Why do grasses become seasonally dormant in winter despite the continued availability of water and sunlight?

(writing in progress)

The Highveld clearly turns brown in winter, with the dormancy, and above-ground drying-out, of the grasses. However, this phenological pattern cannot be ultimately explained by either desiccation or frost.

Throughout the dry season in the Highveld, soils remained wet in bottomlands. The dry season is in winter, but temperatures are not extreme, and then days remain sunny. Ambient temperatures in the Highveld during winter remain above those of the peak growing season in the tundra (Bonan and Shugart, 1989),

This indicates that the winter dormancy of the dominant grasses is a phenological tactic rather than a reflection of absolute physiological constraints imposed by air temperature.

suggesting that interrupted photosynthesis is part of a life-history strategy of grasses as opposed to being an inevitable consequence of the seasonal drought and cold in the Highveld.

In seasonal marshes in the Highveld, the topsoil in vleis remained moist in the middle of winter, when all the grasses are dry and brown. What can be seen repeatedly, in the treeless grasslands of southern Africa, are situations in which winter dormancy in grasses cannot be explained by the seasonal drought of the winter season.

just because no rain falls in winter; the dormancy of grasses at vlei edges proves this because the topsoils remain moist there.
 
So what could cause the dormancy? Another possible cause is the seasonal cold. However, this too makes little sense, because grasses actually grow in the Arctic summer at temperatures lower than those prevailing in the Highveld winter. With appropriate adaptation, grasses should be able to grow all winter long in the Highveld because there is plenty of moisture in the soils in that season, and grasses elsewhere on Earth are known to grow despite the cold.
 
One example taxon: Arctic cottongrasses (Cyperaceae: Eriophorum spp.) are known to be able to maintain growth and positive photosynthesis at temperatures as low as –4 degrees Celsius, i.e. 4 degrees below freezing!
 
Perhaps fire is involved, but perhaps there is a World War 1 trench warfare ‘mentality’ or ‘tactic’ at play here. The grasses/sedges are perhaps resting to save their reserves for the intense battle that plays out in spring and summer when conditions are optimal for growth (and war against competing entities). I am suggesting here that the battle between grasses and trees is not necessarily a continual one; there are peaks and troughs in the battle; and the plants need to have sufficient energy available for the peaks. I am suggesting further that grasses/sedges would lose condition, so to speak, if they grew in winter, relative to grasses/sedges that ‘choose’ not to.
 
When grasses in the Highveld go dormant in winter, what they are doing could either be

  • suffering an environmental limitation, or
  • exhibiting a strategic choice.

Most naturalists in South Africa, if asked to explain the seasonal behaviour of the vegetation, would assume that environmental limitation is the correct answer.

However, ifnstrategic choice is the right answer, what are the costs and benefits of the choice to enter seasonal dormancy despite the possibility of continuing growth? Why do grasses not just grow throughout the year in the Highveld (obviously, more slowly in winter than in summer, but growing nonetheless in the dry and cold season, which is far warmer than the growing season of Arctic grasses)?
 
This is where it gets conceptually interesting. Just as the dominance of herbaceous plants means that the tree niche is empty here, so the dominance of winter-dormant herbaceous plants means that the ‘Arctic grass’ niche is empty here.
 
Put a different way: the fact that Highveld does not support native trees, despite their being ample soil, water, and nutrients for trees here, means that what prevails is a plant community that chooses to fall short of its maximum potential biomass/woodiness/height. Something else is instead maximised, possibly overall rate of catabolism, or biological energy intensity. And the fact that Highveld does not support winter-growing grasses, despite their being more than enough sunshine, ample water (at least in bottomlands), and adequate temperatures (cold but not prohibitively so for grasses as a life-form), means that what prevails is a plant community that chooses to fall short of its maximum potential period of activity. In the niche hyperspace, evolutionary/adaptive decisions have been taken to ‘switch off’ or ‘decline’ certain opportunities despite the resources being available for exploiting those opportunities. The option of growing into tall, woody plants has been declined, and this growth-form has been ‘switched off’; and the option of growing all year long has been declined, and the phenological behaviour of growth during winter has been ‘switched off’ as well.
 
My point: by this thinking the Highveld is doubly ‘self-restrained’: it has opted to forgo a whole ‘spatial stratum’ of plants, namely trees and tall shrubs (which are elsewhere deemed to be the superior competitors for light), and it has opted to forgo a whole ‘temporal stratum’ of plants, namely winter-growing herbaceous plants (or even trees for that matter).
 
Now, the theoretical juice in this is as follows.
 
If opting for treelessness means that the grassland achieves greater power without trees than with trees, and if this is because the most powerful way of using the available resources is expressed by small plants as opposed to big ones, then could it be that the winter dormancy of the grassland also somehow achieves greater power than a theoretical grassland that grows all year long? If so, how exactly does a winter-dormant grass ‘overpower’ a theoretical competitor in the form of an ‘Arctic grass’? What exactly is the payoff in desisting from growth in winter, such that this payoff is more profitable than the alternative payoff of continuing to photosynthesise and grow? And what is the resource parameter in which this payoff should be measured?
 
One possible line of thought: by choosing winter dormancy, the grasses promote seasonal fire, which pays off by .....?

(writing in progress)

Posted on 03 July, 2022 03:40 by milewski milewski

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Eriophorum is a grass-like cype (sedge).

Posted by milewski 11 months ago (Flag)

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