LB 9-100 - Opposing effects of agriculture and urbanization on growing season length across global biomes

Background/Question/Methods:

Changes in vegetation growing season lengths have important, downstream effects on resource availability, nutrient cycling, and ecosystem services. While studies of shifting phenology have focused largely on the indirect effects of greenhouse gas emissions, humans change growing season lengths directly by altering species composition and the abiotic factors that influence phenology. Regional studies have demonstrated that land-use modification alters phenology, but the generality of these patterns across global biomes is unclear. Using phenology estimates derived from EVI (enhanced vegetation index), we analyzed how urbanization and conversion to agriculture affect growing season length across biomes globally, and compared the magnitude of their effects to those of climate change.

Results/Conclusions:

Our results show that, on average, urbanization lengthens the growing season up to 13 days, and conversion to agriculture shortens the growing season as much as 30 days, but the strength of these trends varies by biome. By contrast, temporal trends in growing season length (2001-2016) due to climate change were more biome-specific than general trends seen with changes in land use, with the biggest impacts seen in temperate grasslands (+13 days/decade) and tropical dry broadleaf forests (-6 days/decade). Estimates of human impacts on global phenology should consider land-use in addition to climate change.