Fossil leaf economics quantified: calibration, Eocene case study, and implications

Abstract

Abstract,•Leaf mass per area (M^) is a central ecological trait that is intercorrelated with leaf life span, photosjnthetic rate, nutrient concentration, and palatability to herbivores. These coordinated variables form a globally convergent leaf economics spectrum, which represents a general continuum running from rapid resource acquisition to maximized resource retention. Leaf economics are little studied in ancient ecosystems because they cannot be directly measured from leaf fossils. Here we use a large extant data set (65 sites; 667 species-site pairs) to develop a new, easily measured scaling relationship between petiole width and leaf mass, normalized for leaf area; this enables M^ estimation for fossil leaves from petiole width and leaf area, two variables that are commonly measurable in leaf compression floras. The calibration data are restricted to woody angiosperms exclusive of monocots, but a preliminary data set (25 species) suggests that broad-leaved gymnosperms exhibit a similar scaling. Application to two well-studied, classic Eocene floras demonstrates that M^ can be quantified in fossil assemblages. First, our results are consistent with predictions from paleobotanical and paleoclimatic studies of these floras. We found exclusively low-M^ species from Republic (Washington, U.S.A., 49 Ma), a humid, warm-temperate flora with a strong deciduous component among the angiosperms, and a wide M^ range in a seasonally dry, warm-temperate flora from the Green River Formation at Bonanza (Utah, U.S.A, 47 Ma), presumed to comprise a mix of short and long leaf life spans. Second, reconstructed M^ in the fossil species is negatively correlated with levels of insect herbivory, whether measured as the proportion of leaves with insect damage, the proportion of leaf area removed by herbivores, or the diversity of insect-damage morphotypes. These correlations are consistent with herbivory observations in extant floras and they reflect fundamental trade-offs in plant-herbivore associations. Our results indicate that several key aspects of plant and plant-animal ecology can now be quantified in the fossil record and demonstrate that herbivory has helped shape the evolution of leaf structure for millions of years.

Remnants of an ancient forest provide ecological context for Early Miocene fossil apes

“…Wet-month precipitation, which is the total precipitation for all months exceeding 50 mm of rainfall, was estimated using the leaf area analysis regression of Jacobs and Herendeen 50 (Supplementary Table 3). Leaf mass per area (M A ) and leaf life span was estimated using the methods of Royer et al 51 Table 4). Fossils collected from 2011 to 2013 were mapped using a differential global positioning system and total station to determine their precise location.…”

Palaeoecology of Macroneuropteris scheuchzeri, and its implications for resolving the paradox of ‘xeromorphic’ plants in Pennsylvanian wetlands

“…The less well defended species also are engaging in a classic ecological "trade-off", sacrificing biomass to predation for high benefits derived from that biomass while functioning (reviewed in Royer et al, 2007aRoyer et al, , 2007b. These modern findings have been extended to the fossil record, where similar patterns have been identified and a means of calibration developed empirically for angiosperm leaves (Royer et al, 2007a(Royer et al, , 2007b.…”

“…The late Palaeozoic was a time of very low CO 2 , b400 ppm (e.g., McElwain and Chaloner, 1995;Ekart et al, 1999;Beerling, 2002;Royer et al, 2007aRoyer et al, , 2007b) and very high O 2 (Scott and Glasspool, 2006;Berner, 2009;Glasspool and Scott, 2010), estimated by Scott and Glasspool (2006) to have been continuously above 26%. Under these atmospheric conditions, the high, saturating levels of light to which canopy and open-grown plants are exposed would have had potentially severe physiological consequences.…”

“…This has been described as the "leaf economics spectrum" (Wright et al, 2004). The more heavily constructed leaves are more expensive to produce, tend to be biomechanically tougher or thicker, and generally have lower nutrient content per unit volume, which leads to reduced insect predation as a consequence (Wilf et al, 2001;Royer et al, 2007aRoyer et al, , 2007b. The less well defended species also are engaging in a classic ecological "trade-off", sacrificing biomass to predation for high benefits derived from that biomass while functioning (reviewed in Royer et al, 2007aRoyer et al, , 2007b.…”

“…The more heavily constructed leaves are more expensive to produce, tend to be biomechanically tougher or thicker, and generally have lower nutrient content per unit volume, which leads to reduced insect predation as a consequence (Wilf et al, 2001;Royer et al, 2007aRoyer et al, , 2007b. The less well defended species also are engaging in a classic ecological "trade-off", sacrificing biomass to predation for high benefits derived from that biomass while functioning (reviewed in Royer et al, 2007aRoyer et al, , 2007b. These modern findings have been extended to the fossil record, where similar patterns have been identified and a means of calibration developed empirically for angiosperm leaves (Royer et al, 2007a(Royer et al, , 2007b.…”