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Coniferous Trees Response to Climate Change and Conservation

The last environmental change to discuss, and perhaps the greatest threat to conifer diversity in the future is that in climate. We are already feeling the effects of change, but even if we drastically reduce our production of carbon dioxide and other greenhouse gases, Earth’s climate will continue to warm for the foreseeable future.

The consequences for conifers are difficult to predict accurately, but they are not likely to be beneficial.

On average, wherever conifers grow, they favor cooler sites than nearby flowering plants so that warming trends may tip the balance toward angiosperms, keeping in mind that low nutrient status of soils and a landscape-level fire regimen are other environmental factors favorable to conifers.

Of course, conifers, like most plants, are adaptable and resilient and have survived much climatic change in the past, especially during the rapid glacial-interglacial fluctuations of the past 1.6 million years.

One thing about the present warming, however, that differs from conditions during the repeated abandonment and reoccupation of Canada, the northern United States, and northern Eurasia is that human land use covers much of the surface needed for migration and establishment.

Even if today’s conifers are capable of establishing new populations by northward dispersal of propagules generation after generation, there may be few available places for the seeds to germinate.

That consideration, of course, does not apply to the presently treeless arctic tundra, where human influences (besides climate change) are very low. Even if temperatures become warm enough to support trees north of the arctic circle, any conifers establishing there would still face increasingly long seasons of 24-hour nights northward into the Canadian Arctic Archipelago.

This was truly the last time the high arctic was clothed with forests, during the very warm Eocene, about 50 million years ago. Axel Heiberg Island (and presumably other arctic islands) then supported forests of Metasequoia (Dawn redwood), Glyptostrobus (Chinese swamp cypress), Picea (Spruce), Pseudolarix (Golden larch), and Larix (Larch), among others, in an environment often referred to as the tropical arctic.

This is obviously an exaggeration, and most of the trees endured the dark, if not bitterly cold, winters never experienced in the tropics by being or becoming deciduous. It is not clear how spruces survived these Eocene arctic winters nor how today’s evergreen conifers of the boreal and temperate zones would fare under similar circumstances.

Invasion of newly climatically suitable areas, when possible, is now and will continue to be more obvious than retreat from the southerly portions of their range. Very few species extend to the geographic limits of their full climatic tolerances either to the north or south (except for species at treeline) because, as they approach these limits, they are more limited by competition with other species.

So, as climates warm, the southern edge of the range (or northern edge in the southern hemisphere) is not immediately thrust into unfavorable climate since it will take a while for the northern edge (or southern edge in the southern hemisphere) of unfavorable climate to actually reach the present southern range limit.

Add to this the fact that many conifers are very long-lived trees and it becomes clear that they are likely to persist at their southern limits through several human lifetimes.

It may thus seem, for a time, as if they are actually expanding their ranges with global warming, even if they are in trouble in both the south and the north.

The collapse of populations and vegetational turnover at the warmer edge of the range may then be either gradual or sudden as older trees begin to die off and are not replaced by seedlings and saplings, which may have ceased to become established decades before.

This kind of resistance to change followed by sudden conversion to new forest types has been documented many times in the fossil record and is particularly clear in the record of Carboniferous coal swamps in the eastern United States and elsewhere.

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Conservation in Situ

While all these threats to conifers should be of real concern to everyone, and if human population growth continues unabated not even the most abundant and widespread conifer species may remain safe from the changes of population decline and extinction, at the moment only about 20 conifer species (4% of the total) are critically endangered. A great many of the rare species around the world have at least some populations within national parks, nature reserves, and other more or less protected areas.

The vast majority of these refugees were established around scenic wonders or threatened animal populations rather than the plants, including conifers, that sustain them. Nonetheless, the conifers that happen to be in these parks and preserves are accorded a measure of protection that is of real consequence for many of them.

A few parks and reserves were established explicitly to preserve populations or tracts of conifers. Torreya State Park in Florida houses the rare and local Florida yew (Taxus floridana) as well as Stinking cedar (Torreya taxifolia). It was established before disease virtually eliminated the latter from its slopes. California has many state and national parks and other protected areas centered on conifers.

These include the Redwood (Sequoia sempervirens) and Giant sequoia (Sequoiadendron giganteum) parks, the Schulman Grove in the White Mountains, housing the world’s oldest trees (Methuselah and other Great Basin Bristlecone Pines, Pinus longaeva), and the Torrey Pines State Reserve, housing the tiny mainland population of Pinus torreyana.

Numerous botanical reserves, some of which were established to protect conifer species or communities, are found in places like New Caledonia and China.

While these reserves do not ensure the continued survival of endangered conifer species, they certainly help, now and at least into the near future.

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Conservation ex Situ

Conservation away from natural stands (ex situ conservation) is also extremely important. While dedicated conservation programs in botanical gardens are useful, having a species in general cultivation is fundamentally better insurance.

There are only a few cases as yet of the benefits of being in cultivation for conifer conservation. The most obvious is Stinking cedar (Torreya taxifolia), already mentioned.

The only individuals of this species free from disease and capable of producing seed are found in cultivation far from the natural stands.

There has also been a program of vegetative propagation by rooted cuttings of genetically diverse individuals from the natural range (the disease affects only the roots). Maintenance of genetic diversity is the essential foundation for all conservation programs.

Luckily, most conifers start out at the higher end of the genetic diversity spectrum, at least among the relative handful studied.

The distribution of that diversity is such that a good sampling of a single population will capture much of the allelic diversity in a whole species (although additional variation will be captured with more populations).

This applies primarily to allozymes and other molecular markers while morphological and physiological diversity will often have a strong geographic component (related to adaptation to differing environments) and will require more extensive sampling to preserve it effectively.  

Even species like Red pine (Pinus resinosa), Torrey pine (Pinus torreyana), and Western red cedar (Thuja plicata) that buck the conifer trend by having essentially no electrophoretic (allozymes) variation (indicating that they went through “recent” bottlenecks of drastically reduced population size) usually show variation (albeit harder to measure) among populations in ecologically significant characteristics.

One exception may be Wollemi pine (Wollemia nobilis), whose wild individuals are, to all intents and purpose, a single clone in a single population. Thousands of propagules of this species were produced by seeds, cuttings, and micropropagation (tissue culture, which is widely used in orchid propagation).

As this species spreads across the globe in cultivation, it will be unlikely to become extinct, even if a disease emerges that is lethal to all individuals. It will be interesting to monitor these far-flung individuals to see at what rate new genetic diversity may arise in their offspring, whether as sports (somatic mutations) or as seedling variation.

The goal of all conservation programs must be the maintenance of viable, genetically diverse populations in the wild. This will become increasingly challenging as land available for natural vegetation continues to shrink and global change alters the climates of those areas that remain.

Let us hope that we can meet these challenges and that future generations will continue to learn from and be inspired by these fascinating and often magnificent trees and shrubs.

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