Giant sequoia  (Sequoiadendron giganteum)

Classified as Vulnerable (VU) on the IUCN Red List 2007 (1).

The giant sequoia is world-renowned as the largest living thing on the planet, and these majestic trees continue to inspire wonder. Although not the tallest trees, their sheer volume, with the possible exception of colonial organisms such as corals, make giant sequoias the largest living things on earth (3). Also known as 'big tree' in California, the giant sequoia lives up to its name, reaching up to 95 meters in height and 11 meters in diameter (2). The massive, tapering trunk is a characteristic reddish-brown colour; the bark is extremely thick, sometimes up to 60 centimetres, and deeply furrowed (4). In mature trees the first half of the trunk is clear of branches, they form a rounded crown towards the top with individual branches sweeping downward with upturned ends (3). The small, scale-like leaves are green and spirally arranged (4). Both male and female cones are carried on the same tree; female cones are up to 7.5 centimetres long and four centimetres wide, composed of spirally arranged scales. They are reddish-brown when mature and contain numerous, flattened, winged seeds (4).

The giant sequoia is found on the western slopes of the Sierra Nevada mountain range, California, United States (4). Today the range is much more discontinuous than it once was, and the species is restricted to around 75 distinct groves (2).

Inhabits mixed conifer woodlands, which are dominated by the California white fir (Abies concolor var. lowiana) (2). Found in protected areas where there are deep, moist soils, at altitudes between 1,100 and 1,500 metres above sea level (4).

Giant sequoias take around 20 years to reach maturity and start bearing cones, and the oldest known individuals are over 3,000 years old (2). Pollination occurs between December and May and cones develop during the spring and summer months (4). Seeds are only released as the cones dry out, shrinking and thus revealing gaps from which the seeds can fall; the process is therefore dependent on particular conditions and cones can lay dormant for many years (5). Each cone contains roughly 230 seeds and each tree will have around 11,000 cones at any one time; these tiny winged seeds are dispersed away from their parent tree by the wind, insects and rodents (2). The germination of seeds is, however, also dependent on particular conditions and these tiny, thin seeds require highly favourable soils with no overlying vegetation into which they can bury easily (4). As with other long-lived trees, fire seems to pay an important role in the life of a giant sequoia. A relatively high frequency of low intensity fires helps to rid the area of competitors whilst providing rich soils for the germination of seedlings. The heat generated by fires also helps to dry out the cones and open them. Mature trees are fairly indestructible; the loosely packed fibres in the thick bark are very poor conductors of fire (3).

Giant sequoia forests were massively logged from the time they were first discovered in the mid 1800s until the 1950s (1). The resistant nature of the wood made it a favourable timber and it was used to make a wide variety of items from fence posts to patio furniture (2). Roughly 34 percent of the original range of the giant sequoia was lost to timber extraction (4). Ironically, a further threat to sequoia groves came from fire prevention strategies imposed by forest managers; this strategy prevented sequoias from regenerating successfully, whist allowing competitor species to proliferate (4).

The giant sequoia is now recognised as a national treasure and as much as 90 percent of the population is protected (1). The best-known groves are found in Yosemite, Sequoia and Kings Canyon National Parks (3); attracting tourists from far and wide to view these magnificent trees. The National Parks Service now practices controlled burning as part of its management strategy, although further research is needed into the natural cycle of disturbance in order to better understand these processes (4). Giant sequoias have a vital role within the Sierra Nevada ecosystem and their majesty has also provided an aesthetic and cultural role within society; thus making their future survival extremely important (6).

For further information on the giant sequoia see:

• NatureServe Explorer:
  www.natureserve.org/explorer/servlet/NatureServe?searchName=Sequoiadendron+giganteum