In the course of climate change, a change of tree species can help to stabilise forest stands. By supplementing the existing range of tree species, the risk of failure can be spread across different admix tree species.
The tree species and its characteristics
Fig. 1: Individual populations in the natural range of the species with selected stands for seed harvesting. Source: Dr. Muhidin Šeho (AWG)
Fig. 2: A possible seed crop stand of Turkish hazel in Surdulica, Serbia. Photo: Dr. Muhidin Šeho (AWG)
The natural range of the Turkish hazel is on the Balkan Peninsula, in northern Turkey, and in the Caucasus. Here, however, it does not occur contiguously, but in isolated individual populations (see Figure 1). The area is characterised by climatic conditions that can be expected in many places in Germany over the next 80 years. The average annual temperature here is between 5°C and 13°C, with an average annual precipitation of 570 to 800 mm. The Turkish hazel can withstand temperature extremes from -38 to +40 °C. Due to its modest requirements in terms of soil and climate conditions, it grows on a wide range of possible sites: shallow to deep, dry to almost wet, nutrient-poor to nutrient-rich. The Turkish hazel is generally very stable because of its 3-4 m deep taproot. This root system also allows it to grow even on skeleton-rich soils.
The Turkish hazel can be mixed with other species at stand level. In the mountains it occurs in beech communities, at lower altitudes in oak communities. Its superior growth performance compared to other tree species is only evident on shallow, dry, low-nutrient limestone soils. However, because it lacks competitiveness in comparison with other trees, it is not considered to be an invasive species. Its valuable wood has led to it being over-utilised on most sites. For this reason, this tree species is no longer important for the forestry industry in its countries of origin. In central Europe, the Turkish hazel is known as a park tree.
New tree species - quite a challenge
In general, the introduction of a possible alternative tree species poses major challenges for stakeholders in the forest and forestry industry. It involves a great deal of responsibility - a responsibility that begins with the companies that provide the seed and planting material. It is their task to bring high-quality propagation material of assured provenance to market. The product to be expected is predefined by the selection of the seed and the sale of the planting material. For tree species subject to the [German] Forest Reproductive Material Act, there are designated seed harvesting stands of suitable provenance. For a rare tree species such as the Turkish hazel that is not subject to the act, it is no easy task to find and bring seed of suitable quality to market.
Seeds of unknown provenance
The seed from which the trees have been grown up to now is mostly of unknown provenance and/or comes from individual urban or park trees. No attention was paid to the idea of provenance. And yet the individual occurrences of Corylus colurna in its natural range do have quite different phenotypic and genotypic characteristics. For forestry in particular, it is therefore necessary to harvest seed from several populations and to test it in field trials in order to be able to conclusively assess its suitability for cultivation according to provenance.
Knowledge of the physiological adaptability and genetic make-up of the planted trees and Turkish hazel populations is still limited. The “CorCed” project, funded by the Agency for Renewable Raw Materials, an initiative of the Germany Ministry of Food and Agriculture, was launched in 2015 to carry out an appropriate assessment of this tree species under central European climatic conditions, and to evaluate its suitability for cultivation.
Fig. 3: Fruit of the Turkish hazel. Photo: Klaus Stangl
The project and its focus
Four main areas of focus were defined:
- The establishment of contacts with authorities, institutions and seed suppliers in the natural range of the species.
- The selection of phenotypically suitable seed crop stands in the natural range.
- The documentation of their occurrence and genetic characterisation of the propagation material.
- The carrying out of controlled seed harvests for a provenance trial to assess suitability for cultivation according to provenance on a scientific basis.
In addition to investigating the suitability for cultivation, another important objective is to create favourable conditions for subsequent seed harvesting measures and seed imports. When introducing Turkish hazel seed, attention must also be paid to seed quality and the germination success rate. To avoid high economic losses for the tree nurseries, the authority responsible for forest genetics (Amt für Waldgenetik, AWG) thus carried out various tests and trials with the first seed imports and tested different stratification methods (see details below).
Seed testing of Turkish hazel
Forest seed testing is a standard procedure used to determine the external quality of seed material. The following parameters are determined: Purity, hollow grain content, thousand grain mass, viability as the proportion of viable seeds per kg. The assessment was made in accordance with the rules of ISTA 2014. The maximum number of seedlings to be expected per kg of seed is calculated from the formula:
technical purity∗proportion of viable seeds∗100 / thousand grain mass
The seed tested at the Office for Forest Genetics (AWG) came from Turkey (seed harvests of 2013, 2014, 2015 in two autochthonous stands near the villages of Bolu and Tosya), France (obtained from the Vilmorin company, provenance unknown, collection from park trees) and Germany (collection from 30 individual trees in Gotha Castle Park by colleagues from the Gotha Forestry Competence Centre).
- All seed batches underwent standardised forest seed testing in the AWG's laboratories.
- The technical purity of the Turkish hazel nuts was relatively high at 99 %.
- The proportion of hollow grains was low in all batches, at 1-7% (see Table 1).
Tab 1: Results of seed testing on 5 batches of Turkish hazel.
The results of the testing of the five seed batches reveal clear differences in seed weight and the proportion of viable seeds. According to NINIC-TODOROVIC et al (2005), low temperatures at the beginning of the growing season have a considerable influence on the size and quality of the nuts.
The seed from Tosya in Turkey performed best. Both the thousand grain mass and the proportion of viable seeds are highest in this batch. The correlation described in the literature between the increasing size of the nuts and the proportion of viable seeds applies here (NINIC-TODOROVIC et al. 2012). The nuts are quite large and have the typical characteristics (Figure 4). As Figure 5 shows, the seeds are intact and very healthy. The tetrazolium test revealed 89% (489 pieces per kg) of viable seeds for this batch.
Of the three Turkish seed batches, the seed harvested in 2014 had a thousand grain mass comparable with that of the other two batches, but a comparatively low number of viable seeds at 28%. One possible factor influencing the different qualities is the transport and storage time, which was longest (nine months) for the 2014 delivery. There are still many uncertainties when it comes to the optimal conditions for transporting and storing the Turkish hazel. Both factors must be analysed in relation to harvest date and humidity, as they can have a strong influence on germination success.
Fig. 4: Turkish hazel nuts from Tosya, Turkey. Photo: Dr. Muhidin Šeho (AWG)
Fig. 5: Nut cracked open, with seed intact. Photo: Dr. Muhidin Šeho (AWG)
The German seed from Gotha had a thousand grain mass of 1269 grammes, and the proportion of viable seeds was 51%. The maximum number of seedlings from one kg of seed was 404. This gives this batch an average rating.
The ratio was completely different for the seeds from France. The thousand grain mass was 1404 grammes, but the proportion of live seeds was only 19%. The TZ test was carried out here on the pre-stratified seed supplied. The actual emergence result was also low with this seed, despite the pre-stratification (see Figure 8, variant S5). This may be due to the seed drying out during transport.
Stratification of the Turkish hazel
Stratification is a process for breaking seed dormancy. It is carried out differently for all germination-inhibited tree species. The Turkish hazel has a high level of germination inhibition. To avoid the need for stratification, autumn sowing is recommended. With sowing in October, warmer temperatures prevail for the first four weeks, and are followed by the colder winter months - replicating cold stratification. This process is very similar to the process of natural regeneration, as the ripe fruit falls to the ground in September when the temperatures are still relatively mild. This is followed by winter and the seed is exposed to lower temperatures. Seeds sown in autumn must however be protected from mice, or there may be high losses.
If sowing in the autumn is not possible, the seeds should be sown in early spring (by mid-March), as the Turkish hazel germinates very early. In this case, however, the seed must be stratified beforehand. It is also important to ensure that the seed (mainly from abroad) is kept moist during transport from the harvesting site to the breeding company (see Figure 6). If the nuts of the Turkish hazel dry out during transport or interim storage and are not sown in time, primary dormancy may give way to secondary dormancy. This results in a high proportion of nuts lying dormant until the following year. In order to gain experience with stratification, various stratification processes were tested and optimised at the AWG and at the Staatsklenge Nagold state seed supply in Baden-Württemberg.
Fig. 6: Transport and storage of the seed. Photo: Dr. Muhidin Šeho (AWG)
Fig. 7: Stratification of the seed in moist sand. Photo: Dr. Muhidin Šeho (AWG)
Results of the stratification trials
The following stratification variants were tested:
- S1 - Direct sowing in pots (autumn sowing as a comparative variant)
- S2 - Pre-soaking of the nuts for 14 days in moist peat and a warm environment
- - then refrigeration for at least 6 weeks at 2-4 °C (in the same peat)
- - keeping the nuts moist in a warm greenhouse from February until sowing
- S3 - Storage of the nuts in damp sand at 0-5 °C to protect them from mice (compare Fig. 7)
- - regular watering and turning of the nuts (approx. 7 weeks)
- S4 - Storage of nuts in moist sand at 4 °C (approx. 7 weeks)
- S5 - Pre-stratification by supplier, method unknown, delivered ready for sowing
- S6 - dousing of the nuts every 2 days with cold tap water (9 weeks)
Six stratification variants were carried out, as shown in the overview above. After stratification, the Turkish hazels were sown in the nursery. The emergence results were monitored over two years, as the Turkish hazel is a species that can also lie dormant in the nursery. The results in Figure 8 clearly show that the autumn sowing (variety S1) is by far the best, with the highest total emergence from the first year. Spring sowing after stratification is significantly less successful for all variants, with variant S3 (7 weeks’ cold stratification) and S5 (method unknown) having the worst emergence.
We consider the relatively short stratification period (max. 63 days) to be the reason for the significantly lower emergence after stratification. This view is also supported by the significantly better emergence in the following year after a practically natural “second stratification” caused by the nuts lying dormant in the soil until the following year. Only 21% of the nuts sprouted in the first year (S2). The remaining nuts only emerged in the second year in all stratification variants. The nuts of the Turkish hazel can remain dormant in the soil for up to four years. The bed should therefore not be cleared the very next year.
We cannot assess to what extent the poor result with pre-stratified seeds can be attributed to the stratification process or the transport of the stratified seeds. What is certain that only a total of 19 seedlings per kg emerged with this variant.
The seeds should be sown as early as possible in the year and the beds should be kept moist. If the seeds are sown too late, there is a risk that the bed will dry out and that this will lead to the nuts lying dormant until the following year.
Fig. 8: Emergence of Turkish hazel per kg for seeds sown in April/May 2014.
To further optimise the results, a new stratification trial was started at the AWG with the variants S2 and S3, but with an extension of the stratification period to between 120 and 150 days. The results of the trial speak for a stratification period of at least 120 days. Our results are in line with those of other authors (AYGUN et al. 2009 and NINIC-TUDUROVIC et al. 2012), who also achieved the best results with autumn sowing and a stratification period of 120 days.
Conclusion and recommendations
As there are no large natural forest stands of Turkish hazel in Germany, the seed is usually ordered from the countries of origin of the Turkish hazel. The seed should be delivered to the tree nurseries as soon as possible after harvesting and without interim storage. During transport and storage, the nuts must always be kept moist, but care should be taken to avoid fungal infestation. If the seed dries out, the nuts can lie dormant for one or several years. If conditions are favourable, autumn sowing is recommended. Otherwise cold stratification must be implemented. The minimum duration of stratification is 120 days.
Autumn sowing is often not possible, however, as the seed is usually only delivered in winter, when it is already too late for autumn sowing.
A central issue for tree nurseries is the dormancy of the seeds until the following year. This can result in high costs for the maintenance and provision of the planting material. Certain conditions must be taken into account if dormancy of the seed until the following year is to be avoided:
The seedbed must under no circumstances be allowed to dry out during the germination phase and must be watered if necessary. The soil moisture and soil temperature in the nursery beds must therefore be checked regularly. It is advisable to cover the seedbed to protect the seed from being eaten by birds.
