Application of mixed models and multivariate hypothesis testing to long-term tropical tree assemblage data from a BACI experiment

Background spatial heterogeneity and "random" temporal variability pose major challenges to the analysis of ecological treatments in field experiments, and thus to our mechanistic understanding of ecosystems. Therefore, space-for-time chronosequences have been shown to be of limited use for our understanding of complex tropical secondary forest succession.

I present univariate and multivariate analyses of repeated measures of forest regrowth attributes in response to factorial nutrient addition in permanent plots of a BACI experiment. Shapes of response fits of tree biomass measures over time are examined in the light of multiple possible trajectories, unknown end points, and inevitably too short measurement periods relative to the multi-decade process of forest regrowth. Second order polynomial fits of woody biomass of individual, common species indicate distinct trajectories over time in response to experimental fertilization, which explains poor fit of total biomass of all species pooled. Pooling species based on the type of their individual nutrient response, and subsequent refitting pooled biomass of these 'functional' groups of species reveals more subtle interactions between the effects of time of regrowth, different nutrients added, and covariates, which are detectable neither in total woody biomass, nor individual species biomass fits.

Hellinger-standardization is applied to woody biomass of all species in the tree assemblage to avoid distortion of subsequent ordinations due to high zero-inflation, mainly rare species, and the autocorrelation structure associated with repeated measures. Hellinger-Principal Component Analysis reveals that most of the compositional variance is associated with spatial heterogeneity among plots. After conditioning the variance associated with plot differences, partial Hellinger-Redundancy Analysis indicates clear compositional change over time, but also interactions between time and fertilization, suggesting non-accelerating shifts in tree species biomass composition in response to nitrogen addition.

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