CSIRO PUBLISHING www.publish.csiro.au/journals/ajb Australian Journal of Botany, 2005, 53, 749–755 Seedling growth in conifers and angiosperms: impacts of contrasting xylem structure T. J. BrodribbA,B,D, N. M. HolbrookB and R. S. HillC ADepartment of Plant Science, University of Tasmania, PO Box 252-55, Hobart, Tas. 7001, Australia. BDepartment of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA, USA. CDepartment of Environmental Biology, University of Adelaide, SA 5005, Australia. DCorresponding author. Email: [email protected] 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. Introduction The feature most likely to differentiate conifers and The Cretaceous radiation in angiosperm diversity has been angiosperms in terms of assimilation and growth potential proposed as having a major influence on the subsequent is the absence of vessels in conifer wood. Conifer wood is decline in gymnosperm diversity (Stebbins 1974; Knoll comprised solely of tracheids, generally <30 µm in lumen 1986). A popular theory accounting for gymnosperm decline diameter and ∼1–3 mm in length (Panshin and De Zeeuw invokes displacement of conifers by angiosperms, putitively 1980), whereas angiosperm wood typically contains large superior in both reproductive and vegetative potential (see lumen cells (commonly 100–200 µm in diameter) joined Bond 1989 for review). There are good reasons to suspect in series with degraded end walls that form tubes often that vegetative innovations in the angiosperm lineage have metres in length (Zimmermann and Jeje 1981). Because the contributed to their success, and that these characters hydraulic conductivity of tubes is proportional to the fourth (namely vascular and leaf morphological traits) enable power of their radius, these vessels enable angiosperms to angiosperm seedlings to overwhelm conifer competitors produce wood which is many times more conductive (per (Bond 1989; also see review by Feild et al. 2004). However, cross-sectional area) to water than conifer xylem (Tyree et al. it is important to bear in mind that conifer families dominate 1991). The benefits of highly conductive xylem are clear, many temperate and most boreal regions, and that it given the evidence for a general correlation between xylem is only in lowland subtropical and tropical forests that conductance and photosynthetic capacity (Brodribb and Feild conifers tend to be absent. In order to understand which 2000; Hubbard et al. 2001). However, large conduits can aspects of angiosperm physiology may be responsible for also become a liability under freezing conditions because of conifer displacement, it is most informative to compare freeze–thaw embolism of xylem vessels (Sperry et al. 1994). tropical angiosperms and conifers, and to contrast their This process is highly sensitive to conduit size, meaning performance in an environment where angiosperm traits have that that most (but not all; see Pittermann and Sperry 2003) proven superior. conifer tracheids are immune from freeze embolism during © CSIRO 2005 10.1071/BT05049 0067-1924/05/080749 750 Australian Journal of Botany T. J. Brodribb et al. winter freezing. Angiosperm xylem composed of vessels can a greater optimisation of conductance and support functions suffer almost complete deactivation after freezing, leading to in angiosperms. Such optimisation should become most winter deciduousness in many temperate species. evident in the comparison of sun and shade plants where The low hydraulic conductivity of conifer xylem has the roles of conduction and support are likely to be the been suggested as a primary cause of reduced conifer most variable. competitiveness in the tropics because of the restricted In this study, we compare the growth rate, allometry and distribution of water and nutrients to leaves (Bond 1989). hydraulic properties of conifer and angiosperm seedlings Although this argument oversimplifies the physiological from tropical and temperate climates to determine whether advantages of vessels over tracheids, it is appealing to patterns in stem and leaf-area growth are related to Ksp in imagine that the greater efficiency afforded by vesseliferous these species. We focus on conifer species from the most wood might enable more rapid growth of seedlings, successful conifer family in the tropics, the Podocarpaceae. particularly under carbon-limited (shady) conditions. Podocarps are known for expressing a diversity of The term ‘efficiency’ here refers to the volume of water mechanisms for producing flattened shoots functionally carried per mass of carbon invested in vasculature, and it analogous to angiosperm broad-leaves (Brodribb and Hill should be of great importance to plants as photosynthetic 1997), and two of the species chosen here produce rapid rates among species vary in proportion to the water- growth rates and broad (>5-cm-wide) leaves reminiscent carrying capacity of the xylem (Brodribb and Feild 2000; of angiosperm leaves. For comparison, we used a fast- Hubbard et al. 2001). growing tropical Meliaceae, a primary tropical forest species Given their lower xylem conductivity per cross-sectional (Rubiaceae), and a fast-growing Eucalypus species. Growth area (Ksp), conifers must either (1) carry less foliage of seedlings under contrasting light intensities is compared per branch, (2) produce lower rates of water loss (and in order to test the relative flexibility of seedling allometry in photosynthesis) per leaf or (3) compensate for lower these conifer and angiosperm trees. hydraulic conductivity with a larger water-potential gradient than angiosperms. First, it appears true that conifers Materials and methods compensate for low xylem conductivity by investing more Plant material xylem tissue per leaf, although hydraulic conductivity, when Within the large constraints of seed availability and germinability normalised to leaf area (Kl), is still generally lower in of the tropical Podocarps we were able to find three species that conifers than in associated angiosperms (Becker et al. 1999; served our criteria of fast-growing trees, two of which are tropical Brodribb and Feild 2000). Second, there is some evidence species. The three conifers chosen for this study are all rainforest that the stomatal conductance and photosynthetic rate of taxa, two from tropical South-east Asia (Nageia fleuryi from tropical conifer foliage are lower than those of angiosperms, both southern China and Podocarpus greyii from tropical north-eastern Australia) and one from New Zealand (Dacrycarpus dacrydioides). in comparisons among neighbouring species (Brodribb and Podocarpus was chosen because it is the most diverse conifer genus Feild 2000; Lusk et al. 2003) and in terms of maximum rates in the tropics, whereas Nageia is a fast-growing tree with broad, multi- (Korner¨ et al. 1979). Finally, there are few data to test the veined leaves. Dacrycarpus is also a highly successful tropical genus third hypothesis that conifer leaves operate at lower water and D. dacrydioides, although more temperate in origin than the other potentials (ψ ) than angiosperm leaves, thus enabling them conifers, is the fastest-growing conifer in New Zealand (Ebitt and Ogden l 1998). Both N. fleurii and P.greyii regenerate in the forest understorey, to maintain similar evaporative fluxes by increasing the water- whereas D. dacrydioides tends to regenerate in forest gaps or more potential gradient across the plant. However, there is evidence commonly in open riverine environments. that conifer wood is more resistant to dysfunction at low The three angiosperms chosen were from tropical (Toona australis, water potentials than associated angiosperms (Sperry et al. Meliaceae, and Flindersia breyleyana, Rutaceae) and temperate 1994; Tyree et al. 1998; Brodribb and Hill 1999), suggesting (Eucalyptus regnans, Myrtaceae) forest. Toona is a common pioneer tree in north-eastern Australia from the highly successful tropical that conifer leaves may operate at lower water potentials than family Meliaceae. It produces large compound leaves and rapid growth those of neighbouring angiosperms. (Herwitz 1993). Flindersia is a more shade-tolerant primary forest tree Another advantage of vessel-bearing wood