Measurement methods and variability assessment of the Norway spruce total leaf area: implications for remote sensing

Estimation of total leaf area (LA(T)) is important to express biochemical properties in plant ecology and remote sensing studies. A measurement of LA(T) is easy in broadleaf species, but it remains challenging in coniferous canopies. We proposed a new geometrical model to estimate Norway spruce LA(T) and compared its accuracy with other five published methods. Further, we assessed variability of the total to projected leaf area conversion factor (CF) within a crown and examined its implications for remotely sensed estimates of leaf chlorophyll content (C (ab)). We measured morphological and biochemical properties of three most recent needle age classes in three vertical canopy layers of a 30 and 100-year-old spruce stands. Newly introduced geometrical model and the parallelepiped model predicted spruce LA(T) with an error < 5 % of the average needle LA(T), whereas two models based on an elliptic approximation of a needle shape underestimated LA(T) by up to 60 %. The total to projected leaf area conversion factor varied from 2.5 for shaded to 3.9 for sun exposed needles and remained invariant with needle age class and forest stand age. Erroneous estimation of an average crown CF by 0.2 introduced an error of 2-3 mu g cm(-2) into the crown averaged C (ab) content. In our study, this error represents 10-15 % of observed crown averaged C (ab) range (33-53 mu g cm(-2)). Our results demonstrate the importance of accurate LA(T) estimates for validation of remotely sensed estimates of C (ab) content in Norway spruce canopies.