Abiotic Drivers of Seedling Bank Diversity in Subtropical Forests of Southern China

Abiotic factors are important to shape plant community composition and diversity through processes described as environmental filtering. Most studies on plant diversity in forests focus on adult trees, while the abiotic drivers of forest seedling community characteristics are less understood. Here, we studied seedling banks’ composition, richness, diversity, and abundance, and investigated their relationships with microsite abiotic conditions along a wide elevational gradient. We sampled seedling communities in 312 1-m2 quadrats, distributed in 13 one-ha plots in four subtropical forests in south China, covering an elevation gradient of 1500 m, for 2 years. We measured light availability, slope, and 11 soil nutrients for each seedling quadrat. We used analysis of similarities and multivariate analysis of variance to compare the composition and abiotic drivers of the four forests’ seedling communities. We then used mixed models and structural equation modeling to test the direct and indirect effects of abiotic factors on seedling species richness, diversity, and abundance. The differences in seedling community composition among these forests were mostly explained by differences in elevations and soil nutrients. Seedling diversity as Shannon and Simpson diversity index decreased with increasing elevation and increased with increasing slope, but seedling abundance and species richness did not. Elevation had an indirect effect on Simpson’s diversity index through modulating the direct effects of soil properties. Our findings show that soil properties play a prominent role in favoring differentiation in species composition among the four forests we studied and provide additional evidence to decreasing species diversity with elevation. However, this was reflected in decreasing Shannon and Simpson indices rather than species richness, which is more commonly studied. Whether and to what extent future environmental changes in climate and soil acidification will alter future forest composition and diversity needs to be investigated.

Temporal Development and Regeneration Dynamics of Restored Urban Forests

<p>Urban forest restoration programmes are a key tool used to initiate, re-create or accelerate the succession of forest species; improving ecosystem services, function, resilience and biodiversity. Succession is a temporal shift in species dominance driven by abiotic and biotic influences, but over decadal timescales the trajectory and success of restoration plantings in degraded urban environments can be hindered. To facilitate the successful reconstruction of forest ecosystems from scratch, an understanding of the temporal patterns in planted forest development, dynamics of seedling regeneration and dominant drivers of seedling diversity is required. Using a chronosequence approach, permanent plots were established at 44 restored urban forests aged 5 to 59 years since initial plantings took place, across five New Zealand cities between Wellington and Invercargill. Vegetation surveys were undertaken and data on micro-climate were collected. This study examined the 1) temporal dynamics of restored urban forest development and seedling regeneration and 2) dominant drivers of seedling regeneration. Data were analysed using linear regression models, breakpoint analysis and mixed-effects modelling. Early forest development (<20 years) exhibited the most changes in canopy composition and structure, forest floor dynamics, seedling community and microclimate. This period saw significant increases in canopy stem abundance, height, basal area and leaf litter cover. Significant declines occurred for light transmittance, herbaceous cover and daily soil and air temperature range within the same timeframe. Dominant traits amongst the seedling community included early successional species, tree species, shade and drought tolerant species, insect-pollinated species and frugivory dispersed species. Seedlings with these traits had higher species richness levels across the whole chronosequence. Collectively, five biotic drivers representing forest composition, structure and landscape factors strongly influenced seedling diversity. Seedling diversity increased with the proportion of surrounding natural landcover, sapling diversity, basal area, canopy diversity and herbaceous cover. The influence of these predictors of seedling diversity, was more significant when modelled as a set, than when viewed independently. Geographic location (city) was indicated as a stronger predictor for similarities in canopy and seedling community composition than the age of the restoration planting. This was shown by stronger clustering of sites according to their city, more so than forest planting age, in a non-metric multidimensional scaling analysis. Our results provide valuable insight to restoration practitioners on the outcomes of urban restoration programmes implemented across much of New Zealand and helps close the gap between the science of restoration ecology and the practice of ecological restoration.</p>

Temporal Development and Regeneration Dynamics of Restored Urban Forests

<p>Urban forest restoration programmes are a key tool used to initiate, re-create or accelerate the succession of forest species; improving ecosystem services, function, resilience and biodiversity. Succession is a temporal shift in species dominance driven by abiotic and biotic influences, but over decadal timescales the trajectory and success of restoration plantings in degraded urban environments can be hindered. To facilitate the successful reconstruction of forest ecosystems from scratch, an understanding of the temporal patterns in planted forest development, dynamics of seedling regeneration and dominant drivers of seedling diversity is required. Using a chronosequence approach, permanent plots were established at 44 restored urban forests aged 5 to 59 years since initial plantings took place, across five New Zealand cities between Wellington and Invercargill. Vegetation surveys were undertaken and data on micro-climate were collected. This study examined the 1) temporal dynamics of restored urban forest development and seedling regeneration and 2) dominant drivers of seedling regeneration. Data were analysed using linear regression models, breakpoint analysis and mixed-effects modelling. Early forest development (<20 years) exhibited the most changes in canopy composition and structure, forest floor dynamics, seedling community and microclimate. This period saw significant increases in canopy stem abundance, height, basal area and leaf litter cover. Significant declines occurred for light transmittance, herbaceous cover and daily soil and air temperature range within the same timeframe. Dominant traits amongst the seedling community included early successional species, tree species, shade and drought tolerant species, insect-pollinated species and frugivory dispersed species. Seedlings with these traits had higher species richness levels across the whole chronosequence. Collectively, five biotic drivers representing forest composition, structure and landscape factors strongly influenced seedling diversity. Seedling diversity increased with the proportion of surrounding natural landcover, sapling diversity, basal area, canopy diversity and herbaceous cover. The influence of these predictors of seedling diversity, was more significant when modelled as a set, than when viewed independently. Geographic location (city) was indicated as a stronger predictor for similarities in canopy and seedling community composition than the age of the restoration planting. This was shown by stronger clustering of sites according to their city, more so than forest planting age, in a non-metric multidimensional scaling analysis. Our results provide valuable insight to restoration practitioners on the outcomes of urban restoration programmes implemented across much of New Zealand and helps close the gap between the science of restoration ecology and the practice of ecological restoration.</p>

Seedling Recruitment of Native Tree Species in Active Restoration Forest

Since 1970’s, large are of deforestation and forest conversion to agriculture in the northern mountains of Thailand grew concern of environmental impact. Forest restoration became an important strategy to rapidly increase forest habitat in a wide-scale in Thailand. The Framework Species (FWS) technique is an active forest restoration that has been developed to restored forest habitat and ecosystem on former-agriculture land in the northern Thailand. After planting 20-30 framework species, the method then relies on incoming native tree species to sustain forest succession. However, there has been little known about the recruitment of native tree species and factors limiting the recruitment in this area. The study compared recruit native tree species in the seedling community of the FWS restoration and nearby seed source to assess how many of those are recruit or absent from the community. The factor of seed-dispersed agents and seed sized was investigated as the possible limited factors of the recruitment and compared among 3 restoration periods (at age 6, 10, and 14 years). The result reveals half of native species were absent from the seedling community across all 3 restoration ages. Seed available was a major limitation for the recruitment of native tree species. Big-seeded species had higher chance to be limited than small-seeded species to recruit in the restoration site (p-value = 0.0249 by the Tukey test). whilst seed-dispersed agents were not limited (p-values=0.420 by Chi square). The FWS forests efficiently facilitated seedlings of native tree species to recruit at the similar recruitment rates across all 3 ages of restoration. However, the technique was still limited in regard of seed available. Species of less common or rarer might need to be included for the FWS plantation in the future to enhance species diversity and better outcome of the restoration.

Timing is Not Everything: Assessing the Efficacy of Pre- Versus Post-Harvest Herbicide Applications in Mitigating the Burgeoning Birch Phenomenon in Regenerating Hardwood Stands

Sweet birch (Betula lenta L.) is aggressively recruiting in temperate forest understories of the eastern United States and often dominates the post-disturbance seedling community, diminishing diversity and hindering sustainable silviculture. The type and timing of silvicultural actions affect birch recruitment via their effects on seedling recruitment, survival, and growth. Here, we examine birch regeneration under two contrasting treatment sequences: pre- versus post-shelterwood harvest herbicide application (H–S vs. S–H) in combination with white-tailed deer (Odocoileus virginianus Zimmerman) browsing (fenced vs. unfenced) at 22 sites in northwestern Pennsylvania, USA. Additionally, we examine how treatments interact with additional site factors, including potential propagule sources and site productivity (i.e., integrated moisture index). We found the S–H sequence initially reduced birch density by 71% relative to the H–S sequence; however, the magnitude of this reduction waned over five growing seasons. Furthermore, birch proliferated following the H–S sequence only where mature birch were present. Deer browsing reduced birch height by 29% relative to fenced areas protected from browsing; however, by the fifth growing season birch seedlings were over twice as tall as other hardwood species across all treatments. Finally, increasingly mesic sites enhanced birch height growth. In sum, although post-harvest herbicide (S–H) provides short-lived control over birch, land managers should also consider browse pressure, seed source, and site productivity, as these may enhance or diminish the efficacy of post-shelterwood herbicide sequence effects on birch.

Plant–soil feedbacks between arbuscular- and ecto-mycorrhizal communities

Soil microbiomes of adult trees exert species-specific effects on the survival and growth of seedlings1-6, yet empirical evidence that such plant–soil microbiome interaction drives seedling community assembly remains scarce. Here we show that mycorrhizal fungal communities determine seedling community assembly by controlling how resident plant communities alter the growth of newly established seedlings. We reciprocally introduced seedling communities of arbuscular-and ecto-mycorrhizal plant species to replicated mesocosms to follow the effects of mycorrhizal type match/mismatch with resident plant communities on seedling growth rates. The growth rates of recruited seedlings were generally higher under resident trees of the same mycorrhizal types than under those of different mycorrhizal types, generating positive plant–soil feedbacks through mycorrhizal-type matching. Such positive effects of matching were linked with seedlings’ greater acquisition rates of mycorrhizal symbionts from matched resident plants than from mismatched plants, and such linkage was pronounced for ecto-mycorrhizal plant species. In contrast, under the condition of mycorrhizal-type matching between resident plants and seedlings (i.e., within-mycorrhizal-type comparison), plant–soil feedback effects varied considerably in their sign and strength among resident–seedling species combinations. Consequently, the assembly of a temperate tree seedling community is driven by a combination of species-specific plant–soil feedbacks and the match/mismatch of mycorrhizal type between resident plants and seedlings.

Effects of bamboo dominance and palm-heart harvesting on the phylogenetic structure of the seed and seedling communities in an old-growth Atlantic Forest

A comprehensive assessment of the effect of disturbances on tropical and subtropical forests is needed to better understand their impacts on forest structure and diversity. Although taxonomic and functional diversity measures have been successfully adopted in this context, phylogenetic diversity metrics are still poorly explored. We compared the phylogenetic structure of the seed rain and regenerating seedling community in patches of an old-growth Atlantic Forest remnant dominated or not by a ruderal bamboo species, Guadua tagoara. We sampled those patches before and after illegal harvesting of the palm Euterpe edulis thus assessing if the harvesting led to changes in the phylogenetic structure of the seed rain and the regenerating community in both patches. Bamboo-dominated patches showed a significantly higher presence of species in the seed rain that were more distantly related to each other in the phylogeny than expected by chance compared with patches without bamboos, but this difference disappeared after palm-heart harvesting. Contrary to what we expected, we did not find significant changes in the phylogenetic structure of seedlings before or after palm-heart harvesting. The phylogenetic structure at the tips of the phylogeny was random overall. The maintenance of a higher presence of far relatives in the phylogeny of the seedling community suggests, assuming trait conservatism, that despite bamboo dominance and palm-heart harvesting, functional diversity is being preserved at least in the early regenerating stages and in the time frame of the study. However, higher presence of pioneer taxa after palm-heart harvest indicates that this disturbance may lead old-growth areas to earlier successional stages.