Tree species traits affect which natural enemies drive the Janzen-Connell effect in a temperate forest

A prominent tree species coexistence mechanism suggests host-specific natural enemies inhibit seedling recruitment at high conspecific density (negative conspecific density dependence). Natural-enemy-mediated conspecific density dependence affects numerous tree populations, but its strength varies substantially among species. Understanding how conspecific density dependence varies with species’ traits and influences the dynamics of whole communities remains a challenge. Using a three-year manipulative community-scale experiment in a temperate forest, we show that plant-associated fungi, and to a lesser extent insect herbivores, reduce seedling recruitment and survival at high adult conspecific density. Plant-associated fungi are primarily responsible for reducing seedling recruitment near conspecific adults in ectomycorrhizal and shade-tolerant species. Insects, in contrast, primarily inhibit seedling recruitment of shade-intolerant species near conspecific adults. Our results suggest that natural enemies drive conspecific density dependence in this temperate forest and that which natural enemies are responsible depends on the mycorrhizal association and shade tolerance of tree species.


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To identify the groups of natural enemies that may be responsible to community-wide conspecific density dependence , we manipulated the access of three groups of natural enemies, large herbivores (using fences), insect herbivores (using insecticide) and fungal pathogens (using fungicide), to seedlings in 180 1m × 1m quadrats in an old growth temperate forest in Northeast China. Further, we examined whether variation in the strength of conspecific density dependence among tree species is associated with two important plant traits: type of mycorrhizal association and shade tolerance.
We established three 55 m × 50 m blocks, separated by at least 200 m within an old growth temperate forest. Each block was split into two equally-sized fenced and unfenced plots. Within each plot, thirty 1m × 1m seedling quadrats were equally divided and randomly allocated to each of the following treatments: fungicide (F), insecticide (I) and water (control of pesticide treatment; W). Across the 180 1 m × 1 m quadrats, 3929 individual seedlings recruited from 16 species in the censuses from 2015 to 2017.
We focused on the community-scale results, thus all 3929 individuals were tested as a whole.
All woody plants less than 1 cm DBH were tagged, mapped and identified to species in all quadrats. In September of each year, we checked the status (survival/dead) of existing seedlings. In June 2017, we identified all adult trees (DBH > 5cm) within each block and recorded their species identity, DBH and distance to each quadrat within 20 m.
We calculated recruitment as the number of new seedlings > 1 cm tall in each quadrat in June each year. Seedling survival was checked in September, the late growing season in our temperate forest. Censuses were conducted across three years and across all 180 quadrats in three blocks.
No data were excluded from the analyses.
We conducted sensitivity analyses by randomly removing 25%, 50% and 75% of individuals of two dominant species (accounting for 74% seedlings) from the community data set, repeating this procedure 999 times. We found that our community-wide results were insensitive to removing random fractions of individuals of these two dominant species. Thus, we believe our results are robust and could be repeated in other temperate forests.
In our experiment, all treatments were randomly allocated to each quadrat. Meanwhile, we considered quadrat and species identity as random intercepts and allowed the effect of census to vary among quadrats as a random effect.
Blinding is not relevant to our study. We collected field data by recording the status, species identity, number of individuals, and DBH of woody plants, and did not exclude any data for analysis.
This study was conducted in an old-growth temperate forest. The climate is characterized by an annual mean temperature of 2.8°C (-13.7-19.6°C) and average annual precipitation of~700 mm, mostly as rain from June to September.
We can access the study area freely, and do not need to get permission to collect data.
The study area is located in the Changbai Mountain Nature Reserve, and has been spared from severe distances for about 300 years.