Cluster planting facilitates survival but not growth in early development of restored tropical forest

Microhabitat Conditions and Inter-Species Competition Predict the Successful Restoration of Declining Relict Species Populations

The local populations of relict plant taxa living near the limits of their geographical range are particularly vulnerable to extinction. For example, Salix lapponum is one of the rarest and most endangered glacial relicts in Western and Central Europe. In Poland, the number of its sites has dramatically decreased over the past few decades, prompting us to take conservation measures focused on saving endangered populations. During a field experiment aimed at the reconstruction of the downy willow population in the Knyszyn Forest (NE Poland), 730 individuals of the species were planted in four different natural sites. The seedlings were obtained by micropropagation from parts of vegetative individuals taken from the most abundant population of this species in eastern Poland (Lake Bikcze). The success of the reintroduction, measured by the number of individuals that survived 2-3 years in the wild and took up growth, was about 67%, however, with low flowering efficiency (7.5%). Additionally, monitoring showed significant differences in plant survival and growth rates under different habitat conditions prevailing at the site and with different cover from competing species, especially tall grasses. However, the restoration projects for relict shrub species should include periodic removal of competing plants and protection of plants from trampling and browsing by herbivorous mammals to increase reintroduction success.

The role of nurse successional stages on species-specific facilitation in drylands: Nurse traits and facilitation skills

Plant establishment is a challenge in semiarid environments due to intense and frequent drought periods. The presence of neighboring trees (nurses) can increase the establishment of seedlings (targets) by improving resource availability and microclimate. The nurse effect, however, might vary depending on nurse-target species combinations but factors that predict this specificity are poorly known. We used a multispecies experiment to investigate the facilitation potential of trees from a range of successional stages, focusing on how nurse functional traits can predict species-specific interaction outcomes. We conducted a factorial field experiment in a Brazilian semiarid tropical forest during a severe drought period. Sixty pairs of interacting tree species, 20 potential nurses, and three targets were used. Seedlings of all targets were planted both under and far from the nurse canopy, in a randomized block design replicated five times. Target growth and survival were monitored for 275 days from the beginning of the dry season, and interaction outcomes were calculated using the Relative Interaction Intensity (RII) index. Nurse functional traits such as successional stage, height, wood density, and canopy diameter were used as explanatory variables to predict RII values. The average effect of nurse species on target plants was in general positive, that is, seedling survival and growth increased under the nurse canopy. However, for growth pairwise interactions were significantly species specific. Successional stage was the only functional trait explaining RII values, with pioneer tree species being stronger facilitators than later successional trees. However, the explanation power of this variable was low, and positive, negative, or neutral interactions were found among nurse trees of all successional stages. Because seedling mortality during drought in semiarid systems is high, future studies should investigate how nurse traits related to water use could influence nurse facilitation skills.