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Trees: Pinyon Pine

Single leaf Pinyon Pine - Pinus Monophylla

Regeneration Processes:

-- Fertilization
Reproduction in singleleaf pinyon is by seed and does not occur naturally by vegetative means. It bears both male and female flowers and is wind pollinated. Cone and seed development require 3 seasons and about 26 months. The seed bearing cones begin growing the season before the spring they appear and are dormant through the preceding winter.



Pollen release is controlled by local conditions. Once pollinated, growth of the pollen tube undergoes another winter dormancy period prior to fertilization. After fertilization, seeds develop rapidly, mature, and disperse about 6 months later, the 2nd autumn after pollination. With so much time between so many stages, cone and seed crops are exposed to a number of variables that affect the seed crop, such as weather, predation, and internal competition for resources between the previous and current year's cones; therefore, seed production is highly variable from tree to tree, year to year, and place to place.

Bearing Cones

Singleleaf pinyon generally begins bearing cones at about 35 years of age, begins producing good seed crops at about 75 to 100 years, and reaches maximum production at about 160 to 200 years. Singleleaf pinyon exhibits region-wide synchrony in cone production, masting every 2 to 3 years. Some seeds may be produced every year, and good seed crops occur somewhere over a geographical area (e.g. the Great Basin) nearly every year.






A 5-year study reported per acre cone production as follows: 1975, 765 cones; 1976, 0 cones; 1977, 2,560 cones; 1978, 2,325 cones; 1979, 585 cones. Mast years in singleleaf pinyon may be related to the polar front jet stream.
Singleleaf pinyon is said to be more productive and predictable than Colorado pinyon, with seed production being predicted fairly accurately 2 years in advance and more accurately 1 year in advance. Seed production and survival may be affected by several insect pests of cones and seeds. For example, the pinyon cone beetle can destroy more than 50% of the crop of its host pinyon, while coneworms tunnel in cones and shoots but are of minor importance. For other pests, such as the pinyon cone borer, the magnitude of the effects is unknown.


Pinyon Seeds

Because singleleaf pinyon seeds are totally wingless, seed dispersal is dependent on vertebrate dispersers that store seeds in food caches, where unconsumed seeds germinate. This dispersal mechanism is a good example of a co-evolved, mutualistic, plant-vertebrate relationship. The cone scales of singleleaf pinyon have a membranous tissue that holds the seed in place after the cones open, protecting them from ground-foragers and keeping them available to avian dispersers.


Membranous tissue holding seeds in place

Several corvids are responsible for dispersal of singleleaf pinyon seeds over distance. Scrub and Steller's jays forage alone or in pairs, carrying 1 to a few seeds less than a mile before burying them. Pinyon jays forage in flocks of hundreds, carrying about 40 nuts each up to 5 miles (8.3 km) before caching them in the soil. Clark's nutcrackers forage in somewhat smaller flocks, each carrying several dozen seeds a distance of up to 13 miles (22 km) and burying 1 to 15 seeds per cache, 1 to 3 cm deep in gravelly soil, mineral soil, or duff. Seed caching by Clark's nutcrackers begins late August to early September. In a good seed crop year, an individual Clark's nutcracker may scatter-hoard 17,900 singleleaf pinyon seeds. The large range of singleleaf pinyon may be attributable, in part, to seed dispersal by these birds. Seed dispersal by humans in the past has also been suggested.
Chipmunks, squirrels, deer mice, pinyon mice, Great Basin pocket mice, and Panamint kangaroo rats all scatter-hoard singleleaf pinyon seeds locally. These animals consume most of the seed, but some is left to germinate. Quantitatively, these rodents are less effective than avian dispersers, since it is only in mast years that large numbers of seeds fall to the ground and become available to rodents. The seed characteristics and the microhabitats in which seeds are placed are important in determining their fate after dispersal. Rodents are qualitatively effective dispersers of singleleaf pinyon seed since they tend to bury seeds under and adjacent to shrubs, whereas avian dispersers tend to cache seeds in interspace environments, a less suitable environment for singleleaf pinyon seedlings.

Vertebrate disperser

In general, pinyon seeds are short-lived with little innate dormancy and thus form only a temporary seed bank. Fresh seeds have 85 to 95% viability, but this decreases in a year or less. Most seeds germinate the spring following dispersal, requiring 28 to 90 days of cold stratification for germination. Germination and establishment are most likely when favorable moisture conditions follow a mast year. Seeds may germinate in the open, but seedling establishment in the open is rare.


Pinyon Seedlings

Singleleaf pinyon seedling establishment is episodic. Population age structure is affected by drought, which differentially reduces seedling and sapling recruitment more than other age classes. Top-growth of seedlings is slow (1.0 inch (2.5 cm) per year in height, and 0.012 inch (0.3 mm) per year in diameter). Root growth is more rapid, with the taproot reaching 6 inches (15 cm) 10 days after germination. Seedlings can thereby withstand soil water below the wilting point for about 2 weeks. However, field drought conditions are often more severe than this, and so seedlings only survive in the most favorable microenvironments. Singleleaf pinyon seedlings survive best in the microhabitat provided by nurse plants, where organic matter, nutrient concentrations, relative humidity, water infiltration, and water holding capacity tend to be higher, and irradiance and soil temperatures tend to be lower. However, nurse plants also compete for water and nutrients, so the trade-off is slower seedling growth rate. Seedlings maintain a more favorable water status and have greater drought avoidance than shrub nurse plants. The complex interaction between seedlings and nurse plants is a balance between facilitation and competition on moisture and light gradients.


Pinon Seedling

Ecotones

The ecotones between singleleaf pinyon woodlands and adjacent shrublands and grasslands provide favorable microhabitats for singleleaf pinyon seedling establishment since they are active zones for seed dispersal, nurse plants are available, and singleleaf pinyon seedlings are only affected by competition from grass and other herbaceous vegetation for a couple of years. This facilitates expansion of woodlands along these ecotones, with singleleaf pinyon seedlings eventually overtopping and shading out the shrubs. Conversely, singleleaf pinyon seedlings establishing under adult trees have little chance of maturing unless the adult tree is removed or dies.


Pinyon Growth

Singleleaf pinyon is slow growing. A dominant tree requires about 60 years to reach 6.6 feet (2 m) in height, and about 150 years to attain 28 feet (8.5 m) in height and a stump height diameter of about 12 inches (30 cm). Average annual height and diameter growth of immature dominants is about 2 inches (5 cm), and 0.04 to 0.20 inch (1 to 5 mm), respectively. Growth rates vary considerably even among trees on identical sites, and are greatly influenced by competition for severely limited water supplies. Dominant trees may maintain constant diameter growth rates for more than 200 years. Observed reductions in growth rates with age are likely caused by increasing competition as stands develop, and no definite age of culmination of growth has been determined.


Pinon, 7-8' feet tall


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