ASSESSING PLANT MICROENVIRONMENT AND FOREST STRUCTURE FROM FOSSIL LEAF ANATOMY

Because leaves are the primary interface between plant and environment, fossil leaf morphology has been widely proven to be a valuable source of information regarding past climates and atmospheric compositions. From the leaf’s perspective, however, the plant to which the leaf is attached is as much a part of its environment as regional climate. Thus, fossil leaves may also preserve an untapped wealth of information concerning the habit and ecology of the parent plant as well as the vegetation structure of the landscape. Previous modeling and laboratory experiments indicated that--regardless of leaf venation pattern--hydraulic capacity is maximized when the distance between veins (d) equals that from vein to leaf surface (δ). This relationship was born out by measurements made in diverse extant plants: d = δ for plants of exposed environments. Having d < δ, i.e. high vein density in a thick leaf, would provide no benefit for the costs associated with the extra vein production and has not been seen in nature. Having d > δ, i.e. low vein density in a thin leaf, would lead to rapid overheating or desiccation in an exposed environment because hydraulic supply would be unable to accommodate demand, but can be tolerated in a plant as long as it is in a sheltered environment with a low vapor pressure deficit. Thus the d/δ ratio provides an indicator of microhabitat readily accessible from anatomically preserved fossils. Fossil leaves that have been sampled occupy a similar range of d-δ combinations as living plants. Where whole-plant habit is known, the fossil plants that fall well off the d = δ line are reasonable candidates for sheltered understory environments, suggesting this proxy can provide useful information regarding the many other fossil plants for which whole plant reconstructions are not available and only the leaf is known.