Specific Leaf Area Drives Tropical Forest Seedling Diversity

Several forest restoration strategies are commonly attempted today, such as monocultures, mixed-species plantations, and passively regenerating forests. Scientists are also looking at an increasingly wider array of parameters to find out more about the processes that drive seedling diversity in recruitments in different forest restoration methods. Specific leaf area was found to be crucial in this novel study on functional traits and phylogenetic diversity.

What Happens During Forest Restoration?

In 2020 alone, 12.2 million hectares of tree cover was lost in the tropical forests, according to the World Resources Institute. Tree cover includes not only loss of primary and secondary forest, but also trees from plantations. Primary forest loss in the humid tropics was 4.2 million hectares, up by 12% from the previous year.

A concerted effort in forest restoration is underway to reverse the rate of deforestation. The number and kind of species recruited can have different benefits for ecosystem functioning and can influence future recruitment of seedlings.

The role of abiotic environmental factors, such as rainfall, soil fertility, and light conditions, in recruiting species is well known. Species with similar plant traits are chosen due to environmental filtering. However, biotic interactions between plants, like competition, favors species with differing traits so that they can benefit from niche differentiation.

It is difficult to tease apart how the two opposing forces of environmental filtering and species interaction influence understory seedling recruitment in forest restoration methods. A multidisciplinary team of environmental, soil, and biological scientists—Wills, Herbohn, Wells, Moreno, Ferraren, and Firn—decided to tackle this problem anyway.

Could Phylogeny and Plant Traits Drive Seedling Recruitment?

According to phylogenetics, plant functional traits are conserved among related species during evolution. The multidisciplinary team decided to use the seedling community’s phylogenetic and functional structure to find out what was driving species recruitment in forest restoration: environmental filtering or species interaction.

The scientists checked the phylogenetic and functional diversity of seedling communities under actively restored monocultures of exotic Swietenia macrophylla and multi-species plantation (rainforestation) and passively regenerating/selectively logged forests in the Philippines. The scientists expected to find closely related seedling communities in monocultures, which are driven by environmental filtering and limited dispersal. While at the other end, regenerating forests would be shaped by a diverse seedling community, competition, and long-distance dispersal types.

To test this hypothesis, the scientists chose five sites in each forest restoration type that were between 13-18 years old. In each site, they laid 2-3 circular plots to get a total of thirty-five plots.

The functional trait they focused on was intraspecific (SLA) specific leaf area measurement, as the scientists wanted to find out if this parameter was phenotypically plastic. In addition, they also studied leaf nutrients, life forms, plant height, and seed dispersal type.

To get specific leaf area measurements, the scientists first estimated leaf area. They took at least two fully expanded leaves from each species and scanned them using CI-203 Handheld Laser Leaf Area Meter, manufactured by CID Bio‐Science. The scanning was done either on-site or elsewhere in the afternoon of the same day. The Leaf Area Meter is a portable and light device that can be easily handled on the field to give rapid and precise readings. The device calculated the leaf area, using preloaded formulae. Later, to get the weight of the leaf, the leaf samples were oven-dried to calculate SLA as cm2/g. Nitrogen and phosphorus content were the other leaf traits estimated.

Specific leaf area measurements were made for 856 plants, belonging to 91 species from the total of 219 species sampled. To study intraspecific differences in SLA comparison, only 39 species could be considered, as specific leaf area measurements of at least five individuals per species were needed.

By reviewing previous studies, data for three other functional traits were obtained for 123 species:

• Depending on plant height, species were classified as understory, mid-canopy, canopy, and emergents layer.
• The 123 species were grouped into five types of life forms:  93 trees, 13 shrubs, 13 herbs, 3  woody vine/liana, and 1 palm species.
• Dispersal types were grouped as biotic or abiotic.

Community phylogeny was pieced together for 125 species, common to all three forest restoration types. Of these, 95 species were native and 30 were exotics.

Environmental Filtering Or Interactions

The scientists asked the following questions to decipher ecosystem dynamics in forest restoration:

1. How do the functional traits vary in the seedling communities under the three forest restoration methods?
2. What is the intraspecific variation in SLA between the three forest types and in species common to all forest restoration methods?
3. How do phylogeny, functional structure, and intraspecific variation in SLA shape seedling communities?

Phylogenetic and Functional Diversity

Seedlings recruited in monocultures were phylogenetically clustered and closely related species. Environmental filtering is the dominant factor in monocultures, and species that can thrive in similar conditions exist in the seedling community. Seedlings were phylogenetically overdispersed, meaning that the species were not closely related in passively restored forests, as the scientists had hypothesized, due to competition. As shown in Figure 1, mixed-species plantation/rainforestation falls in between.

Monocultures had a higher number of exotics and small-seeded species, such as Moraceae family members that are dispersed by general birds. Many species dispersed by humans were also found here.

The trend for functional traits in the three restoration forests is not so straightforward. Leaf traits showed a similar trend to phylogenetics, especially SLA and leaf nitrogen content, from clustering in monocultures to overdispersion in rainforestation and regenerating forests. However, when all plant traits are combined, there was overdispersion in monocultures and random to clustering in rainforestation and passive regenerating sites, as shown in Figure 2a. When only native species are considered, the rainforestation seedlings show overdispersion in traits; see Figure 2b.

This shows that diversity in plant height, life form, and dispersal type is less in species-rich forests compared to monocultures. Among all restoration forest types, wind dispersed species were found to be the least.

Specific Leaf Area Matters

The variation (CV) in specific leaf area was the highest in regenerating forests, moderate in mixed-species plantations, and least in monocultures; see Figure 3a.

The intraspecific difference in 39 species shows overdispersion or that individuals with a less and more intraspecific difference in SLA are found in regenerating forests. The other two forest types had seedlings with less diversity, as can be seen in Figure 3b & c. The scientists concluded that both competition and environmental filtering were important to explain the seedlings recruited in regenerating forests, while in the two plantations it was only environmental filtering.

The diversity in species in regenerating forests creates a more complex and diverse plant canopy. The structural diversity and complexity in regenerating forests create more niches in the understory. The diverse phylogenetics of seedlings is due to these niches and competition between the dense population in the understory. In turn, the diversity in plant canopy explains the varying SLA found in regenerating forests. The understory has differences in environmental and biotic conditions, so in response SLA within a species varies. For example, species from the genus Ficus (Moraceae), with a high SLA variation, are common in regenerating forests.

Regenerating forests also recruit seedlings that have low variation in SLA, characteristic of later successional species, which thrive under more homogenous environmental conditions. This includes, for example, the wind-dispersed native species from Dipterocarpaceae with low SLA variation.

Monocultures have a simpler plant canopy and offer the least number of niches and, therefore, have closely related species. The clustered phylogenetics could also occur due to dispersal limitations. Since the number of seedlings is less there is also little competition. The plant canopy in mixed-species plantations is not as complex as the regenerating forests, showing that 18 years is not enough to develop and perform all ecosystem functioning of a mature forest.

Rare Insights into Forest Restoration

The scientists concluded that increasing the number of species alone is not enough. Including species with more phylogenetic and functional diversity will quicken the development of more ecosystem functions, which will, in turn, create more niches and lead to better future species recruitment. The scientists also suggest that artificial forest restoration efforts should take care to include emergent, native, wind-dispersed, and large-seeded species. Since SLA plays an important role, the species selection should be those that have a broad range of specific leaf area and various levels of intraspecific SLA. As phylogenetics have rarely been studied during restoration, this study provides important insights into ecosystem processes during forest restoration.

—

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Sources

Wills, J., J. Herbohn, J. Wells, M. O. Maranguit Moreno, A. Ferraren, and J. Firn. 2021. Seedling diversity in actively and passively restored tropical forest understories. Ecological Applications 31(3): e02286. 10.1002/eap.2286

Weisse, M., & Goldman, E. (n.d.). Forest Pulse: The Latest on the World’s Forests: World Resources Institute Research. The Latest Analysis on Global Forests & Tree Cover Loss | Global Forest Review. Retrieved from  https://research.wri.org/gfr/forest-pulse.