Romain Lehnebach

The pipe model theory half a century on: a review

Background More than a half century ago, Shinozaki et al. (Shinozaki K, Yoda K, Hozumi K, Kira T. 1964a. A quantitative analysis of plant form - the pipe model theory. I. Basic analyses. Japanese Journal of Ecology B: 97-105) proposed an elegant conceptual framework, the pipe model theory (PMT), to interpret the observed linear relationship between the amount of stem tissue and corresponding supported leaves. The PMT brought a satisfactory answer to two vividly debated problems that were unresolved at the moment of its publication: (1) What determines tree form and which rules drive biomass allocation to the foliar versus stem compartments in plants? (2) How can foliar area or mass in an individual plant, in a stand or at even larger scales be estimated? Since its initial formulation, the PMT has been reinterpreted and used in applications, and has undoubtedly become an important milestone in the mathematical interpretation of plant form and functioning. Scope This article aims to review the PMT by going back to its initial formulation, stating its explicit and implicit properties and discussing them in the light of current biological knowledge and experimental evidence in order to identify the validity and range of applicability of the theory. We also discuss the use of the theory in tree biomechanics and hydraulics as well as in functional-structural plant modelling. Conclusions Scrutinizing the PMT in the light of modern biological knowledge revealed that most of its properties are not valid as a general rule. The hydraulic framework derived from the PMT has attracted much more attention than its mechanical counterpart and implies that only the conductive portion of a stem cross-section should be proportional to the supported foliage amount rather than the whole of it. The facts that this conductive portion is experimentally difficult to measure and varies with environmental conditions and tree ontogeny might cause the commonly reported non-linear relationships between foliage and stem metrics. Nevertheless, the PMT can still be considered as a portfolio of properties providing a unified framework to integrate and analyse functional-structural relationships.

Origin and Dispersal of Domesticated Peach Palm

Peach palm (Bactris gasipaes Kunth) is the a Neotropical palm domesticated by Native Americans. Its domestication resulted in a set of landraces (var. gasipaes), some with very starchy fruit used for fermentation, others with an equilibrium of starch and oil used as snacks. Which of the three wild types (var. chichagui) was involved and where the domestication process began are unclear, with three hypotheses under discussion: an origin in southwestern Amazonia; or in northwestern South America; or multiple origins. We reevaluate one of the wild types, defining it as the incipient domesticate, and then evaluate these hypotheses using the Brazilian peach palm Core Collection and selected herbaria samples to: 1) model the potential distributions of wild and domesticated populations; 2) identify the probable origin of domestication with a phylogeographic analysis of chloroplast DNA sequences; and 3) determine the dispersal routes after domestication using spatial analysis of genetic diversity based on 17 nuclear microsatellite loci. The two very small-fruited wild types have distinct distributions in the northern Andes region and across southern Amazonia, both under moderately humid climates, while the incipient domesticate, partly sympatric with the southern wild type, is also found along the Equatorial Andes, in a more humid climatic envelope, more similar to that of the domesticated landraces. Two distribution models for Last Glacial Maximum conditions (CCSM4, MIROC) also suggest distinct distributions for the two wild populations. The chloroplast DNA phylogeographic network confirms the area of sympatry of the incipient domesticate and the southern wild type in southwestern Amazonia as the origin of domestication. The spatial patterns of genetic diversity confirm the proposal of two dispersals, one along the Ucayali River, into western Amazonia, northwestern South America and finally Central America; the other along the Madeira River into central and then eastern Amazonia. The first dispersal resulted in very starchy fruit for fermentation, while the second may have been later and resulted in snack fruits. Further explorations of southwestern Amazonia are essential for more precise identification of the earliest events, both with new archaeological methods and genetic analyses with larger samples.

Interlocked grain and density patterns in Bagassa guianensis: changes with ontogeny and mechanical consequences for trees

Key messageInterlocked grain and basic density increase from pith to bark in Bagassa guianensis and greatly improve trunk torsional stiffness and wood tenacity in the radial plane.AbstractTrees modulate their building material, wood, throughout their lifetime to meet changing mechanical needs. Basic density, a widely studied wood property, has been proved to be negatively correlated to growth rate and is then considered to reflect the diversity of species growth strategies. An alternative way for trees to modulate growth strategy at constant construction cost is changing the organisation of their fibre network. Interlocked grain, the result of a periodic change in the orientation of the fibres in the tangential plane, is found in numerous tropical tree species. In this study, we first describe the variations in basic density and interlocked grain occurring during ontogeny of Bagassa guianensis, a fast-growing Amazonian species, and analyse their influence on the local mechanical properties of wood at the tissue level. The observed radial patterns and properties are then incorporated in a finite element model to investigate their effect on mechanical properties of the trunk. We report extreme and highly reproducible concomitant radial variations in basic density and interlocked grain in all the sampled trees, with grain angle variations ranging from − 31° to 23°. Such changes in wood during ontogeny allows trees to tailor their growth rate while greatly improving resistance to torsion and reducing the risk of splitting.