Abstract
The canopy photosynthesis and carbon balance of the subtropical forests are not well studied compared to temperate and tropical forest ecosystems. The main objective of this study was to assess the seasonal dynamics of Normalized Difference Vegetation Index (NDVI) and potential canopy photosynthesis in relation to seasonal changes in leaf area index (LAI), chlorophyll concentration, and air temperatures of NE Argentina subtropical forests throughout the year. We included in the analysis several tree plantations (Pinus, Eucalyptus and Araucaria species) that are known to have high productivity. Field studies in native forests and tree plantations were conducted; stem growth rates, LAI and leaf chlorophyll concentration were measured. MODIS satellite-derived LAI (1 km SIN Grid) and NDVI (250m SIN Grid) from February 2000 to 2012 were used as a proxy of seasonal dynamics of potential photosynthetic activity at the stand level. The remote sensing LAI of the subtropical forests decreased every year from 6 to 5 during the cold season, similar to field LAI measurements, when temperatures were 10 °C lower than during the summer. The yearly maximum NDVI values were observed during a few months in autumn and spring (March through May and November, respectively) because high and low air temperatures may have a small detrimental effect on photosynthetic activity during both the warm and the cold seasons. Leaf chlorophyll concentration was higher during the cold season than the warm season which may have a compensatory effect on the seasonal variation of the NDVI values. The NDVI of the subtropical forest stands remained high and fairly constant throughout the year (the intra-annual coefficient of variation was 1.9%), and were comparable to the values of high-yield tree plantations. These results suggest that the humid subtropical forests in NE Argentina potentially could maintain high canopy photosynthetic activity throughout the year and thus this ecosystem may be a large carbon sink.
Highlights
Compared to tropical and temperate forests, the carbon balance of subtropical forests is not well studied
The cold season decline in normalized vegetation index (NDVI) suggests a depression in canopy photosynthesis, which is related to the direct low temperature effects on leaf photosynthetic enzyme activities [38] and the seasonal decrease in photosynthetic leaf surface area due to a Leaf Area Index (LAI) decrease
Our results suggest that the NDVI values, as a measure of potential canopy photosynthetic activity of the Atlantic subtropical forests, remains fairly high throughout the year, comparable to high-yield tree plantations
Summary
Compared to tropical and temperate forests, the carbon balance of subtropical forests is not well studied. Trees in humid subtropical forests may maintain relatively high photosynthetic rates even during the cold periods owing to relatively mild seasonal low temperatures. This pattern was observed for some East Asian subtropical evergreen forests [1,2]. Leaf level results show that net CO2 assimilation in dominant evergreen species of an East Asian subtropical forest never dropped below. The forest sites had considerably high values of carbon sequestration, as respiratory losses declined strongly during the cold season [1].
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