Over the past ten years, with the support of the Swiss Federal Office for the Environment (FOEN), the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) has been conducting genetic paternity analyses on various tree species in Switzerland. This entails collecting seeds from some trees and leaves from all trees of a particular species within a specific stand. Hence, all mother trees and their offspring, as well as all potential father trees within a given stand, are characterized.
Genetic and statistical methods can then be used to ascertain which seeds from a particular mother tree have been fertilised by which father tree, or whether none of the trees within that particular stand is the father tree. This information can be used to determine which father trees are pollinating a mother tree, over which distances pollination is occurring and thus the range within which genetic intermixing is taking place, and whether pollen is being brought in from outside the stand and, if so, at what proportion.
As part of this genetic paternity testing programme, we have studied both common and rare tree species, species which are pollinated by insects and those by wind, species from the lowlands and mountain species: oak, wild pear, wild service, true service, black poplar and Swiss stone pine.
Examples: Wild pear
The wild pear is a rare species in Switzerland. It generally occurs only in small populations of fewer than 50 trees (Fig. 1). It flowers in the spring (Fig. 2) and is pollinated by a wide variety of insects. The sample results obtained concerning pollen dispersal for this species of tree were recorded for a wild pear mother tree in a stand near Effingen in the canton of Aargau in the Jura (Fig. 3).
This particular mother tree was pollinated from various other wild pears within the same stand. Many of the father trees are located close to it, while some are further away. A total of 36% of the seeds of this mother tree were pollinated from father trees outside the local stand, since no father trees were found that matched the respective offspring in the Effingen stand. The closest wild pears are located anywhere up to several hundred metres from the stand studied. T
aking an average of all the mother trees studied in the Effingen stand, 51% of the pollen originating from father trees within the same stand was dispersed over a distance of less than 10 m, 28% over a distance of between 10 and 40 m, and 21% over a distance of between 40 and 120 m. The total percentage of pollen entering the stand from outside it was 38%.
We studied pollen dispersal among the wind-pollinated sessile and pedunculate oak (Fig. 4) in a stand near Büren an der Aare. Our findings indicate that in the area of the forest dominated by pedunculate oak, successful fertilisation occurred primarily within that particular species (Fig. 5).
Only a small proportion of offspring (< 2%) arose from crosses between species, whereby pedunculate oak was generally the father species. The latter could be due to an uneven representation of the two species within the stand studied, in which pollen from the common oak is overrepresented. However, the proportion of hybrids in a single tree’s progeny varied substantially among mother trees.
As with wild pear trees which are pollinated by insects, our findings showed that in general the father trees which fertilised a mother oak’s seeds were generally situated close by. However, we observed pollen-dispersal of up to 346 m within the stand. The average percentage of pollen entering the stand from outside it was 53%, which likely implies long-distance dispersal over several hundred metres. However, a proportion of this pollen may not have travelled very far since other oak trees were present adjacent to the study area.
What are the consequences?
One positive finding is that many trees are involved in producing the next generation within their particular stand, which ensures a good mix of genes even within small stands. Accordingly, genetic diversity and the adaptive potential are maintained in both naturally regenerating areas and in seed stands. The continued ability to adapt through the immigration of new genes is crucial for a tree population’s survival if environmental conditions change rapidly, as is currently occurring with climate change.
A good genetic mix can further be facilitated through free thinning of individual trees in order to foster flower and seed production. Account must be taken of the various proportions of father-tree pollination of a mother tree’s seed so as to maintain genetic diversity within a local stand, be it via harvesting seed (from many mother trees across the entire stand) or through promoting natural regeneration (several small areas instead of one large one).
There is also a down side to this mixing of genes, though. Where two species of tree are able to hybridise to produce offspring, i.e. they fertilise each other, it is important to remember that this can also happen over large distances. Well known are the naturally occurring hybridisations between the wild service tree and the common whitebeam, and between the pedunculate, sessile and downy oaks. Hybridisation can also become a problem when non-native species of tree are introduced. Where these are able to procreate with native tree species, this will likely occur sooner or later.
A considerable proportion of successful pollen originates from outside a stand, which frequently results in genes being exchanged between stands that are geographically isolated. It might be supposed that in such cases it is unlikely that pollen and genes will be exchanged and will only occur occasionally and by chance. However, this is not necessarily true: even stands which are relatively remote from one another can become linked by pollen dispersal. In the case of the insect-pollinated true service tree, for example, we could show in the canton of Schaffhausen that pollen is being exchanged between trees more than 16 km apart. Even rare forest trees are therefore much more genetically connected than we tend to think. As such, in the case of many small populations of forest trees the problem is not so much a lack of connectivity but rather the frequent lack of regeneration. However, this issue can be addressed by promoting silvicultural measures.
This type of genetic intermixing can have a negative aspect, though. Firstly, in the case of seed stands which are geographically isolated from one another, a significant proportion of seeds will be pollinated by trees outside the stand in question. The seeds harvested will therefore contain a combination of the desired characteristics of the seed stand as well as potentially unwanted characteristics from outside that stand. In such cases, the seed will seldom be “pure”. The same is true of gene conservation stands: it is impossible to protect them from genes entering the stand from outside it.
This interlinking of trees and populations thanks to pollen and seed dispersal, the latter of which is not investigated here, thus promotes broad-scale genetic mixing. Genes are travelling across the landscape to sites and stands in which they would not previously have been found. In view of changing environmental conditions, this means that genes are being transported to areas in which, in the future, they may find an environment which suits them. This is thus the precondition for selection: where genetic diversity is present, natural selection can occur and the appropriate tree species and genes can prevail. It is important that this process is maintained.
Summary of key findings
- Within a stand, many trees contribute to reproduction. Mother trees are generally, although not exclusively, pollinated from nearby trees, but the father trees may also be dispersed across the entire stand. This is true both of species pollinated by wind and those pollinated by insects.
- The seeds of an individual mother tree indicate the uneven reproductive success of individual father trees: few father trees are dominant pollinators, while many father trees will produce few offspring. This imbalance in reproductive success has the effect of the seeds of one tree being pollinated by multiple father trees, but of the latter passing on their genes unevenly to future generations.
- A large percentage of seeds within a stand are pollinated by trees outside that stand. These father trees may be located several hundred metres or even kilometres away. Of course, this percentage will vary depending on individual situations, but it is generally high and, surprisingly, is true of both stands seemingly connected and of those which are geographically isolated. Exactly the same conclusions as those detailed here in terms of long-distance pollen travel have been drawn in numerous studies worldwide. Accordingly, it must be expected that a considerable amount of pollen, and hence genes, will always be introduced into a stand from outside it.