Seedling growth in conifers and angiosperms: impacts of contrasting xylem structure

Abstract

Competitive interaction between conifers and angiosperms has moulded the structure of global vegetation since the Cretaceous. Angiosperms appear to enjoy their greatest advantage in the lowland tropics, an advantage often attributed to the presence of vessels in their xylem tissue. By monitoring the seedling growth of three members of the pan-tropical conifer family Podocarpaceae and three tropical angiosperm tree species, our aim was to determine whether these conifer and angiosperm seedlings showed distinct patterns of growth and light adaptation that might be attributed to the presence/absence of vessels. Angiosperm seedlings were consistently more efficient in terms of leaf area carried per unit stem investment, as well as more responsive to light climate than the conifer seedlings. Apparently linked to this were larger growth rate, stem hydraulic conductivity and stomatal conductance in the angiosperm sample. Stem hydraulic conductivity and maximum stomatal conductance were highly correlated among species and light treatments explaining the association between highly conductive vessel-bearing wood and high rates of gas exchange. We conclude that xylem vessels contribute to higher rates of gas exchange and more efficient production of leaf area in our sample angiosperms than in conifers. However, this advantage is limited by shade.

Conifers, Angiosperm Trees, and Lianas: Growth, Whole-Plant Water and Nitrogen Use Efficiency, and Stable Isotope Composition (δ 13C and δ 18O) of Seedlings Grown in a Tropical Environment

“…Measurements of whole-plant hydraulic conductance per unit plant mass would be necessary to confirm this hypothesis. However, it would be consistent with differences in hydraulic conductivity observed previously between gymnosperm and angiosperm seedlings (Brodribb et al, 2005). As shown in Equation 1, the term f c , the proportion of net C fixation used for respiration, has potential to influence r. It was previously observed for 24 herbaceous species that f c ranged from about 0.5 to 0.3, and that r was negatively correlated with f c , as predicted by Equation 1 ).…”

“…However, conifers are largely absent from the lowland tropical and subtropical forests of today. It has been suggested that one means by which angiosperm tree species are able to out-compete gymnosperm tree species in tropical environments is through faster seedling growth caused by improved hydraulic efficiency (Bond, 1989;Brodribb et al, 2005). Angiosperm xylem tissue contains vessels, specialized waterconducting cells that are generally larger in diameter, and therefore more conductive to water, than conifer tracheids (Sperry et al, 2006).…”

Ecology and Distribution of the Malesian Podocarps

“…This has limited the range of possible leaf sizes and shapes in conifers and, they argued, helps to explain why conifers are rarely prominent in shaded habitats. Subsequent experimental studies and reviews by Brodribb and colleagues (Brodribb and Hill, 1998Brodribb et al, 2005), Becker (2000), and others (Pammenter et al, 2004;Coomes et al, 2005) support this contention, illustrating that conifers generally have lower hydraulic efficiency and so are disadvantaged under high potential productivity conditions (see Brodribb, this volume). However, under low-light conditions this competitive disadvantage relative to angiosperms declines (but is not eliminated), and under a combination of low-light and low-nutrient conditions, such as in some tropical forests, the disadvantage may disappear altogether (Becker, 2000;Pammenter et al, 2004).…”

Conifer–Angiosperm Interactions: Physiological Ecology and Life History

“…The intervening 20 years have witnessed tremendous growth of plant physiological ecology; although only a handful of empirical papers have directly addressed the issue of conifer-angiosperm interactions (e.g., Becker et al, 1999;Lusk et al, 2003;Brodribb et al, 2005), a good deal of relevant data and ideas have nevertheless been published. Although both lineages encompass a wide range of maximum growth rates, comparative studies of seedlings confirm that even relatively "harelike" conifers such as Pinus species are unable to match the performance of the fastestgrowing early successional angiosperm trees (e.g., Cornelissen et al, 1996;Reich et al, 1998).…”