THE CAMBRIDGE STATISTICS DISCUSSION GROUP



                  Thursday 5th March 2009 7:15 for 7:45




                        Lecture Theatre,
                    Department of Plant Sciences,
                          Downing Site,
                            Cambridge






  Mapping the spatial change of soil phosphorus in the northern Everglades




                         Ben Marchant
                     Rothamsted Research




Abstract: 
In recent decades the Florida Everglades have been impacted by runoff 
from neighbouring agricultural areas. Nutrients - particularly phosphorus -
  within this runoff have caused a change to the predominant plant communities.
Indigenous vegetation communities have been replaced by bulrushes. These 
bulrushes grow rapidly in the nutrient rich soil and block the sunlight 
required by the indigenous wildlife. Thus the biodiversity of the Everglades 
has been reduced.

Attempts have been made to restore the Everglades ecosystem. The Everglades 
have been divided into a number of water conservation areas which are 
bounded by canals. Since the mid-1990s agricultural runoff has been 
diverted or treated. Thus the inputs of phosphorus have been decreased. 
Soil surveys have been conducted to determine whether this strategy has been 
effective at decreasing the amount of phosphorus available for plants.

This talk is concerned with the analysis of soil surveys conducted in 1991 
and 2003. Geostatistical methods are employed to map the change in the 
concentration of phosphorus in the soil between these dates. Conventional 
geostatistical methods assume that the variable of interest is second order 
stationary across the study region. However this assumption is not valid 
because the variation of soil phosphorus is larger adjacent to the bounding 
canals than in the interior of the conservation areas.

Therefore a novel and more general geostatistical model is devised which 
permits different behaviour adjacent to the boundary than in the interior. 
This model is expressed as a linear mixed model, the parameters of which 
are fitted by residual maximum likelihood. One of these parameters describes 
the extent of the boundary region.

The effectiveness of this model is confirmed via cross-validation and 
likelihood ratio tests. The resulting map of change shows that a front 
of soil phosphorus advanced from the boundary region into the interior of 
the conservation areas between 1991 and 2003. This front is not evident when 
conventional models are used because these models are dominated by the 
variation in the boundary region.

Further soil surveys are being conducted to determine whether this front 
continued to advance after 2003 when the restoration efforts were fully 
implemented. Based upon the novel geostatistical model the sites at which 
soil phosphorus is to be measured have been optimized using an algorithm 
known as spatial simulated annealing. The first phase of this optimized 
survey was conducted in December 2008.

Speaker:
Ben Marchant is a senior post-doctoral scientist at Rothamsted Research, 
Harpenden, UK. His current research is concerned with the development and 
application of geostatistical methods for the analysis of spatial soil data. 
He trained as a mathematical modeller and received his D. Phil. from the 
University of Oxford in 2000 having developed a model describing the 
invasion of malignant tumours. From Oxford he moved to Silsoe Research 
Institute before joining Rothamsted Research in 2004. He has developed 
methods for analyzing non-stationary, contaminated and coregionalized 
data and for the optimization of spatial surveys. Dr Marchant has applied 
these methods to a diverse range of problems such as the spatial monitoring 
of soil phosphorus in the Florida Everglades and the identification of 
sites contaminated with heavy metals around Swansea.



Directions: 
The Department of Plant Sciences is located on the Downing site situated 
between Downing Street and Tennis Court Road. Go through the second door you 
come to in the building immediately on your left as you enter from Tennis 
Court Road. Once inside the building follow the arrows. 
It is not possible to park on the Downing site in the evening. The nearest 
car park is at Lion Yard. Arrivals after 7:45pm can gain admittance by 
contacting the secretary on 07761769436.




Next Meetings:
2nd April - Phil Dawid (Statistical Laboratory) on 'Statistics and the Law'. 
7th May - Jenni Barclay (UEA).


Supper: Some members eat regularly in the University Centre before
each talk meeting in the downstairs bar at 5-45pm. Feel free to join them.

Subscriptions:  of 1 pound are now due for attending the 2008-2009 session.

Secretary: Peter Watson, MRC Cognition and Brain Sciences Unit, 
15 Chaucer Road, Cambridge CB2 7EF;telephone 01223 355294 Extension 801; 
E-mail [log in to unmask]

Take a look at our website: 
http://www.mrc-cbu.cam.ac.uk/people/peter.watson/csdg.html