Larger presence of ectomycorrhizae detected from pygmy pine ecotype in the fire-frequent pine barrens ecosystem

Pine barrens ecosystem has acidic, sandy, and nutrient-poor soil and is prone to drought and fire. In the New Jersey Pine Barrens, the predominant pitch pine (<i>Pinus rigida</i>) consists of two ecotypes: the regular pitch pines with heights of 4.6–12 m, and the pygmy pines of low stature (1.2–1.8 m) in the New Jersey Pine Plains. Previous ecological studies suggested that the dwarf pines in the Pine Plains that are embedded within the Pine Barrens were an evolutionary adaptation to frequent fire. Pines are obligate ectomycorrhizal (EcM) mutualists, and their root mycobiota may contribute to stress protection and plant health. However, information on the mycobiota associated with plants in the pine barrens ecosystem is lacking. To have a holistic understanding of the evolution and adaptation in this stressed environment, we used both culture-independent metabarcoding and culture-based method to characterize the mycobiota from soil and root of the two ecotypes and to identify core mycobiota. We found that Agaricomycetes, Leotiomycetes, and Mucoromycotina are predominant fungi in the New Jersey Pine Barrens ecosystem, which is rich in root mutualistic fungi. We observed that the pygmy pine roots had significantly higher density of EcM tips than the regular pine roots. This was corroborated by our metabarcoding analysis, which showed that the pygmy pine trees had higher ratio of ectomycorrhiza-forming fungi than the regular-statured pines. We hypothesize that symbiotrophic EcM fungi associated with pygmy pines are capable of mitigating high fire stress in the Pine Plains.

松瘠荒生态系统（Pine barrens ecosystem）的土壤呈酸性、沙质且养分贫瘠，易发生干旱与火灾。在新泽西松瘠荒区域，优势物种刚松（Pinus rigida）存在两种生态型：一种是株高4.6–12米的普通刚松，另一种是分布于新泽西松原（New Jersey Pine Plains）的矮生型刚松，株高仅1.2–1.8米。既往生态学研究表明，嵌入松瘠荒区域的松原中的矮生松树，是对频繁火灾的进化适应结果。松树为专性外生菌根（ectomycorrhizal, EcM）共生体，其根部真菌群落可助力植株抵御胁迫、维持健康。但目前针对松瘠荒生态系统中植物相关真菌群落的研究信息仍较为匮乏。为全面理解该胁迫环境下的进化与适应机制，本研究采用非培养型宏条形码技术（culture-independent metabarcoding）与培养法相结合的手段，对两种生态型松树的土壤与根部真菌群落进行表征，并鉴定核心真菌群落。研究发现，伞菌纲（Agaricomycetes）、锤舌菌纲（Leotiomycetes）和毛霉亚门（Mucoromycotina）是新泽西松瘠荒生态系统中的优势真菌类群，该生态系统中根际共生真菌资源丰富。我们观察到，矮生刚松的根部外生菌根根尖密度显著高于普通刚松。这一结果通过宏条形码分析得到了验证：分析显示矮生刚松的外生菌根形成真菌占比高于普通株高的刚松。我们推测，与矮生刚松共生的外生菌根真菌，能够缓解松原区域的高强度火灾胁迫。