A Functional Analysis of Podocarp Ecology

Viewed in terms of species diversity, the Southern Hemisphere conifer family Podocarpaceae is equivalent to its northern sister the Pinaceae. However, despite their similar age and common heritage these two families diverge strongly in morphology, physiology, and ecology. Both Pinaceae and Podocarpaceae are successful families of vascular plants, and in both cases this success has been built upon contrasting and rather canalized adaptive differentiation. Pinaceae thrive in virtually all Northern Hemisphere forest environments where freezing limits the success of evergreen angiosperm competitors. In contrast, the Podocarpaceae are most successful in the tropics, where they compete directly with broadleaved angiosperms, usually in montane rainforest. This chapter discusses some of the key adaptations that have enabled tropical Podocarpaceae to remain competitive during the rise of angiosperms. I also examine the hydraulic “Achilles heel” that limits all members of the Podocarpaceae to a mesic rainfall envelope. ADAPTATION TO LIfE IN TROPICAL fOREsTs Large, bilaterally flattened leaves are more efficient at harvesting light than spirally arranged needle or scale leaves (Leverenz, 1995; Pickup et al., 2005). This general rule is highly significant when considering the outcome of conifer – angiosperm competition in the period since angiosperm diversification in the Late Cretaceous. With the exception of Gnetum, gymnosperms appear to be incapable of producing large laminate leaves, and as such, they cannot benefit from the economic advantages of efficient light harvesting and minimal selfshading that broad leaves offer (Falster and Westoby, 2003). The overwhelming dominance of broadleaved taxa in most productive regions of the tropics indicates that the benefits of large leaf size reach their peak in this region (Webb, 1968). Warm temperatures and an absence of stresses that could potentially limit leaf size, including cold, soil water deficiency, and abiotic mechanical damage in tropical rainforests, contribute to the predominance of large, flattened leaves. Given the adaptive bias toward large leaves in the tropics and the limited Timothy J. Brodribb, School of Plant Science, Private Bag 55, University of Tasmania, Hobart, Tasmania 7001, Australia. Correspondence: [email protected]. Manuscript received 13 April 2010; accepted