Showing papers on "Leaf area index published in 2004"

Annual Q10 of soil respiration reflects plant phenological patterns as well as temperature sensitivity

Abstract: The temperature sensitivity of soil respiration (SR) is often estimated from the seasonal changes in the flux relative to those in soil temperature, and subsequently used in models to interpolate or predict soil fluxes. However, temperature sensitivities derived from seasonal changes in SR (from here on denoted seasonal Q10) may not solely reflect the temperature sensitivity of SR, because seasonal changes in SR can also be affected by other seasonally fluctuating conditions and processes. In this manuscript, we present a case study of how the seasonal Q10 of SR can be decoupled from the temperature sensitivity of SR. In a mixed temperate forest, we measured SR under vegetations with different leaf strategies: pure evergreen, pure deciduous, and mixed. Seasonal Q10 was much higher under deciduous than under evergreen canopies. However, at a shorter time scale, both vegetation types exhibited very similar Q10 values, indicating that the large differences in seasonal Q10 do not represent differences in the temperature sensitivity of the soil metabolism. The seasonal Q10 depends strongly on the amplitude of the seasonal changes in SR (SRs), which, under the particular climatic and edaphic conditions of our forest study site, were significantly larger in deciduous forest. In turn, SRs was positively correlated with the seasonal changes in leaf area index (LAIs), a measure of the deciduousness of the vegetation. Thus, in this temperate maritime forest, seasonal Q10 of SR was strongly influenced by the deciduousness of the vegetation. We conclude that the large differences in seasonal Q10 were not entirely due to different temperature sensitivities, but also to different seasonal patterns of plant activity in the evergreen and deciduous plants of this site. Some coniferous forests may be more seasonal than the one we studied, and the deciduous–evergreen differences observed here may not be broadly applicable, but this case study demonstrates that variation of plant phenological process can significantly contribute to the seasonality of SR, and, hence, calculated Q10 values. Where this occurs, the seasonal Q10 value for SR does not accurately represent temperature sensitivity. Because the strong seasonal correlation between SR and temperature does not necessarily imply a causal relationship, Q10 values derived form annual patterns of SR should be used with caution when predicting future responses of SR to climatic change.

Spectral and Structural Measures of Northwest Forest Vegetation at Leaf to Landscape Scales

Abstract: We report a multiscale study in the Wind River Valley in southwestern Washington, where we quantified leaf to stand scale variation in spectral reflectance for dominant species. Four remotely sensed structural measures, the normalized difference vegetation index (NDVI), cover fractions from spectral mixture analysis (SMA), equivalent water thickness (EWT), and albedo were investigated using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data. Discrimination of plant species varied with wavelength and scale, with deciduous species showing greater separability than conifers. Contrary to expectations, plant species were most distinct at the branch scale and least distinct at the stand scale. At the stand scale, broadleaf and conifer species were spectrally distinct, as were most conifer age classes. Intermediate separability occurred at the leaf scale. Reflectance decreased from leaf to stand scales except in the broadleaf species, which peaked in near-infrared reflectance at the branch scale. Important biochemical signatures became more pronounced spectrally progressing from leaf to stand scales. Recent regenerated clear-cuts (less than 10 years old) had the highest albedo and nonphotosynthetic vegetation (NPV). After 50 years, the stands showed significant decreases in albedo, NPV, and EWT and increases in shade. Albedo was lowest in old-growth forests. Peak EWT, a proxy measure for leaf area index (LAI), was observed in 11- to 30-year-old stands. When compared to LAI, EWT and NDVI showed exponentially decreasing, but distinctly different, relationships with increasing LAI. This difference is biologically important: at 95% of the maximum predicted NDVI and EWT, LAI was 5.17 and 9.08, respectively. Although these results confirm the stand structural variation expected with forest succession, remote-sensing images also provide a spatial context and establish a basis to evaluate variance within and between age classes. Landscape heterogeneity can thus be characterized over large areas—a critical and important step in scaling fluxes from stand-based towers to larger scales.

The slow reversibility of photosystem II thermal energy dissipation on transfer from high to low light may cause large losses in carbon gain by crop canopies: a theoretical analysis

Abstract: Regulated thermal dissipation of absorbed light energy within the photosystem II antenna system helps protect photosystem II from damage in excess light. This reversible photoprotective process decreases the maximum quantum yield of photosystem II (Fv)/Fm) and CO2 assimilation (phiCO2), and decreases the convexity of the non-rectangular hyperbola describing the response of leaf CO2 assimilation to photon flux (theta). At high light, a decrease in phiCO2 has minimal impact on carbon gain, while high thermal energy dissipation protects PSII against oxidative damage. Light in leaf canopies in the field is continually fluctuating and a finite period of time is required for recovery of phiCO2 and when light drops below excess levels. Low phiCO2) and can limit the rate of photosynthetic carbon assimilation on transfer to low light, an effect prolonged by low temperature. What is the cost of this delayed reversal of thermal energy dissipation and phiCO2 recovery to potential CO2 uptake by a canopy in the field? To address this question a reverse ray-tracing algorithm for predicting the light dynamics of 120 randomly selected individual points in a model canopy was used to describe the discontinuity and heterogeneity of light flux within the canopy. Because photoprotection is at the level of the cell, not the leaf, light was simulated for small points of 10(4) micro m rather than as an average for a leaf. The predicted light dynamics were combined with empirical equations simulating the dynamics of the light-dependent decrease and recovery of phiCO2 and and their effects on the integrated daily canopy carbon uptake (A'c). The simulation was for a model canopy of leaf area index 3 with random inclination and orientation of foliage, on a clear sky day (latitude 44 degrees N, 120th day of the year). The delay in recovery of photoprotection was predicted to decrease A'c by 17% at 30 degrees C and 32% at 10 degrees C for a chilling-susceptible species, and by 12.8% at 30 degrees C and 24% at 10 degrees C for a chilling-tolerant species. These predictions suggest that the selection, or engineering, of genotypes capable of more rapid recovery from the photoprotected state would substantially increase carbon uptake by crop canopies in the field.

Short communication Estimation of leaf area index and covered ground from airborne laser scanner (Lidar) in two contrasting forests

Abstract: Two canopy properties, leaf area index (LAI) and covered ground (CoverGnd), were estimated using hemispherical photography of three oak (Quercus pyrenaica) and eight pine (Pinus sylvestris) forest plots in Sierra de Guadarrama (central Spain). Pulses from airborne laser scanner (Lidar) that hit the surface on the exact location (within centimeter resolution) of the photographs were analyzed and separated by different radius size (from 0.5 to 20 m). The correlation between Lidar and hemispherical photography estimates of canopy properties was highly significant, but was affected by the type of forest and the radius size. CoverGnd was better estimated using a small radius size (2.5 m, equivalent to one fourth of canopy height), while LAI was better estimated using a larger radius size (7.5–12.5 m, equivalent to the entire canopy height). In general, the smaller the tree, the shorter the radius was that must be used to select Lidar data, and the best Lidar estimator of canopy properties was the percentage of canopy hits. Overall oak canopies showed better results than pine forest. The poorer estimation in pine forest plots was probably due to the larger foliage and branch clumping of pine versus oak canopies. Lidar data could be used to produce high-resolution regional maps of the canopy properties studied.

Carbon dioxide and water vapor exchange in a warm temperate grassland

Abstract: Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and its components, ecosystem carbon assimilation (Ac) and ecosystem respiration (RE), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled Ac and evapotranspiration (ET) using a big- leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excursions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when integrated over the year, this grassland ecosystem was a net C source (97 g C m �2 a �1 ) due to a minor imbalance between large Ac (�1,202 g C m �2 a �1 ) and RE (1,299 g C m �2 a �1 ) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (θ<0.2 m 3 m �3 ), which influenced Ac and thereby NEE by decreasing stomatal conductance. For this experiment, low θ had minor impact on RE. Thus, stomatal limitations to Ac were the primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of �65 g C m �2 a �1 assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems.

Reduced simple ratio better than NDVI for estimating LAI in Finnish pine and spruce stands

Abstract: Estimation of leaf area index (LAI) using spectral vegetation indices (SVIs) was studied based on data from 683 plots on two Scots pine and Norway spruce dominated sites in Finland. The SVIs studied included the normalised difference vegetation index (NDVI), the simple ratio (SR), and the reduced simple ratio (RSR), and were calculated from Landsat ETM images of the two sites. Regular grids of size 1 km2 with gridpoints placed at 50 m intervals were established at the sites and measurements of LAI using the LAI-2000 instrument were taken at the gridpoints. SVI-LAI relationships were examined at plot scale, where the plots were defined as circular areas of radius 70 m around each gridpoint. Plotwise mean LAI was computed as a weighted average of LAI readings taken around the gridpoints belonging to the plot. Mean LAI for the plots ranged from 0.36 to 3.72 (hemisurface area). All of the studied SVIs showed fair positive correlation with LAI but RSR responded more dynamically to LAI than did SR or NDVI. Especially NDVI showed poor sensitivity to changes in LAI. RSR explained 63% of the variation in LAI when all plots were included (n = 683) and the coefficient of determination rose to 75% when data was restricted to homogeneous plots (n = 381). Maps of estimated LAI using RSR showed good agreement with maps of measured LAI for the two sites.

Landscape patterns of sapling density, leaf area, and aboveground net primary production in postfire lodgepole pine forests, Yellowstone National Park (USA)

Abstract: Causes and implications of spatial variability in postfire tree density and understory plant cover for patterns of aboveground net primary production (ANPP) and leaf area index (LAI) were examined in ninety 11-year-old lodgepole pine (Pinus contorta var. latifolia Engelm.) stands across the landscape of Yellowstone National Park (YNP), Wyoming, USA. Field studies and aerial photography were used to address three questions: (1) What is the range and spatial pattern of lodgepole pine sapling density across the burned Yellowstone landscape and what factors best explain this variability? (2) How do ANPP and LAI vary across the landscape and is their variation explained by abiotic factors, sapling density, or both? (3) What is the predicted spatial pattern of ANPP and LAI across the burned Yellowstone landscape? Stand density spanned six orders of magnitude, ranging from zero to 535,000 saplings ha )1 , and it decreased with increasing elevation and with increasing distance from unburned forest (r 2 = 0.37). Postfire densities mapped

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

Abstract: 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.