Chao Guo and Hans Cornelissen, Vrije Universiteit Amsterdam, discuss their article: Using traits to integrate non-additive effects of species mixtures across ecosystem processes
Imagine you want to make a campfire outside your tent in the woods. You have some big wood blocks available, but they are hard to get going. You know from your parents’ fireplace that you need to add some fine fuel in the form of dead plant material; this will create an initial fire front that produces enough heat to get the wood starting to burn. You collect a lot of dead pine and spruce needles around your tent. But which of these will work best to ignite the wood blocks? You decide to put this question to the test, knowing you still have two weeks of campfires ahead. You first build a fuel bed of spruce litter adjacent to the wood blocks and ignite it. Result: nothing happens and the fire stops before it gets properly started. Next you try the same with the pine litter: it ignites and burns like mad and soon sets the wood blocks alight. Finally, you try a mixture of spruce and pine needles. The result is the same as for spruce needles alone: barely any ignition, no campfire, no heat.
This very simple test by a shivering camper in the woods is where this paper about traits, non-additivity, and ecosystem functioning starts. The traits involved are needle size and shape. Spruce needles fall off the tree as individual small needles, which together form very dense litter beds. Fire needs plenty of oxygen, which is not in great supply in a dense litter bed. Pine needles, on the other hand, stack very loosely because they are long and fall off in pairs. This is why pine litter beds ignite and burn beautifully, perhaps also helped by the resins in the needles. However, when you add spruce needles to a litter bed of pine needles, the spruce needles fill up the spaces between the pine needles, resulting again in a dense fuel bed with little oxygen available. Based on the average flammability (percentage of litter burnt) of the pine litter bed (100 %) and the spruce litter bed (0 %), you would have expected the percentage of the mixture to be burnt to be halfway between those of the single species, i.e. at 50 %. The fact that the mixture burns differently from expected, i.e. not at all, is a non-additive effect of mixing two species.
Experiments such as the one above have also been done in more standardized ways in fire labs by the community of fire ecologists. There is also a separate community of ecologists doing experiments comparing the decomposition of single species litter with those in mixture, often finding strong interactions, i.e. non-additivity, between litter types. And then there is yet another separate community of researchers who find that, because of ‘complementarity’, two species growing in a mixed community often have higher productivity than predicted from the productivities of each species growing with neighbours of the same species. Such non-additive effects of species mixtures can be important for the carbon and nutrient cycles in ecosystems. But how can we find out how the non-additive effects of species mixtures on fire, decomposition and productivity, and perhaps other ecosystem processes, relate to each other? If we get negative effects for fire, like in the camper’s test, but positive effects for decomposition of the same species mixture, these effects may cancel each other out when we are interested in CO2 released back to the atmosphere. In other words, we have to get the different research communities on non-additivity to talk with each other; and in the same language.
With this ambitious aim, the author team got together to brainstorm for three days on the beautiful Dutch island of Schiermonnikoog, hoping they would find a way to connect the traits of the plant species in a community to non-additivity in their mixtures, irrespective of the ecosystem processes studied; and thereby to facilitate non-additivity linkages between different ecosystem processes. They made a first conceptual breakthrough on the final day of this get-together that stands as the basis of the paper now published. Hopefully the conceptual framework will inspire and help many researchers to put their own species mixture experiments in this broader context. It should really help our understanding of how biodiversity, via the traits of the species in a community, affects ecosystem functioning.
