Data and code for "Resource gradients drive the distribution and photobiont characteristics of two lichen species in a subarctic lava field"

This project described how the distribution and morphology of two species of lichen varied along a transect perpendicular to Lake Myvatn in Iceland. All data were collected in a recent (1729) lava field and we focused on two species Flavocetraria cucullata and Peltigera leucophlebia. Lake Myvatn is famous for large midge (Diptera: Chironomidae) emergences of which many are deposited on land. Thus, the transect reflects variation in midge deposition. P. leucophlebia contains both green algae and cynaobacterial endosymbionts and thus is able to fix atmospheric nitrogen, while F. cucullata contains only green algae and is therefore dependent on environmental nutrients. We suspected that these species differed in their response to the midge-derived nutrient gradient.

We measured the distribution of each lichen along a 400 x 15 m transect perpendicular to the lake, characterized their surrounding vegetation, and took thin sections of thalli on which we characterized photobiont thickness and chlorophyll-a concentrations using images taken with a near infrared camera.

The data are housed in the Data folder. FLG_Transect_Points.csv contains the location and names of each lichen plot. FLG_Surrounding_Vegetation.csv contains the vegetation inside each plot. Lichen_IR.csv contains the ENDVI, our metric of chlorophyll-a concentration, and photobiont thickness, in units of pixels taken using a standardized zoom.

All scripts associated with the project are housed in the Analysis folder. IRProcessing.R contains the code used to calculate the chlorophyll-a concentration and thickness from each of the photographs of the lichen thin sections. Myv_analysis.R contains all the code used for the statistical analyses and to generate figures. GOLichenAnalysis2019.Rproj is the RStudio project, which sets the relative locations of files. See https://support.posit.co/hc/en-us/articles/200526207-Using-RStudio-Projects for more information on .Rproj files.

本研究阐明了冰岛米湖（Lake Myvatn）沿岸垂直样带中两种地衣的分布与形态变化特征。所有数据采集于1729年形成的熔岩场，研究聚焦于挂膜黄髓地衣（Flavocetraria cucullata）与白脉皮叶地衣（Peltigera leucophlebia）。米湖以大规模摇蚊（双翅目：摇蚊科）羽化事件闻名，大量羽化的摇蚊会沉降至陆地，因此该样带可反映摇蚊沉降量的空间变异。白脉皮叶地衣同时包含绿藻与蓝细菌内共生体，因此能够固定大气氮；而挂膜黄髓地衣仅含有绿藻，因此依赖环境中的营养物质。我们推测这两种地衣对摇蚊来源的营养梯度响应存在差异。
我们沿垂直于米湖的400×15米样带调查了每种地衣的分布情况，对样地周边植被进行了表征，并制备了地衣体（thallus）的薄切片；通过近红外相机拍摄的切片图像，我们分析了光合共生体（photobiont）的厚度与叶绿素a浓度。
数据集存储于Data文件夹中。其中FLG_Transect_Points.csv包含各地衣样地的位置与编号信息；FLG_Surrounding_Vegetation.csv记录了每个样地内的植被情况；Lichen_IR.csv包含本研究用于表征叶绿素a浓度的指标ENDVI，以及以标准化变焦拍摄的像素为单位的光合共生体厚度。
本项目相关的所有代码脚本存储于Analysis文件夹中。其中IRProcessing.R包含用于从地衣薄切片的照片中计算叶绿素a浓度与光合共生体厚度的代码；Myv_analysis.R包含所有统计分析与图表生成的代码；GOLichenAnalysis2019.Rproj为RStudio项目文件，用于设置文件的相对路径。有关.Rproj文件的更多信息，请参阅https://support.posit.co/hc/en-us/articles/200526207-Using-RStudio-Projects。