Forest food web dynamics with special emphasis on soil biota / Dinamika gozdne prehranske mreže s poudarkom na življenjski združbi tal

author: Heikki Setälä, University of Helsinki
published: Oct. 21, 2011,   recorded: October 2011,   views: 137
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The dynamics of forests are commonly considered from the above-ground point of view, where plants, mostly trees, are believed to play a regulative role. Accumulating amount of evidence suggests, however, that the organisms of the below-ground food webs can pose a strong control over the various phenomena in the forests, most importantly forest productivity. As soil saprophytic microbes, such as fungi and bacteria, comprise up to 98% of the total biomass of the forest soil food webs they are often considered to play a decisive role as affecting the decomposition rate of dead organic matter and liberating growth-limiting nutrients, such as nitrogen and phosphorus, for the uptake by forest trees. This belief is, however, not necessarily true and it has been challenged by a multitude of experiments conducted in small-scale experiments under laboratory conditions. Based on the studies that I have conducted with my research group since late eighties, I will present evidence showing that, by feeding upon soil saprophytic microbes, soil fauna stimulate the activity of soil microbes and thereby pose a strong effect on decomposition processes in forest soils. I will also show that soil fungal-feeding fauna can affect the growth and activity of ectomycorrhizal fungi on the plant roots and that this interaction can be beneficial for tree growth. Results of these studies suggest that it is the functional group (such as fungal feeders, bacterial feeders, detritus feeders, predators) diversity of soil fauna and microbes, rather than species diversity of the two groups, that control below-ground processes and plant growth. However, some species, such as the enchytraeid worm Cognettia sphagnetorum, are more influential than others and can deserve the status of a key-stone species in the forest. The absence of such taxa can lead to changes in the nutrient content of plants which can reflect in the performance of plant foliage-eating insects above the ground. It is concluded that to fully understand the dynamics and well-being of forest ecosystems it is essential to recognise that these ecosystems are driven by the mutualistic feedback apparatus existing between soil biota and the organisms living above the ground.


Dinamiko gozdnih ekosistemov navadno obravnavamo s stališča površinskega dogajanja in procesov, v katerih odigrajo rastline, predvsem drevesa, ključno vlogo pri uravnavanju sistema. Vendar pa so nedavne intenzivne raziskave pokazale, da imajo predvsem organizmi v prehranskih mrežah (food web) pod zemljo še veliko večji vpliv na procese v gozdovih, najpomembneje na produktivnost dreves. Glede na to, da predstavljajo saprofitski mikrobi, zlasti bakterije in glive, kar 98 % vse biomase v gozdnih tleh, lahko upravičeno pričakujemo njihovo odločilno vlogo pri razgradnji odmrle organske mase in pri sproščanju sicer za rast dreves ključnih, a pogosto v količinah omejenih hranil, dušika in fosforja, v tleh. Ta domneva pa ni nujno resnična in je bila preverjena v mnogih manjših laboratorijskih poskusih. Na podlagi lastnih raziskav, ki so se začele že v osemdesetih letih, bom predstavil dokaze o ključni vlogi talne favne v gozdnih tleh. S hranjenjem s saprofitskimi mikrobi talna favna pravzaprav spodbuja aktivnost mikrobov in ima tako izjemno velik vpliv na proces razgradnje v tleh. Videli bomo tudi, da talna favna z obžiranjem ektomikoriznih gliv na koreninah vpliva na rast in aktivnost teh gliv in da je ta splet povezav ključen za drevesno rast. Pokazalo se je, da je za podzemeljske procese in rast rastlin pomembna pestrost funkcionalnih skupin znotraj talne favne (npr. mikrobivori, ki se hranijo z glivami ali bakterijami, detritivori, plenilci) in ne dejanska biotska pestrost, torej število vrst, skupin. Kljub temu so se posamezne vrste, denimo beli deževnik vrste Cognettia sphagnetorum, izkazale za bolj vplivne od drugih, zato jih lahko upravičeno štejemo za ključne vrste gozdnega ekosistema. Odsotnost takih vrst v tleh ima namreč za posledico spremembe v vsebnosti nekaterih za rastline ključnih nutrientov v tleh, kar se odrazi tudi na populacijah rastlinojedih žuželk na površju. Zaradi tega je za razumevanje dinamike in stabilnosti gozdnega ekosistema neobhodno potrebno poznavanje pozitivne povratne zanke (mutualistic feedback apparatus) med življenjsko združbo tal in površinskimi organizmi.

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