Quantifying the contribution of mass flow to nitrogen acquisition by an individual plant root.

The classic model of nitrogen (N) flux into roots is as a Michaelis-Menten (MM) function of soil-N concentration at root surfaces. Furthermore, soil-N transport processes that determine soil-N concentration at root surfaces are seen as a bottleneck for plant nutrition. Yet, neither the MM relationship nor soil-N transport mechanisms are represented in current terrestrial biosphere models.

Clinical Outcomes of Computed Tomography-Based Volumetric Brachytherapy Planning for Cervical Cancer.

Purpose/objectives: A report of clinical outcomes of a computed tomography (CT)-based image guided brachytherapy (IGBT) technique for treatment of cervical cancer.

Methods And Materials: Seventy-six women with International Federation of Gynecology and Obstetrics stage IB to IVA cervical carcinoma diagnosed between 2007 and 2014 were treated with definitive external beam radiation therapy (EBRT) with or without concurrent chemotherapy followed by high-dose-rate (HDR) IGBT. All patients underwent planning CT simulation at each implantation.

New insights into carbon allocation by trees from the hypothesis that annual wood production is maximized.

Allocation of carbon (C) between tree components (leaves, fine roots and woody structures) is an important determinant of terrestrial C sequestration. Yet, because the mechanisms underlying C allocation are poorly understood, it is a weak link in current earth-system models. We obtain new theoretical insights into C allocation from the hypothesis (MaxW) that annual wood production is maximized.

Plant root distributions and nitrogen uptake predicted by a hypothesis of optimal root foraging.

CO(2)-enrichment experiments consistently show that rooting depth increases when trees are grown at elevated CO(2) (eCO(2)), leading in some experiments to increased capture of available soil nitrogen (N) from deeper soil. However, the link between N uptake and root distributions remains poorly represented in forest ecosystem and global land-surface models. Here, this link is modeled and analyzed using a new optimization hypothesis (MaxNup) for root foraging in relation to the spatial variability of soil N, according to which a given total root mass is distributed vertically in order to maximize annual N uptake.

Why does leaf nitrogen decline within tree canopies less rapidly than light? An explanation from optimization subject to a lower bound on leaf mass per area.

A long-established theoretical result states that, for a given total canopy nitrogen (N) content, canopy photosynthesis is maximized when the within-canopy gradient in leaf N per unit area (N(a)) is equal to the light gradient. However, it is widely observed that N(a) declines less rapidly than light in real plant canopies. Here we show that this general observation can be explained by optimal leaf acclimation to light subject to a lower-bound constraint on the leaf mass per area (m(a)).

Modeling carbon allocation in trees: a search for principles.

We review approaches to predicting carbon and nitrogen allocation in forest models in terms of their underlying assumptions and their resulting strengths and limitations. Empirical and allometric methods are easily developed and computationally efficient, but lack the power of evolution-based approaches to explain and predict multifaceted effects of environmental variability and climate change. In evolution-based methods, allocation is usually determined by maximization of a fitness proxy, either in a fixed environment, which we call optimal response (OR) models, or including the feedback of an individual's strategy on its environment (game-theoretical optimization, GTO).

Leaf-trait variation explained by the hypothesis that plants maximize their canopy carbon export over the lifespan of leaves.

Measured values of four key leaf traits (leaf area per unit mass, nitrogen concentration, photosynthetic capacity, leaf lifespan) co-vary consistently within and among diverse biomes, suggesting convergent evolution across species. The same leaf traits co-vary consistently with the environmental conditions (light intensity, carbon-dioxide concentration, nitrogen supply) prevailing during leaf development. No existing theory satisfactorily explains all of these trends.

Rooting depth explains [CO2] x drought interaction in Eucalyptus saligna.

Elevated atmospheric [CO(2)] (eC(a)) often decreases stomatal conductance, which may delay the start of drought, as well as alleviate the effect of dry soil on plant water use and carbon uptake. We studied the interaction between drought and eC(a) in a whole-tree chamber experiment with Eucalyptus saligna. Trees were grown for 18 months in their C(a) treatments before a 4-month dry-down.

CO2 enhancement of forest productivity constrained by limited nitrogen availability.

Stimulation of terrestrial plant production by rising CO(2) concentration is projected to reduce the airborne fraction of anthropogenic CO(2) emissions. Coupled climate-carbon cycle models are sensitive to this negative feedback on atmospheric CO(2), but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO(2) fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO(2) concentration over 11 y in a free-air CO(2) enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee.

Low temperature studies of the excited-state structure of negatively charged nitrogen-vacancy color centers in diamond.

We report a study of the 3E excited-state structure of single negatively charged nitrogen-vacancy (NV) defects in diamond, combining resonant excitation at cryogenic temperatures and optically detected magnetic resonance. A theoretical model is developed and shows excellent agreement with experimental observations. In addition, we show that the two orbital branches associated with the 3E excited state are averaged when operating at room temperature.

Why is plant-growth response to elevated CO amplified when water is limiting, but reduced when nitrogen is limiting? A growth-optimisation hypothesis.

Experimental evidence indicates that the stomatal conductance and nitrogen concentration ([N]) of foliage decline under CO enrichment, and that the percentage growth response to elevated CO is amplified under water limitation, but reduced under nitrogen limitation. We advance simple explanations for these responses based on an optimisation hypothesis applied to a simple model of the annual carbon-nitrogen-water economy of trees growing at a CO-enrichment experiment at Oak Ridge, Tennessee, USA. The model is shown to have an optimum for leaf [N], stomatal conductance and leaf area index (LAI), where annual plant productivity is maximised.

Mechanisms linking plant productivity and water status for a temperate Eucalyptus forest flux site: analysis over wet and dry years with a simple model.

A simple process-based model was applied to a tall Eucalyptus forest site over consecutive wet and dry years to examine the importance of different mechanisms linking productivity and water availability. Measured soil moisture, gas flux (CO, HO) and meteorological records for the site were used. Similar levels of simulated HO flux in 'wet' and 'dry' years were achieved when water availability was not confined to the first 1.

On the validation of models of forest CO2 exchange using eddy covariance data: some perils and pitfalls.

With the widespread application of eddy covariance technology, long-term records of hourly ecosystem mass and energy exchange are becoming available for forests around the world. These data sets hold great promise for testing and validation of models of forest function. However, model validation is not a straightforward task.

Interaction between sapwood and foliage area in alpine ash (Eucalyptus delegatensis) trees of different heights.

In native stands of Eucalyptus delegatensis R. T. Baker, sapwood area (As) to foliage area (Af) ratios (As:Af) decreased as tree height increased, contradicting the common interpretation of the Pipe Model Theory as well as the generally observed trend of increasing As:Af ratios with tree height.

On the importance of including soil nutrient feedback effects for predicting ecosystem carbon exchange.

To grow, plants need both carbon, which is fixed in photosynthesis, and inorganic nutrients, which are generally obtained from the soil. Much interest currently exists in trying to understand the uptake and storage of carbon by terrestrial ecosystems. This paper investigates to what extent carbon gain and storage are modified by soil nutrient availability.

Subarachnoid pneumocephalus: a rare complication of epidural catheter placement.

Several potential complications may occur during identification of the epidural space. We present a case of subarachnoid pneumocephalus as a rare complication of epidural catheter placement.

Increased understanding of nutrient immobilization in soil organic matter is critical for predicting the carbon sink strength of forest ecosystems over the next 100 years.

The terrestrial biosphere is currently thought to be a significant sink for atmospheric carbon (C). However, the future course of this sink under rising [CO2] and temperature is uncertain. Some contrasting possibilities that have been suggested are: that the sink is currently increasing through CO2 fertilization of plant growth but will decline over the next few decades because of CO2 saturation and soil nutrient constraints; that the sink will continue to increase over the next century because rising temperature will stimulate the release of plant-available soil nitrogen (N) through increased soil decomposition; that, alternatively, the sink will not be sustained because the additional soil N released will be immobilized in the soil rather than taken up by plants; or that the sink will soon become negative because loss of soil C through temperature stimulation of soil respiration will override any CO2 or temperature stimulation of plant growth.

Aboveground net primary production decline with stand age: potential causes.

Aboveground net primary production (ANPP) commonly reaches a maximum in young forest stands and decreases by 0-76% as stands mature. However, the mechanism(s) responsible for the decline are not well understood. Current hypotheses for declining ANPP with stand age include: (1) an altered balance between photosynthetic and respiring tissues, (2) decreasing soil nutrient availability, and (3) increasing stomatal limitation leading to reduced photosynthetic rates.

Foliage, fine-root, woody-tissue and stand respiration in Pinus radiata in relation to nitrogen status.

We measured respiration of 20-year-old Pinus radiata D. Don trees growing in control (C), irrigated (I), and irrigated + fertilized (IL) stands in the Biology of Forest Growth experimental plantation near Canberra, Australia. Respiration was measured on fully expanded foliage, live branches, boles, and fine and coarse roots to determine the relationship between CO(2) efflux, tissue temperature, and biomass or nitrogen (N) content of individual tissues.

Declining forest productivity in aging forest stands: a modeling analysis of alternative hypotheses.

Several explanations have been advanced to account for the decline in forest net primary productivity (NPP) with age in closed-canopy stands including the hypotheses that: (1) sapwood maintenance respiration rate increases, reducing the availability of carbon to support new growth; (2) stomatal conductance and hence photosynthetic efficiency decline; and (3) soil nutrient availability declines. To evaluate these hypotheses we applied the ecosystem model G'DAY to a 40- and a 245-year-old stand of lodgepole pine (Pinus contorta Dougl. ex Loud.

Modeling productivity and transpiration of Pinus radiata: climatic effects.

Climatic effects on annual net carbon gain, stem biomass and annual transpiration were simulated for Pinus radiata D. Don at Canberra and Mt. Gambier.

Sustainable stemwood yield in relation to the nitrogen balance of forest plantations: a model analysis.

We used an existing analytical model of stemwood growth in relation to nitrogen supply, which we describe in an accompanying paper, to examine the long-term effects of harvesting and fire on tree growth. Our analysis takes into account the balance between nitrogen additions from deposition, fixation, and fertilizer applications, and nitrogen losses from stemwood harvesting, regeneration burning, leaching and gaseous emissions. Using a plausible set of parameter values for Eucalyptus, we conclude that nitrogen loss through fire is the main factor limiting sustainable yield, defined as the maximum mean annual stemwood volume increment obtained in the steady state, if management practices are continued indefinitely.

Analytical model of stemwood growth in relation to nitrogen supply.

We derived a simplified version of a previously published process-based model of forest productivity and used it to gain information about the dependence of stemwood growth on nitrogen supply. The simplifications we made led to the following general expression for stemwood carbon (c(w)) as a function of stand age (t), which shows explicitly the main factors involved: c(w)(t) = eta(w)G*/ micro (w)(1 - lambdae(- micro (w)t) - micro (w)e(-lambdat)/lambda - micro (w)), where eta(w) is the fraction of total carbon production (G) allocated to stemwood, G* is the equilibrium value of G at canopy closure, lambda describes the rate at which G approaches G*, and micro (w) is the combined specific rate of stemwood maintenance respiration and senescence. According to this equation, which describes a sigmoidal growth curve, c(w) is zero initially and asymptotically approaches eta(w)G*/ micro (w) with the rate of approach dependent on lambda and micro (w).

Long-Term Response of Nutrient-Limited Forests to CO"2 Enrichment; Equilibrium Behavior of Plant-Soil Models.

Established process-based models of forest biomass production in relation to atmospheric CO"2 concentration (McMurtrie 1991) and soil carbon/nutrient dynamics (Parton et al. 1987) are integrated to derive the @'Generic Decomposition and Yield@' model (G'DAY). The model is used to describe how photosynthesis and nutritional factors interact to determine the productivity of forests growing under nitrogen-limited conditions.