Leaf Area Index Estimation Using Three Distinct Methods in Pure Deciduous Stands

The leaf area index serves as the interface between leaves and the atmosphere in which many physiological processes are implemented, especially photosynthetic uptake. The main advantages of this method for leaf area index estimation are the straightforward applicability, short measurement time requirements, and the precision of the acquired data. Given that these methods have already been used for ecophysiological studies in crops, grasslands, conifers, and hardwood forest stands, they should be widely applicable to most other plant ecosystems.

All of the described methods for leaf area index estimation are straightforward and simple to use. Researchers new to the methods should not encounter difficulties with implementing the measurements. Visual demonstration of these leaf area index estimation methods is essential for their comprehension, design, and implementation.

Demonstrating the procedure will be Pavel Haninec, a PhD student from the Department of Forest Botany, Dendrology, and Geobiocoenology. For leaf area index estimation using litter traps, place 15 to 25 traps per investigated stand with a capturing area ranging from 0.18 meters squared up to 0.5 meters squared or more at the beginning of the growing season. Place the traps at regular spacing throughout the studied stand within one or two mutually perpendicular transects or a regular grid, a minimum of 0.1 meter above the ground surface to enable air to blow beneath the collecting part of the traps.

Firmly fix each of the traps above the ground surface and below the stand canopy so that there are no changes in the capturing area. At each measurement time point, place the collected litter from each of the traps into appropriately labeled paper bags for transport to the lab. Back at the lab, separate the assimilation apparatus from the other litter components.

To perform a specific leaf area estimation, thoroughly mix the sample from each trap before separating a subsample of at least 100 to 200 leaves from all of the used traps. After briefly soaking any dried leaves in 60 to 70 degrees Celsius water to keep the leaves from folding or curling, place the leaves from the subsample in a flat, straight manner onto a scan board without overlapping. Dry the subsample for 48 hours at 80 or 105 degrees Celsius to attain a constant weight in a ventilated oven with a thermostat to homogenize and maintain the internal temperature.

At the end of the drying period, weigh the dry mass of the subsample on a lab scale with a minimum degree of accuracy of one gram and calculate the specific leaf area as the fresh projected area of leaves from the subsample designated for the specific leaf area estimation divided by the dry mass weight. Then, oven dry the rest of the sample from each trap for 48 hours at the same temperature that was used for the specific leaf area estimation and multiply the dry mass weight of the rest of the sample for each particular litter trap by the correct specific leaf area value to reach the total projected leaf area index per trap. To estimate the leaf area index using the needle technique, immediately after a complete leaf fall use a sufficiently long, sharp, metallic needle with no more than a two-millimeter diameter to puncture leaves at a more or less similar angle through the layer of freshly fallen leaves lying on the ground surface at each of at least 100 probed sampling points.

Check to make sure that only freshly fallen leaves are present on the needle, removing any partially decomposed leaves from the previous year as necessary. Then, count the number of leaves pierced by the needle from each stab at each sampling point before repeating the needle technique leaf collection at subsequent sampling points. For leaf area index estimation using a plant canopy analyzer optical device, locate a suitable open clearing with identical sky conditions as above the observed plot, a maximum distance of one kilometer.

Apply the same cap and orientation for both above-and below-canopy readings and slowly move with the sensor below the canopy in transect, watching the variability of the most upper ring readings. Using the same view cap for the both above-and below-canopy readings is necessary for correct leaf area index estimation using a plant canopy analyzer. The number and spacing of the transects depends upon the particular canopy structure of the stand.

Perform the above-canopy readings as the first measurement of each stand transect or grid in a sufficient open area and perform the below-canopy readings in one to three transects and between five to 36 sampling points from 0.5 to two meters above the ground. Use the restriction view caps if the sensor is held below two meters to exclude the operator from the field of view and use a minimum distance between the sensor and the nearest element of the aboveground parts of the plant of at least four times the diameter or width of the component. Estimate the woody area index during the leaf-off period, both before bud breaking in early spring and after complete leaf fall in late autumn.

To calculate the woody area index values from the field-measured raw data, use the the leaf area index 2200 File Viewer freeware and estimate the plant area index using the same procedure as for making the woody area index estimation. Then calculate the actual leaf area index value at the stand level as the difference between the mean plant area index and woody area index values. In this representative experiment, on Plots B, C, and D, the needle technique significantly underestimated the leaf area index obtained from the litter traps.

Conversely, on Plot A, this technique overestimated the leaf area index measured using the litter traps at a not-significant level. Further, significant differences among the leaf area index values estimated by the plant canopy analyzer and the needle technique were found in all cases. The woody area index can be readily measured using a plant canopy analyzer after the complete leaf fall and before bud break.

In this representative experiment, the most rapid leaf area index development was noted during the period from bud break occurring in April until the beginning of May. From May until the end of June, a continuation of the fast leaf area index development of leaves was observed, although with a lower intensity compared to the previous period. From the second half of June until the end of July, the leaf area index value declined in Plot B and the stagnation of the leaf area index was more evident during this period on Plot A.In all of the studied forest stands, the leaves started to fall at the end of September, as illustrated by the decrease in the leaf area index curve.

The most critical part of the litter trap method is the accurate estimation of the specific leaf area from a sufficiently large subsample of leaves. For the plant canopy analyzer, it is essential to perform the measurements under standard overcast and windless conditions. Digital hemispherical photographs and laser scanning technology can also be used to determine the leaf area index and the spatial distribution of the leaves within the canopy.

Currently, the most significant challenge in leaf area index estimation is using remote sensing to determine the foliage quality.