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

Abstract

Terrestrial plants have transformed Earth's surface environments by altering water, energy, and biogeochemical cycles. Studying vegetation-climate interaction in deep time has necessarily relied on modern-plant analogs to represent paleo-ecosystems—as methods for reconstructing paleo- and, in particular, extinct-plant function were lacking. This approach is potentially compromised given that plant physiology has evolved through time, and some paleo-plants have no clear modern analog. Advancements in the quantitative reconstruction of whole-plant function provide new opportunities to replace modern-plant analogs and capture age-specific vegetation-climate interactions. Here, we review recent investigations of paleo-plant performance through the integration of fossil and geologic data with process-based ecosystem- to Earth system–scale models to explore how early vascular plants responded to and influenced climate. First, we present an argument for characterizing extinct plants in terms of ecological and evolutionary theory to provide a framework for advancing reconstructed vegetation-climate interactions in deep time. We discuss the novel mechanistic understanding provided by applying these approaches to plants of the late Paleozoic ever-wet tropics and at higher latitudes. Finally, we discuss preliminary applications to paleo-plants in a state-of-the-art Earth system model to highlight the potential implications of different plant functional strategies on our understanding of vegetation-climate interactions in deep time. ▪ For hundreds of millions of years, plants have been a keystone in maintaining the status of Earth's atmosphere, oceans, and climate. ▪ Extinct plants have functioned differently across time, limiting our understanding of how processes on Earth interact to produce climate. ▪ New methods, reviewed here, allow quantitative reconstruction of extinct-plant function based on the fossil record. ▪ Integrating extinct plants into ecosystem and climate models will expand our understanding of vegetation's role in past environmental change.

Functional traits of fossil plants

“…Pyrolysis experiments have been used to simulate the chemical changes that take place within leaves due to diagenesis (Mosle et al ., 1997) and would be equally valuable to assess the fate of LNC during fossilization. Pilot LNC measurements on compression fragments of Late Pennsylvanian fossil taxa have yielded promising results that plot within the trait values of modern LNC (Matthaeus et al ., 2023) and are in line with expectations based on their other paleo‐leaf economic spectrum traits but further systematic investigation is required. A likely future challenge in establishing direct protocols for measuring fossil LNC will be to collect adequate amounts of compression fossil material (a minimum of 1 mg of ground‐up sample material is required; Aslam et al ., 2012).…”

“…Plant functional traits: broadly defined as any measurable morphological, anatomical, physiological, biochemical, or phenological trait of an individual plant that potentially affects its environment or its fitness (from Pérez-Harguindeguy et al, 2013). For the purpose of this review, we focus more on plant functional traits which affect their local, regional and/or global environment (Chapin 3rd's (2003) 'effect' traits) as these are important for Earth system modelling (sensu Lavorel et al, 2007) in the present and past (Matthaeus et al, 2023).…”

“…Water use efficiency is a measure of how water saving a plant is in relation to photosynthesis. Low values in the 20 to 40 μmol mol À1 range are typically observed in modern angiosperm trees of tropical everwet forest biomes while higher values (60 to 80 μmol mol À1 ) are recorded from desert plants and those from seasonally dry biomes (Soh et al, 2019).Water use efficiency is an informative functional trait in relation to the hydrological cycle and hydroclimate generally in the deep past (White et al, 2020) and allows broad characterization of paleo-biomes from fossil iWUE estimates of dominant taxa within paleo-ecosystems (Matthaeus et al, 2023). Intrinsic water use efficiency (iWUE) is expressed as the ratio of photosynthesis (A) and leaf conductance to water vapor transfer (g) of which g s is the dominant component.…”

“…Current Earth system models (ESMs) incorporate vegetation, and biosphere feedbacks and drivers, but generally have not considered how plant‐driven feedbacks and forcing over time may have changed with the emergence of new plant evolutionary groups and their associated functional traits (Matthaeus et al ., 2023). On geological timescales, the primary drivers of plant trait selection and filtering such as Earth's global mean annual temperature and precipitation, atmospheric composition, wildfire ecology, biota of herbivores, pathogens, symbionts, mutualists, dispersers, pollinators have all changed dramatically.…”

“…Leaf lifespan covaries with LMA, A mass , and g s (Wright et al ., 2004; Poorter et al ., 2009). The duration of green foliage is important in the hydrological cycle, for productivity and Earth albedo effects, and should be included where possible within paleo‐ecosystem models (Matthaeus et al ., 2023). Shorter‐lived leaves tend to show resource allocation toward high photosynthetic rates and have lower investment in C‐rich lignified tissues, whereas longer‐lived leaves are often lignin‐rich and tend to allocate resources toward leaf protection (Reich et al ., 1991).…”