Robert J. Luxmoore

Discerning the forest from the trees: an essay on scaling canopy stomatal conductance

Abstract Stomata are major conduits for the diffusion of many trace gases between leaves and the atmosphere. The role of the stomata in controlling gas exchange between the terrestrial biosphere and the atmosphere at the landscape, meso- and global scales has only recently been appreciated. Further advances in modeling trace gas exchange will depend on our ability to provide realistic information on stomatal mechanics at the sub-grid scale of landscape and meso scale models; in other words, information is needed at the canopy scale. This paper describes two approaches for estimating canopy stomatal conductance. These are the ‘bottom-up’ and ‘top-down’ scaling methods. The bottom-up method entails computing canopy stomatal conductance by integrating the response of individual leaves to controling biotic and abiotic factors, which are determined by the micrometeorology of the canopy. The top-down approach entails inverting a descriptive stand-level, trace gas exchange model to estimate canopy stomatal conductance. The model is driven with abiotic variables measured at a reference point above the canopy. The strengths and weaknesses of these two approaches are discussed, and recommendations for future research are presented.

Tree-ring analysis and conifer growth responses to increased atmospheric CO2 levels

SummaryTree-ring data of naturally grown connifers were analyzed to evaluate the possibility of enhanced tree growth due to increased atmospheric CO2. Tree cores were obtained from 34 sites in four different climatic regions in the northern hemisphere. In each of the four regions, the sampling sites were located along ecological gradients between the subalpine treeline and low elevations and, sometimes, the arid forest border. Growth trends after 1950, when the atmospheric CO2 concentration increased by more than 30 μl·l-1 indicate an increase in ring-widths at eight of the 34 sites. These chronologies were from sites which moderate temperature or water stress. In four cases the growth increase in the post-1950 period coincided with favorable climatic conditions. In the remaining four cases, the growth increase exceeded the upper bound response expected from CO2 enrichment experiments with seedling conifer species. Therefore, increased growth in any of the tree-ring chronologies examined could not be solely attributed to higher atmospheric CO2 concentrations.

Signal-transfer modeling for regional assessment of forest responses to environmental changes in the southeastern United States

Stochastic transfer of information in a hierarchy of simulators is offered as a conceptual approach for assessing forest responses to changing climate and air quality across 13 southeastern states of the USA. This assessment approach combines geographic information system and Monte Carlo capabilities with several scales of computer modeling for southern pine species and eastern deciduous forests. Outputs, such as forest production, evapotranspiration and carbon pools, may be compared statistically for alternative equilibrium or transient scenarios providing a statistical basis for decision making in regional assessments.

Addressing multi-use issues in sustainable forest management with signal-transfer modeling

Management decisions concerning impacts of projected changes in environmental and social conditions on multi-use forest products and services, such as productivity, water supply or carbon sequestration, may be facilitated with signal-transfer modeling. This simulation method utilizes a hierarchy of simulators in which the integrated responses (signals) from smaller-scale process models are transferred and incorporated into the algorithms of larger spatial- and temporal-scale models of ecological and economic phenomena. Several innovative procedures germane to multi-issue sustainable forest management have been initiated in our signal-transfer modeling development for forests of the southeastern United States. These developments include response surface interpolation for multi-factor signal-transfer, use of loblolly pine modeling to infer the growth of other southern pines, determination of soil nutrient limitations to productivity, multivariate clustering as a spatial basis for defining land units relevant to forest management, and variance propagation through the modeling hierarchy. Algorithms for larger scale phenomena are shown to constrain the variance introduced from a smaller-scale in a simulation of ambient ozone exposure effects on loblolly pine timber yield. Outputs of forest variables are frequency distributions that may be statistically compared for alternative environmental or management scenarios.

Infiltration and runoff predictions for a grassland watershed

Abstract Field-determined infiltration characteristics used as input to an infiltration equation combined with adjusted parameters in a variable contributing area (VCA) algorithm of a hydrologic model were used to provide agreement between the annual simulated runoff and measurements from the grassland watershed site. Any discrepancy in Hortonian runoff from the infiltration calculations was compensated for by VCA runoff in the hydrologic simulation. Evaluation of the VCA parameterization suggested that contributions of VCA runoff to total streamflow were unreasonably high and that the infiltration parameters overpredicted annual infiltration. It is recommended that field measurements of infiltration characteristics for hydrologic studies be obtained during the season with predominant Hortonian runoff.

Urea fertilization effects on nutrient uptake and growth of Platanus occidentalis during plantation establishment.

SummaryThe benefit of fertilizer application during establishment of a tree plantation depends on effective nutrient uptake and the utilization of the nutrients in growth. Five urea treatments (0, 50, 75, 150, and 450 kg N/ha) were applied in a completely randomized plot design to a field planted with American sycamore (Platanus occidentalis L.) seedlings to evaluate growth responses and nitrogen use efficiency during the first season of plantation establishment. The site was in the Oak Ridge Reservation in eastern Tennessee on a highly weathered soil. Harvests were conducted on 3 occasions during a 22 week experimental period, and dry weights of stems, leaves, and large and small roots were measured. Chemical analyses were conducted on plant tissues from the 0, 75, and 450 kg N/ha treatments. Plant dry weight increased with urea application and growth analysis showed that this was mainly associated with increase in leaf area and to a minor extent with increase in net assimilation rate. Root weight increased significantly with urea application. The specific absorption rate of roots for several nutrients was greater at higher urea levels for the first 2 harvest periods, but this pattern reversed during the 3rd growth period. Surprisingly, manganese uptake and the specific absorption rate for manganese were enhanced with higher urea application. The acidifying effect of urea nitrification is a likely explanation for the increased Mn availability, and nitrate leaching and/or nitrogen immobilization contributed to low uptake of urea-N by the seedlings. The proportion of the applied nitrogen incorporated into the seedlings was 1.5 and 0.6% for the 75 and 450 kg N/ha urea treatments, respectively. Broadcast fertilizer application is not an effective way of supplying nutrients to seedlings during plantation establishment.

Modeling moisture and thermal transport in unsaturated porous media

Abstract This paper presents the development of a multidimensional model for simulating moisture and thermal transport in an unsaturated aquifer system using the i ntegrated c ompartment m ethod (ICM). The entire unsaturated zone is divided into a number of compartments with different sizes and shapes. The Philip-de Vries equations governing moisture movement and heat transfer are integrated over each of the compartments to yield a system of nonlinear ordinary differential equations. The system is then solved by one of three optional time integration schemes. These include the split explicit, the implicit pointwise iteration, and the matrix inversion iteration schemes. The model is verified by comparing its simulations with those of known analytical computations for simplified cases. Results indicate that the ICM algorithm can indeed simulate moisture movement and heat transfer in accordance with the Philip-de Vries equations. The validation of the model is under further investigation.