VUCETICH AND PULLAR: SOILS 65

The Rotomahana shower was a most un- Salisb.); III, Effect of temperatuJ'e and soH usual volcanic event, but nevertheless strik- conditions. Aust. J. Bot. 7: 279-294. ingly demonstrated the resilience of indigen- CRANWELL,L. M., and MOORE,L. 8., 1936.1 The occur.' ous vegetation. rence of kauri in montane forest on Te Moe- hau. N.z. J. Sci. Tech. 18: 531-543. I Ash showers may not always have becn KIRK. T., 1872. Notes on the flora of the Like district catastrophic for vegetation, and even where of the"North' Island. Trans. N.Z. Il1st. 5: 322- forest may have been obliterated by ash 345. I flows a return may not invariably have MASTERS, S. E., HOLLOWAY,J. T., and MsKELVEY"P. involved a long time and protracted plant J., 1957. The national forest survey (1 New Zea- successions. Long-lasting changes may be land, 1955, Vol. 1. Gavt. Printer, Wellington. considered probable only where soils werc MILLENER,L. H., 1953. How old is the velgetation on radically changed. Rangitoto Island? Rept. 2nd Ann.1 Mtg., N.z. Ecot. Soc. 17-18. REFERENCES NICHOLLS,J. L., 1959. The volcanic erupiions of Mt. BIELESKI,R. L., 1959. Factors affecting growth and Tarawera and Lake Rotomahana Imd effects distribution of kauri (Agathis australis on surrounding forests. N2. J. For. ,8: 133-142.

ASH BEDS AND SOILS IN THE DISTRICT

C. G. VUCETICH and W. A. PULLAR

Soil Bureau, Department of Scientific and Industrial Research, Christchurch: and Whakatane

INTRODUCTION Named Beds , Tarawera scoria (and Rotomahana mud) erupted During the Late Quaternary, volcanic 1886 Ash 810+ 70' eruptions of the explosive or paroxysmal Taupo Pumice 1700+ 1501 type (Taylor, 1953) occurred in the central Taupo Subgroup, members 9-13 North Island about centres, which for con- Waimihia Ash 3420:t:70' Rotokawau Ash venience, are designated Okataina, Waita- Whakatane Ash hanui, Maroa, and Tokaanu (Fig. 1). The Ash resulting ejectamenta formed thick layered Rotoma Ash beds largely of rhyolitic pumice. Most is Taupo Subgroup, members 16--18 8850+ 1000' Waiohau Ash known about the distribution of beds from Rotorua Ash the Okataina and Waitahanui cruptive Rerewhakaaitu Ash centres where named beds have been Unnamed Beds "'- mapped in the Rotorua, , and "'X' Beds"; (Ash and lapilli) three in number. in Gisborne districts (Vucetich and Pullar, in Rerewhakaaitu locality and believed of restricted distribution; not mapped because of I few expo- press). Little is known about the Maroa and sures. Tokaanu centres except that they are con- *"Pinkish-brown Beds"; multiple, of lash grade, sidered to have been important contributors weakly weathered; mainly northern p:trt of Kai~ earlier in the Holocene and that they were ngaroa Forest, Wairakei, and :~otorua. also active contemporaneously with the *"Mauve Beds"; three in number' TIainly ".on

Mamaku Plateau. . , Taupo Pumice eruptions. , . . ""'f'.,., "Yellow-block Bed"; mainly along Bay of Plenly and. in ,Gisborne District; not traced . south and STRATIGRAPHY west of Rotorua. I * The "pinkish-brown" and "mauve" bids are not The following ash beds have been recog- seen in stratigraphical order in one section and nised; the age where known is given in may be coeval. Mapping of these together with years before 1950: lower.-beds is proceeding.. ,;. . I 66 VUJ.l"'A!\JTrTTV Af\JO

,"Whlte--block. Bed"; mainly along Bay uf Plt::Jlty; Produt.:lS of ash-flow uroppcd by glowing not traced west and south of Rotorua. (Both of the clouds or nuees ardentes are often wide- CiUUVC lUUY be a correlative of Ka:wcrnu Brcccit1.) "Grey-banded Bed"; wide distribution from west spread and more difficult to plot because of of Tauranga to Gisborne 'but southernmost limits violent eruption occurring simultaneously not kn01Nn.' (CoJ:'J:"elative of Rotoiti Bre<'c-i~.)'~[ trom a number of vents:. The cloud~ tr~vPllf"CI ,"Red Beds" or "ginger tuffs" '(Hamilton correla- long distances, transporting vast quantities tive); cOrI'Hated with known Hamilton beds west of Taurang;l and at Matamata; traced along Bay of of pumice and ash at temperatures sufficient- Plenty to Ciisbomei perhaps the most widespread ly high to char wood in their passage across

, of aU a~h-fijll beds, I the landscape. Deposits from one eruptive

I episode vary according IO LOpography; In 1. N.Z. lAC. 10. N.Z.J. ScL Tech., B36; 371-4 depressions and on valley floors, particu- ,(1954). I larly towards the foot of hill slopes. they l. N.£.. HC. 1. N.L.J. 5C1. Tech., 113.:5; 127-5 (19.:'13). 3. N.Z. 14C.179 N.Z.J. Geol. Geophys., 2: 208-41 are characteristically thick pumice breccias N.Z. ItC. 165 } (1'15'1) froquontly containing charco~l, ~nd on clopoc, they arc ach brocciac Q good dQ~l thinner but of irregular thickness. ERUPTIVE EVENTS I I Vegetatjon wa~ a]~o drowned or buried The ash column records events separated by water and alluvium impounded in tem- by considerable time intervals. Each event porary lakes. I c.i1u~cd c(n:nplctc or portia.! {' dC:5truction; of From ash-fall plans prepared by Vucetich the vegetation within an inner zone and, and Pullar (in pres:s:) the area covered by dm1ng the ensuing period I of quiescence, veg,etatioll established itself and soils began ash more than 24 in. thick is 5,000 square n'lilc.3 j 1TIOrc than 18 in., 7,000 .3quarc milc,:,; to lorm. ] n the preceding cold period (Last Glaciation) the ash column shows erosion and nJ.on; than 12 in., 12,000 "quarc milc", breaks mid it would appear that a number I in the combined districts of Rotorua, Taupo, of orupti~)n.c took plo.cQ ,"vhon tho. upland~ Bay of Plenty, and Northern Hawke's Bay were bad, of vegetation and there was ex- (FIg. I). tcn3ivc clo~ion. The critical thickness of cold ash required I to destroy vegetation cannot be given with Detailed studies of the Taupo Pumice and precision. On Mt. Egmont A. P. Druce (pers. :soils dtai;/cd frOID it are uscd as a bash for comm.) observes that the Burrell Ash the pattei'n of forest destruction and for a deposited cold some 300 years ago had the suggested history of the forest during the following effect on cedar and Hall's totara; 13 In. [lUck compleIe desIrucIlon Late Quaternary. 12-15 in. almost complete destruction 9-12 in. partial destruction

hnC<"J.OI of '\'ULO UIT nnd. '\':COC'ThTIOH ThUUli:l:S (1000) levun:s ..hi:l.. ..ltt::: fUlt:::Sl Wi:lS DESTRUCTION not destroyed on Mt. Edgecumbe where Tarawera scoria and ash' fell 14 in. thick. In Ihe viclnlIy of the vents, blast ami If a conservative estimate of ash thickness glowing, avalanches' were the principal for complete destruction is 18 in., then des- agents of destruction and shower-bedded truction on at least one occasion occurred depo!i:itli: beyond are cnnc,;:idprprl to h~vf" I una iUI ct:u::::ct uf 7,000 ~y'UctH::: utik:s. f~1l9n l"l:\lA Qnrt tn h~"p Idl1Prtvpgpt~tinn hy ,burial. This principle is well illustrated in TAUPO PUMICE ERUPTIONS the products of Jhe Kaharoa eruption where , t.:harrt::u lug5 al~ :st:::eu in locali5cd ash.flow A :scdc:s of cl-uptions:: frol11 ccntl.cs aL deposits dropped by glowing avalanches on Waitahanui, Maroa, and' Tokaanu and the' lower slopes of Mt. Tarawera" but the occurrtng at short mtervals are known as ash-fall deposits lurther away do not con- the Taupo Pumice Eruptions. They termin- tain charcoal. Charcoal occurs sparingly in ated with particularly violent explosions ash-fall deposits and is usually to be found from vents which appearito have extended in contact with or within the buried soil beyond the centres named and with ash- rather thi~n in the overlying ash bed5. flQW material pend.5ting for long d15tal1ct:::t. , VUCETICH AND PULLAR: SOILS 67

ERUPTIVE CENTRES

12" lSOAIrICH ...-18" ISOPACH

FIGURE 1. Area in which forest was destroyed by vulcanicity on at least one occasion during the Holocene. Destruction is postulated to be complete with deposits greater than 18 in. thick and partial with those 12-18 in. thick.

The timetable, form of eruption, and pat- Phase 2 tern of vegetation destruction is given below Widespread deposits of Taupo Lapilli with particular reference to the Waiotapu- mainly from Waitahanui are shower bedded Oranui locality. and caused localised complete de,;truction by burning and blast, and by burial in an Phase 1 outer zone. Large trees remained standing Initial ash-fall deposits from centres near through 18 in. thick deposits. Waitahanui; Hatepe Ash, "putty coloured layer", and Rotongaio Ash caused localised Phase 3 destruction mainly by burial. After a short interval a violent eruption 68 VULCANICITY AND VEGETATION

of the ":hyolitic block member" gave rise to , TEMPERATURE.,., . OF ASH-FLOW , . a wide, pread thin bed of rhyoliticlap,illi. " This eruption ~ppears to have its sequeni1' " ,Ash-flows must have had a substantial the Maroa centre where, on slopes near burning effect on the, vegetation, but while Orakei{orako, rhyolite ash and lapilli and the minimum temperature required to char pumice lapilli form thick, mainly loose, wood is 250"C, there is no sign of baking deposit,! of irregular thickness. In valley of a buried soil and this would suggest bottom,' massive breccias up to 15 ft thick lowest temperatures at the base of deposits are cha:acterised by discontinuous vertical and a temperature gradi,ent within the over- fissures with inclusions of loose ash and' lying ash. If the buried soil was not appreci- chalazo dites ( volcanic hailstones). "..c'- This ably heated seeds, rhi>:omes''.' and fungal eruptioll caused localised forest destruction hyphae may be expected' to' survive and by bias c and burning as well as by burial. become important plant:;material to colonise those,hill slopes which, although overrun by nuees, are thinly ash c'oated. These views Phase" may have substance from the, observations Uppe;' Taupo members include multiple of Miller et al ( 1955) who recorded tempera- flow bEds of indurated ash (and pumice tures as high as 600"C during scrub fires at lapilli) )n the uplands thinning off as slope Taita Experimental Station. increaSE s, and thick pumice breccias of the " valley f: oors. Some hill slopes in the lee' of SOILS the bel'eved direction of blast have little, e e, ' Profiles of the buried soils are compared or no a:;h deposit. with present day soils, and matched for The pattern of breccias indicates that development of pedologic horizons and nuees,Nhich were to some extent chan- degree of weathering as ,indicated by colour nelled t y the Paeroa scarp into the Waikite and content of amorphous clay minerals. and Whirinaki valleys, were partially With few excepMons the evidence suggests obstruc,ed by the orographic barriers of that'forest reclothed. each successive ash rhyolite domes and Horohoro Bluff and deposit but there is little indication as to deflecte 1 to the west. The source of the the rate of recovery. nuees j,; not known but the occurrence of thick heccias containing pumice blocks Soils derived from Kaharoa Ash

near Ol'akei Korako suggests ee that there were eruptions from this placee'e The limit After 800 years, subsoil characters are of breccia and ash-flow beds is also the limit very weakly expressed in the loose, coarse of fore,t destruction. Forest would be des- Kaharoa Ash. Under podocarp-dominant forest along Hongi's Track ne"";m Lake troyed by blast, ,eby_bmning or charring e '" where ,engulfed by nuees, and by the im- Rotoiti, 'subsoilsee are hiimus stained and poundir g of valleys for a short period when lenses of fine ash are weakly cemented' with drainag:,e was obstructed 'by pumice humus and iron. In the :>Kaingaroa : Forest breccia,; for example, valley, and Galatea basin (Vucetich et aI, 1960) pro- which ,':as a lake below the 1,070 ft contour. files are more strongly developed on hill The area of destruction is roughly 4000 sq. slopes under a manuka scrub community miles, but it is believed that remnants of (Lepto.spermu111,:scoparium) and least so on forest nay have survived on thinly coated flattish sites in both basin and plateau under slopes v,'ithin this area and so.provided plant a manoao (lJracophyllum;suljulatum) and tussock (Poa caespitosa) cover. Differenoo materia, for eventual colonisation...- ':'..,,_; ..1.":'';1';,,, ' may be explained by a relatively, unfavour- e able microclimate. .' , Phase 5! Unfortunately, .firing of thevegetatio(l,by A final phase with source vents in the the,early ,Maori makes :interpretation difli' Maroa-1\[okai-Oranui area locally produced ctilt:.: This point is well illustrated near thick pumice breccias with restricted distil: Kafngaroa Forest Headquarters, where cadre"~

butionJmt, with more widespreadash-fall."" Rere subsoil from "Kaharoa Ash shows dis-

'c, ,-r" VUCETICH AND PULLAR: SOIl.S 69

rupted shower bedding about a site once Profiles suggest a good recovery of the occupied by a tree with a basal diameter of forest after the Taupo eruptions, beijIgI most about 10 in. A thin weakly-stained lens of rapid on the hills where pockets 'jf plant fine ash in the subsoil with further iron material survived and, then e panded staining in the buried Taupo Pumice soil slowly across lowlands and plateaux It may below indicates that a podzolising tree sur- be inferred that recovery wouldha(e ~ been vived a 12 in. thi2k deposit of Kaharoa Ash quicker after the ash-fall type of eJuption, and, furthermore, continued to podzolise which destroyed smaller areas of for\,st, and the new material. But this forest was des- which provided a more suitable loose parent troyed by a fire prior to European settle- material for forest growth. " ment. , Buried Soils and Past Vegetation Soils from Taupo Pumice Present day podocarp-dominant forests Information gained from prof les of have left a marked imprint in Taupo present-day surface soils and from ~inds of pumice soils particularly on the hills. Soil vegetation growing on them is use\l for a ~rovisional interpretation of vegeta- horizons are best exhibited in a 'saucer- . the J shaped soil, podzolised under rimu (Dacryd- tlpnal and climatIc history dunng tjIe Late Quaternary. ium cupressinum) which has, under,its , " mor-litter, it pale brown, bleached A2 and a Apart from the black scrub soils (Kaha- dark brown, humus illuvial, B2 horizon. roa Ash) buried beneath TaraweraI or These features show up well in sOme 24 in. Rotomahana ashes, almost all buri~d soils of ' indurated silty sand, representative of are brown or yellowish brown in Jcolour. the ash-flow type of deposit, and the podzol Distinctive A horizons are not gcnerally B 'horizons are well preserved where the found and the buried soils comprl se one foresfhas been destroyed and where a black main horizon over yellowish brown! parent topsoil, enriched with bracken fern or scrub material. Most of the buried soi~s have humus, has subsequently formed. By com- irregular thickening of the old B horizons parison, broadleaved trees produce mull with tongues of similar material extending litters arid a humus-rICh' A horizon over a 10--12 in. below. The best example of thin B except where grown on sites pre- 'tonguing' occurs in soils bearing fine-tex-

viously occupied by rimu or similar pod- tured, indurated lenses, and having I a pale zolising trees, e.g., where rata supplants a yellowish-brown colour penetrattd by host rimu, a mull litter and a humus-rich strongly coloured dark brown tlmgues. A overlie the podzol B ,horizon. , - , These features together with the infl equent " ' occurrence of charcoal fragments in ],ockets " Soils derived from thick ash-flow beds on flattish basin sites are commonly";' shallow are indicative of forest. The followipg is a brief, inventory of characters for I buried with a pumice pan restricting plant roots.' A ,- -1,;01-, . common profile under manoao or manuka soils: Rotokawau Ash; basic, but containing rhyolitic scrub shows a black topsoil resting on a ash; one of the most weathered of I-~olocene dark brown subsoil stained reddish-br.9wn a~n~s (sandy loam texture), and tHe most and weakly cemented, and passing abruptly strongly and uniformly coloured; comm9.nly red- to a massive silty sand at 10--12 in. dish yellow; very limited area; stratigraphic relationship to Waimihia Ash not known I On the uplands, near Waimihia, scrub Waimihia Ash; more uniformly humus Jnd iron stained than overlying present dayl Taupo soils have similar characteristics also attri- Pumice soils and considered to br more butable to podzolisation. Taylor (1953) did weathered but not yet confirmed by clay mineral not consider these shallow rooting soils to analyses (note that comparison IS ma;:leI with soils derived from ash-fall' deposits o~ Taupo be normal "pumice podzols". But the weight Pumice east of the limit of ash-flow deposits). of indirect evidence strongly suggests a pre- The climate may have been slightly warn1ler than vious shallow-rooting forest. Such a forest after the Taupo Pumice eruptions. would be vulnerable to fire and may explain Whakatane Ash; yellow brown silt~,I sand and sandy loam; texture distinct from grey complete destruction before European shower beds at base. - - t settlement. Mamaku Ash: sirong brown or brighil yell()w. 70 VULCANtCtTY AND VEGETATiON

brown coloured soil, more weathered than either "Red beds" (correlative 'of Hamilton beds): Whakatane above or Rotoma below; soil colour pedolo~ic features strong induding colour of distinctive for separating these beds. yellowIsh red and dark red, clayey textures, and Rolome Ash; blackish brown soil contrasting strongly developed blocky structures with clay sharply with marked shower bedding at base> skins on faces of aggregates: slippery and non- Taupo subgroup members 16--18; str

eruptive events, and a date previously dis- the same time of soil formation would indi- cussed establishes a tentative chronology for cate a slight decrease in the intens'ty of soil the older beds. A zero rating (0) for "pink- weathering after,the Waimihia eruption. ish-brown", "mauve", "yellow-block" and "white block" beds implies slight weather- ing but not sufficient to rate as (1). CONCLUSIONS The impact of Holocene and Late Quater- nary vulcanicity on the vegetation i 1 the cen- tral region is summed up as follow, : 1. Has speeded the establishment "f vegeta- tive cover on the, eroded uplands after the Last Glaciation by providir g a root------.-Rotoma ing medium over fretted ignimhrite, and likewise, speeded colonisatior of the hg 16-18 greywacke axial ranges. Waiohau 2. Caused subsequent short-lived destruc------.. tion of forests around eruptiv" centres. An area of 5000 square miles has been ------Rotorua affected at one or more times during the R~rewhokaai------Holocene. 'x'be~ ------'x' be 3. Each eruptive event has mantled a large 'x' bed -----.---_.- zone beyond the central zone OJ'destruc- inkisn-brown' .-rriou.Ye----- tion with an accretion of fresh minerals --ello'W.block' and so offset the progressive degrading ~-fi.C-Dl--1i''w I eo OC' -feYMDon(le:r of soils by weathering, lead ing, and podzolisation. Benefit is optimL m where the accretion is 12 in. or less oi' fine ash and where the vegetation is m,t greatly disturbed. " 4 . Each accretion increased the thickness of ash mantle on the steep slop es of the greywacke ranges and has assured pro- tection from erosion and so provided watershed control. 5. Taupo Pumice eruptions were the most devastating to vegetation. FIGURE Z. Trends in climate expressed relative to weatheredness of Late Quarten- 6. Thick deposits of Taupo Pumice on the ary ash beds. A weathering index is based Kaiweka Range, for example, are highly on 14 C dating of four volcanic eruptions. susceptible to actual or potentid erosion (A. P. Druce pers. comm.). Warmer temperatures are marked by diS' 7. Indirectly, the effect of vulcanici ty is also tinct soil formation on the unnamed "X" demonstrated in the widespread destruc- beds and a further increase in temperature tion of forest following the intlOduction by stronger weathering of the Rerewhaka-" of fire by man. Yellow-brown pumice aitu soil. An interval for a "thermal maxi- soils, unlike the more weath"red and mum" is acknowledged during the Rotorua more moisture-retentive yellcw-brown episode extending with perhaps decreasing loams and yellow-brown 'earth" dry out temperature to the time of eruption of Taupo periodically to a depth of abc ut 12 in. Subgroup members 16-18. From the Rotoma (Will, 1962) and at these times their episode to the present day the record shows vegetation is susceptible to lire. The relatively little change although a compari- scrub vegetation at the time of European son of Taupo and Waimihia soils each with settlement was largely inducer. by fire 72 VULCANICrTY AND VEGETATION

andnct by vulcanicity. Succession,from pattern. Rep. 2nd Ann. Mtg. N.l. Ecol. Soc. 11-22. THOMAS,A. P. W., 1888: Report on the eruption of fire-induced. scrub to forest" ,has 'Tarawera and Rotomahana, . Govt. has been prevented or slowed down by .'Printer, WeHington. soil d,'gradation including both strong VUCETICH, C. G., et aI, 1960: Soils, forestry, and leachirg and unfavourable physical pro- agriculture of the northern part, Kaingaroa State . Foresl and the Galatea Basin. NZ. Soil Bur. Bull. pertIes. 18. ' REFERENCES " VUCETICH, C. G., and PULLAR, W. A. (in press): MILLER,R. B., STOUT,J. D., and LEE" K. E.; 1955: The stratigraphy of Holocene ash in the Rotorua Biologica and chemical changes following scrub and Gisborne district. Part II. N.l. Geol. Surv. burning (In a New Zealand hill soil. N.Z.J.Sci. Bull. n.s. 73. , Tech., B37; 295. ' WILL, G. M., t962: Soil moisture and tempera lure TAYLOR,N. H., 1953: The ecological significance" of studies under radiata pine, Kaingaroa State the central North Island ash showers - The soil Forest, 1956-8. N.l. J. Agric. Res. 5; 111-20.