Dieback in New Zealand Nothofagus Forests!

J. A. WARDLE 2 and R. B. ALLEN 2

ABSTRACT: Dieback has been observed in New Zealand Nothofagus forests for some time, and a number of causal factors have been recognized. Some understanding of the effects of dieback on forest structure has been gained in a study of events after snowfalls had caused partial damage to an area of mountain beech forest. The results of this study are used to interpret the structure of beech forests elsewhere in New Zealand.

Nothofagus FORESTS in New Zealand are branch ofthe Rakaia River in inland Canter generally associated with mountain environ bury. These forests, which clothe the upper ments and are consequently subjected to valley between 650 m altitude and the sub frequent disturbance, often ofclimatic origin. alpine timberline at about 1350 m, cover an The effects of these are usually local and area of 5000-6000 ha. The forest is simple contained, but sometimes a relatively minor monotypic Nothofagus solandri var. cliffort event may predispose the forest to damage by ioides (Hook. f.) Poole (mountain beech) and other, often biotic, agents causing extensive there are virtually no other tree species dieback of the forest canopy. As yet, there is (Figure 1). little information on the long-term effect of Two hundred and seventeen permanent 2 canopy dieback on forest structure or on the plots, each 400 m , were established through characteristics that make one stand vulner out the Harper forests during the 1970-1971 able and another not. An understanding of summer (before the second snow storm in these factors should make it possible to 1973). In each plot, all stems of mountain predict the susceptibility ofstands to dieback. beech were measured for diameter at breast Such an understanding is important in the height and permanently tagged with numbers management of these forests. for identity, giving a total of about 21,000 tagged stems. Seedling subplots were estab lished within each permanent plot to estimate HARPER STUDY seedling densities by height classes. These Two heavy, moist snowfalls in 1968 and permanent plots provided an opportunity to 1973 caused extensive damage to the beech gauge the impact of the snow storms, partic forests in some parts of Canterbury, New ularly the 1973 storm, on the forests of the Zealand. Up to 35% of the trees on the catchment as a whole, and to study the southern facing slopes in certain areas were long-term effects on forest structure. Each of seriously damaged. Most of this damage the plots has been remeasured four times at involved canopy breakage, but uprooting, intervals of about 2 yr. tilting, and main stem breakage occurred as Significant structural damage from the well. Main stem breakage involved trees up two snow storms was recorded from nearly to 20 cm or sometimes even 30 cm diameter. 30% of the 217 plots. The mean basal area of Both snow storms caused extensive damage the Harper forests decreased from 51.9 m2 /ha to the forests of the Harper catchment, a to about 50.7 m2 /ha between 1970 and 1974, a l~ss of about 2.3%. Much of this initial loss could be attributed directly to mechanical I N.Z.F.S. No. I685jODC 182.2. Manuscript accepted 5 October 1983. damage from the 1973 snow storm, but there 2 Protection Forestry Division, Forest Research Insti would also have been continuing mortality tute, P.O. Box 31-011, Christchurch, New Zealand. associated with the 1968 event. By the time 397 398 PACIFIC SCIENCE, Volume 37, October 1983

FIGURE l. Interior of mountain beech forest. Note general absence ofother species. of the last measurement in the 1980-1981 nounced in the forests on the lower slopes summer, the basal area was still continuing and river terraces, and many of the stands to decrease substantially (Figure 2) and the near timberline apparently escaped un mean had been reduced to 46.2 m2/ha. This scathed. Subsequent mortality was mostly represents a net loss, over and above incre associated with the areas where snow damage ment, of nearly 6m2/ha, or, in other words, was most severe, but there are recent indica II%over the whole catchment, and it is ex tions of increasing death of canopy trees in pected from present trends that the basal area stands that escaped the initial damage. The ofthe forest will continue to decrease further. mechanisms involved here have yet to be Relatively few of the trees that escaped elucidated. discernible snow damage were responding in A large increase in seedling density has terms of growth to the reduced competition occurred in the catchment since the snow associated with these lower basal areas. storms (Figure 2). This increase has been Indeed, by 1980-1981, many of them were especially pronounced in the larger seedling also dying. The incidence of wind damage categories. In 1970-1971, seedlings ofmoun throughout the catchment has increased tain beech between 76 and 135 cm high were since the snow break occurred, and un rare, averaging only about I/ha. By 1980 doubtedly this has accounted for some ofthe 1981, these had increased to about 43/ha. losses. Seedlings between 46 and 75 cm high in The initial snow damage was most pro- creased tenfold from about 34/ha to 390/ha,

I lQ3N",s'a:uaml @ltIUlilil1lll1iiJDg@;!iiid'.· Dieback in New Zealand Nothofagus Forests-WARDLE AND ALLEN 399

1973 Snow-fall

100 ,!' 4000 .

>- ~ III , c:: 3000 Q) c .: 0 CI OJ .;: c:: 0 '0 .. Q) 0 Q) - 90 2000 CIl ~ .... c:: .,' , III .. Q) Gl III .. C1l 0( .... Q) .. ' 1000 ..u III .. ' II) ...... • ..: III ... 0·········· al I ••••• • . • •••.0•••••••••••••• ~ .- , . .... ,::...:.:*:.::.----.-----.-----..----_. 80 0 1981 Year

76-135cm Seedlings 46-75cm Seedlings 15-45cm Seedlings

FIGURE 2. Structural changes associated with heavy snow storms in the Harper forests. The diagram shows the mean reduction in basal area ofthe forests over the whole catchment and growth responses in three height categories of mountain beech seedlings. while the smaller seedlings (from 15 to 45 cm be attributed directly to the mechanical high) increased by little more than double, effects of the 1973 snowfall. In the un from 670 to 1680jha. damaged plots there was loss during this Twenty-eight of the Harper plots suffered period from the smaller size classes, but a notable damage only during the 1973 snow slight increase in the larger ones. This loss storm. These, with a further 28 randomly can be attributed partly to competition selected from the plots which showed no related mortality, and partly to through damage either in 1968 or 1973, were studied growth to the larger size classes. There was further to evaluate changes in stand structure no compensating recruitment into the smaller resulting from the snow damage. Final size classes from seedlings, since mountain evaluation awaits more detailed analysis of beech is a light-demanding species and stands the plot data. have a tendency to assume a normal bell The damaged plots showed a reduction in shaped diameter distribution as they develop. all stem diameter classes, particularly in the The smallest class (2-1O-cm diameter) gen smaller classes, between the 1970 and 1974 erally continued to show an annual loss of measurements (Figure 3). Much ofthis could between 4% and 8% throughout the study 400 PACIFIC SCIENCE, Volume 37, October 1983

Plots with No Record of Damage. 10 ....:...... ::...... -----~~., •••~~.~~~~n:~~...... ::"':"'9 ----- • .--~----_• o ••••:. ~-' ·1····- •• ••• .....:.r...r-;* ·-~ ... -. ....----- '" ... CIl .-..7 CIl -E -10 .!!! o "- III -204------r-----,..-----..----.------r------...., E CIl en- 1970-74 -o 1974 -76 ... 1976 -78 CIl J:l 1978-80 E zj Plots with Record of Damage. 10 CIl en c co ~ t> --....:~'. ••r:-~~~~_ -':-::;Tw.~ .....-:: _ _~ - o .. -.. ..- ~...... , .fIIII"""'-- -. . ~.". .. ~ ...... -. -.. ~ co ~~ j -10 .- c ..~- c .... -- ...... « ...... c '. co -20 CIl ~ ...... •.....

-30 2 10 20 30 40 50 80 Diameter Class (em)

FIGURE 3. Comparison of structural changes that have occurred in snow-damaged and undamaged forests in 2 the Harper branch of the Rakaia River. The data on each graph are based on 28 permanent plots, each 400 m . period in both damaged and undamaged of measurements (1978-1980). By this time, stands, presumably because of through the surge of seedlings released by the loss of growth and intraspecific competition. How basal area due to the snow break and related ever, the number of stems in the damaged events was beginning to appear as ingrowth plots actually increased between the last pair into the 2-1O-cm class.

@ Ji& Dieback in New Zealand Nothofagus Forests-WARDLE AND ALLEN 401

The maindifferences between damaged and severe where the trees were oflarge diameter, undamaged plots occurred in the > 40 cm and the only areas offorest that escaped were size classes. The number of stems in these some isolated pockets that had originated larger classes showed little change between after recent disturbances. measurements in the undamaged plots, with Similar catastrophic mortality in mountain 50.9 stems/ha in 1970-1971 and 53.6/ha in beech forest occurred in Tongariro National 1980-1981 on the average. However, the Park, and circumstantial evidence linked this number oflarger stems surviving in the snow with a period of relatively low precipitation damaged plots changed dramatically. During (Skipworth 1981). There is also an example the same IO-yr period, the rate of mortality of extensive recent dieback in Nothofagus of these trees has increased progressively, fusca (Hook. f.) Oerst (red beech) after a until at the time of the last remeasurement, period of low rainfall in the Maruia and only 21.4 of the original 61.6 stems/ha were Inangahua valleys near Springs Junction, alive. Many, in fact probably most, of these west of the Main Divide (c. Gleason, larger trees that have died showed minimal unpublished). evidence ofdirect mechanical snow damage. These are recent examples, but the dieback This study indicates that a disturbance phenomenon in New Zealand beech forests such as the 1973 snow storm can cause a has long been recognized. Early records of reduction in the diameter range, and presum apparentlyexcessive mortalityin beech forests ably therefore the age range, of the stems in date back at least to the first decade of this the affected forest. The damage synchronizes century (Cockayne 1908). Extensive dieback mortality in the larger, older cohorts of the affecting over 2000 km2 of the beech forests population, and also encourages synchronous in the Buller area was noted after the regeneration. In this way, the range of ages Murchison earthquake of 1929 (Rawlings in anyone stand periodically becomes re 1953). Extensive dieback in beech forests was duced, so that there is a tendency toward the also reported after droughts in the 1945-1946 development and perpetuation of an even summer in the central North Island (Elder aged, or partial-aged, structure. 1962, Grant 1963, Hocking 1946). Many of our beech forests, especially in the drier mountain ranges, show structural evidence of synchronous regeneration. Many BEECH FORESTS IN OTHER AREAS are composed of even-aged, or possibly two Where events in the past have led to a aged, mosaics. Frequently, the sizes of these forest structure with stems uniformly large uniform stands are quite large, and some in diameter over extensive areas, we might times there has been more or less synchronous anticipate that an event such as a heavy snow development over entire mountain ranges or storm could lead to mortality rates of valley systems. catastrophic proportions. Apparently, this The mean structure of the mountain beech has happened in another tributary of the forests of the entire Wairau and Waitaki Rakaia, the Moa catchment. In this catch catchments (two large catchments in the ment, most of the trees in the 1000 + ha of eastern South Island) was calculated from mountain beech forest have died. From large numbers of sample plots of the type various unpublished accounts, death in the described for the Harper study. The distribu standing trees seemed to originate in the tion of basal area by diameter size classes for upper reaches of the valley and then pro these mountain beech forests were compared ceeded downstream over a number of years. with those for mountain beech forest of the Extensive areas of wind-damaged forest in Harper catchment (Figure 4). Quite obvi the upper valley is the probable originating ously, the structure of the mountain beech point for the dieback. The average stem size forests in these three areas was very different. in the Moa catchment was large, indicating From the data collected in the Harper catch that the forests were old. Mortality was most ment, a plausible explanation is that the dif- 20 Mountain Beech Silver Beech

16 16 Wairau

-iUIV •.•• Harper 12 "'1"'1I ••• ~ -'00 12 ~ - r L.....::7.:!-!-,, -0 I : I " k" ~ J : L:IWaitakiWalta I I- 1:L:, 1 ...... 8 ...... !;II - ....,...JJ toIV I G)CIl .... I L, -1 ...L- .... I I ,...1,-:-I~.. et< _J .....! L, I ITararua I I toIV 4 I I i ,-.r·!.-.r·! III I -, toIV J '--1....-1 ! InCD I s 1..""-,'-"'L. , ...J..l .... -,...... O-F....l0 .... 0 20 40 60 80 100 o 20 40 60 80 100 120 Diameter Class (em)

FIGURE 4. Distribution of basal area by diameter classes for all mountain beech stems in the Wairau (249 plots), Harper (214 of the 217 plots were used), and Waitaki (158 plots) catchments; and for all silver beech stems in the Wairau catchment (127 plots), Waitaki catchment (lOS(105 plots), and Tararua mountain range (85 plots). In each case, the structure is based on analysis of diameter class frequency distributions from 400 m2 permanent sample plots. " Dieback in New Zealand Nothofagus Forests-WARDLE AND ALLEN 403 ferences were associated with synchronous while in another forest, similar or even mortality of canopy trees after disturbance greater damage has relatively little effect? to the forest. The Waitaki forests were biased First, the stand must be at a susceptible toward the larger size classes and were stage of development. It is suggested here probably relatively old, whereas, the Wairau that old, even-aged stands are particularly forests had a greater representation in the susceptible, while younger stands, or those smaller size classes. These forests probably with a greater mixture of age classes are originated relatively recently. The Harper relatively immune. There is some evidence forests were intermediate in structure, and from beech thinning trials that dense, com probably also age. peting pole stands represent another suscep Data from the Harper study, as well as the tible stage in the developmental history of observations from the Moa, indicate that it the stand. Late thinning often leads to wind was the larger-diameter and presumably older instability, poor growth rates on the retained trees in the stand that were most susceptible trees, and excessive mortality. to the dieback phenomenon. Therefore, we The second requirement is some form of might prediCt that the Waitaki mountain disturbance or stress·. Examples include the beech forests are particularly susceptible snow damage in the Harper forests, possible because they are generally old, and that some wind damage in the Moa, drought in the physical or climatic disturbance might well Maruia and central North Island, and earth lead to extensive dieback. The Wairau forests quake damage in the area surrounding are relatively young and therefore not so Murchison. There are others. Dieback has susceptible. been associated with rising water tables after Nothofagus menziesii (Hook. f.) Oerst damming of lakes for the production of (silver beech) does not usually show the hydroelectric power (Mark, Johnson, and degree ofstructural differences between areas Wilson 1977) and with felling operations. shown by mountain beech. The distributions The third ingredient seems to be a popula of basal area by diameter classes of silver tion outbreak of some pests or pathogens in beech forest in the Waitaki .and Wairau response to the stressed condition ofthe trees, catchments were compared wIth those for the buildup of dead or dying plant material, the Tararua range in the southern North or for some other reasons. Island (Figure 4). The structure in all three The beech, buprestid, Nascioides enysii, areas was remarkably similar. Silver beech and the Armillaria root rot fungi have been is relatively shade-tolerant in contrast to the most frequently cited pest and pathogen mountain and red beech, which are strongly associated with beech mortality (Morgan light-demanding species. Silver beech tends 1966, Rawlings 1953), but it has subsequently to develop mixed-aged structure, even in been found that outbreaks of Nascioides are stands that have originated at one time secondary (J. S. Dugdale, unpublished). This following disturbance, so that there is not insect is an inner bark feeder which, as far as the same potential for dieback to occur is known, does not transmit toxic substances synchronously over large areas. Or pathogens to trees. It requires the death of the host tree to complete its life cycle, but does not seem capable of killing the trees itself. It now appears that most ofthe mortal DISCUSSION ity formerly attributed to Nascioidesin A number of examples of canopy dieback beech forest was caused by the fungal in New Zealand beech forests have been pathogen, Sporothrix, which is often intro described, and an attempt has been made to duced to the tree by ambrosia beetles of the illustrate its significance in stand dynamics. genus Platypus (Faulds 1977, Milligan 1972, What ingredients must be present for dieback 1979). This combination ofinsect and fungal to occur? Why does relatively minor damage pathogen has been demonstrated experi lead to extensive tree mortality in one forest, mentally to be capable ofkilling even healthy 404 PACIFIC SCIENCE, Volume 37, October 1983 beech trees, and to be attracted to attacked GRANT, P. J. 1963. Forests and recent trees by aggregating pheromones. It is often, climatic history of the Huiarau Range, if not usually, associated with the dieback Urewera region, North Island. Trans. R. phenomenon. Soc. New Zealand Bot. 2: 143-172. Undoubtedly, other pests and diseases may HOCKING, G. H. 1946. Drought in Hawkes be involved. Armillaria has been demon Bay. New Zealand J. For. 5:230-231. strated to attack beech (Campbell 1962), and MARK, A. F., P. N. JOHNSON, and J. B. high concentrations of fruiting bodies have WILSON. 1977. Factors involved in the been observed after forest disturbances (L. H. recent mortality of plants from forest and Roth, unpublished; W. Silvester, personal scrub along the Lake Te Anau shoreline, communication). The scale insect Inglesiafagi Fiordland. Proc. New Zealand Ecol. Soc. has also been associated with mortality 24:34-42. (D. Kershaw, unpublished). However, the MILLIGAN, R. H. 1972. A review of beech evidence for the association ofeither of these forest pathology. New Zealand J. For. genera is still circumstantial. 17: 201-211. ---. 1979. Platypus apicalis White, Pla typus caviceps Broun, Platypus gracilis Broun (Coleoptera: Platypodidae). The native pinhole borers. Forest Research LITERATURE CITED Institute, New Zealand Forest Service, Forest and timber insects in New Zealand CAMPBELL, E. O. 1962. The mycorrhizae of No. 37. Gastrodia cunninghamii. Trans. R. Soc. MORGAN, D. F. 1966. The biology and New Zealand Bot. 1: 289-296. behaviour of the beech buprestid, Nas COCKAYNE, L. 1908. Report on a botanical cioides enysii (Sharp) (Coleoptera: Bupre survey of the Tongariro National Park. stidae) with notes on the ecology and Govt. Printer, Wellington, New Zealand. possibilities for its control. Trans. R. Soc. ELDER, N. L. 1962. Vegetation of the New Zealand Zool. 7: 159-170. Kaimanawa Ranges. Trans. R. Soc. New RAWLINGS, G. B. 1953. Insect epidemics on Zealand Bot. 2: 1-37. forest trees in New Zealand. New Zealand FAULDS, W. 1977. A pathogenic fungus J. For. 6:405-412. associated with Platypus attack on New SKIPWORTH, J. P. 1981. Mountain beech Zealand Nothofagus species. New Zealand mortality in the West Ruapehu Forests. J. For. Sci. 7: 384-396. Wellington Bot. Soc. Bull. 41: 26-34.

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