ECOGRAPHY 23: 651–658. Copenhagen 2000 Is there a higher risk for herbivore outbreaks after cold mast years? An analysis of two plant/herbivore series from southern Norway Vidar Sela˚s Sela˚s, V. 2000. Is there a higher risk for herbivore outbreaks after cold mast years? An analysis of two plant/herbivore series from southern Norway. – Ecography 23: 651–658. Historical data on two plant-herbivore interactions from southern Norway were used to test the hypothesis that the degree of herbivore outbreaks in post-mast years is negatively related to summer temperatures in the mast year, because plants are more depressed after a high seed production if temperatures and thus the photosynthetic activity is low. The plant species were the sessile oak Quercus petraea and the bilberry Vaccinium myrtillus. For the former species post-mast years were identified from reports given by the local forest authorities for the period 1930–48, and from acorn export curves for the period 1949–98. For the latter species, post-mast years were identified mainly from bilberry export curves for the period 1920–31, from game reports for the period 1932–78, and from diary notes for the period 1979–87. The herbivore species used were the green oak leaf roller moth Tortrix 6iridana and the capercaillie Tetrao urogallus. Eight moth outbreaks on oak forests were reported by the forest authorities in the period 1930–98, and they all started in a post-mast year of the sessile oak. There were however also eleven post-mast years without moth outbreaks. According to game reports, observations and autumn counts, all increases in the autumn population size of capercaillie during 1920–88 occurred in or after a year with high bilberry production. Among 18 post-mast years, there were seven with strong increase, seven with slight or moderate increase, and four with no increase. For both herbivore species, post-mast years with marked population increases had significantly lower summer temperatures in the preceding (mast) year than had post-mast years with no or slight increases. For moth populations there also was a negative effect of high temperatures in April, possibly because moth eggs tend to hatch too early relative to budburst if spring temperatures are high. For the capercaillie, high amount of precipitation in June–July seemed to have some negative impact on the autumn population sizes, as also found in previous studies. V. Sela˚s ([email protected]), Dept of Biology and Nature Conser6ation, Agricul- tural Uni6. of Norway, P.O. Box 5014, N-1432 A,s, Norway. For some long-lived plant species, the annual number 1996, Zangerl et al. 1997, Baldwin et al. 1998), popula- of seeds produced varies from huge amounts to almost tions of masting plants might be expected to exhibit zero. When this temporal variation in seed production annual variations also in the latter activities. Support is synchronised within a population, the phenomenon is for this view is that vegetative growth is usually low in termed mast seeding (Kelly 1994). Assuming that there mast years (e.g. Silvertown and Doust 1993), and that in general is a trade-off between the three essential population outbreaks of some cyclic herbivore species activities seed production, growth and chemical defence coincide with post-mast years of their host plants (Sela˚s (e.g. Bazzaz et al. 1987, Herms and Mattson 1992, 1997, see also Tast and Kalela 1971, Laine and Hent- Silvertown and Doust 1993, Bergelson and Purrington tonen 1983). Accepted 25 February 1999 Copyright © ECOGRAPHY 2000 ISSN 0906-7590 Printed in Ireland – all rights reserved ECOGRAPHY 23:6 (2000) 651 According to the mast depression hypothesis, cyclic ulation outbreaks of both the green oak leaf roller populations of herbivores are a result of mast seeding moth and capercaillie have occurred at irregular inter- of their food plants, because masts are assumed to be vals in these areas during the 20th century. produced at the expence of chemical defence against Climatic data were taken from Byglandsfjord Meteo- herbivores (Sela˚s 1997, 1998a). If the level of chemical rological station, situated 50 km from the coast, in the defence compounds is significantly reduced in a mast central part of Aust-Agder county. As an index of year, plants will not be able to recover until the next summer temperatures in the mast year I used the mean summer. The impact of masting on chemical defence temperature for the four months June–September, may however depend not only on the level of resources which by forest scientists has been assumed to be of allocated to seed production, but also on the level vital importance for both growth and seed production stored prior to the masting and on photosynthesis in of forest trees in Norway. the mast year. The latter should depend mainly on solar radiation and temperature, variables which in general are positively correlated. Thus, if photosynthetic activ- Data on acorns and moth outbreaks ity in the mast year is important for the ability of plants to respond to subsequent herbivore attacks, then there During the 1930s and 1940s, annual evaluations of the should be a negative relationship between summer tem- acorn production were given by the local forest author- perature in the mast year and the rate of population ities in Agder to the National Works of Forest Seed change of herbivores in the post-mast year. (2–5 reports each year, mean 3.2). An index for the In this paper I test the prediction of a relationship annual access of acorns (range 0–1) was calculated as between mast seeding, temperature and herbivore out- described by Sela˚s (1997), and years with an index breaks by comparing temperatures in mast years with equal to or higher than 0.5 were assumed to have a high post-mast year levels of herbivore populations. The acorn production and were termed mast years. Acorns species used are the green oak leaf roller moth Tortrix of sessile oak have been exported from Agder to Den- 6iridana and its host tree, the sessile oak Quercus pe- mark since 1946 (data from 1949 onwards provided by traea, and the capercaillie grouse Tetrao urogallus and the National Works of Forest Seed). According to G. its most important food plant during summer, the Oveland (pers. comm.), who was responsible for most bilberry Vaccinium myrtillus. The bilberry is important of this export, the annual variation in the access of not only as food for the capercaillie, but also as food acorns was very similar for the two Agder counties. The for caterpillars preyed upon by capercaillie chicks (Atle- demand for acorns in Denmark varied throughout the grim and Sjo¨berg 1995). study period, and the annual export value therefore has to be compared with that of the nearest 2–3 yr. How- ever, by comparing the export curve with subjective evaluations of the acorn production (given in annual Material and methods reports from the Forestry Societies in Agder, and in Study area letters from G. Oveland to the National Works of Forest Seed), years with high as well as low acorn The data on moth populations were obtained from the production could easily be recognised. coastal part of Aust-Agder and Vest-Agder counties The oak commonly produced high seed crops in two (Agder) in southern Norway, where sessile oak is a or three succeeding years, indicating that the trees were common tree species and often occurs in pure forest not always seriously depressed after a high seed produc- stands. Most of the data on capercaillie autumn popu- tion. However, two or more mast years in a row could lation size were obtained from a more restricted coast- also be caused by an asynchronous flowering in the near area in the south-eastern part of Aust-Agder population. Anyway, as long as the reason for succes- county (Vega˚rshei and Tvedestrand municipalities), sive mast years in the past is not known, I chose the where forester T. Grasaas studied capercaillie popula- conservative approach, and used only post-mast years tion dynamics. For a few years, however, information with a low or a relatively low seed production in the from nearby areas had to be used. The coastal areas of current analyses. Agder are situated within the boreonemoral zone. The The oak forests in Agder are sometimes subject to landscapes are dominated by forests, usually character- serious defoliation by caterpillars in May–June. The ised by a fine-grained mosaic of young, medium and most important species among moths responsible for old-aged coniferous, mixed and deciduous stands. Scots such damage is the green oak leaf roller moth, but also pine Pinus sil6estris, Norway spruce Picea abies, sessile other species, such as Cacoecia xylosteana, Erannis oak, aspen Populus tremula and birch Betula spp. are defoliaria and Agriopis aurantiaria, may contribute to the dominant tree species, whereas bilberry is a com- the defoliation. Reports on forest damage caused by mon species in the field layer. The occurrence of oak pest species were given in annual reports from the decreases with increasing distance from the coast. Pop- Norwegian Forestry Directorate and in annual reports 652 ECOGRAPHY 23:6 (2000) from the Forestry Society in each county. I also con- authorities in Norway each autumn (2–29 reports each tacted the forest authorities and the forest owner asso- year, mean 9.1). These reports are available at the ciations in Aust-Agder and Vest-Agder, and the Norwegian Public Record Office. For each year, I Norwegian Forest Research Institute at A,s, in case calculated a bilberry index (range 0–2) as described by some recently recorded moth attacks were not referred Sela˚s (1997), and years with an index equal to or higher to in any of the available reports.