Trees have genes? Why do they matter?
Establishing the right trees in the right places is essential to successful forest regeneration, health, growth, and survival. In this context, the ‘right’ tree is genetically suited to the current and future site conditions where it is established. Foresters are acutely aware that management decisions around tree establishment made in the past and present have long-term implications for the services and products that trees provide in future decades or centuries. Healthy, productive trees are the foundation of ecosystem services, socioeconomic benefits including amenity value, and a sustainable, profitable timber yield. Foresters in both rural and urban environments therefore need to choose the best genetic stock, but how do they select the right tree?
In this post, my objective is to give a brief overview of the genetic variation of forest trees, outline how ‘traditional’ foresters working in rural environments utilize this genetic variation, and finally to introduce the challenges of managing the genetic variation of trees in urban forests.
What is genetic variation?
All organisms have a genetic ‘barcode’ contained in their DNA. Variation within DNA causes consistent physical and physiological genetic differences in traits among species, populations within species, and individuals within populations. In the absence of genetic variation there would be no evolutionary basis for differences in survival and reproduction among individuals in response to natural selection. Life as we know it would probably not have progressed much beyond the unappealing, muddy, mineral-rich primordial soup that life emerged from approximately 4.25 billion years ago.
Genetic variation in forest trees:
Obvious genetic differences exist among tree species, and within each species, subtle yet very important differences exist among populations that have become adapted to their local landscapes over thousands of years. Local adaptation is necessary for many widespread temperate tree species because they have geographic ranges that span hundreds or thousands of kilometers across climatically different regions or localities. Large amounts of genetic variation also typically exist among individual trees within any locally adapted population. Genetic variation within populations is the basis for local adaptation of the whole population because some trees are better adapted to their local environment than others. On average, the better adapted trees are likely to grow more rapidly, produce more offspring, and thus transfer their genes to future generations. Therefore, natural selection of suitably adapted individuals leads to long-term genetic adaptation of a whole tree population to the context of local environmental conditions.
Sitka spruce is a fantastic example of local adaptation in a geographically widespread tree species. Large amounts of variation in growth and cold tolerance correspond to genetic differences among populations across its 4000 km species range from California to Alaska. Each local population has become adapted to local environmental conditions over successive generations.
How do foresters use genetic variation?
Foresters use their understanding of genetic variation among tree species to ensure they establish mixed-species stands that are similar to local natural forests. This is the most coarse-grained level of genetic variation typically used to manage forests. It forms the foundation of ecologically sustainable forest function, including resilience to disturbance events such as drought, fire pests and disease.
Genetic variation among populations within species is important because it means that foresters can choose to replant forest stands by selecting trees from populations within a species range that are adapted to local conditions. These adapted trees are likely to remain healthy, provide ecosystem services and produce timber reliably over their lifetime. This approach is enshrined in the forester’s mantra ‘local is best’. For several hundred years it has been effectively applied in temperate forest regions to manage local adaptation despite little knowledge of quantifiable genetic differences among tree populations became available in the mid-20th Century. The Province of British Columbia natural seed deployment zones are an example of how local adapted populations are operationally defined and maintained (found here). The challenge for this approach to managing forest genetic resources is that climate change disassociates locally adapted populations from the climates they are historically adapted to. It means that trees become stressed, leading to reduce growth and timber production, as well as greater vulnerability to disturbances such as pests, disease and drought. These effects had substantial negative economic and environmental consequences.
Trees naturally colonize new landscapes and adapt to the prevailing climate, but this process takes many generations and occurs far more slowly than rate of climate change some regions of the planet currently experience. Assisted migration and assisted gene flow are techniques that relocate genetic entities (e.g. individuals or populations) to achieve defined population management and conservation objectives. By recognizing and understanding genetic variation among tree populations, foresters have a basis to employ assisted gene flow and assisted migration to mitigate the negative effects of climate change on tree health and productivity. These tools are being used to relocate locally adapted forest tree seedlings to sites within regional landscapes that better match the future distribution of climates they are historically adapted to (further information on assisted migration in forestry can be found here).
Lastly, foresters use natural variation within populations by selecting individual trees that have desirable traits such as growth, wood quality, cold tolerance or disease resistance. This is the finest level of genetic variation used to manage forests. This type of preferential selection and propagation of trees is documented as early as 1664 by John Evelyn in his book Sylva. During the 20th Century, tree breeding programs that select natural variation became recognized as the most effective way to increase timber yields, while managing genetic diversity and trade-offs among other ecologically important traits.
How do urban foresters use genetic variation?
It appears that the management of urban forest genetic resources is far less structured than in traditional forestry, and arguably less resilient to disturbance events or environmental change. Urban landscape planning favors trees that meet aesthetic requirements and the nursery production efficiencies of clonal propagation at the expense of genetic variation. As a result, urban forestry genetic management emphasizes deploying a relatively large diversity of non-native species and cultivated varieties to ensure a diverse and resilient urban forest. This represents the highest level of genetic variation used in traditional forestry. It means proactive management strategies that use genetic variation of locally adapted populations or individuals within species cannot usually be implemented in urban forests to mitigate the effects of future environmental change. An exception may be found in semi-natural or urban parkland areas that emphasize the use of native species and sourced from local populations.
I intend these issues of genetic management in urban forests to be the subject of a series of blog posts. They underlie the need for a new, adaptive genetic management paradigm in urban forestry that is appropriate for the environmental challenges of urban environments and future climates.