Jim Dalling (University of Illinois, USA) attended the joint BES/gtö tropical ecology symposium in Edinburgh last month. UTE2019 (Unifying Tropical Ecology) celebrated collaboration across the entire tropics and promoted the work of a diverse range of topics and research groups. Jim reports back on one of the sessions.

The productivity of tropical forests is affected by a variety of potentially limiting resources: light, water and mineral nutrients. However, the degree to which nutrient availability constrains plant growth in different ecosystem contexts, the components affected (radial growth, root and leaf production), and even the identity of these nutrients (nitrogen (N), phosphorus (P) and base cations) remains highly contentious. One approach to gain insights into nutrient limitation is through nutrient addition treatments applied to replicate forest plots. Although this is a classical approach, with experiments in tropical forests carried out since the 1980s, most have been short-lived, potentially capturing only transient responses, or missing effects that may take a decade or more to materialize.

At UTE2019, Kelly Andersen (Nanyang Technological University), Laëtitia Bréchet and Lucia Fuchslueger (both University of Antwerp) organized a session titled “Nutrient limitations in tropical forests – do expectations match results? From soil microbes to plant productivity”.

Kelly Andersen, Laëtitia Bréchet, and Lucia Fuchslueger (L to R).

This session marked a renaissance in nutrient addition experiments and was the first time research groups from the majority of the on-going or recently-established experiments convened. Representatives, particularly in the neotropics, presented results from several wet montane forests in Ecuador and Panama, lowland wet forests in French Guiana, Brazil and Panama and dry forests from Costa Rica.

Joe Wright (Smithsonian Tropical Research Institute) set the scene with a meta-analysis of the 48 studies that have been done in natural forests (excluding mangroves). Most of these (32) have been done in the neotropics, SE Asia (8) or Hawaii (8), with none in Africa or Australia. While the meta-analysis strongly supported significant growth responses to nutrient addition, there was no evidence for the paradigm that P limitation is replaced by N limitation with increasing elevation. Another notable feature of the review is that the potential impact of other nutrients such as calcium and potassium, which has clear effects on fine root production, have been largely ignored.

Joe Wright summarizes results of 48 nutrient addition experiments carried out in tropical forests.

Following this, I presented a talk which highlighted differential responses of trees with arbuscular mycorrhizal (AM) versus ectomycorrhizal (EM) associations to N addition in the NITROF experiment in lower montane forest in western Panama, with higher litterfall and radial growth responses in AM species. Jürgen Homeier (University of Göttingen) also highlighted functional group responses to N and P addition in the NUMEX experiment in montane forest in Ecuador, with stronger growth responses of species with more acquisitive functional traits. Similarly, at the FERTEX experiment in seasonally dry forest, Jennifer Powers (University of Minnesota) showed that N and P fertilization generally did not increase above ground productivity, but P addition had a strong impact on growth and nodule production in N-fixing legumes.

Finally, two of the newest experiments in lowland forests highlighted contrasting responses to nutrient addition. At the AFEX experiment in central Amazonia Kelly Andersen (Nanyang Technological University) showed that P effects appeared within a year of addition with significant effects on foliar P in litterfall, and on soil CO₂ efflux. In French Guiana, at the IMBALANCE-P experiment, Laëtitia Bréchet (University of Antwerp) showed only modest effects of N and P after two years on soil C dynamics, with significant effects on CO₂ efflux in particular, compared to two other soil greenhouse gases (CH₄ and N₂O).

Summary of measured responses to nutrient addition (from Ana Carolina Moraes). 

The session ended with a broad discussion of the utility of nutrient addition experiments – can they be used to provide predictions of the effects of anthropogenic nutrient deposition, or are they more valuable in elucidating fundamental biogeochemical processes and their response to perturbation?

There was also broad agreement that the time is now right to meet regularly as a network of nutrient addition experiments to coordinate activities and for comparative studies taking advantage of newly developed tools and approaches, particularly to determine the role of soil microbial communities in mediating nutrient limitation. As such, the network of nutrient addition experiments in tropical forests is set to create a new framework for nutrient limitation in contrasting tropical forests.

Jim Dallling, University of Illinois, USA, and Associate Editor for Journal of Ecology

Read more about UTE2019 and other events organized by the British Ecological Society.