How do we keep our forests resilient? Part 11 Assisted Migration of Trees
In Ontario, there are 35 static seed zones. These were established to ensure that planting stock was climatically adapted to the region of planting. This supported the approach of ‘ local is best ‘ where locally adapted seeds would be more acclimatized to the site. Recommended distances were no more than 50 kilometers from the parent stand and, if possible, less than 30 kilometers.
So what has been the success of these static Ontario seed provenances? Provenance refers to the geographical area and environment to which the parent trees are native and have developed through natural selection. Interestingly enough, scientists found that forest success rates were declining and low genotypic diversity present. Though we assumed ‘ local is best ‘, we forgot Mendel’s genetic rules in our attempt to maintain static seed zones. The broader and more diverse the genetic base, the more parents sampled from, the more resilient the population. This is absolutely fundamental. The term, ‘ composite provenancing ‘ has been conceived and we will discuss this, just a little later.
What is strikingly clear is that static provenances are no longer valid with a changing climate. Climatic envelopes, areas of suitable climatic habitat for tree species, are shifting north. This change will be ongoing and unrelenting. It is this unrelenting change that has us paralyzed.
Since 1963, we have known that our forests would be affected by climatic change. Historically, foresters relied on prediction models to plan for growth and yields of trees. These prediction models were based on historic conditions and static climate and, now, are invalid. To date, most forest management is still business as usual. There is uncertainty of action, and understandably so. We want to create adaptation with care and, we certainly, do not want to risk ecosystems.
By delaying action we may also create risk. We have not really moved sufficiently and are still debating proposals for assisted migration since 2007. While we are waiting for the science to implement change ; the science is changing. Climate change is causing high levels of uncertainty because of our limited ability to predict future impacts of climate change.
So what are some issues concerning assisted migration? It is the intent of assisted migration to push seed provenances north or even introduce new species north and accomplish climatic adaptation in 1 generation in what would have taken nature several generations to achieve. One of the biggest risks of planting stock north of its current zone, is freezing damage. Natural selection has resulted in species aligning their growing cycles to avoid damage from late spring and early fall frosts. Events such as breaking dormancy, bud burst and flowering are carefully timed for tree species adapted to a local environment.
On an even larger scale, we could be mismatching tree species to photo period. By moving seed sources north, species are no longer matched to local day length. Longer photo periods experienced at more northerly latitudes may cause trees to be more susceptible to all frosts. By mid century, it is estimated that most of Ontario’s tree species will have to move 400 to 600 kilometers north to keep withing their climatic envelopes. This will, indeed, cause mismatching of tree species to photo period.
Decisions will have to be made at every level – government, conservation authorities, nurseries, foresters and the general public. Do we keep our static provenances and, ‘ local is best ‘ mandate or do we attempt assisted migration with its risks. Don’t forget, not doing anything also carries risk. Most tree species naturally distribute seeds and pollen over relatively short distances, less than 100 meters. Longer distance seed dispersal is hindered in southern Ontario by habitat fragmentation and lack of long distance vectors – such as a lack of pollinators. Scientists believe that without adequate long distance seed dispersal it is predicted that many tree species will be unable to naturally disperse seeds far enough for successful migration to future suitable habitats.
We have decided, at Puslinch Naturally Native Trees nursery, to take the plunge. We will be launching an assisted migration program. A huge undertaking but we have always been committed to a sustainable environment and forests. Now the hard work begins where we build the program and try to introduce it to our clients and general public.