Relationships between root exudation and root morphological and architectural traits vary with growing season.

Plants allocate a substantial amount of C belowground for root exudates and for the construction and adjustment of root morphological and architectural traits. What relationships exist between root exudates and other root traits and these relationships change with growing season, however, remain unclear. We quantified the root exudation rate and root morphological traits, including total root length (RL), total root surface area (RS), root diameter (RD), specific root length (SRL), specific root area (SRA) and root tissue density (RTD), and architectural traits, such as branching intensity (BI), and investigated their associations during the rapidly growing season (April and August) and the slowly growing season (December) of three common native tree species, Liquidambar formosana, Michelia maudiae and Schima superba, in subtropical China. We found that the linkages of RD, SRL, SRA, RTD and BI did not change with the growing season, reflecting their highly conservative relationships. The root exudation rate varied significantly with growing season (P < 0.05) and produced various associations with other root traits at different growing seasons. During the rapidly growing season (i.e., April), the exudation rate was the highest and was positively correlated with RL. The exudation rate was the lowest during the slowly growing season (i.e., December) and was negatively associated with RL, RS and RTD. Our findings demonstrate the seasonality of the linkages of root exudation rate with other root traits, which highlights the highly plastic and complex associations of belowground root traits. These findings help to deepen our understanding of plant nutrient acquisition strategies.

Exploring carbon sequestration in broad-leaved Korean pine forests: Insights into photosynthetic and respiratory processes.

A comprehensive understanding of carbon assimilation and sequestration in broad-leaved Korean pine forests is crucial for accurately estimating this significant aspect of temperate forests at a regional scale. In this study, we introduced a high-temporal resolution model designed for carbon assimilation insights at the plot scale, focusing on specific parameters such as leaf area dynamics, vertical leaf distribution, photosynthetically active radiation (PAR) fluctuations, and the photosynthetic traits of tree species. The findings reveal that most tree species in broad-leaved Korean pine forests exhibit an inverted U-shaped pattern in leaf area dynamics, with shorter leaf drop periods than leaf expansion events. Leaf distribution varies significantly among different canopy heights, with approximately 80 % of the leaves above 15 m. PAR decreases as canopy height decreases, with PAR at 25 m accounting for about 60 % of the PAR above the canopy. Our framework incorporates a leaf-scale light-response curve and empirical photosynthesis-temperature relationships to estimate forest carbon assimilation on daily and hourly scales accurately. Using the model, we assess the gross primary productivity (GPP), leaf net photosynthetic assimilation (LNPA), and carbon increment (ΔC) of broad-leaved Korean pine forests from 2017 to 2020. The results demonstrate GPP, LNPA, and ΔC values of 21.4 t·ha-1·a-1, 17.4 t·ha-1·a-1, and 4.0 t·ha-1·a-1, respectively. Regarding efficiency, GPP, LNPA, and ΔC per square meter of leaf per year are 179 g, 146 g, and 33 g, respectively. Notably, tree species in the canopy layer of the forest exhibit significantly higher efficiency than those in the understory layer. This research significantly contributes to our understanding of carbon cycling and the responses of forest ecosystems to climate change. Moreover, it provides a practical tool for forest management and the development of carbon sequestration strategies.

Plant-soil feedback regulates the trade-off between phosphorus acquisition pathways in Pinus elliottii.

Plant-soil feedback (PSF) is conventionally characterized by plant biomass growth, yet it remains unclear how PSF affects plant nutrient acquisition strategies (e.g., nutrient absorption and nutrient resorption) associated with plant growth, particularly under changing soil environments. A greenhouse experiment was performed with seedlings of Pinus elliottii Englem and conditioned soils of monoculture plantations (P. elliottii and Cunninghamia lanceolata Hook). Soil sterilization was designed to test plant phosphorus (P) acquisition strategy with and without native soil fungal communities. Soils from P. elliottii and C. lanceolata plantations were used to explore the specific soil legacy effects on two different P acquisition pathways (absorption and resorption). Phosphorus addition was also applied to examine the separate and combined effects of soil abiotic factors and soil fungal factors on P acquisition pathways. Due to diminished mycorrhizal symbiosis, PSF prompted plants to increasingly rely on P resorption under soil sterilization. In contrast, P absorption was employed preferentially in the heterospecific soil, where species-specific pathogenic fungi could not affect P absorption. Higher soil P availability diluted the effects of soil fungal factors on the trade-off between the two P acquisition pathways in terms of the absolute PSF. Moreover, P addition plays a limited role in terms of the relative PSF and does not affect the direction and strength of relative PSF. Our results reveal the role of PSF in regulating plant P acquisition pathways and highlight the interaction between mycorrhizal and pathogenic fungi as the underlying mechanism of PSF.

Complementary belowground strategies underlie species coexistence in an early successional forest.

Unravelling belowground strategies is critical for understanding species coexistence and successional dynamics; yet, our knowledge of nutrient acquisition strategies of forest species at different successional stages remains limited. We measured morphological (diameter, specific root length, and root tissue density), architectural (branching ratio), physiological (ammonium, nitrate, and glycine uptake rates) root traits, and mycorrhizal colonisation rates of eight coexisting woody species in an early successional plantation forest in subtropical China. By incorporating physiological uptake efficiency, we revealed a bi-dimensional root economics space comprising of an 'amount-efficiency' dimension represented by morphological and physiological traits, and a 'self-symbiosis' dimension dominated by architectural and mycorrhizal traits. The early pioneer species relied on root-fungal symbiosis, developing densely branched roots with high mycorrhizal colonisation rates for foraging mobile soil nitrate. The late pioneer species invested in roots themselves and allocated effort towards improving uptake efficiency of less-mobile ammonium. Within the root economics space, the covariation of axes with soil phosphorus availability also distinguished the strategy preference of the two successional groups. These results demonstrate the importance of incorporating physiological uptake efficiency into root economics space, and reveal a trade-off between expanding soil physical space exploration and improving physiological uptake efficiency for successional species coexistence in forests.

Understory plant removal counteracts tree thinning effect on soil respiration in a temperate forest.

Elucidating the response mechanism of soil respiration (Rs) to silvicultural practices is pivotal to evaluating the effects of management practices on soil carbon cycling in planted forest ecosystems. However, as common management practices, how thinning, understory plant removal, and their interactions affect Rs and its autotrophic and heterotrophic components (Ra and Rh) remains unclear. Therefore, we investigated Rs, Ra and Rh by the trenching method from 2011 to 2015 in a Pinus tabuliformis plantation in northern China, subjecting to four treatments (intact control plots [CK], thinning [T], understory removal [UR], and thinning with understory removal [TUR]). Mean annual Rs was significantly increased by thinning (by 15.3%), whereas decreased by UR (by 17.4%), compared with CK. These variations in Rs were mainly attributed to changes in Ra. The increments of Ra were caused by the enhanced growth of fine root biomass after thinning. However, UR led to lower Ra compared with CK (p < .05), indicating that understory growth is inadequate to compensate for the decreased respiring root biomass induced by understory removal. Rs was unchanged between TUR and the intact control plot due to the opposite effects of thinning and UR on the Ra. Changes in Rh exhibited no significant differences among the treatments, partly because of the stable microbial biomass carbon (MBC) and forest floor mass (litter and fine woody debris). No interaction effect between thinning and understory removal was detected on Rs, Ra, and Rh. The lowest temperature sensitivity (Q10 ) value of Ra was found in CK. This study highlights the necessity of incorporating understory plant effects on soil CO2 efflux in assessing forest management practices on soil carbon cycling.

Mycorrhizal symbiosis pathway and edaphic fertility frame root economics space among tree species.

The root economics space (RES) is multidimensional and largely shaped by belowground biotic and abiotic influences. However, how root-fungal symbioses and edaphic fertility drive this complexity remains unclear. Here, we measured absorptive root traits of 112 tree species in temperate and subtropical forests of China, including traits linked to functional differences between arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) hosts. Our data, from known mycorrhizal tree species, revealed a 'fungal-symbiosis' dimension distinguishing AM from ECM species. This divergence likely resulted from the contrasting mycorrhizal evolutionary development of AM vs ECM associations. Increased root tissue cortical space facilitates AM symbiosis, whereas increased root branching favours ECM symbiosis. Irrespective of mycorrhizal type, a 'root-lifespan' dimension reflecting aspects of root construction cost and defence was controlled by variation in specific root length and root tissue density, which was fully independent of root nitrogen content. Within this function-based RES, we observed a substantial covariation of axes with soil phosphorus and nitrate levels, highlighting the role played by these two axes in nutrient acquisition and conservation. Overall, our findings demonstrate the importance of evolved mycorrhizal symbiosis pathway and edaphic fertility in framing the RES, and provide theoretical and mechanistic insights into the complexity of root economics.

[Radial growth dynamics of Chinese fir and its response to seasonal drought]. / æ‰æœ¨å¾„å‘ç”Ÿé•¿åŠ¨æ€åŠå ¶å¯¹å­£èŠ‚æ€§å¹²æ—±çš„å“åº”.

We explored the seasonal dynamics and climate responses of radial variation of Chinese fir, we continuously monitored intra-annual stem radial size changes with the automated dendrometers in central Jiangxi Province in 2016 and 2017. We analyzed daily and seasonal dynamics of radial variations, and their Spearman correlations with climatic factors. We found that the diurnal radial variations had a pattern of contraction during the day and expansion at night. The growth onset in 2017 was one month earlier than that in 2016, whereas the continuous tree water deficit in the dry season caused the cessation one month early. During the main growing season (from April to September), the radial increment was significantly positively correlated with precipitation and relative humidity but negatively correlated with photosynthetically active radiation and vapor pressure deficit in both wet and dry seasons. However, the correlation for tree water deficit was opposite to stem radial increment. Severe water deficit significantly enhanced the impacts of soil water content on stem radial variation in the dry season. Moisture condition was always the key factor affecting stem radial variation of Chinese fir. Appropriate measures such as increasing soil water content could promote stem radial growth during summer drought.

Return Period Evaluation of the Largest Possible Earthquake Magnitudes in Mainland China Based on Extreme Value Theory.

The largest possible earthquake magnitude based on geographical characteristics for a selected return period is required in earthquake engineering, disaster management, and insurance. Ground-based observations combined with statistical analyses may offer new insights into earthquake prediction. In this study, to investigate the seismic characteristics of different geographical regions in detail, clustering was used to provide earthquake zoning for Mainland China based on the geographical features of earthquake events. In combination with geospatial methods, statistical extreme value models and the right-truncated Gutenberg-Richter model were used to analyze the earthquake magnitudes of Mainland China under both clustering and non-clustering. The results demonstrate that the right-truncated peaks-over-threshold model is the relatively optimal statistical model compared with classical extreme value theory models, the estimated return level of which is very close to that of the geographical-based right-truncated Gutenberg-Richter model. Such statistical models can provide a quantitative analysis of the probability of future earthquake risks in China, and geographical information can be integrated to locate the earthquake risk accurately.

[Response of radial growth of Pinus sylvestriformis and Picea jezoensis to climate warming in the ecotone of Changbai Mountain, Northeast China]. / 长白山群落交错带长白松和鱼鳞云杉径向生长对气候变暖的响应.

Changbai Mountain is a typical distribution area of temperate coniferous and broad-leaved mixed forests, with significant influence of global climate change. In order to understand the responses of forest ecosystem to climate change, we examined the responses of dominant arbor species in the community ecotone of broad-leaved Korean pine forest and spruce-fir forest (also known as dark coniferous forest), Pinus sylvestriformis and Picea jezoensis. The standard chronologies were established by obtaining tree ring width data in order to identity the key climatic factors that confine the radial growth of both species. The responses of P. sylvestriformis and P. jezoensis to climate factors were different.P. sylvestriformis was more sensitive than P. jezoensis, indicating that P. sylvestriformis was more suitable for dendroclimatological analysis. The radial growth of P. sylvestriformis was consistent with the increases of mean temperature, while the radial growth of P. jezoensis showed a "divergence problem" which decreased with the increases of mean temperature. The radial growth of P. sylvestriformis was mainly limited by temperature, especially the mean temperature in last July and August and current September. However, there was a negative correlation between standard chronologies of P. jezoensis and mean temperature in most months, which was limited by both temperature and precipitation. The correlation between radial growth of both species and climate factors after sudden temperature rise, was weaker than that before sudden temperature rise. The correlation between radial growth and climate factors changed from positive to negative in some months. Current temperature rise might not exceed the critical threshold of the radial growth of P. sylvestriformis, which could promote the radial growth. In addition, the wavelet analysis showed that the radial growth of trees in this area might be affected by large-scale coupling effects of atmospheric-ocean-land changes. In conclusion, climate warming was beneficial to the radial growth of P. sylvestriformis, while drought stress caused by warming was the main factor limiting the radial growth of P. jezoensis. If the global temperature continues to increase in the future, it will have an adverse impact on P. jezoensis. The results would help improve our understanding of the responses of radial growth of P. sylvestriformis and P. jezoensis to future climate change, and provide some basic data for climate reconstruction using both species.

[Soil enzyme activities and their stoichiometry of typical plantations in mid-subtropical China].

We measured the activities of six kinds of enzyme, including ß-glucosidase (BG), ß-N-acetyl-glucosaminidase (NAG), leucine aminopeptidase (LAP), acid phosphatase (AP), polyphenol oxidase (POX), peroxidase (POD), as well as enzyme stoichiometric ratios and soil physical and chemical properties at 0-10 and 10-20 cm layers across typical Pinus massoniana plantation, Pinus elliottii plantation and mixed plantation of P. massoniana and Schima superba (broadleaved-conifer mixed plantation) in mid-subtropical China. Key factors driving the variation in soil enzyme activity and stoichiometry among different stand types were investigated. The results showed that the activities of soil BG and LAP were significantly affected by stand type. Soil BG activity at 10-20 cm soil layer was significantly higher in P. elliottii plantation than in P. massoniana plantation, while the activity of LAP was highest in the P. massoniana plantation. Soil BG/(NAG+LAP) and BG/AP at 10-20 cm layer of P. elliottii plantation were significantly higher than those of P. massoniana plantation, while (NAG+LAP)/AP of P. massoniana plantation was significantly higher than those of P. elliottii plantation and mixed plantation. The vector length of enzyme stoichiometry at 10-20 cm soil layer was significantly different among stand type, with an order of P. elliottii plantation > broadleaved-conifer mixed plantation > P. massoniana. The vector angles of enzyme stoichiometry in the three plantations were greater than 45°, with the vector angle in the P. elliottii plantation at 10-20 cm soil layer being significantly greater than that of the P. massoniana plantation. Results from redundancy analysis showed that soil carbon quality index and the ratio of soil organic carbon to total phosphorus (C/P), soil water content and C/P were the key factors affecting soil enzyme activity and stoichiometry at 0-10 and 10-20 cm soil layers, respectively. The quantity and quality of soil carbon and phosphorus, and soil water content played a key role in regulating nutrient cycling in mid-subtropical plantation ecosystem.