Harsh environmental conditions promote cooperative behavior in an epiphytic fern

Harsh, unpredictable environments are known to favor cooperative groups in animals. Whether plants exhibit similar relationships is unknown. Staghorn ferns (<i>Platycerium bifurcatum</i>, Polypodiaceae) are epiphytes that form cooperative groups which build communal water and nutrient ‘nests’ at the tops of trees, a habitat characterized by water and nutrient stress. We conducted field observations to test whether staghorn ferns continue to live in large, reproductively active groups after they become dislodged from the canopy and fall to the forest floor, where they are less limited by water and nutrient deprivation. To rule out the potentially confounding effects of light limitation on the forest floor, we also conducted a multi-year glasshouse experiment where we transplanted individual plants into soil and onto vertically oriented boards under standardized light conditions. Results from field observations showed that dislodged colonies formed smaller groups that reproduced less than epiphytic colonies. Results from the glasshouse experiment showed that even when growing in full sun, terrestrial individuals tended to remain solitary, while epiphytic individuals tended to recruit new individuals into colonies. Results also showed that plants growing in potting soil and exposed to full sunlight sporulated more heavily than plants growing epiphytically. However, localities that are characterized by both elevated soil and light resources are generally not available to staghorn ferns in the wild, perhaps with the exception of large, epiphytic colonies with well-developed nests at the top of tree canopies. Overall results indicate that the harsh environmental conditions at the tops of trees trigger the formation of colonies in staghorn ferns, similarly to group living animals.

严酷且不可预测的环境通常被认为会促进动物形成合作群体。而植物是否也存在类似的群体协作关系，目前尚不清楚。鹿角蕨（*Platycerium bifurcatum*，水龙骨科Polypodiaceae）为附生植物（epiphyte），可形成合作群体并在树木顶端构建兼具储水与储养功能的公共巢室，该生境以水分与养分胁迫为典型特征。本研究通过野外观测，探究鹿角蕨从林冠脱落并坠落至森林地表后，是否仍能维持大型且具有繁殖活性的群体——森林地表的水分与养分匮乏胁迫程度相对更低。为排除林地表光照限制可能带来的混淆效应，我们还开展了多年期温室实验：在标准化光照条件下，将单株鹿角蕨分别移栽至土壤基质与垂直放置的木板上。野外观测结果显示，脱落的鹿角蕨群体形成的规模更小，繁殖效率低于附生状态的群体。温室实验结果表明：即便处于全日照环境，陆生个体仍倾向于单独生长，而附生个体则会招募新个体融入群体。此外，栽培于盆栽土壤且接受全日照的鹿角蕨，其产孢量显著高于附生状态的个体。然而，在自然环境中，同时具备充足土壤资源与光照条件的生境通常并不为鹿角蕨所利用，仅树木林冠顶端拥有发育完善巢室的大型附生群体可能属于例外。综合所有实验结果可知，树木顶端的严酷环境会诱导鹿角蕨形成群体，这与群居动物的群体形成机制具有相似性。