The manner in which woody plants grow vegetatively is impacted by many different environmental qualities – light availability, water supply, frequency of flooding, ambient temperature, soil fertility, soil salinity, soil bulk density, atmospheric and soil pollution, wind speeds, frequency of fire, and pests and diseases. The focus of this post will be exclusively on how wind affects vegetative growth.
First and foremost, we must recognise that wind can be both good and bad for vegetative growth. For example, whilst the wind can aid with seed dispersal to ensure the species can continue to exist, the wind may also be the cause of major limb failure, entire windthrow, or soil erosion – additionally, the wind can be the harbinger of arboreal pestilence (as we have seen, to some degree, with Hymenoscyphus fraxineus). Furthermore, not all tree species are equal in their response to wind – conifers are more likely to suffer from the wind than are broadleaved trees. To complicate matters here however, it is also important to understand that fast-growing pioneer species (such as birch, pine, poplar, and willow) will be more adversely impacted than later-successional species (such as beech and oak), given they are far more exposed as a result of both their quick growth rate and unestablished environmental surrounds (these species create woodland; they don’t enter into stabilised woodlands).
In addition, strong winds following periods of heavy rainfall, or a marked lack of rainfall (drought), are likely to have significant adverse impacts – windthrow at the root plate and stem failure are the most notable impacts, respectively. Not only this, but disease and dysfunction manifesting within the roots and stems of trees will leave them more susceptible to windthrow when compared to the susceptibility of a healthy tree. This may be particularly apparent after a dense stand has been thinned, exposing once-sheltered trees to wind gusts they have not adapted to (in both their rooting and aerial structures).
Building on the concept of exposure, forests that reside along the coastline have their ‘edge trees’ damaged by the wind. For example, exposed sycamores suffered damage to 46% of their leaves as a result of strong coastal winds. Damage manifested as a result of foliar tearing, the collapse of epidermal and mesophyll cells, and dehydration via the disruption of protective foliar waxes. Even mild winds of 6m per second will oft dehydrate leaves (such as of aspen).
Mild winds will also bring about a decrease in primary elongation whilst initiating increased secondary thickening, thus altering the form of the main stem. Trees in exposed settings and therefore shorter, thicker, and more tapered than sheltered counterparts, in order to resist swaying violently and to retain uniform stress throughout. In conifers, the adaptive secondary thickening (xylem increment) actually is more discernible on the leeward (compression) side – this may be exacerbated when the very same conifer leans with the wind, given conifers lay down reaction wood on the compression side in an attempt to self-right themselves (becoming a ‘sabre tree’, as penned by Mattheck).
In essence, all the above changes can be attributed to three different types of change brought about by wind – a change in water relations, food relations, and hormone relations.
Looking firstly at water relations, wind has been shown to actually have varying effects depending upon species. To demonstrate, Norway spruce and Swiss stone pine are observed to transpire less during wind, whilst larch and alder transpire more. It is considered that this is due to varying stomatal responses, with stomatal size being a particular determinant – smaller stomata dehydrate quicker and thus closer faster. However, as a general rule of thumb, woody plants exposed to wind will initially have their transpiration rates increase rapidly, with rates then tailing-off gradually (dependent upon the species) as the stomata close because of dehydration.
Turning towards food relations, exposure to strong winds will usually lead to reduced photosynthetic rates. This is because the strong winds cause foliar injury and / or foliar shedding. However, the effect wind has upon leaf temperature also drives the change in photosynthesis. As winds will cool leaves, the conductance of leaves will alter – this will impact upon photosynthesis. Strong winds may also increase respiration rates of leaves, with the consequence of reduced carbohydrate supplies.
Lastly, hormone production and translocation is markedly impacted by wind, with auxin and ethylene being the main hormones affected. This drives the physiological changes mentioned earlier – namely a decrease in primary elongation and an increase in secondary thickening. For example, the reaction growth laid down on the compression side of conifers is attributed to a high auxin gradient, which prompts carbohydrates to be mobilised and used in the leeward region for growth. Conversely, tension wood laid down by broadleaved trees in identical conditions is linked to a deficiency in auxin.
Source: Kozlowski, T. & Pallardy, S. (1997) Growth Control in Woody Plants. UK: Academic Press.
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