Rof í gróðurbreiðu nema þúsundum kílómetra hér á landi. Óvíða sjást merki um, að unnið sé að því að loka rofabörðum. Ljósm. ÁHB
Inngangur
Mönnum verður tíðrætt um það, hvort ofbeit eigi sér stað hér á landi eða ekki. Sitt sýnist hverjum og verður úr þessu oft langvinnt karp á milli manna.
Fáir hafa sinnt rannsóknum á gróðri af alvöru og því er almenn þekking á honum næsta lítil. Steindór Steindórson er sá maður, sem mest hefur lagt af mörkum á því sviði, auk nokkurra útlendinga, sem hafa komið að vissum verkefnum. Starf Steindórs var einkum fólgið í því að greina gróðurfélög og samræma flokkun á þeim. Þetta var viðamikið starf en því miður aðeins lítið brot af því, sem vert er að kanna.
Í raun og veru er afskaplega lítið vitað um áhrif beitar á gróður. Margir hrossabændur skáka í skjóli þess. Ljósm. ÁHB.
Það er efni í langa grein að gera rannsóknum Steindórs skil, sem vert er. Af þeim má draga margvíslegan lærdóm. Eitt af því eftirtektarverðasta við niðurstöðurnar er sú staðreynd, að skil á milli einstakra gróðurfélaga eru óeðlilega lítil í samanburði við aðrar norrænar athuganir á þessum tíma. Þetta kemur skýrast fram í gróðri á þurrlendi, en gildir síður um votlendi og snjódældir. Ætla má, að þar taki Steindór einkum mið af rannsókum Rolfs Nordhagens (1894-1979), sem hann greinilega hefur tekið mikið mark á og haft að fyrirmynd.
Aldur gróðurs
Gróðurfélög á Íslandi eru ekki nema um tíu þúsund ára gömul eða frá lokum ísaldar. Þetta þykir ekki hár aldur, en til samanburðar má nefna gróðurfélög um miðbik jarðar, sem eru um 40 milljón ára að aldri. Það tekur plöntur og aðrar lífverur æði langan tíma að laga sig að öllum aðstæðum á hverjum stað og mynda tiltekið líffélag, sem er í bezta samræmi við ytra umhverfi. Um þetta samspil og framvindu þess eru til nokkrar kenningar, sem allar hafa sitthvað til síns ágætis. Hér er um geypiflókið ferli að ræða, þar sem hver þáttur, lífrænn sem ólífrænn, hefur sitt að segja, ýmist beint eða óbeint. Hér gilda engin meðaltöl hvers þáttar, heldur miklu heldur hámarks- og lágmarksgildin.
Að þúsundum ára liðnum næst ákveðið jafnvægi á milli lífvera á hverjum stað, sem þolað hafa bæði súrt og sætt saman. Slík gróðurfélög ættu að vera gríðarlega þolin gagnvart ytri öflum. Þau hafa verið nefnd „hástigs-líffélög“ eftir kenningum Clements frá 1916. (Reyndar eru til nýjar og betri útlistanir á „hástigs-líffélögum“, sem litið er framhjá að sinni, sbr. „poly-climax-theory.)
Hver svo sem þessi „hástigs-líffélög“ eru eða hafa verið, er þess ekki að vænta, að menn geti ræktað eitthvað eða komið á fót gróðurfélagi, sem jafnast á við náttúrleg gróðurfélög, sem eru margreynd í tímans rás. Þetta ættu menn ávallt að hafa í huga, þegar gróðri er spillt við verklegar framkvæmdir.
Hér á landi virðist það vera trúa manna, að unnt sé að bæta fyrir gróðurspjöll með tilbúnum áburði og grasfræi. Öðrum læt eg eftir, að ætla hverjir bera ábyrgð á slíkum áróðri.
Áhrif beitar
Loðvíðir var talinn ein bezta búfjárhagaplanta, enda rífur fé hana í sig mjö gráðugt. Ljósm. ÁHB
Nú skal aftur vikið að einsleitni gróðurfélaga hér á landi í samanburði við önnur lönd. – Sannast sagna munu fáir staðir vera til hér á landi, sem eru algjörlega ósnortnir af völdum manna í víðustu merkingu orðsins. Til eru nokkrar illkleyfar torfur og eyjar, sem komast næst því að bera upprunalegan gróður. Þessir staðir eru litlir að umfangi og veita því takmarkaða vitneskju. Næst þessu komast staðir með birkikjarri, þar sem beit hefur verið mjög takmörkuð og lítil sem engin grisjun átt sér stað. Nefna má Teigsskóg sem dæmi og kemst hann næst því, sem kalla mætti frumskóg. Út frá því sjónarmiði er hann einstakur og þar þyrfti að fara fram mjög vönduð og yfirgripsmikil rannsókn.
Víða á afréttum er ekki mikið að hafa. Ljósm. ÁHB.
Helztu ástæður fyrir einsleitni gróðurfélaga geta einkum verið að tvennum toga. Annars vegar getur skýringin verið sú, að tegundafæð íslenzku flórunnar valdi því, að hvert gróðurfélag er öðru líkt. Slíkt má svo vera í nokkrum tilfellum, en þó er ljóst, að innan friðaðra svæða verður aðgreining gróðurfélaga miklu skýrari en utan þeirra.
Hins vegar er önnur skýring mun sennilegri og hún er sú, að einhver einn eða nokkrir þættir hafi slík áhrif, að þeir jafni út samkeppnisstöðu einstakra tegunda. Og sá þáttur, sem er sameiginlegur í þessum gróðurfélögum er einmitt beit og taðk. Hvoru teggja veldur því, að áhrif annarra umhverfisþátta mega sín lítils. Á beittu landi ná engar tegundir undirtökunum til þess að móta gróðurfélagið, heldur er því ávallt haldið á einhverju óstöðugu framvinduskeiði, sem meðal annars sést af því, að jafnan eru fleiri tegundir á beittu landi en óbeittu og hlutdeild (þekja) flestra er tiltölulega lítil.
Það er ótvírætt, að búsetan hefur haft veruleg áhrif á gróður. Ekki aðeins til að rýra gildi gróðurfélaga, heldur hefur beit gengið svo nærri landi, að það er örfoka.
Á þessum bæ, Jarlsstöðum í Bárðardal, var rekið myndarlegt sauðfjárbú um langan aldur. Engum blöðum er um það að fletta, að búreksturinn tók drjúgan toll af gæðum landsins. Gróðurinn á jarðvegstorfunum mun ná sér á skrið, þó að hann fari ekki í sama horf og var í árdaga. Á hinn bóginn munu melkollarnir seint skrýðast gróðri nema mannshöndin komi til hjálpar. Ljósm. ÁHB.
Niðurstaðan er sú, að meginhluti þurrlendis á afréttum landsins er mjög markaður af beit. Beitarþolið er mjög takmarkað og í raun eru öll þessi lönd í stórhættu. Það eina rétt, sem unnt er að gera í núverandi stöðu, er að koma upp beitarhólfum, svo að stór svæði fái notið friðunar í nokkur ár. Síðan mætti taka hluta þeirra til beitar og hlífa beittum svæðum. Með skipulagðri beitarstjórn er möguleiki á því, að land nái sér að einhverju marki, og þá fyrst er hægt að tala um, hve mikla beit land þolir. Á meðan engin stjórn er höfð á beit, liggur allt land undir skemmdum og mun halda áfram að ganga úr sér.
ÁHB / 12. sept. 2014
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