Data from: Including tree spatial extension in the evaluation of neighbourhood competition effects in Bornean rain forest

Classical tree neighbourhood models use size variables acting at point
distances. In a new approach here, trees were spatially extended as a
function of their crown sizes, represented impressionistically as points
within crown areas. Extension was accompanied by plasticity in the form of
crown removal or relocation under the overlap of taller trees. Root
systems were supposedly extended in a similar manner. For the 38 most
abundant species in the focal size class (10 - <100 cm stem girth)
in two 4-ha plots at Danum (Sabah), for periods P1 (1986-1996) and P2
(1996-2007), stem growth rate and tree survival were individually
regressed against stem size, and neighbourhood conspecific (CON) and
heterospecific (HET) basal areas within incremented steps in radius. Model
parameters were critically assessed, and statistical robustness in the
modelling set by randomization testing. Classical and extended models
differed importantly in their outcomes. Crown extension weakened the
relationship of CON effect on growth versus plot species’ abundance,
showing that models without plasticity over-estimated negative density
dependence. A significant negative trend of difference in CON effects on
growth (P2 − P1) versus CON or HET effect on survival in P1 was strongest
with crown extension. Model outcomes did not then support an explanation
of CON and HET effects being due to (asymmetric) competition for light
alone. An alternative hypothesis is that changes in CON effects on small
trees, largely incurred by a drought phase (relaxing light limitation) in
P2, and following the more shaded (suppressing) conditions in P1, were
likely due to species-specific (symmetric) root competition and
mycorrhizal processes. The very high variation in neighbourhood
composition and abundances led to a strong ‘neighbourhood stochasticity’,
and hence to largely idiosyncratic species’ responses. A need to much
better understand the roles of rooting structure and processes at the
individual tree level was highlighted.