Plant paleoecophysiology traits in deep time: hydraulic conductivity and drought resistance in late Carboniferous Period plants

Plants have been a key interface in the global carbon and water cycles for nearly 475 million years. The magnitude of vegetational effects has waxed and waned dynamically because plant abundance and community composition have changed over time. Unravelling how plant communities have shaped, and been shaped by, global biogeochemical cycles relies upon reconstructing the paleoecology and paleoecophysiology of plants, and this process can be challenging in deep time, when plant communities contained organisms with traits that are rare in—or absent from—present-day ecosystems. Fortunately, the archive of how plants have shaped and responded to environmental change is preserved in the fossil record, because the traits and properties of extinct plants can be interpreted from fossilized anatomy in a qualitative, semi-quantitative, and quantitative way. Traits related to water transport in plants. including drought resistance and hydraulic supply to leaves, are particularly useful and important because these traits link individual plant performance to the water and carbon cycles.

The collapse of tropical everwet rainforests end of the Carboniferous Period (~300 Ma) provides an illustration of how plant water transport traits influenced, and were shaped by, the water and carbon cycles. These traits are quantified by combining mathematical models of stem hydraulic conductivity and drought resistance with anatomical measurements from scanning electron and light microscopy images of fossilized plant water transport cells, called xylem. Analysis of stem hydraulic traits in five lineages of extinct Carboniferous plants—arborescent lycophytes, stem group seed plants, stem group tree ferns, coniferophytes, and sphenophytes—reveals differential hydraulic capacity and drought resistance among these plants, despite their simultaneous presence in tropical everwet ecosystems. Significant differences in these two traits are not only present between these five lineages, but can also be observed within several of these plant groups: for example, key parameters may vary by more than an order of magnitude in related plants. High hydraulic capacity and low drought resistance traits were associated with a decline in relative abundance toward the close of the Carboniferous Period, whereas plants with lower hydraulic capacity and higher drought resistance traits increased in relative abundance and survived this floral transition. This change in relative abundance within these communities shaped the hydrologic and carbon cycles which, in turn, amplified environmental stress that, consequently, further altered plant community composition. Implementing this analysis in trait-aware paleoecosystem models illustrates the effect of plant traits on global environments, and vice versa, yielding insight into plant performance during extreme environmental change that is analogous to anthropogenic impacts predicted for the late 21^st century and beyond.