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A mesocosm experiment was carried out in a Norwegian fjord near Bergen in May 2006, with the main objective being the study of the effects of increasing concentrations of atmospheric CO2 (and associated effects such as increased acidification) on blooms of natural marine coastal plankton. Three mesocosms were bubbled with CO2(g) to achieve a high (~700ppm) CO2 concentration (pH ~7.8) to simulate predicted future conditions as a result of rising atmospheric CO2 concentrations. Another three mesocosms were treated as controls and bubbled with ambient air to represent a near pre-industrial scenario (atmospheric CO2 concentration ~300ppm, surface seawater pH ~8.15). Blooms in the mesocosms were stimulated by the addition of nutrients at a near-Redfield ratio ([N:P] ≈ [16:1]), and scientific measurements and analyses were carried out over the course of the blooms for approximately one month. Of particular interest in this study were the autotrophic plankton. The diversity and activities of these microorganisms under the two treatments was therefore investigated. By designing and using new degenerate primers specifically targeting ‘Green-type’ (Form IA and IB), ‘Red-type’ (Form IC and ID) and Form II RuBisCO, analysis of primary producers was carried out using PCR and either gDNA or cDNA (mRNA) templates from key time points spanning the complete duration of the blooms throughout the mesocosm experiment. Over 1250 novel RuBisCO large subunit sequences have been fully annotated and deposited in the NCBI GenBank® database. These sequences revealed distinct changes in the diversity of primary producers both over the courses of the blooms and between treatments. Particularly striking was the effect of acidification on the community structure of the eukaryotic picoplankton, Prasinophytes. A clade of prasinophytes closely related to Micromonas pusilla showed a distinct preference for the high CO2 conditions; a laboratory-based experiment confirmed the high tolerance of Micromonas pusilla to lower pH. Conversely, a clade related to Bathycoccus prasinos was almost entirely excluded from the high CO2 treatments. Clades of form II RuBisCO-containing dinoflagellates were also abundant throughout the experiment in both treatments. The high similarity of some of these clades to the toxin-producing species Heterocapsa triquetra and Gonyaulax polyedra, and apparent high tolerance of some clades to high CO2 conditions, is perhaps cause for concern in a high CO2 world and demands further research. In parallel with the RubisCO work, new primers were designed that target the gene encoding the Fe protein of nitrogenase (NifH). 82 Bergen genomic nifH sequences have been annotated and submitted to GenBank®. These sequences include those from organisms related to Alpha, Beta, and Gammaproteobacteria, and Cluster II and Cluster III sequences that align most closely with anaerobic Bacteria, Gram positive, and/or sulphur-reducing Bacteria. The biggest surprise, however, was the apparent abundance and significance of a Rhodobacter sphaeroides-like microorganism throughout the duration of the experiment in both treatments. Whilst this clade was unsurprisingly absent in the RuBisCO cDNA libraries, all but two of 128 nifH cDNA clones analysed were identical to the gene from Rhodobacter sphaeroides. This shows that this clade was potentially fixing N2 throughout the entire experiment, even in the presence of combined N added to both sets of mesocosms at the start of the experiment. A group of Rhodobacter sphaeroides-like microorganisms present at Bergen may therefore have been an unexpected source of new N during the experiment and contributed to the maintenance of the mesocosm communities as nutrients became depleted. One organism dominated the autotrophic communities after the blooms in both treatments. Synechococcus spp. Form IA rbcL clones most closely related to the coastal strain Synechococcus sp. strain CC9902 were recovered throughout the experiment but were particularly numerous toward the end of the experiment and dominated the “Green-type” libraries at this time. Initially, rbcL clones from these cyanobacteria were mostly derived from the ambient CO2 mesocosms but were equally distributed between treatments by the end of the experiment. This suggests that cyanobacteria related to strain CC9902 may be less tolerant of elevated CO2 (which was greatest at the beginning rather than the end of the experiment). However, despite the mesocosms being Pi-limited at the end of the experiment, several Synechococcus species (including those related to strain CC9902 and another coastal strain, CC9311) thrived. Following on from this observation, Pi uptake and assimilation mechanisms in a Synechococcus species were investigated in the laboratory. This led to the sequencing and characterisation of a pstS gene from the marine cyanobacterium Synechococcus sp. WH 8103. Unlike conventional pstS, it was discovered that the pstS II gene in this organism is constitutively expressed and unresponsive to or only weakly regulated by Pi supply. The use of PstS/pstS as a marker for P-limitation in natural samples, therefore, should be interpreted with caution.

The market demand together with the need for alternatives to withstand climate change led to the recovery of autochthonous grapevine varieties. Under climate change, the summer pruning of vineyards may lead to an increase of vegetative residuals of nutritional and medicinal interest. The objectives of our study were (1) to evaluate the nutritional properties of the leaves of three local Spanish grapevines (Tinto Velasco, TV, Pasera, PAS, and Ambrosina, AMB) when grown under climate change conditions, and (2) to test the potentiality of these grapevines as suitable candidates to be cultivated under climate change scenarios based on the quality of their must. Experimental assays were performed with fruit-bearing cuttings grown in temperature gradient greenhouses that simulate rising CO2 (700 µmol mol−1 ) and warming (ambient temperature +4 ◦C), either acting alone or in combination. TV and AMB were the most and the least affected by air temperature and CO2 concentration, respectively. The interaction of elevated CO2 with high temperature induced the accumulation of proteins and phenolic compounds in leaves of TV, thus enhancing their nutritional properties. In PAS, the negative effect of high temperature on protein contents was compensated for by elevated CO2 . Warming was the most threatening scenario for maintaining the must quality in the three varieties, but elevated CO2 exerted a beneficial effect when acting alone and compensated for the negative effects of high temperatures. While TV may be a candidate to be cultivated in not very warm areas (higher altitudes or colder latitudes), PAS behaved as the most stable genotype under different environmental scenarios, making it the most versatile candidate for cultivation in areas affected by climate change.

Nutrient inputs and climatic conditions can significantly impact soil phosphorus (P) bioavailability and dynamics by influencing chemical, biological, and biochemical processes. This research aimed at investigating the effects of nitrogen (N) and P inputs and elevated atmospheric carbon dioxide (CO2) on soil P bioavailability and dynamics under New Zealand pasture soils. Inorganic P (Pi) and organic P (Po) fractions, together with biological and biochemical properties linked to P cycling, were assessed. In P-deficient soil (4 mg kg-1 Olsen P) with high organic matter content (7.2 %), P addition alone (45 kg P ha-1) or in combination with N (45 kg P ha-1 + 200 kg N ha-1) increased plant growth, total P and N uptake, microbial biomass P, organic anion release, and the depletion of moderately labile Pi and stable Po, whereas N addition alone (200 kg N ha-1) had no impact on plant growth and soil P fractions under legumes (Lupinus angustifolius and Trifolium repens L.) and grasses (Lolium perenne L. and Triticum aestivum L.). In N-limited soil with an Olsen P of 10 mg kg-1, N addition alone (250 kg N ha-1) or in combination with P (250 kg N ha-1 + 50 kg ha-1) increased shoot biomass and shoot P uptake of Italian ryegrass (Lolium multiflorum Lam.) by an average of 1.6-fold compared to the control. In contrast, P addition alone (50 kg P ha-1) had no impact on shoot biomass and shoot N uptake but significantly increased Olsen P and shoot P uptake. Acid and alkaline phosphatase activities increased in summer, especially under N addition treatments, which was attributed to increased plant P demand, alleviation of carbon limitation, and higher temperature. Nitrogen addition alone decreased readily available Pi, labile Pi, labile Po, and moderately labile Pi by 55, 19, 28, and 7 %, respectively, compared to the control treatment. On the other hand, the combined addition of N and P promoted labile Po mineralisation (28 %) and the depletion of labile P fractions but did not mobilise the moderately labile Pi pool compared to P addition alone. In a long-term P fertilisation experiment (0, 188, and 376 kg superphosphate ha-1 yr-1 for more than 65 years) under grazed pastures, soil P availability increased with increasing P inputs, but microbial biomass P was similar under 188 and 376 kg superphosphate ha-1 yr-1. The latter result was attributed to microbial biomass need to maintain nutrient stoichiometry. Alkaline phosphatase activity increased in response to long-term P inputs, whereas acid phosphatase activity decreased. This finding supported the differentiation in origin and nutrient demand between acid and alkaline phosphatase enzymes, where acid phosphatase enzymes are released from plant roots and inhibited by high P availability, whereas alkaline phosphatase enzymes are derived from soil microbes and driven by C availability rather than P. In P-deficient soil (7 mg kg-1 Olsen P), plant biomass, P uptake, and rhizosphere properties did not respond to elevated CO2 (700 ppm) but were significantly impacted by plant species (Lupinus angustifolius, Lolium perenne L., and Triticum aestivum L.). Hence, soil P availability and P fractions distribution were similar under ambient and elevated CO2 due to the negative feedback of P deficiency on photosynthesis. Twenty-two years of CO2 enrichment (500 ppm) under grazed pasture decreased labile and moderately labile Pi by 39 and 15 %, respectively, while promoting the accumulation of moderately labile and stable Po by 26 and 17 %, respectively, compared to ambient CO2. Decrease in soil Pi was attributed to plant P uptake and immobilisation of Pi in the microbial biomass. Accumulation of Po was linked to enhanced biological activity, increased inputs of Po from root detritus, and adsorption onto reactive mineral surfaces of aluminium, iron, and calcium. Plant species investigated in this study and their associated rhizosphere microbes were able to mobilise sparingly available and recalcitrant soil P fractions. This result challenges the recalcitrance of some soil P fractions, which is defined by resistance to extraction and clearly not to action by plants or microbes. Legumes, especially blue lupin, have shown higher ability in releasing organic anions and phosphatase enzymes, which concurred with higher mineralisation of organic P. Under real soil conditions, organic anions played a minor role in Pi acquisition under both legumes and grasses. Nutrient addition experiments and long-term studies revealed that besides P, C and N availability played a critical role in P dynamics and cycling by impacting microbial P transformations and phosphatase enzyme activity. Applications of both N and P in excess of plant requirements cannot accelerate soil P cycling but, in the contrary, can cause soil P accumulation and eutrophication of waterways. Phosphorus deficiency can limit plant ability to grow and acquire P under elevated CO2 conditions. Elevated CO2 can decrease soil P availability under permanent agroecosystems characterized by organic matter accumulation and high availability of free cations (Al, Fe and Ca). Further research is needed to evaluate changes in soil P fractions in response to N and P inputs under N and P co-limited soils. Investigation of the combined effects of elevated CO2, increased temperature, and water stress on soil P availability and dynamics is warranted.

The associated increase in global mean surface temperature together with raised atmospheric carbon dioxide (CO2 ) concentration is exerting a profound influence on grapevine development (phenology) and grape quality. The exploitation of the local genetic diversity based on the recovery of ancient varieties has been proposed as an interesting option to cope with climate change and maintaining grape quality. Therefore, this research aimed to characterize the potential fruit quality of genotypes from seven local old grapevine varieties grown under climate change conditions. (2) Methods: The study was carried out on fruit-bearing cuttings (one cluster per plant) that were grown in pots in temperature gradient greenhouses (TGG). Two treatments were applied from fruit set to maturity: (1) ambient CO2 (400 ppm) and temperature (T) (ACAT) and (2) elevated CO2 (700 ppm) and temperature (T + 4 ◦C) (ECET). (3) Results: Results showed that some of the old genotypes tested remained quite stable during the climate change conditions in terms of fruit quality (mainly, total soluble solids and phenolic content) and of must antioxidant properties. (4) Conclusion: This research underlines the usefulness of exploiting local grapevine diversity to cope with climate change successfully, although further studies under field conditions and with whole plants are needed before extrapolating the results to the vineyard.

A key uncertainty in ecology is how concurrent global change factors will interact to affect terrestrial ecosystems. Humans have altered Earth’s carbon dioxide (CO2) concentrations, climate, nutrient levels, and biodiversity, all of which affect plant communities and ecosystem function. Yet, few multi-factor field studies exist to examine interactive effects of global changes on plants and ecosystems. I characterized the physiological responses of perennial grassland species from four plant functional groups (C3 grasses, C4 grasses, nitrogen-fixing leguminous forbs, and non-leguminous forbs) to single and interactive global changes including elevated carbon dioxide, increased soil nitrogen supply, reduced rainfall, and climate warming. I also determined how elevated CO2, increased soil nitrogen supply, and planted species richness affected total ecosystem carbon (C) storage over 19 years. These studies took place in the open-air, global change grassland ecosystem experiment, BioCON (Biodiversity x CO2 x Nitrogen), located at the Cedar Creek Ecosystem Science Reserve in Minnesota, USA. I present evidence that (1) the ability of plants to capture additional C as atmospheric CO2 rises via photosynthesis may be more limited than traditionally believed; (2) substantial, sustained declines in stomatal conductance and increases in water-use efficiency under elevated CO2 occur widely among grassland species; (3) global change factors interact in complex ways to affect photosynthesis, and how factors interact varies among grassland species; and (4) declines in biodiversity may influence ecosystem C storage more than a 50% increase in CO2 or high rates of nitrogen deposition in perennial grassland systems. These findings show that simple predictions of plant physiological responses to global changes based on theoretical expectations of isolated effects and on functional classifications of species are not sufficient – global changes and other environmental factors interact in complex ways to impact responses of species. These results also highlight the importance of biodiversity in promoting ecosystem function and call into question whether elevated CO2 will increase the C sink in grassland ecosystems and help to slow climate change.

Environmental stress factors caused by climate change affect plant growth and crop production, and pose a growing threat to sustainable agriculture, especially for tree crops. In this context, we sought to investigate the responses to climate change of two Prunus rootstocks (GF677 and Adesoto) budded with Catherina peach cultivar. Plants were grown in 15 L pots in temperature gradient greenhouses for an 18 days acclimation period after which six treatments were applied: [CO2 levels (400 versus 700 mol mol-1), temperature (ambient versus ambient + 4°C), and water availability (well irrigated versus drought)]. After 23 days, the effects of stress were evaluated as changes in physiological and biochemical traits, including expression of relevant genes. Stem water potential decreased under drought stress in plants grafted on GF677 and Adesoto rootstocks; however, elevated CO2 and temperature affected plant water content differently in both combinations. The photosynthetic rate of plants grafted on GF677 increased under high CO2, but decreased under high temperature and drought conditions. The photosynthetic rates of plants grafted onto Adesoto were only affected by drought treatment. Furthermore, in GF677-Catherina plants, elevated CO2 alleviated the effect of drought, whereas in those grafted onto Adesoto, the same condition produced acclimation in the rate. Stomatal conductance decreased under high CO2and drought stress in both graftedrootstocks, and the combination of these conditions improved water-use efficiency.Changes in the sugar content in scion leaves and roots were significantly differentunder the stress conditions in both combinations. Meanwhile, the expression of mostof the assessed genes was significantly affected by treatment. Regarding genotypes,GF677 rootstock showed more changes at the molecular and transcriptomic level thandid Adesoto rootstock. A coordinated shift was found between the physiological statusand the transcriptomic responses. This study revealed adaptive responses to climatechange at the physiological, metabolic, and transcriptomic levels in twoPrunusrootstocks budded with 'Catherina'. Overall, these results demonstrate the resilient capacity andplasticity of these contrasting genotypes, which can be further used to combat ongoingclimate changes and support sustainable peach production.

Elevated concentrations of CO2 (CO2) in plants with C3 photosynthesis metabolism, such as wheat, stimulate photosynthetic rates. However, photosynthesis tends to decrease as a function of exposure to high (CO2) due to down-regulation of the photosynthetic machinery, and this phenomenon is defined as photosynthetic acclimation. Considerable efforts are currently done to determine the effect of photosynthetic tissues, such us spike, in grain filling. There is good evidence that the contribution of ears to grain filling may be important not only under good agronomic conditions but also under high (CO2). The main objective of this study was to compare photoassimilate production and energy metabolism between flag leaves and glumes as part of ears of wheat (Triticum turgidum L. subsp. durum cv. Amilcar) plants exposed to ambient [a(CO2)] and elevated [e(CO2)] (CO2) (400 and 700 µmol mol−1 , respectively). Elevated CO2 had a differential effect on the responses of flag leaves and ears. The ears showed higher gross photosynthesis and respiration rates compared to the flag leaves. The higher ear carbohydrate content and respiration rates contribute to increase the grain dry mass. Our results support the concept that acclimation of photosynthesis to e(CO2) is driven by sugar accumulation, reduction in N concentrations and repression of genes related to photosynthesis, glycolysis and the tricarboxylic acid cycle, and that these were more marked in glumes than leaves. Further, important differences are described on responsiveness of flag leaves and ears to e(CO2) on genes linked with carbon and nitrogen metabolism. These findings provide information about the impact of e(CO2) on ear development during the grain filling stage and are significant for understanding the effects of increasing (CO2) on crop yield.

Atmospheric CO2 enrichment is expected to benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants procure most of their N as two inorganic forms, nitrate (NO3-) or ammonium (NH4+), using membrane-localized transport proteins in roots which are key targets of improving N-use efficiency in plants. Though it is expected that there will be interactive effects of elevated CO2, chronic warming, and N form on root N relations, this has not been studied. In this study, tomato (Solanum lycopersicum L. cv. Big Boy) plants were grown at two levels of CO2 (ambient=400 ppm vs. elevated=700 ppm) and under two temperature regimes (30oC vs. 37oC), while providing either NO3- (1.5 mM) or NH4+ (1.5 mM) as the sole N source. To assess plant N relations under these conditions, plant biomass (shoot, root, total), %N (shoot, root, total), root N-uptake rate, the concentration of total root proteins, and the concentration of N uptake and assimilatory proteins in roots per unit total root protein and per gram of dry root mass were measured. Elevated CO2 and chronic warming acted synergistically to severely inhibit the growth of plants, regardless of N form, while individually, these factors slightly increased growth. Although %N in roots among all treatments was similar, the combination of elevated CO2 and warming decreased (1) the rate of N uptake by roots, (2) the concentration of total protein in roots, indicating an inhibition of N assimilation (especially in NH4+-treated plants), and (3) shoot %N, indicating a possible negative effect on N translocation from roots to shoots (especially in NO3--treated plants). However, for the most part, the concentrations in roots of the main N-uptake and N-assimilation proteins were little affected by elevated CO2 and warming, and when affected, were not correlated with root total protein or N-uptake rate; hence, N uptake and assimilation in roots were controlled by the activities of these proteins rather than their concentrations. Taken together, these results indicate that elevated CO2 and chronic warming will act synergistically to decrease plant N uptake and assimilation, thereby decreasing growth and protein concentration; thus, future global warming may potentially decrease both crop production and its food quality.

Humans are altering tropical ecosystems and their biotic and biogeochemical processes with unprecedented scale and severity. The increasing size and abundance of tropical lianas (woody climbing plants) relative to trees may be the result of global anthropogenic change, and may further alter forest function. Yet the mechanisms responsible for this reported phenomenon are unclear, and the scale at which it occurs has been unexamined. In this dissertation, I use a combination of empirical experimentation, ground-based forest censuses, and advanced airborne remote sensing imagery and analysis to investigate the question of why lianas are increasing and at what spatial scale. First, I tested the hypothesis that elevated CO2 gives species of lianas a growth advantage relative to trees, especially during periods of seasonal drought. In the first experiments to directly compare the relative response of tropical liana and tree species to elevated CO2, I found no significant differences between the two growth forms. Both lianas and trees responded equally well to elevated CO2, even when soil water was limited by seasonal drought. Second, I extended the tests of liana-tree response to CO2 to include the effects of soil nutrient availability. No interactions between elevated CO2 and either soil nitrogen or phosphorus availability were found for lianas. Instead, changes to soil nutrient availability or CO2 alone had strong and significant effects on lianas. Finally, I used data collected from my field censuses to train machine learning algorithms to detect severe liana coverage in tree canopies using high-resolution hyperspectral imagery. This method proved to be very accurate at distinguishing severe liana cover from liana-free cover in tree canopies, and quantified severe liana infestation as 11.9%-18.0% of the total canopy cover over a 600-ha tropical forest. The results of the experiments and the development of landscape-scale liana detection methods are key steps toward a full understanding of the mechanisms and scope of the liana increase.

Ectomycorrhizal communities may play a major role in preventing decreases in forest productivity associated with the depletion of nutrients caused by anthropogenic nitrogen and in facilitating increased productivity in response to elevated atmospheric CO2 concentrations. In my doctoral research I attempted to shed some light on the following question: As nutrient demand by forest trees is altered by human induced global change how will the functioning of ectomycorrhizal communities respond? In chapter 1, I present the results of a study of the effects of nitrogen fertilization on ectomycorrhizal communities in an Eastern US hardwood forest. I found the ectomycorrhizal communities of the organic and mineral horizon to be quite distinct, and that high, but not moderate levels of nitrogen fertilization altered the ectomycorrhizal community composition and decreased ectomycorrhizal species richness. I also found that ectomycorrhizal colonization intensity increased in the mineral soil, and when considered in conjunction with other studies conducted in the same research forest, this may indicate that the ectomycorrrhizal community is shifting in accordance with the shifting nutrient demands of the forest. In Chapter 2, I attempted to elucidate the role of soil heterogeneity in shaping fungal community composition in the mineral soil. Depth and soil carbon content were consistently correlated with fungal community composition. The parent material from which the overlying soil is derived and soil calcium content may be important in determining fungal community composition but our sampling scheme did not allow us to isolate these chemical factors from the potential influence of geographic location on fungal community. In chapter 3, I present a literature review that endeavored to determine whether there is potential for ectomycorrhizal communities or individuals to alter their mineral weathering capabilities and nutrient provision to host plants in response to altered nutrient demand from their host plants. Knowledge of the mechanisms that control belowground carbon allocation by plants in response to nutrient demand is limited, but there is a potential for plants to respond to increased demand of phosphorous and potassium by allocating more carbon to the fungal symbionts most adept at providing these nutrients. Future studies on ectomycorrhizal weathering should explicitly test the role of host nutrient demand in stimulating fungal weathering. In chapter 4, I investigated the role of plants, ectomycorrhizae, and low molecular weight organic acids in stimulating mineral weathering, as well as the potential for elevated carbon dioxide to affect biotic weathering. We found that plants, but not their associated ectomycorrhizae, stimulated weathering. Elevated CO2 did not affect weathering rates. The lack of an effect of ectomycorrhizal colonization may have been due to low levels of mycorrhizal colonization. The biotic weathering observed in this study was driven by the uptake of nutrient cations, and not by substrate acidification or root exudation. My doctoral research suggests that ectomycorrhizae may play an important role in mineral nutrient provision.

The concentration of atmospheric carbon dioxide (CO2) in the atmosphere has increased by 35% since pre-industrial levels. Projections for the next 100 years indicate an increase to levels between 490 and 1260 parts per million by volume (ppm) of CO2, equating to a 75 % to 350 % increase in concentration since the year 1750. Associated with this increase in [CO2] will be a 1.4 to 5.8º C increase in lower atmospheric temperature. While past research has attempted to address the effects of such climatic changes on individual plant responses, predictions of plant responses at the ecosystem level are still highly uncertain. Difficulties lie in the enormous variation of plant responses to climate change variables among and within species, and between and within environmental conditions. Past research assumed that plants using either the C3 or C4 metabolic pathways would respond differently but predictably to climate-change variables based on their metabolic pathway. Recent evidence has suggested however, that the added interactions of external environmental variables and species-specific sensitivities to climate change make it difficult to predict plant and ecosystem responses to climate change. To investigate the mechanisms behind responses of Australian grasses to climate change, 2 pot experiments was conducted using growth cabinets to compare the effect of elevated CO2 and water-limitation on the invasive C4 grassland plant, Eragrostis curvula (E. curvula), native Australian C3 grassland plant, Austrodanthonia racemosa (A. racemosa), and wheat species, Triticum aestivum (T. aestivum). The experiment was run at ambient levels of CO2 maintained at 390 ppm compared to elevated levels of 740 ppm. Imposed restrictions to water supply consisted of gradually drying the soil down to 30 % available soil water (ASW) followed by re-wetting to 50 % ASW. Well-watered conditions for the experiment consisted of gradually drying the soil down to 50 % ASW, followed by rewetting to 95 % ASW. Plants were grown in mixtures and monocultures, consisting of 9 plants equally spaced in a grid design. The three significant findings of the thesis were that: 1) the metabolic pathway (C3 versus C4) was not always an accurate predictor of biomass accumulation under elevated CO2 in the plants studied. Previous research suggested that CO2-stimulation of photosynthesis in C3 plants would lead to greater increases in biomass under elevated CO2 compared to C4 plants, though both C3 and C4 plants could benefit from any reduction in stomatal conductance under dry conditions at elevated CO2. The results from the experiments in this thesis showed a strongly significant biomass response to elevated CO2 in both dry and wet conditions for C4 grass E. curvula. The C3 grass A. racemosa in dry conditions, did not. It was speculated that without the CO2-induced water conservation effect, the C3 grass experienced photosynthetic down-regulation and this precluded a positive biomass response under elevated CO2. 2) the magnitude and direction of biomass response to elevated CO2 was dependant on factors such as resource-availability and the phenotypic variability of the plants species. 3) critical analysis of results from this thesis, combined with past research on plant responses under elevated CO2 showed a tendency for researchers to repeatedly test plants from the Poaceae family, or close relatives of the Poaceae family. As a result, when past data were corrected for this lack of independence, there was no relationship between the evolution of the C3 and C4 metabolic pathway and biomass response to elevated CO2. Instead, other factors (such as growth rate, plant height, leaf number, etc) were presented as being more important in determining biomass response. These observations were supported by results found in this thesis.

Rising atmospheric CO2 and the need to understand potential impacts on terrestrial ecosystems has become increasingly recognized. Models can play a beneficial part in this research to enhance understanding of ecosystem responses to changing conditions like elevated CO2. In this study, data from a long term elevated CO2 experiment in a native forested ecosystem in east central Florida were employed to assess the utility of a multi-layer canopy photosynthesis model as a tool to better understand the responses to elevated CO2 in this ecosystem. Model results compared satisfactorily with the canopy gas exchange measurements in this ecosystem for the period modeled. Sensitivity analyses were used to evaluate the robustness of the model and understand the effects that changing model parameters had on model results, i.e. carbon assimilation in the system. The parameters evaluated included canopy height, leaf area density profile, number of canopy layers, maximum rate of carboxylation (Vcmax), and canopy species composition. Results of the sensitivity analyses point to structure and species as being important to carbon assimilation in this ecosystem. Although only an initial examination, this model could be a valuable tool to further understanding of the response of this important ecosystem to increasing CO2 and indicates that further work is certainly warranted.