Modern foliar nitrogen-climate relationships in select Cupressaceae used to model middle Eocene high latitude foliar nitrogen

As global climate rapidly changes, investigating nitrogen in the past is important in understanding how plants affected and will affect the global nitrogen cycle. Nitrogen is primarily found in photosynthetic proteins and thus determines photosynthetic capacity and primary productivity (PCPP). Foliar nitrogen in particular has been shown to increase with latitudes and lower temperatures. Here, we use nitrogen content (%N)-climate general linear models (GLM) for insights into the past. We chose the middle Eocene, ~50 Ma, a period of global warmth our planet is projected to reach by 2100. The middle Eocene supported Metasequoia (Cupressaceae) dominated forests in the Arctic Circle, an extant deciduous conifer. Metasequoia and closely related Sequoia and Taxodium leaves were collected (n=115) for %N analysis across North America, Europe and Asia. Biodiversity Climate Change Visual Laboratory was used to model the relationship between %N and Bioclim layers: mean annual temperature (R²=0.085, p<0.001) and mean temperature of coldest quarter (R²=0.216, p<0.001). %N and climate are weakly correlated however %N increases with decreasing temperature. Previously published climate model was combined with fossil localities to extract Eocene climate data. We found modern Metasequoia had the highest range, 1.1 - 2.9 %N (mean=1.9 ± 0.4 %N) and Sequoia the lowest, 0.7 - 2.0 %N (mean=1.1 ± 0.3 %N). Four analyses (Metasequoia, Taxodium, Sequoia, and all taxa models) showed Akaike Information Criteria was lowest in Sequoia, followed by Taxodium, and highest for all taxa. Pairwise T-tests comparing raw and modeled modern samples showed Metasequoia and all-taxa models performed the best and reconstructed an average of 1.17 ± 0.24 %N and 1.24 ± 0.18 %N, respectively, suggesting a decrease in middle Eocene Arctic photosynthetic capacity. Using %N-latitude relationship (R²=0.104, p<0.001), our model estimates %N values for middle Eocene forests similar to our modern samples at 37 ºN. Modern mid-latitude forest PCPP (measured as leaf litter or biomass) aligns with the estimated biomass of middle Eocene Arctic Metasequoia forests. Testing our method with angiosperms and other fossil localities will refine our ability to reconstruct foliar nitrogen in the past using GLM.