Sensitivity of leaf size and shape to climate: global patterns and paleoclimatic applications

Abstract

Summary• Paleobotanists have long used models based on leaf size and shape to reconstruct paleoclimate. However, most models incorporate a single variable or use traits that are not physiologically or functionally linked to climate, limiting their predictive power. Further, they often underestimate paleotemperature relative to other proxies.• Here we quantify leaf-climate correlations from 92 globally distributed, climatically diverse sites, and explore potential confounding factors. Multiple linear regression models for mean annual temperature (MAT) and mean annual precipitation (MAP) are developed and applied to nine well-studied fossil floras.• We find that leaves in cold climates typically have larger, more numerous teeth, and are more highly dissected. Leaf habit (deciduous vs evergreen), local water availability, and phylogenetic history all affect these relationships. Leaves in wet climates are larger and have fewer, smaller teeth. Our multivariate MAT and MAP models offer moderate improvements in precision over univariate approaches (± 4.0 vs 4.8°C for MAT) and strong improvements in accuracy. For example, our provisional MAT estimates for most North American fossil floras are considerably warmer and in better agreement with independent paleoclimate evidence.• Our study demonstrates that the inclusion of additional leaf traits that are functionally linked to climate improves paleoclimate reconstructions. This work also illustrates the need for better understanding of the impact of phylogeny and leaf habit on leaf-climate relationships.

Developmentally based scaling of leaf venation architecture explains global ecological patterns

“…1; Table 1). These trends were robust across the full range of leaf sizes (leaf area ranging from < 1 to 280 cm 2 ; Supplementary Table S1, Supplementary Data 1), though our sampling included more small leaves than large leaves, reflecting the greater commonness of smaller leaf sizes globally 7 . Indeed, the trends were so strong that subsampling of species showed the same relationships, that is, within the given genera and families (Methods).…”

“…For estimation based on a subsampled region of lamina adjacent to the midvein, which would enable the 2° veins to be decisively located (that is, from fragments including the midvein), log (leaf area) = 1.62-1.82×log (subsample 2° vein density) (R 2 = 0.74; P < 0.001). These trends enable the estimation of intact leaf size from fossil fragments to improve resolution of leaf size, and potential palaeo-estimation of rainfall and of potential ecosystem biodiversity 7,17,18 .…”

“…The general linkage of major vein density with leaf size provides a mechanism to explain perhaps the most famous of global plant biogeographical trends, the predomination of small leaves in drier and more exposed habitats 7,44 . Indeed, the variation in leaf size across environments varying in moisture has appeared in the literature going back to Theophrastus, von Humboldt and Raunkiaer 45 .…”

“…Across species, leaf venation shows enormous structural diversity, the more remarkable because of its shared development and functions. The functional consequences of leaf venation have attracted increasing interest in a widening range of fields, because vein traits, including vein diameters and densities ( = length per leaf area), as well as leaf size are key determinants of plant performance and indeed of shifts in the lineages that dominated the world vegetation through deep time [1][2][3][4][5][6][7][8][9] .…”

“…In compiling the database, we included as many families and genera as possible towards maximising species diversity, aiming to determine trends robust across species with leaves that vary widely not only in size, but also in shape and also in texture, in internal and surface anatomy, in rates of photosynthesis and respiration, and in shade and drought tolerance 4,15,16 . Additionally, we determined whether vein scaling could allow a new method for estimation of intact leaf size from small leaf fragments, which would be valuable given that fossil deposits are rich in fragments, and leaf size can be used as a palaeo-proxy for rainfall and of potential ecosystem biodiversity 7,17,18 . Finally, we determined the degree that the global scaling of leaf venation with size could be explained on the basis of detailed understanding of leaf anatomy and development during leaf formation and expansion.…”