LEAF MORPHOLOGICAL FEATURES OF TREES WITH RING POROUS WOOD ARE PREDICTIVE OF DECIDUOUSNESS IN THE FOSSIL RECORD

Paleobotanists study fossil dicot leaves using techniques that estimate mean annual precipitation (MAP) and mean annual temperature (MAT) as indicators of paleoclimate. The functional morphology of leaves has been approached predominately in the framework of productivity. However, during their lifetime leaves are not isolated entities, they are attached to plants. As such they are a central component of the vascular system that pulls the water column from roots and stems up through successively negative water potentials via the driving force of evapotranspiration.

Plants adapting to cold temperate deciduous or dry seasonal environments need to withstand the harsh conditions of water stress caused by drought or winter freezing. Many of them do so by dropping their leaves during adverse conditions and producing new ones seasonally. Distinct patterns of leaf morphology occur in combination with a plant's functional wood anatomy and phenology. Typically, early-leafing plants have diffuse porous wood and plants with later-forming leaves may have ring porous wood, however there are exceptions, such as certain species of oaks and elms with ring porous wood that leaf early. All extant trees with ring porous wood are known to be deciduous, assuring that this feature is predictive of deciduousness in the fossil record. We discovered the following suite of foliar characters, of early leafing ring porous species, that predict a deciduous habit include 1) craspedodromous secondary venation; 2) a primary vein with a distinctively wide base; 3) marginal teeth that develop early; 4) a high density of secondary veins.

Several hypotheses have addressed the significance of leaves with craspedodromous secondary venation (Zohner et al. 2019). The support supply hypothesis correlates craspedodromy with thin lamina (Givnish 1978, 1979; Givnish and Kriebel, 2017), whereas the bud-packing hypothesis suggests a positive relationship of toothed margins with winter buds (Edwards et al. 2017). We conclude that there is also selection for connecting the secondary xylem of woody species with leaf venation. Selection for this pattern occurs in deciduous species and is reflected by characteristic wood and leaf morphology.