Competition in the regeneration niche between conifers and angiosperms: Bond's slow seedling hypothesis

Abstract

What features of angiosperms have enabled them to dominate the biomass of much temperate and tropical vegetation despite the high productivity of many adult gymnosperms, especially conifers? Bond (1989) argued that slow-growing gymnosperm juveniles are poor competitors with angiosperms regenerating in forest gaps and other well lit, well watered habitats. Consequently, gymnosperms are restricted to areas where growth of angiosperm competitors is reduced, for example by cold or nutrient shortages. A historical implication of Bond's hypothesis is that angiosperms overran gymnosperms during the Cretaceous and Tertiary by invading their regeneration niche. Bond's (1989) 'slow seedling' hypothesis has considerable appeal because it seeks to explain major phytogeographical patterns and evolutionary trends in terms of observable vegetative traits. Data suitable for testing Bond's plausible arguments were then quite limited, and what was proposed as hypothesis has often been cited as fact in recent reviews of conifer biology (Enright & Hill 1995; Richardson 1998; Smith & Hinckley 1995). Studies of large-scale trends may be exploratory (generating hypotheses) or sceptical (testing hypotheses); both modes of inquiry are essential and indeed complementary (McShea 1998). This review emphasizes the sceptical mode, and aims to test some of the main premises and generalizations of Bond's (1989) slow seedling hypothesis. Although Bond's arguments embraced the polyphyletic gymnosperms, relevant data are mostly limited to the monophyletic conifers, also constraining this review. A key element of Bond's (1989) hypothesis is that conifer seedlings cannot match the fastest growth rates of angiosperms. This is supported here for earlysuccession angiosperms, but not for late-succession, north-temperate trees. Few comparative data are available from regions where both angiosperms and conifers have long-lived leaves, a trait that is correlated with reduced seedling growth rate (Reich 1998). Whether Bond's hypothesis can explain the rarity of conifers and gymnosperms in lowland tropical forests therefore remains uncertain. Bond (1989) attributed the slow growth of juvenile conifers to an inefficient transport system and a low