Monitoring of Dactylorhiza sambucina (L.) Soó (Orchidaceae)—Variation in Flowering, Flower Colour Morph Frequencies, and Erratic Population Census Trends

Abstract

Central questions of reproductive research on Dactylorhiza sambucina (Orchidaceae) are, if and how pollinator-mediated negative frequency dependent selection might maintain its flower colour polymorphism. As this hypothesis was based on artificial populations, it needs to be verified under natural conditions. Therefore, we demonstrate and discuss spatial-temporal variation of flowering, flower colour morph frequencies and population fluctuations in D. sambucina as prerequisites for understanding its population and reproduction dynamics. Knowledge of these dynamics is also crucial for the species’ conservation. We investigated colour morph frequencies for ten Austrian populations of D. sambucina over maximum time series of 18 consecutive years. We report repeated changes in the dominance of red- or yellow-flowering morphs in at least three populations during these time series. Even though being basically erratic (e.g., switches in different years), we identify smaller populations as being more prone to changes of flower colour dominance. Conversely, constant morph frequencies and the dominance of one flower colour morph is demonstrated for individual-poor and large populations. As previous large-scale (continental) analyses failed to identify environmental factors explaining the distribution of differing morph frequencies across Europe, we strongly argue for local approaches by investigating such factors at the micro-habitat scale.