Stoichiometry in Sulawesi

My job during my PhD was to work out how heavy metal rich ultramafic soils affect Sulawesi’s tree communities. To do this we looked at soil stoichiometry (i.e. chemical elements) and observed how this influenced leaf stoichiometry. We sampled communities on ultramafic and non-ultramafic (limestone, mafic and sand) soils for comparison.

The results of this have just been published in Plant and Soil – available OA here.

We first summarised all the relationships between soil and leaf elements – we didn’t pick up too many clear relationships, especially for the heavy metals found in ultramafic soil – though soil Cr did have a negative effect upon leaf P.

Ultramafic influence was most obvious when looking at a metric borrowed from the functional trait literature known as functional distinctiveness. What this metric does is take many variables – which could be plant height, SLA, wood density etc or each leaf element for our data and simplify them to a single measure that tells us how different species are from all others in a community.

Increasing stoichiometric distinctiveness of tropical tree leaves with increasing soil metal concentrations.
Figure: Increasing foliar stoichiometric distinctiveness of tropical tree species with increasing soil metal concentrations.

Measurement of distinctiveness showed us that species on ultramafic soils were more different from one another – largely because more species were accumulating metals not available in non-ultramafic soil. This is interesting because the potentially toxic heavy metals in ultramafic soils are stressful for plants and stress tends to limit how varied species are. Think of the stresses upon high mountain plant communities – limited to hardy herbs – compared to the forests of the comfy wet tropics that have a herb layer, multiple understorey layers, the canopy and emergents (plus epiphytes). So, whereas there are less distinct plant traits on stressful mountaintops there is more distinct plant stoichiometry on stressful ultramafic soils. Something different therefore seems to be going on in ultramafic rich areas – stress seems to be offering more viable strategies.


In 2016 I pottered around Sulawesi setting up forest plots. The first results of which have been published in Biotropica (link to pdf below). The paper describes the tree flora of our sites in eastern Sulawesi.

The first site is Mount Tompotika, the highest point on the exposed central eastern peninsula. It’s a basalt mountain with cloud forest at about 500 m and an area of grassland and large boulders at a similar elevation. The surrounding foothills are ultramafic giving way to limestone.

The next location was Wawonii, a small island off Sulawesi’s southeastern peninsula – where despite some flooding (and Brexit and being knocked out of the footy by Iceland) we setup plots on sand and ultramafic soils. Ultramafic soils are heavy metal rich – see photo below of ultramafic rock.

The final sites were in Morowali Nature Reserve in the armpit of the central and south-eastern peninsulas where we established plots in the ultramafic foothills rising out of a large alluvial flatland.

Across six months I collected specimens which we shipped backed to the herbarium in Bogor. Duplicate specimens were then shipped on to Kew, where I was able to pick the brains of taxonomists – who pointed me toward decent species identifications.

It was hard to draw firm conclusions on how the environment shapes the flora with just our ten plots. However, the data we collected alongside other datasets should give us a chance to get to grips with how the environment affects tree communities in the Indonesian archipelago.

I’m enormously indebted to the many field guides I worked alongside – they made fieldwork both possible and a laugh.

The ms is here: trethowan_et_al_floristics_sulawesi


Whilst in Sumbawa (see previous post) our story about the rediscovery of a legume genus endemic to Sulawesi was published in Ecology (link below). The paper has some added details on soil and biome affiliations of the genus and it’s relatives. As well as some interesting anther morphology and a preliminary extinction risk assessment. Below is briefly how the story came about.

In the summer of 2015 I was perched on the Kew herbarium balcony with Gwil Lewis and newly minted Professor Domingos Cardoso. We were discussing the continued search for long lost Legume genera (somewhat a forte of Domingos’). I was being told about the Dialioideae, a subfamily with three missing genera. One in the Amazon (Domingos is based in Salvador), one in Kalimantan and one on Sulawesi (where I was about to begin working for my PhD). The challenge, with the encouragement of Gwil was to go after and find these genera that hadn’t been seen for 40 plus years.

I began my search for the Sulawesi genus Kalappia as with most enquiries using google and happened across some work by Dr Rosmarlinasiah from Halu Oleo University in Southeast Sulawesi. The indication was that the single species in the genus Kalappia celebica was often a conduit for honey collection from bees that made the trees their home. This location was over 100 km south of the previously known and potentially extinct populations. The original populations are around a heavily mined area. The timber of the species is also super valuable.

Having failed in my attempts to find ‘kelapi’ (the local name for the tree) elsewhere, a team including Rosmarlinasiah and other academics and students from Halu Oleo went to Abuki, the home of Indriawan one of the students. With permission from his father we visited a grove of trees retained for the honey they provide and some heavily logged forest where we found over 20 individuals of the species. Crucially a couple of trees were in flower so with the help of Gwil and a microscope back at Kew we could be sure that these trees were indeed Kalappia.

Regarding the Brazilian genus Androcalymma, the hunt continues.

As for the genus Uittienia in Kalimantan there have been some recent reports of its presence in floristic inventories. There is also an individual tagged as the species in Bogor Botanic Gardens.


Kalappia photo in header taken by Alvin student at Halu Oleo university.