The growth of tropical rain forests’ trees is limited by soil nutrients, particularly tropical heath forests that have among the least fertile and most acidic tropical soil. Trees smaller than 10 cm DBH are often ignored by forestry and ecological studies. This cohort includes forest understory specialists, that reproduce without receiving direct sunlight, and saplings of non-understory species. Here, we propose that the response of tropical heath forest trees to nutrient addition might differ across life history strategies. For example, nutrient addition increased growth of canopy species, but understory specialists’ growth rate decreased after long term N and P addition in a sub-tropical Chinese forest. Furthermore, allometric equations are widely used to calculate forest biomass but the response of trees’ allometry to nutrient addition is unknown, and nutrient deposition is increasing throughout the tropics.

In 2016 we started a factorial Nitrogen and Calcium Carbonate fertilization experiment in a tropical heath forest in Sabah, Malaysian Borneo. We tested the impact of nitrogen deposition and change in soil pH on tree growth rate between understory specialist species and overstory species by measuring the DBH of 3200 stems under the four different treatments and tree allometry for 1600 trees throughout three years.

Understory specialists species made up only 16 % of the total species number, but they represented 49 % of stem density in this forest understory. The saplings of overstory species significantly increased their DBH growth rate after N addition and significantly increased their crown volume growth rate after Nitrogen and Calcium Carbonate addition, highlighting a greater crown structural plasticity of overstory species in comparison to understory specialists. After the treatments, there was no significant difference in the allometric relationship between tree diameter and height across both understory specialists and overstory treelets. This highlights that height – diameter allometric relationships appear stable in a framework of increased nutrient availability. The allometric relationship between tree diameter and crown volume, instead, show clear differences across life history strategies, with understory specialists having a wider crown, but with a similar crown volume, than overstory treelets for a given DBH across all the treatments. We speculate that future nutrient deposition by anthropic pollution may increase growth rate of topical heath forests and favour treelets of canopy species. This might decrease overall species diversity through the loss of understory specialist plant species. Nonetheless, biomass estimates relying on current allometric equations will remain reliable even under increased nutrient deposition.