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Background for monitoring of orchids

Introduction to monitoring

In the early summer of 1943, Buderupholm National Forest District carried out the first census of the population of lady’s-slipper orchids in the enclosure in Buderupholm Bjergeskov. The census was repeated the following year and has continued ever since. Thus, the census thus constitutes the longest-running, continuous monitoring of an orchid population in Denmark and, perhaps, in the world.

About the monitoring programme

In 1982, several counties started a regular annual census of selected orchid populations, which led to the Danish Nature Agency developing a joint programme for monitoring Danish orchids in 1987. In 1994, the monitoring programme was transferred to the Danish Environmental Research Institute, and from 2011 to Aarhus University. Ever since the beginning of the programme, approximately 565 populations have been monitored throughout the period or during parts of the period. The monitoring of some populations has had to be discontinued because the species has disappeared from the site, e.g. as a result of a landslide, because the monitor was not able to retrieve the  markings in the field or because the species in question grew outside the field markings set by the monitor. A population may also have been wiped out as a result of human intervention, e.g. in the form of having been dug up or as a result of overgrowth.

How many species are found in Denmark?

According to the book “Danmarks vilde orkidéer” ("Denmark's Wild Orchids",) 37 species, 10 subspecies and 4 varieties of orchids are found or have been found in Denmark. Danish orchids are native, and many of them have been in Denmark for a long time, while others have only appeared within the past 200 years. Three orchids have disappeared from the country. These are Traunsteiners' hawkweed (dactylorhiza majalis subsp. lapponica), which was found once in 1894 in Bredemose in Bjergsted forest in Northwest Zealand, the long-bracted green orchid  (coeloglossum viride), most recently registered 1950 in Veddinge Bakker in Odsherred, and autumn lady’s-tresses (spiranthes spiralis), most recently observed in 1981 on Bornholm.

How does monitoring take place?

Monitoring is carried out by one annual visit to the growing sites  during the orchids' flowering period, which, depending on the species, is from the end of April to August. Among others, monitoring is carried out by volunteers who are interested in following the developments in orchid populations. Local authorities and the Danish Nature agency also participate. Monitoring of orchid populations takes place across most of the country.

 

Of the more common species, only a small portion of the populations is monitored. However, for the rare orchids with less than 10 populations, the aim is to monitor all populations.

In practice, monitoring is a two-tiered process.

  1. A census is made of the size of the population, which is determined by counting the number of flowering shoots and, where possible, the number of vegetative shoots, i.e. shoots that do not bloom. In some cases, the above-ground shoots cannot be counted because the species in question has remained in the soil that year.
  2. An assessment is made of the state of the growth site, and the following issues are examined: Is the growing site being grazed? How heavy is the grazing pressure? Are there changes in the water level? Is the growth site overgrown by trees and bushes? Are there any changes in the growth site compared to the previous year?

The information is submitted on a reporting template to Aarhus University, which is responsible for ensuring that the information is available on the orchid website.

Results from the monitoring

When one of the Danish species names is activated on the orchid list, you will find a list of locations where the orchid in question is being monitored. When the name of a location is activated, a bar chart will appear showing the annual counts. The bars show the number of flowering shoots for the period for which Aarhus University has information. For some species, the number of vegetative shoots is also shown.

There may be temporal ' gaps ' in some bar charts due to the fact that the population has not been counted for one or more years or that the above-ground shoots were not produced.

Location of the sites

Most orchid populations grow far off the beaten track. For this reason, the names of the sites are given by nearby cities, which can be found on a common map of Denmark.

Some orchids are only found in small numbers. In addition, many populations grow in places that do not have public access. For this reason, Aarhus University has decided to ensure that the exact growth sites of the populations are not disclosed on the website.

Orchids are protected

All Danish orchids are protected. This means that they should be observed in nature and may be drawn, photographed, etc. But they must remain untouched. The plants must not be damaged, picked or dug up, and seeds may not be collected when they grow in the wild outside gardens, parks, cemeteries or horticulture. Conservation has been introduced as general protection of all Danish orchids because many rare and vulnerable species can be mistaken for the more common ones and because many orchids are threatened in their last habitats. Conservation is also a consequence of Denmark's accession to the Bern Convention, the purpose of which is to protect European species of wild plants and animals as well as their habitats. The Convention places particular emphasis on the protection of endangered and vulnerable species. In addition, two Danish orchid species, lady’s-slippers (cypripedium calceolus) and yellow widelip orchid (liparis loeselii), are covered by the EU Habitats Directive. This means that the species and their habitats require strict protection.

Status of orchids in Denmark

The status of the species has, among other things, been assessed on the basis of prevalence and number. Some orchid species are widespread and occur in all parts of the country, while others have only one or a few habitats. The current prevalence of the individual species within Denmark's borders is reported according to the following division: Northern Jutland, Western Jutland, East Jutland, Southern Jutland, Funen, Lolland, Falster, Møn, Zealand and Bornholm. Their relative frequency is illustrated by the number of habitats.

According to the Red List, of the 51 Danish orchids, 33, or 67%, have been assessed  as either disappeared, threatened or almost endangered in 2020. The assessment has been made on the basis of criteria drawn up by the International Union for the Conservation of Nature and Natural Resources (IUCN). Of 324 Danish endangered taxis, orchids constitute 10%. The Danish orchids' distribution in categories are with IUCN's category designations in brackets: Eight orchids are categorised as Critically Endangered (CR), seven as Endangered (EN), seven as Vulnerable (VU) and eight in Near Threatened (NT). The category Least Concern includes 16 orchids, while two others have not yet been red-listed. Two species and one subspecies are no longer found in the Danish flora and are therefore in the category disappeared (RE).

Flowering individuals are counted

The flowers contain the reproductive organs of the plants. In the orchids that grow in Denmark, the dust leaves (male organs) and fruit nodules (female organs) are close to each other in the flowers on a column, but are normally separated from each other by a beak to avoid self-fertilisation. For this reason, pollination often requires help from animals, such as bees, butterflies, moths or, in the tropics, hummingbirds. Many orchids produce nectar, which lures pollinators when the flowers are ripe for fertilisation. The fertilisation results in seeds to safeguard the future of the species.

There are, thus, several reasons why the flowering shoots are counted:

  1. they contain the orchids' reproductive organs, which produce the next generations, and therefore indirectly measure the population's relative reproductive capacity,
  2. they measure the extent and size of the population, even though there is no linear relationship between the number of flowering and vegetative individuals, and 3) they are often easy to observe in other vegetation when the orchid is in bloom.

However, if the aim is to measure the reproductive and propagation capacity of a population, the vegetative individuals must also be counted. Counting vegetative individuals may be more difficult and, thus, takes more time, as these plants can be difficult to distinguish from other vegetation or from other orchids when several species grow at the same site. Finally, many vegetative shoots, especially in young plants, have a shorter growing season and wither earlier than the flowering shoots.

Rejuvenation

All Danish orchids are soil-dwelling, unlike those in the tropics, which mainly grow in the crown layer of the rainforest, with the exception of the yellow widelip orchid (liparis loeselii) and bog adder’s mouth (hammarbya paludosa), which can be perceived as epiphytes growing in the moss layer. To be soil-dwelling means that the plants’ storage and rejuvenation bodies (not to be confused with the reproductive organs in the flowers) are underground or on the ground surface. The rejuvenation body contains the essence of next year's above-ground shoots. At the same time, a vegetative division of the body may take place, so that one parent plant can become two or more independent plants. The explanation for the name of the 'rejuvenation body' is due to the fact that when next year's above-ground shoots are planted, the old part withers.

There are several types of rejuvenation bodies: 1. The soil stem, which is a stem that can grow either vertically or horizontally, 2. the root tuber, which is a swollen part of the root, and 3. the stem tuber, which is a swelling of the lower part of the above-ground stem. The table shows how the 37 Danish orchid species are distributed among the different types of rejuvenation bodies.

TypeStem tuberSoil stemRoot tuber
Number of species21619

Immigration

All wild Danish orchids are naturally indigenous, i.e. they have come to the country without direct help from humans. A comparison with other Danish plant families of the same size shows that the orchid family is the only one without imported or introduced species. “Introduced”  in the botanical context means that an organism has come passively to the country, e.g. by means of human transport, such as by car, ship or train. Most native species have presumably immigrated along with the landscape development that followed the retreat of the ice after the ice age approx. 15,000 years ago. However, the western heart-leaved twayblade (listera cordata) and creeping lady’s tresses (goodyera repens) were not registered until 1700 and 1878, which is primarily attributed to planting with conifers.

The fact that the number of Danish species continues to change is due, among other things, to the intensive field studies and the research that takes place. For example, it should be mentioned that violet helleborine  (epipactispurpurata) was first acknowledged in 1953, and that the bee orchid (Ophrys apifera) was discovered as newly immigrated in 2004. Both have undoubtedly grown in the country prior to this.

Orchids and fungi

The majority of the 37 orchid species contain chlorophyll (leaf green) and produce nutrient and build-up substances using photosynthesis. Three species, https://www.dmu.dk/1_Om_DMU/2_Tvaer-funk/3_fdc_bio/datasamlinger/orkide/orkidedata_NH.asp?ID=17  ghost orchid (epipogiumaphyllum), northern coral root (corallorhiza trifida) and bird’s-nest orchid (Neottia nidus-avis), hardly have any chlorophyll - if at all any. Their nutritional needs are met by establishing close cohabitation with underground fungi. This cohabitation means that individuals of these species, in particular, adapt to a fungal way of life. They can live underground for several years and build up a new, powerful soil stem, which, in turn, collects enough energy to grow flowering shoots. Bird’s nest orchid can even grow underground, self-pollinating flowers on the actual soil stem.

In addition to being able to form organic matter through photosynthesis, most of the 34 'green' orchid species also have a complex interaction with soil fungi. This means that many species are more or less specialised when it comes to choosing habitats. Therefore, it is often difficult or impossible to move an orchid from one habitat to another. As the fungal partner often is sensitive to the addition of fertiliser and toxins, this means that the cohesive orchids also are sensitive.

The presence of soil fungi also has significance for the orchids' chances of success in propagation. The orchids produce the smallest and lightest seeds in the plant kingdom, as they are not provided with albumen. For the fragrant orchid, the seed weight has been calculated to be 0.000008 grams.

When a seed is to germinate, it is dependent on a suitable fungal partner that supplies the nascent orchid with albuminoidal substances. Together with the soil fungus, the burgeoning orchid produces a small tuber-shaped or worm-like stem formation, the outer layer of which is covered with fungal strands, a protocorm. It may take several years before the prototype forms roots and above-ground shoots. Therefore, often several years pass between seed germination and flowering. In cultivation experiments with seeds of the yellow widelip orchid (liparis loeselii), it took four years for the first leaves to appear.

Habitat requirements

Some orchid species are specialists living in specific soil conditions, in which their fungal partner can thrive. Others have broader ecological requirements and therefore occur in several vegetation types; some are particularly dispersive and may therefore occur in many locations, even where the soil has been altered by cultivation. Several species can act as pioneers, i.e. in places with sparse plant cover, either as a result of soil erosion or excavation. Examples of this are yellow widelip orchid (liparisloeselii), northern marsh orchid (dactylorhiza majalis subsp. purpurella var. cambrensis) and marsh helleborine (epipactis palustris). A common characteristic of all Danish orchids is their requirement for unfertilized and unsprayed conditions at their growing sites. For example, the tubers of the last specimens of the fragrant orchid (gymnadenia conopsea subsp. conopsea) on Funen rotted around the 1990s due to the addition of fertiliser-enriched water from a nearby trench.

If the orchids are distributed according to their preferred nature type, 12  can be considered as forest orchids, 13 as marsh orchids and 9 as pasture orchids. The remaining 11 species are less habitat-specific, as they can fall into several categories.

Way of life

The height of the fully grown orchids varies from a few cm up to 1.5 m; the smallest individuals are found in the https://www.dmu.dk/1_Om_DMU/2_Tvaer-funk/3_fdc_bio/datasamlinger/orkide/orkidedata_NH.asp?ID=41 bog adder’s mouth (hammarbyapaludosa) and musk orchid  (herminiummonorchis), while the tallest must be found among species of helleborine (epipaactis spp.). Many species, especially among marsh, pasture and the heath orchids, also require light and therefore only tolerate overgrowth of high-growing, perennial herbs, shrubs and trees to a limited extent. Conversely, many forest orchids, e.g. more helleborine (epipactis spp.), are adapted to varying degrees of shade. Orchids without chlorophyll can complete their life cycle in the shade in places such as willow scrub or closed beech forests.

Pollination

The majority of Danish orchids require pollination, and several have developed complex pollination mechanisms to avoid self-pollination. Other orchids produce fruit upon self-pollination. Finally, there are a few species where both pollination methods are at play: If pollination is absent, fertilisation is ensured by self-pollination.

In most species, the gendered propagation (i.e. the formation of fruit upon pollination) is important in connection with spreading seeds on the site itself and is the only source of dispersion over longer distances. In addition, some orchids can spread at the site by vegetative formation of underground offshoots or splitting of soil stems. This can also be done by creating more than one new tuber each year to replace the old one in tuber orchids. Vegetative propagation with the help of organs other than those underground occurs in bog adder’s mouth, which as the only Danish orchid can reproduce by breeding buds from the leaf tips.

Variation in numbers

The mechanisms that regulate population fluctuations in orchids are only known to a lesser extent. A dry versus a wet spring influences the number of flowering individuals in a population. Spring 2018 came early and was dry, and the number of early flowering orchids in many pasture sites was lower than in previous years. On the other hand, there were many flowering orchids, both in 2019 and 2020, where the spring followed a mild, wet winter.

Another significant factor for the number of orchids is human impact, both conscious and unconscious, on the habitats of the orchids. The choice of grazing animals, their number and species, the timing of harvesting, the supply of nutrients, the degree and type of forestry and human traffic on the site all impact the composition of the rest of the vegetation and, thus, the living conditions of the orchids. At the same time, the hereditary material of each species plays a significant, but hitherto unknown, role for the individual's germination, flowering frequency, fruiting and life duration. In addition, the significance of the combination with fungi on the orchids ' life cycle is virtually unknown, disregarding studies in the laboratory re the relationships between fungi and the emergence of the orchids.