Sex-specific competition differently regulates ecophysiological responses and phytoremediation of Populus cathayana under Pb stress

Lead (Pb) contamination seriously threatens agroforestry production and safety. We aim to determine the interactive influence of Pb and sexual competition on the growth performance, photosynthetic and biochemical traits, ultrastructure and phytoremediation-related parameters of males and females. In the present study, eco-physiological responses and phytoremediation traits of Populus cathayana females and males were evaluated under interactive treatments of Pb and competition. There were significant sex-specific competition effects on biomass partition, photosynthetic activities, carbohydrate contents, nitrogen and phosphorus use efficiencies, ultrastructure and phytoremediation under Pb stress. When competition within the same sex was compared, females were more sensitive to Pb stress, while males possessed greater Pb contents, and a higher bioconcentration factor and tolerance index. Under inter-sexual competition, males alleviated competition effects through greater Pb absorption, and lower photosynthetic rates, nutrient use efficiencies and biomass accumulation. Moreover, Pb stress altered competition intensities of both sexes. Sex-specific competition and neighbor effects may regulate responses and phytoremediation under heavy metal stress in dioecious plants. In the future, more attention should be paid on the effects of inter- and intra-sexual competition on dioecious species in the process of forestation and restoration of contaminated soil.Peer reviewe

Asymmetric pruning reveals how organ connectivity alters the functional balance between leaves and roots of Chinese fir

Following asymmetric pruning of leaves and/or roots, the functional balance of distribution of carbon, but not of nitrogen, in Cunninghamia lanceolata is more readily achieved for the roots and leaves on the same side of the pruning compared with those on the opposite side. Abstract The functional balance between leaves and roots is believed to be mediated by the specific location of shoots and roots, i.e. differences in transport distances and degrees of organ connectivity. However, it remains unknown whether the adaptive responses of trees to biomass removal depend on the relative orientation of leaf and root pruning. Here, we applied five pruning treatments to saplings of Cunninghamia lanceolata (Chinese fir) under field and glasshouse conditions, namely no pruning (control), half of lateral branches pruned, half of lateral roots pruned, half of the branches and roots pruned on the same side of the plant, and half of the branches and roots pruned on opposite sides of the plant. The effects of pruning on the growth, carbon storage and allocation, and physiology of leaves and fine roots on the same and opposite sides of the plant were investigated. Compared with the effect of root-pruning on leaves, fine roots were more limited by carbon availability and their physiological activity was more strongly reduced by shoot pruning, especially when branches on the same side of the plant were removed. Pruning of branches and roots on the opposite side of the plant resulted in the lowest carbon assimilation rates and growth among all treatments. The results of a stable-isotope labeling indicated that less C was distributed to fine roots from the leaves on the opposite side of the plant compared to those on the same side, but N allocation from roots to leaves depended less on the relative root and leaf orientation. The results collectively indicate that the functional responses of C. lanceolata to pruning are not only determined by the source-sink balance model but are also related to interactions between leaves and fine roots. We argue that the connectivity among lateral branches and roots depends on their relative orientation, which is therefore critical for the functional balance between leaves and fine roots.Peer reviewe

Nitrogen balance along a northern boreal forest fire chronosequence

Fire is a major natural disturbance factor in boreal forests, and the frequency of forest fires is predicted to increase due to climate change. Nitrogen (N) is a key determinant of carbon sequestration in boreal forests because the shortage of N limits tree growth. We studied changes in N pools and fluxes, and the overall N balance across a 155-year non stand replacing fire chronosequence in sub-arctic Pinus sylvestris forests in Finland. Two years after the fire, total ecosystem N pool was 622 kg ha(-1) of which 16% was in the vegetation, 8% in the dead biomass and 76% in the soil. 155 years after the fire, total N pool was 960 kg ha(-1), with 27% in the vegetation, 3% in the dead biomass and 69% in the soil. This implies an annual accumulation rate of 2.28 kg ha(-1) which was distributed equally between soil and biomass. The observed changes in N pools were consistent with the computed N balance +2.11 kg ha(-1) yr(-1) over the 155-year post-fire period. Nitrogen deposition was an important component of the N balance. The biological N fixation increased with succession and constituted 9% of the total N input during the 155 post-fire years. N2O fluxes were negligible (<0.01 kg ha(-1) yr(-1)) and did not differ among post-fire age classes. The number and intensity of microbial genes involved in N cycling were lower at the site 60 years after fire compared to the youngest and the oldest sites indicating potential differences in soil N cycling processes. The results suggest that in sub-arctic pine forests, the non-stand-replacing, intermediate severity fires decrease considerably N pools in biomass but changes in soil and total ecosystem N pools are slight. Current fire-return interval does not seem to pose a great threat to ecosystem productivity and N status in these sub-arctic forests.Peer reviewe

Nitrogen resorption in Acer platanoides and Acer saccharum : influence of light exposure and leaf pigmentation

We investigated the effects of leaf color change in the fall on photosynthetic production and nitrogen resorption. Seedlings of Acer platanoides L. and A. saccharum Marsh. were grown in a shade house for 5 months in either 21 % (intermediate light, M) or 4.9 % (low light, L) of incident irradiance. After this period, a subset of the intermediate-light grown seedlings was transferred to a high-light stress treatment (H). Gas exchange, chlorophyll fluorescence, pigments, antioxidant activity, and nitrogen (N) resorption were examined at three leaf senescence stages during September and October. Our results show that plants of both species produce more anthocyanins in the H treatment. In comparison with plants grown in the L and M treatments, plants of both species in the H treatments had lower chlorophyll, carotenoid and chlorophyll fluorescence parameters (F (v)/F (m), I broken vertical bar (PSII), NPQ and ETR) at the third sampling date (October 12-18), and indicating higher levels of photoinhibition in the seedlings exposed to high light. Our results imply that autumn leaf redness is inducible and closely linked to photo-oxidative stress. However, anthocyanins did not enhance antioxidant capacity in red leaves in either species, when exposed to high light. For both species, our results showed a higher N-resorption for high-light stressed plants. We also observed that the number of abscised leaves at the second sampling dates (September 10) was higher than at the third sampling dates. The intra-leaf distribution of anthocyanin, the association between anthocyanin production and the high-light environments, the retention of red leaves, the substantial physiological gain of photosynthetic activity, as well as the links between anthocyanins and increased N resorption led us to assume that one primary role of autumn anthocyanin could be to protect the photosynthetic apparatus from photo-oxidative damage as light filters rather than as antioxidant. Another major role is to extend carbon capture and help supply the energy needed for N resorption from senescing leaves in both A. saccharum and A. Platanoides during high-light stress. Nevertheless, photoprotective capacity of anthocyanins was not able to fully compensate for photoinhibitory stress as the anthocyanins are not optimally located to efficiently reduce light within the leaves.Peer reviewe

Effects of phosphorus availability on later stages of primary succession in Gongga Mountain glacier retreat area

Intra- and interspecific competition and modifications in environmental characteristics are the main drivers of plant community dynamics, but few studies have investigated the combined effects of competition and phosphorus (P) availability on ecological succession. Seedlings of conifers Abies fabri and Picea brachytyla were collected from the late-stage Hailuogou glacier retreat area and grown under different P regimes (control and P fertilization) to investigate the impact of intra- and interspecific competition on photosynthetic capacity, resource (water, N and P) use efficiency and growth performance in two types of native soil. In the control treatment, there were no differences in the total biomass of A. fabri between the two competition patterns under either type of soil, whereas interspecific competition decreased the total biomass of P. brachytyla grown in the soil collected from A. fabri plots. However, under P fertilization, A. fabri individuals exposed to interspecific competition showed a stronger competitive ability, as their total biomass, absolute height growth rate, net photosynthetic rate, water use efficiency (delta C-13) and leaf P content were significantly higher under interspecific competition compared to intraspecific competition. No differences in these traits were detected in P. brachytyla between the two competition patterns. The results indicated that P plays an important role in determining asymmetric competition patterns among Pinaceae species. The interactive effect of interspecific competition and P availability highlighted here could influence the community composition and dynamics of plants during late stage primary succession in a glacier retreat area.Peer reviewe

<i>Salix myrtillacea</i> Female Cuttings Performed Better Than Males under Nitrogen Deposition on Leaves and Drought Conditions

Drought and nitrogen (N) deposition are major threats to global forests under climate change. However, investigation into how dioecious woody species acclimate to drought and N deposition and how this is influenced by gender has, so far, been unexplored. We examined the phenotypic and physiological changes in Salix myrtillacea females and males under 60 d drought, and wet N deposition on leaves’ treatments. Drought inhibited their growth by limiting water acquisition, photosynthesis, and increasing oxidative stress, especially in males. However, females exhibited greater drought resistance than males due to their better water acquisition ability and instantaneous water use efficiency (WUEleaf), higher foliar abscisic acid (ABA) and auxin (IAA) levels and greater antioxidase activities. N deposition increased foliar ABA, H2O2 accumulation, and reduced N distribution to the leaves, causing restricted photosynthesis and aerial growth in males. Interestingly, N deposition improved biomass accumulation in both the genders under drought, with greater positive effects on drought-stressed males by increasing their radial growth and causing greater N distribution to the leaves, increased foliar IAA and reduced oxidative stress. Regardless, S. myrtillacea females still showed better growth and drought resistance than males under both drought and N deposition. The females’ superior performance indicated that they are more appropriate for forestation, thus supporting the dominant gender’s selection in the afforestation of unisexual S. myrtillacea in drought and severe N deposition regions

Sex-specific responses of Populus deltoides to defoliation

There has been an increasing interest in understanding the differential effects of sexual dimorphism on plant stress responses. However, there is no clear pattern in the responses of the sexes to defoliation. In this study, the effects of different severity of artificial defoliation on biomass production, total nonstructural carbohydrate (NSC) concentration, and photosynthetic rate (PN) of male and female Populus deltoides were examined. We used half and full defoliation to observe the differences between the sexes in three harvest dates (1 week, 4 weeks, and 8 weeks after treatments). We hypothesized that female and male P. deltoides compared with an undefoliated control would have compensatory growth in response to defoliation treatments. Results showed that half and full defoliation reduced the growth of both sexes. Following half defoliation, root growth was reduced, especially in males, at T2 (4 weeks after defoliation) and T3 (8 weeks after defoliation), while males showed an increase in height increment under the half defoliation compared with the nondefoliation treatments. By contrast, females were more negatively affected by defoliation than males in terms of biomass after 8 weeks. One week after defoliation, PN increased significantly in females and males under half defoliation (+30%, +32%, respectively) and full defoliation (+58%, +56%, respectively). However, 8 weeks after defoliation, there was little difference in PN between defoliated and undefoliated female cuttings. Increases in stomatal conductance (gs) and leaf nitrogen were observed under fully defoliated female and male cuttings. Moreover, males had less NSC concentrations following half defoliation compared with females. Our results indicate that leaf compensatory growth in male cuttings of P. deltoides was maintained by obtaining greater photosynthetic capacity, higher leaf nitrogen, and lower NSC concentration following half and full defoliation. Our results highlight that females suffered from greater negative effects than did males following half defoliation, but under full defoliation, the differences between both sexes were subtle

Warming Affects Soil Nitrogen Mineralization via Changes in Root Exudation and Associated Soil Microbial Communities in a Subalpine Tree Species Abies fabri

The objective of this study was to quantify the responses of root exudates and nitrogen (N) mineralization of Abies fabri to warming and to identify links between root exudation and microbial N mineralization via interactions with the soil microbial characteristics. We conducted a 2-year study to assess the effects of warming on root traits, root exudation, soil microbial community, net N mineralization, and soil N availability within the rhizosphere of A. fabri. Results showed that warming enhanced the exudation rate of total organic carbon (REC) in both years, while warming tended to increase the exudation rate of total N (REN) in the first year, but decrease it in the second year. At the end of the second year, the C/N ratio of root exudates was higher under elevated temperature than under ambient (control) conditions. Warming increased the net N mineralization and net nitrification rates during the first year, but these effects were not observed in the second year. Although warming showed no significant effect on the content of bacteria and fungi in the first year, warming significantly increased the fungi, Gram-positive bacteria, and fungi/bacteria ratio in the second year. Warming effects on rhizosphere soil N mineralization were mainly positively correlated with the REC and REN. These results suggest that net N mineralization rates may be controlled by the quantity and C/N ratio of root exudates, rather than by the simple enhancement in root exudation rates. Our results suggest that warming could promote soil N cycling in cold regions via increased quality and quantity of root exudates