Relating leaf traits to seedling performance in a tropical forest: building a hierarchical functional framework

Bornean tropical forests recovering from logging at risk of regeneration failure

Active restoration through silvicultural treatments (enrichment planting, cutting climbers and liberation thinning) is considered an important intervention in logged forests. However, its ability to enhance regeneration is key for long-term recovery of logged forests, which remains poorly understood, particularly for the production and survival of seedlings in subsequent generations. To understand the long-term impacts of logging and restoration we tracked the diversity, survival and traits of seedlings that germinated immediately after a mast fruiting in North Borneo in unlogged and logged forests 30-35 years after logging. We monitored 5119 seedlings from germination for ~1.5 years across a mixed landscape of unlogged forests (ULs), naturally regenerating logged forests (NR) and actively restored logged forests via rehabilitative silvicultural treatments (AR), 15-27 years after restoration. We measured 14 leaf, root and biomass allocation traits on 399 seedlings from 15 species. Soon after fruiting, UL and AR forests had higher seedling densities than NR forest, but survival was the lowest in AR forests in the first 6 months. Community composition differed among forest types; AR and NR forests had lower species richness and lower evenness than UL forests by 5-6 months post-mast but did not differ between them. Differences in community composition altered community-weighted mean trait values across forest types, with higher root biomass allocation in NR relative to UL forest. Traits influenced mortality ~3 months post-mast, with more acquisitive traits and relative aboveground investment favoured in AR forests relative to UL forests. Our findings of reduced seedling survival and diversity suggest long time lags in post-logging recruitment, particularly for some taxa. Active restoration of logged forests recovers initial seedling production, but elevated mortality in AR forests lowers the efficacy of active restoration to enhance recruitment or diversity of seedling communities. This suggests current active restoration practices may fail to overcome barriers to regeneration in logged forests, which may drive long-term changes in future forest plant communities.

Seedling growth and photosynthetic response of Pterocarpus indicus L. to shading stress

In tropical forests, the shade provided by tree canopies and extreme climate causes inhibition of plant seedling growth due to the lack of light. However, the plants can acclimate to such environmental stress by generating specific responses. The present study aimed to investigate the effects of shading conditions on ecophysiological performance of Narra seedlings (Pterocarpus indicus L.) via a mesocosm experiment. A pot experiment was conducted for 20 weeks in a greenhouse with different shading treatments, 75% (control), 25%, and 4% of full sunlight (FS). As a result, the photosynthetic rate (PN), Rubisco enzyme activity, maximum carboxylation rate (VCmax), and maximum electron transport rate (Jmax) in 25% FS treatment were higher or similar to those in control after three weeks of the beginning of shade treatment, whereas the highest values after ten weeks were observed in control. In contrast, the photosynthetic pigments were highest in control after three weeks, while the values were highest in 25% FS treatment after ten weeks. The growth parameters, such as biomass and leaf area, were highest in 75% FS treatment. The expression of Rubisco, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, and fructose-1,6-bisphosphatase were up-regulated in 4% FS treatment compared to control after ten weeks, contributing to tolerating the shade stress. Our findings indicated the capacity of P. indicus seedlings to tolerate and acclimate low light conditions causing shade stress by generating specific physiological and morphological responses, especially Rubisco enzyme activity as well as gene expression related to photosynthetic activity. The present study will improve our understanding of the tolerance mechanism of Narra plant under light-deficient conditions, thereby providing a better strategy for efficiently growing seedlings of this species in tropical rainforests.

Functional traits and habitat heterogeneity explain tree growth in a warm temperate forest

To explore how traits determine demographic performance is an important goal of plant community ecology in explaining the assembly and dynamics of ecological communities. However, whether the prediction of individual-level trait data is more precise compared to species average trait data is questioned. Here, we analyzed the growth and trait data for 11 species collected from October 2018 to October 2020 in a temperate forest, Donglingshan, Beijing. To quantify the relationships between traits and growth rate, we conducted linear regression models at both the species and individual levels, as well as developed structural equation models at both levels. We found there was a clear difference in growth between the warm and cold seasons, with tree growth mainly concentrated in the warm season. Growth rate was positively correlated with the specific leaf area, while negatively correlated with leaf thickness and wood density without considering environmental information. Adding important contextual information in the analysis of species-level structural equation modeling, growth rates were positively correlated with specific leaf area and leaf thickness. However, in the individual-level, there was a negative correlation between growth rate and wood density. Our study showed that individual-level trait data have better predictions for individual growth than species-level data. When we use multiple traits and establish links between traits and tree size, we generated strong predictive relationships between traits and growth rates. Furthermore, our study highlighted that the importance of incorporating topographical factors and considering different seasons to assess the relationship between tree growth and functional traits.

Leaf hydraulics coordinated with leaf economics and leaf size in mangrove species along a salinity gradient

Independence among leaf economics, leaf hydraulics and leaf size confers plants great capability in adapting to heterogeneous environments. However, it remains unclear whether the independence of the leaf traits revealed across species still holds within species, especially under stressed conditions. Here, a suite of traits in these dimensions were measured in leaves and roots of a typical mangrove species, Ceriops tagal, which grows in habitats with a similar sunny and hot environment but different soil salinity in southern China. Compared with C. tagal under low soil salinity, C. tagal under high soil salinity had lower photosynthetic capacity, as indicated directly by a lower leaf nitrogen concentration and higher water use efficiency, and indirectly by a higher investment in defense function and thinner palisade tissue; had lower water transport capacity, as evidenced by thinner leaf minor veins and thinner root vessels; and also had much smaller single leaf area. Leaf economics, hydraulics and leaf size of the mangrove species appear to be coordinated as one trait dimension, which likely stemmed from co-variation of soil water and nutrient availability along the salinity gradient. The intraspecific leaf trait relationship under a stressful environment is insightful for our understanding of plant adaption to the multifarious environments.