Paleobotanical Evidence for the Origins of Temperate Hardwoods

Abstract

Premise of research. Several key morphological and anatomical features are found in temperate deciduous hardwood taxa that undergo seasonal dormancy, including long-shoot/short-shoot differentiation, heterophylly, and diffuse-porous wood. These features, when found together in the fossil record, can serve as indicators suggesting that fossil plants had physiological responses to changing photoperiod and chilling similar to those present today in temperate deciduous forests. We measured heterophyllous leaves on branches of extant Cercidiphyllum trees and used these data as a profile with which to compare fossil leaves of the family Trochodendraceae. Methodology. We recorded frequency distributions of L∶W ratios of leaves from short and long shoots on branches of live Cercidiphyllum japonicum (Cercidiphyllaceae) trees and compared them with fossil leaves of late Paleocene Zizyphoides flabellum of Almont, North Dakota, of Zizyphoides type 1 from the late early Eocene “upland” Okanogan Highlands locality at Republic, Washington, and Trochodendron nastae, also from Republic. We reviewed the literature on seasonal dormancy and on the correlative distribution of leaf phenology and wood porosity type. Pivotal results. Extant Cercidiphyllum and Z. flabellum both showed a bimodal distribution of frequency distribution of L∶W ratios in leaves; Trochodendron did not. Zizyphoides sp. also showed a trend toward bimodality but had less conclusive results. Conclusions. We suggest that on the basis of morphological features, temperate deciduous forest trees demonstrating seasonal heterophylly can be detected in the fossil record. We suggest that triggers such as photoperiod and changing temperature that today influence these plants were pivotal to the evolution of modern temperate deciduous trees. This response may have been first to the trigger of photoperiod change in high-latitude plants and later combined with lower-temperature regimes of upland regions such as the Okanogan Highlands. The development of preformed leaves on plants with diffuse-porous wood allowed for early leaf emergence in the temperate environment.