Cations limit tree growth of a keystone pioneer species in Central Africa

Tropical forests are increasingly subjected to land-use change with important implications for biodiversity conservation and global carbon-, energy-, and water cycling. Particularly in Central Africa, slash-and-burn agriculture is the main source of forest losses, and the projected increase in the African population is only to exacerbate these dynamics. The imminence of this social-ecological front is already reflected in the decreasing length of fallow periods, combined with an increase in the number of clearing cycles.

Studying regrowth forests in a thorough and comprehensive way entails including not only biodiversity- and carbon stock recovery, but also the recuperation of nutrient cycling. The current paradigm on tropical secondary forest succession states that these forests move from a nitrogen (N) to a phosphorus (P) limitation during ecosystem recovery. However, recent research has shown that cations become scarce and potentially limiting in later successional stages in Central African forests. Additionally, in a context of repeated forest clearing, soil total and available cation stocks have been shown to decrease each clearing cycle, potentially affecting the regrowth of secondary forests.

An essential phase in the recovery of forests on fallows in Central Africa is the establishment of the pioneer tree species Musanga cecropioides R.Br. ex Tedlie, often exhibiting a temporary monodominance on abandoned fallows and creating a microclimate that facilitates the establishment of other tree species. To study the effects of nutrient losses due to land-use intensification on forest regrowth, our team installed a pot experiment with M. cecropioides close to Kisangani in the Democratic Republic of the Congo, with the following nutrient treatments: control, nitrogen (225 kg ha^-1 yr^-1), phosphorus (75 kg ha^-1 yr^-1), and a combined cation treatment (calcium, magnesium, and potassium; each 75 kg ha^-1 yr^-1). These four treatments were then combined in a full factorial setup with 15 replicates. 

Plant diameters and -heights were measured biweekly for one year. Plants that received the combined cation treatment quickly overtook the ones that only received single treatments of N and P, as well as the controls, both in terms of measured diameter and height. The height measurements showed a steep increase when N was combined with cations, however, the addition of P did not show any additional effect. Plant diameter measurements showed a first increase when cations were added solely and a second increase when the cation addition was combined with nitrogen. Again, the additional provision with P did not add to the observed diameter increase, emphasizing the importance of cations as potentially limiting nutrients for forest regrowth.

A final complete harvest of the plants will allow for total biomass quantification per tissue, as well as destructive sampling for chemical analysis. Analysis of tissue concentrations and stoichiometries of C, N, P, and cations will importantly supplement the growth rates by providing more insight into the relative effects of the added nutrients and will help to disentangle the effects of the separate cations that were combined in the aggregated ‘cation’ treatment.