The greening effect characterized by the Normalized Difference Vegetation Index was not coupled with phenological trends and tree growth rates in eight protected mountains of central Mexico

Abstract

In temperature-limited ecosystems such as high-elevation forests, rising temperatures and increasing carbon dioxide can result in a greening effect and changes in the phenology of the forests. Up to now, phenological metrics of subtropical high-elevation forests remain primarily unquantified, which strongly limits a global comprehension of climate change impacts. We assessed the long-term trend dynamics (greenness and land surface phenology) using the Normalized Difference Vegetation Index (NDVI) and compared them with data from an existing dendrochronological network that came from eight protected mountains in the Trans-Mexican Volcanic Belt in central Mexico during the last 20 yr (2000–2019). We found three main NDVI clusters reflecting a longitudinal geographical pattern (Pacific, Center, and the Gulf of Mexico). A notable greening effect at the canopy level was seen for all mountains (up to 23.8% increase concerning the year 2000). Nevertheless, the NDVI trends were not uniform, and they differed along an elevation gradient with the highest increases at the bottom. Minimum temperature and soil moisture positively impacted the NDVI time series and explained up to 87% of the variance, while maximum temperature negatively affected. Regarding phenological metrics, the start, and end of the growing season were the day of the year 148 ± 20 (late-May) and 9 ± 12 (early-January), respectively, with an average length of 226 ± 33 d, differing from previous studies related to annual tree-ring formation. We did not see a later or earlier start of either the spring or the fall, which partially refutes the hypothesis that the growing season extended despite the increase in temperature. Finally, our investigation revealed that the common greening effect was not parallel with radial growth trends, except for the wettest site from the Gulf of Mexico cluster, and partially related to the forest management strategies. Further modelling of regional phenology coupled with more intensive tree-ring monitoring is needed to explain the factors that mediate the processes of photosynthesis (NDVI) and storage of carbon (tree-ring formation) under a global warming outlook.