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.
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.