Opposite-sign influences of canopy structural and physiological dynamics on water–carbon biogeography

Abstract. The terrestrial water and carbon cycles are strongly coupled through both vegetation canopy extent (structural) dynamics and canopy-intensive, physiological water-use dynamics. Although landscape vegetation productivity is directly proportional to both canopy extent and water use efficiency by definition, they are negatively correlated with each other at monthly scale, and their roles in connecting carbon uptake with water variables are in opposition. This study separates the roles of structural and physiological dynamics in driving a functional biogeographical pattern. We show that the relationship between Gross Primary Production (GPP) and Evaporative Fraction (EF, the normalized ratio of evaporation to available energy) more strongly links local temporal behavior with spatial biogeographical patterns than do rain-use efficiency relationships. This GPP–EF relationship has opposite correlation at daily (n = 325,374) and monthly timescales (n = 11,776): daily dynamics evolve toward a monthly “attractor” curve, while monthly correlations are positive and align with an across-site, across-biome, biogeographical relationship. Similarly, vegetation canopy extent and EF show the same timescale-dependent pattern: canopy extent increases with daily drying (negative correlation), then co-evolves positively with EF at monthly scales. The relationship between EF and physiological, canopy-averaged water-use efficiency is negatively-correlated at all scales. At monthly and longer timescales, structural canopy dynamics dominate water–carbon coupling over physiological water-use responses.

These results imply that modeling water–carbon feedbacks at timescales beyond daily requires accurate representation of both (a) dynamic canopy extent and (b) canopy-averaged water-use efficiency, or risk conflating opposite-sign correlations with changes in productivity. Specifically, vegetation growth, mortality, and responsive canopy phenology (structural processes) dominate water–carbon coupling over physiological water-use efficiency at timescales beyond daily.