Chronic anthropogenic disturbance mediates the biodiversity-productivity relationship across stand ages in a large temperate forest region

Temperate forests, especially those in the densely populated regions of the world, are experiencing increasing levels of habitat degradation and biological impoverishment due to subtle but pervasive chronic anthropogenic disturbances including frequent and continuous grazing and extraction of non-timber forest products. However, the effects of these subtle, chronic disturbances on the biodiversity-productivity relationship have rarely been examined especially in forests at different development stages. Accordingly, this study explores how chronic anthropogenic disturbance affects the relationship between tree species diversity and forest productivity at different stand development stages in a large temperate forest region.

We used the human footprint index as a proxy for chronic human disturbance. Hierarchical Bayesian models were employed to assess the effects of chronic human disturbance on the relationship between tree diversity and forest productivity across different stand age. Several measures of diversity were employed, including taxonomic, functional and phylogenetic diversity.

Forest productivity consistently increased with taxonomic, functional and phylogenetic biodiversity; these biodiversity facets were the main drivers of forest productivity compared to stand age, chronic human disturbance, and climate. However, the magnitude at which productivity increases with the increments of taxonomic and functional diversity diminishes with the increasing chronic disturbance, especially in younger stands. The effects of phylogenetic diversity on productivity did not vary with chronic disturbance, regardless of stand age.

Synthesis and applications: Chronic human disturbance in a large temperate forest region reduces the increase in community productivity due to different facets of biodiversity, especially in young forests. The evidence suggests that the mitigation of chronic human disturbance and the conservation of biodiversity will be effective in sustaining essential ecosystem functions.
Methods
The study area (c. 700,000 km2) is a large temperate forest region situated in northeastern China (latitude 39°42’48”-53°19’21” N, longitude 119°48’12”-134°01’01” E) where coniferous and broadleaved forests occur. The climate is temperate continental with significant temperature seasonality (average annual temperature ranging from -5.1 to 8.8°C) and precipitation seasonality (rainfall ranging from 404 to 985 mm). Altogether 379 circular field plots, each measuring 0.1 ha, were systematically established in 2017, across the study region. The minimum and average distance between plots is 10 km and 19 km, respectively. All individual trees with a diameter greater than 5 cm at breast height (DBH) were tagged, mapped, and recorded. Each plot was equally divided into ten subplots of 0.01 ha. Wood cores were extracted from the northern side of each tree at breast height using increment borers (5.15 mm) in 2017. The collected wood core samples were subjected to analysis to directly ascertain past diameters by subtracting the radial increments of each tree for the previous five-year interval (from 2012 to 2017). We calculated the biomass accumulation of all trees with a DBH > 5 cm based on the available species-specific allometric equations in the various regions. Then, the biomass of each subplot was estimated for the years 2012 and 2017. Forest biomass productivity (BP) of each subplot was quantified as biomass increments per hectare from 2012 to 2017.