Coarse woody debris (CWD, dead wood greater than 10 cm in diameter) is
a large stock of carbon in tropical forests, yet the flux of carbon
out of this pool, via respiration, is poorly resolved (Chambers,
Schimel and Nobre, 2001). The heterotrophic process involved in CWD
respiration should respond to reductions in moisture availability,
which occurs during dry season (Chambers, Schimel and Nobre, 2001).
CWD respiration measurements were taken in a tropical forest in west French
Guiana, which experiences extreme contrasts in wet and dry season (Bonal
et al., 2008). An infrared gas analyzer and a clear chamber sealed to
the wood surface were used to measure the flux of carbon out of the wood
(Stahl et al., 2011). Measurements were repeated 13 times, from July
to November 2011, on six pieces of wood during the transition into and out of
the dry season. The aim is to assess if there were shifts in the CWD
respiration of any of the pieces in response to the transition into (early
August) and out of (late October) the dry season.
Zeileis and Hothorn (2013) investigated the six-variate series of CO_2
reflux, aiming to find out whether the reflux had changed over the sampling
period in at least one of the six wood pieces.
Source
The coarse woody debris respiration data were kindly provided by Lucy Rowland
(School of GeoSciences, University of Edinburgh).
References
Bonal, D., Bosc, A., Ponton, S., Goret, J.-Y., Burban, B., Gross, P.,
Bonnefond, J.-M., Elbers, J., Longdoz, B., Epron, D., Guehl, J.-M. and
Granier, A. (2008). Impact of severe dry season on net ecosystem exchange in
the Neotropical rainforest of French Guiana. Global Change Biology14(8), 1917–1933.
Chambers, J. Q., Schimel, J. P. and Nobre, A. D. (2001). Respiration from
coarse wood litter in central Amazon forests. Biogeochemistry52(2), 115–131.
Stahl, C., Burban, B., Goret, J.-Y. and Bonal, D. (2011). Seasonal
variations in stem COmathrm{_2} efflux in the Neotropical rainforest of
French Guiana. Annals of Forest Science68(4), 771–782.
Zeileis, A. and Hothorn, T. (2013). A toolbox of permutation tests for
structural change. Statistical Papers54(4), 931–954.
Examples
## Zeileis and Hothorn (2013, pp. 942-944)
## Approximative (Monte Carlo) maximally selected statistics
CWD[1:6] <- 100 * CWD[1:6] # scaling (to avoid harmless warning)
mt <- maxstat_test(sample2 + sample3 + sample4 +
sample6 + sample7 + sample8 ~ trend, data = CWD,
distribution = approximate(B = 100000))
## Absolute maximum of standardized statistics (t = 3.08)
statistic(mt)
## 5 % critical value (t_0.05 = 2.86)
(c <- qperm(mt, 0.95))
## Only 'sample8' exceeds the 5 % critical value
sts <- statistic(mt, "standardized")
idx <- which(sts > c, arr.ind = TRUE)
sts[unique(idx[, 1]), unique(idx[, 2]), drop = FALSE]