Climate change – are forest trees adaptive? (1)

2013.06.03

Mechanisms

The most basic mechanism for adaptation is selection. Generations of trees are genetically shaped by climate and other environmental conditions during the course of time, favouring some genes and discarding others. Thus, tree populations occupying different environmental habitats usually have contrasting phenotypes due to selection. For instance, boreal conifers have a systematic trend (cline) in both growth start and growth cessation from south to north and from low to high altitudes. Another means of adaptation is phenotypic plasticity, which is the ability of a genotype to produce different phenotypes in different environments. A clone of a tree species being able to survive large amplitudes in spring temperatures, including frost events, has a high level of plasticity. Although plasticity is defined at the clonal level, it also applies at the population and species level. Plasticity is particularly important in long-lived species and has probably evolved in response to the large variation in climate during the life span of trees. More recently, yet another mechanism for adaptation has been studied. It turns out that the environment may influence the expression of genes - certain environmental conditions may turn on genes, other conditions may turn them off. This is called epigenetics. In Norway spruce a well-known example is that the growth rhythm of the seedling is affected by the temperature during seed maturation. Low temperatures during the early childhood of seeds tend to produce plants with early growth start in spring and early bud set in late summer, whereas the late spring flush and bud set in summer are associated with high temperatures during seed maturation. The epigenetic way of adaptation in Norway spruce implies that the phenotype may change drastically from one generation to the next. Among forest trees epigenetic adaptation has only been studied thoroughly in Norway spruce. Selection, plasticity and epigenetic modulation work in concert, but not necessarily in complete harmony. Selection works on the phenotype, and in the case of extensive plasticity a greater variation of genotypes may remain reproducing in the population than if plasticity is low. Similarly, an epigenetic modulation of the phenotype is a quick process that does not involve any changes in the DNA sequence neither in the allelic frequencies. As such epigenetic modulation of the phenotype and plasticity may oppose adaptation by selection. The evolution rewards any process that promotes survival and reproduction, irrespective of the mechanism behind, and recent research has shown that decreased selection due to plasticity is more than compensated by the increased phenotypic match with the environment allowed by plasticity. Accordingly, adaptation to environmental heterogeneity based only on selection would probably be too slow and insufficient, and phenotypic plasticity and epigenetic modulation probably add indispensable layers of robustness in long lived trees. In addition to local adaptation trees may migrate, both as seeds and pollen. It has been shown that even extensive gene flow has not prevented local adaptation. Substantial gene flow can also compensate for the long generation time of trees.

Future

Is the combination of these mechanisms sufficient to cope with future climatic changes? It might be that the rate of environmental change exceeds the adaptive capacity of tree populations, particularly at high latitudes where the mean temperature is expected to rise most extensively. Numerous research projects address these issues worldwide. In the meantime we should address mitigating options, and take advantage of the adaptive mechanisms described above. Associated with earlier springs it is likely that the growth start of trees will advance. Networks of phenological gardens have shown that the growing season in Europe increased by about 11 days during 1960-1990, mostly due to earlier bud burst, and climate change is expected to advance the spring even more in the future. One obvious concern is increased frost damages associated with premature budburst. For certain commercial species, such as Scots pine and Norway spruce, bud burst is a trait with high heritability, which implies that this trait can be effectively selected during breeding. Thus, premature budburst may be counteracted by using late breeding material with late growth onset. Similarly, by using plant material with high level of phenotypic plasticity it is possible to increase the survival and viability of the planting stock. Again it turns out that plasticity is genetically controlled, e.g. in Norway spruce some families display highly plasticity for important adaptive traits, whereas other families do not. If there is no suitable breeding material at hand it is also possible to make use of assisted migration, i.e. use provenances with later growth start. In many species southern provenances initiate growth later than northern provenances. There are, however, large differences between species with respect to how far they can be moved along latitudinal and longitudinal gradients. Whereas Scots pine is rather sensitive to transfers, Norway spruce is quite robust. A general guideline for assisted migration would be to use information from previous translocation experiments for the species in question. The preparedness of forestry to climate change may depend on how well scientific information is transmitted to the different stakeholders and users. NordGen Forest will therefore undertake a survey to investigate the awareness of climate change, possible strategies for adaptation, scientific information needs, and possible scientific knowledge deficits among breeding and forestry organisations, and other stakeholders. Based on the results it may be possible to tune the information better with the needs.