Climate and nutrient effects on Arctic wetland plant phenology observed from phenocams

This study explores how climate and nutrients influence productivity of arctic wetland plants. The Green-excess Index (GEI) derived from Red, Green and Blue digital image brightness values from digital repeat photography (a.k.a. phenocams) was used to track the inter-annual variability in seasonal greening and above ground biomass for two dominant aquatic emergent graminoids on the Arctic Coastal Plain of northern Alaska: Carex aquatilis and Arctophila fulva. Four years of seasonal and inter-annual greening trends show strong differences in timing and intensity of greenness among species. Thawing degree-days (TDD, days above 0 °C) was a good predictor of GEI in both A. fulva and C. aquatilis. Employing regression tree analyses, we found a greening threshold of 46 TDD for A. fulva, after which GEI increased markedly, while C. aquatilis greened more gradually with a greening mid-point of 31 TDD. Based on long-term climate records and TDD thresholds, greening date has begun 16 thawing degree-days earlier over the past 70 years. To understand the effects of latitude and nutrients on seasonal greening, we compared southern sites and nutrient enriched sites with reference sites. We found statistically higher greenness in southern sites and enriched sites compare to reference sites in both plant species, supporting the role of nutrients and warmer temperatures as key factors enhancing productivity in arctic wetlands. In addition, this study provides an inexpensive, alternative method to monitor climate and nutrient effects at high frequency in arctic aquatic systems through camera-derived GEI greenness and has the potential to bridge the gap between plot level and satellite based observations given its strong relationships with biomass and NDVI.