Standardized response signatures of functional traits pinpoint limiting ecological filters during the migration of forest plant species into wooded corridors

Tree-lines and alleys are expected to operate as migration enhancing corridors for habitat-demanding species, but their functionality is limited by the set of ecological fillers. We use multiple plant traits related to dispersal and establishment to identify the limiting filters for forest plants in the rural landscape of Estonia. We develop a set of quantitative metrics to rank indicator traits according to their distributional changes along migration distance. We implement a trait comparison to the potential optimum level suggested by two ecological reference groups of species to interpret these responses as filter driven convergence or divergence. The suggested set of metrics provided a clear ranking of traits and showed interpretational limitations of widely used short list of traits (e.g. seed weight, plant height and SLA). Results also demonstrate that there is no consistency between indicator metrics based on the shift in trait mean and those based on the reduction of variability, instead they provide complementary information. Unexpectedly, many characteristic traits of forest-specialist plants do not exhibit the expected responses. The response signal of many dispersal traits is too ambiguous to interpret because either (1) they do not have one clear optimum level, or (2) they indicate an establishment/persistence limitation instead, such as seed weight and flowering duration. Establishment traits indicate filtering clearly by improved light conditions. The pattern of trait means demonstrates that the dispersal filtering incrementally intensifies with distance, while establishment filtering occurs sharply at the forest-corridor ecotone. Consolidated results underscore that the migration of forest-restricted plants into corridors is limited by the habitat quality for dispersal vectors (e.g., for myrmecochores and zoochores), the scarcity of suitable microsites for seedling establishment, and the competition for light. A single optimal structure of wooded corridors cannot be suggested as forest-dwelling species exhibit various adaptations. Forest-biodiversity-enhancing wooded corridors should incorporate a diversity of shade levels and have structures that facilitate visits of forest insects, birds and mammals. We show that biased conclusions about the functional efficiency of habitats and limiting ecological filters can be avoided when indicator analytics include (1) multiple response metrics and analytical methods, (2) multiple seemingly redundant traits, and (3) several reference groups for interpretation. The proposed analytical approach adjusts for (1) the indictor choice subjectivity, and (2) the scaling bias implemented in several ecological indicator trait systems, which over-emphasise qualitative preferences of species rather than reflect their real niches.