Diameter growth performance varies with species functional-group and habitat characteristics in subtropical rainforests

We examined tree diameter growth in 20 plots subjected to various disturbance intensities (natural, low, moderate and intensive logging) in a bid to understand the general tree growth responses in relation to habitat characteristics in subtropical rainforests of north-eastern NSW, Australia. Species-specific regeneration strategy, maximum size and level of shade tolerance were used to classify species into five groups: emergent and shade tolerant main canopy (group 1), shade tolerant mid canopy (2), shade tolerant understoreys (3), moderate shade tolerant (4) and shade intolerant (5) tree species. Data series for trees ≥10 cm diameter at 1.3 m above the ground level (dbh) providing observations spanning over 36 years were used in multilevel regression analyses. The results showed that spatial and temporal effects in tree growth at the stand level are a combination of the differences between species functional-group compositions and environmental gradients. High growth responses were observed in the shade intolerant species while increasing level of shade tolerance and decreasing maximum size decreased trees growth rates. Tree growth increased with altitude on a large scale across regions, and with disturbance intensity on a small scale at the plot (stand) level. Increase in northness (south through flat to north facing sites) increased growth in species group 1 for trees <67 cm dbh, but beyond this dbh threshold the opposite was true. These showed that saplings of species group 1 may require increased illumination to reach the forest canopy, but once in the canopy, low soil water availability may be limiting to tree growth in the north facing sites. Decrease in northness was associated with increased growth in species group 2 indicating that reduced illumination and improved soil moisture in the south facing sites were conducive for maximum growth in this species group. Maximum growth potential in species groups 4 and 5 increased with decrease in eastness, suggesting that the increased afternoon solar radiation and temperature were conducive for high growth rates in these species. Although topographic gradient may determine the spatial and temporal variations in tree growth where growth appeared to increase from the crest down the slope into the creek/gully, its effects on soil fertility and water availability, and interactions between these and other factors may make it difficult to discern clear growth patterns.