In a previous tech tip, we used digital photography to show how canopy gap fraction measurements can be over-estimated if measurements are taken when foliage is brightly lit (i.e. when measurements are taken in direct sunlight). The over-estimation of gap fraction leads to under-estimation of leaf area index (LAI), typically by about 10% or more. The underestimation of LAI depends on factors such as sun angle and leaf angle distribution. To avoid this, the LAI-2200 should ideally be used in overcast conditions. But sometimes, there is no choice but to take measurements in sunny conditions (if for example, there are many plots to measure and you are working at a location with a high proportion of sunny days).
In this tech tip, we present some steps you can follow to help ensure accurate LAI estimates, even if you must use the LAI-2200 in direct sunlight. Watch this video or read on to learn more about it.
If you must use the LAI under direct sunlight, measure the canopy as you normally would. Make the measurements when the sun is high in the sky, between 10 am and 2 pm. Next, re-measure a subset of points or transects to find the correct LAI during ideal conditions (early morning, early evening, or overcast). Be sure that these measurements are taken at the same locations and with the same procedure as the first measurements. Divide each of these LAI measurements by the previous measurement made at each respective location to compute the “sun effect.” Then find the average “sun effect” for all points. Multiply all LAI values by the “sun effect” to account for the underestimate resulting from direct sunlight.
And, remember these tips:
- This procedure works best if the foliage is dense and the canopy lacks significant gaps. The more sunlit leaves the sensor can see, the larger the underestimate of LAI.
- Be sure the sun does not shine directly onto the sensor.
- If sky conditions are changing, make associated A and B readings as close to each other as possible, both in time and space.
- Use a view cap to block foreign objects from view.
- Use a narrow view cap and a larger sample size if there are significant gaps in the canopy.
See the LAI-2200 Instruction Manual, Chapter 4, for additional information about using the LAI-2200 under different sky conditions.