Old-growth boreal forests and niche wood-decay ecosystems

Within old-growth boreal forests, which are found at the northern-most latitudes of the world, decaying tree trunks are a key micro-habitat. Within the tree trunks, decay is very slow, and the forests therefore have high levels of both standing and fallen deadwood that exist in ideal conditions for long periods of time. In Fennoscandia, around 1,000 species of beetle rely on either decaying wood, or the macro wood-decay fungi themselves (their sporophores), for habitat. However, because many old-growth boreal forests have come under some form of management, which typically sees deadwood cleared, there are perhaps untold extinctions that have taken -and are still taking – place.

In this research study, the authors focus on the sporophores of the wood decay fungus Fomitopsis rosea (which induces a brown rot), found in old-growth spruce swamp forests upon stumps and fallen trunks, and assess whether its decline across Finland (due to logging) has lead to a change in population dynamics of insects reliant upon the fungus’ sporophore (once it is partly dead) in the remaining isolated patches of old-growth spruce swamp forest. The isolated patches surveyed included five fragments isolated for between 2-7 years, and an additional ten fragments isolated for between 12-32 years, whilst the control areas were large patches of old-growth forest not isolated due to logging. All sites were however equal, in the sense that they had similar tree species composition, a similar number of dead stumps and fallen trunks, and were of similar age.

A group of sporophores on a fallen stem. Source: Mycoweb.

At each site, fruiting bodies were located and samples were taken – a total of 251 were taken from control sites, 60 from sites isolated for 2-7 years, and 44 from sites isolated for 12-32 years. These samples were then taken back to the laboratory, where they were placed in cloth-covered plastic boxes in outdoor conditions for just over a year. Every month, the boxes were checked to ascertain whether any insects had emerged from the sporophores, and any that had emerged were taken and stored either in alcohol or as dry samples for identification.

From the samples taken, a total of 33 insect species were identified. Many of the species found are classed as rare across Fennoscandia. The most dominant (33%) insects identified were the larvae of the moth Agnathosia mendicella, which eat the fungal tissues, and the parasitic fly Elfia cingulata that specialises in parasitising on the moth larvae. This fly had not, at that time, been recorded in any other fungal species’ sporophore, and nor was it found in any of the sporophores sampled that contained other moth species in place of Agnathosia mendicella.

This sporophore is partly dead, and may very well be the type of sporophore that insects utilise. Source: Mycoweb.

The presence of the moth Agnathosia mendicella was most abundant in the control groups that were not isolated, and the parasitic fly Elfia cingulata fared similarly – as did the abundance of the fungus Fomitopsis rosea. However, Elfia cingulata was not found at all in patches isolated for more than 12 years, and the more isolated patches of 2-7 years that contained the fungal sporophores were host to fewer Agnathosia mendicella and Elfia cingulata. In fact, the presence of the moth Agnathosia mendicella was significantly lower in the isolated old-growth fragments, as was the presence of Fomitopsis rosea in patches isolated for 12-32 years – particularly when the forest fragments were small and the decaying trunks were exposed to sunlight (the fungus rarely grows in sun-exposed settings).

In light of the data, the authors suggest that fragmentation of old-growth forest, and the amount of time the fragments have been isolated for, is directly related to the declining presence of the Fomitopsis rosea –Agnathosia mendicella – Elfia cingulata trophic relationship. Other insect species observed suggested similarly. Therefore, it is important that not only is habitat fragmentation reversed over time, but patches of old-growth forest are allowed to persist or increase in size. Currently, the isolated fragments simply cannot provide the right conditions for such niche and specialised species, from the fungus itself all the way up the trophic levels to insect parasitoids. Changes in forestry practice are thus necessary, else local extinctions of niche ecosystems (not just those relating to Fomitopsis rosea) may more frequently occur.

Source: Komonen, A., Penttilä, R., Lindgren, M., & Hanski, I. (2000) Forest fragmentation truncates a food chain based on an old-growth forest bracket fungus. Oikos. 90 (1). p119-126.

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