Canopy crown dynamics, competition and diameter growth in an old-growth tropical forest

Background/Question/Methods

The majority of carbon uptake in forests occurs in the canopy, here defined as the layer of trees with direct sun access.   Thus, access to direct light is a major resource driving tree competition and underlies patterns of succession.  Modeling methods that quantify the levels and types of competition experienced by canopy trees are often based on the sizes and distance of neighboring stems.  Aboveground competition occurs from crowding and shading which occurs via the location, shapes and leaf densities of tree crowns not via the stems upon which the competition indices are determined.  Tropical tree crowns are often highly irregular and displaced from the tree stems, reflecting the range of tropical tree architectures as well as past competitive interactions.  To understand how tree crowns are shaping competition requires tracking individual crown changes through time.  Here I use a time-series of stereophotographs to quantify the dynamics of individual tree crowns and link them to diameter growth.  Stereophotographs were taken in 2000 and 2009 of the same 4-ha area of the 50-ha Barro Colorado Island, Panama forest dynamics plot.  In both years, digitized crown boundaries were linked to individual tags, providing species identification and diameter history for each crown.  I identified trees that entered and exited the canopy and determined the change in sun-exposed crown area of trees that remained in the canopy.

Results/Conclusions

Many areas of the forest, including forest 20-30 meters in height, experienced large changes in crown configurations, including crown growth, complete or partial overtopping and displacement of where the crowns were exposed to direct light.  Tree growth reflected not just the position of neighboring stems, but the locations of surrounding tree crowns.  Crown loss on surviving trees is a major factor in crown shape and potentially tree growth.  Crown area change was positively, but weakly, correlated with diameter change.  I used a model of transition of trees into and out of the canopy to determine the rates of canopy dynamics as a function of surrounding canopy height and tree density.