Fossil leaf economics quantified: calibration and Eocene case study.

ROYER, D; SACK, L; WILF, P; LUSK, C; JORDAN, G; NIINEMETS, U; WRIGHT, I; WESTOBY, M; CARIGLINO, B; COLEY, P; CUTTER, A; JOHNSON, K; LABANDEIRA, C; MOLES, A; PALMER, M; VALLADARES, F

Chicago

Congreso; Contributed paper. Botanical Society of America and American Society of Plant Biology Joint Congress; 2007

Leaf mass per area (LMA) is a central ecological trait that is intercorrelated with leaf lifespan, photosynthetic rate, nutrient concentration, and palatability to herbivores. These coordinated variables form a globally convergent leaf economics spectrum, which represents an adaptive 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 LMA estimation for fossil leaves from petiole width and leaf area, two variables which are commonly measurable in leaf compression floras. Application to two well-studied, classic Eocene floras demonstrates that LMA 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-LMA species from Republic (Washington, USA, 49 Ma), a warm-temperate flora with a strong deciduous component among the angiosperms, and a wide LMA range in a seasonally dry flora from the Green River Formation at Bonanza (Utah, USA, 47 Ma), presumed to comprise a mix of short and long leaf lifespans. Second, the fossil species exhibit significant negative correlations between reconstructed LMA and insect herbivory. These correlations are consistent with herbivory observations in extant floras that reflect fundamental tradeoffs in plant-herbivore associations. Our results indicate that several aspects of plant and plant-animal ecology can now be quantified in the fossil record and demonstrate that herbivory has shaped the evolution of leaf structure for millions of years.