His­tory leaves its fin­ger­print in the whorls of Ir­ish tree rings

The Irish Times - Weekend Review - - ENVIRONMENT - Michael Viney

Our young beech tree, clearly drunk on ex­cess car­bon diox­ide, flings its branches ever sky­ward. But the yew in­side the gate pur­sues its destiny with mea­sured con­sol­i­da­tion. Bought as a pros­trate tree in a gar­den-cen­tre pot, it has since built the base of a dark tower, sev­eral me­tres across and head high. At its heart, where black­birds hide, an in­ac­ces­si­ble briar has sprung up to crown the canopy with thorns.

Our yew is thus the very op­po­site of the form that stands sen­try in so many Ir­ish church­yards, its branches clus­tered into a dark and som­bre spire.

This cul­ti­var has been cloned by the mil­lion from seedlings brought down from a lime­stone crag on the Fer­managh side of Cuilcagh Moun­tain some 250 years ago. The mother tree of Taxus bac­cata ‘Fasti­giata’ still stands in the Florence Court demesne, a rather for­lorn old bun­dle when I sought it out years ago. “No other mu­ta­tion of tree,” judged Thomas Pak­en­ham, “has achieved this feat of mul­ti­pli­ca­tion.”

The oc­ca­sional grandeur of ven­er­a­ble yews, with their fluted, ma­hogany glow, has en­cour­aged many myths about their age. The old­est ones are of­ten hol­low; their rings of new growth can be­come pa­per thin, and the den­sity of their tim­ber adds to the chal­lenge of tak­ing cores to at­tempt their dat­ing.

Yews can, in­deed, live for 1,000 years or more, as the long­est-liv­ing plant species in these is­lands. The largest and prob­a­bly old­est known tree in Ire­land is at Bal­lyrankin, Co Wex­ford, with a girth of 7.64m, found by the hunter of Ire­land’s cham­pion trees, Aubrey Fen­nell (Per­rin TCD the­sis 7341).

Wild yews were part of Ire­land’s first forests, flour­ish­ing, for ex­am­ple, on the lime­stone pave­ments of the Bur­ren. On sim­i­lar, mossy pave­ment be­tween the lakes in Kil­lar­ney Na­tional Park grows the largest yew wood in these is­lands, Reenadinna, dated by an­cient pollen to some 5,000 years ago.

Most yews of Reenadinna to­day, how­ever, are still fairly slim and straight at up to 300 years old, and there are no re­ally large and an­cient trees to speak for the wood’s con­ti­nu­ity. They have, how­ever, had their uses in a very cur­rent branch of sci­ence: den­drochronol­ogy and the his­tory of cli­mate.

Each year’s growth of a tree forms, in ef­fect, a new tree around the old one, so the thick­ness of the rings of new cells record the tree’s re­sponse to tem­per­a­ture, wind and rain dur­ing the grow­ing sea­son: the thin­ner the rings, the worse the weather.

A slice of an old tree can look quite beau­ti­ful, as in one I once ad­mired in the Na­van Fort vis­i­tor cen­tre in Co Ar­magh. It had fig­ured in the re­mark­able work of Mike Bail­lie, pro­fes­sor of palaeoe­col­ogy at Queen’s Univer­sity Belfast.

Over 40 years of re­search , Bail­lie com­pleted a 7,400-year en­vi­ron­men­tal his­tory of Ire­land, back to 5400 BC, as re­vealed in the tree rings of na­tive Quer­cus oaks. Its se­quences were pieced to­gether from sam­ples of an­cient trees, some sal­vaged from bogs and me­dieval build­ings.

Bail­lie be­came one of the world’s most re­spected author­i­ties on tree-ring dat­ing and, in his own term, “a den­dro-catas­trophist”. He was es­pe­cially con­cerned with tree-ring “sig­na­tures” in bog oak of stunted growth fol­low­ing two ma­jor dis­as­trous en­vi­ron­men­tal events.

His be­lief that these nar­row­est tree rings may have co­in­cided with comet ac­tiv­ity, rather than vol­canic erup­tions, led him into con­tro­versy.

But he also came to ac­cept that such var­i­ously gath­ered sam­ples of Ir­ish oaks were not re­li­able ma­te­rial for re­con­struct­ing pre­cise tem­per­a­ture and rain­fall records. With his col­league Dr AM Gar­cia-Suarez, Bail­lie “gave up any hope of con­tribut­ing to the is­sue of cli­mate change us­ing tree rings”, as quoted in the Guardian. Re­search has since con­tin­ued, how­ever, with trees such as Ir­ish beech, ash and Scots pine.

Ir­ish yews came into the pic­ture with the PhD the­sis of Stephen Galvin at NUI Gal­way: The Im­pact of Vol­canic Erup­tions on the Cli­mate and Ecol­ogy of Ire­land Since AD 1800 was pub­lished on­line in 2010.

The oak-ring record pi­o­neered by Mike Bail­lie had al­ready shown down­turns in tree pho­to­syn­the­sis that fol­lowed vol­canic erup­tions. Daily weather records were kept at Ar­magh Ob­ser­va­tory from 1796 on­wards.

Match­ing these and other data with tree-ring cores from yews in Reenadinna Wood, Stephen Galvin stud­ied the re­sponse, in 32 trees, to aerosol veils thrown up by well-doc­u­mented erup­tions.

There were se­vere global ef­fects from five low-lat­i­tude erup­tions, such as Tamb­ora, in In­done­sia, in 1815, which brought failed har­vests to Ire­land and poorer growth in the yews. The much closer erup­tions in Ice­land, how­ever, rarely in­ject mat­ter into the strato­sphere, and their ef­fects can be masked by ma­jor swings of weather in the North At­lantic Os­cil­la­tion.

Galvin’s work es­tab­lished the long-lived yew as a re­li­able barom­e­ter of en­vi­ron­men­tal change. Even in a warm­ing world, dis­tant erup­tions could still leave their fin­ger­print in the whorls of Ir­ish tree rings.

Wild yews were part of Ire­land’s first forests, flour­ish­ing, for ex­am­ple, on the lime­stone pave­ments of the Bur­ren

Michael Viney’s Re­flec­tions on An­other

Life, a se­lec­tion of col­umns from the past four decades, is avail­able from irish­times.com/irish­times­books; viney@anu.ie

Barom­e­ter: the yew re­flects en­vi­ron­men­tal change. PHO­TO­GRAPH: MICHAEL VINEY

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