Masting

Masting is the irregular periodic synchronous production of large seed crops in perennial plants such that the majority of trees produce heavy seed crops in certain years (the mast years) while in other years the crop is small or nonexistent. The pattern of reproduction involving the production of a superabundance of seeds became known as mast seeding from the German word for fattening livestock on abundant seed crops, so that years of high abundance became known as mast years. In many species, including those described below, good seed years are unequivocal and obvious. It should be borne in mind, however, that variability in seed production is a long continuous gradient with strongly masting species at one end and those that produce roughly equal number each year at the other. This does not detract from the interest of masting species but does explain the large number of weakly masting species that fall in between.

Masting occurs in a wide range of temperate and northern forest trees such as beeches Fagus, oaks Quercus, ashes Fraxinus, firs Abies, spruces Picea, pines Pinus and the Douglas fir Pseudotsuga menziesii. In the southern hemisphere masting occurs in a range of woody species. Norton and Kelly (1988) list several in New Zealand: three species of southern beech Nothofagus in the beech family, Fagaceae, four species in the podocarp family (Podocarpaceae) including rimu or red pine Dacrydium cupressinum a needle-leaved evergreen, the tawa tree Beilschmiedia tawa in the Lauraceae and the southern rata Metrosideros umbellata in the Myrtaceae.

4.4.1 Beech masting in England and continental Europe

Masting is particularly well seen in European beech Fagus sylvatica and Douglas fir. The masting phenomenon is essential to the regeneration of common beech; in many forests the nuts of particular years have given rise to a complete new generation of young trees. This is because in mast years seed predators are completely satiated (but see Section 4.4.4), so many seeds survive uneaten and germinate the following spring. In a truly natural situation (virgin forest, called urwald in German) beech seedlings tend to develop most rapidly where old trees have fallen or died in situ, often leaving a magnificent erect hulk. The age mosaic found in virgin forest is to some extent imitated when continuous cover forestry is practised, gaps in the canopy encouraging natural regeneration when masting occurs so that the planting sequence associated with extensive clear felling becomes unnecessary.

Continuous systematic records of masting in common beech have been made at a number of widespread sites in England since 1980 (Fig. 4.9). They show an underlying biennial masting pattern of the kind described below, but in practice really heavy masts in England and many other places are, for various reasons, several years apart and result in very effective predator satiation. Considerable variations occur both between sites and even on a tree to tree basis within a single site. Less precise records of major masting years in England are known for a much longer period. These, together with other records of masting in northern Europe for the past two centuries, are summarized by Hilton and Packham (2003). Their fig. 2, which was derived from amalgamating the records for Denmark, England, Germany, the Netherlands and Sweden, gives the extent of beech masting in northern Europe for the years 1897 to 2001.

In his classic study of Swedish beechwoods, Lindquist (1931) discusses forestry records for four sites from 1895-1928. They show a clear pattern of masting every 2 or 3 years, broken only once. The suggestion is that there is an inherent periodicity which excludes significant flowering every year, but permits it every other year. Failures of the biennial pattern were related to the weather. There is a requirement for high temperatures in the previous June and July, when the flower buds are initiated, together with the absence of severe frost in late April and May in the year of masting when both male and female flowers would be damaged. Very hot summers resulted in good masting the following year, unless they coincided with good masting in the current year.

Maurer (1964) collated results for Lower Franconia, southern Germany, for many years, noting deviations between regions, though the large and famous masts-diegrossen undbekannten Masten-of 1811,1823,1888,1909,1946 and possibly 1958, seem to have been common to most regions. Trees in many German beechwoods date from the 1888 Jahrhundermast mast of the century. The very heavy mast of 1946 did not have the same result as 80% of the nuts were collected for oil production. Figure 4.10 is a model to account for the periodicity and variability of masting in beech. A larva of the moth Cydia fagiglandana, which is responsible for most damage to beech nuts in Britain, is shown in Fig. 4.11 along with typical damage caused both by it and the birds which open the nuts to get at the larvae.

Damage by mould and insect attack has been carefully observed since 1980 as part of the English Beech Masting Survey. During the whole of this period, neither insect damage nor mould seem to have resulted in any significant effects on recruitment after the overwhelmingly important mast years which lead to the majority of seedling development. In 1995, for example, when both sites were high Grade 5 (see Fig. 4.9), 2.4% of full nuts at Patcham Place were tree age, genotype, situation, geometry and photosynthetic production illuminance of branches flower buds initiated when summer temperatures high (buds become reproductive or vegetative)

weather conditions

NEGATIVE FEEDBACK depletion of nutrients and/or inhibition by hormones survival of developing flower buds (catkins visible)

fruit maturation (cupules develop)

viability of seed (pericarps full/empty)

damage caused by frost in late spring nutrient supply extent of pollination seed quality (mould/insect attack)

extent of predation and infection

Figure 4.10 Model to account for the periodicity and variability of beech masting. Operation of the negative feedback loop depends on the extent of masting in the previous year. (Packham and Hilton, 2002. Arboricultural Journal 26.)

mechanically damaged or mouldy while less than 1% of full nuts of the single tree sampled at Withdean were non-viable. Equivalent figures for the mast year of 2000 are 2.1% for Patcham Place and zero for Withdean. The proportion of damaged full nuts is much higher in non-mast years. Regional variation

Infection Holes Hand Caused Insect

Figure 4.11 Four beech nuts damaged by larvae of the moth Cydia fagiglandana. A perfect exit hole caused by the departing adult moth is shown by the nut, bottom right. The other three show exit holes that have been enlarged by birds endeavouring to reach a larva, one of which is shown bottom left. Fungal mycelium is present on the two nuts on the right hand side. Note the millimetre scale. (Photographs by Malcolm Inman. From Packham and Hilton, 2002. Arboricultural Journal 26.)

Figure 4.11 Four beech nuts damaged by larvae of the moth Cydia fagiglandana. A perfect exit hole caused by the departing adult moth is shown by the nut, bottom right. The other three show exit holes that have been enlarged by birds endeavouring to reach a larva, one of which is shown bottom left. Fungal mycelium is present on the two nuts on the right hand side. Note the millimetre scale. (Photographs by Malcolm Inman. From Packham and Hilton, 2002. Arboricultural Journal 26.)

remains important. At Buckholt in the mast year of 2000 the proportion of damaged nuts was much higher, being 14.9% for Cydia damage and 2.9% for mould, but this still left a mean of 113.7 viable full nuts per tree in each 7-minute sample. Assessments are made shortly after sampling, so the figures obtained give minimum estimates of such losses. Some seeds left on the ground will be consumed or rot during the winter, and there have been a few occasions when Cydia larvae have later bored their way out of seeds judged to be viable.

Empty pericarps are likely to result from a lack of cross-pollination. Nielsen and de Muckadeli (1954) enclosed branches on 28 beech trees to allow only self-pollination and recorded just 8% full nuts, whereas 58% were full on control trees allowed to cross-pollinate. At the Himley site (Fig. 4.9) in 1987, 29 well-spaced parkland trees bore 32% full nuts, while 20 adjacent woodland trees bore 77% full nuts.

Studies of masting in American beech Fagus grandifolia at a site in Michigan showed a similar basic pattern, broken by failure in pollination which led to large numbers of empty pericarps. As in the English beech mast survey, insect predation of the nuts was irregular, being most noticeable in poor mast years when a higher proportion of the already very small crops were destroyed.

Figure 4.12 Rating of cone crop abundance for Douglas fir Pseudotsuga menziesii in western Oregon and Washington, USA, for the years 1909-1956. This species is the most abundant tree in the USA and has been much planted elsewhere since its seed was first collected by the Scot David Douglas in 1827; its timber is extremely valuable. The small drawing shows the distinctive cone. (Drawn from data from Lowry, 1966. Forest Science 12. These ratings were obtained from the US Forest Service and the Weyerhauser Timber Company. Cone drawn by John R. Packham.)

Figure 4.12 Rating of cone crop abundance for Douglas fir Pseudotsuga menziesii in western Oregon and Washington, USA, for the years 1909-1956. This species is the most abundant tree in the USA and has been much planted elsewhere since its seed was first collected by the Scot David Douglas in 1827; its timber is extremely valuable. The small drawing shows the distinctive cone. (Drawn from data from Lowry, 1966. Forest Science 12. These ratings were obtained from the US Forest Service and the Weyerhauser Timber Company. Cone drawn by John R. Packham.)

4.4.2 Apparent influence of meteorological conditions on cone crop production in Douglas fir

Perhaps the most comprehensive attempt to investigate the meteorological requirements for masting is that made by Lowry (1966), who took the 48-year record of cone crop abundance in Douglas fir illustrated in Fig. 4.12 and correlated it with records of mean monthly temperature and mean monthly precipitation at Salem, Oregon, USA, which is central to the region concerned. He concluded that in this region an abundant cone crop in a given October required a warm January in that year, a high precipitation in March-April the year before, and a cool July 2 years before the harvest.

As with European beech, Douglas fir may go for several years without appreciable seed production; in 1924-1929 coning was never more than light. If October 1923 is taken as an example of a time of abundant coning, the theory is that the January of that year was warm, there was high precipitation in March-April 1922 and that July 1921 was cool. How could these meteorological conditions influence coning? A cool July 2.5 years before coning may be thought of as being too early to influence the reproductive physiology of the trees. The clear statistical link led the author to speculate that other long-term processes might be involved, possibly an adverse effect of a cool summer on pest build-up. The positive influence of high March-April precipitation in the year before coning may be related to the imposition of a suitably optimal intermediate temperature range in the meristematic tissue during a period when the cone primordia (the first cells of the cone) are being laid down. A warm January in the year of coning would avoid frost damage and might enhance formation of microspores and megaspores (pollen and seeds).

4.4.3 Masting in other tree species

Oaks, like beeches and ashes, are also masting trees, though in some species their acorns take 2 years to develop, although those ofthe white oak group develop in one. Their mast years do not necessarily coincide with those of beech. In Britain, moderate acorn crops of pedunculate oak Quercus robur and sessile oak Q. petraea occur at intervals of 3-4 years compared with 2-3 years in beech; even in years of general failure there can be abundant seed in some areas. Years in which there are uniformly and exceptionally heavy crops over considerable areas are not more frequent than every 6 or 7 years in southern England (Jones, 1959), but still more frequent than the 5-12 year periodicity seen in beech. The common pattern in masting species is for seed production to be more uniform between trees in years of high seed production. Abundant crops produced by isolated trees suggest that cross-pollination is not essential to oak. Although the underlying reasons are not fully understood, they are probably related to climate; in many continental parts of Europe 20-25 years may pass without appreciable production ofacorns, though a few trees may fruit heavily. Masting can have wide ramifications on the forest fauna. For example, Jones et al. (1998) highlight a link between oak masting, gypsy moth outbreak and Lyme disease in eastern USA. Mast years in the oaks, particularly the red oak Quercus rubra with its large acorns, led to increases in the number of white-footed mice Peromyscus leucopus the following summer since acorns are an important food source for them. Mice are also important predators of the pupae of the gypsy moth Lymantria dispar, a serious pest that defoliates millions of hectares of oak forest, so poor-acorn years may be an important factor in controlling gypsy moth outbreaks. The abundance of acorns also attracts white-tailed deer Odocoileus virginianus to the oak forests; in the autumn of mast years the deer spend more than 40% of their time in oak stands compared with less than 5% in non-mast years. Both the mouse and the deer are the main hosts of the black-legged tick Ixodes scapularis which also increases in density during and immediately after mast years. This tick is the vector of the bacterium Borrelia burgdorferi which causes Lyme disease in humans (see Section 5.7.1). Since deer numbers lead to high tick densities in the autumn of mast years, and the mice, which are primarily responsible for infecting the ticks with the disease, are abundant the following summer, the probability of humans being bitten by a tick, and of that tick carrying Lyme disease, increases dramatically 1-2 years after a mast year. Control is difficult since removing acorns would aid the gypsy moth and leaving the acorns would promote Lyme disease.

Not all mammals respond equally to a mast year, depending upon their food supply. In the hardwood forests of north-eastern USA, mast crops of red oak resulted in increased numbers of white-footed mice and deer mice Peromyscus maniculatus but did not affect red-backed voles Clethrionomys gapperi. In contrast, large red maple seed crops led to higher numbers of the vole but not the mice (Schnurr et al, 2002).

The amount of fruit produced by the European small-leaved lime Tilia cordata, like that of many forest trees, varies greatly from year to year. The direct influence of climate is most marked at the northern limit of distribution, which is reached in northern England but extends further north in Finland and Sweden and is likely in future to be modified by the effects of global warming. At its northern English limit the species flowers in July and August, but produces only small quantities of fertile seed and then only in very warm summers such as that of 1976. This is caused by a failure of fertilization in the present cool oceanic climate of Britain. The more continental climate of Finland allows fertilization in most years, but the seeds of northern trees fail to complete their development by autumn. In central Europe fruiting is prolific and much more frequent, commencing when the trees are more than 25 years old. In Russia, small-leaved lime produces the best quality seed in the upper part of the crown; initiation of inflorescences and formation of fruit occur only on unshaded branches. This is also true even of trees in the Polish BialowieZa Forest (pronounced Bee-ow-a-vey-sha), where July and August are normally warm and sunny with mean air temperatures of 19-20 °C. In the unbroken forest, production of fertile fruit was in 1973 almost confined to the emergent crowns of old trees, around which quite high densities of seedlings were visible even in August.

Growth and behaviour interact with the environment and there are considerable genetic differences within species, so studies of the flowering, fruiting, seed losses and germination of tree species in particular sites are of great value.

Numbers of fruits recorded at the study site in each of four seasons

Figure 4.13 A young seedling of common ash Fraxinus excelsior and diagrams showing fates of ash fruit and seed up to the time of germination (numbers per m2) in Meadow Place Wood, Lathkilldale, Derbyshire, UK, for the crops of 1966-1969. Most of the infested seed was spoilt by caterpillars of the moth Pseudargyrotoza conwagana. The largest seed loss was due to small mammals, notably wood mice Apodemus sylvaticus and bank voles Clethrionomys glareolus. A very small crop was produced in 1967, of which no seedlings survived, and frosting diminished the crop of 1968. Following these two very poor years, the crop of 1969 was very large. Seeds usually germinated in April or early May, about 20 months after falling in autumn (see Section 4.6.2). The values for seeds germinating are therefore based on the seedlings present in the quadrats two years after flowering. (Drawn from the data of Gardner, 1977. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Numbers of fruits recorded at the study site in each of four seasons

Figure 4.13 A young seedling of common ash Fraxinus excelsior and diagrams showing fates of ash fruit and seed up to the time of germination (numbers per m2) in Meadow Place Wood, Lathkilldale, Derbyshire, UK, for the crops of 1966-1969. Most of the infested seed was spoilt by caterpillars of the moth Pseudargyrotoza conwagana. The largest seed loss was due to small mammals, notably wood mice Apodemus sylvaticus and bank voles Clethrionomys glareolus. A very small crop was produced in 1967, of which no seedlings survived, and frosting diminished the crop of 1968. Following these two very poor years, the crop of 1969 was very large. Seeds usually germinated in April or early May, about 20 months after falling in autumn (see Section 4.6.2). The values for seeds germinating are therefore based on the seedlings present in the quadrats two years after flowering. (Drawn from the data of Gardner, 1977. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Observations of common ash at Meadow Place Wood (Fig. 4.13) demonstrated the very large proportions of the seed infested by caterpillars or eaten by small mammals. Although the proportion of seed that germinated was not high in the crop of 1969, a strong mast year, it was so prolific that the number of seedlings that resulted in 1971 was very great. Gardner (1977) found that fruit fall was evenly dispersed throughout the wood and continued from September until the following August. Each ovary contains four ovules of which one usually develops; some fruits contained no seed, 1.1% contained two seeds, and in one case all four ovules were seen to develop.

Common ash is a pioneer species whose seedlings have been recorded 125 m distant from the parent tree. Each tree may produce 10 kg of winged fruits, containing over 100 000 seeds in a mast year. Observations showed an alternation of good and bad years, a basically biennial fruiting pattern. This tree flowers on the wood of the previous year, so lack of carbohydrate or other nutrient reserve during that previous year could prevent initiation of flower buds. In 1968 flowering was considerably less than expected because the flowers of some trees were destroyed by frost and the drain of carbohydrate reserves was low. The fruit crop of 1969 was exceptionally good, presumably because photosynthate for 1969 was supplemented by reserves saved from both 1967 and 1968. Geographical position may also affect masting: seeding of Scots pine Pinus sylvestris is more irregular in the north of Sweden than in the south, where mast years are hardly discernible.

Different species of trees in temperate woodlands may mast together in the same season, but mast fruiting at the community level is especially spectacular in certain evergreen rain forests on nutrient-poor white sand soils in Borneo and Malaysia. Here various species of dipterocarp fruit synchronously over areas of several square kilometres at intervals of 5-13 years. Masting is normally rare in the tropics, presumably because seed predators can move and change diets in high enough numbers to eat excess seeds in a mast year. But the dipterocarp forests of south-east Asia are animal-poor and the extraordinary seed falls in mast years undoubtedly overwhelm the meagre seed eaters (see Section 4.4.4).

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    What is masting in a woodland mean?
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