Abstract
The present study reports results of qualitative melissopalynological analyses of Finnish honey between the years 2000–2007 and changes in its pollen content from the period 1960–2007. Altogether the pollen content of 734 honey samples was analysed with an average of 415 pollen grains counted from a sample. Pollen of Trifolium repens type, Rubus spp., Salix spp. and the Brassicaceae family were present in more than 90% of the samples, and these pollen types were also found in the highest proportions. Annual variation in the relative amounts of the most numerous pollen types could be as high as 10%. On the basis of the pollen spectra of the honey samples, four regions of forage plants for bees could be identified in Finland. In the period between 1960 and 2007, the most marked change observed was that the percentage of the Trifolium spp. pollen type had decreased from 70% to 10%, while the proportions of Brassicaceae and Rosaceae pollen types showed a corresponding increase.
Pollen grains are always found in natural honey processed by standardised methods. The pollen content of the honey not only reflects regional agricultural practices and forest vegetation, but also the floral diversity and species composition of the plants foraged by the honey bees and, as such, the melliferous plant species available in the vicinity of the apiary (Louveaux et al., Citation1978). Honey bees select their forage plants primarily on the basis of the sugar content of the plant nectar, the raw material of honey (Crane, Citation1980). Pollen grains are usually present in floral nectar and thus are considered to be the primary source of pollen in honey. Moreover, exogenous pollen may be introduced into a beehive in numerous ways: bees carry pollen to the hive in their pollen baskets, pollen grains may fall from bees' body parts into the nectar-filled combs, airborne pollen may enter the hive via air currents, used wax combs are added to hives, or imported pollen is fed to bees.
There are around 2000 beekeepers in Finland maintaining about 50000 bee colonies; total honey yield ranges from 1 to 3 million kilos annually. On an average, one colony produces 26–56 kg of honey per annum (Anonymous, Citation2008). The northernmost bee colonies in Finland are found in Inari (69º N), but more than 65% of the honey is produced in the southern and western parts of the country (Regions 9, 10, and 13–19 in ), where cultivated Brassicaceae species are the main honeybee plants.
Figure 1. The forage plants regions (Region I–IV) and the administrative regions (with numbers) – referring to .
Table I. Administrative regions of Finland (number refers to ) and the number of honey samples obtained for the pollen analyses of the present study. Regions grouped to forage plant regions (I – IV; in bold), as in
Finland is situated on the boreal coniferous zone of Scandinavia. Phytogeographic regions of Finland are hemi-, southern-, middle- and northern boreal zones and predominant forest types range from mesic nutrient-rich to oligotrophic dry heath coniferous forests (Ahti et al., Citation1968). In the eastern and northern Finland the main honey yield comes from wild forest plants. The pollen types of wild growing plants are derived from typical boreal coniferous zone flora. These species are, for example, Salix spp., Epilobium angustifolium L., Calluna vulgaris (L.) J. Hull, Menyanthes trifoliata L., Anthyllis vulneraria L., Knautia arvensis (L.) T. Coulter, Tussilago farfara L., Astragalus spp., Trifolium spp., Rhamnaceae spp. and Geranium spp. as well as wild berries Rubus idaeus L., R. chamaemorus L., R. arcticus L., Fragaria vesca Coville, Vaccinium myrtillis L., V. vitis-idaea L., V. uliginosum L. and V. oxycoccos L.
To date, melissopalynological studies in the European boreal coniferous zone have been conducted only in Finland. Results of honey pollen analyses are available from as early as 1937 and 1960 (Martimo, Citation1945 and Aarnio, Citation1961, respectively). These studies indicated that the proportion of Trifolium repens type pollen was about 50%. Later studies by Varis et al. (Citation1982), consisting of 120 honey samples from 1977–1978, revealed a marked shift in the main forage plants, as Brassicaceae pollen became the dominant type in the total pollen content of honey. Moreover, Varis (Citation2000) found that in 1997 the proportion of Brassicaceae pollen had increased to 60%, accompanied by a simultaneous decrease in the Trifolium type species. The most likely explanation for this is that the cultivation area of the oil-plant species of the Brassicaceae family (Brassica rapa ssp. oleifera and B. napus ssp. oleifera) has steadily increased in Finland since the 1970s (Anonymous, Citation2008).
Systematic annual pollen analyses were started by the Finnish Beekeepers Association in 1995 with the aim of observing the quality of the honey sold in Finland, and the qualitative pollen content of approximately 100 samples are analysed annually.
The aim of the present study is to report the results of qualitative pollen analyses from 734 honey samples collected in Finland during 2000–2007. The main focus is in the floristic spectrum of plants foraged by honey bees and in the identification of the most important plant sources for honey pollen. Also considered are the geographical variability of forage plants in Finland and changes in the forage plant composition between 1960 and 2007.
Material and methods
Honey samples were collected by the Finnish Beekeepers Association from different honey selling enterprises in Finland or assigned for quality control analyses by beekeepers during 2000–2007. Honey samples came from different hives and different locations each year and the samples were purchased or received during autumn and winter. Most Finnish beekeepers collect honey from their colonies only once a year, therefore these honey samples present more or less the whole yield of one year. The geographical origins of the honey samples are shown in and . There were no honey samples from the Aland Islands.
Qualitative melissopalynological analyses were conducted by slightly modified methods recommended by the International Commission for Bee Botany (Louveaux et al., Citation1978). The honey samples for microscopic pollen analysis were prepared in the laboratory as follows: ten grams of honey were dissolved in 20 ml of distilled water and centrifuged (10 minutes, 3500 r/min). The supernatant was disposed of and the residue washed again with 20 ml of water. The supernatant was again removed after the second centrifugation (5 min, 3500 r/min) and the residue transferred using a Pasteur pipette onto a microscope slide, which was left to dry, covered by a piece of paper, until the next day. The sample area was subsequently covered with Kaiser's glycerol gelatine and a cover slip, and again left to dry under paper for 24 hours.
Pollen identification was conducted according to pollen types described in . Juniperus type includes Juniperus spp. and Taxus spp., Picea type includes Picea spp. and Abies spp. Pollen type Asteraceae/Liguliflorae includes taxa Achillea spp. and Leucanthemum spp., and Asteraceae/Tubuliflorae type includes Aster spp., Senecio spp., Erogeron spp. and Solidago spp. Cirsium type includes Cirsium spp. and Carduus spp., where as Malus type includes Malus spp. and Pyrus spp. Nymphaea type includes Nymphaea spp. and Nuphar spp. Syringa type includes Syringa spp. and Ligustrum spp. and Taraxacum type includes Taraxacum spp. and Sonchus spp. Trifolium pratense type includes Trifolium pratense L., T. medium L., and T. incarnatum L. Trifolium repens type includes T. repens L., T. resupinatum L., and other T. hybridum L., but not Trifolium hybridum ‘Frida’ L., which can be distinguished from other Trifolium species. Vicia type includes Vicia spp., Lathyrus spp., and Pisum spp.
Table II. The analysed pollen grains were classified into 116 pollen types, 88 of which are from melliferous plants and 28 from non-melliferous plants (marked with an asterisk)
If bees use honey dew as the raw material of honey, honey dew elements like fungal spores, hyphae and microscopic algae may occur in honey. The amount of honey dew elements was also counted, if they were observed in the honey samples.
Frequency of occurrence denotes the relative amount (%) of samples in which a certain pollen type was encountered. It was calculated by dividing the number of samples in which a pollen type occurred by the total number of samples and then multiplying the quotient by 100. For the counting of the relative proportion of a pollen type we formed a huge honey sample by summarising the calculations of each pollen type from one year. Then the relative proportion was calculated by dividing the sum of one pollen type by the total number of all counted pollen grains in that year and then multiplying the quotient by 100.
In order to find out the variability of the plants foraged by bees in provinces of Finland, all the honey samples were classified according to their respective geographical origin. The numbers of each pollen type of one geographical area were summarised and the relative proportion of a pollen type was calculated as described above.
The percentage distribution of the most common pollen types in Finnish honey samples from the years 1960, 1962, 1963, 1977 and 1978 was compared by Varis et al. (Citation1982) (). The corresponding pollen data of the present study (from the years 2000 earliest, 2003 middle, and 2007 latest) were added to the data set of Varis et al. (Citation1982). The pollen types for the comparison were the same as those used in 1982 by Varis: Brassicaceae, Rosaceae (including Fragaria spp., Rubus chamaemorus, Rubus spp. and Sorbus spp.), Apiaceae, Trifolium repens type, Filipendula spp., Asteraceae (including Asteraceae/Liguliflorae, Asteraceae/Tubuliflorae, Tussilago farfara, Cirsium spp., Carduus spp., Centaurea cyanus L., C. jacea L., C. montana L., Taraxacum spp. and Sonchus spp.) and a seventh pollen type for the others.
Figure 5. Percentage distribution of eight pollen types in Finnish honey samples in 1960, 1962, 1963, 1977 and 1978 (Varis et al., Citation1982) and in 2000, 2003 and 2007 (current study).
Results
A total of 734 honey samples were analysed from the years 2000–2007. All observed melliferous and non-melliferous pollen grains and honey dew elements were counted. An average of 415 pollen grains were counted from the samples (). Altogether, 116 different pollen types were analysed from the honey samples. A pollen type may represent a family, a genus or a species. Altogether, 23 types were identified to species level, 78 to genus level and 15 to family level (), 9–47 of which could be found in one honey sample (). On average, the samples contained 27.3 pollen types, 21.1 of which originated from melliferous and 6.3 from non-melliferous plant sources (). Unidentified pollen taxa occurred only in five samples.
Table III. Total number of pollen grains counted, average number of pollen grains per sample, number of samples and average number of melliferous and non-melliferous pollen types in samples per year (S.D. – standard deviation)
Honeydew honey is not collected by bees in Finland every year. Thus honeydew elements were found in only 28 honey samples, from the years 2000, 2002, 2003, 2006 and 2007.
Frequency of occurrence of the pollen types
Trifolium repens type, Rubus spp., Salix spp., Brassicaceae and Apiaceae pollen types were encountered in more than 90% of the honey samples (). Commonly occurring types (> 75% frequency) were also those of Trifolium pratense type, Vaccinium spp., Taraxcum type, Epilobium spp. and Trifolium hybridum ‘Frida’. Pollen of Hieracium spp., Knautia arvensis, Rubus arcticus, R. chamaemorus, Tussilago farfara and Sedum spp. were found in less than 10% of the samples, although these are relatively common plants in Finland.
Figure 2. Frequency of occurrence of melliferous pollen types (> 30% frequency) in the years 2000–2007.
Among the non-melliferous plants, Filipendula spp. and Poaceae pollen types were present with the highest frequencies (92% and 87%, respectively, ). Wind pollinated pollen of Betula spp. and Pinus spp. were quite commonly found in honey samples (45% and 44%, respectively; ). Pollen of Clematis spp., Fraxinus spp. and Juglans spp. types were not found in the honey samples during the years 2000–2007.
Figure 3. Frequency of occurrence of non-melliferous pollen types (> 30% frequency) in the years 2000–2007.
Relative proportions of pollen types
Of all the counted pollen grains, 75%–85% belonged to the four most numerous melliferous pollen types (Brassicaceae, Rubus spp., Trifolium repens type and Salix spp.) (). Other pollen types with a proportion higher than 2% were Apiaceae, Fragaria spp., Menyanthes trifoliate, Myosotis spp., Sorbus spp., Trifolium hybridum ‘Frida’, Trifolium pratense spp. and Vaccinium spp. (). About 95% of all the counted pollen grains of melliferous plants belonged to the 12 most numerous pollen types, and only less than 5% of the pollen grains were represented by the 76 remaining types. The yearly variation of the relative proportion of a pollen type was approximately 10%.
Table IV. Relative pollen values from melliferous and non-melliferous pollen types (types with participation greater than 2%)
Of the non-melliferous plants, the Filipendula spp. pollen type was the most common, representing 62.3% of all the non-melliferous pollen grains in the data (). The relative proportions of Anemone spp., Ranunculaceae and Rumex spp. pollen types show a marked variation between different years. The Ranunculaceae pollen variation was 4.2% – 12.9% in the data set (calculated from ). The annual range of windborne Poaceae pollen type percentages fluctuated between 6.8% – 15.4% ().
Geographical variation of bees' forage plants in Finland
In order to find out the variability of the plants foraged by bees in different parts of Finland, the honey samples were classified according to their geographical origin. The seven pollen types included in the pollen type spectra were those that were either the most numerous in the samples (Brassicaceae, Rubus spp., Salix spp. and Trifolium spp.) or are considered important plants as a unifloral honey source (Epilobium spp. and Vaccinium spp.). According to the similarity of the pollen type spectra, four source regions of supply (Regions I – IV) could be outlined with reference to the plants foraged by the bees ( and ).
Figure 4. The forage plants' regions according to pollen type spectra in honey samples in the different administrative regions of Finland.
The honey originating from Region I had low quantities of Brassicaceae pollen type while Salix spp., Rubus spp. and Trifolium spp. occurred in high percentages, and the proportion of Epilobium spp. pollen type was higher than in other regions. Honey from Region II was characterised by very high values (41% – 56%) of the Rubus spp. pollen type. Rubus spp. and Brassicaceae pollen types occur in more or less equal proportions in Region III, where the Trifolium spp. pollen type was also abundant. In Region IV, Brassicaceae pollen was by far the most common type (42% – 54%).
In the northern part of Finland (Lappi and Kainuu), the proportion of Salix spp. pollen type was relative high (27% – 31%), and here the proportions of other wild plant pollen (e.g. Vaccinium spp.) were also higher than in the other regions. The proportion of the Trifolium spp. pollen type was highest in Northern Ostrobothnia (31%).
Changes in the forage plants of bees since 1961
The change in the pollen spectrum of the most common pollen types in Finnish honey samples from the years 1960, 1962, 1963, 1977 and 1978 was compared by Varis et al. (Citation1982). They found that the proportion of Trifolium spp. pollen had decreased (from 60% to 7%) in parallel with the increase in the proportion of Brassicaceae pollen (from 3% to 20%). Furthermore, proportions of Rosaceae, Filipendula spp. and Apiaceae pollen grains showed slight rising tendencies. In this study, the corresponding pollen data (from the years 2000, 2003, 2007) were added to the data set of Varis et al. (Citation1982) (). The results indicate that the proportion of Trifolium spp. pollen type has decreased markedly, from 70% to 13%, during the last five decades and has been largely replaced by Brassicaceae and Rosaceae pollen (). In the 1970s Brassicaceae pollen was clearly the dominant type, but it declined by the 2000s to values almost equal to those of Rosaceae (). The proportion of the Rosaceae pollen has increased by 15% and the proportions of Asteraceae and Apiaceae pollen have declined markedly from the late 1970s to 2006 ().
Discussion
The present study provides new insights into the pollen composition of Finnish honey. The honey samples that were used in this study were collected for controlling the quality and origin of honey sold in Finland. Therefore samples were not taken from some apiaries or at the same time of the year, and the number of samples varied yearly. However, the abundant number of the samples gives an adequate picture of the pollen content of Finnish honey.
Pollen in honey originates from three main sources: wild plants, those that are cultivated in crops, or those planted in gardens. The pollen types of wild growing plants are derived from typical Finnish flora like Rubus arcticus, R. idaeus, R. chamaemorus, Epilobium angustifolium, Calluna vulgaris, Fragaria vesca, Menyanthes trifoliata, Anthyllis vulneraria, Knautia arvensis, Tussilago farfara and species of Vaccinium spp., Astragalus spp. and Salix spp. and Rhamnaceae. Cultivated plants that are commonly foraged and pollinated by bees in Finland are the Brassicaceae family oil plants (B. rapa ssp. oleifera and B. napus ssp. oleifera), Trifolium and Phacelia species, Fagopyrum esculentum C. Moench, and cultivated berries and fruit plants, such as Rubus idaeus, R. arcticus, Fragaria vesca and the Malus spp. and Pyrus spp. species. Phacelia spp. is cultivated by beekeepers in order to provide forage plants for bees during Autumn, and its pollen was found in 40% of the honey samples. Some pollen types include pollen of both cultivated and wild plant species. The most important species are Brassicaceae, Rubus spp., Fragaria spp. and Trifolium spp. Salix spp. pollen occurs commonly in honey samples, but never in high proportions. Four of the pollen types observed in the analyses (Acacia spp., Castanea sativa P. Mill, Myrtaceae and Robinia pseudoacacia L.) are neither native nor cultivated in Finland. These pollen types most probably originate from imported pollen fed to bees in cold Springs.
Based on frequency of occurrence and relative proportions, the four most important plant groups or species for bees in Finland are the Brassicaceae family, Rubus spp. and Salix spp. species and Trifolium spp. It was an unexpected finding that Trifolium repens type had the highest frequency percentage and that Fragaria spp. and Rubus spp. pollen is substantially more frequent in the honey samples from the 2000s than has been observed in earlier studies. On the other hand, we expected to find high values for Epilobium spp., Vaccinium spp. and Calluna vulgaris pollen types, because Finnish beekeepers know that these plants produce large amounts of nectar for honey bees in the eastern and northern parts of Finland. However, the pollen grains of these plants were present only in proportions of 0.35, 2.88 and 0.07% (respectively).
The nectar of Rubus idaeus attracts bees very effectively because of its high sugar concentration and the high quantity of the nectar (Crane et al., Citation1984). It has also been noticed that, especially in those years when the honey yield is good (2003, 2005 and 2006) (Anonymous, Citation2008), there are abundant Rubus spp. pollen grains in the honey samples. The frequency percentage of Rubus spp. was higher and its proportion in the honey samples was markedly higher than in the earlier pollen studies conducted in Finland (Martimo, Citation1945; Aarnio, Citation1961; Varis et al., Citation1982; Varis, Citation2000). This could be explained by three factors. First, cultivation areas of Rubus idaeus have increased since 1995 (Anonymous, Citation2008). Berry growers commonly keep bees near cultivated fields because of the importance of bee pollination to berry yields. Second, the increase of wild Rubus idaeus growing in forests may explain the increased proportion of Rosaceae pollen types since 1960 (), as wild Rubus idaeus has become more common in Finnish forests from the 1950s to 1995 (Hotanen et al., Citation2000; Lampinen & Lahti, Citation2008). The explanation lies in changes in silvicultural practices such as clear-felling creating favourable habitat for pioneer plant species like R. idaeus (Hotanen et al., Citation2000). Third, the beekeeping area has spread to new regions in the eastern and northern parts of the country. Samples from these areas contain more pollen of wild plant species, as Brassica spp. is not cultivated in these areas.
Pollen analyses do not reveal all aspects of the botanical origin of honey, because the pollen grains found in honey samples rarely correspond one-to-one to the vegetation composition around the apiaries or the floral types they represent (Bryant & Jones, Citation2001). For example, Myosis spp., with its minute and numerous pollen grains, and Brassicaceae pollen are known to be over-represented in honey samples (Van der Ohe et al., 2004). Apiaceae pollen type seems to be over-represented in honey samples as well. Apiaceae pollen was found in over 90% of the samples, but it represented a proportion of only 3% in the samples. Apiaceae taxa flower with large populations at the end of June, when bees visit them actively and collect large amounts of their pollen grains. However, Apiaceae flowers produce only small quantities of nectar (Ruottinen, Citation2005). On the other hand, pollen of Epilobium spp. and Calluna vulgaris seems to be under-represented in honey samples (Van der Ohe et al., 2004; Bryant & Jones, Citation2001). In many areas of eastern and northern Finland, bees produce large amounts of honey collected from Calluna vulgaris and Epilobium angustifolium. However, pollen from these taxa occurs in small amounts even in unifloral honey (Ruottinen, Citation2005). The structure of the Epilobium angustifolium flowers is such that bees are able to collect nectar without touching the pollen bearing anthers, and the pollen grains are so large that bees are easily able to filter them out from the nectar (Bryant & Jones, Citation2001). The pollen content of these unifloral honeys requires further investigation.
In the present study, one of the main interests was to distinguish geographical regions in relation to the forage plants of bees in Finland. About 70% of the Finnish honey is produced in southern Finland, Regions III and IV (), (Heikki Vartiainen, pers comm., 17/9/2008). In these two regions cultivated Brassica rapa ssp. oleifera forms the basis for the effective honey production. This is clearly reflected in the pollen assemblages of honey from these areas. The importance of Rubus idaeus as a forage plant for bees can be seen in Region II, but the amount of the pollen of Epilobium spp. was lower than expected. One of the surprises was that high proportions of Trifolium spp. pollen could be found only in Northern Ostrobothnia. It was expected that Trifolium spp. pollen would be found in high number in the eastern part of Finland as well.
Marked changes have taken place in the relations between Trifolium spp., Brassicaceae and Rosaceae pollen in honey since the 1960s. At the end of the 1990s the amount of Brassicaceae pollen had increased while the amount of Trifolium spp. type pollen had decreased in samples. This phenomenon is connected with changes in the cultivation areas of these plants (Varis, Citation2000). The cultivation area of Brassica rapa ssp. oleifera has increased by 40% from 1997 to 2006 (Anonymous, Citation2008). However, for unknown reason, this increase was not observed in the proportion of Brassicaceae pollen type in our study in the 2000s ().
The nearest and ecologically similar areas in the neighbourhood of Finland where similar melissopalynological studies have been contacted are in northern Central Europe. Von der Ohe, W. and von der Ohe, K. (Citation1996) and Wróblewska and Stawiarz (Citation2004) have presented results of melissopalynological analyses from Germany and Poland, respectively. Of the ten most frequent pollen types observed in these studies, five are the same as in our study (Trifolium repens type, Rubus spp., Brassicaceae, Salix spp. and Taraxacum spp.). On the other hand, Sorbus spp., Salix spp., Vaccinium spp., Rubus spp. and Epilobium spp. pollen types are more frequent in Finland than in Germany and Poland.
Conclusions
There have been profound changes in agri- and silviculture in Finland since the 1960s. These are, for example, the increased Brassica spp. cultivation in the southern part of Finland and the more intense clear-cuts of Pinus and Picea-forests all over in Finland. These changes have markedly affected floristic diversity, which is in turn reflected in the composition of honey. A follow-up study is a good way to follow these changes, to prevent honey adulteration and to improve the quality of honey. Furthermore, more research is needed on the unifloral honeys typical for the boreal coniferous zone in order to investigate their melissopalynological, organoleptical, physic-chemical and production capacity properties.
Acknowledgements
The permission of the Finnish Beekeepers Association is gratefully acknowledged for making the results of the quality analyses of Finnish honey available to us. The English usage was kindly revised by Ms. Rosemary Mackenzie.
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