Divergent impacts of temperature and precipitation changes on leaf area index of deciduous broadleaf and needleleaf forests in the Northern Hemisphere

Abstract

Vegetation coverage significantly affects the ecological functioning of forest ecosystem. While deciduous forests with various leaf forms employ distinct ecological strategies, their vegetation coverage dynamics across the growing season and intra-seasonal periods, as well as their responses to climatic change, remain unclear. Leaf Area Index (LAI) serves as a primary parameter of physiological functional status for canopies. Using MODIS LAI data, this study analyzed variations in LAI of deciduous needleleaf forests (LAI DNF ) and deciduous broadleaf forests (LAI DBF ) across the Northern Hemisphere and assessed their climatic drivers. This study revealed that from 2003 to 2021, LAIDBF and LAIDNF during the growing season exhibited increased rates of 0.91·10 –2 and 0.92·10 –2 m 2 m -2 yr -1 , respectively; however, their intra-seasonal patterns differed significantly. LAI DBF increased significantly in both June–August and September, whereas the significant increase in LAI DNF was primarily concentrated in June–August. The increase in precipitation was associated with a general decrease in LAI for both forest types during the growing season and in varied periods; conversely, rising temperatures increased LAI across the growing season, in both May and June–August across all leaf forms. Nevertheless, it should be emphasized that the responses of LAI DBF and LAI DNF to temperature changes in September diverged to some extent: warming was positively associated with an increased LAI DBF but showed no statistically significant change in LAI DNF . Interestingly, we identified asymmetric associations of LAI with diurnal warming: warming during the day had a notably more pronounced positive impact on both LAI DBF and LAI DNF in most regions, whereas the response of LAI DBF to nighttime warming showed spatial heterogeneity. Specifically, nighttime warming was positively associated with LAI DBF in areas with favorable thermal conditions (eastern United States), but negatively associated with LAI DBF in cold, high-latitude regions (northeast China). Our findings highlight the seasonally distinct responses of two deciduous forest types to climatic change and suggest that the asymmetric impacts of diurnal warming on different leaf forms should be incorporated into forest canopy leaf area dynamic prediction models.