A thousand little mosses …

Castle_Eden_Burn_July19

Nature doth thus kindly heal every wound. By the mediation of a thousand little mosses and fungi, the most unsightly objects become radiant of beauty. There seem to be two sides of this world, presented us at different times, as we see things in growth or dissolution, in life or death. And seen with the eye of the poet, as God sees them, all things are alive and beautiful.
Henry David Thoreau (journal entry, March 13, 1842)

I was back in Castle Eden Dene earlier this week for my regular visit and, once again, encountered a dry stream bed.  This was no great surprise but, having written about the algae of dry river beds in earlier posts from Castle Eden Dene (see “When the going gets tough“ for the most recent instalment), I thought that I would focus on some of the other vegetation that I could see in and around the stream and, in particular, the bryophytes.   I asked Gaynor Mitchell, who wrote her MSc thesis on the bryophytes of the Dene, to come along and help me with these as my skills never really extended beyond those mosses and liverworts that live permanently submerged in streams and, as we have seen, there is rarely enough water in the burn here for such species to thrive.

There is a rich carpet of mosses on the woodland floor in much of Castle Eden Dene but, in the stream bed and its immediate environs, it is thalloid liverworts that are the most conspicuous bryophytes. Two species, in particular, stand out: the first is Conocephalum conicum, which has broad ribbon-shaped branches and an upper surface covered with pores – which just visible as light coloured dots to the naked eye.   The other is Pellia epiphylla, which was particularly noticeable on the top surface of boulders that are, I suspect, rarely covered, even when the burn is very full.   P. epiphylla had smaller thalli than C. conicum and, importantly, lacked the distinct pores on the upper surface.

Conocephalum_conicum_CED

Conocephalum conicum from Castle Eden Dene, July 2019. The pores are clearly visible on the thallus in the lower image.

Pellia_epiphylla_CED

Pellia epiphylla from the top of a boulder in Castle Eden Burn, July 2019

Alongside Pellia epiphylla on the boulder tops were shoots of the moss Thamnobryum alopercum.  The populations on top of the stones were rather non-descript to the naked eye, being stems growing horizontally across the rock surface. However, amidst these, we found a few of the upright stems which have a distinctly tree-like appearance.   We found more characteristic growths on the woodland floor nearby and my now-dated copy of Watson does, in fact, comment that this species has these two distinct habitats and also that it is a good indicator of calcareous conditions (for anyone who had not noticed the towering limestone cliffs in Castle Eden Dene, I presume?).   Lower down (and, thus, more frequently submerged), we saw Rhynchostegium confertum though this, too, is a species more often associated with terrestrial rather than aquatic habitats.  More significantly, the mosses I associate with streams in north-east England – Rhynchostegium riparioides, Fontinalis antipyretica and Leptodictyon riparium – are all missing from Castle Eden Burn.

Thamnobryum_allopercum_CED

Tree-like shoots of Thamnobryum alopercum from the forest floor in Castle Eden Dene in July 2019.  Growths on rocks in Castle Eden Burn were smaller but there were enough upright stems for it to be recognisable with the naked eye. 

Gaynor’s sharp eye spotted many other mosses and liverworts, though more in the woodland around the stream than in the stream bed itself.  As well as mosses and liverworts, the stream’s vegetation also consisted of a number of grasses and patches of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage.

The story that the vegetation is telling is, I would venture, that Castle Eden Burn is a shaded terrestrial habitat that is occasionally wet, rather than an aquatic habitat that is often dry.  I dug out an old account of the Winterbourne Stream, an intermittent stream in the chalk downlands of southern England for comparison, and found little overlap in the species recorded.   Care is needed for this comparison as the focus of the surveys is different (the Winterbourne account, for example, includes no bryophytes and spans perennial as well as intermittent sections) but there was a mix of genuinely aquatic and amphibious species, including Callitriche sp. and aquatic Ranunculus, which I did not see in Castle Eden Burn.    I suspect that Castle Eden Burn spends longer as a dry stream bed than the upper parts of the Winterbourne.  However, we also must remember that the Winterbourne data are now almost 50 years old, so that stream, too, may have changed much in the interim.

All this adds to my opinion that Castle Eden Burn – and the streams flowing through the other coastal denes in County Durham – are a unique and understudied habitat.  And that’s before I start thinking about the animal life here…

Chrysoplenium_alterniflorum_CED

A patch of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage, on the bed of Castle Eden Burn, July 2019.

Reference

Berrie, A.D. & Wright, J.F. (1984).  The Winterbourne Stream.   pp.179-206.  In: Ecology of European Rivers (edited by B.A. Whitton).  Blackwell, Oxford.

Mitchell, G. (2015).  Bryophytes: changes in diversity and habitat in Castle Eden Dene (1975-2011).   Northumbrian Naturalist: Transactions of the Natural History Society of Northumbria 79: 39-66.

Watson, E.V. (1981).  British Mosses and Liverworts. Third Edition.  Cambridge University Press, Cambridge.

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