Bringing Color into the Picture: Using Digital Repeat Photography to Investigate Phenology Controls of the Carbon Dioxide Exchange in a Boreal Mire

摘要

Mire vegetation phenology is closely linked to the ecosystem carbon cycle but rarely monitored and quantified with high temporal resolution. In this study, we use digital repeat photography to explore phenology as a control of the carbon dioxide (CO2) exchange measured by eddy covariance (EC) in a minerogenic boreal mire in northern Sweden over 2 years (2011-2012). Strong correlations and seasonal hysteresis effects were observed between the green chromatic coordinate (g (cc)) derived from the digital image archive and leaf area index, day length, and growing degree-day sum (GDDS). Differences in GDDS between the 2 years were the main control on the between-year variations in the spring patterns of g (cc). Periods with lower water table level coincided with an increase of the red chromatic coordinate. The onset and magnitudes of EC-derived photosynthetic CO2 uptake (that is, gross ecosystem production, GEP) and net ecosystem CO2 exchange (NEE) during the spring green-up of vascular plants were more closely related to those of g (cc) than to those of air temperature and photosynthetically active radiation. In contrast, abiotic variables controlled GEP during the summer period when vascular plant canopy cover was fully developed. Stepwise regression analysis suggested that g (cc) contributed substantially in explaining variations in GEP during spring and autumn. Over both growing seasons, g (cc) was well correlated with GEP (r (2) = 0.68), NEE (r (2) = 0.58), and ecosystem respiration (r (2) = 0.50). Overall, we show that digital repeat photography provides an inexpensive and effective method for the continuous quantification of the phenological patterns of the vascular plant community in mire ecosystems. Our results suggest that vegetation phenology is an important control of the mire CO2 exchange and should be considered in both experimental and modeling studies to better account for the separate effects of phenology and abiotic drivers on mire carbon dynamics.