Heikki Smolander

A new parameterization of canopy spectral response to incident solar radiation: case study with hyperspectral data from pine dominant forest

A small set of independent variables generally seems to suffice when attempting to describe the spectral response of a vegetation canopy to incident solar radiation. This set includes the soil reflectance, the single-scattering albedo, canopy transmittance, reflectance and interception, the portion of uncollided radiation in the total incident radiation, and portions of collided canopy transmittance and interception. All of these are measurable; they satisfy a simple system of equations and constitute a set that fully describes the law of energy conservation in vegetation canopies at any wavelength in the visible and near-infrared part of the solar spectrum. Further, the system of equations specifies the relationship between the optical properties at the leaf and the canopy scales. Thus, the information content of hyperspectral data can be fully exploited if these variables can be retrieved, for they can be more directly related to some of the physical properties of the canopy (e.g. leaf area index). This paper demonstrates this concept through retrievals of single-scattering albedo, canopy absorptance, portions of uncollided and collided canopy transmittance, and interception from hyperspectral data collected during a field campaign in Ruokolahti, Finland, June 14–21, 2000. The retrieved variables are then used to estimate canopy leaf area index, vegetation ground cover, and also the ratio of direct to total incident solar radiation at blue, green, red, and near-infrared spectral intervals. D 2003 Elsevier Science Inc. All rights reserved.

Leaf area index estimation of boreal forest using ENVISAT ASAR

A method for retrieval of leaf area index (LAI) using ENVISAT Advanced Synthetic Aperture Radar vertical/horizontal (VV/HH) polarization ratio was derived for boreal forests. Five alternating polarization single-look complex images of the test site were acquired in summer 2003. The swath range from IS1-IS6 was studied. The VV/HH polarization ratio correlated quite well with the ground truth LAI values. The mean error of the LAI estimates was 0.27 for the test site with mixed forest when data from all images and stands were used without separating between species (or swaths). The respective mean LAI estimation error was 0.3 for Norway spruce (Picea abies (L.) Karst.) and 0.07 for Scots pine (Pinus sylvestris L.) dominated stands.

Autumn colouration of first year Pinus sylvestris seedlings during frost hardening

Autumn colouration, dry matter content and frost hardiness of Pinus sylvestris seedlings were monitored during their first autumn. Seedlings of southern (62° N, 24° E) and northern (67° N, 26° E) Finland origin were grown under two fertilization regimes in Suonenjoki (63°N, 27°E). Northern seedlings turned purple earlier than southern ones, and low fertilized earlier than normal fertilized. Northern seedlings also hardened earlier than southern ones, while fertilization had no effect on hardening. Neither purple autumn colour nor dry matter content was clearly related to frost hardiness of seedlings when treatment means were compared. Comparison of seedlings within treatments suggested, however, that purple seedlings had higher dry matter content than green seedlings at the beginning of hardening period but the relationship between colouring and frost hardiness was not clear.

Photosynthesis of a Scots pine shoot: The effect of shoot inclination on the photosynthetic response of a shoot subjected to direct radiation

Abstract A set of photosynthetic responses of a Scots pine ( Pinus sylvestris L.) shoot to light was derived from the shoot geometry and the photosynthetic response of a single needle. Computations showed that the shape of the photosynthesis light-curves varies substantially depending on the direction of radiation relative to the shoot position. Differences in the initial and maximum rates of photosynthesis were due to changes in the effective projection area and the irradiated fraction of the shoot, respectively.

Photosynthesis of a scots pine shoot: Test of a shoot photosynthesis model in a direct radiation field

Abstract The photosynthetic light curves of eight Scots pine ( Pinus sylvestris L.) shoots, each with a different structure, were measured in a direct radiation field at different positions relative to the radiation beam. Using a model of shoot geometry, the irradiance distributions on the needle surface area of the same shoots were simulated in each position. Using measurements of shoot photosynthesis, the photosynthetic light curve for a needle surface area element (a small area on the needle surface) was estimated indirectly for each shoot. Shoot photosynthesis was then calculated from the estimated photosynthetic light response curve of a needle surface area element and the irradiance distribution on the surface area of needles. Calculated rates for shoot photosynthesis agreed well with the measured rates. The mean irradiance on the needle surface area was the major component causing variation in the light response of a shoot irradiated from different directions, but the shape of the irradiance distribution on the needle surface area had a considerably smaller effect.

Integration of a nonlinear function in a changing environment: Estimating photosynthesis using mean and variance of radiation

Abstract If the mean (integral) of a convex or concave function is estimated by evaluating the response function at the mean of the environmental variable, the estimate is necessarily biased. This study computes from empirical radiation data the errors obtained when mean photosynthesis is estimated using mean radiation. Furthermore, three different methods of estimating mean photosynthesis are compared in a case where the second power of the irradiance is also integrated and the variance is thus known. The smallest errors are obtained when the irradiance distribution is approximated by a two-point distribution: the bias is reduced to one-tenth and the root mean square error to one-third compared to the situation when only mean radiation is used. The results indicate that, if accurate estimates are needed for integrals of nonlinear responses, the second power of radiation or any other fluctuating environmental variable should also be measured.

Application of a forest reflectance model in estimating leaf area index of Scots pine stands using Landsat-7 ETM reflectance data

We tested the Kuusk‐Nilson forest reflectance model in simulating the relationship between leaf area index (LAI) and spectral reflectances of 23 pure Scots pine plots located in southeastern Finland. Landsat-7 enhanced thematic mapper (ETM) reflectance data for channels ETM 3 (red spectral band) and ETM 4 (near-infrared (NIR) band) were used for the analysis. LAI was determined by inversion from canopy gap fraction data measured by the LAI-2000 plant canopy analyzer, using a theoretical forest gap fraction model parameterised for the plots. An independent estimate of LAI was obtained from data on plotwise tree variables using an allometry-based regression developed for Scots pine. The Kuusk‐Nilson model can be run in direct and inverse modes. First, the model was applied in direct mode, in which LAI and stand data were used to simulate reflectances in the red (661 nm) and NIR (838 nm) bands. The model-simulated reflectances in the red band were within approximately the same range as the ETM 3 reflectances. In the NIR band, though, a majority of the simulated values were higher than the ETM 4 values. Empirical and simulated relationships between LAI and reflectance in the red and NIR bands showed similar, weakly negative trends in reflectance with increasing LAI. Second, the model was run in inverse mode, i.e., Landsat ETM reflectance data and stand parameters were used for LAI retrieval. The inversion requires as input an initial estimate of LAI and the uncertainty of it. We used as input the allometric estimates of LAI, with two levels of uncertainty (20 and 50%). As the uncertainty increased, correlation between retrieved and initial LAI decreased.

A METHOD FOR APPROXIMATING THE PHOTOSYNTHETIC PRODUCTION OF STAND MEMBERS INSIDE THE CANOPY

SUMMARY (1) A method is described which estimates the photosynthetic products which are available for the growth of a plant within a canopy. (2) The basic assumptions are: (i) a plant can be treated as an integrator of the curvilinear relationship of photosynthetic rate on light, and (ii) variation in photosynthetic rate of the leaves of community members can be reduced to the variation in the light intensity. (3) Equipment is described which integrates light intensity, simulating the curvilinear relationship between photosynthesis and light intensity. (4) A strong correlation was found between actual dry matter production and the photosynthetic production estimated by the present method.

Root Egress and Field Performance of Actively Growing Betula pendula Container Seedlings

The objective of this study was to evaluate the possibility of enlarging the planting window of container-grown silver birch seedlings from spring and autumn to summer by planting young, actively growing seedlings. For over 3 yrs the study investigated the growth, survival and damage of silver birch seedlings grown in containers and planted at different times during the growing season on 18 sites in fields and forest sites in central Finland. In a pot experiment in the greenhouse, the root-egress ability of seedlings planted at different times during the growing season was monitored. Root egress of seedlings was rapid from the beginning of July to the middle of August. Actively growing seedlings planted in summer on a site suitable for silver birch grew and survived at least as well as seedlings planted in spring during the dormant stage. The risk of drought stress increased when seedlings were planted on easily drying, coarse sandy soils. The risk was also high in soils with high levels of fine soil fractions that become hard when dry and when the dry period continued for several weeks before planting. In conclusion, it is possible to enlarge the planting window of birch seedlings to July and the beginning of August by using actively growing container seedlings.

Height variation of transplanted seedlings of Scots pine in Finnish nurseries

The height variation of Scots pine (Pinus sylvestris L.) seedling stock was studied using data from annual inventories of seedling crops. The heights of 240 batches of 2-year-old and 3-year-old transplanted seedling from the years 1973–1986 grown at three nurseries in southern Finland were examined. From each batch, 100 clusters of three successive seedlings were systematically located and measured. The mean heights of the seedling batches ranged from 7.8 to 27.0 cm, with a mean of 14.1 cm. Within batch standard deviations ranged from 1.7 to 7.8 cm, with a mean standard deviation of 3.7 cm. There was considerable variation in the mean heights of seedling batches both within a given year and between years. However, estimated variance components of seedling height showed that year, batch and cluster effect each accounted for about 20 percent of the total variance, while seedling effect explained the remaining 40 percent. The implications of results for single tree and batch grading are discussed.