Comment on gmd-2021-202

Abstract

Lycophytes (club mosses) represent a distinct lineage of vascular plants with a long evolutionary history including numerous extant and extinct species which started out as herbaceous plants and later evolved into woody plants. They enriched the soil carbon pool through newly developed root-like structures and promoted soil microbial activity by providing organic matter. These plants enhanced soil carbon dioxide (CO2) via root respiration and also modified soil hydrology. These effects had the potential to promote the dissolution of silicate minerals, thus intensifying silicate weathering. The weathering of silicate rocks is considered one of the most significant geochemical regulators of atmospheric CO2 on a long (hundreds of thousands to millions of years) timescale. The motivation for this study is to achieve an increased understanding of the realized impacts of vascular plants, represented by modern relatives to the most basal plants with vascular tissue and shallow root system, on silicate weathering and past climate. To this end, it is necessary to quantify physiological characteristics, spatial distribution, carbon balance, and the hydrological impacts of early lycophytes. These properties, however, cannot be easily derived from proxies such as fossil records, for instance. Hence, as a first step, a process-based model is developed here to estimate net carbon uptake by these organisms at the local scale, considering key features such as biomass distribution above and below ground, root distribution in soil regulating water uptake by plants besides, stomatal regulation of water loss and photosynthesis, and not withholding respiration in roots. The model features ranges of key physiological traits of lycophytes to predict the emerging characteristics of the lycophyte community under any given climate by implicitly simulating the process of selection. In this way, also extinct plant communities can be represented. In addition to physiological properties, the model also simulates weathering rates using a simple limit-based approach and estimates the biotic enhancement of weathering by lycophytes. We run the Lycophyte model, called LYCOm, at seven sites encompassing various climate zones under today’s climatic conditions. LYCOm is able to simulate realistic properties of lycophyte communities at the respective locations and estimates values of Net Primary Production (NPP) ranging from 126 g carbon m−2 year−1 to 245 g carbon m−2 year−1. Our limit-based weathering model predicts a mean chemical weathering rate ranging from 5.3 to 45.1 cm ka−1 rock with lycophytes varying between different sites, as opposed to 0.6–8.3 cm rock ka−1 without lycophytes, thereby highlighting the potential importance of such vegetation at the local scale for enhancing chemical weathering. Our modeling study establishes a basis for assessing biotic enhancement of weathering by lycophytes at the global scale and also for the geological past. Although our method is associated with limitations and uncertainties, it represents a novel, complementary approach towards estimating the impacts of lycophytes on biogeochemistry and climate.