54-7 Constraining the Evolution of Nitrogen-Fixing Symbioses in Alders and its Influence on Herbivory Using Foliar C:N Ratios

Session: New Approaches to Old Fossil Collections

Presenting Author:

Alexandra Grajales

Authors:

Grajales, Alexandra M¹, Johnson, Benjamin W², Wing, Scott³, Currano, Ellen⁴, Kipp, Michael⁵, Hall, Grace⁶, Purdue, Catherine⁷, Purcell, Ashlyn⁸

(1) Iowa State Univeristy, Ames, IA, USA; Smithsonian, National Museum of Natural History, Washington DC, USA, (2) Iowa State University, Ames, Iowa, USA, (3) Smithsonian, National Museum of Natural History, Washington DC, USA, (4) University of Wyoming, Laramie, Wyoming, USA, (5) Duke University, Durham, North Carolina, USA, (6) Duke University, Durham, NC, USA, (7) Iowa State University, Ames, IA, USA, (8) Iowa State University, Ames, IA, USA,

Abstract:

Foliar carbon: nitrogen ratios (C:N) reflect species and ecosystem variability. This ratio changes in response to N limitation and atmospheric CO₂ fluctuations, generally increasing with N limitation and/or elevated CO₂. Since N acquisition is key for protein production, several vascular plant lineages, such as legumes and Alnus, evolved specialized organs (root nodules) for symbioses with diazotrophs. This symbiosis leads to lower foliar C:N in N-fixing plants compared to non-N-fixing plants, as they are not solely dependent on environmental N.

Foliar C:N may also influence herbivory and is thus a key factor in (paleo)ecology. Previous research has shown that insect herbivore damage on fossil floras tends to increase as the proportion of taxa whose modern relatives are N-fixers increases (Currano and Jacobs, 2021). However, it remains unclear when different lineages evolved this trait, and whether it contributes to site-level herbivory patterns. To further test this hypothesis and provide insight into how foliar C:N varies with atmospheric CO₂ , we present a suite of C and N measurements from Eocene fossils and herbarium specimens of taxa that today have (Alnus) and do not have (e.g., Acer, Platycarya) N-fixing symbionts. This work provides geochemical constraints on the evolution of N-fixing and leaf nutrient content.

We hypothesize that if N-fixing symbionts were present during the Eocene, fossil Alnus will yield lower C:N than non-N-fixers, following modern patterns. Yet, C:N ratios from Eocene Alnus and Platycarya (non-N-fixer) from the Bighorn Basin, WY, show similar foliar C:N values (20.7 and 23.2, respectively, p=0.1). In contrast, herbarium specimens from the last 150 years have statistically different foliar C:N between Alnus (19.8) and Acer (29.7, p=0.01). Mean Eocene N isotopic values are not statistically distinct between Alnus and Platycarya (3.4±1‰ and 3.7±0.7‰, p=0.23, Kipp et al., this session), in contrast to modern samples which are distinct (Alnus, -0.9±0.7‰, Acer, 1.1±3‰, p=0.01). The cause of similarity between the C:N and isotopic signatures of the fossilized Alnus and Platycarya is unknown and remains as an area of exploration of this research. Further analyses to test potential diagenetic and/or preservation alterations, and a larger sample set are also needed to confirm these findings and contextualize broader herbivory patterns during periods of changing atmospheric conditions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7028

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