Abstract
Karst systems represent an important carbon sink worldwide. However, several phenomena such as the CO2 degassing and the exchange of cave air return a considerable amount of CO2 to the atmosphere. It is therefore of paramount importance to understand the contribution of the ecosystem to the carbon budget of karst areas. In this study conducted in a mid-succession ecosystem developed on abandoned karst grassland, two types of model were assessed, estimating the gross primary production (GPP) or the net ecosystem exchange (NEE) based on seven years of eddy covariance data (2013-2019): (1) a quadratic vegetation index-based empirical model with five alternative vegetation indices as proxies of GPP and NEE, and (2) the vegetation photosynthesis model (VPM) which is a light use efficiency model to estimate only GPP. The Enhanced Vegetation Index (EVI) was the best proxy for NEE whereas SAVI performed very similarly to EVI in the case of GPP in the empirical model setting. The empirical model performed better than the VPM model which tended to underestimate GPP. Therefore, for this ecosystem, we suggest the use of the empirical model provided that the quadratic relationship observed persists. However, the VPM model would be a good alternative under a changing climate, as it is rooted in the understanding of the photosynthesis process, if the scalars it involves could be improved to better estimate GPP.
Highlights
Within the context of global changes, the exchanges of carbon between the atmosphere and ecosystems are of high importance for a better understanding of the biosphere’s carbon balance
High photosynthetic activities occur in the studied ecosystem between May and July and is translated into large negative values of net ecosystem exchange (NEE) (Fig 2A), large values of gross primary production (GPP) and Reco (Fig 2B) and generally high values of Vegetation indices (VI) (Fig 2C)
Low photosynthetic activity could be observed from October to March where GPP values are low and Reco reflects significantly in NEE values that become mostly positive
Summary
Within the context of global changes, the exchanges of carbon between the atmosphere and ecosystems are of high importance for a better understanding of the biosphere’s carbon balance. When grasslands used as pasture are abandoned, they naturally evolve into woody systems and into forests [2], which leads to a change in the ecosystem carbon balance. This change is more beneficial in karst ecosystems where the CO2 originating from the degassing of caves is absorbed more efficiently by plants, improving the carbon budget of such ecosystems [3].
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