No Consistent Shift in Leaf Dry Mass per Area Across the Cretaceous—Paleogene Boundary

Abstract

The Chicxulub bolide impact has been linked to a mass extinction of plants at the Cretaceous—Paleogene boundary (KPB; ∼66 Ma), but how this extinction affected plant ecological strategies remains understudied. Previous work in the Williston Basin, North Dakota, indicates that plants pursuing strategies with a slow return-on-investment of nutrients abruptly vanished after the KPB, consistent with a hypothesis of selection against evergreen species during the globally cold and dark impact winter that followed the bolide impact. To test whether this was a widespread pattern we studied 1,303 fossil leaves from KPB-spanning sediments in the Denver Basin, Colorado. We used the relationship between petiole width and leaf mass to estimate leaf dry mass per area (LMA), a leaf functional trait negatively correlated with rate of return-on-investment. We found no evidence for a shift in this leaf-economic trait across the KPB: LMA remained consistent in both its median and overall distribution from approximately 67 to 65 Ma. However, we did find spatio-temporal patterns in LMA, where fossil localities with low LMA occurred more frequently near the western margin of the basin. These western margin localities are proximal to the Colorado Front Range of the Rocky Mountains, where an orographically driven high precipitation regime is thought to have developed during the early Paleocene. Among these western Denver Basin localities, LMA and estimated mean annual precipitation were inversely correlated, a pattern consistent with observations of both fossil and extant plants. In the Denver Basin, local environmental conditions over time appeared to play a larger role in determining viable leaf-economic strategies than any potential global signal associated with the Chicxulub bolide impact.

A Systems Approach to Understanding How Plants Transformed Earth's Environment in Deep Time

“…Other fossil leaf traits have not been integrated into whole-plant functional strategy studies but play recognizable roles in ecosystem processes. For example, leaf morphology influences wildfire behavior, intensity, and fire recurrence rates (Baker et al 2022, Belcher et al 2021, and leaf nutrient economy [e.g., LMA (Butrim et al 2022, Royer et al 2007] plays a role in biogeochemical cycling. Additionally, undulation index-measured from leaf fossils and phytoliths-and LMA both inform canopy cover and surface energy balance [i.e., open versus closed vegetation canopy (Carvalho et al 2021, Cheesman et al 2020, Dunn et al 2015, Kürschner 1997, Milligan et al 2021].…”

Functional traits of fossil plants

“…High LMA leaves have a greater investment in structural tissues, higher densities of smaller cells, larger petioles to mechanically support leaves that have thicker tissue layers (including cuticle), and/or more dense tissue. Application of paleo‐LMA methods to fossil plants has enabled functional classification of extinct genera (Soh et al ., 2017; Wilson et al ., 2017), assessment of extinction selectivity associated with mass extinction events (Blonder et al ., 2014; Soh et al ., 2017; Butrim et al ., 2022), and appraisal of herbivore‐plant interactions (Currano et al ., 2008) among many others. Not surprisingly, therefore, LMA scored highly as a paleo‐functional trait (8) and ESE Trait (6.4) and yielded high ESI scores in our evaluation as it is among a few functional traits currently parameterized in paleo‐ecosystem models (Fig.…”

Trait‐based approaches as ecological time machines: Developing tools for reconstructing long‐term variation in ecosystems

“…In response, proxies that do not rely on the preservation of entire leaves have been developed for certain traits (e.g. Butrim et al, 2022;Soh et al, 2017). more closely resembled modern rainforests with a closed canopy and multi-stratal structure (Carvalho et al, 2021).…”

“…In response, proxies that do not rely on the preservation of entire leaves have been developed for certain traits (e.g. Butrim et al, 2022; Soh et al, 2017). For example, Soh et al (2017) reconstructed the LMA of fossil leaves preserved in a series of discrete fossiliferous horizons in East Greenland spanning the period of major climate warming ca.…”