Backing the broadleaves to save our soil

 

Another Life Michael VineyWatching the steady hosing of the hills this summer, their daily gloom threaded with a filigree of swollen cascades and waterfalls, I have wondered what nourishment could possibly be left in the upland soils of the west.

We have already seen how unconscionable rain can rip the peaty skin from a mountain (as on the slopes of Killary Harbour, round the corner). We know that our western conifer forests have helped to boost damaging acid in a great many trout and salmon streams. What are the prospects, at this late stage, of starting to mend the hills and the sour chemistry of their soils by planting the right kind of trees?

The conifers versus broadleaves issue has been strongly argued for decades, with commercial priorities slowly abandoning unproductive, gale-swept peatland to the causes of biodiversity. To champions of broadleaves, the virtues of woodland planted "for nature" with oak, birch and rowan must be self-evident. But since many of the landscapes they would like to see used in this way are in acid-sensitive areas, would broadleaves actually do a better job of looking after the life of the soil and the streams?

The answers, in a new report from Coford, the National Council for Forest Research and Development, are broadly positive. But there is surprisingly little specific research on the subject, and none in Irish conditions. Decisions may need to be taken on the consensus of best-informed scientific opinion if we are not to wait on through decades of experiment.

"Acid-sensitive areas" are regions such as Connemara, much of upland Connacht and the Wicklow mountains, where bedrock of shale, schist, gneiss, granite or sandstone already gives the streams an acid tang, especially where soil is thin and rainfall heavy. Conifers such as Sitka spruce grow well in such areas, but their year-round interception of polluting particles from the rain can damage the soil's microbial life and tip the chemistry of so-called "poorly buffered" hill-streams into conditions hostile to insects and aquatic life in general.

All afforestation has some acidifying effect, so planting no trees at all, especially on upland moors, would probably be the safest option. But the authors of the Coford report, Marcus Collier and Edward Farrell, both of UCD, have found enough in their favour to consider five trees, four of them native (or originally so) to Ireland, for planting in mixed stands on acid-sensitive land - even, perhaps, for "continuous cover" forestry. They are choices not only for new native woodlands and national parks, but many of the clear-felled slopes of Coillte's western province.

Unlike the shallow-rooting Sitkas, often toppled like ninepins by Atlantic gales, the deep roots of broadleaved forests not only stabilise the trees but help to buffer acidification by bringing neutralising molecules such as calcium, magnesium and potassium into the shallower soil.

A ready choice is the native sessile oak, still foresting much of Killarney and defying sheep and wind in the ravines of Connacht's hill-streams. Its deep-rooting habit is matched by a generous production of leaf-litter, and this, unlike the sterilising, phenol-laden carpet of spruce needles, rots down fast for the use of plants and soil insects.

The native downy birch, too, is a trusted "soil improver", raising the pH of acid soils so that other species can follow. It is already under trial by Teagasc as a forest tree, cloned from outstanding Irish specimens, but its value on the poor, acid slopes of the west must be mainly environmental.

Rowan, too, will grow on acid soils and fertile peats, offers berries to birds and a buffering layer of fast-rotting fallen leaves.

Two conifers complete the chosen species. One made up the native canopy on many of our western hills, if only up to the first millennium AD. The Scots pine (so named because it still grows there as a native) is actually high on the acidifying list, but has several compensations: it "interacts well" with birch, is high in biodiversity, and "may be important for the survival of rare invertebrates". The European larch was never a native, but is deep-rooting and deciduous, and has produced a modest rise in soil pH in some Scottish forests.

Among the native wisdom consulted for the Coford report was a paper in 2003 by three TCD woodland botanists, George Smith, Daniel Kelly and Fraser Mitchell. In Establishing Native Woodlands in Former Conifer Plantations in Ireland, they recommended, for upland acid soils, woods dominated by sessile oak and birch, with holly and rowan following on as their usual understorey, and Scots pine as a suitable punctuation.

Ecologist Marcus Collier and Prof Ted Farrell, who leads UCD's forest ecosystem research, suggest planting a mix of birch and rowan, with Scots pine and oak as "companion species", and leaving the high moorlands bare.

The Environmental Impact of Planting Broadleaved Trees on Acid-Sensitive Soils, available from Coford at Arena House, Arena Road, Sandyford, Dublin 18 or www.coford.ie

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