Bayesian Mapping of Lichens Growing on Trees

Abstract

Suitability of trees as hosts for epiphytic lichens are studied in a forest stand of size 25 ha. Suitability is measured as occupation probabilites which are modelled using hierarchical Bayesian approach. These probabilities are useful for an ecologist. They give smoothed spatial distribution map of suitability for each of the species and can be used in detecting high‐ and low‐probability areas. In addition, suitability is explained by tree‐level covariates. Spatial dependence, which is due to unobserved spatially structured covariates, is modelled through an unobserved Markov random field. Markov chain Monte Carlo method has been applied in Bayesian computation. The extensive spatial data consist of the occurrences of eight lichen species and one bryophyte on all of the 1253 potential host trees. In addition, coordinates of the trees and several tree characteristics have been recorded. The data have been analysed for four most abundant species: Lobaria pulmonaria, Nephroma bellum, Nephroma parile and Peltigera praetextata. The tree level parameters, subject to estimation, consist of the occurrence probabilities for each tree and for each lichen species. Model validation is discussed in detail and, in addition to Bayesian validation tools, the autologistic model and case‐control design based on logistic regression have been suggested for validation of covariate effects. As a result we present suitability maps for the four lichen species. We observed, that among the observed tree covariates, the diameter at breast height (DBH) correlates with lichen occurrence. Our modelling approach has close connections to disease mapping in spatial epidemiology.

Fragment quality and matrix affect epiphytic performance in a Mediterranean forest landscape

“…Another measure of the habitat quality was the tree diameter, which refl ects how long the tree has been available for colonization ( Riiali et al, 2001 ). Many epiphytic lichens and bryophytes need old trees for their persistence, because the colonization process is slow ( Kuusinen, 1996 ;Sn ä ll et al, 2003 ).…”

Dispersal ecology of the endangered woodland lichen Lobaria pulmonaria in managed hemiboreal forest landscape

“…The difference in trunk diameter between neighbouring trees was defined to be different in ecological sense in case the difference in diameter among the trees exceeded 25%. Such a diameter threshold is based on the generalized knowledge gained from other studies (Riiali et al 2001;Ö ckinger et al 2005;Belinchón et al 2009). …”

Effects of Stand-Level Disturbances on the Spatial Distribution of a Lichen Indicator

“…These vegetative soredia are not likely to disperse as far as ascospores (Levin et al 2003) owing to their larger size. Since we know that soredia production is poor in younger stands (Hilmo 2002) and that successful establishment of vegetative propagules is positively correlated with host tree diameter at breast height (Riiali et al 2001), it seems plausible that a stand-level disturbance event would have a persisting effect on L. pulmonaria.…”