Aims Our objective was to examine the underline mechanisms on the driving factors of eco-hydrological processes and identify the limiting factors through both path analysis and piecewise regression. Methods The eddy covariance and meteorological data of a plantation in Chongling watershed in Northern China over the period from August 2012 to August 2013 were used for analyzing the relationships between flux indices and environmental factors. The flux indices include sensible heat, latent heat, net ecosystem production, gross ecosystem production, and ecosystem respiration, and the environmental factors include soil water content, vapor pressure deficit, air temperature, soil temperature, net radiation and photosynthetically active radiation. The direct and indirect effects of dominant and secondary factors were determined through the path analysis, and the control of secondary factors on dominant factors were analyzed using the piecewise regression. Important findings We found that the primary factor affecting sensible heat and water use efficiency was vapor pressure deficit, while latent heat and carbon fluxes were mainly controlled by radiation and temperature respectively. There also appeared significant influences from secondary variables on those fluxes. The correlations between latent heat and net radiation, ecosystem respiration and soil temperature, and water use efficiency and vapor pressure deficit were all strong when soil water content was between 0.20 m³·m^-3 and 0.35 m³·m^-3. The correlations between ecosystem production (both gross and net) and photosynthetically active radiation was strong when vapor pressure deficit was ≤1.0 kPa.

Aims The objective of the study was to evaluate the dynamics of recalcitrant components during foliar litter decomposition at different forest gap size in Pinus massoniana plantation in the low hilly land, Sichuan basin. Methods The experiment was set up by thinning P. massoniana plantation to establish seven different gap sizes (G1: 100 m², G2: 225 m², G3: 400 m², G4: 625 m², G5: 900 m², G6: 1225 m², G7: 1600 m²). The contents of four recalcitrant components (condensed tannins, total phenol, lignin, cellulose) in foliar litter of two native species (Cinnamomum camphora and Toona ciliata) placed in litterbags at different locations in the forest gaps were evaluated. The litterbags placed under closed canopy were used as the control. Litterbags with air-dried leaves of C. camphora and T. ciliata were placed at center, edge of the gap and under the closed canopy in November 2013, and collected in December 2013, February 2014, May 2014 and August of 2014 for lab analysis. Important findings The results showed that: 1) Forest gap size had significant effect on the content of condensed tannins, total phenol and lignin for T. ciliata in gap center. However, the forest gap size had no significant effect on the content of recalcitrant components in the litters of C. camphora and cellulose content of T. ciliata. With the increase of gap size, except for cellulose content, the other three recalcitrant components content in small and medium sized gaps (G1-G5) were significant lower than in large gaps (G6, G7). 2) The condensed tannis content of T. ciliata at the gap center were significant lower than at the gap edge. The lignin contents at gap center of G3 was significant reduced in the C. camphora litter. The condensed tannins, total phenol, and lignin contents of T. ciliata litter in small and medium gaps significantly decreased. 3) The contents of the four recalcitrant components in both species’ foliar litter changed with time. The contents of condensed tannins and cellulose decreased and the content of lignin increased significantly with time, however, the total phenol content increased initially, and then decreased. Therefore, small and medium sized gaps (100-900 m²) could be the optimal gap sizes to promote the degradation of litter recalcitrant components for two native species in P. massoniana plantations.

Aims Soil respiration (R_s) is the largest fraction of carbon flux in forest ecosystems, but the effects of forest understory removal on R_s in Chinese fir (Cunninghamia lanceolate) plantations is poorly understood. In order to quantify the effects of forest understory removal on R_s and microbial community composition, a field experiment was conducted in a subtropical Chinese fir plantation. Methods Forest understory was removed manually in June 2012. R_s was measured monthly using a LI-COR 8100 infrared gas analyzer from July 2012 through July 2014. Soil temperature and moisture were also measured at 5 cm depth at the time of R_s measurements. Surface soil (0-10 cm) samples were collected in July 2013 and 2014, respectively, and the soil microbial community structures were determined by phospholipid fatty acids (PLFAs) analysis. Important findings R_s decreased by 32.8% over a two-year period following understory removal (UR), with a greater rate of decrease in the first year (42.9%) than in the second year (22.2%). The temperature sensitivity of R_s was affected by UR, and was 2.10 and 1.87 in the control and UR plots, respectively. UR significantly reduced the concentration of fungal PLFAs by 18.3%, but did not affect the concentration of bacterial PLFAs, resulting in an increase in the fungal:bacterial ratio; it significantly increased the concentration of gram-positive bacterial PLFAs by 24.5%, and the ratio of gram-positive to gram-negative bacterial PLFAs after one year of treatment, but decreased the concentration of gram-positive bacterial PLFAs by 9.4% and the ratio of gram-positive to gram-negative bacterial PLFAs after two years of treatment. The results suggested that R_s and microbial community composition were both affected by UR in Chinese fir plantation, and the effects were dependent of the duration following the UR treatment.

Aims The micro-elemental stoichiometry as well as nitrogen (N) and phosphorus (P) plays an important role in ecosystem process. However, the drivers of the variations in these stoichiometric ratios in plants are less explored in compared with N and P. Plant productivity and plant stoichiometry can response simultaneously to environmental changes, such as water and nutrient supply levels. However, the relationships between the changes in plant stoichiometry and biomass were unclear yet although both of them play important roles in ecosystem functioning. Our object was to investigate the changes in plant stoichiometry (including multiple macro- and micro-elements) and in biomass under different nutrient and water supply. Methods We collected seeds from six grass species in an arid-hot valley and performed a nutrient-water addition experiment in 2012 with a complete factorial design (nutrient × water). The concentrations of N, P, K, Ca, Mg, Zn and Mn in different organs and plant biomass were measured. The effects of species, water and nutrient on element concentration and plant biomass were analyzed by three-way ANOVA. Linear regressions were used to test the relationships between changes in plant stoichiometry and changes in biomass after nutrient and water addition. Important findings Nutrient addition increased plant biomass by 32.55% compared with control. High-level water supply increased plant biomass by 31.35% and the combination of nutrient and high-level water addition increased plant biomass by 110.60%. Nutrient, water, species identity and their two-way interactions significantly affected plant biomass. Changes in total plant K:Ca, K:Mg, K:Mn, K:Zn and Mg:Mn were significantly and positively related to changes in plant biomass. The ratio between the concentrations of macro-elements and micro-elements tended to increase with biomass. Species identity and treatment had no effects in most of these relationships, suggesting that the changes in stoichiometry were mostly driven by the variations in biomass. The relationships between changes in stoichiometry and in biomass also occurred in leaves, stems and roots. The covariation between plant stoichiometry and biomass can have profound effects on ecosystem functioning under the global environmental changes.

Aims Fractal root system is phenotypic plasticity result of plant root architecture to respond to environmental heterogeneity, may reflect the growth strategy of plants to adapt to environmental conditions. Our objective was to explore the relationship between root fractal dimension and fractal abundance of fractal root system of Melica przewalskyi population in response to aspect variation in the northwest of China. Methods The study site was located in a degraded alpine grassland on the northern slope in Qilian Mountains, Gansu Province, China. Survey and sampling were carried out at 40 plots which were set up along four slope aspects transects with 20 m distance between adjacent plots. Handheld GPS was used to determine the elevation, longitude and latitude of each plot. ArcGIS was used to set up digital elevation model (DEM). Community traits were investigated and six individuals roots of M. przewalskyi were collected randomly at each plot. The samples were cleaned and divided into different organs, then scanning the root with the Win-RHIZO for measurements of fractal dimension and fractal abundance in laboratory, and their biomass were then measured after being dried at 80 °C in an oven. Important findings With the slope aspect turned from north to east, west, and south, the density, height and soil moisture content of the plant community displayed a pattern of initial decline, the height, density, root fractal abundance of M. przewalskyi increased and the root fractal dimension decreased. The root fractal dimension was negatively associated with the fractal abundance in all aspects, but the relationship varied along the slope aspects gradient; there was a highly significant negative correlation (p < 0.01) between the root fractal dimension and fractal abundance at north slope and south slope aspect, whereas the correlation only reached a significant level (p < 0.05) at the east slope aspect and west slope aspect; indicating that there is a trade-off between the root fractal dimension and fractal abundance. In addition, when the slope aspect changed from north to east, west and south, the standardized major axis (SMA) slope of the regression equation in the scaling relationships between root fractal dimension and fractal abundance increased (p < 0.05), indicating that the roots of M. przewalskyi at the droughty southern slope have less branch and more sparse in the same soil volume of root exploitation and utilization. Consequently, the resource allocation pattern on reasonable trade-off between root fractal dimension and fractal abundance in different slope aspect of M. przewalskyi, reflects the relationship between the income and the cost of construction of plant root architecture.

Aims Irrigation and fertilization have great potentials to enhance yield in forest plantations. The integrated effect of water and nitrogen management on fine roots morphology and distribution of Populus × euramericana ‘Guariento’, however, remains unclear. The objective of this study was to evaluate the effect of water and nitrogen addition on fine root morphology and distribution in poplar plantations for developing the best water and nitrogen strategy for promoting fine root.
Methods The soil core method was used to quantify the morphology and distribution of fine roots in the 0–60 cm in a poplar plantation with surface dripping irrigation and fertilization technologies. The experiment included nine treatments, which were a combination of three irrigation treatments where dripping irrigation was applied when soil water potential (ψ_soil) reached –75, –50, or –25 kPa, and three fertilization treatments at nitrogen additions of 150, 300, or 450 g·tree–1·a–1, respectively). A control plot with non-irrigation and non-fertilization treatment in growing season (CK) was also included in the study.
Important findings The fine roots biomass density, fine root surface area density, average root diameter in all treatments were mainly found at 0–10 cm and 10–20 cm depths, with root biomass density in the 10–20 cm of 1.03 to 1.21 times of that in the 0–10 cm, 1.25 to 1.80 times of that in the 20–30 cm, 1.62 to 22.10 times of that in the 30–40 cm, 2.77 to 54.35 times of that in the 40–50 cm, and 6.48 to 293.09 times of that in the 50–60 cm. The root biomass density in the 10–20 cm accounted for 27%–37% of the total biomass density in the top 60 cm. For root biomass density and average diameter, there were no significant differences between 0–10 cm and 10–20 cm depths, and between 40–50 cm and 50–60 cm depths. Fine roots in the irrigation and fertilization treatments were significantly higher than that of the CK, except the D1F1 treatment (i.e., with low water and low nitrogen level). Additionally, fine roots in the D2F3 treatment (i.e., with intermedia irrigation and high nitrogen level) and the D3F3 treatment (i.e., with high water and high nitrogen level) were significantly higher than those in other treatments, but not significantly different between D2F3 and D3F3. Compared with the CK, the fine roots biomass density in six soil layers were significantly enhanced at 359%, 388%, 328%, 3823%, 4774% and 2866%, respectively, for the treatment with high water and high nitrogen levels. The vertical distributions of fine roots appeared not affected by the interaction of irrigation and nitrogen addition. However, the surface dripping irrigation and fertilization treatments increased fine roots significantly. Finally, we found that the response of fine root growth and distribution was stronger to fertilization than to the irrigation in this poplar plantation.

Xylem cavitation/embolism is the blockage of xylem conduits when woody plants suffer from water stress under drought and other environmental conditions, the study of embolism has become a hot and key topic under global climate change. Recent researches on the relationship between the vulnerability of xylem embolism and hydraulic architecture/drought tolerance have made some progress, however, scholars reached different conclusions based on results from different regions or different materials. This paper reviews the current achievements and controversial viewpoints, which includes indicator of xylem embolism vulnerability (P₅₀), method of vulnerability curve establishment, the relationship between embolism vulnerability and hydraulic architecture (vessel diameter, vessel length, pit area, wood density, fiber and fiber tracheid) and the relationship between embolism vulnerability and drought tolerance of woody plants. Future studies should use Cochard Cavitron centrifuge and Sperry centrifuge coupled with traditional methods to establish vulnerability curves, calculate P₅₀, analyze the difference among different organisms (root, stem, leaf), and measure physiological and ecological indexes. Future studies should be aimed to explore the relationship between the vulnerability of xylem embolism and hydraulic architecture/drought tolerance and to assess drought tolerance ability of different species under future climate change.