William E. Winner

Foliage physiology and biochemistry in response to light gradients in conifers with varying shade tolerance

Abstract To examine the predictability of leaf physiology and biochemistry from light gradients within canopies, we measured photosynthetic light-response curves, leaf mass per area (LMA) and concentrations of nitrogen, phosphorus and chlorophyll at 15–20 positions within canopies of three conifer species with increasing shade tolerance, ponderosa pine [Pinus ponderosa (Laws.)], Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], and western hemlock [Tsuga heterophylla (Raf.) Sarg.]. Adjacent to each sampling position, we continuously monitored photosynthetically active photon flux density (PPFD) over a 5-week period using quantum sensors. From these measurements we calculated FPAR: integrated PPFD at each sampling point as a fraction of full sun. From the shadiest to the brightest canopy positions, LMA increased by about 50% in ponderosa pine and 100% in western hemlock; Douglas fir was intermediate. Canopy-average LMA increased with decreasing shade tolerance. Most foliage properties showed more variability within and between canopies when expressed on a leaf area basis than on a leaf mass basis, although the reverse was true for chlorophyll. Where foliage biochemistry or physiology was correlated with FPAR, the relationships were non-linear, tending to reach a plateau at about 50% of full sunlight. Slopes of response functions relating physiology and biochemistry to ln(FPAR) were not significantly different among species except for the light compensation point, which did not vary in response to light in ponderosa pine, but did in the other two species. We used the physiological measurements for Douglas fir in a model to simulate canopy photosynthetic potential (daily net carbon gain limited only by PPFD) and tested the hypothesis that allocation of carbon and nitrogen is optimized relative to PPFD gradients. Simulated photosynthetic potential for the whole canopy was slightly higher (<10%) using the measured allocation of C and N within the canopy compared with no stratification (i.e., all foliage identical). However, there was no evidence that the actual allocation pattern was optimized on the basis of PPFD gradients alone; simulated net carbon assimilation increased still further when even more N and C were allocated to high-light environments at the canopy top.

Stress Physiology and the Distribution of Plants

C. B. Osmond is a professor in the Department of Environmental Biology, Australian National University in Canberra City, Australia, and formerly was director of the Biological Sciences Center, Desert Research Institute, University of Nevada, Reno, NV 89506. M. P. Austin is a visiting fellow in the Department of Environmental Biology, Australian National University, Canberra City, Australia. J. A. Berry is a faculty member in the Department of Plant Biology, Carnegie Institution of Washington, Stanford, CA 94305. W. D. Billings is a professor in the Department of Botany, Duke University, Durham, NC 27706. J. S. Boyer is a professor in the Department of Soil and Crop Sciences, Texas A & M University, College Station, TX 77843-2474. J. W. H. Dacey is an associate scientist in the Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. P. S. Nobel is a professor in the Department of Biology, University of California, Los Angeles, CA 90024. S. D. Smith is an assistant research professor at the Department of Biological Sciences, University of Nevada, Las Vegas, NV 89514. W. E. Winner is an assistant professor and director of the Laboratory of Air Pollution, Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. ? 1987 American Institute of Biological Sciences. Nearly every perturbation of a plant community results in stress

Carbon Dioxide Exchange Between an Old-growth Forest and the Atmosphere

Eddy-covariance and biometeorological methods show significant net annual carbon uptake in an old-growth Douglas-fir forest in southwestern Washington, USA. These results contrast with previous assumptions that old-growth forest ecosystems are in carbon equilibrium. The basis for differences between conventional biomass-based carbon sequestration estimates and the biometeorologic estimates are discussed. Annual net ecosystem exchange was comparable to younger ecosystems at the same latitude, as quantified in the AmeriFlux program. Net ecosystem carbon uptake was significantly correlated with photosynthetically active radiation and air temperature, as well as soil moisture and precipitation. Optimum ecosystem photosynthesis occurred at relatively cool temperatures (5°–10°C). Understory and soil carbon exchange always represented a source of carbon to the atmosphere, with a strong seasonal cycle in source strength. Understory and soil carbon exchange showed a Q10 temperature dependence and represented a substantial portion of the ecosystem carbon budget. The period of main carbon uptake and the period of soil and ecosystem respiration are out of phase, however, and driven by different climatic boundary conditions. The period of strongest ecosystem carbon uptake coincides with the lowest observed values of soil and ecosystem respiration. Despite the substantial contribution of soil, the overall strength of the photosynthetic sink resulted in the net annual uptake. The net uptake estimates here included two correction methods, one for advection and the other for low levels of turbulence.

Three-dimensional Structure of an Old-growth Pseudotsuga-Tsuga Canopy and Its Implications for Radiation Balance, Microclimate, and Gas Exchange

We describe the three-dimensional structure of an old-growth Douglas-fir/western hemlock forest in the central Cascades of southern Washington, USA. We concentrate on the vertical distribution of foliage, crowns, external surface area, wood biomass, and several components of canopy volume. In addition, we estimate the spatial variation of some aspects of structure, including the topography of the outer surface, and of microclimate, including the within-canopy transmittance of photosynthetically active radiation (PAR). The crowns of large stems, especially of Douglas-fir, dominate the structure and many aspects of spatial variation. The mean vertical profile of canopy surfaces, estimated by five methods, generally showed a single maximum in the lower to middle third of the canopy, although the height of that maximum varied by method. The stand leaf area index was around 9 m2 m−2, but also varied according to method (from 6.3 to 12.3). Because of the deep narrow crowns and numerous gaps, the outer canopy surface is extremely complex, with a surface area more than 12 times that of the ground below. The large volume included below the outer canopy surface is very porous, with spaces of several qualitatively distinct environments. Our measurements are consistent with emerging concepts about the structure of old-growth forests, where a high degree of complexity is generated by diverse structural features. These structural characteristics have implications for various ecosystem functions. The height and large volume of the stand indicate a large storage component for microclimatic variables. The high biomass influences the dynamics of those variables, retarding rates of change. The complexity of the canopy outer surface influences radiation balance, particularly in reducing short-wave reflectance. The bottom-heaviness of the foliage profile indicates much radiation absorption and gas exchange activity in the lower canopy. The high porosity contributes to flat gradients of most microclimate variables. Most stand respiration occurs within the canopy and is distributed over a broad vertical range.

Ecology of SO2 resistance: I. Effects of fumigations on gas exchange of deciduous and evergreen shrubs

SummaryA unique gas exchange system is described in which photosynthesis, transpiration, and stomatal conductance can be measured on leaves during SO2 fumigations. SO2 concentrations can be continuously monitored and manipulated between 0 and 2.0 ppm. Rates of total SO2 uptake and SO2 absorption through stomates of a fumigated leaf can also be determined.Using this system we compared the effects of SO2 on the gas exchange rates of two shrub species that co-occur in the Califormian chaparral. Diplacus aurantiacus, a deciduous shrub, was more sensitive to SO2 fumigation than Heteromeles arbutifolia, an evergreen shrub. The differences in photosynthetic sensitivity could be attributed, in large part, to differential SO2 absorption rates.

Ecology of SO2 resistance: II. Photosynthetic changes of shrubs in relation to SO2 absorption and stomatal behavior

SummaryIn an effort to predict SO2 sensitivity of plants from their morphological and physiological features, the effects of SO2 on photosynthesis were partitioned between stomatal and nonstomatal components for a drought deciduous shrub, Diplacus aurantiacus, and an evergreen shrub, Heteromeles arbutifolia. As predicted, the drought deciduous shrub had the higher gas conductance, and hence SO2 absorptance. However, nonstomatal components also play a role in determining SO2 sensitivity. Apparently a plant with a high intrinsic photosynthetic capacity will be more sensitive to SO2 than one with a lower capacity.

Compensating effects to growth of carbon partitioning changes in response to SO2-induced photosynthetic reduction in radish

SummaryExposure of plants to SO2 reduced their photosynthetic performance due tio reductions in carboxylating capacity. Although the reduced carbon gain resulted in a lower growth rate of SO2-exposed plants over that of controls, their loss of potential growth was minimized because of proportional increases in allocation to new leaf material.

Ecology of SO2 resistance - IV. Predicting metabolic responses of fumigated shrubs and trees

Summary10 broadleafed trees and shrubs native to the mediterranean climactic zone in California were surveyed for their photosynthetic and stomatal responses to SO2. These species ranged from drought deciduous to evergreen and had diverse responses to SO2. These results suggest an approach for predicting SO2 resistances of plants.We found that conductance values of plants in SO2-free air can be used to estimate the quantity of SO2 which plants absorb. These estimates are based on conductance values for plants in non-limiting environmental conditions. SO2 absorption quantities are then used to predict relative photosynthesis following the fumigation. Thus, relative photosynthesis of plants following fumigation can be predicted on the basis of conductance in SO2-free air. This approach to predicting SO2 resistances of plants includes analysis of their stomatal responses to fumigation, their characteristics of SO2 adsorption and absorption, and their change in photosynthesis resulting from SO2 stress.

Ecology of SO2 resistance: III. Metabolic changes of C3 and C4 Atriplex species due to SO2 fumigations

SummaryThe photosynthetic processes of two ecologically-matched, herbaceous Atriplex species differed in their response to SO2 fumigations. Atriplex triangularis, a C3 species, was more sensitive than the C4 species, A. sabulosa. This difference in sensitivity can be attributed in part to the higher conductance of the C3 species in normal air and saturating light as well as greater stimulation of stomatal opening following exposure to SO2. In addition, photosynthetic mechanisms of the C3 species had higher intrinsic SO2 sensitivity than the C4 species. Differences between photosynthetic responses of these two species may also reflect differences in morphological configuration of mesophyll tissues and greater SO2 sensitivity of the initial photosynthetic carboxlating enzyme of the C3 species. It is likely that certain of the differences in photosynthetic SO2 sensitivity of these contrasting C3 and C4Atriplex species are characteristic of C3 and C4 plants in general.

Stable sulfur isotope analysis of SO2 pollution impact on vegetation

SummaryThe δ34S value of SO2 emitted by natural gas refineries is about +25, which is higher than that for non-industrial sulfur sources in our study areas. Terrestrial mosses absorb SO2 from the atmosphere and have a δ34S value which is directly related to the degree of SO2 stress to which they are subjected. The δ34S values for conifer needles are lower than for mosses at the same collection site, which indicates that trees obtain sulfur from both atmospheric and soil sources.Potted conifers were transferred to sites differing in their degree of SO2 stress. This difference is reflected by the change of δ34S values of their needles. SO2 absorbant pot covers, such as charcoal and moss, reduce the amount of airborne sulfur which is available to tress. Moss also may reduce SO2 absorbed by soils in forest stands. We have used analysis of δ34S values to (1) help define SO2 dispersion patterns; (2) reveal the rates at which plants accumulate this pollutant; and (3) associate suspected SO2 injury more closely to an emission source.