot diameter and shoot length) were measured monthly, while the phenological responses (e.g. timing of leaf discolouration and fruit presence) were monitored every two weeks. The effects of disturbances were investigated in two separate experiments, in which cut trees were exposed to a five month browsing period and different cutting frequencies, respectively. Coppice regrowth variables were monitored for 12 months in trees exposed to browsing and for five months in trees exposed to different cutting frequencies. The effect of multiple cutting cycles on total non-structural carbohydrates and leaf chemistry (carbon, nitrogen and phosphorus) was measured. There was evidence of self-thinning of coppice shoots within cut trees in all experiments in the second year of growth, with supplemented trees also recording lower shoot numbers. With shoot production higher in trees that received no resource addition, water and nutrients interacted synergistically, doubling shoot diameter and shoot length for supplemented trees after 12-months of addition. The majority of the phenophases monitored peaked in the wet growing season. Supplemented trees changed colour from the typical green to senescent yellow later in the growing season than unsupplemented trees. Fruit presence occurred in the second year after cutting for supplemented trees. Shoot length and shoot diameter for unbrowsed trees were twice those for browsed trees, with leaf nitrogen and phosphorus content significantly higher for browsed trees compared to unbrowsed trees. Trees subjected to multiple cutting cycles recorded half the TNC levels, and half the resprout shoot diameter and shoot length of trees cut only once. Results from this study demonstrate that self-thinning (i.e., negative change in shoot number) is not primarily under resource control. By contrast, the findings suggest that shoot growth characteristics, the timing and duration of phenological stages in coppicing trees are resource-limited in savannas. Browsing induces an initial compensatory response through higher shoot production in browsed trees and should be kept minimal because, in the long run, browsed cut trees would take longer to recover lost biomass compared to unbrowsed cut trees. Repeated cutting significantly depleted non-structural carbohydrate reserves in stems, implying that repeatedly cut trees rely heavily on non-structural carbohydrate reserves for regrowth. The significantly lower shoot diameter and shoot length in repeatedly cut trees imply that the coppicing ability of a tree reduces as the cutting frequencies increase. Such information can be vital in establishing the competitive growth ability of T. sericea in a multiple-species ecosystem affected by changes in resource availability as well as natural disturbances. With reference to multiple cut trees, a form of cutting or harvesting strategy should be in place that allows for sustainable regeneration of the study species. A threshold in terms of number of cutting cycles a tree can tolerate based on either maximum or minimum levels of reserve carbohydrates should also be established, as this has a direct effect on the coppice growth and survival.

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The development, yield and water use of six sorghum (Sorghum bicolor (L.) Moench) hybrids and their respective male and female parents were evaluated under stressed and well irrigated conditions during 1980 and 1981 at Tucson, Arizona. Changes in soil moisture storage were measured by neutron modulation on a semiweekly schedule. Transpiration, diffusive resistance and leaf-ambient temperature differentials were monitored biweekly using a steady state porometer. Meteorological data was collected on a daily basis. The 1980 season had higher maximum temperatures and pan evaporation than the 1981 season. Differences in soil moisture extraction among sorghum entries were not apparent within water treatments. Mean cumulative evapotranspiration (ET) for the stressed treatment was 270 and 261 mm, for 1980 and 1981, respectively. Mean cumulative ET for irrigated treatment was about twice that at 520 and 648 mm during 1980 and 1981, respectively. There were no apparent differences in cumulative ET for entries in the stressed treatment while genotypic differences were manifested under well irrigated conditions. Temperature differential demonstrated a significant and negative correlation with diffusive resistance especially under stressed conditions (r = -.64 in 1981). Temperature differential was positively correlated with transpiration (r = .70 in 1980 stressed treatment). Plant height was significantly affected by water level both years while stem weight was affected by water level only in 1980. Soil moisture treatments did not affect leaf area either year and genotypic differences were demonstrated only in 1981. Hybrids produced greater grain yield than their male parents under both water treatments. This was due to greater seed number for hybrids. Seed number was also more stable for hybrids under both moisture levels. Hybrids four and seven had the greatest grain yield in 1980 and 1981, respectively. Harvest index was improved with increased water application due to increased seed number. Hybrid four in 1980 and hybrid seven in 1981 were extremely efficient in water use exhibiting ET ratios of 283 and 378, respectively under high water application. Reduced water application had little affect on the performance of these entries.

Recently, canopy temperature measurements and atmospheric vapor pressure deficits have been used to determine water stress in numerous plant species. A linear regression of the two parameters yields a crop water stress index (CWSI) baseline capable of determining a fraction of water stress between 0 (wet) and 100 (dry). Such a baseline was determined for one line of guayule (N396) in the spring of 1983 and used to determine time of irrigation for field plots. Three irrigation treatments were chosen based on CWSI measurements to aquire a relationship between rubber yield and water Stress. Treatments were irrigated when their respective CWSI measurements reached 0.30 (wet=1), 0.60 (medium=2), and 0.90 (dry=3). Duplicate tests were planted to determine if water delivery by drip irrigation differed from furrow irrigated plots. Very good correlations exist between rubber yield and seasonally averaged CWSI (r = 0.85). The interrelationship of rubber yield and total water applied was also high with r = 0.87. The highest rubber yield occurred in March in treatment 1 (wet) in the furrow irrigated field. Rubber yield was positively correlated with total water applied and inversely correlated with CWSI. Small differences were observed between the type of water delivery system used with similar treatments from both fields not being significantly different. The wet and medium treatments in both fields were about the same with the dry treatment being significantly different from the wet and medium treatments for total water applied. This indicates that guayule is not as sensitive to changes of 0.30 CWSI units as other species measured. Correlations between the CWSI and soil moisture deficits were very good with r=0.83 for 1983 and r=0.91 for 1984 indicating the CWSI can be used accurately to determine soil moisture deficits and vice-versa. A 0-stress moisture deficit range (0-SMDR) was determined for guayule where 0.0-2.26 and 0.0-4.58 cm (1983 and 1984, respectively) of water could be depleted from the soil profile before the plants showed stress as indicated by the CWSI. The 0-SMDR appears to be constant for a particular plant species in the same field for a particular year and may further define and standardize plant available soil moisture.

Axelrod (1958) suggested that the late Tertiary shift in regional climate -- the elimination of summer rains -- had a profound influence on the evolution of biotic provinces in the American southwest. In particular, the taxa endemic to biotic provinces characterized by summer drought, e.g., the Mojave Desert, should be derived from ancestors that likely inhabited regions of summer rain, e.g., the Chihuahuan Desert. Further, the derived features of summer-drought taxa should be related to water stress. I examined Axelrod's thesis, using a combination of phylogenetic systematics, physiological ecology, and vicariance biogeography. The first chapter is a cladistic study of the pinyon pines, 13 taxa of small trees that range from the summer-wet regions of Mexico to the summer drought regions of Nevada and California. A parsimony analysis using twenty morphological characters showed that the most recently derived pinyons are from regions of summer drought. The "summer-drought" taxa are characterized by relatively few needles per fascicle. Because fewer needles per fascicle results in a reduction in the needle surface-to-volume ratio, Haller (1965) hypothesized that fewer needles in pines is an adaptation to reduce transpirational water loss. The second chapter reports on a two year study of the xylem pressure potentials of single- and double-needled fascicles of hybrid pinyons in central Arizona. The results showed no significant differences between single- and double-needles. I concluded that either needle morphology does not effect water relations, or that the relatively high precipitation during the study did not allow significant water stress to occur. The third chapter uses the methods of vicariance biogeography to search for a common pattern of relationship between southwestern biotic provinces, as indicated by the relationships of their endemic taxa. Using a biogeographic parsimony analysis, I compared the area cladograms of six taxa -- junipers, pinyon pines, the composite Palafoxia, hedgehog cactus, desert tortoises, and gecko lizards. The most parsimonious area cladogram supports Axelrod's (1958) hypothesis, but also shows that some taxa, notably the junipers, support other patterns of area relationships, e.g., summer-drought primitive. I suggest that there is no single pattern of area relationships because of the effects of the Pleistocene (including dispersal and extinction) and vicariance events other than the Tertiary climatic change, e.g., the separation of the Baja peninsula from mainland Mexico during the Miocene.

A Coulostatic Impulse Generator (CIG) is an electronic device that transfers electrical charge to and from a pair of electrodes inserted in plant tissue. Six discrete charge transfers can be implemented in any desired sequence. The major purpose of the CIG is to determine the electrochemical constituents of the plant apoplast electrolyte. The objective of this thesis is threefold: (1) to design, construct and test the supervisory circuitry of the CIG, (2) to design, construct and test the interface between the NEC portable computer and the CIG, (3) to generate utility software to control each circuit board in the system. To handle the extreme difference in the timing of the charge transfer (microseconds) and the subsequent plant response a three step timing sequence is employed which permits an independent range of sample times and sample numbers. Data acquired is first stored in RAM in the computer within the CIG and then transferred to the external computer. (Abstract shortened with permission of author.)

The performance of three maize (Zea mays L.) hybrids, grown under two irrigation levels, was used to investigate the effects of soil moisture competition between adjacent plots, the transmission of these effects into multi-row adjacent plots and types of multi-row plots and plot borders most effective in shielding from these interplot competition effects. On the basis of grain yield, competition effects intended to the second rows of five-row plots necessitating more than five-row plots to accurately evaluate the full transmission of interplot competition effects into adjacent plots. Evaluation of genotypes in one-row plots all with the same common border row genotype to make them three-row plots would be more suitable for evaluation of relative competitiveness for soil moisture under soil moisture stress conditions compared to no border rows or border rows of the same genotype being evaluated.