etiology of forest dieback areas within the kaimai range, north …€¦ · waterlogging; symptoms;...

New Zealand Journal of Botany, 1986, Vol. 24: 513-5270028-82 5X/86/2404-0513$2.50/0 © Crown copyright 1986

513

Etiology of forest dieback areaswithin the Kaimai Range, North Island, New Zealand

G. T. JANEN.Z. Forest Service, P.O. Box 25022Christchurch, New ZealandT. G. A. GREENDepartment of Biological SciencesUniversity of WaikatoPrivate Bag, Hamilton, New Zealand

Abstract Severe forest decline exists in uplandareas of the Kaimai Range, North Island, NewZealand. Stand structure of major affected vege-tation types is described. As stand dominants arewell represented in induced serai vegetation, theoverall species composition of the upland forestsis not likely to change following decline. Within thedecline zone a considerable range of forest damageis found. Characteristic damage types are describedincluding effects on seedling vigour, root systemdevelopment, and shoot phenology. The declineappears to result from a sequence of natural phe-nomena. High fog occurrence coincides with thedecline zone producing soil waterlogging andgenerally poor growth conditions. This predisposesthe forests to periodic drought damage.

Keywords forest decline; forest dieback; soilwaterlogging; symptoms; roots; stand structure;phenology; fog

INTRODUCTION

Jane & Green (1983a, b) discuss location and extentof forest mortality in the Kaimai Range but pro-vide only a limited description of the nature ofaffected stands. World literature on forest pathologycontains references to many similar problems usu-ally called diseases of complex or unknown causesbut includes few descriptions of symptoms orinvestigations into site ecology.-Notable exceptionsare descriptions of diebacks in Australia (Weste1981), ohia dieback in Hawaii/(Mueller-Domboiset al. 1981), and oak diebacks in southern Europe(Vajda 1951). These dieback syndromes show manyresemblances to the current situation in-the Kaimai

Received 27 February 1985; accepted 24 April 1986

Range. Diebacks are often referred to as diseasesor mortality events, with the inference that a path-ogen is probably involved. Manion (1981) makesthe point that diebacks are the result of a sequenceof often abiotic events and the term disease maybe inappropriate.

Mortality in the Kaimai Range is concentratedabove a critical altitude, which ranges from 550 mto 700 m a.s.l., dependent on locality, and whichcorresponds with the fog zone (Jane & Green1983b). Mortality, therefore, may extend from semi-lowland tawa (Beilschmiedia tawa) forests to uplandsilver beech (Nothofagus menziesii) forests (Jane &Green 1983a) where it may occur on slopes of upto 35°. It affects a wide range of forest communities(including induced serai communities) and species.In order to understand further the nature of theproblem, selected stands are described in detailtogether with some growth characteristics of pre-dominant species.

METHODS

The principal study sites within the range at "TeHunga", "Te Rere", and Te Aroha are describedin Jane & Green (1983a) and shown in Fig. 1.Height intercept and distance measurements alongline transects were used to compile vegetation pro-files, details of which may be found in Jane (1983).Nomenclature follows Allan (1961) and Moore &Edgar (1970). Partial soil profile descriptions weremade on at least five cores removed with a Dutchclay auger along profile transects to identify themain soil types as recognised in the earlier land-slide study (Jane & Green 1983c). Soil profiledescriptions were also made at a few soil pits toconfirm characteristics determined by soil auger.

The main species studied were silver beech(Nothofagus menziesii), kamahi (Weinmanniaracemosa), kaikawaka [Libocedrus bidwillii), quin-tinia (Quintinia acutifolia), and tawari (Jxerbabrexioides). These are the principal dominants inupland forests. Bud break and leaf developmentwere monitored through summer 1981-82 and1982-83 on tagged branches. Sample trees on threestudy ridges were visited weekly or fortnightly insummer, and monthly during winter between Sep-tember 1980 and March 1983. Records were madeof leaf size, leaf number by age classes, and visible

514 New Zealand Journal of Botany, 1986, Vol. 24

KEYRegular sample routes

— Vegetation transects

• Other sample sites

n Soil sample sites

Scaleo 1 2 km

Fig. 1 Location of the studyareas. Vegetation transects arenumbered as in text as 1 — TeRere; 2 — Silver beech (TeHunga); 3 — Silver beech/kaika-waka (Te Hunga). Other samplesites are 1 — 950 m, 2 — 850 m,3 — 850 m altitude sites at TeAroha (Table 2); 4 — Poor site800 m, Te Hunga, 5 — Good site,750 m, Te Hunga (Table 4). Sites3 and 4 are also used for Table 5.Soil sample sites (Table 1) at TeHunga are unnumbered. Regularsample routes had soil moistureand temperature sites at 50 m or100 m altitude intervals and werevisited weekly or fortnightly.

damage on leaves. Leaf age was determined fromobservations on tagged branches and supple-mented by estimation on other plants and olderbranches using leaf bud scars.

Root development patterns were examined onwindfalls and excavated seedlings. Stumps up to7 cm basal diameter were also forcibly uprootedalong a track cut two years previously through poorsites on "Te Hunga" with typical peaty soils. Sur-face root distribution was mapped at a few local-ities and root/shoot ratios were determined onseedling material returned to the laboratory. Rootgrowth activity was noted in soil samples collectedat the weekly or fortnightly visit to study sites (Jane& Green 1983c).

RESULTS

Stand structureBecause subcanopy species frequently survive ascanopy plants in the fog zone (Jane & Green 1983a,b, e), on what can be regarded as poor sites, mor-tality produces serai stands of Melicytus ramiflorusand Hedycarya arborea shrubland in tawa forests;dense horopito (Pseudowintera axillaris) or Cyatheasmithii in tawari forests; and dense quintinia/ka-mahi shrubland in silver beech forests (Jane &Green 1983a). All forest types are characterised bya range of damage intensity resulting in unthriftyplants showing similar symptoms. Hence it will suf-fice to describe three localities in detail.

Jane & Green—Forest dieback 515

20n

0 20

Distance (m)

Fig. 2 Schematic cross-section of affected stands at Te Rere Bald. Bearing 160° from left to right;altitude 750 m. (a) mature tawari forest, (b) affected forest, (c) grassland, (d) recently affected forest.Symbols are Ib, tawari; Nm, silver beech; Pc, Pseudowintera colorata; Cs, Cyathea smithii; Pf, rniro;Ud, Uncinia distant, An, Astelia nervosa; Gl, broadleaf; Dl, neinei; Q, quintinia; Ms, toro; Wr, kamahi;Cf, Cyathodes fasciculata ; Lb, kaikawaka; Bf, Blechnum fluviatile, Di, Dacrydium intermedium.

1. Tawari forest — "Te Rere" baldAt "Te Rere" bald two ages of mortality are evi-dent on aerial photographs and, until recently, goats(Capra hircus) have prevented forest regeneration(Jane & Green 1983a). These age classes are shownalong the first transect as scattered emergents overmature stands and low grassland and shrublandtowards the end of the transect (Fig. 2). Vegetationtypes before dieback commenced were probablysimilar on the two pairs of sites (a) with (d), inslope areas protected from prevailing winds, and(b) with (c) on flatter areas.

Soils throughout the transect were yellow-brownloams about 1 m deep but were between 10 and30 cm deep on the high knoll. Gleying was com-mon and local podsols were present under silverbeech.(a) Mature forestTawari forms a dense canopy about 10 m tall withscattered emergent silver beech and miro (Podo-carpus ferrugineus). Scattered horopito, quintinia,toro (Myrsine salicina), and Cyathea smithii forma poorly denned shrub tier at 2-3 m height whichmerges with a low shrub tier at 1-1.5 m containingabundant Alseuosmia macrophylla and scatteredAstelia solandri. Ground cover is dense and con-tains a variety of ferns and sedges. Gahnia spp. andBlechnum capense are particularly abundant inmore open areas.

(b) Older affected forestDead kaikawaka (Libocedrus bidwillii) and scat-tered miro, tawari, and silver beech of about 10 mheight, with partially dead crowns, are present overa dense shrubland 2-4 m tall. The canopy com-prises tawari, quintinia, toro, horopito, broadleaf(Griselinia littoralis), and Coprosma foetidissima.Quintinia or horopito from 1-2 m tall may formsmall dense stands in the general matrix. In moreopen areas a variety of ferns are present and Asteliaspp. or Gahnia spp. may dominate. A well-definedboundary occurs at the grassland.(c) Open grasslandsGrassland is dominated by Uncinia distans, Hei-rochloe redolens, and Microlaena avenacea up to50 cm tall. Spars of kaikawaka ranging from 3-5 mtall are prominent and scattered, windblown neinei(Dracophyllum latifolium and D. pyramidale) reach1-1.5 m. Rubus cissoides, seedlings of kamahi,quintinia, horopito, pate {Schefflera digitata) andmany other species are present within grassland.(d) Unthrifty tawari forestA gradual transition to heavily damaged tawariforest occurs in areas protected from prevailingwesterly winds. The canopy of old stands rangesfrom about 4 m tall in exposed areas to over 10 min protected areas and contains dead and unthriftytawari and miro. Shrubland emerges gradually outof grassland and reaches about 3 m high, 5 m from


0"I"

20Distance (m)

40

Fig. 3 Schematic cross-section of affected and unaffected silver beech stands at Te Hunga. Bearing345° from left to right; altitude 850 m. (a) thrifty, (b) partial mortality, (c) serai stand. Abbreviationsare as for Fig. 2, but add Am, Alseuosmia macrophylla; Bm, Blechnum minus; Ca, Coprosma aus-tralis; Gp, Gahnia pauciflora.

the margin. Horopito and Cyathea smithii form themain canopy but pate, Coprosma australis, Hedy-carya arborea, Rubus cissoides, and Melicytus ram-iflorus are locally common. All species show somesigns of damage. Some ground ferns are present,particularly Blechnum fluviatile.

2. Mature silver beech — "Te Hunga"Almost pure silver beech stands are widespread at"Te Hunga". Mature healthy stands often show agradual transition to unthrifty stands through a zoneof partial mortality (Fig. 3). Soils appear betterdrained in mature stands but localised podsols arecommon in a predominantly gleyed yellow-brownloam up to 1.5 m deep.(a) Mature silver beechThe mature canopy of silver beech is about 25 mtall and somewhat open. Tawari, quintinia, miro,and small silver beech form a sparse subcanopy12 m tall and a sparse ill-defined shrub tier at 2-3 m. This merges with a low shrub tier at 1-2 mheight that includes Cyathea smithii, Alseuosmiamacrophylla, Coprosma australis, C. tenuifolia,numerous sedges and ferns, and abundant bry-ophytes and liverworts.(b) Open standMature forests show a gradual transition to severelydamaged forest. The canopy cover becomes moresparse and trees appear unthrifty, many have deadbranches and broken crowns. Subcanopy tiers arealso more open and ground cover is often domi-nated by dense Gahnia pauciflora or Blechnumminus.

(c) Serai shrublandIn open areas a dense shrubland dominated byquintinia, kamahi, tawari, and Alseuosmia macro-phylla about 1.5-2.5 m tall prevails. Patches ofGahnia pauciflora are common and large clumpsof Astelia spp. occupy canopy openings whereasSphagnum, other mosses and liverworts fill wetterhollows.

3. Silver beech/kaikawaka — "Te Hunga"Stands with a high proportion of tawari are com-mon on flatter areas of "Te Hunga". They have asharp boundary with unthrifty serai stands (Fig. 4).

Soils throughout this transect are waterloggedyellow-brown loams 1-1.5 m deep. For most of theyear surface water appears more common in seraiscrub. However this impression is false since thewatertable is just as high in mature stands as shownby high soil moisture contents (Table 1) but is hid-den by development of perched root systems.Windfallen silver beech often display localisedpodsols.(a) Mature standEmergent kaikawaka, pink pine (Dacrydiumbiforme), and silver beech over shorter statured,subdominant tawari form a dense canopy about 15-20 m tall of elfin nature with no subcanopy. Asparse shrub tier at 2 m height contains seedlingsof tawari, horopito, quintinia, kamahi, and toro. Adense carpet of Schistochila spp. with other liver-worts and mosses frequently covers the ground. Incanopy gaps Pseudopanax colensoi, P. simplex, toro,and tawari form prominent groups.


20i

10

An Q

20Distance (m)

40

Fig. 4 Schematic profile of a silver beech/tawari stand at Te Hunga, bearing 285° from left to right,altitude 850 m. (a) thrifty mature stand, (b) unthrifty serai stand. Abbreviations as for Fig. 2 andFig. 3 but add Db Dacrydium biforme.

Table 1 Soil moisture content and mid-morning temperatures of soils frompairs of thrifty closed mature stands and adjacent unthrifty serai stands at 850 mon Te Hunga taken on 15 Febuary 1982, at the end of 4 weeks without rain.

Stand type

Kaikawaka/tawari12

Means.d.

Silver beech3456

Means.d.

Thiftymoisture

(%)

129.3145.2137.2511.24

66.778.658.762.964.96.5

closed maturetemperature

CO

12.112.212.150.07

12.112.212.612.112.30.4

Unthrifty seraimoisture temperature

(%)

93.961.877.8522.70

54.541.7

_48.953.98.5

(°Q

13.514.013.750.35

12.513.4

-14.013.30.3

- = no data.Soil moisture content of specimens from 20 cm depth, oven dried at 105°C.Soil temperatures taken at 20 cm depth between 10 a.m. and 12 noon. Differencesbetween groups are significant. For soil moisture content F = 70.3, for temper-ature F = 10.0. Of these groups, soil moisture contents in similar vegetation aredifferent between thrifty and unthrifty stands at /><0.5; and temperatures aresimilar for thrifty or unthrifty stands but differ between these groupings at P < 0.05.

(b) Serai shrublandMature forests give way with little transition to adense shrubland 2-3 m high dominated by tawari,silver beech, quintinia, toro, Coprosma dodonaei-folia, and kaikawaka. Spars of kaikawaka and raresilver beech about 15 m tall may be present. A widerange of seedlings species are abundant; Astelia spp.and Gahnia paucijlora fill canopy gaps and Sphag-num and other mosses and liverworts fill wethollows.

Stand dynamicsSurviving mature stands are perched at high pointson the range crest in a zone of high fog occurrence.They provide some information on factors influ-encing stand stability and dynamics. Survivingstands have dense even canopies and soil watercontent is high (Fig. 5, Table 1). The closed standstructure (Fig. 5), maintains low soil temperatures(Table 1) and minimises evaporative losses, andmay be important in maintaining stands. For this


Fig. 5 Structure of fully maturedense stand. Note lack of under-storey and dense tawari canopyabove. View is to the edge of thestand on the transect Fig. 4 andthus has back lighting.

reason, stands, particularly those of dense silverbeech/kaikawaka, may have a high chance of com-plete collapse when mortality begins becausemutually protective effects of stand structure arethen lost.

Mature stands have abundant seedlings butsaplings and poles are rare. In more open standsof silver beech, poles of canopy species are uncom-mon although serai species are present as long-livedshrubs, less than a metre in height. Goats have hada limited impact in these stands (Jane & Green1983c), consequently lack of understory in densestands is probably caused by low light intensities,but in more open beech stands (such as Transect

2a) reasons for a weak shrub tier are obscure. Seed-lings are restricted to better aerated sites on oldstumps and logs raised above the terrain.

Surviving trees are scattered and exposed, as aresult, transpiration rates are probably high andtrees are prone to wind clipping and windthrow.Redevelopment of stands following major damagecommences from fresh seedlings since saplings arelargely absent and changes in the environment ofestablished seedlings may cause severe mortality inthat tier. Partial stand breakdown has a similareffect since established seedlings apparently do nottolerate increased light intensities and water stressfollowing the canopy breakdown.


Table 2 Comparison of leaf damage on silver beech shoots of current and pre-vious years growth between sites 1-3 on Te Aroha on 24 January 1982.

800 mThrifty stand950 mThrifty stand850 mUnthrifty stand

% leaves lostCurrent Previous

nil 6

4 70

22 95

ScorchCurrent

nil

5

12

Necrotic spots(number/leaf)

Current Previous

13 5

92 25

252 230

Counts made on 2 shoots from each of 5 plants from each site.

Composition of stands following disturbance doesnot change radically unless browsing mammals arepresent at damaging densities (as at "Te Rere", Fig.2), since all the major species are present in seraistands. Serai stands contain silver beech, kaika-waka and tawari, dominants in unaffected stands,at densities above, or close to those in mature stands(Jane 1983). Providing mortality is low, this assuresan adequate representation in mature forest. Con-versely, serai species (horopito, quintinia, andkamahi) are present as understory plants or seed-lings in mature stands and provide a nucleus forrapid colonisation following stand damage.

Broken canopy in unthrifty stands and presenceof older emergent kaikawaka and silver beech indi-cate that damage may be repeated with differingintensities on the same site. Plants of slower grow-ing, longer lived species of lesser susceptibilityappear able to attain dominance after each periodof mortality and before serai plants become re-established. Gradual re-establishment of formerforest structure is therefore achieved throughpreferential survival of the main canopy speciesduring minor disturbances to form a new emergenttier.

Characteristic damage on shoots and leavesAt all localities thrifty plants of kamahi, toro, andquintinia appear to retain leaves for two years, andtawari or silver beech for about five years. Unthriftyplants shed old leaves not long after bud break and,in extreme cases, retain leaves for less than oneyear.

Thrifty forest and shrubland stands are quitedense and contain trees with leaves of normal sizeand show no evidence of reduced vigour. Unthriftyforest and shrubland stands contain scatteredemergent gymnosperms and broadleaved treeswhich have many dead twigs and branches. In lowstature, serai vegetation up to 3 m tall, saplings ofkaikawaka and miro normally appear thrifty butbroadleaved plants exhibit varying intensities ofdamage.

Damage to plants on poor sites can be one of thefollowing characteristic types:(1) Severely affected individuals have a stragglyappearance since old leaves are shed rapidly, oftenjust before or soon after bud break, and few leavesmay be retained for more than one year. Damageof this type is typically found in silver beech. Manyleaves were lost from the new shoots of silver beechwithin a few weeks of bud break in the 1981-82summer (Table 2, Fig. 6). Similar heavy leaf falloccurs in red and hard beech (Nothofagus fusca andN. truncata) on Te Aroha. Small leaves with redblotches or necrotic spots are also a common fea-ture of unthrifty silver beech in the study areas.(2) Leaves have reduced size and branches haveshort internodes which concentrate current foliageat branch tips. Damage of this type is typical ofunthrifty kamahi (Table 3, Fig. 6) and also occursin five-finger and sometimes in tawari or neineiwhere it emphasises the usual growth form.(3) Shoots from lower stems may have leaves ofnormal size (similar to that on thrifty plants in thesame locality) and few red blotches, whereas shootsin the upper canopy show progressive reduction ininternode length and leaf size, so that finally newgrowth cannot be initiated and a branch dies. Eachsummer, the first leaf on lower branches of a plantis of normal size but succeeding leaves are smaller.This can be seen when shoots from poor and goodsites are compared (Fig. 6) and, more particularly,when leaves are dissected from a shoot (Fig. 7).This type of damage occurs in unthrifty quintinia,toro, and Coprosma austmlis on poor sites.

Some damage appeared to develop during leafmaturation. Scorch and necrotic spots were evidenton new leaves of silver beech (Table 2), toro, andquintinia in all localities immediately after a periodof foggy weather in January 1982 and 1983. Similardamage was not evident on tawari or kamahi.Scorch resulted in considerable loss of new leavesin silver beech before development was complete(Table 2) and left conspicuous damage on remain-ing leaves. During the period when damage


Fig. 6 Thrifty and unthrifty shoots of the main species. Upper row, thrifty shoots; lower row, unthriftyshoots from nearby sites. Species from left to right are: kamahi, quintinia, tawari, and silver beech.

Table 3 Leaf size, leaf number, and foliated length of shoots from a good anda poor site selected from the upper crowns of all plants. Values are the mean ofcounts on ten shoots of each species/site combination. Samples collected Feb-ruary 1982.

TawariKamahiQuintiniaSilver beechToro

Leaf length(cm)

poor

7.43.84.81.07.6

good

12.18.4

10.71.2

12.2

Numberold

poor

12.411.83.5

27.17.4

good

24.114.814.193.4

9.9

of leavescurrent

poor

21.57.21.23.21.5

good

58.819.522.8

106.77.4

Foliated length(cm)

poor

11.510.3

1.54.52.0

good

26.319.021.315.78.8

occurred leaf cuticle in many plants was glossy andin quintinia was also sticky to touch, compared withthe scaly cuticle of mature leaves, suggesting thatcuticular waxes were incompletely hardened.

Seedling vigourIn mature upland stands seedlings are often clus-tered around old stumps and on logs (Fig. 8). Thisprobably indicates a better potential for survivalabove the wet soil surface in an aerated, but sod-den, bryophyte cover. Seedlings present in poorlydrained thrifty and unthrifty stands are often top-pled by moss and liverwort epiphytes and become

layered, producing a number of vertical shoots.These plants have reduced root systems (Fig. 9).

Fine roots are of strikingly different sizes in spe-cies studied (Fig. 9). Kamahi has thin fibrous rootsconcentrated close to the stem. Quintinia and sil-ver beech have well developed fine roots of moder-ate size and tawari and kaikawaka have stoutprimary roots. Root systems on silver beech seed-lings are well developed and roots are prolific onkaikawaka seedlings (Fig. 9). Root systems on allseedling specimens of tawari in both thrifty andunthrifty stands were much smaller than in theother species (Fig. 9) and when compared with the


Fig. 7 Leaf size variation on a toro plant from a poor site through two growing seasons. The leafon the left is the oldest and the smallest 'leaf to the left of the shoot is a stipule which marks thebeginning of the second year growth.

Fig. 8 Contorted root systems in an open stand. Root systems such as that to the left or bottomright suggest that seedlings originated on stumps or logs.


Table 4 Root/shoot ratios and other characteristics of seedlings from good andpoor sites on Te Hunga.

Poor site 850 mFine root/leafRoot/shootLeaf/stem

Good site 750 mFine root/leafRoot/shootLeaf/stem

Quintinia

1.170.360.58

1.340.470.63

Tawari

0.070.041.58

0.630.241.38

Beech

0.160.091.50

0.680.361.45

Kamahi

0.070.050.39

1.080.480.90

Kaikawaka

---v

1.400.370.36

Mean

0.430.151.49

1.000.390.97

Values are the mean of 10 seedlings of each species, 15-30 cm in height, fromeach site. Parts of each plant were divided into three groups. Shoot = leaf + stem;and root = tap root + fine roots. The poor site at Te Hunga was at the rangesummit and the good site was chosen at a lower altitude to obtain maximalvalues for all species growing in the locality. At the summit site insufficient plantsof kaikawaka could be found at any point to obtain an adequate sample.

Fig. 9 Root systems of seedlings of the main study species. From left to right are: kaikawaka, tawari,silver beech, quintinia, and kamahi. Only a small part of the kaikawaka root system is shown. Thekamahi plant has a strong coppice root to the right, which was cut when collected.


Table 5 Timing of the main bud break and leaf development during the summers of 1981-82 and1982-83.

Summer

800 m Te ArohaSilver BeechTawariKamahiQuintiniaMiro

800 m Te HungaSilver beechTawariKamahiQuintiniaMiro

Bud break1981-82

7/11/8120/11/8120/11/8114/10/8112/12/81

7/11/8129/11/8114/10/8114/10/819/12/81

1982-83

9/11/8219/11/8229/9/822/9/82

19/12/82

20/10/8219/11/8229/9/8229/9/82

19/12/82

Fully developed1981-82

14/12/8114/2/817/11/817/11/8119/1/82

21/11/8114/2/827/1/81

14/10/8114/1/82

1982-83

19/12/822/83

19/12/8219/12/82

19/1/83

19/11/8210/1/83

19/12/8229/9/8219/1/83

Fully hardened1981-82

2/823/82

progressiveprogressive

3/82

3/822/82

progressiveprogressive2/82

1982-83

2/833/83

for seasonfor season

3/83

3/833/83

for seasonfor season

3/83

other species appear to be inadequate to providenutrients to the attached shoot.

Root/shoot ratios of seedlings at upper altitudesites were found to be low for all species exceptquintinia (Table 4), apparently a result of restrictedroot development coupled with heavily woodedstems carrying few leaves. However, in quintinia,high root/shoot and fine-root/leaf ratios were pro-duced by woody shoots with few or small leavesand few fine roots in proportion to primary roots.These quintinia plants appeared to have died backfrom a more vigorous state and a balanced reduc-tion in crown size and fine roots had occurred butnot in the woody material of taproot and mainstems. Disparity in leaf/shoot ratio of kamahibetween good and poor sites resulted from differ-ences in foliage vigour and consequent leaf size.This was readily seen when leaves of plants fromthe lower altitude site were compared with thosefrom a good or poor plateau site (Fig. 6).

Root systems and patterns in mature plants ofupland standsPrimary laterals of most species examined are con-fined to the top 2-5 cm of peaty soil. Root systemsof tawari and silver beech trees are often at the soilsurface and lenticels are frequent where roots areexposed. Trees and shrubs of kamahi, toro, andquintinia are more deeply rooted so that few sur-face or fine roots are present. However, basal swell-ing and enlarged lenticels are quite evident on lowerstems. Conifer saplings are difficult to uproot andthose of kaikawaka have deep, extensive rootsystems.

Stems of broadleaved species within serai standsare clumped, and excavation of saplings frequentlyshows that they have originated from prostratestems which have produced fresh shoots in the deep

bryophyte ground cover and upper peaty soil hori-zons. Older thrifty trees of mature stands are alsoclumped on raised mounds and logs which in somecases have rotted away to leave roots perched abovethe ground (Fig. 8). Between mounds, roots aresparse and soil pits or Dutch auger cores reveal fewroots below the superficial layer. However, even inwetter areas, live roots of kaikawaka can be foundnear bedrock at depths of about 80 cm.

PhenologyBud break typically begins in kamahi and quintiniain early October and a month later in tawari andsilver beech at 500 m altitude sites (Green & Jane1983b). In each species, bud break begins at thehighest altitude about 4 weeks later. Bud break onadjacent trees of the same species at higher alti-tudes occurs over a period of several months andat least some trees may take advantage of delayedonset to put on increment in adverse years.

A large difference in bud break date at the samesites was noted between tawari and silver beech inboth 1981-82 and 1982-83 (Table 5). On Te Arohathe flush on all trees occurred more or less uni-formly and was completed by early December butat "Te Hunga", it was less well defined and for afew trees it occurred well into February. Delayedand irregular flush at "Te Hunga" appeared to berelated to lower temperatures recorded during a 6week period of wet and cloudy weather from mid-November 1981 to early January 1982 and cloudyweather in the summer 1982—83 (Jane & Green1984) whereas Mt Te Aroha remained clear formuch of this period.

The sequence of leaf production in the speciesstudied partly explains the damage patterns char-acteristic of the species. Miro, toatoa (Phyllocladusglaucus), tawari, and silver beech produce a single


whorl, or a short shoot of ten or more pairs of lea-flets which develop within a short period and sothere is no variation in leaf size within a shoot.Kamahi, toro, and quintinia produce single leavesor pairs of leaves throughout the season, and growthmay be affected by varying environmental factorsthrough the season and modified by the contrastbetween good and poor sites.

At Te Aroha leaf development and hardening insilver beech is rapid, but on Te Hunga it is slowerparticularly on poor sites. This makes the thinleaves particularly susceptible to adverse climaticconditions. Leaf maturation on vigorous plants ofsilver beech is followed by progressive elongationof leading shoots. Leaf development in tawari ismuch slower, so that elongation and maturationmay not be complete until late February (Table 5).The slow leaf maturation in these thick leaves maymoderate the adverse effects of climatic variations.

The main leaf fall of old leaves on poor sites at"Te Hunga" summit was observed in a wide rangeof species in January 1982 within two weeks of budbreak, and before new leaves were mature. Spec-tacular leaf falls were observed on red beech at TeAroha in both years. Here whole trees were den-uded at bud break. Leaf fall in most species wasnot conspicuous at altitudes below 700 m in thestudy area. Tawari was the exception. Leaf fall inthis species occurred in November 1982 and March1983 following several weeks of dry weather.

New root growth began at different times in thetwo study years on both sites. New root growthbegan between February and March 1982 (earlyautumn), but was evident in all species and on allsites by January 1983 (early summer). The differ-ence could have arisen from lower soil moisturecontents following a dry winter and spring in 1982as shown in soil moisture content change in asso-ciated projects (Jane & Green 1983d, Jane & Green1984).

DISCUSSION

Distribution of vegetation damageManion (1981) has produced a general outline ofdecline syndromes in forests. Both biotic and abioticfactors may be involved but an important featureof the development of the syndrome is the sequencein which factors affect the forests. In particular,droughts are regarded as important inciting factorswhich precipitate a mortality event in forests thathave already been predisposed by other site factors.

Examination of particular stands in the KaimaiRange showed that no consistent link is presentbetween dieback areas and stand composition orstructure. Dieback occurred on flat ground (Tran-sect 3) or on slopes (Transect 1) and without any

evidence of a change in species composition (Tran-sect 2). Similarly, soils types vary between andwithin affected areas both in depth and type. Thus,on Transect 1, mortality occurs on soils both 1-1.5 m deep at the slope foot and soils 0.1-0.3 mdeep on the knoll with a transition from moreaffected areas to areas now in grassland both onthe slope and at the slope foot. It occurs on deepsoils (Transect 3) and shallow soils (Transect 2).The main difference between transects lies in dam-age intensity. In tawari forests boundaries of affectedareas are sharp whereas a buffer of partially affectedvegetation is present in silver beech forest (Tran-sect 2). Distinctness of damage boundaries is oftena characteristic of drought-damaged areas.

Soil water contentForests in the dieback zone appear to be sensitiveto even slight water stress (Jane & Green 1985), asensitivity which is almost certainly a consequenceof reduced root systems in waterlogged soils. It isalso in this zone that fog both increases precipi-tation and reduces evapotranspiration, makingvegetation particularly sensitive to droughts.

Study of soil moisture content in waterloggedsoils when drought was absent but during a rela-tively prolonged dry spell yielded limited infor-mation on soil moisture differences between thriftyand unthrifty stands (Table 1), probably becauseimportant differences are only evident at times ofextreme water stress, during drought. However,longer-term data over two study years showed thatupland soils in the fog zone are almost perpetuallywaterlogged but were below field capacity at loweraltitudes (Jane & Green 1983c). The impact of soilwaterlogging can be seen in growth characteristicsof the species.

Marked reduction in leaf size, prominent lenti-cels, and basal swelling in seedlings of kamahi andquintinia are characteristic of plants undergoing, oradapted to, prolonged flooding or waterlogging andhave been reported from many species (Hook et al.1972, Kozlowski 1976, Coutts & Armstrong 1975,Bradford & Yang 1981, Kawase 1981). Large num-bers of lenticels in tawari and silver beech and thinbark in kaikawaka are also important adaptationsseen in other species growing under waterloggedconditions (Kozlowski 1982). In many specieswaterlogging may intensify symptoms of waterstress (Kawase 1981), reduce overall vigour, reduceroot/shoot ratios, and produce marked differencesbetween adjacent trees because of minor site differ-ences (Levitt 1972). All these features are noted inserai species such as quintinia, kamahi, and torostudied here.

General poor vigour and high susceptibility towater stress in kamahi and quintinia on upland sites


(Green & Jane 1983b) and intensification of symp-toms in more foggy areas, as noted here, suggest alow tolerance to flooding or waterlogging.Kozlowski (1982) recognised a close link betweenwaterlogging and water stress resulting from lowrates of water transport from reduced root systemsand poor root growth limited by anaerobic soilconditions. In physiological studies it was shownthat there is a strong sensitivity to water stress inserai species at the height of a normal summer, asmight be expected from plants undergoing water-logging (Green & Jane 1983a, b; Jane & Green1983e).

Two strategies to minimise the effects of water-logging appear to be present. First, plants such askaikawaka, show deep vigorous root growth becauseof tolerance to anaerobic conditions; and second,shallow rooted plants, such as silver beech andtawari, avoid anaerobic conditions. Serai speciesare deep rooted but intolerant of anaerobic con-ditions. This leads to poor root development inwaterlogged soils and higher drought sensitivity.Dendrochronological data suggest that growth ofthese plants improves rapidly as watertables fall,in drier conditions, but is halted when watertablesrise again (Jane 1983).

Although trees of mature stands are small (Tran-sect 3) and appear to be young and vigorous, infact, they are several hundred years old and slowgrowing. They are probably protected from severewater stress both by closed stand structure, withconsequent cooler stand interior and reduced windturbulences, and by location in areas of higher fogfrequencies.

Role of fogThe altitudinal zone in which forest damage isfound has been shown to coincide with the fog zone(Jane & Green 1983b). Fog would lead to lowerlight intensities and may reduce total carbon fixa-tion to marginal levels and limited seedling sur-vival in mature upland stands (Huber 1978,O'Rourke & Tejung 1981). It may also affect leafdevelopment. Leaf damage and leaf fall, particu-larly on silver beech during spring, appeared to berelated directly to long periods of foggy weather.Development of new leaves in fog forests alsoappeared to be completely halted particularly asnoted in January 1982 and 1983 at "Te Hunga".Lowered temperatures and light levels slow leafdevelopment and maturation (Kozlowski 1971), andmay impair stomatal development (Brainerd &Fuchigami 1982). Prolonged fog and cloud followedby fine weather produces strongly contrasting envi-ronmental conditions (Leigh 1975). Fine hot daysin January, exposing new leaves with unhardenedcuticles to the full force of almost overhead mid-

summer sun, accentuates water stress (Green & Jane1983b) and may cause scorch and leaf fall in thinleaves. Water stress has frequently been noted tocause leaf scorch and leaf fall in waterlogged plants(Pereira & Kozlowski 1977). Reduction in leaf size,observed in silver beech, kamahi, toro, and quin-tinia, is also similar to that resulting from waterstress in plants growing on waterlogged soils (Pal-lardy 1981, Syvertsen 1982). Water stress similarto that noted in stomatal conductance (Jane &Green 1985) and water potential measurements inthe study area (Green & Jane 1983b) may also trig-ger a major leaf fall as in other species under water-logged conditions (Kozlowski 1971). Shoots ofplants such as kamahi and quintinia, with damagednew leaves, have reduced photosynthate reserves(Green & Jane 1983a). There are also few leavesto sustain growth and produce further new leaves(Fig. 7). As a result, the shoot may show theobserved decreasing leaf size through the season.Low soil temperatures in denser stands, as observedin 1981-82, may intensify damage by delaying rootgrowth until after bud break (Tesky & Hinckley1981) and could produce the reported root/shootimbalance (Table 4).

Red beech, which is common only on Mt TeAroha, serves as an example of severe fog impactin the study area. It occurs at 600-750 m a.s.l.,below the fog zone, on Mt Te Aroha, the study areawith lower fog frequencies (Jane & Green 1983d),and it was noted to suffer severe spring leaf fall.Red beech was not noted elsewhere in the KaimaiRange. It appears possible that high fog frequenciesat the same altitude in other study areas at "TeRere" and "Te Hunga" prevent its presence byaffecting leaf growth and survival and reduces itscompetitive ability. Similarly, Elder (1965) notesthat red beech is confined to areas with lower fogfrequency in Ruahine Ranges.

CONCLUSIONS

The common factor linking dieback areas appearsto be water stress. Forests in the mortality zone aresensitive to even slight water stress (Jane & Green1985), almost certainly a consequence of reducedroot systems in waterlogged soils. Increased pre-cipitation and reduced evapotranspiration, conse-quences of high fog frequency, also make plantsparticularly sensitive to drought.

The role of soil moisture stress in mortality iswell recognised but the association of dieback syn-dromes with soil/water problems is less commonlyrecognised. More often the dieback syndrome is firstrecognised as a pathological problem associatedwith root saprophytes and weak pathogens such asArmillaria mellea or Phytophthora cinnamomi.


Work by Newhook (1959), Podger et al. (1980),Weste (1981), and Helliwell (1983) and many othershas gone far towards recognising the more complexecological links in forest dieback syndromes.

A firm general framework for these syndromes,involving previous forest history, intricate ecolog-ical links between climate, soil moisture and dis-ease, has been succinctly enunciated by Manion(1981). Both biotic and abiotic factors may beinvolved but the time sequence of their occurrenceis very important. In particular, droughts areregarded as common inciting factors precipitatingmortality events in forests already predisposed byother site factors.

The data, taken together, suggest that for forestmortality to occur in the Kaimai Range severalconditions must coincide:

(1) There must be a high watertable producingwaterlogging.(2) There must be a slow fall in watertable, per-haps over several years (in widespread mortalityepisodes). During the early phase of a dry periodplant growth may be vigorous, particularly insensitive serai species.(3) A subsequent rise in watertable produces soilflooding and waterlogging resulting in consider-able root death in sensitive species, a conse-quent decline in vigour, and formation ofmaladjusted root systems.(4) A further dry period will then produce severewater stress and widespread mortality.

Such a sequence would give rise to the wide-spread mortality which occurred in 1914-20 and1940-48 and possibly 1807-1814 (Jane 1983). Ashorter sequence of dry years would cause mortal-ity in sensitive serai species and produce the symp-toms already outlined for unthrifty plants.

ACKNOWLEDGMENTSThe authors particularly wish to thank the referees forconstructive comment. Thanks are also due to R. Julianthe photographer and Mrs D. Whitehead the draughts-women. Dr T. G. A. Green received support from theUniversity Grants Committee for equipment purchase.

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etiology of forest dieback areas within the kaimai range, north …€¦ · waterlogging; symptoms;...

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