Introducing wood anatomical and dendrochronological aspects of herbaceous plants: applications of the Xylem Database to vegetation science
Although more than half of the Earth's landmass is covered by non-forest vegetation and forests only account for ca. 20%, the majority of wood anatomical and dendrochronological studies still centre on trees and their timber products of secondary meristem growth. Comprehensive assessments of the anatomy, taxonomy, physiology and dendrochronology of herbs are, however, still missing. We therefore aim to evaluate the importance of stem anatomical and dendrochronological aspects of herbaceous plants for vegetation science, and further intend to explore if species-specific differences in xylem anatomy can be linked with patterns of life form, plant size and site ecology.
Numerous extra-tropical sites within six biogeographic regions on six continents, spanning an elevational gradient of 4400 m from sea level to the alpine zone.
We employ the Xylem Database to investigate well-established and newly defined wood anatomical and dendrochronological traits in 3347 dicotyledonous tree, shrub and herb species from 140 families and 952 genera. The obtained site information is placed in a global context of vegetation cover.
We demonstrate that plants included in the Xylem Database broadly represent the diversity of growth forms, habitats and life histories of extra-tropical dicots, thus validating its potential for a range of uses within vegetation science. An unexpectedly rich catalogue of xylem anatomical and dendrochronological features is found in the stems of almost all dwarf shrub and herb species, with the quantity and diversity of their characteristics being comparable to larger plants, including most tree species. Annual rings are formed independently of life form, plant size and site ecology. Secondary growth, which is an expression of plant age, defines the main principle of terrestrial angiosperm development.
Based on a first assessment of the Xylem Database, we suggest merging the traditionally separated research communities of wood anatomy and dendrochronology (including dendroclimatology and -ecology), as well as expanding their joint application to all non-forest, extra-tropical vegetation types. Moreover, we recommend overcoming the theoretical life form separation of trees, shrubs and herbs. This conceptual advancement will likely provide annually resolved insight on the past dynamics of dry matter production and carbon allocation for many marginal vegetation zones beyond the geographic limits of forest growth, which are particularly vulnerable to environmental change. Our study may further stimulate timely efforts in comparing plant morphological and wood anatomical characteristics with phylogenetic reconstructions.