PlantLife Volume 51.8, July 2021. Thigmomorphogenesis - Stunted plant species at Mkambati Nature Reserve

 Thigmomorphogenesis – what?  The answer is blowing in the wind.

Stunted plant species on the wind-swept coastal grasslands of the Mkambati Nature Reserve

by Graham and Kate Grieve

 

Introduction

Like much of the northern Wild Coast, grasslands in the Mkambati Nature Reserve in Pondoland stretch right down to the sea with only the odd rock or small shrub to break the force of the strong winds that blow parallel to the coastline. Many of the plants in these highly exposed areas have a stunted appearance, with stems and branches hugging the ground. This is an example of thigmomorphogenesis, that is, a plant’s response to mechanical sensation (in this case wind) by altering its growth pattern. This is actually an adaptive response because the overall shape of the plant offers least resistance to the wind and it develops shorter and stronger stems.  However, wind is not the only factor influencing the growth pattern as exposure to salt spray and sandy soil also play a role. Just a short distance inland where the topography provides more protection and there is a different substrate, the same species are able to grow to their normal height.

Despite their stunted stature, plants in the highly exposed area still seem capable of reproducing successfully, flowering, getting pollinators to visit (how flying insects manage to provide their pollination services under these conditions is another conundrum) and producing fruits to provide the source of seed for the next generation.

The photographs in this article were taken along a particularly exposed section of grassland just north of the mouth of the Msikaba river, sometimes juxtaposed with photographs of “normal” specimens.

Lasiosiphon triplinervis

This was the first species to bring this phenomenon to our attention. Typically, Lasiosiphon triplinervis is a small shrub standing about 1,5 m tall, and indeed, there are large stands of this species growing to this height in more benign conditions a short distance inland of the coast. In many cases the L. triplinervis in the exposed areas grow to a height of 50 to 150 mm tall, with branches flat on the ground but still managing to flower normally.

 

Typical sized shrub of Lasiosiphon triplinervis

Stunted Lasiosiphon triplinervis with flowers only just protruding over the very short grass at Mkambati

Carissa macrocarpa

The Natal plum or Amatungulu is a well-known plant and garden subject in coastal areas. It has large, sweet-scented white flowers and produces delicious tasting fruits that turn red when ripe. In less wind-swept areas these shrubs can grow to well over 3 m tall. On the exposed grasslands at Mkambati we found specimens barely 200 mm tall. These plants were still flowering and fruiting despite their dwarf stature.

 

Typical robust shrub of Carissa macrocarpa

Stunted Carissa macrocarpa at Mkambati barely 150 mm tall

Eugenia capensis subsp. capensis

The Dune Myrtle, Eugenia capensis subsp. capensis, as the common name implies, has a coastal distribution along the east coast from the Eastern Cape into Mozambique. When provided with shelter, for example in dune forest, the plant can grow as a small tree up to 4 m tall. In exposed locations it tends to grow much shorter, as was the case at Mkambati. (The other two subspecies grow as geoxylic suffrutices, so perhaps subspecies capensis has a species tendancy).

 

A tree-sized Eugenia capensis

Eugenia capensis subsp. capensis in exposed conditions at Mkambati; height about 200 mm tall.

 

Diospyros scabrida

The Coastal Bladder-nut, Diospyros scabrida, in sheltered conditions in riverine forests can grow to several metres tall. In more exposed situations it grows as a stunted shrub, as is the case at Mkambati.

 

Diospyros scabrida growing as a small tree along a cliff edge

 

A stunted Diospyros scabrida sheltering in the lee of a small rock at Mkambati

Thunbergia atriplicifolia

The Natal Primrose, Thunbergia atriplicifolia, even under the best conditions seldom grows taller than 350 mm. However, along the Mkambati coastline, it hunkers down amongst surrounding grassland plants, only allowing its flower to protrude to attract pollinators.

 

A typical shrublet of Thunbergia atriplicifolia, a common grassland plant

A small Thunbergia atriplicifolia plant sheltered below the short grass and showing a single flower at Mkambati

Psychotria capensis

Black bird-berry, Psychotria capensis is a mist-belt to coastal species with a distribution from the Western Cape through to Mozambique. In protected conditions such as dune forests, it can grow to 8 m tall. Where the grassland meets the sea at Mkambati, the plants managed to gain a foothold utilising the protection of rocks and other vegetation but remained generally about 250 mm tall.

A typical small tree-sized Psychotria capensis

A stunted Psychotria capensis in the lee of a rock at Mkambati together with Diospyros scabrida

Tephrosia grandiflora

Grand Hoarypea, Tephrosia grandiflora, given sufficient protection, can grow to a well-branched shrub about waist height and is often covered in its large pink pea-flowers. At Mkambati we found a plant that preferred to secret its leaves below the surrounding vegetation, only allowing its flower to protrude slightly. 

 

Typical shrub-sized Tephrosia grandiflora

With its leaves sheltered below the surrounding grass, this Tephrosia grandiflora at Mkambati has pushed up a single flower for pollinators to find.

 

Seeing these familiar species in different guises was a good test of our identification skills. We continue to be amazed at the tenacity of our flora and the extraordinary adaptability to variations in habitats and conditions.

 

References:

Elsa Pooley. A Field Guide to the Wild Flowers of KwaZulu-Natal and the Eastern Region. Natal Flora Publications Trust, Durban. 1988

Richard Boon. Pooley's Trees of Eastren South Africa. A Complete Guide. Natal Flora Publications Trust, Durban. 1993.

About the Authors: Kate and Graham Grieve retired to the lower south coast of KwaZulu-Natal in 2009 and took advantage the opportunity to learn about the plants of Pondoland through regular exploration of the nearby Umtamvuna Nature Reserve. They had the privilege of enjoying a number of trips to Mkambati Nature Reserve on the Wild Coast.