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Why does the abundance of Erica tetralix decrease on wet heathlands?

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Why does the abundance of Erica tetralix decrease on wet heathlands?. Christian Damgaard Aarhus University. Why does the abundance of Erica tetralix decrease on wet heathlands?. Erica tetralix, which is the characteristic species of wet heathlands, is decreasing in abundance

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Why does the abundance of Erica tetralix decrease on wet heathlands?

Christian Damgaard

Aarhus University

why does the abundance of erica tetralix decrease on wet heathlands
Why does the abundance of Erica tetralix decrease on wet heathlands?

Erica tetralix, which is the characteristic species of wet heathlands, is decreasing in abundance

We are unsure what is causing this change and what management actions are needed to reverse the change

Until now we have only been able to explore the phenomenon using monitoring data

monitoring of danish terrestrial habitats
The Danish monitoring programe NOVANA was initiated as a response to the EU-Habitat directive with the objective to:

Describe sources of pollution and other pressures and their effects on the status and trend regarding terrestrial natural habitats

Document the overall effect of national action plans and measures directed at terrestrial natural habitats – including whether the objectives are attained, and whether the trend is in the desired direction

Help fulfill Denmark’s obligations under EU legislation, international conventions and national legislation

Help strengthen the scientific foundation for future international measures, national action plans, regional administration and other measures to improve terrestrial natural habitats, including helping to develop various tools

Monitoring of Danish terrestrial habitats
danish monitoring program novana
Hierarchical sampling:

768 sites from 28 different terrestrial habitat types

18 open habitat types (dune, heath, grassland, bogs, …)

10 forest types

At each site 20 – 60 randomly positioned plots.

the plots at some stations are revisited with GPS accuracy

Several variables are measured at the plot

plant cover (pin-point)

soil pH

Danish monitoring program (NOVANA)
novana measurements
Plot 0.5x0.5 m:

Cover of plant species (pin-point)

pH in soil/water

Conductivity

C/N - ratio

Phosphorous

Nitrate in soil/water

N in shoots, mosses and lichens

5 m circle:

Presence of other species

Vegetation height

Cover of woody species

Grazing or moving

Gaps in vegetation/bare soil/sand

Cover of plants affected by herbivory

Hollows in bog structure

NOVANA - measurements
the pin point point intercept method
Objective method for measuring plant cover

Place a frame with a grid pattern

A pin is inserted vertically through one of the grid points into the vegetation

The pin will typically touch a number of plants and the different species are recorded

This procedure is repeated for each grid point

The pin-point (point-intercept) method
distribution of pin point cover data within a site
The distribution of plant cover is modelled using a generalised binomial distribution with two parameters

q : mean plant cover

d : intra-plot correlation

The probability density function of the generalised binomial distribution (=Pólya-Eggenberger distribution) is equal to the beta-binomial mixture distribution but is somewhat more general in that negative intra-plot correlation is allowed (Qu et al. 1993)

Distribution of pin-point cover data within a site
generalised binomial distribution
Generalised binomial distribution

q : mean cover at the site

d : intra-plot correlation

analysing trends in plant cover
Framework for analysing trends of hierarchical plant cover data has been developed

Bayesian state-space model

Separation of process and sampling error

Efficient treatment of missing values

Effect of treatment or co-variable on change in cover

Possible to account for auto-correlation

Ecology 93(6), 2012, pp. 1269–1274

Analysing trends in plant cover
separation of process and sampling variance
Separation of process and sampling variance
  • Process equation (solid error)
    • Dxi =a + eiei ~ Normal(0,sp2)
  • Measurement equation (dashed error)
    • yi,j ~ Generalised Binomial Distribution (n, xi, d)
erica tetralix among site variation
Erica tetralix - among site-variation

Although Erica tetralix is the characteristic species of wet heathlands there is a large among-site variation in cover (Damgaard, submitted)

sem of hierarchical data
SEM of hierarchical data
  • The site means are modelled by latent variables
  • Nitrogen deposition (model calculated) and pH (measured in the top soil) are assumed to be normally distributed
  • The pin-point cover measures of E. tetralix are assumed to distributed according to a generalised binomial distribution
      • Important to include the effect of inter-specific aggregation!!
  • The model is parameterised using a Bayesian MCMC approach
why does the abundance of erica tetralix decrease on wet heathlands1
Why does the abundance of Erica tetralix decrease on wet heathlands?

We are keen to investigate the causal relationships using experimental manipulations

Observation: Erica tetralix seems to be dying due to toxicity - not outcompeted (Strandberg et al. 2012)

Working hypothesis: soil with low pH in combination with low base cation concentrations reach toxic levels under certain environmental conditions

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