Assessing earth system model predictions of C₄ grass cover in North America: From the glacial era to the end of this century

Abstract

Aim C4 grasses are distinct from C3 grasses, because C4 grasses respond in a different manner to light, temperature, CO2 and nitrogen and often have higher resource‐use efficiencies. C3 and C4 grasses are typically represented in earth system models (ESMs) by different plant functional types (PFTs). The ability of ESMs to capture C4 grass biogeography and ecology across differing time periods is important to assess, given the crucial role they play in ecosystems and their divergent responses to global change. Location North America. Time periods Last Glacial Maximum (LGM), historical modern period (ca. 1850) and end of this century. Major taxa studied C4 grasses. Methods Proxy data representing relative cover and productivity of C4 grasses were collated, including carbon isotope ratios of soil carbon and animal grazer tissue, and vegetation plot data in undisturbed grasslands. We selected available model predictions of C4 PFT percentage cover. Models were compared against one another and assessed against proxy data at key time points: the LGM, the historical modern period before widespread grassland conversion to agriculture, and the end of this century. Results We highlight large differences among model predictions of percentage C4 grass cover across North America: all pairwise combinations have correlations < .5, and most are < .2. Models also do not capture spatial patterns of the percentage C4 grass cover from proxy data, during either the LGM or the historical modern period. Models generally under‐predict percentage C4 grass cover, particularly during the historical modern period. Main conclusions Earth system models do not accurately represent the biogeography of C4 grasses across a range of time‐scales, and their outputs do not agree with one another. We suggest model improvements to represent this crucial functional type better, including more collection and greater integration of C3 and C4 grass trait data, explicit representations of tree–grass competition for water, and a greater focus on disturbance ecology.

Problems with inferring a lack of competition between Rancho La Brea dire wolves and sabertooth cats based on dental enamel

“…Earlier work on bone collagen from La Brea yielded  13 C values consistent with a predominantly C 3 ecosystem (i.e., plant  13 C values  -25‰) with little or no evidence of C 4 plants [2,4,5]. This agrees well with most proxy data and current climate models for this area in the late Pleistocene [6]. Enamel  13 C values for most herbivores and both species of canid, on the other hand, are higher than expected for pure C 3 consumers and suggest signifi cant consumption of C 4 plants (or the animals consuming C 4 plants) by these species.…”

Lineage‐based functional types: characterising functional diversity to enhance the representation of ecological behaviour in Land Surface Models

“…Dominance by C 4 vs C 3 grasses has major influences on gross primary productivity and ecosystem structure and function (Still et al ., 2003) and strongly influences interannual variability of the global carbon cycle, due to a combination of ecological and climatic factors (Poulter et al ., 2014; Griffith et al ., 2015). Dynamic vegetation models largely fail to reproduce spatial patterns of grass cover – both past and present – and productivity at regional to continental scales, limiting ability to predict future plant community changes (Fox et al ., 2018; Still et al ., 2018). As a consequence, LSMs require significant improvement to adequately represent vegetation responses to increasing CO 2 (De Kauwe et al ., 2016; Smith et al ., 2016).…”

“…Many models also miss key transitions between biome states (e.g. Still et al ., 2018) that exist as a result of disturbance or biogeographic history (e.g. Staver et al ., 2011; Dexter et al ., 2018).…”

The photosynthetic pathways of plant species surveyed in Australia’s national terrestrial monitoring network

“…Global warming is expected to alter the competitive advantage of plants with different photosynthetic pathways [14][15][16] , changing species distributions and community composition, and leading to significant bottom-up effects on the structure, diversity and function of terrestrial communities [17][18][19] . Thus, the ecology and evolution of these different pathways has become a focus of recent botanical research [20][21][22] . Australia is an ecologically diverse continent that includes a wide variety of habitats and climatic zones [23][24][25] , making it an ideal environment to examine trends in C 3 , C 4 and CAM distribution 23,26 .…”