Digging for plant gold: Understanding the role of soil in structuring tropical forests

This summer, I returned to Sepilok in Malaysian Borneo to collect more data towards my PhD. This time, I went digging for nutrients in the soil.

Following a successful field campaign in 2018 where we measured a suite of plant functional traits in the giant dipterocarp trees, I returned with my post-doc colleague, Paulo Bittencourt, to understand the soil environment of the Sepilok forest reserve. During the previous year’s fieldwork, we had noticed distinct patterns between the functional traits of the different forests and fine scale patterns in dipterocarp species distributions. This year, we wanted to discover if the soil drives these patterns.

Sepilok provides the perfect study site to understand the role of soil in species distributions patterns. Within 10km, the Sepilok forest shows dramatic turnover in its species composition and aboveground biomass. Here, four distinct forest types can be found: mangrove, alluvial, sandstone and kerangas. The mangrove forest is situated at the lowest elevation close to the coast. The alluvial forest is situated in the low floodplains of the reserve, with mudstone hills dotting its landscape. This forest is dominated by tall dipterocarp trees reaching 70m in height, including Shorea johorensis, Shorea smithiana and Parashorea tomentella. Situated at a slightly higher elevation, you find the sandstone forest, characterised by its steep valleys and narrow ridges. This forest is dominated by species of Dipterocarpus and Shorea. At the highest elevation, the canopy begins to get shorter (still an impressive 40m) and more open. This is the kerangas (or heathland) forest, which is characterised by sandy soils and a dominance of Myrtaceae. Here, dipterocarps still remain abundant with species such as Shorea multiflora, Cotylelobium melanoxylon and Hopea beccariana common, but they no longer dominate like in the alluvial and sandstone forests. The short spatial distance between these forests provides a unique opportunity to study the role of soil in structuring tropical forests as we can control for the climate in our analysis.

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Giant 70m tall Shorea smithiana trees found in Sepilok’s alluvial forest. Photo copyright: David Bartholomew

In order to understand how soil structures these forests, we set out on an ambitious plan to collect over 1000 soil samples for nutrient and granulometry (grain size) analysis. Focusing on the alluvial, sandstone and kerangas forests, we sampled every 40m across nine 4 ha plots (3 plots per forest type). At each location we collecting soil from three depths (0-5, 5-15 & 15-30cm). In addition to the systematic grid samples, we collected five samples across three elevation gradients in each plot to understand how the soil changed with topography. Combined, we will have one of the finest scale resolution soil nutrient datasets ever collected in a tropical forest. Visual inspection of the soil promises exciting results with high heterogeneity within and between forests apparent.

We collected over 1000 soil samples in under a month for nutrient analysis, sampling at three depths: 0-5cm, 5-15cm and 15-30cm. Photo copyright: David Bartholomew
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Drying our soils in the Sepilok Forest Research Centre Chemistry Lab. Photo copyright: David Bartholomew

Asides from collecting samples for soil nutrient analysis, we continued our campaign to measure soil water content. For this, we used a soil moisture probe to measure volumetric water content at saturation every 20m across the same 4ha plots. This data will replicate the dataset on soil moisture we recorded in 2018. After collecting another 500kg of soil, we were able to calibrate the sensor back in the lab and make direct comparisons between the plots.

Calibrating the soil moisture probe was a demanding challenge requiring a full week of labwork: 1) We collected 500kg of soil from the forest – three buckets of soil per plot; 2) We dried the soil so it had 0% moisture; 3) Removing the soil from the oven; 4) Preparing the soil for calibration by transferring it back into its bucket; 5) Breaking the soil up into fine particles to allow it to be fully mixed; 6) We created a calibration curve for each of the soils by sequentially adding water, mixing and measuring it with the probe. Photo copyright: David Bartholomew

During our trip to Sepilok, we also managed to install a new meteorological station in the Forest Research Centre on the edge of the forest. This, combined with long term soil moisture sensors we installed in the forest, will reveal seasonal variations in water availability across the reserve.

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Installing our new meteorological station in the Forest Research Centre. Photo copyright: David Bartholomew
Re-installing our long-term soil moisture probes to 5cm and 30cm. Photo copyright: David Bartholomew

A final component of our field campaign was to measure the height of the trees we had functional trait data for. This extra dataset will allow us to determine whether leaf and wood traits vary with tree height. Measuring tree height in the dense rainforest is not easy though as we have to be able to see and identify the highest branch. Once identified, we used a laser rangefinder to estimate its height.

Measuring tree heights and diameters to pair with our functional traits dataset. Measuring the diameter for trees with large butresses required a tall ladder. Photo copyright: David Bartholomew

Overall, we had a very succesful trip collecting lots of exciting new data. However, as always, this work would not be possible without an amazing team of research assistants and collaborators. We all quickly discovered working with soils is a heavy, dirty and sweaty endeavour. We all hope though that this effort will help reveal exciting new findings about the rainforests of Borneo.

The rainforests of Borneo are home to some beautiful wildlife, including orchids and kingfishers. Photo copyright: David Bartholomew
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It was a tough month of fieldwork, but we squeezed in a climb of one of the giant trees.
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Fieldwork is a massive team effort. We would not be able to collect our data without our amazing team of field assistants and ranger.

For more photos, check out my portfolio.

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