Data from: Combined observational and experimental data provide limited support for facilitation in lichens

It is increasingly recognized that facilitative interactions can shape communities. One of the mechanisms through which facilitation may operate is when one species facilitates the colonization of another through the exchange of shared symbionts. Lichens are symbiotic associations composed of a mycobiont (lichenised-fungus) and one or two photobionts (algae or cyanobacteria). Different lichen species may have overlapping specificity for photobionts, creating the possibility that facilitation drives lichen community assembly. To investigate whether facilitation occurs in lichens, we combined an observational study (a) with a manipulative field experiment (b). For (a), we quantified the effect of local patch conditions, facilitation and the size of the surrounding metapopulation on colonizations of an epixylic lichen species (Cladonia botrytes) in an area of managed boreal forest. This was done by twice surveying lichens on 293 stumps, located in stands of three age classes. For (b), we treated unoccupied surfaces of 56 cut stumps with algal mixtures of an Asterochloris photobiont and recorded C. botrytes colonizations over three years. In (a), colonization rates of C. botrytes increased with increasing abundance of other lichen species with specificity for Asterochloris photobionts, consistent with an effect of facilitation. However, in the field experiment (b), colonizations of the focal species did not provide support for facilitation. We conclude that our study provides limited support for facilitation in green-algal lichens, underscoring the importance of combining observational studies with experiments when studying species interactions.

学界日益认识到，促进性相互作用（facilitative interactions）能够塑造生物群落。促进作用可通过多种机制发挥功能，其中一种是：某一物种通过共享共生体（symbionts）的交换，促进另一物种的定殖。地衣是由菌共生体（mycobiont，地衣型真菌（lichenised-fungus））与一种或两种光合共生体（photobionts，即藻类或蓝细菌（cyanobacteria））构成的共生联合体。不同地衣物种对光合共生体的特异性可能存在重叠，这为促进作用驱动地衣群落组装提供了可能。为探究地衣中是否存在促进作用，本研究将观测性研究（a）与控制性野外实验（b）相结合。针对（a），我们量化了局域生境斑块条件、促进作用以及周边集合种群（metapopulation）规模对腐木生（epixylic）地衣物种Cladonia botrytes定殖过程的影响。该研究通过对3个林龄等级林分中的293个伐桩开展两次地衣调查，完成了上述量化工作。针对（b），我们使用星球藻属（Asterochloris）光合共生体的藻类混合液，处理了56个未被占据的伐桩表面，并在3年时间内记录了Cladonia botrytes的定殖情况。在观测性研究（a）中，Cladonia botrytes的定殖率随其他对Asterochloris光合共生体具有特异性的地衣物种的丰度上升而升高，这与促进作用的效应相符。然而在野外实验（b）中，目标物种的定殖情况并未为促进作用假说提供支撑。本研究最终得出结论：我们的研究仅为绿藻地衣中的促进作用提供了有限的证据，这凸显了在研究物种相互作用时，将观测性研究与实验相结合的重要性。