A Latitudinal Gradient of Wood and Litter Production, and Its Implication Regarding Competition and Species Diversity in Trees

Abstract

Rates of wood and litter production in mesic forests along a latitudinal gradient are surveyed, and hypotheses concerning the eco- logical and evolutionary implications of emergent patterns are presented. Wood production is relatively constant along a latitudinal gradient, whereas total amount of light available during the growing season de- creases with increasing latitude. Therefore, efficiency of wood production is higher at temperate latitudes. We hypothesize that the higher wood production efficiencies at temperate latitudes came about as a result of selection for tree species with wood growth rates that are high during the growing season. Since rapid wood growth provides a competitive advan- tage in trees, we hypothesize that competition among trees is greater at higher latitudes. High efficiency of wood production apparently is a prerequisite for survival of forest trees at high latitudes. Only those tree taxa that had high wood production efficiency, or were able to evolve a high efficiency, survived when the primarily tropical or subtropical environment at higher latitudes turned much colder during the late Tertiary. This situation contributed to the present low diversity of trees at high latitudes relative to that at lower latitudes.