Bioscience – Aarhus University

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

Christian DamgaardAarhus University

Bioscience – 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 abundanceWe are unsure what is causing this change and what management actions are needed to reverse the changeUntil now we have only been able to explore the phenomenon using monitoring data

Bioscience – Aarhus University

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

Bioscience – Aarhus University

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

accuracySeveral variables are measured at the plot

plant cover (pin-point)soil pH…

Bioscience – Aarhus University

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

Bioscience – Aarhus University

The pin-point (point-intercept) methodObjective 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

Bioscience – Aarhus University

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)

Bioscience – Aarhus University

Generalised binomial distribution

n

ynqyq

yn

qnyfY,11

,)1)(1(,11

),,;(

d

dd

d

d

)1()1()()(),( nxxxxnxnx

qnYE )(

))1(1()1()( nqqnYVar d

q : mean cover at the sited : intra-plot correlation

Bioscience – Aarhus University

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

Bioscience – Aarhus University

Separation of process and sampling variance

Process equation (solid error) Dxi =a + ei ei ~ Normal(0,sp

2) Measurement equation (dashed error)

yi,j ~ Generalised Binomial Distribution (n, xi, d)

Bioscience – Aarhus University

Erica tetralix on wet heathlands

a sp2

Bioscience – Aarhus University

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)

Bioscience – Aarhus University

Is the cover of Erica tetralix regulated by nitrogen deposition or pH or both?

Bioscience – Aarhus University

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

Bioscience – Aarhus University

Is the cover of Erica tetralix regulated by nitrogen deposition or pH or both?

Bioscience – Aarhus University

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

Bioscience – Aarhus University