Parasitism modifies the direct effects of warming on a hemiparasite and its host

Climate change is affecting interactions among species, including host-parasite interactions. The effects of warming are of particular interest for interactions in which parasite and host physiology are intertwined, such as those between parasitic plants and their hosts. However, little is known about how warming will affect plant parasitic interactions, hindering our ability to predict how host and parasite species will respond to climate change. Here, we test how warming affects aboveground and belowground biomass of a hemiparasitic species (Castilleja sulphurea) and its host (Bouteloua gracilis), asking whether the effects of warming depend on the interaction between these species. We also measured how warming affected the number of haustorial connections between parasite and host. We grew each species alone and together under ambient and warmed conditions. Hosts produced more belowground biomass under warming. However, host biomass was reduced when plants were grown with a hemiparasite. Thus, parasitism negated the benefit of warming on belowground growth of the host. Host resource allocation to roots versus shoots also changed in response to both interaction with the parasite and warming, with hosts producing more root biomass relative to shoot biomass when grown with a parasite and when warmed. As expected, hemiparasite biomass was greater when grown with a host. Warmed parasites had lower root:shoot ratios but only when grown with a host. Under elevated temperatures, hemiparasite aboveground biomass was marginally greater, and plants produced significantly more haustoria. These findings indicate that warming can influence biomass production, both by modifying the interaction between host plants and hemiparasites and by affecting the growth of each species directly. To predict how species will be affected, it is important to understand not only the direct effects of warming but also the indirect effects that are mediated by species interactions. Ultimately, understanding how climate change will affect species interactions is key to understanding how it will affect individual species.

气候变化正影响物种间的相互作用，包括宿主-寄生生物互作（host-parasite interactions）。增温对那些寄生虫与宿主生理紧密交织的互作尤为受关注，例如寄生植物与其宿主的相互作用。然而目前对增温如何影响植物寄生互作的认知仍十分有限，这阻碍了我们预测宿主与寄生生物物种如何响应气候变化的能力。本研究探究了增温对半寄生植物（hemiparasitic species）硫磺色火焰草（Castilleja sulphurea）及其宿主细叶格兰马草（Bouteloua gracilis）的地上与地下生物量的影响，旨在明确增温的效应是否依赖于二者的种间互作。我们同时测量了增温如何影响宿主与寄生生物之间的吸器连接（haustorial connections）数量。我们在环境温度（ambient conditions）与增温条件（warmed conditions）下，分别对各物种进行单种种植，并将二者混合种植。宿主植物在增温条件下会产生更多的地下生物量。但当宿主与半寄生植物共同种植时，其生物量会显著降低。由此可见，寄生作用抵消了增温对宿主地下生长的益处。宿主对根系与地上部分的资源分配也会因寄生互作与增温而发生改变：当与半寄生植物共同种植，或处于增温环境时，宿主的根生物量相对于地上生物量的占比会更高。正如预期，半寄生植物在与宿主共同种植时生物量更高。增温后的半寄生植物根冠比（root:shoot ratios）更低，但这一现象仅出现在与宿主共同种植的组别中。在升温环境下，半寄生植物的地上生物量略有提升，且其产生的吸器数量显著增加。上述研究结果表明，增温可通过两种途径影响生物量生产：一是改变宿主植物与半寄生植物之间的种间互作，二是直接影响各物种的生长。若要预测物种将受到的影响，我们不仅需要理解增温的直接效应，还需厘清由种间互作介导的间接效应。归根结底，理解气候变化如何影响物种间互作，是探明其如何影响单个物种的关键所在。