Wood density and functional differentiation of axial and radial parenchyma

The functional relationship between cell types and three xylem functions of conduction, support and storage using approximately 800 tree species from China.
Functional differentiation of axial and radial parenchyma
Functional differentiation of axial and radial parenchyma

Wood performs three critical functions: mechanical support of the photosynthetic surface; storage of water, sugar and other nutrients; and conduction of water and other substances from the soil to the photosynthetic surface. In angiosperms, each of these functions is generally carried out by particular cells types so that mechanical support is primarily determined by fibres, storage by living cells such as parenchyma, and water conduction by xylem vessels. One cell type can perform more than one function. For example, living fibres can be the storage compartment in woods with scanty parenchyma and also function as support cells. Because wood carries out all these different tasks simultaneously, environmental demands that require a prominent role for a particular function can create a trade-off and/or positive interaction with other functional axes of variation – increased mechanical support often decreases water conduction efficiency but increases resistance to cavitation.

Wood density is a key functional trait that has been the focus of extensive research in the last few years. Since wood density describes the amount of carbon invested in support, it is related to a variety of ecological dimensions linked to life history traits (e.g. growth rate, survival or life span). For instance, wood density is inversely related to growth rates and successional position. Species with low wood density tend to be comparatively short-lived, fast-growing pioneers, while species with high wood density tend to be long-lived climax species. High disturbance and turnover rates favour fast-growing species with low wood density.

Most of the studies dealing with the anatomical determinants of wood density, especially those including large samples from global databases, lack information on the third functional dimension, storage. As axial and radial parenchyma are associated with stem water and nutrient storage, we expect a trade-off between mechanical strength and storage because increased area of parenchyma in stems should be achieved at the expense of other cell types (i.e. fibres), especially if conductance is to be maintained.

A new paper in Annals of Botany examines a large database of approximately 800 tree species from China to analyse patterns of correlated evolution between proportions of different tissue types and functional characteristics of the stem related to support (wood density), water conduction (potential conductivity, vessel composition and fraction) and storage. While other analyses at global scales have focused on vessel traits to explain variation in wood density and theoretical conductivity, the authors question whether there is a clear trade-off among anatomical traits that hypothetically represent mechanical, conduction and storage functions, and how early these axes were established in the history of the lineages. There is intraspecies and intra-tree radial variation in wood traits associated with ontogenetic changes that might affect the strength of trade-offs. The aim of this study was to determine whether such trade-offs are generally recognized across angiosperm lineages at the species level.