Persistence variation and adaptation within the genus Pinus

Pinus has been accurately described as the 'most ecologically and economically significant tree genus in the world' (Richardson and Rundel, 1998, p. 3). It has 111 species, mostly restricted to the northern hemisphere, and this ancient genus, of which the earliest representatives date from the Lower Cretaceous around 120 Ma, is still remarkably successful. Indeed several species, often through the intervention of humans, are currently increasing their range. By the end of the Mesozoic 65 Ma Pinus had diverged into two groups: (1) the hard or white pines which normally have 2 or 3 needles in a bundle (officially called the diploxylon pines with two veins or fibrovascular bundles per needle); and (2) the soft or yellow pines with 5 needles per bundle (the haploxylon pines with one vein and little resin in the wood). The names hard and soft refer to the density of the wood.

The geographical range of pines varies tremendously. The Scots pine P. sylvestris, an example of the first group, has the widest distribution of any

Fibrovascular Pinus

Beneath it is a 2-needled shoot and two diminutive winged seeds of mountain pine Pinus mugo. Next to it is an acorn and the underside of the leaf of pedunculate oak Quercus robur, which has short leaf stalks and auricles - shown in heavy line - at the base of the leaf blade. The adjacent sessile oak Q. petraea (under the beech leaf) has much longer leaf stalks but lacks auricles and its acorns are not borne on long stalks like those of pedunculate oak. Foliage of the common beech Fagus sylvatica is shown on the right along with its nuts viewed from the outside and in section. (Drawn by John R. Packham.)

Beneath it is a 2-needled shoot and two diminutive winged seeds of mountain pine Pinus mugo. Next to it is an acorn and the underside of the leaf of pedunculate oak Quercus robur, which has short leaf stalks and auricles - shown in heavy line - at the base of the leaf blade. The adjacent sessile oak Q. petraea (under the beech leaf) has much longer leaf stalks but lacks auricles and its acorns are not borne on long stalks like those of pedunculate oak. Foliage of the common beech Fagus sylvatica is shown on the right along with its nuts viewed from the outside and in section. (Drawn by John R. Packham.)

pine, being found from the Scottish Highlands along the Atlantic seaboard to the Pacific coast of Eastern Siberia, with relict populations throughout the northern Mediterranean Basin. Conversely, the Canary pine P. canariensis (a hard pine) is found naturally on just five of the Canary Islands off the African coast with a natural range of probably less than 400 km2. Size also varies greatly. The sugar pine P. lambertiana of the western USA (a soft pine) has the greatest height and girth of any pine, reaching more than 75 and 5 m, respectively. In contrast the dwarf mountain pine P. mugo of Europe (a hard pine) is no more than a shrub. Pines can also be long-lived: bristlecone pines P. aristata (a soft pine) are the oldest living organisms on Earth, growing on the White Mountains of California where some individuals have reached ages of nearly 5000 years (Section 1.3.2).

Pines, like many other conifers, typically grow in harsh environments that are cold and/or dry, from near the arctic timberline to acidic nutrient-poor soils in the tropics, perhaps driven to these environments by competition with the later-evolving and very successful angiosperm trees (see Section 3.7.4). The Siberian dwarf stone pine Pinus pumila, is extremely tolerant of cold and so is an important species covering large areas of the arctic and alpine belts in eastern Asia. It reaches a height of only 0.5-5 m. The main cold-adaptive feature of this species, which lacks a central trunk and forms clumps of connected branches, is its ability to lie prostrate beneath snow cover in winter. In order to flourish, this species also needs full sun in summer, good drainage and adequate aeration for its roots. When well-lit, dwarf stone pine produces very adequate crops of seed, but when shaded - as it is when growing in the lower canopy of larch forests - it develops vegetatively, aided by its adventitious roots on prostrate branches (Khomentovsky, 1998). With its strong reproductive potential, the Siberian dwarf stone pine is a most valuable pioneer species on a wide variety of substrates including fresh volcanic ash, burnt tundra and coastal dunes of climatically adverse north-eastern Eurasia. The North American equivalent of this species is the whitebark pine P. albicaulis, a closely related haploxylon pine. Further south in North America very dry soils of near deserts are the home of such species as ponderosa pine P. ponderosa, and the various pinyon pines such as P. edulis and P. monophylla. This continues onto the dry tropical soils of the savannas of Central America and the Caribbean where 47 species of pine form the greatest diversity of pines of any area in the world. Pines resemble other conifers in their tolerance of low-fertility stress, but are marked out by their ability to act as aggressive post-disturbance invaders.

Fire was (and is) a major feature of the surroundings in which the pines evolved and closely influenced the development of their life histories. Some pines meet repeated fires over very long periods; there are records of a ponderosa pine trunk showing 21 dated fire scars between 1659 and 1915. Fire regimes of low-, medium- and high-severity have led to the evolution of different physical and ecological characteristics in many species. Several species reproduce by basal resprouting after fire; the cut stumps of the canary pine P. canariensis produce numerous resprouts on the volcanic soils of Tenerife. Serotinous cones which remain closed for sometimes decades after the seeds mature, but release seeds once high temperatures have melted the resin sealing them shut, are important in the survival strategies of several species such as Monterey radiata pine P. radiata and knobcone pine P. attenuata (see also Section 4.6.2). The lodgepole pine P. contorta var. latifolia and a number of other trees possess both serotinous and non-serotinous populations.

Lodgepole Pine Serotinous Cones

Figure 3.22 Variation in cone size amongst pines. Sugar pine P. lambertiana (a) native to California is a five needle pine with a typical cylinder-shaped cone. Cones of three-needle pines such as the coulter or big-cone pine P. coulteri (b), gray or digger pine P. sabiniana (c) and Monterey pine P. radiata (d), all from California, tend to be large and globular while those of two needle pines, including dwarf mountain pine P. mugo (e) from Europe and jack pine P. banksiana (f) from Canada, are usually much smaller. Note that Monterey pine and jack pine are both serotinous and remain closed until heated. There is also enormous variation in wing size, seed weight and seed size within the genus. Sugar pine, Coulter pine, and gray pine have some of the largest winged seeds while jack pine has one of the smallest. (Photograph by Peter A. Thomas.)

Figure 3.22 Variation in cone size amongst pines. Sugar pine P. lambertiana (a) native to California is a five needle pine with a typical cylinder-shaped cone. Cones of three-needle pines such as the coulter or big-cone pine P. coulteri (b), gray or digger pine P. sabiniana (c) and Monterey pine P. radiata (d), all from California, tend to be large and globular while those of two needle pines, including dwarf mountain pine P. mugo (e) from Europe and jack pine P. banksiana (f) from Canada, are usually much smaller. Note that Monterey pine and jack pine are both serotinous and remain closed until heated. There is also enormous variation in wing size, seed weight and seed size within the genus. Sugar pine, Coulter pine, and gray pine have some of the largest winged seeds while jack pine has one of the smallest. (Photograph by Peter A. Thomas.)

Pines are amongst the most genetically diverse of organisms, showing great variations in form and stature as well as a more than 300-fold variation in seed weight. Cone size is also immensely variable and although there is often little correlation between seed size and cone size, ecological patterns can be picked out (Fig. 3.22). The soft pines (with 5 needles per bundle) tend to have long, soft cylinder-shaped cones, reaching up to 25-50 cm and even 60 cm in the sugar pine. The hard pines typically grow in the driest habitats (or bogs where water is physiologically hard to get due to restricted root growth) and produce hard, more globular cones. Two needle pines tend to produce small cones (often golf ball-sized such as in Scots pine) while the 3-needle pines produce the heaviest cones, typically bigger than a tennis ball and in the case of the coulter or big-cone pine P. coulteri, 25-30 cm in length and weighing up to 2.3 kg each. Species with the biggest cones tend to live in arid environments where food for herbivores is in short supply; it could well be that intense herbivore pressure on the large seeds needed for successful pine establishment in dry areas has made it cost-effective in evolutionary terms to invest in large expensive cones.

Pines are diploid (2n = 24), respond rapidly to selection, and have a mating system which promotes outcrossing (cross-pollination) and the consequent development of diversity while allowing self-pollination, enabling isolated individuals to reproduce and colonize. Seed dispersal is diverse. Initial dispersal by normal winds leaves a high concentration of seeds close to the seed source. If taken, moved, stored but left uneaten by rodents, these seeds can germinate later at more distant sites. On relatively rare occasions winged seeds may be carried long distances by turbulent winds, sometimes leading to new populations in fresh areas. In pines with wingless seeds, birds may disperse and store seeds over both long and short distances (see Section 4.2.2).

Although pines vary greatly in stature according to species and environment, they mostly have a rather similar basic narrow, pyramidal structure whether they are found on an arctic timberline or in the tropics. The pyramid shape helps intercept light from a low angle in high-latitude summers, and in southern dry areas reduces the interception of sun at midday which lowers water stress. Variations are, of course, numerous, such as the broad spreading canopy of the stone pine Pinus pinea of the Mediterranean, sometimes aptly called the umbrella pine. Here the shape (similar in many savanna trees) acts like an aerofoil, diverting winds around the canopy rather than through it, again reducing water loss but allowing vertical convective heat loss. Environmental differences have, however, led to considerable eco-physiologi-cal differences both between and within species, while individuals also adapt to changes in conditions. Pine seedlings growing in low-light conditions show a dramatic reduction in root growth, while older pines in shaded environments are more likely to suffer injury or death due to drought. Although the immediate cause of death is desiccation, the ultimate cause is the reduction in photosynthesis caused by low light. Other conifers such as Norway spruce, silver fir and western hemlock are much more shade-tolerant. There is considerable variation within the genus regarding cold-tolerance and water relations, while many recent studies have been concerned with air pollution (in the Jeffrey pine P. jeffreyi, elevated levels of ozone cause chlorosis) and increased concentrations of atmospheric CO2.

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