Experimental Center of Tropical Forestry

Abstract. Increased nitrogen (N) availability influences soil phosphorus (P) cycling through multiple pathways. Soil microorganisms are essential facilitating a wide range of ecosystem functions. However, the impact of how mixed plantations of Eucalyptus and N-fixing tree species affect P transformation and microbiota interactions remains unknown. Therefore, we conducted a 17 y field experiment comparing pure Eucalyptus plantations (PPs) and mixed plantations (MPs) with Eucalyptus and an N-fixing tree species to assess their effects on soil P transformation, using data collected from two soil layers (0–10 and 10–20 cm). The results showed that α-diversity indices (ACE, Chao1, and Shannon indices) were significantly higher in MPs than in PPs for both bacteria and fungi. Furthermore, MPs exhibited significantly higher relative abundances of bacterial phyla Proteobacteria (0–10 cm), Verrucomicrobia, and Rokubacteria, as well as fungal phyla Mortierllomycota, Mucoromycota, and Rozellomycota. Conversely, MPs showed lower abundances of the bacterial phyla Chloroflexi, Actinobacteria, and Planctomycetes and the fungal phylum Ascomycota. Gene copy numbers of functional genes were also elevated in MPs, including 16S rRNA, internal transcribed spacer (ITS), N functional genes [nifH (0–10 cm), AOB-amoA, narG, nirS, and nosZ (0–10 cm)], and P functional genes [phoC, phoD (0–10 cm), BPP, and pqqC]. The findings indicate that MPs can enhance soil microbial diversity, network complexity, and the relative abundance of functional genes, which involved N and P transformation, by optimizing soil nutrient levels and pH, thereby facilitating P transformation. Therefore, MPs of Eucalyptus and N-fixing tree species may represent a promising forest management strategy to improve ecosystem P benefits.

Due to climate effects and human influences, wildfire regimes in boreal forests are changing, leading to profound ecological consequences, including shortened fire return intervals and elevated tree mortality. However, a critical knowledge gap exists concerning the spatiotemporal dynamics of fire-induced tree mortality specifically within the vast North American boreal forest, as previous studies have predominantly focused on Mediterranean and tropical forests. Therefore, in this study, we used satellite observation data obtained by the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra MCD64A1 and related database data to study the spatial and temporal variability in burned area and forest mortality due to wildfires in North America (Alaska and Canada) over an 18-year period (2003 to 2020). By calculating the satellite reflectance data before and after the fire, fire-driven forest mortality is defined as the ratio of the area of forest loss in a given period relative to the total forest area in that period, i.e., the area of forest loss divided by the total forest area. Our findings have shown average values of burned area and forest mortality close to 8000 km2/yr and 40%, respectively. Burning and tree loss are mainly concentrated between May and September, with a corresponding temporal trend in the occurrence of forest fires and high mortality. In addition, large-scale forest fires were primarily concentrated in Central Canada, which, however, did not show the highest forest mortality (in contrast to the results recorded in Northern Canada). Critically, based on generalized linear models (GLMs), the results showed that fire size and duration, but not the burned area, had significant effects on post-fire forest mortality. Overall, this study shed light on the most sensitive forest areas and time periods to the detrimental effects of forest wildfire in boreal forests of North America, highlighting distinct spatial and temporal vulnerabilities within the boreal forest and demonstrating that fire regimes (size and duration) are primary drivers of ecological impact. These insights are crucial for refining models of boreal forest carbon dynamics, assessing ecosystem resilience under changing fire regimes, and informing targeted forest management and conservation strategies to mitigate wildfire impacts in this globally significant biome.

Xylem-associated fungus can secrete many secondary metabolites to help Aquilaria trees resist various stresses and play a crucial role in facilitating agarwood formation. However, the dynamics of endophytic fungi in Aquilaria sinensis xylem after artificial induction have not been fully elaborated. Endophytic fungi communities and xylem physio-biochemical properties were examined before and after induction with an inorganic salt solution, including four different times (pre-induction (0M), the third (3M), sixth (6M) and ninth (9M) month after induction treatment). The relationships between fungal diversity and physio-biochemical indices were evaluated. The results showed that superoxide dismutase (SOD) and peroxidase (POD) activities, malondialdehyde (MDA) and soluble sugar content first increased and then decreased with induction time, while starch was heavily consumed after induction treatment. Endophytic fungal diversity was significantly lower after induction treatment than before, but the species richness was promoted. Fungal β-diversity was also clustered into four groups according to different times. Core species shifted from rare to dominant taxa with induction time, and growing species interactions in the network indicate a gradual complication of fungal community structure. Endophytic fungi diversity and potential functions were closely related to physicochemical indices that had less effect on the relative abundance of the dominant species. These findings help assess the regulatory mechanisms of microorganisms that expedite agarwood formation after artificial induction.

Multiple facets of global change affect the earth system interactively, with complex consequences for ecosystem functioning and stability. Simultaneous climate and biodiversity change are of particular concern, because biodiversity may contribute to ecosystem resistance and resilience and may mitigate climate change impacts. Yet, the extent and generality of how climate and biodiversity change interact remain insufficiently understood, especially for the decomposition of organic matter, a major determinant of the biosphere-atmosphere carbon feedbacks. With an inter-biome field experiment using large rainfall exclusion facilities, we tested how drought, a common prediction of climate change models for many parts of the world, and biodiversity in the decomposer system drive decomposition in forest ecosystems interactively. Decomposing leaf litter lost less carbon (C) and especially nitrogen (N) in five different forest biomes following partial rainfall exclusion compared to conditions without rainfall exclusion. An increasing complexity of the decomposer community alleviated drought effects, with full compensation when large-bodied invertebrates were present. Leaf litter mixing increased diversity effects, with increasing litter species richness, which contributed to counteracting drought effects on C and N loss, although to a much smaller degree than decomposer community complexity. Our results show at a relevant spatial scale covering distinct climate zones that both, the diversity of decomposer communities and plant litter in forest floors have a strong potential to mitigate drought effects on C and N dynamics during decomposition. Preserving biodiversity at multiple trophic levels contributes to ecosystem resistance and appears critical to maintain ecosystem processes under ongoing climate change.

BackgroundThe desert shrub Nitraria tangutorum Bobr. is important for its resistance to salt and alkali in Northwest China. It is an ecologically important species in this region and provides edible and medicinal berries. This study showed a mutant of N. tangutorum (named Jincan, JC) that has a strong yellow pericarp vs red in a wild type (represented by NT).ResultsIn this study, the secondary metabolic and molecular mechanisms responsible for Nitraria fruit coloration were investigated using LC–MS-based widely targeted metabolomics and transcriptomics data. As a result of our study, 122 and 104 flavonoid metabolites were differentially expressed throughout the mature and transition stages between JC and NT, respectively. Furthermore, two cyanidin derivatives (cyanidin 3-O-glucoside and cyanidin-3-O-(2''-O-glucosyl) glucoside) and one pelargonidin derivative (pelargonidin-3-O-glucoside) were identified only in the NT phenotype. The functional genes F3H (flavanone 3-hydroxylase), F3'H (flavonoid 3′-hydroxylase) and UFGT (flavonoid 3-O-glucosyltransferase) and the transcription factors MYB, bHLH, NAC and bZIP were significantly downregulated in JC. Meanwhile, the activity of UFGT was extremely low in both periods of JC, with a five-fold higher enzymatic activity of UFGT in RT than in YT. In summary, due to the lack of catalysis of UGFT, yellow fruit of JC could not accumulate sufficient cyanidin and pelargonidin derivatives during fruit ripening.ConclusionTaken together, our data provide insights into the mechanism for the regulation of anthocyanin synthesis and N. tangutorum fruit coloration and provide a theoretical basis to develop new strategies for developing bioactive compounds from N. tangutorum fruits.

Introducing N2 -fixing tree species into Eucalyptus plantations could replace nitrogen (N) fertilization to maintain high levels of N consumption and productivity. However, N enrichment may exacerbate phosphorus (P) limitation as Eucalyptus robusta Smith is extensively planted in P-poor tropical and subtropical soils. We conducted a field experiment in a pure plantation of Eucalyptus urophylla × grandis to investigate the impacts of N fertilization and introduced an N2 -fixing tree of Dalbergia odorifera T. Chen on soil P transformation. Nitrogen fertilization significantly enhanced soil occluded P pool and reduced the other P pools due to acidification-induced pH-sensitive geochemical processes, lowering Eucalyptus leaf P concentration with higher N:P ratio. By contrast, introduced N2 -fixing tree species did not change soil pH, labile inorganic P pool, and Eucalyptus leaf N:P ratio, even enhanced organic P pools and reduced occluded P pool probably due to altering microbial community composition particularly stimulating arbuscular mycorrhiza fungal abundance. Our results revealed differential responses and mechanistic controls of soil P transformation in Eucalyptus plantations with N fertilization and introduced N2 -fixing tree species. The dissolution of occluded P pool along with organic P accumulation observed in the mixed plantations may represent a promising future to better manage soil P availability.

BackgroundNitraria sibirica Pall. is a halophytic shrub with strong environmental adaptability that can survive in extremely saline-alkali and drought-impacted environments. Gene expression analysis aids in the exploration of the molecular mechanisms of plant responses to abiotic stresses. RT–qPCR is the most common technique for studying gene expression. Stable reference genes are a prerequisite for obtaining accurate target gene expression results in RT–qPCR analysis.ResultsIn this study, a total of 10 candidate reference genes were selected from the transcriptome of N. sibirica, and their expression stability in leaves and roots under different treatment conditions (salt, alkali, drought, cold, heat and ABA) was evaluated with the geNorm, NormFinder, BestKeeper, comparative ΔCt and RefFinder programs. The results showed that the expression stability of the candidate reference genes was dependent on the tissue and experimental conditions tested. ACT7 combined with R3H, GAPDH, TUB or His were the most stable reference genes in the salt- or alkali-treated leaves, salt-treated roots and drought-treated roots, respectively; R3H and GAPDH were the most suitable combination for drought-treated leaves, heat-treated root samples and ABA-treated leaves; DIM1 and His maintained stable expression in roots under alkali stress; and TUB combined with R3H was stable in ABA-treated roots. TBCB and GAPDH exhibited stable expression in heat-treated leaves; TBCB, R3H, and ERF3A were stable in cold-treated leaves; and the three most stable reference genes for cold-treated roots were TBCB, ACT11 and DIM1. The reliability of the selected reference genes was further confirmed by evaluating the expression patterns of the NsP5CS gene under the six treatment conditions.ConclusionThis study provides a theoretical reference for N. sibirica gene expression standardization and quantification under various abiotic stress conditions and will help to reveal the molecular mechanisms that confer stress tolerance to N. sibirica.

We investigated the effects of thinning intensity on the carbon allocation of Cunninghamia lanceolata Lamb. Hook by analyzing the stand growth and carbon content of a plantation under three thinning intensities (I: 70%; II: 50%; III: 30%) and with no thinning treatment. Using the carbon balance framework of the CROwn BASe (CROBAS) model and multi-source inventory data, we calibrated the parameters of the CROBAS-C. lanceolata (CROBAS-CL) model to simulate the carbon content in the plantation. We validated the CROBAS-CL model by comparing the predicted stand diameter at breast height (DBH) and stand height (H) with the measured values. Finally, the predicted stand carbon was compared with the soil carbon to assess the dynamics and allocation of ecosystem carbon content. Overall, our findings suggest that the predicted stand carbon of CROBAS-CL satisfies the statistical test requirements: the deviation of height and DBH predicted by the CROBAS-CL model from the measured height and DBH are less than 0.087 m and 0.165 cm, respectively. These results confirm that the model is useful for a dynamic prediction of stand carbon in C. lanceolata plantations. Based on the results of the proposed model, we determine that Thinning III (30% thinning intensity) is beneficial for the growth of C. lanceolata plantations and improving soil carbon sequestration. Additionally, the simulated carbon storage of an individual tree in the C. lanceolata plantation gradually increased with the tree age. Our study provides a strong reference for the efficient operation and management of C. lanceolata plantations in southwestern China.