Showing papers on "Leaf area index published in 1999"

Biomass allocation in plants: ontogeny or optimality? a test along three resource gradients

Abstract: We examined biomass allocation patterns throughout the entire vegetative growth phase for three species of annual plants along three separate gradients of resource availability to determine whether observed patterns of allocational plasticity are consistent with optimal partitioning theory. Individuals of the annual plant species Abutilon theophrasti, Chenopodium album, and Polygonum pensylvanicum were grown from locally field-gathered seed in controlled greenhouse conditions across gradients of light, nutrients, and water. Frequent harvests were used to determine the growth and allocation (root vs. shoot, and leaf area vs. biomass) responses of these plants over a 57-d period. Growth analysis revealed that each species displayed significant plasticity in growth rates and substantial amounts of ontogenetic drift in root:shoot biomass ratios and ratios of leaf area to biomass across each of the three resource gradients. Ontogenetically controlled comparisons of root:shoot and leaf area ratios across light ...

Agronomic changes from 58 years of genetic improvement of short-season soybean cultivars in Canada

Abstract: In Canada, yield of short-season soybean [Glycine max (L.) Merr.] cultivars has increased by approximately 0.5% per year since they were first cultivated in the early 1930s. Future yield gains may be dependent on an understanding of the changes made to soybean cultivars by breeding and selection. Our objective was to examine physiological differences associated with seed yield increase within a group of historical cultivars. At Ottawa. Ontario, we grew 14 cultivars representing seven decades of breeding and selection (1934-1992) in a randomized complete block design with four replications, across 4 years. Growth analysis provided data on leaf area and dry weight. Photosynthetic rate per leaf area was measured at several stages of development each year. Yield and harvest index were determined at maturity. The number of days to maturity and the total plant dry weight were not affected by the year of cultivar release. Seed yield, harvest index, and photosynthetic rate were found to have increased by 0.5% per year, while leaf area index decreased by 0.4% per year. The increase in seed yield with year of release was significantly correlated with an increase in harvest index, photosynthesis, and stomatal conductance and a decrease in leaf area index. Today's cultivars are more efficient at producing and allocating carbon resources to seeds than were their predecessors.

Characterization of radiation regimes in nonrandom forest canopies: theory, measurements, and a simplified modeling approach.

Abstract: We used field measurements and Monte Carlo simulations of canopy gap-size distribution and gap fraction to examine how beam radiation interacts with clumped boreal forest canopies of aspen (Populus tremuloides Michx.), black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.). We demonstrate that the Beer-Lambert law can be modified to accommodate transmission of radiation through a clumped forest canopy as a function of path length or sun zenith angle. Multiband Vegetation Imager (MVI) measurements and Monte Carlo simulations showed that values of the zenith element clumping index (Omega(e)(0)) are typically between 0.4 and 0.5 in jack pine and black spruce and 0.65 in aspen. Estimates of LAI obtained from MVI measurements of the canopy gap fraction and adjusted for canopy clumping and branch architecture yielded LAI values of 3.0 in jack pine, 3.3 in aspen, and about 6.0 in black spruce. These LAI estimates were within 10-25% of direct measurements made at the same sites. Data obtained with the MVI, along with numerical simulations, demonstrated that assumptions of random foliage distributions in boreal forests are invalid and could yield erroneous values of LAI measured by indirect techniques and false characterizations of atmosphere-biosphere interactions. Monte Carlo simulations were used to develop a general equation for beam radiation penetration as a function of zenith angle in clumped canopies. The essential measurements included stem spacing, crown diameter, crown depth, and within-crown gap fraction.

Net primary productivity distribution in the BOREAS region from a process model using satellite and surface data

Abstract: The purpose of this paper is to upscale tower measurements of net primary productivity (NPP) to the Boreal Ecosystem-Atmosphere Study (BOREAS) study region by means of remote sensing and modeling. The Boreal Ecosystem Productivity Simulator (BEPS) with a new daily canopy photosynthesis model was first tested in one coniferous and one deciduous site. The simultaneous CO2 flux measurements above and below the tree canopy made it possible to isolate daily net primary productivity of the tree canopy for model validation. Soil water holding capacity and gridded daily meteorological data for the region were used as inputs to BEPS, in addition to 1 km resolution land cover and leaf area index (LAI) maps derived from the advanced very high resolution radiometer (AVHRR) data. NPP statistics for the various cover types in the BOREAS region and in the southern study area (SSA) and the northern study area (NSA) are presented. Strong dependence of NPP on LAI was found for the three major cover types: coniferous forest, deciduous forest and cropland. Since BEPS can compute total photosynthetically active radiation absorbed by the canopy in each pixel, light use efficiencies for NPP and gross primary productivity could also be analyzed. From the model results, the following area-averaged statistics were obtained for 1994: (1) mean NPP for the BOREAS region of 217 g C m−2 yr−1; (2) mean NPP of forests (excluding burnt areas in the region) equal to 234 g C m−2 yr−1; (3) mean NPP for the SSA and the NSA of 297 and 238 g C m−2 yr−1, respectively; and (4) mean light use efficiency for NPP equal to 0.40, 0.20, and 0.33 g C (MJ APAR)−1 for deciduous forest, coniferous forest, and crops, respectively.

Seasonal variations in leaf area index, leaf chlorophyll, and water content; scaling-up to estimate fAPAR and carbon balance in a multilayer, multispecies temperate forest.

Abstract: Seasonal differences in phenology between coniferous and deciduous tree species need to be considered when developing models to estimate CO(2) exchange in temperate forest ecosystems. Because seasonal variations in CO(2) flux in temperate forests are closely correlated with plant phenology, we quantified the phenology of forest species in a multilayered forest with patches of Scots pine (Pinus sylvestris L.) and oak (Quercus robur L.) in Brasschaat, Belgium. A scaling-up modeling approach was developed to simulate reflectance at the leaf and canopy scales over a one-year cycle. Chlorophyll concentration, water content, specific leaf area and leaf area index of the forest species were measured throughout an entire year (1997). Scaling-up from the leaf to canopy was achieved by linking the PROSPECT and SAIL models. The result is the annual progression of the fraction of absorbed photosynthetically active radiation (fAPAR) in a 1 km(2) forest area, which can be directly related to high-resolution, remotely sensed data.

Large-scale modelling of forest hydrological processes and their long-term effect on water yield

Abstract: A water balance model was used to simulate the long-term increases in water yield with forest age which are observed in the mountain ash (Eucalyptus regnans) forests of Victoria, Australia. Specifically, the hypothesis was tested that water yield changes could be explained by changes in evapotranspiration resulting from changes in leaf area index (LAI). A curve predicting changes in the total LAI of mountain ash forest was constructed from ground-based observations and their correlation with Landsat Thematic Mapper measurements of the transformed normalized difference vegetation index (TNDVI). A further curve for mountain ash canopy LAI was constructed from destructive LAI measurements and stem diameter measurements. The curves were incorporated within Macaque, a large-scale, physically based water balance model which was applied to three forested catchments (total area 145 km2). The model was used to evaluate the effect of changes in LAI on predicted stream flow over an 82-year period spanning the 1939 wildfires which burnt most of the area. The use of the LAI curves induced improvement in the predicted hydrographs relative to the case for constant LAI, but the change was not large enough to account for all of the difference in water yield between old-growth and regrowth forests. Of a number of possibilities, concomitant changes in leaf conductance with age were suggested as an additional control on stream flow. These were estimated using data on stand sapwood area per unit leaf area and coded into Macaque. The hydrograph predicted using both the LAI curves and a new leaf conductance versus age curve accurately predicted the observed long-term changes in water yield. We conclude that LAI is a partial control on long-term yield changes, but that another ‘water use efficiency per unit LAI’ control is also operative. Copyright © 1999 John Wiley & Sons, Ltd.

Carbon gain in a multispecies canopy: the role of specific leaf area and photosynthetic nitrogen‐use efficiency in the tragedy of the commons

Abstract: summary Models have been formulated for monospecific stands in which canopy photosynthesis is determined by the vertical distribution of leaf area, nitrogen and light. In such stands, resident plants can maximize canopy photosynthesis by distributing their nitrogen parallel to the light gradient, with high contents per unit leaf area at the top of the vegetation and low contents at the bottom. Using principles from game theory, we expanded these models by introducing a second species into the vegetation, with the same vertical distribution of biomass and nitrogen as the resident plants but with the ability to adjust its specific leaf area (SLA, leaf area:leaf mass). The rule of the game is that invaders replace the resident plants if they have a higher plant carbon gain than those of the resident plants. We showed that such invaders induce major changes in the vegetation. By increasing their SLA, invading plants could increase their light interception as well as their photosynthetic nitrogen-use eciency (PNUE, the rate of photosynthesis per unit organic nitrogen). By comparison with stands in which canopy photosynthesis is maximized, those invaded by species of high SLA have the following characteristics: (1) the leaf area index is higher; (2) the vertical distribution of nitrogen is skewed less; (3) as a result of the supra-optimal leaf area index and the more uniform distribution of nitrogen, total canopy photosynthesis is lower. Thus, in dense canopies we face a classical tragedy of the commons: plants that have a strategy to maximize canopy carbon gain cannot compete with those that maximize their own carbon gain. However, because of this strategy, individual as well as total canopy carbon gain are eventually lower. We showed that it is an evolutionarily stable strategy to increase SLA up to the point where the PNUE of each leaf is maximized.

Net CO2 and H2O fluxes of terrestrial ecosystems

Abstract: Using 139 flux studies, we addressed the variability of net ecosystem surface assimilation (Asmax), net ecosystem surface respiration (Rsmax), as well as net surface evapotranspiration (Esmax) among and within vegetation types. While forests and C3 crops, particularly in the northern hemisphere, have been preferentially investigated, information on tropical forests, C4 grasslands or wetlands is rather limited. Almost no data are available for disturbed sites. Despite large variations within a vegetation type, enclosure studies tended to give highest Asmax rates compared to micrometeorological techniques. Excluding enclosure studies, we tested the effect of stand age and leaf area index (LAI) on net ecosystem gas exchange. For grasslands, Asmax increased by 7 μmol m−2 s−1 per unit LAI, for C4 crops by 11 μmol m−2 s−1, and for coniferous forests by 0.9 μmol m−2 s−1 per unit LAI. In contrast, Asmax of broad-leaved forests and C3 crops as well as Rsmax stayed constant over a wide range of LAI. Asmax and Rsmax of forests were lowest in young stands ( 160 years) was within the same range as those of 30- to 80-year-old forests, and always higher than those of regenerating stands. Rsmax seemed to decrease with age. Asmax increased linearly with ecosystem surface conductance for all vegetation types (r2 = 0.65). Asmax of forests and grasslands was closely related to Esmax (r2 = 0.87), with a slope of 0.082 μmol CO2 m−2 s−1/mmol H2O m−2 s−1. The results clearly illustrated where gaps in our knowledge exist and how ecosystem properties affect the capacity of net ecosystem gas exchange.

Radiation use efficiency and leaf CO2 exchange for diverse C4 grasses.

Abstract: Biomass accumulation of different grass species can be quantified by leaf area index (LAI) development, the Beer–Lambert light interception function, and a species-specific radiation-use efficiency (RUE). The object of this field study was to compare RUE values and leaf CO 2 exchange rates (CER) for four C 4 grasses. Biomass, LAI, and fraction of photosynthetically active radiation (PAR) intercepted were measured during three growing seasons. CER was measured on several dates and at several positions in the canopies. Switchgrass ( Panicum virgatum L.) had the greatest RUE whereas sideoats grama [ Bouteloua curtipendula (Michaux) Torrey] had the lowest. Big bluestem ( Andropogon gerardii Vitman) and eastern gamagrass [ Tripsacum dactyloides (L.) L.] values were intermediate. The two species with the greatest differences in RUE, switchgrass and sideoats grama, had similar relative amounts partitioned to roots. Likewise differences among species in the accumulation of soil carbon showed trends similar to total shoot biomass production. The light extinction coefficients ( k ) of switchgrass, big bluestem, and eastern gamagrass were smaller than for sideoats grama, implying that light was more effectively scattered down into the leaf canopy of the first three grasses. Whole canopy CER values were calculated with a stratified canopy approach, using LAI values, the Beer–Lambert formula with appropriate extinction coefficients, and CER light response curves. Differences among species in RUE were similar to these values for estimated whole-canopy CER divided by the fraction of light that was intercepted. High LAI along with low k contributed to higher RUE in switchgrass, in spite of lower values for single-leaf CER.

Light environment, sapling architecture, and leaf display in six rain forest tree species.

Abstract: Architecture and leaf display were compared in saplings of six rain forest tree species differing in shade tolerance. Saplings were selected along the whole light range encountered in a forest environment. Species differed largely in realized height and crown expansion per unit support biomass, but this could not be related to differences in shade tolerance. The results demonstrate that there exist various solutions to an effective expansion of plant height and crown area. It is argued that choice of the study species and the ontogenetic trajectory regarded determine to a large extent the outcome of interspecific comparisons. No evidence was found that pioneers were characterized by a multilayered and shade tolerants by a monolayered leaf distribution. Yet, sun plants had a similar crown area, a deeper crown, and a higher leaf area index compared to shade plants and their leaves were more evenly distributed along the stem. This suggests that differences in leaf layering are found between plants growing in different light environments, rather than between species differing in shade tolerance.

Vegetation characteristics and primary productivity along an arctic transect: implications for scaling‐up

Abstract: Summary 1 Arctic terrestrial ecosystems, which are important components of the global carbon (C) cycle, are expected to undergo considerable future climate change. However, predicting arctic C budgets is complex because the landscape is highly diverse and plant biomass very variable. 2 We investigated the diversity of vegetation at 14 sites along an arctic transect in northern Alaska, including sites on wet coastal tundra, tussock tundra, heath and shrub tundra. We applied these data to a model of the soil–plant–atmosphere (SPA) continuum, to investigate how variations in vegetation structure might affect landscape patterns of gross primary productivity (GPP). 3 Although the dominant species varied, there was a highly significant relationship for vascular plants between the leaf area index (LAI) and total foliar nitrogen (N; g m–2 ground area) at each site. 4 A modelled response surface of GPP to changes in both N and LAI indicated that for tundra plants there is an almost constant ratio between the sensitivity of productivity to the two variables. 5 The conservative nature of canopy LAI–N ratios simplifies the task of generating regional predictions of C assimilation by vascular plants (for which LAI and N are key requirements) because measurements of LAI can be used to infer N. 6 Spatial and temporal patterns of LAI and N must be determined if the photosynthetic component of regional C budgets is to be modelled accurately for diverse arctic ecosystems. Patterns of solar irradiance, together with air temperature and soil moisture constraints, are also influential but less important. The contribution of bryophytes must be accounted for as well.

Light distribution and foliage structure in an oak canopy

Abstract: Leaf angle distribution and shoot bifurcation ratio were measured and related to photon flux density (PFD) distribution in an oak canopy. Leaf angle distribution deviated substantially from random and changed markedly throughout the canopy. The observed leaf angle distribution was described by an ellipsoidal function with the single parameter of the distribution, x, changing from 1.6 at the top of the canopy to 3.2 in the lowest branches. In vertically homogeneous canopies, the extinction coefficient for diffuse radiation is expected to decline with increasing leaf area index (LAI). However, in the canopy studied here, the leaf angle distribution changed with height such that the effective extinction coefficient remained constant. Both shoot bifurcation ratio and leaf number per shoot declined with decreasing PFD inside the canopy. Based on these observed relationships, a simple canopy growth model that assumes horizontal homogeneity of the canopy was constructed. Calculations showed that a steady state, when growth in the upper of the canopy is in equilibrium with decline of lower canopy, the total canopy LAI should equal to 4.3. This predicted value of equilibrium LAI is larger than that estimated from measurements of PFD transmission (LAI=3.3), but smaller than that directly determined by litter collection (LAI=6.2 in 1998). Possible reasons for these discrepancies are discussed.

GOTILWA: An Integrated Model of Water Dynamics and Forest Growth

Abstract: GOTILWA is a simulation model of forest growth. Its name, GOTILWA, is an acronym for Growth Of Trees Is Limited by WAter. The name itself defines the main characteristic of the model. Water is, very often, the limiting factor for plant growth (Pinol et al. 1991; Sala 1992; Chap. 13) and thus it constitutes a key factor in the model (Tello et al. 1994). In a standard simulation, daily climatic data are analyzed. From the interaction between daily rainfall and the forest structure, the amount of intercepted water by the canopy layer, throughfall and stemflow are estimated. This effective rainfall increases the water stored in the soil which is used by the trees. The proportion of sapwood to heartwood, the leaf area of each tree and, consequently, the leaf area index (LAI) of the forest are all highly dependent on water availability in the model.

Is Potential Evapotranspiration and Its Relationship with Actual Evapotranspiration Sensitive to Elevated Atmospheric CO2 Levels

Abstract: The possibility is examined that potential evapotranspiration values may be sensitive to changes in atmospheric carbon dioxide content. Enhanced levels of atmospheric CO2 increase water use efficiency of vegetation by improving growth rates and suppressing transpiration per unit leaf area. Highly cultivated crops without water or nutrient constraints are able to show the greatest growth improvements. In many natural or semi-natural ecosystems, under enhanced atmospheric CO2 concentrations, limits on the availability of soil nutrients severely constrains the possibility of improvements in growth and significant increases in leaf area index that could compensate for a decrease in transpiration per unit leaf area. Thus, in many natural or semi-natural ecosystems, which often form water gathering grounds in river basins, enhanced levels of CO2 will suppress transpiration and perhaps increase the proporation of precipitation that forms runoff or ground water. In low vegetation covers, such as grassland, the rates of transpiration and also evaporation from canopies that are wet after rainfall (interception loss) are very similar. In these canopies, evapotranspiration is unlikely to be significantly increased by small increases in leaf area index. It is suggested that the suppression of potential evapotranspiration by enhanced CO2 levels will be small, but that actual transpiration from tall, slow growing vegetation covers may be significantly suppressed. Thus for some vegetation covers the relationship between actual and potential evapotranspiration may be sensitive to CO2 levels. If this is so, it could be of importance to many water balance calculations. The suppression of evapotranspiration by enhanced CO2 levels will be most noticeable in dry climates where interception loss is insignificant and largely masked in very wet climates where a large proportion of evapotranspiration consists of interception loss.

Practical field methods of estimating canopy cover, PAR, and LAI in Michigan Oak and pine stands

Abstract: With the increased use of variables such as canopy cover photosynthetically active radiation (PAR) and overstory leaf area index (LAI) in forestry research, relationships between these variables and traditional forestry variables must be defined before recommended levels of these research variables can be achieved by forestry practitioners on the ground. We measured basal area, canopy cover, Ozalid percent full light, PAR, and overstory LAI in thinned and unthinned plots within oak and pine stands with the objectives of: 1) determining the relationships between these variables in two common forest types, (2) investigating the feasibility of using basal area to estimate and achieve recommended levels of canopy cove, PAR, and LAI in the field, and (3) examining the possibility of using direct canopy cover and Ozalid light measurements for estimating PAR and LAI. Very strong relationships (r2 > 0.90 and P < 0.0001 ) were indicated between basal area and canopy cover, PAR, and LAI. Direct canopy cover and Ozalid light measurements were also strongly related to PAR and LAI. It is likely that the even-aged structure of the stands studied contributed to these results. The strength of the relationships between the measures examined suggest that ptactical variables such us basal area could potentially be used by forestry practitioners to estimate and achieve recommended levels of canopy cover, PAR, and LAI in similar oak and pine stands. The possibility also exists for strong relationships between these variables in other stand types that resemble those studied in terms of overstory structure.