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

INTRODUCTION: 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.

METHODS: Using MODIS LAI data, this study analyzed variations in LAI of deciduous needleleaf forests (LAIDNF) and deciduous broadleaf forests (LAIDBF) across the Northern Hemisphere and assessed their climatic drivers.

RESULTS: 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 m2 m-2yr-1, respectively; however, their intra-seasonal patterns differed significantly. LAIDBF increased significantly in both June-August and September, whereas the significant increase in LAIDNF 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 LAIDBF and LAIDNF to temperature changes in September diverged to some extent: warming was positively associated with an increased LAIDBF but showed no statistically significant change in LAIDNF. Interestingly, we identified asymmetric associations of LAI with diurnal warming: warming during the day had a notably more pronounced positive impact on both LAIDBF and LAIDNF in most regions, whereas the response of LAIDBF to nighttime warming showed spatial heterogeneity. Specifically, nighttime warming was positively associated with LAIDBF in areas with favorable thermal conditions (eastern United States), but negatively associated with LAIDBF in cold, high-latitude regions (northeast China).

DISCUSSION: 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.