Biomedical subjects

Michael G. Ryan

Publications and source records attributed to Michael G. Ryan.

8 recordsLinked to original sources

PubMed2000

Introduction to BOREAS special issue.

Carbon flux attributed to the boreal forest biome significantly impacts the global carbon cycle. The multi-scale Boreal Ecosystem Atmosphere Study (BOREAS) was initiated to investigate exchanges between boreal forests and the atmosphere, to examine the effects of increased atmospheric CO(2) concentration on boreal forests, to develop and improve process models to describe exchanges between boreal forests and the atmosphere, and to develop methods for the large-scale application of these models. Field measurements have shown that boreal ecosystems differ significantly from their temperate and tropical counterparts with respect to seasonality, productivity and biosphere-atmosphere interactions.

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PubMed1999

Evidence that hydraulic conductance limits photosynthesis in old Pinus ponderosa trees.

We tested the hypotheses that hydraulic conductance is lower in old (about 250 years old and 30 m tall) compared to young (about 40 years old and 10 m tall) Pinus ponderosa Dougl. ex Laws. trees and that lower hydraulic conductance of old trees limits their photosynthesis. Hydraulic conductance at the end of summer 1995, calculated from leaf water potential and leaf gas exchange measurements on one-year-old needles, was 44% lower in old trees compared to young trees growing in a mixed age-class stand on the east slope of the Oregon Cascades. Whole-tree sapflow per unit leaf area averaged 53% lower in old trees compared to young trees and mean hydraulic conductance calculated from sapflow and water potential data was 63% lower in old trees than in young trees. For the entire summer, stomatal conductance (g(s)) and assimilation (A) declined more steeply with air saturation deficit (D) in old trees than in young trees. For both old and young trees, mean g(s) and A were approximately 32 and 21% lower, respectively, at typical midday D values (2.5-3.0 kPa). We hypothesized that if hydraulic conductance limits g(s) and A, then increasing or decreasing the leaf specific conductance of a branch will result in proportional changes in the responses of g(s) and A with D. Removal of 50% of the foliage from a set of experimental branches on old trees caused g(s) and A to decline less steeply with D in early summer, but values were not significantly different from control values in late summer. Cutting transverse notches in branches on young trees had no effect on the responses of g(s) and A with D. Leaf nitrogen content and photosynthetic capacity were similar suggesting that differences in g(s) and A between old and young trees were not caused by differences in photosynthetic capacity.

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PubMed1986

Physiographic, stand, and environmental effects on individual tree growth and growth efficiency in subalpine forests.

Annual volume growth of subalpine trees in the central Rocky Mountains was studied in relation to site and stand conditions. Growth of individual trees was most strongly influenced by estimated potential absorbed radiation, which varied with physiographic conditions and tree leaf area. Growth efficiency was estimated by the ratio of annual volume growth to potential radiation absorption by the crown. Growth efficiency was higher in young lodgepole pine (Pinus contorta var. latifolia Engelm.) than in Engelmann spruce (Picea engelmannii Parry ex Engelm.) or subalpine fir (Abies lasiocarpa (Hook.) Nutt.). In all species, growth efficiency declined sharply with age, and suppressed and overtopped trees had growth efficiencies equal to the highest efficiencies observed for dominant/codominant or intermediate trees. Lodgepole pine growth was most responsive to site and stand variables, Engelmann spruce was intermediate, and subalpine fir was rather unresponsive.

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PubMedDate not supplied

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.), growing on infertile soils. Net primary productivity estimated from biomass data was 0.47 and 0.25 kg C m(-2) year(-1) and foliar nitrogen/carbon ratio (N/C) was 0.0175 and 0.017 for the 40- and 245-year-old stands, respectively. Productivities of the young and old stands were derived from a graphical analysis of the G'DAY model. The graphical analysis also indicated that the observed age-related decline in NPP can be explained in terms of interacting processes associated with Hypotheses 2 and 3. However, the relative importance of these two hypotheses differed depending on key model assumptions, in particular those relating to variation in soil N/C ratio. Thus, if we assumed that soil N/C ratio can vary significantly during stand development, then Hypotheses 2 and 3 jointly explain the decline in NPP, whereas if we assumed that soil N/C ratios are constant, then Hypothesis 3 alone explains the decline in NPP. The analysis revealed that only a small fraction of the decline of NPP can be explained in terms of increasing sapwood respiration.

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PubMedDate not supplied

A physiological basis for biosphere-atmosphere interactions in the boreal forest: an overview.

Interdisciplinary field experiments for global change research are large, intensive efforts that study the controls on fluxes of carbon, water, trace gases, and energy between terrestrial ecosystems and the atmosphere at a range of spatial scales. Forest ecophysiology can make significant contributions to such efforts by measuring, interpreting, and modeling these fluxes for the individual components of forest ecosystems and then integrating the results into holistic ecosystem process models. The Boreal Ecosystem-Atmosphere Study (BOREAS) was undertaken because of the importance of the boreal forest biome to various global change issues. The study was conducted from 1993 to 1996 at sites in Saskatchewan and Manitoba, Canada. Results have shown that physiological processes of plants in the boreal forest can have large-scale consequences. For example, the composition of tree species strongly influences flux rates, with deciduous species having much higher carbon and water fluxes than coniferous species. Additionally, physiological limitations to transpiration in boreal conifers, even when soil water is abundant, reduces latent heat flux and increases sensible heat flux over large regions. This physiological control of transpiration can increase the depth of the atmospheric boundary layer on warm spring days to a level similar to that found in desert biomes. This special issue features 10 articles that address various aspects of the physiological basis of biosphere-atmosphere interactions in the boreal forest. The articles emphasize the environmental controls on water flux, carbon flux, and ecosystem productivity.

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PubMedDate not supplied

A general biogeochemical model describing the responses of the C and N cycles in terrestrial ecosystems to changes in CO(2), climate, and N deposition.

A model that simulates carbon (C) and nitrogen (N) cycles in terrestrial ecosystems is developed. The model is based on the principle that the responses of terrestrial ecosystems to changes in CO(2), climate, and N deposition will encompass enzymatic responses, shifts in tissue stoichiometry, changes in biomass allocation among plant tissues, altered rates of soil organic matter turnover and N mineralization, and ultimately a redistribution of C and N between vegetation and soils. The model is a highly aggregated, process-based, biogeochemical model designed to examine changes in the fluxes and allocation of C and N among foliage, fine roots, stems, and soils in response to changes in atmospheric CO(2) concentration, temperature, soil water, irradiance, and inorganic nitrogen inputs. We use the model to explore how changes in CO(2) concentration, temperature, and N inputs affect carbon storage in two ecosystems: arctic tundra and temperate hardwood forest. The qualitative responses of the two ecosystems were similar. Quantitative differences are attributed to the initial distribution of C and N between vegetation and soils, to the amounts of woody tissue in the two ecosystems, and to their relative degree of N limitation. We conclude with a critical analysis of the model's strengths and weaknesses, and discuss possible future directions.

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PubMedDate not supplied

Performance of a canopy light interception model for conifer shoots, trees and stands.

A hierarchical approach was used to evaluate the behavior of a canopy light interception model using submodels for interception at levels of the shoot, crown and stand. Mean silhouette areas of individual shoots of lodgepole pine (Pinus contorta var. latifolia Engelm.) calculated assuming spherical shoot orientation agreed well with silhouette areas measured with a video projection system. A theoretical mean silhouette to total leaf area ratio (STAR) calculated using measured shoot characteristics and a model of shoot geometry agreed reasonably well with STAR values from measured shoot silhouette and leaf areas, although the range of calculated STAR values was slightly lower than that of the measured values. The shadow areas of tree crowns were underestimated for lodgepole pine; however, a model using regular shoot dispersion in the crown gave better agreement than a model based on random shoot dispersion. The underestimates may have been caused by underestimation either of leaf or branch area for the crowns or of the extinction coefficient. Two equations used to calculate leaf area from sapwood area measurements both underestimated leaf area. Accordingly, there are too many uncertainties to determine whether crown interception is best represented by a random or regular distribution of shoots. Mean canopy transmission in the Engelmann spruce (Picea engelmannii Parry ex Engelm.) stand was estimated very well by a random (Poisson) model of the spatial distribution of trees, whereas, in the lodgepole pine stand, a regular (hexagonal) model gave the best results. The Engelmann spruce stand had less regular tree spacing than the lodgepole pine stand. Taken as a whole, the weakest submodel for calculating canopy interception was the model for individual crown transmission, which underestimated crown shadow areas. However, the problem may be in leaf area estimates rather than in model performance.

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PubMedDate not supplied

A simple method for estimating gross carbon budgets for vegetation in forest ecosystems.

Gross carbon budgets for vegetation in forest ecosystems are difficult to construct because of problems in scaling flux measurements made on small samples over short periods of time and in determining belowground carbon allocation. Recently, empirical relationships have been developed to estimate total belowground carbon allocation from litterfall, and maintenance respiration from tissue nitrogen content. I outline a method for estimating gross carbon budgets using these empirical relationships together with data readily available from ecosystem studies (aboveground wood and canopy production, aboveground wood and canopy biomass, litterfall, and tissue nitrogen contents). Estimates generated with this method are compared with annual carbon fixation estimates from the Forest-BGC model for a lodgepole pine (Pinus contorta Dougl.) and a Pacific silver fir (Abies amabilis Dougl.) chronosequence.

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