Ross E. McMurtrie

Progressive Nitrogen Limitation of Ecosystem Responses to Rising Atmospheric Carbon Dioxide

Abstract A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental results and to help researchers integrate models and data, direct tests of the PNL hypothesis remain a great challenge to the research community.

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. Recent empirical and modeling studies reveal that mechanisms (2) and (3) are largely responsible for age-related decline in ANPP for forests in cold environments. Increasing respiratory costs appear to be relatively unimportant in explaining declining productivity in ageing stands.

Climatic factors controlling the productivity of Norway spruce: A model-based analysis

The process-based growth model, BIOMASS, was modified to incorporate low-temperature effects on photosynthetic production in Norway spruce (Picea abies) stands growing in northern Sweden. The low-temperature features incorporated in BIOMASS made it possible to simulate and estimate the reduction in photosynthetic rates caused by boreal conditions. The following four simulation-scenarios were used: (i) ‘potential’ photosynthesis without boreal restrictions; (ii) reduction caused by a frozen soil; (iii) reduction caused by incomplete recovery of photosynthetic capacity during spring as a result of damage caused by low winter temperatures; and (iv) reduction as an effect of frost-induced autumn decline. Annual photosynthetic production (or gross primary production (GPP)) was simulated for three calendar years, 1990‐1992, for stands with low (control) and high (irrigated and fertilized) nutrient availability. The reduction of ‘potential’ GPP, caused by the lowtemperature effects, ranged from 35‐44% for control (C) and from 34‐42% for irrigated-fertilised (IL) stands, respectively. The most pronounced loss of ‘potential’ GPP originated from reduced photosynthetic capacity, in spring and early summer, which led to losses of 21‐28% for C and 19‐26% for IL stands. The variation between years differed mainly as an effect of differences in spring temperatures, which resulted in different rates of recovery of photosynthetic capacity. Reductions caused by frozen soil and low photosynthetic capacity during winter were similar in C and IL stands (12‐13%), as were the losses resulting from severe autumn frosts (3‐4%). It is concluded that, unless the effects of frozen soils and reduced photosynthetic capacity during spring and early summer are considered, large errors (ca. 40%) will be introduced into estimates of the annual photosynthetic production of boreal conifer forests. # 1998 Elsevier Science B.V.

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 individuals strategy on its environment (game-theoretical optimization, GTO). Optimal response models can predict allocation in single trees and stands when there is significant competition only for one resource. Game-theoretical optimization can be used to account for additional dimensions of competition, e.g., when strong root competition boosts root allocation at the expense of wood production. However, we demonstrate that an OR model predicts similar allocation to a GTO model under the root-competitive conditions reported in free-air carbon dioxide enrichment (FACE) experiments. The most evolutionarily realistic approach is adaptive dynamics (AD) where the allocation strategy arises from eco-evolutionary dynamics of populations instead of a fitness proxy. We also discuss emerging entropy-based approaches that offer an alternative thermodynamic perspective on allocation, in which fitness proxies are replaced by entropy or entropy production. To help develop allocation models further, the value of wide-ranging datasets, such as FLUXNET, could be greatly enhanced by ancillary measurements of driving variables, such as water and soil nitrogen availability.

A model of canopy photosynthesis and water use incorporating a mechanistic formulation of leaf CO2 exchange

Abstract A model of the carbon uptake and water balance of forest stands is described where leaf photosynthesis is represented by a mechanistic model of photosynthesis by C3 plants. Data are presented to support an empirical relationship linking stomatal conductance, photosynthesis, relative humidity and ambient CO2 concentration. The model was applied to stands of Pinus radiata subject to extremes of water and nutrient availability. Simulated water storage in the root zone agreed with measurements conducted over a 5 year period. Simulated and measured seasonal patterns of water use reached maximum rates of approximately 7 mm day−1 in summer for irrigated stands with projected leaf area indices of approximately 8. Simulated annual net photosynthesis (net of photorespiration and day-time foliar respiration) ranged from approximately 17 Mg C ha−1 year−1 for control stands to approximately 45 Mg C ha−1 year−1 for irrigated and fertilised stands.

Why is plant-growth response to elevated CO2 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 CO2 enrichment, and that the percentage growth response to elevated CO2 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 CO2-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. The optimisation is represented in terms of a trade-off between LAI and stomatal conductance, constrained by water supply, and between LAI and leaf [N], constrained by N supply. At elevated CO2 the optimum shifts to reduced stomatal conductance and leaf [N] and enhanced LAI. The model is applied to years with contrasting rainfall and N uptake. The predicted growth response to elevated CO2 is greatest in a dry, high-N year and is reduced in a wet, low-N year. The underlying physiological explanation for this contrast in the effects of water versus nitrogen limitation is that leaf photosynthesis is more sensitive to CO2 concentration ([CO2]) at lower stomatal conductance and is less sensitive to [CO2] at lower leaf [N].

Using a simulation model to evaluate the effects of water and nutrients onthe growth and carbon partitioning of Pinus radiata

Abstract The model BIOMASS calculates the water and carbon balances of tree stands. Parameters for the model were estimated using data from an experiment in which a wide range of water and nutritional conditions were imposed on plots in a stand of Pinus radiata . The treatments caused considerable differences in the growth of trees. Net Primary Production (NPP) was estimated from total CO 2 uptake by the stand, minus the respiration rates of the component parts of the trees. CO 2 uptake was calculated from intercepted radiation and the photosynthetic properties of the foliage. Because of the close similarities between the processes involved in transpiration and CO 2 uptake, excellent correspondence ( r 2 = 0.92−0.94) between calculated and measured daily soil water balances over 4 years, gave confidence in the total CO 2 uptake calculations. There were differences in the relationships between NPP and above-ground production in the irrigated trees, where internal nutrient levels fell significantly, compared with trees in the other treatments. NPP was partitioned to foliage stems and roots using these relationships to estimate the partitioning coefficients. Above-ground production was 60–80% of NPP, stem growth was about 40% and root growth varied from 20 to 40% of NPP, with the higher value in the irrigated (low nutrient) treatment. The correspondence between observed and simulated growth patterns of trees — particularly those of stem and foliage — was excellent, indicating that BIOMASS can reproduce differences in growth caused by water and fertility.

Optimal function explains forest responses to global change

Plant responses to global changes in carbon dioxide (CO2), nitrogen, and water availability are critical to future atmospheric CO2 concentrations, hydrology, and hence climate. Our understanding of those responses is incomplete, however. Multiple-resource manipulation experiments and empirical observations have revealed a diversity of responses, as well as some consistent patterns. But vegetation models—currently dominated by complex numerical simulation models—have yet to achieve a consensus among their predicted responses, let alone offer a coherent explanation of the observed ones. Here we propose an alternative approach based on relatively simple optimization models (OMs). We highlight the results of three recent forest OMs, which together explain a remarkable range of observed forest responses to altered resource availability. We conclude that OMs now offer a simple yet powerful approach to predicting the responses of forests—and, potentially, other plant types—to global change. We recommend ways in which OMs could be developed further in this direction.

Dynamics of Pinus radiata foliage in relation to water and nitrogen stress: II. Needle loss and temporal changes in total foliage mass

Abstract The pattern of production and fall of needles was measured over a 4 year (1983–1987) period in 10- to 14-year-old stands of Pinus radiata near Canberra. Australia which were subjected to markedly varying degrees of water and N stress. Annual needle loss (death of green needles) was estimated as annual needle fall plus the increment (or minus the decrement) in the mass of dead needles held in the crown between successive winter measurements. Monthly estimates of needle production and loss were used to calculate seasonal (3-month) changes in total live foliage mass between annual winter measurements of foliage biomass. Annual needle fall ranged from about 1.5 to 5.0 t ha−1, and although well correlated with needle loss, the two parameters differed by up to 2 t ha−1 as stands approached canopy closure. In stands closing canopy. 2–3 t ha−1 of dead needles were retained in the crown. Loss in mass of needles due to leaching and decomposition during the period between senescence and fall results in needle fall being an underestimate of the mass of needles senescing in any year. Annual needle loss was positively linearly correlated with total foliage biomass (r2 = 0.70) or stand basal area (r2=0.75) measured in the previous winter. The average life of needles declined from about 4 years in open stands not suffering severe water stress, to about 2 years after canopy closure or where water stress induced significant loss of foliage. Monthly needle fall varied from less than 100 to greater than 500 kg ha−1 and was positively linearly correlated with a measure of cumulative tree water stress (the water stress integral, Sψ) during the same month. The correlation was highest (r2=0.68) for fertilised stands which were most water stressed, and declined with declining water stress, Sψ useful as a guide to the timing of needle fall, but not as a predictor of total annual needle fall which was mainly determined by foliage biomass of the stand. Older (mostly more than 2 years old) needles were shed largely in response to water stress, and in most years needle fall peaked in the summer-autumn period. In wet years and in irrigated stands the peak in needle fall was delayed by 3–6 months.

Energy Conversion and Use in Forests: An Analysis of Forest Production in Terms of Radiation Utilisation Efficiency (ɛ)

The linear relationship between the photosynthetically active solar radiation (PAR) absorbed by forest canopies (APAR) and the production of dry mass by forests provides a simple, robust model with only one parameter for the estimation of forest production. The slope of the relationship is normally denoted ɛ. The ɛ model has been developed from plant production studies and is soundly based physiologically. It has also evolved from remote sensing studies. Although the relationship between APAR and canopy photosynthesis may be highly variable over short periods, it remains constant over longer periods, such as months or seasons.