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. 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).

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. For the period than 1.0 were assumed to have a high bilberry produc- 1961–90, heavy outbreaks of insect pests in the Nordic tion. There were no game reports from 1943, 1944 and countries were reported also by Christiansen (1969), 1975. In 1943, both the Norwegian Agricultural Price Ehnstro¨m et al. (1974, 1998), Loÿttyniemi et al. (1979), Reporting Office and one representative for the agricul- Austara˚ et al. (1983) and Harding et al. (1998). tural authorities in Aust-Agder reported that the bil- It has been assumed that summer drought makes berry production was higher than in a ‘‘normal’’ year, trees more vulnerable to insect attacks (e.g. Wargo while in 1944 the same two authorities reported the 1996, Cobb et al. 1997), and therefore I also used the bilberry production to be lower than in a normal year. amount of precipitation during June–August in the In 1975, two representatives for the forest authorities in mast year as an explanatory variable in the moth-oak Aust-Agder, both living within the study area, reported analysis. For moth species responsible for oak defolia- that the bilberry production was lower than in a tion in spring and early summer, it seems to be impor- medium year. I therefore used these statements as tant for the caterpillars to hatch as soon after budburst ‘‘game reports’’ for 1943, 1944 and 1975. as possible (Feeny 1970, Hunter 1992). Sometimes, In 1978, only one game report was available. Accord- however, they hatch too early, and in some years this is ing to this report the bilberry production was above an important factor for larval mortality (Feeny 1970). medium. Also one local newspaper reported the bil- Possibly, the timing of budburst depends on both tem- berry production to be high, though not as high as in peratures and day length (Heide 1993a, b), while moth 1977, while another claimed that there were few bilber- eggs which have terminated their diapause in late win- ries. I assumed then that there was a bilberry produc- ter respond mainly to temperature. If so, then spring tion corresponding to a game report index of ca 1.0. temperatures may be critical for this plant-herbivore For the period 1979–88, I used the annual amount of interaction. Thus, I used the mean temperature in April bilberries picked by one family living in Vega˚rshei as an (prior to budburst and egg hatching in May) as an index of the bilberry production. According to diary explanatory variable. Because temperatures may also notes of one of these family members, bilberries were directly affect larval growth or the activity of their abundant both in 1977 and 1978, but the quantities of enemies, the last variables used were the mean tempera- berries picked were not given for these years. ture in May–June (which is the time of larval develop- I also searched for statements on bilberry production ment) of both the mast and the post-mast year. in reports from the Norwegian Agricultural Price Re- porting Office, in annual reports from the Agricultural Society in Aust-Agder and in local newspapers for the Data on bilberries and capercaillie populations entire study period 1919–88. As there is a long tradi- tion for bilberry utilisation in the study area, several From 1923 to 1939, bilberries were exported from evaluations of the bilberry yield were usually found for Aust-Agder to England under the direction of Aust- each year. Some information was also given by T. Agder Berry Company (data given in annual reports Grasaas (references in Sela˚s 1997), Myrberget (1982) from the Agricultural Society in Aust-Agder). There and Frøstrup and Vigerstøl (1992). The bilberry indices was a general increase in the activity of the company calculated (1932–78 and 1979–88) were mostly in ac- during their three first years, and a decrease from the cordance with the evaluations found, and all years with mid 1930s onwards. Even though the export was influ- high bilberry production were certainly identified by the enced also by prices and weather during harvesting, methods used. Post-mast years were defined in the same statements given by the manager of Aust-Agder Berry way as for the sessile oak. Company, gardener H. Hansen, confirm that the an- Due to the high reproductive capacity of the caper- nual quantities exported during 1925–31 mostly caillie, I assumed that all years with favourable condi- reflected the annual variation in the access of bilberries. tions with regard to reproduction should give a rise in For the years 1920–24, years with high bilberry pro- the autumn population size compared to that of the duction were identified from statements given by H. previous year. I also assumed that annual fluctuations Hansen and others in the annual reports from the in the autumn population size is due mainly to varia- Agricultural Society in Aust-Agder (1923–24), and in tions in breeding success, and not to variations in adult local newspapers (1920–24). mortality (Rolstad et al. 1991). In the analyses I used For most years during 1932–77, game reports includ- three categories of years, those with no increase (or ing information on bilberry production were returned poor chick production), those with a slight or moderate from the municipalities to the central forest or game increase (or good chick production), and those with a

ECOGRAPHY 23:6 (2000) 653 considerable increase (or very good chick production). The analyses The breeding success was expected to be highest in To test for effects of the selected explanatory variables post-mast years of bilberry. Any positive effects of on the population growth of moths and capercaillie in masting, such as high access of berries as food for post-mast years, I conducted likelihood ratio tests in chicks (Spidsø and Stuen 1988) or high quality of logistic regression models with different categories of bilberry leaves in late summer, may however be seen post-mast years as response (two categories for moths also in the mast years. and three for capercaillie). Test results are given for Prior to 1940, years with increase in the capercaillie each explanatory variable (i.e. as the contribution of autumn population were identified from game reports each variable) in the whole model, and for the model published annually by the Norwegian Hunter and selected in a backward elimination procedure. At each Fisher Union. Some supplemental information was step in this procedure, the candidate for elimination taken from Hegge (1955), Grasaas (1977), Hjeljord was the variable with the largest non-significant p- (1980) and Frøstrup and Vigerstøl (1992). For the value. period 1940–44, information on capercaillie population size was taken from Anon. (1945), Krefting (1945), Hegge (1955) and Hjeljord (1980). For the years 1940– 42 and 1945–79, I used forester T. Grasaas’ articles Results (references in Sela˚s 1997), unpublished research reports There were reports on moth attacks on oak forests and game reports to identify increase years. For the somewhere in Agder in eight of the 19 post-mast years period 1980–88, autumn counts of capercaillies along with low or relatively low acorn production (Fig. 1). line transects were conducted by Vega˚rshei Hunter and Each attack lasted for 1–6 yr. No other outbreaks were Fisher Union. recorded. The outbreaks in 1940 and 1951–54 were For the period 1945–77, I also calculated a capercail- restricted to Vest-Agder. The most serious attack was lie index from game reports completed by the local the one reported from both counties in 1955–60, when forest or game authorities (2–17 reports each year, large areas of oak forests were defoliated, especially in mean 13.8). In these reports the autumn population size Aust-Agder in 1956. In 1967–69 there were local out- of capercaillie was designated as lower than (value breaks in both counties, and in 1972 in the eastern part −1), equal to (value 0) or higher than (value 1) that of of Aust-Agder. A new attack was reported from the the preceding autumn. For each year from this period I area along the border between the two counties in 1975. calculated a capercaillie index by taking the mean value In 1977 ‘‘heavy outbreaks’’ (Austara˚ et al. 1983) were from all reports from that year. A post-mast year was recorded in Vest-Agder. No attacks were reported from regarded as having increase in the autumn population Vest-Agder in 1978, but in 1979–80 there was a new, size if the index was equal to or higher than 0.0, and a local attack in the southern part of this county. For capercaillie, all increase years were reported in or strong increase if the index was equal to or higher than after a year with high bilberry production (Fig. 2). 0.5. The results were in accordance with information Increasing population levels were reported in 14 of the given by Grasaas. Grasaas (1963) used the number of capercaillie hunted in Vega˚rshei municipality as an index for the autumn population size during 1953–62. The first difference of this log-transformed hunting index was significantly correlated with the index calculated from game reports (r=0.78, DF=8, p=0.012). Weather conditions assumed to have impact on the reproduction of capercaillie are snow depth in spring, which influences the availability of high quality food prior to egg laying, and precipitation and temperatures in summer, which influence chick survival (e.g. Fig. 1. Annual index of acorn production in Agder calculated Slagsvold and Grasaas 1979, Marcstro¨m and Ho¨glund from reports given by local forest authorities during 1930–49 (solid line), annual amount (metric tons) of acorns exported 1980, Erikstad and Spidsø 1982, Steen et al. 1988, from Aust-Agder to Denmark during 1949–98 (black bars) Pulliainen and Tunkkari 1991). In the analyses, I used and years with serious moth attacks on oak forests somewhere ] two categories for spring conditions, early and late in Agder (open circles). Years with an acorn index 0.5 (horizontal dotted line) were assumed to have a high acorn thaw (categorised by Grasaas for the period 1920–79, production and termed mast years. The demand for acorns in and by the author for 1980–88). As indices for summer Denmark was low during the 1960s, and thereafter increasing. conditions which may directly influence on chick sur- Post-mast years with low or relatively low acorn production are hatched. The size of the moth outbreak symbols indicate vival, I used the mean temperature and the total the extent of the attack in relation to that of the previous year. amount of precipitation in June and July. The most important species was the green oak leaf roller moth.

654 ECOGRAPHY 23:6 (2000) Fig. 2. Annual aumont (metric tons) of bilberries exported from southeastern Aust-Agder to England during 1925–31, annual index of bilberry production in the same area calculated from game reports (solid line, range 0–2, axis not given), annual amount (litre) of bilberries picked by one family living within the study area (broken line) and years with increase (open circles) in the autumn population size of capercaillie. Years with a bilberry index ]1.0 (horizontal dotted line) were assumed to have a high bilberry production and termed mast Fig. 3. Post-mast years of sessile oak with (filled symbols) and years. Post-mast years with low or relatively low bilberry without (open symbols) moth attacks on oak forests, in rela- production are hatched. tion to the mean temperature in June–September in the mast year and the mean temperature in April in the post-mast year. 18 post-mast years with low or relatively low bilberry production, and in seven of these there was a strong associated with low summer temperatures in mast increase. The post-mast years with no increase were years. The analyses thus supported the idea that the 1934, 1970, 1973 and 1981. In 1938, 1948, 1962 and photosynthetic activity in the mast year is important for 1985 the increase was slight. the ability of the plant to allocate resources for chemi- There was a significant negative effect of high sum- cal defence in the post-mast year. The hypothesis is mer temperatures in mast years on population growth supported also by two former studies on cyclic popula- of both moths and capercaillie in post-mast years tions of small rodents in Norway, where a negative (Table 1). Also temperatures in April in the post-mast relationship between the level of the population peak year contributed significantly to explain the occurrence and the temperature in the previous year was found of moth outbreaks (Table 1, Fig. 3). For capercaillie, (Myrberget 1974, Østbye et al. 1989). In these studies there was an almost significant negative effect of precip- there were however no indices of the seed production in itation in June and July (Table 1, Fig. 4). food plants. In addition to high summer temperatures in the mast year, also high spring temperatures in the post-mast year seemed to prevent moth outbreaks. Possibly, moth Discussion eggs are not able to respond to temperatures and day length in exactly the same way as the oak. If moth eggs For both green oak leaf roller moth and capercaillie, are less sensitive to photoperiod than are oak buds, high population levels in post-mast years were usually then they are also more likely to hatch too early relative

Table 1. Results from likelihood ratio tests in logistic regression models. The response of the first test was oak post-mast years with and without moth attacks, and of the second bilberry post-mast years with no increase, increase and strong increase in the autumn populations of capercaillie. The explanatory variables were the summer temperature in the mast year and other weather variables which could be of importance for plant resistance or herbivore development. The data were taken from Fig. 1 and Fig. 2. n=total number of post-mast years used in the analysis. Significant test results are highlighted by an asterisk. By use of a backward elimination procedure, all variables except the significant ones were removed from the models.

Response Explanatory variable x2 p

Moths (n=19) Summer temperature in the mast year (year t−1) 4.17 0.035* Summer precipitation in the mast year (year t−1) 0.44 0.503 April temperature in the post-mast year (year t) 5.47 0.019* May–June temperature in the mast year (year t−1) 0.03 0.861 May–June temperature in the post-mast year (year t) 0.03 0.867 Whole model (R2 =0.52) 13.49 0.019 Selected model (R2 =0.50)12.92 0.002 Capercaillie (n=18) Summer temperature in the mast year (year t−1) 13.00 B0.001* Early or late snow melt in the post-mast year (year t) 0.18 0.673 June–July precipitation in the post-mast year (year t) 3.53 0.060 June–July temperature in the post-mast year (year t) 1.89 0.170 Whole model (R2 =0.37) 14.20 0.007 Selected model (R2 =0.25) 9.79 0.002

ECOGRAPHY 23:6 (2000) 655 to oak budburst if spring temperatures are high. In a voles than for capercaillie chicks. Both in 1970 and five-year study of the winter moth Operophtera bru- 1973 bank vole populations did not crash until late mata, Watt and McFarlane (1991) found that a higher autumn or winter, when bilberry becomes more impor- proportion of the eggs hatched prior to budburst of the tant as a food plant for this species (Hansson and host tree in years with high temperatures in January– Larsson 1978, Hansson 1985). March. The relationship may however be more compli- Even though oak and bilberry did not mast in syn- cated, as also autumn temperatures have some influence chrony, there were some similarities in the population on the date of budburst (Heide 1974, Westergard and fluctuation of the green oak leaf roller moth and the Eriksen 1997). capercaillie. Both species had high population levels in For capercaillie, the only other variable than summer the 1920s (information on moths given by Gløersen temperature in the mast year that tended to influence 1953; no data on acorns from this period) and in the on the population growth in the post-mast year was 1950s, but not in the 1930s or in the 1980s and 1990s. summer precipitation. As young chicks have to be Climatic events seem to be the most likely explanation warmed repeatedly by the hen to maintain their body for this pattern, but also landscape changes due to temperature, rapid cooling by rain will dramatically clear-cutting introduced in the 1960s may have con- reduce the time available for foraging. According to tributed to the lack of strong population increases Swenson et al. (1994), weather conditions in summer during the last two decades. The reduction in the ratio of old young oak forest stands could have influenced are critical for chicks of large grouse species such as the / the risk of moth outbreaks if old trees in general are capercaillie, because of their rapid growth. Pre-laying more depressed after mast years than are young trees. weather, such as snow conditions in spring, should be Theoretically, old trees should invest more in each less critical because of the relatively high endogenous reproduction than young trees due to their lower future reserves of the hens. reproductive value. In outbreak years of the autumnal In three of the four bilberry post-mast years which moth Epirrita autumnata, Ruohoma¨ki et al. (1997) did not give an increase in the autumn population size found that sites with old mountain birches Betula of capercaillie (1934, 1970 and 1973), a high chick pubescens had the highest larval density. production was reported in mid summer, but the chicks It is well known that the capercaillie at least season- apparently disappeared in late summer or early au- ally depends on old forest stands (Rolstad and Wegge tumn. This happened despite high vole populations, 1987, Rolstad et al. 1988, 1991, Storch 1993), where which have commonly been believed to buffer the pre- also the bilberry cover is highest. The production of dation pressure on grouse chicks (e.g. Angelstam et al. bilberries is in general highest in old forest stands with 1984). It therefore seems unlikely that the poor chick a moderate canopy cover, a habitat which seems to be production these years was due to predation. As the preferred by capercaillie broods in summer and au- common vole species in the study area, bank vole tumn. In addition to the direct negative effect of clear Clethrionomys glareolus, depends less on bilberry as cutting on capercaillie habitats, predator densities and food in late summer than does the capercaillie, reduced egg and chick predation appear to be higher in frag- foliage quality in late summer may be less critical for mented than in continuously old forests (Wegge et al. 1990, 1992). The poor reproductive effort of capercaillie from the mid-1960s onwards was by Grasaas (1973) ascribed to increasing predator populations. It should be noted, however, that the increases in the capercaillie populations in the study area in the late 1970s occurred before the epizootic of sarcoptic mange reduced the populations of red fox Vulpes 6ulpes (Sela˚s 1998b), which is probably the most important predator on grouse eggs and chicks (Smedshaug et al. 1999). To my knowledge, there are no studies of the effect of masting on the defence ability of plants. However, induced resistance to herbivory has been documented for several plant-herbivore interactions (e.g. Wold and Marquis 1997, Kaitaniemi et al. 1998), and has commonly been considered as the cause for the decline of Fig. 4. Post-mast years of bilberry with no increase (open cyclic or quasi-cyclic herbivore populations feeding on symbols), slight or moderate increase (partly filled symbols) long-lived plant species (Haukioja 1980, 1991, Lindroth and strong increase (filled symbols) in the autumn population and Batzli 1986, Seldal et al. 1994). The hypothesis of size of capercaillie, in relation to the mean temperature in June–September in the mast year and the total amount of induced defence as a cause for herbivore cycles implies, precipitation during June–July in the post-mast year. however, that the level of chemical defence compounds

656 ECOGRAPHY 23:6 (2000) in the host plant is reduced after some years, allowing Ehnstro¨m, B. et al. 1998. Insect pests in forests of the Nordic for a new outbreak of herbivores. Such a relaxed counties 1968–1986., Rapport fra Skogforskningen. – Suppl. 2: 1–12. defence is difficult to explain evolutionarily, unless Erikstad, K. E. and Spidsø, T. K. 1982. The influence of there are some advantages that compensate for the weather on food intake, insect prey selection and feeding costs connected to new herbivore attacks. As the main behaviour in willow grouse chicks in northern Norway. – Ornis Scand. 13: 176–182. goal for a plant should be to maximise the total number Feeny, P. 1970. Seasonal changes in oak leaf tannins and of descendants produced during its life-time, the repro- nutrients as a cause of spring feeding by winter moth ductive strategy is likely to be the key factor for under- caterpillars. – Ecology 51: 565–581. standing also patterns of fluctuations in chemical Frøstrup, J. C. and Vigerstøl, N. P. 1992. Veiderliv. Glimt fra Aust-Agders jakt og fiskehistorie. – Tvedestrand Bok- defence. trykkeri, in Norwegian. Our knowledge with regard to herbivore resistance in Gløersen, F. 1953. Sma˚fuglene og skadeinsektene. – Tidsskrift masting plants such as oak and bilberry is still limited. for Skogbruk 61: 268–272, in Norwegian. Grasaas, T. 1963. Skogsfuglundersøkelsene i Vega˚rshei. – Nonetheless, it should be possible to test experimentally Jakt, fiske og friluftsliv 92: 544–551, in Norwegian. the impact of summer temperatures on plants’ resis- Grasaas, T. 1973. 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