Showing papers on "Leaf area index published in 1993"

Canopy structure and vertical patterns of photosynthesis and related leaf traits in a deciduous forest.

Abstract: Canopy structure and light interception were measured in an 18-m tall, closed canopy deciduous forest of sugar maple (Acer saccharum) in southwestern Wisconsin, USA, and related to leaf structural characteristics, N content, and leaf photosynthetic capacity. Light attenuation in the forest occurred primarily in the upper and middle portions of the canopy. Forest stand leaf area index (LAI) and its distribution with respect to canopy height were estimated from canopy transmittance values independently verified with a combined leaf litterfall and point-intersect method. Leaf mass, N and A max per unit area (LMA, N/area and A max/area, respectively) all decreased continuously by over two-fold from the upper to lower canopy, and these traits were strongly correlated with cumulative leaf area above the leaf position in the canopy. In contrast, neither N concentration nor A max per unit mass varied significantly in relation to the vertical canopy gradient. Since leaf N concentration showed no consistent pattern with respect to canopy position, the observed vertical pattern in N/area is a direct consequence of vertical variation of LMA. N/area and LMA were strongly correlated with A max/area among different canopy positions (r2=0.81 and r2=0.66, respectively), indicating that vertical variation in area-based photosynthetic capacity can also be attributed to variation in LMA. A model of whole-canopy photosynthesis was used to show that observed or hypothetical canopy mass distributions toward higher LMA (and hence higher N/area) in the upper portions of the canopy tended to increase integrated daily canopy photosynthesis over other LMA distribution patterns. Empirical relationships between leaf and canopy-level characteristics may help resolve problems associated with scaling gas exchange measurements made at the leaf level to the individual tree crown and forest canopy-level.

Forest ecosystem processes at the watershed scale: Sensitivity to remotely-sensed leaf area index estimates

Abstract: Recent research has shown that general trends in forest leaf area index along regional climatic gradients can be adequately characterized by using ratios of near-infrared and red reflectances. However it has proven difficult to represent properly the spatial distribution of Leaf Area Index (LAI) at sub-regional scales such as small catchments. The key problem at Thematic Mapper scale is the variation in canopy closure and understorey contribution, which dramatically influences near-infrared reflectance from conifer forests, [n this paper, a new spectral index is presented to estimate LAI of conifer forests using a combination of Red, NIR and mid-IR reflectances from the Landsat Thematic Mapper (TM). A simulation system (RHESSys) was used first, to generate potential vegetation patterns around a watershed in order to test them against remotely-sensed vegetation patterns, and secondly, to test the sensitivity of forest ecosystem processes to LAT estimated from combinations of the Thematic Mapper da...

Photosynthetic acclimation and nitrogen partitioning within a lucerne canopy. II: Stability through time and comparison with a theoretical optimum

Abstract: Nitrogen redistribution between and within leaves was examined in a plot of lucerne (Medicago sativa L. cv. Aurora) in relation to potential canopy photosynthesis. The canopy was sampled during regrowth after cutting and just prior to flowering. As leaves were progressively shaded by the newly produced leaves, nitrogen content fell and photosynthetic acclimation occurred. The rate of acclimation in the canopy was the same as occurred following a step change to 23 or 6% sunlight. The profile of leaf nitrogen content was stable with respect to leaf area index and independent of time of sampling. Optimal profiles of nitrogen distribution between leaves, photosynthetic rate per unit chlorophyll and nitrogen partitioning within leaves were calculated from the relationships between photosynthesis and nitrogen in conjunction with the light environment of the preceding 3 days. The optimal nitrogen content of the leaves should vary in proportion to the relative daily irradiance at each leaf. The observed distribution achieved 88% of the potential daily photosynthesis, while a uniform nitrogen distribution yielded only 80%. Photosynthetic acclimation and nitrogen partitioning within each leaf both responded to daily irradiance similarly to the calculated optimum except at the two extremes. At the top of the canopy, photosynthetic rate per unit of chlorophyll did not increase as much as the calculated optimum, while at the base of the canopy, nitrogen partitioning failed to fall as much as the calculated optimum. This may reflect the constraints on the flexibility of the photosynthetic system. Nitrogen redistribution between leaves made a dramatic contribution to increasing the potential photosynthesis by the canopy. Although acclimation to low irradiance reduced the photosynthetic capacity per unit nitrogen by 12%, the considerable reorganisation of proteins within the thylakoids increased potential daily photosynthesis by 20% over that which would have been gained by a 'sun' leaf. However, in terms of canopy photosynthesis, which is dominated by leaves intercepting most of the light, acclimation contributed only a few per cent to the potential daily canopy photosynthesis.

Canopy dynamics and aboveground production of five tree species with different leaf longevities

Abstract: Canopy dynamics and aboveground net primary production (ANPP) were studied in replicated monospecific and dual-species plantations comprised of species with different leaf longevities. In the monospecific plantations, leaf longevity averaged 5, 6, 36, 46 and 66 months for Quercus rubra L., Larix decidua Miller, Pinus strobus L., Pinus resinosa Ait. and Picea abies (L.) Karst., respectively. Specific leaf area, maximum net photosynthesis per unit mass (A/mass), leaf N per unit mass (N(leaf)/mass) and maximum net photosynthesis on a leaf N basis (A/N(leaf)) were inversely correlated to leaf longevity (r(2) = 0.92-0.97, 0.91, 0.88 and 0.80, respectively). Maximum net photosynthesis per unit area (A/area) was not correlated to leaf longevity, whereas leaf N per unit area (N(leaf)/area) was positively correlated to leaf longevity (r(2) = 0.95). For a similar-diameter conifer, species with long-lived foliage supported a greater foliage mass than species with short-lived foliage; however, Quercus rubra did not follow this pattern. At the stand level, total foliage mass ranged from 3.3 to 30.5 Mg ha(-1) and was positively correlated (r(2) = 0.97) to leaf longevity. Leaf area index (LAI) was also positively correlated (r(2) = 0.82) to leaf longevity. Production efficiency (ANPP/LAI) was inversely related to leaf longevity and positively related to A/mass. Aboveground biomass and net primary production differed significantly (P < 0.05) among the five species but were not correlated to leaf longevity, total foliage mass or leaf area. In monospecific plantations, stem NPP for Larix decidua was 17% greater than for Pinus strobus and 14% less than for Picea abies, but in mixed-species plantations stem NPP for Larix decidua was 62 and 85% greater than for Pinus strobus and Picea abies, respectively. Similar aboveground net primary production rates can be attained by tree species with different leaf longevities because of trade-offs resulting from different structural and physiological leaf and canopy characteristics that are correlated to each other and to leaf longevity.

Leaf area index and above-ground biomass of terra firme rain forest and adjacent clearings in Amazonia

Abstract: Leaf area index (L*) and above-ground biomass were determined by destructive sampling for a 400m2 area of terra firme Amazonian rain forest and for an adjacent clearing. While L* in the forest varied considerably with height though the canopy for four separately sampled 100m2 subplots, the cumulative L* was similar with a mean value of 5.7 + ou - 0.5. The total above-ground dry biomass of the forest was 265 + ou - 95t ha-1, while the leaf dry biomass was 6.3 + ou - 0.5t ha-1. The specific leaf area (SLA, cm2g-1) was determined, and found to vary linearlywith canopy depth rising from 65cm2g-1 at the canopy top, to 114cm2g-1at 5m above the forest floor. The average for the four subplots was 90cm2g-1. The leaf area index of grass in the clearing (blade only) averaged 0.8 + ou - 0.4, and total green area index (blade + stem) averaged 1.2 + ou - 0.6. Total above-ground dry biomass (blade, stem) was 2.3 + ou - 0.6t ha-1, while including dead plant material gave a total dry biomass of 5.6 + ou - 2.1t ha-1. Specific leaf area of the grass was 79cm2g-1. The results clearly demonstrate the profound change in leaf area and biomass which accompany Amazonian deforestation.

Estimating leaf area index from satellite data

Abstract: A method for estimating leaf area index from visible and near infrared measurements of vegetation above a soil background is applied to a Landsat Thematic Mapper data set. Some constants required for the procedure are inferred from the scattergram of data values. The resulting image illustrates variability of leaf area index over an agricultural area. The mixed-pixel case, corresponding to low-resolution data from the NOAA Advanced Very High Resolution Radiometer (AVHRR) is also discussed, and a vegetation index is suggested for both high- and low-resolution data. Consideration of the two types of data leads to the suggestion that a sampled high spatial resolution sensor (50-100 m) be added to the AVHRR in order to permit accurate inference of vegetation conditions over agricultural areas. >

A sunflower simulation model: I. Model development

Abstract: In dryland farming systems, opportunities to improve sunflower (Helianthus annuus L.) yields are mostly associated with management decisions made at planting. Dynamic crop simulation models can assist in making such decisions. This study reports the structure of QSUN, a simple and mechanistic crop model for sunflower, and how it accounts for the dynamic interaction of the crop with the soil and aerial environment. The model incorporates several recent approaches to simulation of crop growth in dryland conditions. QSUN estimates growth, development, and yield of a sunflower crop. Daily inputs of temperature and photoperiod drive a phenology submodel to predict stages of emergence, bud visibility, 50% anthesis, and maturity. Using these stages, the growth submodel, driven by daily inputs of radiation, rainfall, and temperature, estimates leaf area production and senescence and soil water extraction. Biomass production is calculated from the amount of radiation intercepted by leaves or from the amount of water accessible in the root zone, depending on whether radiation or water is limiting crop growth. Seed yield is calculated from the allocation of biomass to the grain following anthesis. Sensitivity testing of the model under several irrigation regimes indicated that QSUN was most sensitive to the rate at which partitioning of biomass to grain increased, the ratio of biomass produced to water transpired, and the rate of soil water extraction in a water limited situation. The model was tested against independent data, with actual phenological data and was able to satisfactorily predict leaf area index (r2 = 0.65), total biomass (r2 = 0.96), and grain yield (r2 = 0.93), thus providing a tool for use in simulations studies and to assist in management decision making.

The effect of trees on summertime below canopy urban climates: a case study bloomington, indiana

Abstract: The objective of this study is to quantify the effects of trees in a Midwest urban area on air temperature and humidity to determine if the effects are significantly different for: different species of trees, trees of the same species in different environments, and whether the effects can be ex- plained by physical characteristics of the individual trees. Replicate trees in each of five categories were studied: sugar maple, pin oak and walnut individuals overgrass, sugar maple individuals along streets over concrete, and sugar maple clumps over grass. All the trees show a consistent effect: temperatures are reduced and humidities are elevated under the canopies. The greatest cooling effect (0.7 -1.3°C) occurs in the early afternoon. The difference between species is insignificant, but street trees are significantly less effective in reducing temperature than either individual trees or clumps planted over grass. The clumps had no greater effect than the individual trees. The amount of cooling observed in this study was considerably less than that documented in many previous studies. No consistent linear relationships were determined between physical characteristics of the trees, such as leaf area index, and temperature reductions or humidity increases.

Effects of clumping on estimates of stand leaf area index using the LI-COR LAI-2000

Abstract: Eighteen natural second-growth stands of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) from a wide range of stand densities and stocking levels were measured for stand leaf area index (LAI) fro...

LAI inversion using a back-propagation neural network trained with a multiple scattering model

Abstract: Standard regression methods applied to canopies within a single homogeneous soil type yield good results for estimating leaf area index (LAI) but perform unacceptably when applied across soil boundaries. In contrast, the neural network reported generally yielded absolute percentage errors of >

Estimating the characteristics of vegetation canopies with airborne radar measurements

Abstract: Possible use of synthetic aperture radars (SAR) for monitoring agricultural canopies is investigated in this paper. Data have been acquired on the Orgcval watershed during the AGRISCATT'88 campaign. Four radar experiments were carried out with the airborne scattcrometer ERASME (C and X bands, HH and VV polarizations, multi-incidence angles). Simultaneous ground measurements (soil moisture, leaf area index, water content of the canopy) were conducted on 11 wheat fields. Backscattering coefficients of the canopies arc interpreted in the framework of semi-empirical ‘water-cloud’ models. A simple paramctrization of the angular effect of soil roughness is introduced, allowing the simultaneous use of multi-incidence angle radar data. With a unique set of parameters for each radar configuration ‘ frequency and polarization’ the water-cloud model appears to describe adequately the backscattering of all the fields, over the range of incidence angles. It is shown that in this case, attenuation is the domin...

The differential role of mechanisms for drought resistance in a Mediterranean evergreen shrub: a simulation approach

Abstract: Plants have the ability to dampen the effects of variability in water resources. Various mechanisms contribute to these properties: reduction of leaf area, increased rooting depth and stomatal conductance. To evaluate the differential roles and interactions of these mechanisms, we have built a model and simulated flows of water in Mediterranean evergreen scrub. The essential concept of this model is that the water status of the canopy is governed by the water lost by transpiration, the availability of soil water and the hydraulic resistances to water flow in soil and plant. The amount of water supplied by the roots is related to changes in water potential between the soil and the leaf. The amount of water lost to the atmosphere is regulated by an interaction between atmospheric demand and canopy water potential. Water uptake by plant is assumed equal to plant water loss. Leaf area appears to affect largely the annual water balance. The critical leaf water potential required to reduce the maximum stomatal conductance by half has a dominant effect on annual leaf water potential. Reducing rooting depth induces a new functional equilibrium for the plant. This new equilibrium is reached by decreasing leaf area and the critical leaf water potential. Our results show the complexity of interactions of these mechanisms and highlight the importance of the coordination between them. Finally, we suggest a reconsideration of these mechanisms in a context of the survival and long-term persistence of the plant.

Responses of potato genotypes to drought. II. Leaf area index, growth and yield

Abstract: Summary Canopy expansion, growth and yield were examined in 19 genotypes of potato (Solanurn tuberosum L.) grown either with irrigation or droughted from the time of plant emergence. In the irrigated treatment, genotypes differed both in the maximum leaf area index (Lai) achieved and in the duration the canopy was maintained. Drought reduced both the rate of canopy expansion and the maximum Lai achieved. In the droughted treatment, biomass production was correlated with leaf area duration. The relation between the effect of drought on the growth of individual leaves and biomass or yield is examined. It is concluded that selection for early leaf appearance combined with the ability to sustain leaf growth with increasing soil moisture deficit (SMD) would improve productivity in the presence of drought.

Distribution of Nitrogen among Leaves in Soybean Canopies

Abstract: The N distribution among leaves in a plant canopy can theoretically have an important influence on crop photosynthetic activity. However, little detailed information is available on the specific leaf nitrogen (SLN, g N m) distribution in soybean [Glycine max (L.) Merr.] canopies. Experiments were established at Gainesville, FL and in Japan to examine the effects of cultivar, canopy maturity, plant density, and N fertility on SLN distribution. The SLN distribution was measured by dividing canopies by height or node number into relatively thin layers and determining SLN for each layer. Canopies with expanding leaves were found to have a fairly uniform SLN in the top 1.5 to 2.0 LAI. Below this top zone there was a linear decline in SLN with increased cumulative LAI. In contrast, mature canopies with fully expanded leaves only were found to have a continuous linear decrease in SLN with cumulative LAI from the top of the canopy. The SLN of the leaves at the top of the mature canopies were substantially greater than those canopies with expanding leaves. Neither plant density nor N fertility substantially altered the linear decline in SLN with cumulative LAI.

Aerodynamic and canopy resistance of short-rotation forest in relation to leaf area index and climate

Abstract: The aerodynamic and canopy resistances of a willow short-rotation stand were estimated during the course of a growing season on the basis of micrometeorological measurements. The normalized roughness length (z0/h) decreased from about 0.10 at a leaf area index of one to 0.05 at a leaf area index of seven. This implies that the aerodynamic resistance at peak leaf area index is more than twice the value at zero leaf area index, all other variables unchanged. The canopy resistance depended strongly on air water concentration deficit and on leaf area index. The Lohammar equation showed good agreement between estimated and “measured” canopy resistances over the whole course of leaf development. The stand was well-coupled to the atmosphere only for very small values of leaf area indices, less than one, and it was practically de-coupled for leaf area indices above two. From the point of view of factors controlling evaporation, this type of stand acts as a traditional forest at the beginning and end of the season and as an agricultural crop in the middle of the season.

LAI estimation by means of the WDVI: A sensitivity analysis with a combined PROSPECT‐SAIL model

Abstract: The PROSPECT leaf reflectance model and the SAIL canopy reflectance model were combined for quantifying the accuracy of LAI estimation from the Weighted Difference Vegetation Index (WDVI). The WDVI was developed as part of the CLAIR model for correcting the near‐infrared reflectance for soil background. From the PROSPECT‐SAIL simulations, it is concluded that the influence of external factors, such as soil reflectance, ratio diffuse/total irradiation and solar‐view geometry, will not pose a large problem for the relationship between LAI and WDVI. Moreover, leaf variables such as leaf chlorophyll content, leaf mesophyll structure and hot spot size‐parameter have quite a small influence for green leaves at near‐nadir observations (e.g. Landsat and SPOT‐nadir). The main variable influencing the relationship between WDVI and LAI is the leaf inclination angle distribution.

Leaf allometry in young poplar stands: reliability of leaf area index estimation, site and clone effects

Abstract: Different allometric relationships were established at the leaf, branch, stem and whole-tree levels for young poplar (Populus) trees during their first and second year of growth. Five poplar clones were grown under an intensively cultured regime at two different locations, Afsnee (near Gent, Belgium) and Orsay (near Paris, France). Leaf area was related linearly or non-linearly (power or logarithmic functions) to parameters that can be measured easily and non-destructively, such as leaf length or width, stem or branch diameter, stem or tree height, and volume index. Only small differences in the basic allometric relationships were noticed between the two sites and the five clones. A very good agreement was observed between leaf area index (LAI) values estimated from allometric relations and from indirect measurements using a plant canopy analyzer. These observations suggest that simple and general relations can be used to estimate LAI in young, high density poplar stands or leaf area at different organizational levels in trees.

Assessing growth and yield of wheat using remotely-sensed canopy temperature and spectral indices

Abstract: Prediction models were developed for wheat to assess crop growth in terms of leaf area index, dry matter production and grain yield from remotely-sensed temperature and spectral indices. The cumulative stress degree days (SDD) for the period of flowering to grain formation stage showed significantly higher correlation with dry matter (r= — 0940) and grain yield (r= —0-939) whereas that, for the period grain formation to harvest stage, showed significantly higher correlation lpar;r= —0-967) for crop water use. Significant and positive correlations between dry matter, leaf area and grain yield with infrared/red, normalised difference (ND), transformed vegetation index and greenness index were attained with the latter providing the highest degree of predictability. Spectral indices measured between flowering to milking stages gave the best prediction indicating the suitability of this period for crop growth assessment by this technique. Inter-stage sensitivity analysis by using multiple regression a...