Search

Transpiration is a significant part of water cycle in forest ecosystems, influenced by meteorological factors and potentially constrained by soil moisture. We used Granier-type thermal dissipation probes to monitor xylem sap flow dynamics of three tree species ( Quercus liaotungensis , Platycladus orientalis , and Robinia pseudoacacia ) in a semi-arid loess hilly region, and to continuously monitor the key meteorological factors and soil water content (SWC). We established the SWC thresholds delineating soil moisture-limited and -unlimited sap flow responses to transpiration drivers. The results showed that mean sap flux density ( J s ) of Q. liaotungensis and R. pseudoacacia was significantly higher during period with higher soil moisture compared to lower soil moisture, while the difference in J s for P. orientalis between the two periods was not significant. We used an exponential saturation function to fit the relationship between the J s of each tree species and the integrated transpiration variable (VT) which reflected solar radiation and vapor pressure deficit. The difference in the fitting curve parameters indicated that there were distinct response patterns between J s and VT under different soil moisture conditions. There was a threshold in soil moisture limitation on sap flow for each species, which was identified as 0.129 m 3 ·m -3 for Q. liaotungensis , 0.116 m 3 ·m -3 for P. orientalis , and 0.108 m 3 ·m -3 for R. pseudoacacia . Below the thresholds, J s was limited by soil moisture. Above these points, the normalized sensitivity index (NSI) for Q. liaotungensis and P. orientalis reached saturation, while that of R. pseudoacacia did not reach saturation but exhibited a significant reduction in moisture limitation. Among the three species, P. orientalis was the most capable of overcoming soil moisture constraints.

Nitrogen (N) deposition in the context of human activities continuously affects the carbon cycle of ecosystems. The effect of N deposition on soil organic carbon is related to the differential responses of different carbon fractions. To investigate the changes in soil organic carbon fraction and its influencing factors in the context of short-term N deposition, four N addition gradients:0 (CK), 1.5 (N1), 3 (N2), and 6 (N3) g·(m 2 ·a) -1 were set up in acacia plantations based on field N addition experiments, and the soil physicochemical properties, microbial biomass, and enzyme activities were measured in June and September. The results showed that:① exogenous N input reduced soil pH, promoted the increase in soluble organic carbon content, and increased soil nitrogen effectiveness. ② Short-term N addition significantly reduced soil organic carbon content, and the response of each component of organic carbon to N addition was different. Among them, the content of easily oxidized organic carbon was significantly reduced and reached the lowest value under the N2 treatment, with 54.4% and 48.2% reduction compared with that of the control, respectively, and the content of inert organic carbon increased, although the increase was not significant. Nitrogen addition reduced the soil carbon pool activity and improved the stability of the soil carbon pool. Soil carbon pool activity reached its lowest under the N3 and N2 treatments, with a decrease of 53.3% and 52.80%, respectively, compared to that of the control. ③Random forest modeling indicated that the soil microbial biomass stoichiometry ratio, microbial biomass carbon, and AP were the key factors driving the changes in soil organic carbon activity under short-term N addition, explaining 65.96% and 66.68% of the changes in oxidizable organic carbon and inert organic carbon, respectively. Structural equation modeling validated the results of the random forest modeling, and soil microbial biomass stoichiometric ratios significantly influenced carbon pool activity. Short-term nitrogen addition changed soil microbial biomass and its stoichiometric ratio in the acacia plantation forest mainly through two pathways, i.e., increasing soil nitrogen effectiveness and promoting soil acidification and inhibiting extracellular carbon hydrolase activity, thus changing the soil carbon fraction ratio and participating in the soil organic carbon cycling process.

The 9-, 17-, 30- and 37-year-old Robinia pseudoacacia plantations in the loess hilly region were investigated to study the dynamics and allocation patterns of carbon and nitrogen storage. The results showed that the ranges of carbon and nitrogen contents were 435.9-493.4 g x kg(-1) and 6.8-21.0 g x kg(-1) in the arbor layer, 396.3-459.2 g x kg(-1) and 14.2-23.5 g x kg(-1) in the herb and litter layer, and 2.7-10.7 g x kg(-1) and 0.2-0.7 g x kg(-1) in the soil layer, respectively. The branch was the major carbon and nitrogen pool in the arbor layer, accounting for 46.9%-63.3% and 39.3%-57.8%, respectively. The maximum storage values were 30.1 and 1.8 Mg x hm(-2) for carbon and nitrogen, respectively, in the 0-20 cm soil layer in the 37-year-old R. pseudoacacia plantation. The total carbon and nitrogen storage in the R. pseudoacacia plantation ecosystem increased with increasing forest age, and the maximum values were 127.9 Mg x hm(-2) and 6512.8 kg x hm(-2) for carbon and nitrogen storage, respectively, in the 37-year-old R. pseudoacacia plantation. Soil layer was the major carbon and nitrogen pool of R. pseudoacacia plantation ecosystem, accounting for 63.3%-83.3% and 80.3%-91.4%, respectively.

Using indigenous tree species to transform large area pure plantations has been an effective close-to-nature forest management mode in China islands. By using the biomass allometric equation and combined with the investigation data from sampling plots, the carbon storage of the dominant species Pinus thunbergii and Robinia pseudoacacia in the plantations on the Nanchangshan Island of Miaodao Archipelago in Changdao County of Shandong Province was estimated. The average carbon storage in the arbor layer of P. thunbergii and R. pseudoacacia plantations was 56.81 and 37.26 t x hm(-2), respectively, being higher than the average carbon stock (27.62 t x hm(-2)) of tree plantations in Shandong Province. Slope aspect and stand density were the important environmental and biological factors affecting the tree's carbon stock in the Island, respectively. There was no significant correlation between the average cumulative rate of biomass in the arbor layer of P. thunbergi plantation and the stand age. The P. thunbergii with good carbon sequestration function could be an ideal tree species on Nanchangshan Island.

Taking the pot-cultured seedlings of four leguminous tree species (Albizia julibrissin, Robinia pseudoacacia, Sophora japonica, and Gleditsia sinensis) as test materials, this paper studied their growth indices, critical salt concentration (C50), and K+ and Na+ allocation under different levels of NaCl stress, aimed to understand the difference of test tree species in salt tolerance. NaCl stress inhibited the seedling growth of the tree species. Under NaCl stress, the dry matter accumulation decreased, while the root/shoot ratio increased, especially for A. julibrissin and G. sinensis. Quadratic regression analysis showed that the C50 of A. julibrissin, R. pseudoacacia, S. japonica, and G. sinensis was 3.0 per thousand, 5.0 per thousand, 4.5 per thousand, and 3.9 per thousand, respectively, i.e., the salt tolerance of the four tree species was in the order of R. pseudoacacia > S. japonica > G. sinensis > A. julibrissin. In the root, stem, and leaf of the four tree species seedlings, the Na+ content increased with the increase of NaCl stress, while the K+ content (except in the root of A. julibrissin) decreased after an initial increase, resulting in a larger difference in the K+/Na+ ratio in the organs. Under the same NaCl stress, the allocation of Na+ in different organs of the four tree species seedlings decreased in the order of root>stem>leaf, while that of K+ differed with tree species and NaCl stress, and leaf was the main storage organ for K+. The K+/Na+ ratio in different organs decreased in the sequence of leaf>stem>root. R. pseudoacacia under NaCl stress accumulated more K+ and less Na+ in stem and leaf, and had higher K+/Na+ ratio in all organs and higher dry mass, being assessed to be more salt-tolerant. In contrast, A. julibrissin under high NaCl stress accumulated more Na+ in stem and leaf, and had a lower K+/Na+ ratio in all organs and lower dry mass, being evaluated to be lesser salt-tolerant. The K+ accumulation in seedling stem and leaf and the Na+ retention in seedling root could be the main reasons for the salt tolerance of leguminous tree species under NaCl stress.

A field experiment was conducted to study the leaf N, P, and chlorophyll contents, photosynthetic gas exchange parameters, and chlorophyll fluorescence parameters in pure Ulmus pumila forest, pure Robinia pseudoacacia forest, and U. pumila-R. pseudoacacia mixed forests [1:1 (1B1C), 1:2 (1B2C), and 2:1 (2B1C)] in different growth periods. From May to September, the plant leaf N and P contents in pure and mixed forests all presented a decreasing trend. By the end of growth period, the leaf N content of U. pumila and the P content of R. pseudoacacia in 1B2C were obviously higher than those in pure forests. In the mixed forests, the chlorophyll content of U. pumila was obviously higher than that of R. pseudoacacia, and the chlorophyll content of U. pumila in 1B2C reached the highest in July. The photosynthetic rate (Pn) of U. pumila and R. pseudoacacia in mixed forests was higher than that in pure forests, and the Pn of R. pseudoacacia in 1B2C reached the highest (18.54 micromol x m(-2) x s(-1)) in July. The transpiration rate (Tr) and stomatal conductance (Gs) of R. pseudoacacia in mixed forests were higher than those in pure forests, and the Tr and Gs in mixed forests were in the order of 1B2C>1B1C>2B1C. In September, the quantum yield of PSII electron transport (phi(PS II)) of U. pumila in mixed forests was obviously higher than that in pure forest. The photochemical quenching coefficients (q(P)) of U. pumila and R. pseudoacacia in pure and mixed forests had no significant difference, but the non-photochemical quenching coefficient (NPQ) of the two tree species in 1B2C was significantly lower than that in corresponding pure forests. It was suggested that mixed planting U. pumila and R. pseudoacacia could significantly improve the leaf nutrient contents and photosynthetic capacity of the two tree species, and the optimum mixed ratio of U. pumila and R. pseudoacacia was 1:2.

From April 14 to October 10, 2009, the dynamic changes of Robinia pseudoacacia sap flow in the hilly-gully region of Loess Plateau were measured with thermal dissipation probe. Meantime, the photosynthetic active radiation (PAR), air temperature, relative air humidity, water vapor pressure deficit (VPD), wind speed, and rainfall were monitored. The diurnal variation of the sap flow velocity (SFV) displayed a single-peak curve, but the pattern varied among different months. In April, the sap flow started to increase at about 12:00, and reached the peak at about 18:00. From May to August, the sap flow started to increase at 5:30-7:30, and reached the peak at about 15:00. In September and October, the sap flow started to increase at about 8:00, and reached the peak at 11:30-13:00. The monthly average SFV was the highest in August and the lowest in April, with the value being 0.002610 cm x s(-1) and 0.000549 cm x s(-1), respectively. During the monitoring period, the sap flow velocity was significantly correlated with PAR, air temperature, VPD, wind speed, and air relative humidity, and the correlation coefficients declined in the order of air temperature > VPD > PAR > relative humidity > wind speed. The sap flow velocity could be estimated by the linear equation with variables PAR and VPD, and the regression coefficients were highly significant.

In order to understand the carbon sequestration of ecological forests in Loess Plateau, a comparative study was made on the organic carbon density (OCD) of soil, litter, and plant organs in an 8-year-old Robinia pseudoacacia plantation and nearby barren land. Comparing with the barren land, the young R. pseudoacacia plantation had a decrease (0.26 kg x m(-2)) of soil OCD, but the OCD in its litter, root system, and aboveground organs increased by 121.1%, 202.0%, and 656. 7%, respectively, with a total carbon sequestration increased by 3.3% annually, which illustrated that R. pseudoacacia afforestation on Loess Plateau had an obvious positive effect on carbon sequestration.

Aimed to explore the effects of different vegetations and of the years of vegetation restoration on soil microbial biomass carbon and nitrogen, a comparative study was conducted, with the 5 year old Robinia pseudoacacia, Hippophae reamnoide and Prunus armeniaca plantations and the 5, 15 and 25 years old R. pseudoacacia plantation in the Yangjuangou catchment of Yanan City of Shaanxi Province, a typical hilly area of the Loess Plateau, as test objects. The results showed that among the three 5-year old plantations, H. reamnoides plantation had the highest soil organic carbon (SOC) and total nitrogen (TN) contents, while R. pseudoacacia plantation had the highest soil microbial biomass carbon (MBC) (99.56 mg x kg(-1)) and nitrogen (MBN) (28.81 mg x kg(-1)). The MBC was in the order of R. pseudoacacia > H. reamnoides > P. armeniaca, and that of MBN was of R. pseudoacacia > P. armeniaca > H. reamnoides. The MBC/SOC was in the order of R. pseudoacacia > H. reamnoides > P. armeniaca, and that of MBN/TN was of R. pseudoacacia > P. armeniaca > H. reamnoides, with the differences being significant (P < 0.05). With the increasing years of vegetation restoration, the soil pH in R. pseudoacacia plantation decreased, while the SOC, TN, electricity conductance (EC), MBC, and MBN all had an increasing trend, which illustrated that in the hilly area of Loess Plateau, planting R. pseudoacacia was more beneficial to the increase of soil MBC and MBN, and, with the increasing years of this planting, soil MBC, MBN, SOC and TN tended to be increasing.

The study on the root distribution characters of Robinia pseudoacacia and soil moisture status in the Loess Plateau showed that on shady slope, there was no obvious difference in the horizontal distribution of R. pseudoacacia fine roots within 2.0 m from the stem, and the maximal distribution depth of the roots could reach 2.0 m. In sunny slope, the vertical distribution of the roots within 0.5 m from the stem could also reach that depth. The root amount in shady slope was larger than that in sunny slope, and the affected depth of R. pseudoacacia roots on soil moisture status reached 3.3 m in shady slope, while 2.7 m in sunny slope.

With locust (Robinia pseudoacacia), the main tree species in afforestation, as test material, this paper studied the characteristics of its water physiological ecology and productivity under four habitats, i. e. , sunny, shady, semi-sunny and semi-shady hillsides, in the North Shaanxi gully areas of Loess Plateau. The mean water content in 0 - 500 cm soil layer was 8.87% (shady), 8.06% (semi-shady), 7.62% (semi-sunny), and 6.96% (sunny), respectively. There was a significant discrepancy between shady, semi-shady, semi-sunny and sunnyhilisides (a 0.01), and the difference between shady, semi-shady and semi-sunny, as well as between semi-sunny and sunny hillsides was also significant (a 0.05) . Remarkable relationships were found between leaf RWC, locus WSD and soil water content (SWC) . The leaf RWC and locust water potential under sunny hillside were lower, but those under shady hillside were higher. The daily mean transpiration was in order of shady (4.07 microg x cm(-2) x s(-1)) > semi-shady (3.89 microg x cm(-2) x s(-1)) > semi-sunny (3.05 microg x cm(-2) x s(-1)) > sunny (2.70 microg x cm(-2)s(-1)) hillside. The remarkable difference of transpiration appeared at 11:00 and 13:00, and there existed a remarkable relationship between transpiration and light intensity, RH and soil water content. All of these resulted in a diversity of locust biomass under different habitats, the highest under shady hillside, and the lowest under sunny hillside, and the differences between different habitats were all significant. It could be concluded that soil water content was the main factor affecting locust growth.