Seasonal leaf area index variations derived from needle growth and fall measurements in two eastern white pine (Pinus Strobes L.) stands

摘要

Existing global leaf area index (LAI) products have a common problem of too large seasonal variations for conifer stands. The seasonal variation of remotely sensed LAI is often distorted due to many confounding factors. We propose a methodology to quantify the LAI seasonal pattern based on needle lifespan and meteorological conditions. In this paper, we test this methodology in two conifer stands in Ontario, Canada. Needle growth and senescence rates were quantified from needle elongation measurements and litterfall collection. By measuring the average needle lifespan and assuming a dynamic equilibrium of LAI for the full annual cycle, the amount of needles renewed annually can be estimated. Then it is seasonally ‘distributed’ based on these two rates in order to reckon seasonal variations of new and old needle cohorts. These measured LAI seasonal trajectories were compared with combined measurements by TRAC (Tracing Radiation and Architecture of Canopies) and LAI 2000 plant canopy analyzer. The difference between peak LAI from needle growth and fall and that from optical instruments is constrained within 15%. Meanwhile, LAI values from needle growth and fall beyond the growing season are not fallaciously low like LAI from remote sensing data, resulting in a more realistic seasonal variation of needles than that is retrieved from algorithms using remote sensing data. We believe this study can be a positive step towards improving global LAI mapping.