Author Hao Ran Lai discusses recently published Journal of Ecology article: “Functional traits that moderate tropical tree recruitment during post-windstorm secondary succession” by Lai et al.
Find out more about their study on what causes differences in tree recruitment in a tropical lowland forest in Singapore.
Understanding how species respond to environmental changes is becoming an urgent pursuit in ecology amidst rapid global change. How will a species’ abundance change with increasing disturbance to its habitat conditions? What is the abiotic environment under which a species reaches peak recruitment? What is the tolerable range of environmental conditions for a species to avoid extirpation?
These questions can be addressed by quantifying a species’ environmental response as the change in its presence–absence, abundance, or performance along an environmental gradient. For example, along a light-availability environmental gradient, we would expect a light-demanding species to show a positive response to increasing light availability (blue slope in the figure below), whereas a shade-tolerant species would display a less positive (or even negative) response (red slope in the figure below).
The next useful piece of information is to find out if the different environmental responses across many species can be explained by some shared biological attributes, such as functional traits. If the species–environmental relationships are moderated by functional traits, then the results of one study can be generalised across systems that vary in community composition. Although field data of species’ abundances and distributions across environments are often sparse, ecologists today may capitalise on the increasingly available large trait databases to predict the demographic responses of a particular species—given its traits—to fast-paced habitat changes.
When a strong windstorm in 2011 disturbed a tropical lowland forest in Mandai, Singapore, large canopy gaps, high amounts of litterfall and coarse woody debris were created heterogeneously across 40 ha of affected forest. Such a rare event provided an excellent opportunity to study the recovery response of multiple species across light and soil gradients, so we immediately set up 100m2 inventory plots to track the recruitment of tree species across various disturbed environments. We also collected and compiled six widely-used plant functional traits—specific leaf area, leaf dry-matter content, lamina thickness, wood density, seed dry mass, and maximum height—to identify traits that could explain species–environment relationships.
We found that the recruitments of 53 species, across the wind-disturbed environments of 28 plots, were dependent on their wood density, seed dry mass, and maximum height. Although most species unsurprisingly recruited more with increasing light availability or canopy openness, we show that low-wood-density species responded more positively to increasing light than high-wood-density species. This is because the former constructs cheaper stems to attain vertical height as rapidly as possible before canopy closure. As opposed to heavy-seeded species, the light-seeded species decreased in recruitment rapidly with thickening leaf litter depth as their low-energy-reserve seeds failed to germinate against litter burial. Lastly, short-statured species tended to recruit more in historically short, young secondary forest sites, while the opposite was found for tall-statured species. In other words, short- or tall-statured saplings are more likely to be found in areas dominated by short- or tall-statured parent trees respectively, suggesting the importance of considering seed source availability or historical contingency when studying secondary succession.
Put together, this study identified a mixture of functional traits that govern multi-species recruitments across multiple environmental gradients, generated by a single disturbance event. These trait–environment interactions not only highlight the ability of species to coexist by partitioning their environmental niches via multiple axes of trait strategy, but also demonstrate that species–environment relationships are generalisable by a few shared biological attributes that are readily measurable.
Hao Ran Lai National University of Singapore, Republic of Singapore and Yale‐NUS College, Republic of Singapore.
Read the full research article online: Functional traits that moderate tropical tree recruitment during post-windstorm secondary succession
You can also read a related blog post from author Kwek Yan Chong on the Journal of Vegetation Science blog: Trudging through treefalls