Biomedical subjects

John D Marshall

Publications and source records attributed to John D Marshall.

7 recordsLinked to original sources

PubMed2007

Large variation in whole-plant water-use efficiency among tropical tree species.

It is well known that whole-plant water-use efficiency (transpiration efficiency of carbon gain, TE(C)) varies among plant species with different photosynthetic pathways. However, less is known of such variation among tree species within the C(3) group. Here we measured the TE(C) of seven C(3) tropical tree species. Isotopic analyses (delta(13)C, delta(18)O, and delta(15)N) and elemental analyses (carbon and nitrogen) were undertaken to provide insight into sources of variation in TE(C). Plants were grown over several months in approx. 80% full sunlight in individual 38-l containers in the Republic of Panama. Soil moisture content was nonlimiting. Significant variation was observed in TE(C) among the C(3) tree species. Values ranged from 1.6 mmol C mol(-1) H(2)O for teak (Tectona grandis) to 4.0 mmol C mol(-1) H(2)O for a legume, Platymiscium pinnatum. Variation in TE(C) was correlated with both leaf N concentration, a proxy for photosynthetic capacity, and oxygen-isotope enrichment, a proxy for stomatal conductance. The TE(C) varied with C-isotope discrimination within species, but the relationship broke down among species, reflecting the existence of species-specific offsets.

Carbon↗

PubMed2006

Co-occurring species differ in tree-ring delta(18)O trends.

The stable oxygen isotope ratio (delta(18)O) of tree-ring cellulose is jointly determined by the delta(18)O of xylem water, the delta(18)O of atmospheric water vapor, the humidity of the atmosphere and perhaps by species-specific differences in leaf structure and function. Atmospheric humidity and the delta(18)O of water vapor vary seasonally and annually, but if the canopy atmosphere is well mixed, atmospheric characteristics should be uniform among co-occurring trees. In contrast, xylem water delta(18)O is determined by the delta(18)O of water being drawn from the soil, which varies with depth. If co-occurring trees draw water from different soil depths, this soil-water delta(18)O signal would be manifest as differences in delta(18)O among the trees. We examined the variation in tree ring delta(18)O, over eight decades during the 20th Century, among three species co-occurring in natural forest stands of the northern Rocky Mountains in the USA. We sampled 10 Douglas-firs (Pseudotsuga menziesii (Mirb.) Franco var. glauca), 10 ponderosa pines (Pinus ponderosa Laws.) and seven western white pines (Pinus monticola Dougl.). As expected, variation in atmospheric conditions was recorded in the delta(18)O of the cellulose produced in a given year, but observed climatic correlations with delta(18)O were weak. Significant correlations with June climate data included: daily maximum temperature (r = 0.29), daily minimum temperature (r = -0.25), mean temperature (r = 0.20), mean daily precipitation (r = -0.54), vapor pressure deficit (r = 0.32) and solar radiation (r = 0.44). Lagged effects were observed in Douglas-fir and western white pine. In these species, the delta(18)O of a given annual ring was correlated with the delta(18)O of the previous ring. Ponderosa pine showed no significant autocorrelation. Although the species means were correlated among years (r = 0.67 to 0.76), ponderosa pine was consistently enriched in delta(18)O relative to the other species; differences were close to 2 per thousand and they are steadily increasing. Relative to the mean for the three species, ponderosa pine is becoming steadily more enriched (-1.0 per thousand). In contrast, Douglas-fir is being steadily depleted and western pine is intermediate, with an enrichment of 0.5 per thousand. Because all trees were exposed to the same atmospheric conditions, the differences in delta(18)O observed between species are likely due either to differences in the depth of water extraction or leaf function. If the former, presumably ponderosa pine has steadily taken up more water from near the soil surface and Douglas-fir has shifted uptake to a greater depth. If the latter, we suggest the pronounced changes in leaf-water delta(18)O are a result of changes in leaf structure and function with tree size and age.

Carbon Isotopes↗

PubMed2005

Estimating leaf-level parameters for ecosystem process models: a study in mixed conifer canopies on complex terrain.

Ecosystem process models are often used to predict carbon flux on a landscape or on a global scale. Such models must be aggregate and canopies are often treated as a uniform unit of foliage. Parameters that are known to vary within the canopy, e.g., nitrogen content and leaf mass per area, are often estimated by a mean value for the canopy. Estimating appropriate means is complicated, especially in mixed-species stands and in complex terrain. We analyzed sources of variation in specific parameters with the goal of testing various simplifying assumptions. The measurements came from mixed-species forests in the northern Rocky Mountains. We found that, for three important parameters (nitrogen concentration and content, and leaf mass per area), a sample taken near the vertical center of the crown provided a good estimate of the mean values for the crown. Altitude (700-1700 m), solar insolation (4200-5400 MJ m(-2) year(-1)) and leaf area index (1-11) had negligible effects on the parameters; only species differences were consistently detected. The correlation between mass-based photosynthetic rates and mass-based nitrogen concentrations was much weaker than the correlation between area-based photosynthetic rates and area-based nitrogen concentration. Comparison of photosynthesis-nitrogen relationships for a wide variety of conifer species and sites revealed a broad general trend that can be used in models. These results suggest important potential simplifications in model parameterization, most notably that canopy means can be estimated with ease, that complex terrain is a minor source of variation in these parameters and that use of one photosynthesis-nitrogen relationship for conifer species does not result in large errors. Species-to-species variation, however, was large and needs to be accounted for when parameterizing process models.

Altitude↗

PubMed2005

Leaf longevity of western red cedar (Thuja plicata) increases with depth in the canopy.

Leaf longevity determines the annual cost of replacing foliage biomass and influences water and nutrient budgets. Longevity is readily estimated in most evergreen species by annual bud scars along the shoot. However, some species with indeterminate growth do not show these annual markers, making estimation of longevity difficult. One of these species is the widespread and economically valuable western red cedar (Thuja plicata J. Donn ex D. Don), for which no dependable estimates of leaf longevity exist. In this study, we estimated leaf longevity for western red cedar by counting growth rings in shoots at the point of leaf abscission. Estimates were obtained on 26 dominant or codominant trees growing in natural stands in a montane forest in northern Idaho, USA. Leaf longevity averaged 8.9 (SE = 0.2) years, but it strongly increased with depth in the canopy (0.3 year m(-1); mean crown depth was 15 m), increasing from a mean of 6.8 years in the upper third of the canopy to 10.6 years in the lower third. The increase in longevity with depth in the crown is consistent with many reports showing that longevity increases in resource-limiting environments. Longevity did not vary significantly with altitude or solar insolation in these montane forests. Among stand-level variables, longevity was correlated only with leaf area index: it increased slightly in stands with high leaf area indices. This approach to longevity estimation may be useful for any species that produces annual rings but no obvious bud scars, including many Cupressaceae species.

Altitude↗

PubMed2005

A regression-based equivalence test for model validation: shifting the burden of proof.

Model validation is often realized as a test of how well model predictions match a set of independent observations. One would think that the burden of proof should rest with the model, to force it to show that it can make accurate predictions. Further, one would think that increasing the sample size ought to increase the model's ability to demonstrate its utility. Traditional statistical tools are inappropriate for this because they default to the case that the model and the data are no different, and their ability to detect differences increases with the sample size. These traditional tools are optimized to detect differences, rather than similarities. We present an alternative strategy for model validation that is based on regression and statistical tests of equivalence. Equivalence tests reverse the usual null hypothesis: they posit that the populations being compared are different and use the data to prove otherwise. In this sense, equivalence tests are lumping tests, whereas the traditional statistical tests are splitting tests. To date, model validation with equivalence tests has focused on comparisons of means. Our proposed test checks not only for similarity of means, but also for similarity between individual predictions and observations. The strategy is demonstrated using three case studies that differ in their modeling objectives, and for varied sample sizes. The proposed strategy provides a formal means of model validation that is superior to traditional statistical tests in each case.

Forestry↗

PubMed2004

Nitrogen uptake and turnover in riparian woody vegetation.

The nutrient balance of streams and adjacent riparian ecosystems may be modified by the elimination of anadromous fish runs and perhaps by forest fertilization. To better understand nitrogen (N) dynamics within stream and riparian ecosystems we fertilized two streams and their adjacent riparian corridors in central Idaho. On each stream two nitrogen doses were applied to a swathe approximately 35 m wide centered on the stream. The fertilizer N was enriched in 15N to 18 per thousand. This enrichment is light relative to many previous labeling studies, yet sufficient to yield a traceable signal in riparian and stream biota. This paper reports pre-treatment differences in delta15N and the first-year N response to fertilizer within the riparian woody plant community. Future papers will describe the transfer of allochthonous litter N to the stream and its subsequent processing by stream biota. Pre-treatment delta15N differed between the two creeks (P=0.0002), possibly due to residual salmon nitrogen in one of the creeks. Pre-treatment delta15N of current-year needles was enriched compared to leaf litter, which was in turn enriched compared to needles aged 4 years and older. We conclude that fractionation due to retranslocation occurs in at least two phases. The first phase, which optimizes allocation of N in younger needle age classes, is distinctly different from the second, which conserves N prior to abscission. The delta15N difference between creeks was eliminated by the fertilization (P=0.42). In the two dominant conifer species, Abies lasiocarpa and Picea engelmannii, most fertilizer N was found in the current-year foliage; little was found in older needles and none was detected in litter (P=0.53). The only N-fixing shrub species, Alnus incana, took up only a small amount of fertilizer N [mean percent N derived from fertilizer (%Ndff) 5.0+/-1.6% (SE)]. Far more fertilizer N was taken up by other deciduous shrubs (mean %Ndff=33.9+/-4.5%). Fertilizer N made up 25% (+/-4.2%) of the N in deciduous shrub litter. These results demonstrate the feasibility of light labeling with 15N and the potential influence of riparian plant species composition on stream nutrient dynamics via allochthonous leaf litter inputs.

Animals↗

PubMed2003

Sources of variation in ecophysiological parameters in Douglas-fir and grand fir canopies.

Forest process models predict ecosystem responses from climate variables and physiological parameters. The parameters describe key ecosystem attributes, often as lumped averages. However, the sources and magnitude of variation in these physiological parameters are unknown, which complicates sampling if models are to be parameterized with field measurements. We measured several key parameters, which had been identified by sensitivity analyses of three models, in Abies grandis (Dougl.) Lindl. and Pseudotsuga menziesii var. glauca (Beissn.) Franco trees throughout the growing season. Trees were sampled at eight sites across the interior northwest of the USA. At each site, fertilized and control plots were sampled. The design provided statistical replication for the analysis of variance within a site, allowing us to draw inferences about a regional population of stands. Specific leaf area (SLA) varied by canopy position and treatment (P = 0.0003), by date of sampling (P < 0.0001) and by species (P = 0.0188). Mass-based foliar nitrogen concentration (%N) increased during the summer in both species (P = 0.0019), but at a faster rate in P. menziesii var. glauca than in A. grandis. Sun foliage had a higher mean %N (1.00, SE = 0.02%) than shade foliage (0.92 +/- 0.01%). Apparent quantum yield (Phi) varied among treatments, between species and between canopy positions; each of these variables interacted with date of sampling (P = 0.0207, P < 0.0001 and P = 0.0344, respectively). In A. grandis, mean Phi values (+/- SE) were 0.048 +/- 0.006 mol CO2 (mol incident photon)-1 for sun foliage and 0.067 +/- 0.007 mol CO2 (mol incident photon)(-1) for shade foliage. In P. menziesii, the corresponding mean Phi values were 0.032 +/- 0.003 and 0.047 +/- 0.004 mol CO2 (mol incident photon)-1. Parameters SLA, %N and Phi were all influenced by date, fertilizer treatment, species and crown position. We discuss methods of inferring quantum yields from light response curves and their utility for parameterizing process models. Parameter mean values are presented for each site; these tables provide a documented data set for the parameterization of models describing the western interior forests of the USA.

Cell Respiration↗