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Nature conservation

Ecosystem experiments on a large scale

In Denmark and in countries similar to ours, a large experiment is taking place with the aim of supplying nitrogen and sulfur from the air. The experiment is the result of emissions from livestock, industry and traffic. As opposed to traditional experiments, this experiment has no controls or knowledge of the starting point. By contrast, it contains numerous repetitions and habitats. An experiment on such a large scale is, of course, a threat to habitats that cannot tolerate the extra amounts of deposition, which has both acidic and eutrophic effects. The first result that was obvious and resulted in trying to find a solution was the acidification caused, to a large extent, by high levels of sulfur deposition. The solution was to reduce the amount of sulfur deposition, and at certain locations lime has been added in order to elevate the pH. This has e.g. been done in Nordic lakes and pine forests. Ever since the late eighties, acidification has not been a large part of the debate. This does not mean that acidification does not take place, merely that the experiment will proceed with a smaller dose. Instead, nitrogen came in focus, and as for sulfur, the dose was reduced. In this way, much has been done to bring down nitrogen emissions from the various sectors, but despite this, deposition still exceeds what nature can handle. The effect of the eutrophication caused by nitrogen is overgrowth and dominance of unwanted species in terrestrial nature. In order to combat eutrophication, it is necessary to either limit or remove deposition from nature. Much has been achieved to reduce nitrogen emissions by policy measures, but not enough that nature can be said to be at a healthy stage. Consequently, excess nitrogen is removed by intensifying nature conservation; more vegetation is removed and, at times, even the topsoil is removed. Conservation methods include mowing, grazing, turf-cutting and burning. Through these measures, it has e.g. been possible to maintain dense growth of heather on the moors. In this way, conservation has diminished the effect of high levels of nitrogen deposition. In other words, a new ecosystem experiment has been initiated on a large scale, which aims to remove the fertilizer by intensifying conservation and, thus, mitigate the ”experiment” that fertilizes the ecosystems.

If conservation is good, does that mean that more conservation is even better?

In this way, an interesting and unpredictable large scale experiment has been initiated completely without knowing the long term effects of the conservation methods. Apparently, no one has given much thought to the effects of intensifying conservation in the long run. Existing studies have mainly assessed whether conservation has the desired effect on the vegetation in the short term! But what happens over time as a result of so much management at short intervals? How does it affect the environment? As most nature in Denmark is subject to what humans do and have done to it, it can no longer sustain itself without various types of management. The Habitats Directive even states that natural areas that result from conservation must be protected and not be reduced. Thus, due to the effects of nitrogen it is necessary to intensify conservation and maintain it. This is not only costly – it is also impossible to know the effects of conservation in the long term.

 

Wet heaths with Erica tetralix

Wet inland heaths, dominated by Erica tetralix, are one of the human-induced habitats, in which Erica tetralix constitutes the ecosystem and the conditions for the biodiversity it contains. See also Box 1 regarding ecosystem generators. Wet heathlands constitute a habitat that can exist for a long time without management, which may be the reason no clear guidelines exist as to how to manage it in order that it may exist even longer. As with a lot of other open wood habitats, tree growth is removed.

Erica tetralix is disappearing

When it became clear that Erica tetralix was disappearing in 2010, Defence Command Denmark requested that we examine what could be done to preserve Erica tetralix at the shooting range Borris Hede (Borris Heath). As we were not familiar with what caused Erica tetralix to disappear and, consequently, what management measures might help, we agreed with Defence Command Denmark that we would survey some areas at Borris Hede. The surveys included vegetation transect surveys from heaths with heather, heaths with bell heather (Erica tetralix) and heath bog, as well as vegetation chemistry and soil chemistry along these transects. For financial reasons, the measurements were limited to nitrogen and carbon in end shoots of heather and bell heather as well as in their topsoil – the moor layer, and pH in the same soil layers. The surveys were supplemented by a few analyses from a dune heath dominated by healthy Erica tetralix, Stenbjerg Hede. Perhaps by observing differences in the two habitats, we would be able to figure out what was wrong at the wet inland heaths.

Erica tetralix supplanted to a thin band

The two transects at Borris Hede showed that the band with dense bell heather growth is narrow, where previously dense growth took place in a broad band. Areas with bell heather are changing and are now characterized by invasion of e.g. common heather and purple moor grass. A few small areas of a couple of square meters dominated by bell heather still exist. However, these areas are declining, which is evidenced by quite a bit of dead or dying bell heather in these areas.

Low ratio of carbon and nitrogen in the moor layer

Measurements of carbon and nitrogen in the moor layer show an unfavorably low C/N-ratio in the moist areas, where bell heather previously was very dominant. Compared to the C/N-ratio for healthy heaths, which is 30 or above, at moist areas at Borris Hede the C/N-ratio was between 21 and 26. This may indicate that conservation measures at Borris Hede, i.e. burning, have not been sufficient to remove the accumulated nitrogen and maintain the area’s ability to build carbon. This is probably due to the fact that burning has taken place at times when the moist areas have hardly been affected by it. At the same time, it is not given that burning is the most suitable conservation method at wet heaths.

Acidic soils

The most surprising observation was the fact that the soil pH was very low. Heaths are already known to be located on acidic soil with pH values between 4 and 5.5 (Hansen 1976). However, at both Borris Hede and Stenbjerg Hede the pH was between 3 and 3.5, so the concentration of hydrogen ions in the top soil may be 10 times higher than in 1960, when Kjeld Hansen completed the measurements. Results from monitoring terrestrial nature confirm this trend.

Causes of acidification

There may be several reasons that the soil is becoming more acidic. Soil acidification is an inexorable natural process that we can influence, both positively and negatively. The main cause of both natural and man-made acidification is atmospheric deposition in the form of sulfur and nitrogen compounds, growth processes in the vegetation, respiration from roots (roots release organic acid that dissolve minerals), and decomposition of plant litter. Acidification processes can be delayed by ion exchange or weathering and/or liming of minerals. Apart from these direct chemical causes of acidification, the change in use of the areas may also cause acidification, e.g. intensive management such as burning or mowing. Initially, heaths are the result of acidification caused overexploitation of forest. This applies to virtually all of our open habitats, only here the nutrient poor starting point contributed to promoting the vegetation type heath, which accelerated the acidification. The original soil, probably brown soil, was replaced by leached heathland podsolic soil. Initially, there were sufficient minerals and free cations to counteract the leaching processes in the soil, but slowly base cations has leached from the soil , which has resulted in highly acid soils with a very low potential for weathering.

This is the situation:

  1. Heath ecosystems are affected by acidic deposition which causes the soils base cations to leach. Acidification also result in  toxic high concentrations of e.g. aluminum in the soil solution.
  2. There are indications that the ecosystem has received and continues to receive too much nitrogen (read about critical loads). Excess nitrogen stimulates growth, creating an imbalance of nutrients, as the increased growth also causes increased consumption of e.g. cations such as magnesium and potassium, and micronutrients such as selenium, copper and boron.

Aluminum-ions in soil solution

Free aluminum ions are toxic to plants and soil organisms. It is known from the forest dieback in central Europe about 30 years ago that acidification resulted in large areas of forest die-back.. Perhaps we are witnessing something similar to this at the wet inland heaths, as the soil no longer can counteract pressure from acidification. This may cause the soil’s acidity to decrease, which means that the soil becomes toxic to some organisms, apparently including bell heather.

Ammonium in soil solution

Studies of the effects of turf-cutting the heaths in the Netherlands have shown that the soil had a high content of ammonium in the soil solution. Apparently, turf-cutting causes the microbiological “plant” in the moor layer that converts the ammonium in rain water to nitrate to disappear (nitrification). Turf-cutting involves the physical removal of these microorganisms and results in a high degree of drying out, which contributes to the process.

The transformation to nitrate is an acid-forming process that increases the amount of protons. The ammonium concentration in the soil solution was so high that it was toxic to certain plants. Furthermore, it has been found that the nitrate formation in the top soil is inhibited at a low pH level, which also results in ammonium accumulating in the soil. In this way, both radical management measures and the increased acidification contribute to increasing ammonium concentrations in the soil and, thus, result in a form of toxicity to some plants.

Healthy heather on acidic soil

The observation of low pH values in the soil at Stenbjerg heath along with healthy bell heather showed that the acidification hypothesis cannot fully explain the inland decline of bell heather in Denmark. In our search for an explanation for this, we chose to examine the availability of a number of nutrients, which tend to become scarce in nutrient poor ecosystems. These include basic cations such as magnesium and potassium and various micro-nutrients (e.g. Se, Cu, B). Common to these cations is that they may become scarce in connection with acidification and the unbalanced growth stimulation the addition of nitrogen causes. Also common to these cations is that they can be supplied from the ocean through salty spray being blown onto land. The amount of spray naturally declines in proportion to the distance to the coast. As an approximation, we chose to examine the state of bell heather in heaths as a function of deposition of basic cations modeled according to the distance to the coast. This analysis showed a correlation, which makes it probable that deposition of basic cations and other micro-nutrients supplied by salty spray from the ocean compensate the bell heather for the deficiency caused by acidification and the unbalanced growth stimulation caused by nitrogen deposition. However, this effect is not strong enough to compensate inland heaths located furthest from the coast, and thus this hypothesis may explain why bell heather is disappearing from inland wet heaths, but not from coastal heaths. Studies by Härdtle et al (2006) from Lüneburger Heide in North-west Germany, which has a pH value of 3.3 – 3.5 in the top soil, calculated that the amount of calcium, potassium and magnesium removed by mowing is compensated by atmospheric deposition within approximately 6 years (Ca), 3 years (Mg) and 9 years (K), which indicates a relatively high impact from salty spray. On the other hand, the loss of phosphorus was slow to be compensated.

Nature conservation does not counteract acidification

Observations and studies from other Danish inland heaths confirm that bell heather has been declining at least since 1995. NOVANA monitoring indicates that coverage with bell heather has declined significantly in the period 2004-2008 from 28 % to 19 %. Purple moor grass covers approximately 40 %, making it the most dominant species on heaths with bell heather. At the same time, it is clear that reliable experience is lacking as to how to manage wet heath with bell heather without, at the same time, having an acidifying effect.

Conclusions from the study:

Acidification of the top soil layer is the most probable reason that bell heather is dead or dying in the wet heathlands. At the same time, low pH in both wet and dry heaths, together with eutrophication with nitrogen, are the main reasons that the biodiversity is declining in these habitats.

Nitrogen deposition is exceeding the critical load for the habitat, which is leading to overstimulation of growth, which is removing the most important nutrients from the soil. Furthermore, where ever they are currently implemented at wet heaths, current management measures cannot eliminate excess nitrogen, which results in nitrogen being accumulated in the wet areas and, consequently, that the vegetation slowly is changing. A change that is still taking place, which will, most probably, result in vegetation dominated by grasses, most likely with purple moor grass as the dominating species. Acidification apparently creates negative feedback to the nitrogen cycle, which may result in ammonium accumulation that may affect the health of the plants.

Conventionally applied management methods are not suited to counteract soil acidification, neither in wet or dry heaths. On the contrary, these management methods may even increase acidification by not only removing nitrogen, but also removing cations from the ecosystems that otherwise would help to counteract acidification.

For this reason, if these preliminary studies turn out to be correct, current nature conservation at Danish heaths is facing a true dilemma.