RAWLINGS: ECOLOGY OF Pil1uS radiata

ENTOMOLOGICAL AND OTHER FACTORS IN THE ECOLOGY OF A PLANTATION

G. B. RAWLINGS

Forest Research Institute, Rotorua

INTRODUCTION will be described and their effects traccd from thc timc of planting in 1922. The details Kaingaroa Forest contains approximately apply only to arcas of similar age and site 115,000 acres of Pil1us radiata D. Don. Of this and must not be taken to apply to areas acreage there are 15,000 acres on which the on other sites or during diffcrent climatic trees are so poor that the areas have been cycles. Only those factors which appcar to scheduled for conversion to other species. havc been of grcatest importance will be These trees are short and heavily branched dealt with. No doubt many events have and many have multiple leaders; these areas passed unobserved or unrecorded or have were mostly understocked in the early been misunderstood at the time. In retro- stages. The reasons for this condition are spect the exact influence of anyone factor to be found in the climatic factors, such as can only be inferred. No sequence of events frost and wind, and in the soil conditions, has yet been cxactly repeated and probably which result from the high elevation, clima- never will be. The prcsence of thc forest, tic factors and poor drainage. These soil apart [rom all other factors, has so changed conditions were indicated in the original thc site conditions that nothing but rcpeated flora by the presence of Dracophyllum subu- fires, or another eruption, could bring about laturn Hook f. The adverse site factors, par- a return of the conditions present before ticularly frost damage, predisposed the trees planting commenced. Planting has now been to attack by fungi such as Phomopsis strobi replaced, to a large extent, by natural re- Syd. and Diplodia pinea (Desm.) Kickx. It generation and the resulting stands differ might be possible to grow a commercially in many featurcs from the forests first estab- acceptable crop of P. radiata on these sites lished. after they had been drained and treated with The site under consideration is at an ele- fertilisers so as to improve the soil and vation of about 1700ft. at latitude 38deg. increase the frost hardiness of the trees. For 20min. S. a forester however it is easier to change the species than to modify the site. The remaining 100,000 acres consist of SOl L FACTORS about 50,000 acres classed as fairly good and 50,000 acres classed as good. The fairly Before planting commenced, the area good sites contain many trees with double under consideration was covered by man uk a leaders or otherwise of poor form. Trees scrub up to 15ft. high, or by bracken fern. on these sites suffer severely from wind dam- The vegetation was burnt and the soil was age and are liable to attack by needle fungi destroyed where manuka had stood. The such as Lophodermium pinastri (Sch. ex Fr.) following is a soil description of a plot in Chev. and Naemacyclus niveus Sacc.; the Compartment 1125 near the southern limit predisposing cause appears to be excessive of thc basaltic lapilli from the Tarawera humidity, especially during hot wet sum- eruption. mers. Trees on good sites are subject to I. On surface, 1 in. liuer of ncedles, pollen cones, damage by wind and fungi but the resultant twigs and small branches. crop is superior both in quality and volume. 2. 2~3 in. of dense black loamy sand consisting ')f hasa1tic lapilU from the Tarawera eruption of 1886 The main factors influencing the develop- and the remains of soit typical of bracken fern, pH ment of a stand on one of the best sites 4.9. 48 NATIVE AND INTRODUCED SPECIES IN NEW ZEALAND

3. 17 in. of pumice from the Kaharoa eruption in about the year A.D. 1000. There are two distinct zones: first, 9-10 in. dark (reddish) brown, grading into lighter brown. gravelly sand; then 7 in. banded gravelly sand and coarse loamy sand. 4, The fossil soil which formed on the ash beds of the Taupo eruption about] 20 A.D.made up of 5 in. dark brown grading into lighter brown coarse sandy . loam and 5 in. grey coarse sand with Taupo pumice fragments. This appears to be water-carried and may be a post-Taupo lake bcd. There foHows 15 in. consisting of I in. orange red coarse sand, 12 in. yellowish brown fine, grading to coarse, pumice gravel and 2 in. gravelly sand. 5. At greater depths coarse sandy loam of older ash showers continues to the underlying ignimbrite rock. Roots probably penetrate as far as the rock and have been found as far down as holes have been dug. The soil material is all acidic except for the Tarawera lapilli which have not yet commenced to disintegrate. However the soil is reasonably fertile but low in pot- ash and phosphate. Water percolates readily through the soil in most places. The water- holding capacity of pumice is high.

CLIMATIC FACTORS It is a common fallacy to suppose that heavy rainfall and. wet years are good for trees. P. radiata is adapted to a mild climate with a rainfall of 25-30in. and a dry summer. FIGURE 1. P. radiata 42 years old, showing The rainfall at Kaingaroa ranges from 32 to strong effects of wind all crowns iH over- 77 inches with an average of 58 inches, of stocked stands. which it is estimated that only about 30in. can be u tilised by trees; the remaining 30 the trees have little room for crown expan- or 40in. leaches salts from the soil and sion and, as they become taller, the arc favours fungi which attack the foliage. Sum- through which the crown moves, in even a mers having up to 30 days without rain are modcratc wind, becomes considerable. The beneficial for P. radiata. Dry summers with trees do not move together, as a field of the equivalent of up to 50 days without rain wheat, but in such a way that two ncighbour- do no apparent harm, but more intense ing trces generally are moving in opposite drought than this has the effect of intensi- directions and thcrc is a continual clashing fying competition, giving dominant trees together of branches. As a result each crown an advantage over smaller trees. Following occupies the centre of a circular area which the 1946 drought, in which there was the increases each year and which represents equivalent of 60 days without rain, an epi- the area moved over during the frequent demic o[ Sirex noctilio Fab. caused wide- moderate, and the more rare severe, winds spread mortality among the smaller trees which occur in this district. The frequent in the P. radiata stands. In stands of Douglas violent contact between crowns injures fir (Pseudotsuga taxifolia) no killing agent branches and growing points, killing buds was present and no spectacular mortality and giving access to fungi, particularly resulted, but the advantage given to the dom- Diplodia pinea, which kill back the leaders inants by the drought is now evident. and branches. Any tree which has become ovcrtopped by its ncighbour cannot grow Wind has been the most important of thc upwards without having its leader killed climatic factors. Planted at 8 x 8ft spacing back by the lower branches of the taller RAWLINGS: ECOLOGY OF Pi/JUS radiata 4"~ tree. As a result of this sway, crowns in no species has caused appreciable mortality crowded' stands are small, branches tend or damage in Kaingaroa Forest, but some to be upright, and the canopy is light. of the early multiple leaders may be the Wind is an important factor in breaking result of attack by tortricids. apart the leaders of forked trees; trees with Caterpillars of the following 17 species double leaders must ultimately split as their of native have been recorded height increases and as leverage becomes feeding on P. radiata foliage:- greater. On poorly drained areas shallow- Selido.H'f1W .Hlavis (Butl.) rooted trees are uprooted, and on other S. jenera/a (Felder) areas trees may be broken off by severe S. dejec(lIria (Walk.) gales. These losses are important because S.leucclaea (Meyr.) they most frequently affect the dominant floccosa (Walk.) D. Ieptotr/eN/ (Walk.) trees. Double leaders are frequently large- D. hcrmio1Jl' (Hudson) crowned wolf trees which have suppressed Asthe!la Plllchlirill (Dbldy.) their neighbours. When they split they make HelioJ'tibes atychioide.~ But1. a large gap and may break or push over other Tortrix excessana (Walk.) T. (lave.'icens (Butl.) trees as they come down. T. canJi!ona Walk. Capua p/agialliflu (Walk.) Frost is important only in newly planted Ctel10psclIsris ohliql/una Walk. or regenerated areas and snow is almost Alls/rotortrix poslvi!fwllI (Walk.) unknown. Cl1cphasia jaclalanu (Walk.) UeceliclI.\. onmivorl/s (Fercd.). To sum up, wind controls the form of the crown at any spacing and assists in the Of the exotic Pine us laevis Mask. eliminating of suppressed trees; it may may be mentioned. Crawlers of this aphid cause losses through wind throw or break- appear to be spread by wind, the winged age of dominants. Extremes of drought or forms having no apparent function in New wetness assist the growth of dominants Zealand. It is now controlled by Leucopsis and assist in the elimination of suppressed abscura Hal. and seldom becomes very num- trees. Drought favours the development of erous. Hylastes ater is of importance only in outbreaks and high humidity favours its attack on regeneration. attack by fungi. Lightning damage, resulting in killing of areas of , to t acre in area, is THE Sirex COMPLEX sporadic throughout the forest. During the summer of 1960-61 the compartment Sirex l1actilia is normally only capable described was struck by lightning. About two acres of the of killing trees debilitated by some predis- best trees had dead crowns and fOUTacres were felled to posing factor. One Sirex female can cause salvage the timber. death only when the tree is already very sickly. Concentrated attack by many Sir"x . BIOTIC FACTORS can kill progressively less sickly trees as the numbers of Sirex increase. A female Sirex The influence of pigs and deer upon an will lay from 25 to 500 eggs, depending upon established forest does not appear to be as its weight. The killing agent is a fungus great as might be expected, although pigs deposited with the egg in holes drilled in may be an important factor in the develop- the tree by the ovipositor or in holes drilled ment of a tree fern understorey. Birds are but abandoned without an egg being laid. probably important but little is known about Sirex is facultatively parthenogenetic, pro- their exact status in the forest. ducing females from fertilised eggs and males from unfertilised eggs. Eggs are laid Some 137 species of insects have been in living trees and oviposition tends to be- recorded as associated with the P. radiata come concentrated just below the lowest forests. O[ these 37 are defoliators, 33 are green branch. On hatching the larva tunnels predators or parasites, 12 are sap suckers with the tracheids consuming both the wood and 55 are wood borers, scavengers, or of no and the fungus which has invaded the wood known significance. Only 7 are of exotic ahead of it. After about the 7th instar it origin. With the exception of Sirex nactilio turns inwards and later, when fully grown. 50 NATIVE AND INTRODUCED SPECIES IN NEW ZEALAND usually moves outwards to pupate just be- time. lbalia oviposits in the eggs of Sire.\'. neath the surface. It is limited in that it must locate the eggs within ten days of their oviposition and its In normal years the males greatly out- success depends upon the successful devel- number the females, and copulation takes opment of the Sirex. On the other hand place in the tops of trees on hot, sunny days. Ibalia will increase with the Sirex, whereas However the female has no time to waste Rhyssa will always be one year behind. since she must deposit as many eggs as pos- sible before her fat-body is exhausted. She The native Guig/ia schauinsZandi (Ashm.) therefore starts ovipositing without copu- Benson attacks Sirex larvae in the same lating if the weather is not favourable. On manner as Rhyssa but is smaller and keeps an average only one female will complete the mainly to the tops of trees. It has an alter- cycle from each batch of eggs. In the 1946 native host in Stenopotes pallidus Pasco drought summer conditions were favourable for copulation and trees were easily killed. HISTORICAL ACCOUNT Consequently in 1947 there were many more Sirex than normal and a much higher pro- Compartment 1125 was established by portion of females. The average size and planting in 1922 on ground which had been egg-laying capacity of the females was also newly burnt over. Except in bracken-fern greater than normal and a one-year, instead areas, the trees received no competition o[ a two-year, life cycle predominated. The from native vegetation and the crowns summers of 1947 and 1948 were also favour- closed after about 8 years. After about 12 able for copulation and the result was the years the lower branches were dying and epidemic outbreak culminating in 1949. the effect of competition between the trees was becoming evident. Up to this time, 193.. The importance of Sirex, then, depends there had probably been some damage to upon the number of females and their ability leaders by tortricids and fairly widespread to lay fertilised eggs. When adequate thin- attack by Pineus laevis. Unseasonable frosts ning cannot be carried out, Sirex is beneficial in 1932 caused some damage and were fol. because it kills unwanted trees. In epidemic lowed by Diplodia pinea attack. Some sup- numbers it may kill too many trees or pressed trees were being killed by Sirex damage trees it cannot kill. Biological con- but the forest was becoming badly over- trol is therefore important. There are three stocked, with verv small crowns and short introduced and one native parasite, all of upright branches:. which play their part in keeping the numbers of Sirex below the epidemic level. Rhyss

no apparent effect upon the forest, which All tree ferns, except D. fibrosa, and most was outwardly in better health than before of the native shrubs are water-dispersing the drought. Actually the drought had had species, the leaves being resistant to wetting a very drastic effect upon the smaller trees and provided with drip points. D. fibrosa which rendered them susceptible to killing is a water-collecting species, water tending by Sirex attack. This enabled the Sirex popu- to run down the fronds to the crown and lation to build up to epidemic numbers. Thc collect in the matted fibrous roots, conse. Sirex population reached a peak in the quently D. fibrosa is the most plentiful fern summer of 1949 but had reverted almost to on the dry ridges. normal by 1950. Tree mortality also in-. creased yearly and reached a peak in the Since 1951 there has been a steady increase winter of 1949. Total deaths over the four in volume over most areas with a light but winters amounted to 30 per cent. of the continual mortality. Most areas appear to be trees. In 1950 mortality was very slight but a badly overstocked should another drought, further 10 per cent. of the surviving trees similar to that of 1946, occur. What effect died in 1951. After this thinning there were the changes in soil, vegetation, and other many important changes in the forest. factors would have in decreasing or increas- Branches grew out and sagged down, spread- ing drought effects is quite unpredictable. ing drips over the forest floor instead of While the history of this particular forest conducting the rain to the trunk. The litter has been sufficiently well recorded for the decomposed, there was a great improvement salient features and controlling factors to in soil structure, earthworms became plenti- be followed from the time of planting, any ful, and white leached zones in the soil began sequence of factors could give a very similar to disappear. Most important of all was the end result provided that damage and mortal- appearance of a great variety of native plants ity occurred in moderation and at appropri- including over 30 species of ferns. Of the ate times. ferns the most conspicuous were the tree ferns of which there are now from 200 to CONCLUSION 400 per acre over much of the forest. Dick- sonia squarrosa Swartz. is the most common, Up to 1946 no entomological factor had followed by D. fibrosa Co!. and Cyathea been of much importance in the develop- dealbata Swartz. C. cunninghamii Hook. f., ment of P. radiata stands at Kaingaroa. The e. smithii (Hook.) Domin. and C. medullaris chief feature had been the lack of natural Swartz. occur in about equal numbers. It is agents to kill suppressed trees and so to evident that if the forest were not felled or prevent stagnation caused by unrelieved burnt the P. radiata could not reproduce it- competition. The 1946 drought and Sirex self in competition with the invading native noctilio combined to relieve a very danger- flora. Although the present stands frequently ous situation through killing the small and contain a dense understorey of regenerated deformed trees. At present the condition of pine, few of these would survive to produce the P. radiata bears much resemblance to seed, and seed would find no place suitable that of 1946 but differs in the age and size for germination. Possibly would of the trees, much lower stocking and the eventually take over and species such as presence of an understorey of tree ferns and rimu, totara, and matai should eventually shrubs. The effects of another drough t sum- colonise the sites. mer are awaited with considerable interest.