Mast Seeding

Mast seeding in trees, with a synchronous super abundance of fruit and seed production by certain species over an extensive geographic area at irregular intervals, is a familiar concept. Less well known is the fact that masting (or mast fruiting) behaviour can also be found in marginal areas in a number of herbaceous species. Despite the striking floral displays that can be observed in early summer in most alpine habitats not all alpine

Tussock Grass Non Polar Alpine Regions
Fig. 4.29 A stand of the tussock snow grass Chionochloa rubra in autumn at the Lewis Pass, Southern Alps, New Zealand. This genus is noted for flowering at irregular intervals and having mast years with extensive synchronous flowering.

plant species flower with predictable regularity. The high-altitude snow tussock grasses of New Zealand (Chionochloa sp.) are highly variable in their flowering and some species, e.g. C. pallens, may not flower for a number of years and then have a year with synchronous flowering across extensive populations (Fig. 4.29). Synchronous flowering at irregular intervals is also found in other New Zealand alpine genera such as Aciphylla and Celmtsta (Mark, 1970; Fig. 4.30). The essential biological feature of mast years is that they occur synchronously at irregular intervals. This then leads to the synchronous production of large quantities of seed; hence the use of the word mast (Germanic root maat, food). Masting is most commonly observed in wind-pollinated, long-lived plants, and the oaks, beeches and pines of the northern hemisphere have been the most studied species. Early explanations of massive synchronous flowering were based largely on the resource-tracking hypothesis emphasizing the need for resources, and in particular carbohydrate reserves, for sustaining a large reproductive effort. Further discussion of the ecological advantages of mast years has given more prominence to the concept of predator satiation that comes from having a year of super abundant seed

Fig. 4.30 Aciphylla horrida, a New Zealand upland dioecious umbelliferous species which flowers at irregular intervals in mast years with extensive synchronous flowering.

production followed by varying periods with lower starvation levels when the amount of fruit and seed production is only sufficient to support a smaller population of potential predators (Harper, 1977). However, in the last decade a number of additional hypotheses have been suggested. When the various possible causes of masting are considered collectively they can be divided into two classes: first, proximate causes such as weather, stored levels of carbohydrate or environmental signals, and secondly, ultimate causes which include several possible evolutionary advantages of variable seed output.

A study of mast fruiting in the tropical ectomy-corrhizal tree Dicymbe corymbosa (Caesalpiniaceae) has drawn attention once again to a possible link between ectomycorrhizas (EM) and mast fruiting (Henkel et al., 2005). There is no proof of a universal association between masting and EM symbiosis but nevertheless many tree species which exhibit this irregular pheno-logical pattern in fruiting are mycorrhizal, as is seen in the Pinaceae, Fagaceae and Betulaceae of temperate forests, and also for several prominent tree families in the tropics, including the Dipterocarpaceae in South-east Asia and Caesalpiniaceae in Central Africa and northern South America. The argument for a role for EM in masting is largely circumstantial and is based on the need for mineral resources, particularly phosphorous. In tropical regions the forest soils are relatively nutrient poor. In Guyana, where the rainforest canopy tree Dicymbe corymbosa occurs, the amount of litter that falls at the end of mast year has been measured as nearly 3 t ha~\ which implies that a long intermast period is needed to recover from such a loss as well as an efficient ectomycorrhiza-mediated nutrient cycling mechanism to replenish phosphorous and other minerals lost to mast flowering and fruiting (Henkel et al., 2005).

One of the more compelling explanations that still deserves attention for the evolution of mast seeding has been the suggestion that it is associated with wind pollination (Kelly & Sork, 2002). Many of the well-known masting species are wind-pollinated trees. Less well known, however, are the wind-pollinated grasses that also exhibit the masting habit. Synchronous mass flowering at irregular intervals in polycarpic species, or as a terminal event in monocarpic species, will ensure economies of scale in total reproductive effort and ensure fertilization and seed production, particularly where there is a significant resource investment in the female flower. A striking feature of mast years is not just the greater production of flowering but also the more successful production of mature fruits and viable seeds.

Wind-pollinated species differ from animal-pollinated species, as the latter risk lowering their reproductive success by satiating their pollinators if too many flowers are produced at once. This is not the case with wind pollination, as seen in the mast seeding of outcrossing trees, such as oaks and pines where it maximizes pollination efficiency. By flowering at the same time, trees maximize their chances of pollination and minimize waste, an assertion that presupposes that wind-pollinated species are indeed pollen limited, for which there is increasing evidence (Koenig & Knops, 2005).

The synchronization of flowering, followed by increased seed production, is one of the most intriguing aspects of mast seeding, especially as large scale surveys indicate that conifer genera at sites as far as 2500 km apart spatially synchronize their seed production. As a result of a study of Californian oaks over a period of 11 years it has been possible to relate the geographically widespread synchronization of flowering to climatic signalling. In the case of the blue oak a synchrony of acorn production was detected that appeared to operate almost universally on an astonishingly large population of 100 to 200 million individuals (Koenig & Knops, 2005). Such widespread synchrony appears to rule out direct signalling between plants and also local effects such as those that might be attributed to mycorrhizal associations. Instead it would now appear that mast seeding is another example of the synchronization of population dynamics by environmental fluctuations through the operation of the 'Moran effect, so named from work in the 1950s by Patrick Moran, an Australian statistician. Moran showed that an external factor, such as weather, that acts across separated populations with similar ecophysiology would tend to produce correlated changes in their abundance and hence synchronize their population cycles (Post, 2003). Moran effects appear to be particularly noticeable in marginal situations and have been detected in reindeer population dynamics in Spitsbergen (Aanes et al., 2003) as well as in subarctic winter moth populations in coastal birch forests (Ims et al., 2004). In Norway, the timing of flowering in three species of flowering plants in 26 populations spanning several hundred kilometres were found to be synchronized with the Arctic Climatic Oscillation over distances up to 500 km (Post, 2003). It appears also that in California, mean temperature in April is spatially synchronized with acorn production by blue oaks as a result of warm, dry conditions during flowering which generally correlate with a larger acorn crop in several oak species (Koenig & Knops, 2005).

Whatever the ultimate causes for the evolution of the masting behaviour some of the proximate causes have a particular relevance for plants in marginal situations. In Austria spruce mast years are reported as occurring at favourable sites every 3-5 years, whereas at higher altitudes the frequency is reduced to every 6-8 years and at marginal sites at the timberline only every 9-11 years (Tschermak, 1950, see Tranquillini, 1979). In an extensive study of flower and seed production in the mountain beech of New Zealand (Nothofagus solandri var. cliffortiodes; Figs. 4.31-4.32) it was also found that mast years declined in frequency with increasing altitude (Allen & Platt, 1990). The situation at the polar timberline, however, is more extreme and

Fig. 4.31 Nothofagus solandri var. cliffortiodes festooned with Usnea growing at an altitude of 870 m near the Lewis Pass, Southern Alps, New Zealand. Mast years in this species become less frequent with increasing altitude (see Fig. 4.32).

appears to be changing. Prior to the 1920s, mast years for Pinus sylvestris at its northern limit in Finnish Lapland and neighbouring Norway were every 60-100 years. Since then, as a result of the climatic improvement that was first noticed in Finland in the early years of the twentieth century (Erkamo, 1956), the frequency of mast years has increased to about every 10 years (Tranquillini, 1979).

Seed weight and viability is also observed to decrease with altitude. Where this has been studied in spruce seed in Austria weight decreases from 900 mg 1000-1 seeds at 1000 m. a.s.l. to 500 mg 1000-1 seeds at 1700 m (the treeline) while germination capacity also falls to below 5% at the timberline (Holzer, 1973, 1950, see Tranquillini, 1979). A similar effect is seen in rowan (Sorbus aucuparia). In the oceanic climate of western Scotland, the high lapse rate and large increase in exposure with increasing altitude are probably relevant factors in the very marked reduction in rowan seed viability with altitude from 67% at sea level to 33% at the relatively modest elevation of 567 m (Barclay & Crawford, 1984).

In Nothofagus spp. in New Zealand there is a marked improvement in seeding in the year following

Fig. 4.32 Mast seeding in Nothofagus solandri var. cliffortiodes. (Reproduced with permission from Allen & Platt, 1990.)

favourable summer temperatures. This is reflected in a number of unrelated genera (Ogden et al., 1996) and presents a convincing argument that climatic variables are at least a dominant proximal cause of mast years in many species, particularly in the more peripheral regions. Whether or not the climatic cause is due to resource variability or to a specific climatic signal that is sensed by a number of unrelated species is still open to debate.

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