The Masting Behavior of Trees

 

            Mast is a noun of Anglo-Saxon origin (męst) that refers to the accumulation of various kinds of nuts on the forest floor that serve as food for hogs and other animals. The process by which trees produce mast is not surprisingly known as masting. The curious thing about masting is that it is not a continuous process, but rather is cyclic. Approximately every three to five years certain trees produce prodigious quantities of nuts; in between the "masts" they will produce almost none. It is a matter of common experience that different species of trees exhibit this behavior at the same time over a large geographic area. The first question is why individual trees regulate their nut production in a boom or bust manner; the masting behavior of an individual tree is called variability.  The second and more profound question is how masting trees manage to coordinate the same cycle with other trees over a large area; the group behavior of masting trees is called synchrony.

            Variability has had two hypothetical explanations which may be characterized as resource responsiveness and economy of scale.  The basic precept of resource responsiveness is that an individual tree will respond to the resources at its disposal. In a good year with plenty of rain and sunlight, a tree would have more resources with which to manufacture more nuts, which would subsequently be more likely to propagate in a moist, nutrient rich environment. The term economy of scale refers to the general precept that benefits will be magnified by the size of the operation; buying in bulk quantities yields economy of scale cost savings to larger operations.

            Resource responsiveness makes sense; periods of more rain and moderate temperatures should logically result in an increase in reproductive activity. Nature is not that simple, however. The fact is that variations in weather do not correlate with masting; moist and sunny weather does not produce a mast crop any more than dry and overcast weather prevents one. However, there is one aspect of resource utilization by masting trees that does track with mast cycles: the resources expended by the tree. A significant resource investment must be made by a tree to produce a mast crop. What this means is that trees grow slowly during mast years and more rapidly in non-mast years as the resources are shifted from reproduction to growth.   This suggests that masting is a part of a complex evolutionary behavior pattern that must stem from an ecological stimulus which implies the generation of masting as an economy of scale strategy.

            There are two elements of the economy of scale hypothesis for masting variability: predator satiation and pollination efficiency. In predator satiation, masting is theorized to be stimulated by a tree's strategy for survival in a world of nut-eating predators. By producing a gargantuan nut crop, the predators become satiated so that an adequate number of nuts will survive to succeed in propagation. The predator population is held in check during the non-mast years, when the paucity of production is reflected in declining predator populations. In the economy of scale paradigm, one can say that each nut in a mast year has a greater probability of escaping predation.

            Pollination efficiency is the second hypothesis for the economy of scale of the masting behavior of trees. It is based on the notion that it is more efficient from the resource standpoint for a plant to successfully propagate if there are a large number of sites for germination. This is not true for all plants; those that employ insect pollination mete out their attractive flowers with some restraint so that their intended foragers are not overwhelmed. Chicory is a good example; only a few flowers open each day and each expires at day's end. However, masting trees are wind-pollinated, relying on the wind to transport the pollen  from staminate to pistillate flowers, a rather precarious and random process.  It is therefore advantageous for them to fill the air with pollen from many trees at the same time, saving up energy during off-years. Fungi are also mostly wind-dispersed and accordingly produce spores in prodigal proportions; a giant puffball has been estimated to contain about 7 trillion spores.

            Field testing for syllogistic evidence of predator satiation and pollination efficiency as causative factors for the masting behavior of trees is difficult and the results accordingly tenuous. For example, a study of masting trees in a 6 hectare study area estimated pollination efficiency by counting the total number male flowers and the number of nuts produced from1988 to 1993. Testing for predator satiation is even more difficult; one must not only show that predators were satiated but also that the interval between masting events was sufficient to result in a decrease in predator population. The same study utilized the number of nuts that had evidence of insect predation relative to those that were undamaged as a measure of the former and the year to year variance in nuts with evidence of insect predation as a measure of the latter. The study inconclusively found that both effects were observable, pollination efficiency having the greater correlation and thus having some predominance over predator satiation.

            But the real conundrum is not why trees mast as individuals (variability), but how they coordinate their activities over large areas and across different species (synchrony). It is a matter of direct observation and scientific study that they do. A survey of acorn production was initiated in 1994 to quantify acorn production of blue oaks at 10 different sites at separated by up to 700 kilometers in California. The conclusion, after eleven years of study was that acorn production extended to virtually every one of over 100 million trees. A more ecumenical literature survey of relevant references on nut production by various trees was organized by W. D. Koenig, a professor at U C Berkeley. A review of 72 sites and 5,000 data points revealed that synchronization of seed production was statistically significant in populations separated by 2500 kilometers. One may conclude that synchrony occurs over long distances and involves almost every tree. So how do they do it?

            Three mechanisms are germane to any discussion of synchronization of activities among plants or animals: chemical, reproductive and environmental. The use of chemicals to transmit signals among individuals is common; however, it is not likely that this is pertinent to the case of masting as chemicals act over much shorter ranges than is observed in masting tree populations. Reproductive synchronization in arboreal terms is called pollen coupling. The concept is that if a tree depends on the pollen from a second tree to produce the fruit nut, then it must be synchronized with it. Implicit in this is that the tree that is providing the pollen must be at some relevant distance away. The effective distance over which pollen is effective in achieving fertilization is of value in forest management; recently completed studies have revealed that pollen is only effective within a range of about 60 meters, hardly on the order of the observed ranges of masting behavior.  Since it is not likely that chemical or reproductive effects could result in the long distance synchrony of masting, environmental is the only other reasonable choice.

            The notion that resource responsiveness to the environment caused the masting behavior of individual trees (variability) was ruled out above. So one might well ask how environmental resources that could not cause masting variability would nonetheless be the cause for masting synchrony.  It is the difference between the weather and the climate, the former term referring to the short term manifestation of the latter. The idea that the environment can cause synchronous fluctuation in population size is not new. It is called the Moran effect after the Australian statistician who showed that the correlation of two populations at different locations was equal to the correlation in their common environmental influence (if they were subject to the same basic parameters).  It has been demonstrated empirically in many organisms, from viruses in the body to caribou in Greenland.

            It is therefore likely that geographically wide-ranging climate conditions cause trees to mast in synchrony. It is not known at present what aspect of the climate might predominate, if, in fact, it is that simple. There is some evidence that temperature may be a key parameter. The study of the California oaks was correlated with the mean temperature in April over the course of the eleven years of data. April was chosen as the most important month for masting, as it is when the trees produce the male and female flowers that produce the nuts that ultimately fall as mast. The spatial synchronization of April temperatures was found to be even more strongly correlated than the masting of the oaks. The syllogism is clear: the periodic fluctuations of temperatures (perhaps caused by the cyclic El Nino phenomenon) operate in synchronization with masting over the same geographic area.

            The masting of trees is an important phenomenon in the ecological balance of the forest, as the food chain becomes distorted with a surfeit of nutrient resources. For example, high mast production promotes rapid expansion of the populations of predator acorn-eating mice and deer. The white footed mouse is a host for the spirochete Borrelia burgdorferi which is the cause of Lyme disease.  The larvae of the black-legged tick frequently feed on white-footed mice and thus become vectors for the spirochete which they impart to deer and the occasional human. Thus a mast year can also be a year with a high black-legged tick population and a concomitant high incidence of Lyme disease.  Predators may be satiated or starved and the trees may or may not efficiently distribute their pollen according to the efficacy of the mast, which must occur at the same time at the same place for it to work. Its synchrony is apparently caused by the climate; the impact of a changing climate on masting may be just one aspect of the larger problem, but it is a compelling one.