Janzen‐Connell effects in a forest BEF experiment: Strong distance‐dependent seedling establishment of multiple species

Abstract

Citation: Germany, M. S., H. Bruelheide, and A. Erfmeier. 2019. Janzen-Connell effects in a forest BEF experiment: Strong distance-dependent seedling establishment of multiple species.Abstract. The Janzen-Connell (JC) hypothesis is a major ecological explanation for high species richness, in particular in tropical forest ecosystems. Central components of the JC hypothesis are noncompetitive effects of distance and density dependence, two drivers that contribute independently to species coexistence, but are ultimately linked in the field. However, although numerous studies provide evidence for either distance-or density-dependent effects based on observational data, experimental testing of simultaneous and interactive effects of distance and density has rarely been conducted, especially in a comprehensive multispecies approach. Here, we make use of the forest Biodiversity-Ecosystem Functioning project (BEF) -China to estimate distance-and density-dependent effects in a reciprocal tree seedling transplant experiment of 11 tree species. We deployed 13,490 juveniles of all 11 species in their own (home) and in all foreign monocultures (away), as well as at three different levels of planting density, thereby testing for distance and density effects, respectively. In addition, to quantify the amount to which density effects were brought about by potential additional effects of intraspecific competition, we set up a common garden experiment with different levels of planting density, where an additional "shadow" treatment controlled for effects of canopy shading. Although the "away" and "high-density" treatments significantly impaired the performance and productivity of seedlings, leaf damage and survival was exclusively affected by either the home/away or the density treatment, respectively. Negative density-dependent effects on leaf damage were less pronounced in the "home" treatment, showing that the effects were not additive. In addition, results obtained in the Common Garden Experiment showed that negative effects of high density may be also brought about by intraspecific competition as an alternative density-responsive mechanism and less by true JC effects. Overall, our results provide strong support on a multispecies basis for the influence of host-specific effects already operating in early stages of a forest plantation. However, they also emphasize the need to account appropriately for potential additional densityresponsive mechanisms such as intraspecific competition or microenvironmental conditions when addressing the role of JC effects for species coexistence.

Wood species identity alters dominant factors driving fine wood decomposition along a tree diversity gradient in subtropical plantation forests

“…Trees were planted with either monocultures or mixtures of 2, 4, 8, 16, or 24 tree species according to a broken‐stick design (Bruelheide et al., 2014). Emerging herbs and unplanted trees were weeded annually (Germany et al., 2019).…”

Philomatry in plants: why do so many species have limited seed dispersal?

“…In contrast to numerous tests of the well‐known Janzen–Connell hypothesis that provide evidence of distance‐dependent seedling establishment in forest trees (Comita et al, 2004; Germany et al, 2019), there have been few experimental tests of the MSH in herbs. As indicated by the examples presented, even for species with limited dispersal, adaptive advantages to philomatry in locally adapted populations have not been demonstrated conclusively.…”

Light can modify density‐dependent seedling mortality in a temperate forest

“…associational resistance; Barbosa et al, 2009;Champagne et al, 2016). Other density-dependent mechanisms such as intraspecific competition for resources and the pressure from soil-/airborne fungal pathogens are also likely to contribute to establishment of an upper spatial scale for seedling neighbourhoods (Queenborough et al, 2007;Germany et al, 2019), resulting from seedling size variation in resource acquisition (Berntson & Wayne, 2000) or pathogen capacities to spread (Hantsch et al, 2014;Liu & He, 2019), respectively. Moreover, resource transfer via mycorrhizal connections of seedling roots may also drive seedling performance depending on the density and distance between seedlings (Nara, 2006;Teste et al, 2009;Waud et al, 2016).…”