Mast is the "fruit of forest trees like acorns and other nuts". The term "mast" comes from the old English word "mæst", meaning the nuts of forest trees that have accumulated on the ground, especially those used historically for fattening domestic pigs, and as food resources for wildlife. In the aseasonal tropics of South-East Asia, entire forests including hundreds of species are known to mast at irregular periods of 2–12 years.
More generally, mast is considered the edible vegetative or reproductive part produced by woody species of plants, i.e. trees and shrubs, that wildlife species and some domestic animals consume. Mast is found in two forms. Mast is generated in large quantities during mast seeding events (or masting, mast events). Mast seeding is a population-level phenomenon that is hypothesized to be driven by economies of scale, weather, and resources. These pulses of resources drive ecosystem-level functions and dynamics.
Types of mast
Other tree and shrub species produce a soft mast, such as raspberries, blueberries, and greenbriar.
Mast seeding is defined as the highly variable production of fruit by a population. These intermittent pulses of food resources drive ecosystem-level functions and forest dynamics. The difference between a mast seeding year and a non-mast seeding year can be thousands of acorns, hickory nuts, beech nuts, etc. Mast seeding dominantly occurs in wind-pollinated tree species, but has also been observed in grasses and Dipterocarps.
Hypothesis for the evolution of mast seeding can broadly be assigned to: economies of scale, resource matching, and proximate cues (i.e. weather).
Economies of scale
The predator satiation hypothesis states that predator populations are controlled by inconsistent pulses of resources, over satiating seed predators in mast seeding years to allow a proportion of seeds to escape, while a lack of resources keeps predator populations low in intervening years. In plant communities with a local abundance of frugivores, large seed releases can effectively exceed seed predation and improve the chance of successful establishment. The pollination efficiency hypothesis states that mast seeding would be selected for to optimize successful pollination and thus fertilization if all individuals within a population synchronized reproductively. This hypothesis is especially relevant for wind-pollinated species, which many mast seeding species are. Both hypotheses are based on the assumption that variable and large reproductive effort is more efficient than small consistent reproductive effort, which will lead to higher fitness within a population.
Resources and weather
The resource matching hypothesis states that reproduction varies with availability of resources. Main limiting resources include water, carbon in the form of nonstructural carbohydrates, and nutrients such as nitrogen and phosphorus. These resources have been shown to be depleted after mast seeding across multiple species.
Weather is categorized as a proximate driver of mast seeding, meaning that in combination with resources and economies of scale, various weather parameters can have an effect on the probability of mast seeding occurring. The effect of weather on mast seeding is highly variable depending on species and geographic location. For some species of oak, mast seeding was shown to be influenced by regional weather-related cues on phenology. Cues included spring temperature, summer drought, and spring frost. These weather variables are associated with critical times for fruit maturation and fertilization.
Mast seeding provides food for animals like mice, rats and stoats, whose populations can explode during a mast year, having been reduced by a lack of food in previous non-mast years. In turn, this makes it more likely that birds subsequently will be targeted by the pests, or that rats will invade nearby fields in what is called a "rat flood". Mast seeding has been shown to have both positive and negative effects on an ecosystem. An example of this is the white-footed mouse. When a mast seeding event occurs, the population of white-footed mice also increases, which has shown to increase instances of Lyme's disease since these animals are the main vectors. The positive effect of increased white-footed mouse population is that they prey on Gypsy moths, which are a major forest pest in the eastern U.S.
The interaction between disturbance by fire and mast seeding is key to white spruce regeneration and subsequent stand dynamics in the boreal mixed-wood forest. Peters et al. (2005) found significantly higher densities of white spruce in stands originating from fires that coincided with mast years than from fires coinciding with years of low cone crops. While noting that previous studies had assessed a three- to five-year window of opportunity for obtaining white spruce regeneration after fire before seedbed deterioration closed it, Peters et al. (2005) adduced three lines of evidence to support their claim that the importance of the fire × mast-year interaction hinges on the rapid deterioration of the seedbed, even within one year after fire. Rapid seedbed deterioration is likely to augment the mast-year effect for white spruce as compared with species that are less dependent on short-lived, disturbance-created regeneration microsites. Seed limitation, as well as seedbed deterioration, influences age structure in white spruce. Mast-year effects on the density of white spruce are long-lasting; 40 years after fire, mast-year fires still had 2.5 times more spruce regeneration than non-mast-year fires.
Mast seeding under climate change
Many mast seeding species are considered foundational species. Predicting how the intensity and frequency of mast seeding may be altered under climate change will help researchers to determine shifts food resources availability to wildlife and forest dynamics.
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