Tropical forests are globally important for biodiversity, carbon and water, yet are frequently found on highly weathered soils with low phosphorus and cation availability. Phosphorus limitation is being enhanced by shifting the relative balance of carbon, nitrogen, and phosphorus at global scales due to human activities increasing atmospheric CO2 and nitrogen deposition. This imbalance is likely to negatively affect carbon storage and ecosystem function in both natural and human-modified ecosystems. Old-growth tropical forests in Southeast (SE) Asian are dominated by Dipterocarpaceae and are associated with ectomycorrhizal fungi, which have a greater capacity to use organic nutrients. In contrast, secondary forests in SE Asia and nearly all Neotropical forests are dominated by trees species associated with arbuscular mycorrhiza, which explore soil volumes for inorganic nutrients. In this study, we examined fine root and mycorrhizal traits of dominant habitat specialists and generalist tree species distributed along a land-use gradient in a South East (SE) Asian lowland tropical forests with low soil phosphorus availability. We hypothesized that old-growth forest species would have a suite of root traits associated with conservative plant economic strategies and organic nutrient uptake, whereas secondary forest species would have acquisitive root trait strategies associated with inorganic nutrient uptake.
Results/Conclusions
In contrast to our hypothesis, we found no differences in fine root trait strategies between old-growth and secondary forests or between mycorrhizal types. However, we found clear patterns of species differences in root trait strategies within each forest and mycorrhizal type. The dominant tree species along the land-use gradient in this low nutrient tropical forest separate into two nutrient uptake strategies based on principal components analysis of fine root morphological traits. The first principal component accounted for 67% of the variation in root traits and represents the root economic spectrum from acquisitive traits, such as high specific root length, to conservative traits, such as high root tissue density. The second principal component accounted for an additional 20% of the variation and represents a shift from thin (0.40 ± 0.02 mm), highly branched to higher diameter (0.50 ± 0.05 mm), less branched roots. Thus, differences in root functional trait strategies among species with similar distribution along forest succession gradients and mycorrhizal types suggests that belowground resource partitioning may promote species coexistence of the dominant tree taxa at this low phosphorus site. Root traits related to nutrient uptake will give further insight to the role of root ecology in tropical forest succession.