Leaf area index (LAI)

Important ecosystem processes acting at the interface between the land surface and the atmosphere, such as photosynthesis (Bonan 1993), respiration, transpiration, litter fall and energy balance (Running 1992, Bobeva 2003) are primarily determined by the canopy structure of the vegetation. Leaf area index provides important information related to a variety of plant canopy processes, and is a value required when including the above ecosystem processes into quantitative descriptions, i.e. into ecosystem models. LAI can be measured or derived by different methods: by harvesting, by collection and weighing of total leaf litter, by allometry of trees with up-scaling to stand level, and by indirect optical or light interception approaches (hemispherical photography, sunfleck ceptometers, tracing radiation and architecture of canopies (TRAC), optical instruments – LAI-2000, LI-COR; see (Chen and Cihlar 1995)). Direct measurements of canopy structure are difficult, destructive and expensive.

Therefore, other methods such as the use of allometric relationships that relate tree size to leaf area, or satellite remote sensing – must be used to obtain LAI over large areas. In this study, the LAI for spruce and beech sites was estimated with allometric functions. Due to lack of information from harvests,

MATERIALS AND METHODS 31

LAI measurements in the experimental larch stand were estimated with a canopy analyzer (light interception method). Leaf area index of Pinus mugo was estimated by direct harvest of stands adjacent to the measurement branches.

3.2.4.1. Picea abies sites

For estimating the leaf area index in the three investigated spruce stands, an allometric function was applied based on tree harvests by Alsheimer (1997), Burger (1939; 1942; 1953), and Faltin (unpublished). The equation derived is as follows:

LA = 0.1 CBH ^1.72 Eq. 3.1,

where CBH is circumference at the breast height (1.35 m) and LA is the total surface leaf area of individual trees. The LAI of the spruce stand was calculated as the sum of LA for all individual trees growing in the investigated plot and by dividing by plot area:

LAIstand = Σ LAtree/ (PA*2.57) (m² m^-2) Eq. 3.2,

where LAItree is the total leaf area of individual trees (m² m ^-2), PA is the plot area (m²), and 2.57 is the conversion factor for estimating projected leaf area from total surface area (Alsheimer 1997; Oren et al. 1986). Total plot area was determined from the mapping of trees as described in section 3.2.1. The plot boundaries were set at the mid-point between included and adjacent trees as shown in Fig. 3.11.

Fig. 3.11: Determination of plot boundaries at mid-points between included trees and adjacent trees at three spruce sites in Berchtesgaden National Park. Leaf area was determined for all included trees and set in relation to the surface area within the circumscribed polygon. BA = Bartholomä, HG = Hirschengarten, SA = Seeangerl.

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MATERIALS AND METHODS 33

3.2.4.2. Fagus sylvatica site

To estimate leaf area index of the experimental European beech stand, an allometric function was similarly based on harvest data (Bartelnik 1997; Burger 1945; Pellinen 1986). The equation is as follows:

LAtree = 0.118 CBH ^1.565 Eq. 3.3,

where CBH is circumference at the breast height (1.35 m) and LA is the projected leaf area of individual trees.

The leaf area index of the site was calculated as the sum of LA for all individual trees growing in the investigated plot divided by plot area:

LAIstand = Σ LAtree/ PA (m²/ m²) Eq. 3.4, ,

where LAtree is the leaf area of individual trees and PA is the plot area (m²). The plot boundaries were set at the mid-point between included and adjacent trees as shown in Fig. 3.12.

E(x) [m]

E(y) [m]

-55 -50 -45 -40 -35 -30 -25 -20

-35 -30 -25 -20 -15 -10 -5 0

Fig. 3.12: Determination of plot boundaries at mid-points between included trees and adjacent trees at the beech site in Berchtesgaden National Park. Leaf area was determined for all included trees and set in relation to the surface area within the circumscribed polygon.

3.2.4.3. Larix decidua and Pinus mugo sites

The measurements of larch leaf area index (LAI) were carried out in 2001 with the Plant Canopy Analyzer LAI2000, LI-COR Inc., Lincoln, USA. The LAI-2000 measures the effective LAI from canopy gap fraction recorded for different solid angles. It is an optical instrument equipped with five detectors monitoring a series of concentric rings. Detector 1 measures radiation directly overhead (0-13° from vertical), while detector 5 measures incoming radiation of a ring between 61° and 74° from vertical. Two instruments are used, one underneath the forest canopy and the second mounted in a nearby open clearing to provide open-sky reference of radiation conditions. This instrument is designed to be used in diffuse light conditions.

In the context of considering water use by the entire stand, the well-developed understory is an important component. Thus, in 2002, maximum biomass and

MATERIALS AND METHODS 35

LAI of the herbaceous and understory vegetation were estimated by harvesting during mid-July.

Leaf area index, i.e., development of stands of Pinus mugo, appears to be highly variable depending on disturbance, soil depth and exposition. Thus, it was only possible to make some coarse estimates of LAI for this species near to the measurement installations in two different ways. First, the LAI of actual measurement branches was estimated by measuring the total length of needled branchlets. Needle area per length of branchlet was quantified previously at the site by harvesting, removing needles and measuring needle projected area with a leaf area meter (CI-202, CID, Camas, WA). With the estimates of needle area per branch, the measured data for the branch which had needles at all levels within the stand could be expressed as an average flux per needle projected surface area. The plot boundaries were set at the mid-point between included and adjacent trees as shown in Fig. 3.13.

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Fig. 3.13: Determination of plot boundaries at mid-points between included trees and adjacent trees at the larch site in Berchtesgaden National Park. Leaf area was determined for all included trees and set in relation to the surface area within the circumscribed polygon.

In addition, a 2 x 2 meter plot was established adjacent to the measurement site at the end of the season and all biomass was harvested in a series of layers between the top of the stand and the ground surface. The material was separated into branches, needles and cones. Fresh weights for each component were determined and sub-samples were taken for needle surface area determination and for oven drying in Bayreuth. From the sub-samples, total LAI and total biomass of the aboveground components was determined.

For the estimation of total LAI for Pinus mugo, the leaf area (LA) of needles from different layers was measured, summed and divided by ground area. After harvesting of Pinus mugo, the biomass and leaf area of understory herbaceous plants and grasses was also measured.

MATERIALS AND METHODS 37