Abstract
The Leaf-Area-Index (LAI) is commonly used to characterize the plant canopy and is a fundamental indication of plant vitality and photosynthetic activity. The forest health status is not only vital for economical reasons, but also has a significant impact on global carbon sequestration. The LAI has a highly dynamic character among deciduous forests and is prone to significant seasonal fluctuations. Accurate continuous LAI measurements do provide valuable information on growth characteristics, but they require considerable measurement effort. In this study, we tested a novel method that would allow for continuous low-effort LAI parameterizations. For our study we used temperature measurements from 2011 to 2019 obtained at two meteorological stations: Station one is an open land station, station two is located inside a forest stand characterized by European beech (measurements were undertaken as part of the ICP Forests program), both are located in Klausen Leopoldsdorf (Austria). We chose the difference in daily maximum temperature between the two sites for our LAI parametrization (LAIpar) since the forest canopy has a significant impact on local radiation conditions. We were able to identify phenological events such as leaf unfolding, the end of leaf growth, and the beginning and end of defoliation by examining at the average course of the year for LAIpar. The resultant LAIpar values were compared to annual values derived from hemispheric photographs taken near the stand temperature sensor. For the years 2011–2017, we found a strong correlation of 0.93 between LAI measures and LAIpar, which dropped to 0.69 after adding the year 2018 and 0.32 after adding 2019. We further compared the phenological events obtained from LAIpar to phenological observations. The impact of forests on their site climate, according to our findings, can be utilized to identify phenological and growth characteristics. The proposed method, however, is not a replacement for conventional LAI measurements.
Highlights
The forest canopy is an important interface between the terrestrial ecosystem and the atmosphere allowing for an exchange of energy, water, and carbon dioxide (Bonan, 1993)
By analyzing the median course of the year for LAI parametrization (LAIpar) (Figure 2) we were able to identify two constant foliage phases: The “Maximum foliage phase” during which the forest canopy is fully developed and no defoliation has yet taken place and the “No foliage phase” for which the constant low temperature difference can be interpreted as the blocking of solar radiation by the branches when defoliation is completed
Comparing the timing of the end of the no foliage phase with observed leaf unfolding showed a delay, with the no foliage phase ending on day 95 and the observed leaf unfolding being marked as day 108
Summary
The forest canopy is an important interface between the terrestrial ecosystem and the atmosphere allowing for an exchange of energy, water, and carbon dioxide (Bonan, 1993) This interface may be affected by climate change as previous studies have shown that the occurrence of phenological phases has shifted due to temperature changes (Chmielewski and Rötzer, 2001; Cleland et al, 2007). Plant phenology shifts occur at a rate of 1 day per decade on average, the usual uncertainties from satellite LSP are much larger. This suggests that utilizing satellite data to track phenological changes still has considerable limitations
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