In a month about wood, there is certainly one topic which must not be neglected: trees. I would like to close our theme session with this article, which will hopefully answer your most frequently asked questions about trees.
What is a tree?
Even though almost everyone knows what a tree looks like and probably has a pretty accurate idea of what it is, defining the term ‚tree‘ is not so easy. In fact, when you check the most popoular botanic textbooks, there is no chapter about ‚trees‘ in general. Why is that? Trees are not one taxonomic group (like for example moss or ferns). The term ‚tree‘ is rather describing the specific growth habit of perennial woody plants (remember the definiton of wood?). ‚Perennial‘ describes a plant as living for more than two years (including the survival of the winter season) in contrast to annual or biennial plants, which die after one (or sometimes two) year(s) of germination, blooming and seed production. The specific growth form defining a tree (and distinguishing it from a shrub, which can also be perennial and woody) is the monopodial growth. This means that the plant is predominantly growing at the apex (the top of the tree) and the upper or most outerly side branches. This results in a plant habitus with a dominant main stem, growing at the top, and subordinate side axis .
A commonly known example: the English oak (Quercus robur).
What is NOT a tree?
Relying on the previous deifintion one could easily say: everything else. As I already mentioned: shrubs are pretty similar to trees (woody and perennial) but they are missing the monopodial growth form and are instead growing sympodial (meaning that the shrub is growing mostly at its base with new branches developing at the basal part of the plant, instead of elongating at the top). A nice example for a shrub: the blackcurrent (Ribes nigrum). Some common names of plants are misleading when it comes to deciding ‚tree or no-tree… that’s the question‘. Palm ‚trees‘ are perennial and monopodial – but not woody, so they are actually not trees by our definition. Can you think of more examples ?
How tall is the tallest tree in the world?
The tallest tree in the world (which we know of) is an exemplar of the species Sequoia sempervirens (coast redwood). It is situated in the Redwood National Park in California – it is 115,72m tall! It is not possible to quantify the age of a tree neither by its height nor the diameter (continue reading for more info about the age of trees), but this giant is probably about 600 years old. But… being a tree: is that actually old ?
so….how old is the oldest tree in the world?
The (supposedly) oldest tree in the world is also coming from California and is a specimen of Pinus longaeva. It is thought to have germinated approximately 5000 years ago. You probably know (or remember from our introduction article about wood), that the age of a woody plant can be determined by counting its growth rings. The vascular cambium produces different types of wood depending on the season: early wood in spring/ early summer and late wood in late summer. The cells in early wood are bigger and have thinner cell walls, whereas late wood cells are smaller with thicker cell walls. This differentiation leads to a visible boarder between early and late wood and one layer of early and late wood makes up one growth ring and consequently one year in the tree’s life (respect to the guy who counted the 5000 rings of Pinus longaeva) .
How do trees grow in height and width?
Concerning growth in width – you know the answer already from this and our introductory article. But trees are also capable to grow at their tip (apex) and the base (the tip of the roots). Responsible for this growth activity are meristematic cells which are able to divide and differentiate. One can imagine these two regions in the plant as the ‚growth poles‘ at the top and bottom. Additionally, above the leaf insertions along the plant stem or branches are so called ‚dormant‘ meristems, which can be reactivated (for example if the plant is damaged at the apex) and establish new branches if necessary .
(How) can a tree react to its environment?
Even though trees are sessile organisms (meaning they are fixed at one site and can not move around like animals) they are pretty adaptable towards their environment and can react to changing environmental conditions. Plant react mainly via growth processes and change their shape according to outer influences. For example, they grow in height and branch out to outperform their neighbors in competition for light. Their roots grow deeper and wider into the soil to compete for nutrition and water. Areas of high stress (for example stem-branch-attachments) are reinforced via additional layers wood. These changes usually do not happen from one day to another – but plants can be surprisingly fast in order to react to changing conditions and survive.
In Biomimetics, we can learn a thing or two from growth forms of trees, when it comes to designing technical components with load-adapted material distribution (and also saving material at positions where it is not necessary). Plants have to carefully economize their resources and material is only accumulated where needed. This is also desirable for technology!
Can a plant become a ‘tree’ without wood?
As I already mentioned , some plants are perennial and have a tree-like habitus (monopodial growth) but are missing wood. So by definition, they are not trees. But still: they look like it and can even grow as high as real trees. So high so they do this without reinforcing secondary xylem/ wood? An interesting example is the papaya plant (Carica papaya), which can grow up to 10m high without wood. Possible success strategies of the papaya are a very high turgor pressure of the parenchyma (unlignified basic tissue) and/ or lignified phloem fibres (remember what phloem is?) which are forming a flexible but tight mesh around the primary xylem, to compensate for the absence of wood. So you see: you do not necessarily need wood to look like a tree – there are certainly other strategies to be stable enough and become very tall !
Alternative strategies of trees (or tree-like plants) for structural support can also be inspirations for Biomimetics. How to become tall and stable without the usage of strong and stiff material? The papaya plant is only one of many examples – can you think of more?
 Kadereit J W, Körner C, Kost B, Sonnewald U (2014). Straßburger – Lehrbuch der Pflanzenwissenschaften (37. Auflage). Springer Spektrum
 https://www.monumentaltrees.com/en/trees/coastredwood/tallest_tree_in_the_world/ (4.4.2017)
 http://www.geo.de/magazine/geo-kompakt/2020-rtkl-wunder-der-botanik-der-aelteste-baum-der-welt (4.4.2017)
 Kempe A, Lautenschläger T, Lange A, Neinhuis C (2014). How to become a tree without wood – biomechanical analysis of the stem of Carica papaya L. Plant Biology, 16: 264-271. doi: 10.1111/plb.12035.