Size distributions are important emergent properties of forests that support estimates of ecosystem services like carbon storage and can provide signatures of fundamental ecological processes driving forest dynamics. Yet, we are still working to understand this key emergent property of forests.
Recently, we were able to explain the perplexing phenomena of consistent, power-law shaped, size distributions across diverse tropical forests. With a combination of data analysis, mathematical, and simulation models, we were able to show that this consistency is likely the product of the consistency of disturbances and competition for light across tropical forests (Farrior et al. 2016).
Now, we ask the question – can these simple processes of disturbance and subsequent competition for light also explain the size distributions of temperate forests? For this, we use data from the ForestGeo network of forest plots to parameterize this simple model and test their predictions at both the individual and stand scales.
Results/Conclusions
Surprisingly, we find that both temperate and tropical forests' size structure may be accurately predicted from a single model including only small scale (patch-level) disturbances and subsequent height-structured competition for light. We find that the only parameters needed are crown-area allometry of trees in the site, and their average growth and mortality rates of trees in full sun and in shade. All predictions use the assumption that half of canopy tree mortality is due to patch-level disturbance.
This match of a simple model to forest emergent size structure and successional stages in both tropical and temperate forests demonstrates, unsurprisingly, the ubiquity of the importance of competition for light in forests. However, the match also shows the importance of relative timescales of growth and disturbance in determining the scale of the emergent forest distribution. That is, the relative rates of growth and disturbance cause the difference between a tropical forest having three to four layers of overlapping canopies and a temperate forest having one to two layers. Now, questions remain as to the determinants of differences in crown area allometries, and the average growth and mortality rates of trees across forests.