Pinus lambertiana

The sugar pine is the tallest and largest Pinus species, commonly growing to 40–60 meters (130–195 ft) tall, exceptionally to 82 m (269 ft) tall, with a trunk diameter of 1.2–2.5 m (3 ft 11 in – 8 ft 2 in), exceptionally 3.5 m (11 ft 6 in).[2] The tallest recorded specimen is 83.45 m (273 ft 9 in) tall, is located in Yosemite National Park, and was discovered in 2015.[3] The second tallest recorded was "Yosemite Giant", an 82.05 m (269 ft 2 in) tall specimen in Yosemite National Park, which died from a bark beetle attack in 2007. The tallest known living specimens today grow in southern Oregon and Yosemite National Park: one in Umpqua National Forest is 77.7 m (254 ft 11 in) tall and another in Siskiyou National Forest is 77.2 m (253 ft 3 in) tall. Yosemite National Park also has the third tallest, measured to 80.5 m (264 ft 1 in) tall as of June 2013; the Rim Fire affected this specimen, but it survived.

Old sugar pines in the Rogue River – Siskiyou National Forest, southern Oregon

The bark of Pinus lambertiana ranges from brown to purple in color and is 5–10 centimeters (2–4 in) thick.[2] The upper branches can reach out over 8 m (26 ft).[2] Like all members of the white pine group (Pinus subgenus Strobus), the leaves ("needles") grow in fascicles ("bundles") of five,[2] with a deciduous sheath. They are 5–11 cm (2–4+1⁄4 in) long.[4] Sugar pine is notable for having the longest cones of any conifer, mostly 10–50 cm (4–19+3⁄4 in) long,[2][5] exceptionally to 60 cm (23+1⁄2 in) long (although the cones of the Coulter pine are more massive); their unripe weight of 1–2 kilograms (2.2–4.4 lb) makes them perilous projectiles when chewed off by squirrels.[2] The seeds are 1–2 cm (1⁄2–3⁄4 in) long, with a 2–3-centimeter (3⁄4–1+1⁄4-inch) long wing[5] that aids their dispersal by wind. Sugar pine never grows in pure stands, always in a mixed forest and is shade tolerant in youth.[6]

The sugar pine occurs in the mountains of Oregon and California in the Western United States, and Baja California in northwestern Mexico; specifically the Cascade Range, Sierra Nevada, Coast Ranges, and Sierra San Pedro Martir. It is generally more abundant towards the south and can be found growing in elevations between 500 and 1,500 m (1,600 and 4,900 ft) above sea level.[2]

The massive 31 gigabase mega-genome of sugar pine has been sequenced in 2016 by the large PineRefSeq consortium.[7] This makes the genome one of the largest sequenced and assembled so far.[7]

The transposable elements that make up the megagenome are linked to the evolutionary change of the sugar pine. The sugar pine contains extended regions of non-coding DNA, most of which is derived from transposable elements. The genome of the sugar pine represents one extreme in all plants, with a stable diploid genome that is expanded by the proliferation of transposable elements, in contrast to the frequent polyploidization events in angiosperms.[8]

Almost ripe female cones

In late stage of embryonal development, the sugar pine embryo changes from a smooth and narrow paraboloid to a less symmetric structure. This configuration is caused by a transverse orientation of division planes in the upper portion of the embryo axis. The root initial zone is established, and the epicotyl develops as an anlage flanked by regions of that define the cotyledonary buttresses. At this stage, the embryo is composed of the suspensor, root initials and root cap region, hypocotyl-shoot axis, and the epicotyl. The upper (distal) portion of the embryo, which gives rise to the cotyledons and the epicotyl, is considered to be the shoot apex.[9]

The apex has the following four zones:[10]

1. The apical initials produce all cells of the shoot apex through cell division. It is located at the top of the meristem and the cells are larger in size compared to other cells on the surface layer.
2. The central mother cell generates the rib meristem and the inner layers of the peripheral tissue zone through cell division. It presents a typical gymnosperm appearance and is characterized by cell expansion and unusual mitosis that occurs in the central region. The rate of mitosis increases on its outer edge.
3. The peripheral tissue zone consists of two layers of cells that are characterized by dense cytoplasm and mitosis of high frequency.
4. Lastly, the rib meristem is a regular arrangement of vertical files of cells which mature into the pith of the axis.

The large size and high nutritional value of the sugar pine seeds are appealing to many species. Yellow pine chipmunks (Neotamias amoenus) and Steller's jays (Cyanocitta stelleri) gather and hoard sugar pine seeds. Chipmunks gather wind-dispersed seeds from the ground and store them in large amounts. Jays collect seeds by pecking the cones with their beaks and catching the seeds as they fall out. Although wind is a main dispersion factor of sugar pine seeds, animals tend to collect and store them before the wind can blow them far.[13]

Black bears (Ursus americanus) rely on sugar pine seeds for their food source in the fall months within the Sierra Nevada. There is relationship between sugar pine seeds and oak acorns, as the bears will feed preferentially on those that are in a higher supply for that season. Both sugar pine and oak species are currently in decline, which can have a direct effect on black bear food sources within the Sierra Nevada.[14]

Sugar pine trees have been impacted by the mountain pine beetle (Dendroctonus ponderosae) which are native to western North America. The beetles lay their eggs inside of the tree and inhibit the trees ability to defend itself against the invading species. The beetles also feed from the trees nutrients which slowly weakens the trees overall health, making the pines more susceptible to other threats like fires and fungal infections by white pine blister rust.[15] Blister rust can weaken the tree and enable further infestation by mountain pine beetles due to the lack of defense from the sugar pine.[16]

Sugar pine starting to succumb to white pine blister rust

The sugar pine has been severely affected by the white pine blister rust (Cronartium ribicola),[17] a fungus that was accidentally introduced from Europe in 1909. A high proportion of sugar pines have been killed by the blister rust, particularly in the northern part of the species' range that has experienced the rust for a longer period of time. The rust has also destroyed much of the Western white pine and whitebark pine throughout their ranges.[18] The U.S. Forest Service has a program (see link below) for developing rust-resistant sugar pine and western white pine. Seedlings of these trees have been introduced into the wild. The Sugar Pine Foundation in the Lake Tahoe Basin has been successful in finding resistant sugar pine seed trees and has demonstrated that it is important for the public to assist the U.S. Forest Service in restoring this species. However, blister rust is much less common in California, and sugar, Western white and whitebark pines still survive in great numbers there.[19]

The species is generally resistant to fire because of its thick bark and because it clears away competing species.[2] However, its mortality has been directly linked to dryer conditions and higher temperatures. Sugar pine trees grow in western North America, a region already impacted by climate change. Higher temperatures within a sugar pine forest can lower resin levels within the tree which will cause less protection against pathogens. At the same time the warmer winters make the survival of the pests and pathogens more likely. The weakened or dying trees then provide fuel to the forest fires, which may become more frequent and more intense, if the climate change results in warmer temperatures in summer, particularly if coupled with drier conditions and stronger winds.[20]

Sugar pine trees are in a slow decline because of the several threats it faces: white pine blister rust, mountain pine beetles and climate change. Efforts to restore sugar pines and other white pine trees that have been impacted by invasive species, climate change and fires have been undertaken by governmental and non-governmental entities. One of the latter is a non-for-profit organization called Sugar Pine Foundation created in 2004 to plant sugar pine seeds in the Sierra Nevada along the border of California and Nevada.[19] They plant seedlings grown from seeds collected from blister rust resistant trees. In order to identify if the trees resistant to that pathogen, Sugar Pine Foundation tested over 500 sugar pine trees and have found 66 resistant trees.[19] The foundation is building a sugar pine population that is resistant to white pine rust because the fungus is a major threat and will continue to kill sugar pine trees at a very high rate.[21]