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Dieback in New Zealand Nothofagus Forests! Pacific Science (1983), vol. 37, no. 4 © 1984 by the University of Hawaii Press. All rights reserved 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.
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