Tuesday, January 29, 2013

How to estimate LAI in boreal forests?

How should we estimate leaf area index for a stand in a boreal forest? How much do the different LAI estimation methods actually differ? These are questions I’m often asked when people working in other fields need a quick way to get LAI for their study sites.

In an effort to tackle these questions, we pooled together our LAI data for nearly 700 stands measured during the past decade and compared a range of methods such as allometric equations, LAI-2000 measurements and inversion of a canopy radiation model. Even though we did not have the so-called true LAI (from destructive sampling) for these stands, we were still able to get an idea of how the different methods perform. Many ecosystem or growth models need LAI as an input variable, and therefore it is crucial to understand how using LAI data from different sources can affect the modeling results.

 Intercalibrating LAI-2000 units in an open area before making measurements in the forest.

 Titta measuring the LAI of a Scots pine stand using the LAI-2000 PCA instrument.
Which LAI estimation method should you use then? There is no simple, straight-forward recommendation; the optimal LAI estimation method depends on the spatial and temporal extent of the monitoring activity you will be doing. The optical LAI estimation methods are usually superior if your research focus is on seasonal or abrupt changes (e.g. defoliation or insect outbreaks) in foliage mass. In addition, optical measurements are relatively fast, inexpensive and less laborious than conventional forest inventories – if you have a camera with a hemispherical lens or an LAI-2000 Plant Canopy Analyzer at hand and are familiar with the theory behind them. For large area applications (such as regional or national LAI maps) a more suitable ground reference method is to use allometric equations based forestry databases. However, allometric equations cannot (currently) predict the seasonal development of LAI in a forest. They will only provide you with the peak growing season LAI which corresponds to the maximum foliage mass in a forest.

For more quantitative information, check out Titta’ s new paper:
Majasalmi, T., Rautiainen, M., Stenberg, P. & Lukes, P. 2013. An assessment of ground reference methods for estimating LAI of boreal forests. Forest Ecology and Management, 292: 10-18.