A window into the past
Trees – a window to the past
The secondary stems and roots of land plants lay down wood continuously, producing reinforced roots, powerful trunks and resilient limbs that can survive for millennia. A strong central cylinder of wood is characterised by growth rings. Genetics and environment control wood growth and thereby confer the physical and aesthetic qualities of each individual timber. But wood is slow to degrade so growth rings also tell us much about the past.
Dendrochronology [dendron (tree); khronos (time)] is the art of interpreting tree rings to date wood tissues and document past events (see What do tree rings tell us?). This is possible because trees increase their girth by continually adding new cells to their outer circumference. The oldest wood is thus deep within the centre of each axis, sometimes rotten and weak.
What tissues are found in tree stems - Wood is comprised of several cell types, most notably secondary xylem vessels and tracheids that conduct water and minerals from roots to leaves. The tough polymer lignin is a key strengthening compound in all wood. The outer layers, collectively termed the bark, are even more complex, containing phloem elements responsible for conducting organic molecules and minerals to growing tissues such as the roots.
Where do these tissues arise from? Specialised cambial cells divide to generate many cell types - we have little idea how these ‘stem cells’ are programmed. The vascular cambium lies between the bark and the wood. New daughter cells arise radially outwards to produce bark, while cells on the inner side of the vascular cambium produce new wood. Within the bark, some species produce cork arising from a cork cambium. Cork is common in deciduous species from high latitudes.
Seasonal growth patterns - In spring, xylem vessels and tracheids are relatively large and thin walled, surrounded by paler wood termed early wood. Later in the summer, cells become narrower with thick dark walls that give the distinctive wood rings you see in all these samples – we term this latewood. In cool temperate climates, wood rings are distinctive and one is laid down each year whereas in the hot savanna and arid zones, rainfall can be erratic, leading to new rings mainly in the wet years. Because seasonal conditions influence tree growth rates, a permanent record is captured in the appearance and chemical composition of growth rings.