Estimating plant abundances from crown cover and forest structure data reveals size‐dependent patterns of rarity in subtropical Australia

Abstract

AbstractAimsAccurate field quantification of stem counts is time consuming and laborious, especially in ecosystems containing many small individuals where counts are immense. There are many large, long‐term data sets worldwide that include various measures of woody plant abundance, such as crown cover, that are faster and easier to measure than true counts of individuals. Quantifying the number of individuals allows in‐depth research into various topics including functional ecology, carbon accounting, conservation and metabolic theory. Here we test the accuracy of predicting the abundance of individual woody plants from cover and forest structure measures, and explore the implications of refined abundance measures for understanding the rarity of woody plant species across southeast Queensland.LocationSoutheast Queensland (SEQ), Australia.MethodsTo supplement an existing data set of 1251 sites with measured crown covers for woody plants in various strata, we sampled stem counts in an additional 30 sites, forming a calibration data set. We ensured that these 30 sites covered the environmental variation in the study area. Generalized additive mixed effects models were used to predict stem counts for all woody plants in the original data set from their crown cover and forest structure values and a site‐level variable accounting for water balance. We used these refined abundance data to investigate relationships between local and regional abundance for different size classes.ResultsStem densities increased with crown cover in a non‐linear fashion; slowing once crown cover reached ca. 5% for shrubs and ca. 50% for trees, depending on their maximum potential height. Using the predicted stem count data, we found that shrubs were more locally abundant than trees, but less regionally abundant. Larger trees (>20 m maximum potential height) showed a negative relationship between local and regional abundance; there was no significant relationship between other size classes.ConclusionsOur study demonstrates how crown cover can be converted to stem counts using a calibration data set and flexible mixed‐effects models. This enables further ecological analysis at a fraction of the investment required to re‐survey. Analyses of abundances revealed that tree species are less locally abundant than smaller woody species, likely due to space filling constraints, but more regionally abundant.