A dynamic local-scale vegetation model for lycopsids (LYCOm v1.0)

Abstract

Lycopsids represent a distinct lineage of vascular plants with a long evolutionary history including numerous extant and extinct species that started out as tiny herbaceous plants and later went on to grow into forests with tree-like structures. 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 potentially promoted 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 over 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 of the most basal plants with vascular tissues and shallow root systems, 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 lycopsids. These properties, however, cannot be easily derived from proxies such as fossil records. Hence, as a first step, a process-based model is developed to estimate net carbon uptake by these organisms at the local scale. The model includes key features such as the distribution of biomass above and below ground, along with a plausible root distribution in the soil affecting water uptake by plants. The stomatal regulation of water loss and its immediate implications for photosynthesis are considered. Moreover, root respiration plays a crucial role in the model by affecting soil carbon dioxide and weathering rates. The model features ranges of key physiological traits of lycopsids to predict the emerging characteristics of the Lycopsida class community under any given climate by implicitly simulating the process of selection. In this way, extinct plant communities can also 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 these plants. We run the Lycopsid model, called LYCOm, at seven sites encompassing various climate zones under today's climatic conditions. LYCOm can simulate realistic properties of lycopsid communities at the respective locations and estimates values of net primary production (NPP) ranging from 126 to 245 g carbong m-2g yr-1. Our limit-based weathering model predicts a mean chemical weathering rate ranging from 5.3 to 45.1g cmg ka-1 of rock with lycopsids varying between different sites, as opposed to 0.6-8.3g cmg ka-1 of rock without these plants, thereby highlighting the potential importance of such vegetation at the local scale for enhancing chemical weathering. Our modeling study establishes a basis for assessing the biotic enhancement of weathering by lycopsids 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 lycopsids on biogeochemistry and climate.