Cushion plants as nurses? September 26, 2016

Now that I’m back from the Himalayas it’s time to try and label some of the hundreds of plant photos I took and think about how they fit into the bigger picture of the ecology of the region.  It can be hard to know where to start!

Some of the most unusual plants we saw were cushions of Thylacospermum caespitosum, just below Taglang La pass on the road from Leh to Manali.  They look a little like lumps of green, expanded polystyrene foam deposited on the rocky slopes, around 5300 m above sea level. Other high-altitude plants, Delphiniums, Saussureas and Sedums, cluster around the cushions and even grow within the cushions.

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Thylacospermum caespitosum (left) and Delphinium glaciale (right) below Taglang La pass, Ladakh

There is very little sign of any soil but cushions trap heat and water efficiently in this hostile environment and it has long been assumed that they act as ‘nurse plants’, allowing seedlings of other species to germinate and become established in their shelter.   Plants need any help they can get in order to establish themselves in this cold desert environment, with less than 100 mm of annual precipitation and temperatures well below zero, especially at night, for six months of the year.

Other plants growing in and around cushions of Thylacospermum caespitosum

However, under the most extreme conditions, plants might not benefit from being close to their neighbours – they might lose out because of competition with such well-adapted plants for very scarce resources (water and minerals).

A group of Czech scientists carried out research at altitudes of around 5900 m on the mountains east of high altitude lake Tso Moriri to see if they could quantify the effect of cushion plants on those around them (De Bello et al., 2011).

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The Chalung Mountians, east of Tso Moriri, where De Bello and colleagues studied the effects of Thylacospermum caespitosum

They looked at plants of other species found either within, on the edge of, or outside cushions and found, to their surprise, that the number of both individuals and species was higher outside or at the edge of cushions than within them.

A number of things could explain this;  it may be that, at this extreme altitude, any plant which can grow is already well enough adapted to its harsh environment without needing ‘nursing’, or perhaps a soil crust composed of algae and cyanobacteria offers the same support to germinating seeds, without then competing for resources.  A third possibility is that the very short growing season in this part of Ladakh is the main issue for plants and cushions don’t help much with extending this or, maybe, the effect De Bello and colleagues found is specific to this particular cushion plant and other species might have different effects.   Thylacospermum caespitosum cushions are particularly dense, which might make it difficult for seeds to settle and germinate.  Much of the earlier work on nursing effects was done on other cushion species from the Andes mountains.

De Bello, F. et al. (2011) Cushions of Thylacospermum caespitosum (Caryophyllaceae) do not facilitate other plants under extreme altitude and dry conditions in the north-west Himalayas.  Annals of Botany, 108, 567–573.

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One thought on “Cushion plants as nurses? September 26, 2016

  1. Pingback: What do Upper Teesdale and Ladakh have in common? | heatherkellyblog

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