Showing papers by "Mathew Williams published in 1998"

Net primary production of forests: a constant fraction of gross primary production?

Abstract: Considerable progress has been made in our ability to model and measure annual gross primary production (GPP) by terrestrial vegetation. But challenges remain in estimating maintenance respiration (R(m)) and net primary production (NPP). To search for possible common relationships, we assembled annual carbon budgets from six evergreen and one deciduous forest in Oregon, USA, three pine plantations in New South Wales, Australia, a deciduous forest in Massachusetts, USA, and a Nothofagus forest on the South Island of New Zealand. At all 12 sites, a standard procedure was followed to estimate annual NPP of foliage, branches, stems, and roots, the carbon expended in synthesis of these organs (R(g)), their R(m), and that of previously produced foliage and sapwood in boles, branches, and large roots. In the survey, total NPP ranged from 120 to 1660 g C m(-2) year(-1), whereas the calculated fraction allocated to roots varied from 0.22 to 0.63. Comparative analysis indicated that the total NPP/GPP ratio was conservative (0.47 +/- 0.04 SD). This finding supports the possibility of greatly simplifying forest growth models. The constancy of the NPP/GPP ratio also provides an incentive to renew efforts to understand the environmental factors affecting partitioning of NPP above and belowground.

Seasonal variation in net carbon exchange and evapotranspiration in a Brazilian rain forest: a modelling analysis

Abstract: Tropical rain forests account for a significant fraction of global net primary productivity, and are important latent energy (LE) sources, affecting extra-tropical atmospheric circulation. The influence of environmental factors on these fluxes has until recently been poorly understood, largely due to a paucity of data, but in recent years the amount of available data has been increased greatly by use of eddy covariance techniques. In this paper we examine the factors that control daily and seasonal carbon (C) and LE fluxes, by comparing a detailed model of the soil‐plant‐atmosphere continuum against a unique long-term data-set collected using eddy covariance at an undisturbed rain forest site north of Manaus, Brazil. Our initial application of the model was parametrized with simple measurements of canopy structure, and driven with local meteorological data. It made effective predictions of C and LE exchange during the wet season, but dry season predictions were overestimates in both cases. Sensitivity analyses indicated that the best explanation for this behaviour was a seasonal change in soil and root hydraulic resistances ( Rb). An optimization routine was then used to estimate the increase in Rb during the dry season that would be required to explain the reduced dry season fluxes. The local soil, a clay latosol, is typical of much of Amazonia, having very low available water and low hydraulic conductivity. We conclude that an increase in soil‐root hydraulic resistance in the dry season introduces a significant seasonal cycle to carbon and water fluxes from this tropical forest. Furthermore, our model structure appears to be an effective tool for regional and temporal scaling of C and LE fluxes, with primary data requirements being regional and temporal information on meteorology, leaf area index (LAI), foliar N, critical leaf water potentials, and plant and soil hydraulic characteristics.

Photo-oxidized bovine arterial grafts: short-term results.

Abstract: Biologic grafts are the conduit of choice for vascular reconstructive procedures. The short-term thrombogenicity, patency, and stability of bovine arterial grafts, altered by a dye-mediated photo-oxidation process, was evaluated in the canine common femoral vein (CFV) and artery (CFA) model. Modified bovine interposition grafts were implanted in the CFV of 12 dogs and in the CFA of 11 dogs, respectively. Polytetrafluoroethylene (PTFE) implants on the contralateral side served as controls. Patency and histology were assessed at 1, 2, 4, and 6 weeks. In the CFV, patency of photo-oxidized grafts was 83% at 1 week and 71% at 2 weeks, compared to 17% and 0% for PTFE, respectively (p = 0.0033). In CFA, patency was 82% for the photo-oxidized graft and 63% for PTFE at 6 weeks (p = NS). Photo-oxidized grafts were nonreactive, without evidence of degenerative changes or cellular infiltration at all time periods. Compared to commercially available PTFE, photo-oxidized arterial grafts have superior patency in the CFV, and comparable patency in the CFA. Preliminary results demonstrate that these xenografts are stable and without degenerative changes. If corroborated by long-term data, these grafts may be a suitable alternative to currently available prosthetics for peripheral vascular reconstructive procedures.