Impacts of Fine-roots on Terrestrial Net Primary Productivity and Soil Nutrient Cycling

Large uncertainties remain in fine-root longevity, and contribution to terrestrial nutrient cyling. In my dissertation, I utilized a carbon isotope tracer at a long-term Free-Air CO2 Enrichment (FACE) experiment in a Liquidambar styraciflua plantation to examine properties of fine-roots including longevity and sources of carbon for growth and respiration. Soil cores were sequentially sampled for multiple growing seasons following cessation of CO2 fumigation. Fine-roots were extracted from soil and separated by diameter. Newly produced roots were produced from exclusively new photosynthate. Fine-root carbon was replaced more slowly, with about half of the carbon remaining after two full growing seasons. Model fitting found at least two turnover rates for carbon occur in the fine-root population, with 10% of carbon quickly being turned over ( 2 years). In a follow-up study, I utilized a potentially more functional approach by separating roots by root branching order. Results indicate that branching order and root nitrogen concentration correlate with root longevity. Thus, easily measurable traits such as nitrogen concentration may help elucidate root longevity in different species or at larger spatial scales. Knowledge of the longevity and standing biomass of fine-roots is essential for quantifying fine-root contribution to terrestrial NPP and forest nutrient cycling. An extensive literature review was conducted to examine fine-root biomass within branching orders, with just 10 reports in the literature. Even with this sparse data-set, it is clear that environmental conditions such as nutrient and water availability impact fine-root biomass distribution. Increased studies quantifying the amount of biomass in roots by branching order will be needed to fully calculate fine-root contribution to terrestrial carbon and nutrient cycling. A final contribution from my thesis is the creation of a dynamic vegetation model that optimizes both above- and below-ground biomass allocation with respect to changing environmental conditions. Results from empirical research, including that done in my other chapters, conclude that biomass allocation is plastic with respect to abiotic conditions. No current simple modeling scheme adequately captures this plasticity. It is hoped that the model developed here can make progress towards that goal

Plasticity in Bundle Sheath Extensions of Heterobaric Leaves

Premise of the study: Leaf venation is linked to physiological performance, playing a critical role in ecosystem function. Despite the importance of leaf venation, associated bundle sheath extensions (BSEs) remain largely unstudied. Here, we quantify plasticity in the spacing of BSEs over irradiance and precipitation gradients. Because physiological function(s) of BSEs remain uncertain, we additionally explored a link between BSEs and water use efficiency (WUE). Methods: We sampled leaves of heterobaric trees along intracrown irradiance gradients in natural environments and growth chambers and correlated BSE spacing to incident irradiance. Additionally, we sampled leaves along a precipitation gradient and correlated BSE spacing to precipitation and bulk delta C-13, a proxy for intrinsic WUE. BSE spacing was quantified using a novel semiautomatic method on fresh leaf tissue. Key results: With increased irradiance or decreased precipitation, Liquidambar styraciflua decreased BSE spacing, while Acer saccharum showed little variation in BSE spacing. Two additional species, Quercus robur and Platanus occidentalis, decreased BSE spacing with increased irradiance in growth chambers. BSE spacing correlated with bulk delta C-13, a proxy for WUE in L. styraciflua, Q. robur, and P. occidentalis leaves but not in leaves of A. saccharum. Conclusions: We demonstrated that BSE spacing is plastic with respect to irradiance or precipitation and independent from veins, indicating BSE involvement in leaf adaptation to a microenvironment. Plasticity in BSE spacing was correlated with WUE only in some species, not supporting a function in water relations. We discuss a possible link between BSE plasticity and life history, particularly canopy position

Supplemental methods: Detailed model derivation, parameterization and validation from The world's biomes and primary production as a triple tragedy of the commons foraging game played among plants

In the main text we detail a general form of the model. To generate output we used the simplest possible first-order approximations of plant function. These have been previously applied to plants, but we derive these functions below, and provide a rationale for the parameters used. We also discuss the FLUXNET and MOD17 datasets in more detail. Finally, we show how the model can be expanded to include any number of substitutable or essential plant resource