The Conventwald ecosystem study is a long-term project of the Department of Soil and Environment, which was initiated in 1991, and has delivered 25 years of continuous observation about water and element cycles. The goal of the Conventwald study is to answer the questions, whether and to what degree different types of silvicultural management are able to counteract disturbances of ecosystems caused by deposition. The study examines how different silvicultural practices can protect the long-term stability of a site regarding plant available nutrient reserves, as well as surface and groundwater quality.

Monitoring Program

The Conventwald is one of five sites in Baden-Wuerttemberg belonging to the ICP-Forest network, on which comprehensive measurements about the climate, soil water budget, element fluxes (input through precipitation, output through soil water seepage), the chemical status of the soil, the soil gas budget, as well as forest stand nutrition, growth and vitality are carried out based on standards that are uniform throughout Europe.

Infobox "Conventwald"

The avalanche forest in the forest district Kirchzarten, eastward of Freiburg i. Br. is located in the montane altitudinal belt (700-860 mNN) at the southern border of the Central Black Forest. It is climatical based on an atlantic montane climate (mean annual temperature 7,3 °C, mean annual condensation 1490 mm).

The area under investigation comprises a near-natural mixed beech, fir and spruce forest (protected forest), as well as different silviculturally managed areas bordering it. On the one hand, the measurement areas are in closed forest stands, where subareas were created in crown centers, the crown edges and the gaps between crowns. On the other hand, different larger, naturally occurring gaps in the forest stand, several gaps from selective patch logging, as well as a clear-cut were utilized to observe their immediate effects on element fluxes in the soil zone. At the outlet of the catchment area – as an integral measure for the dynamics of the different forest stand structures – the amount and chemical composition of the runoff are being monitored.

The long-term, almost uninterrupted observations of the Conventwald ecosystem study make it possible to see long-term trends, e.g. in climatic conditions and in element input and output, as well as short- and medium-term reactions of the forest ecosystem to disturbances such as logging activities and droughts. A selection of results from the ecosystem study is presented in the following sections.

High Element Output of Spruce Stands

The different forest stands vary significantly in both the element input through precipitation and the output through soil seepage water. A comparison between ion fluxes of a 150-year old mixed beech/fir/spruce stand and a 100-year old spruce stand shows the potential in choosing tree species for preserving closed element cycles (Figure 2). The biggest difference between the two forest stands is found in nitrate fluxes. These are negligible in the mixed beech stand while they dominate the element budget of the spruce stand. Also, high sulfate fluxes below 60 cm soil depth boost the output of base cations in the spruce stand. These fluxes originate from the remobilization of precipitated sulfur from earlier depositions. In the mixed beech stand, the element flux density beneath the root zone is lower than the input through precipitation which causes soil nutrient reserves to remain largely intact. This example shows that the choice of tree species can make a noteworthy contribution to preserving the soils’ role of providing nutrition, as well as other functions of the ecosystem such as acidity buffering and nitrogen storage.

High Nitrate Output after Timber Harvesting

As an example, Figure 3 shows long-term nitrate measurements of the soil water in a small clear-cut, in two selective patch logging areas, and in an area of developing natural beech regeneration. Interestingly Figure 3 shows how nitrate concentrations increase abruptly after logging activities. The selective patch loggings generated nitrate peaks in the soil seepage water like those of the small clear-cut, but had a significantly smaller impact on the nitrate load in the runoff due to their smaller surface area. The effect of the developing natural regeneration is striking. In the patch, where vital natural regeneration was already present at the time of logging, nitrate concentrations decreased to the initial level within a few years after logging, while in the patch without natural regeneration, increased nitrate concentrations were observed up to 10 years after logging.

Balanced Element Budget in Continuous Cover Forest

From the individual measurements, cumulative element balances of base cations were derived for total silvicultural rotation times (Source). Fifty element budgets representing different forest stand phases were available in the area under investigation. They were combined for different silvicultural treatment strategies. The element budgets are the result of element input through deposition and rock weathering on the one hand, and element output through seepage water and timber harvesting on the other. Figure 4 shows the progression of the base cation budgets (Ca, Mg, Na and K) through complete rotation times for different silvicultural procedures that deviate substantially from one another regarding the element budget. The two procedures involving clear-cuts show a net loss in base cations of 5-6 kmolcha-1a-1 during the clear-cutting phase. Twenty-five years after clear-cutting the beech stand, the base cation budgets were balanced again. For the spruce stand, in contrast, a loss of base cations can be seen throughout the entire rotation time. The beech/fir/spruce continuous cover forest proves to be the most conservative treatment strategy that does not show longer periods of extensive base cation losses.

Summary & Prospects

Element flux measurements are suitable tools for understanding changes that are motivated by deposition of acids and nitrogen in ecosystems. The objective of the Conventwald Study is to identify the possibilities that forest management has in terms of controlling sustainability under the given environmental conditions. We showed that silvicultural treatment strategies can make a valuable contribution to preserving site quality. Gap-oriented harvesting practices showed balanced nutrient budgets in the Conventwald. In contrast to that, less gentle silvicultural practices such as pure spruce stands with clear-cut regeneration lead to high base loss on the same site. Also, the choice of tree species has an important potential to counteract soil acidification. In the Conventwald, beech stands tend to show positive base cation budgets, while they are clearly negative for the spruce stands.

The results demonstrate that ecosystem studies such as the Conventwald Study or the ICP-Forest program are important tools for monitoring site quality and sustainability. They also make it possible to assess the value of forest ecosystems to provide ecosystem services such as drinking water security. Future objectives of the Conventwald Study will focus on the reactions of the forest ecosystem to climate change, and the threat to site sustainability because of a continuingly high nitrogen input.