Patterns and environmental drivers of C, N, and P stoichiometry in the leaf-litter-soil system associated with Mongolian pine forests.

Ecological stoichiometry is an important approach to understand plant nutrient cycling and balance in the forest ecosystem. However, understanding of stoichiometric patterns through the leaf-litter-soil system of Mongolian pine among different stand origins is still scarce. Therefore, to reveal the variations in Mongolian pine carbon (C), nitrogen (N), and phosphorus (P) stoichiometry and stoichiometric homeostasis among different stand origins, we measured C, N, and P concentrations of leaves, litter, and soil, and analyzed the nutrient resorption efficiencies of leaves in differently aged plantations and natural forests from semi-arid and dry sub-humid regions. The results showed that (1) the stand origin had a significant effect on the C-N-P stoichiometry, and also significantly affected leaf N and P reabsorption efficiencies. Leaf N/P ratios indicated that Mongolian pine was co-limited by N and P in the NF, HB and HQ, and was mainly limited by P in MU. (2) With increasing stand age, C concentrations in the leaf-litter-soil system initially increased and then decreased, the N and P concentrations and reabsorption efficiencies in the leaf-litter-soil system were gradually increased. Overall, stand age had a significant effect on N concentrations, C/N and C/P ratios in the leaf-litter-soil system. (3) The C and N elements between the leaf-litter-soil system had a strong coupling relationship, and the P element between litter-soil had a strong coupling relationship. In addition, plantations exhibited greater N/P homeostasis than natural forests, and N/P exhibited greater homeostasis than N and P alone, which may be a nutrient utilization strategy for forests to alleviate N or P limitation. (4) Environmental factors have a significant influence on C-N-P stoichiometry in the leaf-litter-soil system, the most important soil properties and meteorological factors being soil water content and precipitation, respectively. These results will be essential to provide guidance for plantation restoration and management in desert regions.

Temporal approach to identifying ectomycorrhizal community associated with Mongolian pine in a desert environment, northern China.

To date, the ectomycorrhizal (EM) fungi community has been widely investigated with aging plantations affected by the pedologic factors. However, knowledge on the effects of phenology induced by climatic factors across the age range remains poorly understood on an intra-annual scale. Here, we sampled the fine roots of Mongolian pine (Pinus sylvestris var. mongolica) plantations at different stages of the growing season (from April to September) across three stand ages (27, 34, and 44 yr) in the Mu Us Desert, Northwest China. We aim to disentangle the community composition and structure of EM fungi, as well as the impact of climate on EM fungi. We observed that the 173 distinct EM fungal operational taxonomic units (OTUs) were identified. Geopora, Inocybe, Tomentella, and Tuber were the most frequent, and their dominance was maintained as stand aging. The richness and community composition were not significantly different with stand aging (P > 0.05). Host phenology and stand age are two important factors that have shaped the EM fungal community. The growing stage affected the beta diversity of the EM fungal community more than stand age, and this variation of the EM fungal community was closely related to seasonal climate, particularly precipitation. This improved information will provide a theoretical basis for the reforestation and rehabilitation of the Mongolian pine plantations using mycorrhizal techniques. IMPORTANCE Ectomycorrhizal (EM) fungi are particularly important for host plants in a desert ecosystem. With a high degree of plasticity, EM fungi are largely influenced by host plant and environmental variables and fundamentally contribute to the ability of individuals to adapt to environmental changes. Therefore, the EM fungi are important for Mongolian pine (Pinus sylvestris var. mongolica) plantation in a desert ecosystem. Although previous studies have concluded that multiple endogenous and exogenous processes ultimately lead to species-specific temporal patterns in EM fungal populations. We still neglect the effect of host phenology on EM fungal activity. The significance of our study is the interplay between climate-driven EM fungi and plant phenology.

Tree characteristics and drought severity modulate the growth resilience of natural Mongolian pine to extreme drought episodes.

Global climate change and the increase in the frequency and intensity of drought have led to widespread forest decline and tree mortality. Studying the resilience components of tree growth to drought, including resistance (Rt), recovery (Rc), and resilience (Rs) and the influencing factors, helps assess forests' production and ecological stability under a changing climate. This study analyzed the responses of three resilience components of natural Mongolian pine (Pinus sylvestris var. mongolica) to drought events by examining individual-tree characteristics in two sites of Hulunbuir using the linear mixed effect model. The result showed that drought severity, diameter at breast height (dbh), pre-drought growth, and growth variability prior to drought had significant effects on the three resilience components of Mongolian pine growth. Specifically, as drought severity, dbh and growth variability increased, the Rt and Rs decreased, but Rc increased, showing a trade-off relationship with Rt. However, the Rt, Rc, and Rs decreased with pre-drought growth. Inter-tree competition and tree age also significantly impacted two resilience components. Besides, the interaction term between tree competition and tree age negatively affects Rt and Rs but positively affects Rc. Our findings highlight the influence of drought severity and individual-tree characteristics on drought resilience components, which can serve the adaptive management of natural Mongolian pine forests in the future.

Long-Term Effects of Climate and Competition on Radial Growth, Recovery, and Resistance in Mongolian Pines.

Understanding the response of tree growth and drought vulnerability to climate and competition is critical for managing plantation forests. We analyzed the growth of Mongolian pines in six forests planted by the Three-North Shelter Forest Program with tree-ring data and stand structures. A retroactive reconstruction method was used to depict the growth-competition relationships of Mongolian pines during the growth period and their climatic responses under different competition levels. Drought vulnerability was analyzed by measuring the basal area increment (BAI) of different competition indices (CIs). In young trees, differences in BAIs in stands with different CIs were not statistically significant. After 15-20 years, medium- and high-CI stands had significantly lower tree-ring widths (TWs) and BAIs than the low-CI stands (p < 0.05). The standardized precipitation evapotranspiration index (SPEI), precipitation, relative humidity, and vapor pressure deficit were major factors affecting tree growth. On a regional scale, climate outweighed competition in determining radial growth. The relative contribution of climatic factors increased with the gap in SPEI between plantation sites and the native range, while the reverse pattern of the competition-growth relationship was observed. Drought reduced TWs and BAIs at all sites. Stands of different CIs exhibited similar resistance, but, compared with low-CI stands, high- and medium-CI stands had significantly lower recovery, resilience, and relative resilience, indicating they were more susceptible to drought stresses. Modeled CI was significantly negatively related to resistance, resilience, and relative resilience, indicating a density-dependence of tree response to drought. After exposure to multiple sequential drought events, the relative resilience of high-CI stands decreased to almost zero; this failure to fully recover to pre-drought growth rates suggests increased mortality in the future. In contrast, low-CI stands are more likely to survive in hotter, more arid climates. These results provide a better understanding of the roles of competition and climate on the growth of Mongolian pines and offer a new perspective for investigating the density-dependent recovery and resilience of these forests.

Effects of endophytic fungi in Mongolian pine on the selection behavior of woodwasp (Sirex noctilio) and the growth of its fungal symbiont.

BACKGROUND: The European woodwasp, Sirex noctilio, is a global invasive pest, attacking a wide variety of pine species by inoculating spores of a symbiotic fungus (Amylostereum areolatum) at oviposition. The woodwasp larvae depend on the growth of the symbiotic fungus to feed. The relationship among host endophytic fungi, symbiotic fungus and woodwasp remain elusive. Here, the effects of endophytes in Mongolian pine on the growth of Amylostereum areolatum and the selection behavior of female woodwasp were investigated by quantifying the mycelium growth rates and olfactometry assays. RESULTS: The endophytic plant fungi, Trichoderma harzianum, Phlebiopsis gigantea, T. viride and T. atroviride, completely killed the mycelia of Amylostereum areolatum. Mycelium fermentation broth of Chaetomium globosum inhibited the growth of the symbiont. Moreover, we observed that volatiles of Ophiostoma minus and Aspergillus niger (acetophenone, acetylacetone, hexadecane, phenylethyl alcohol, and isopropyl myristate) had repellent effects on adult female woodwasp. While volatiles of Amylostereum areolatum ((-)-globulol, 2-hexene, cycloprop[e]indene-1a,2(1H)-dicarboxaldehyde, terpene and cyclopentanone) had a significant attractiveness to adult female woodwasp. CONCLUSIONS: Some species of the host endophytic fungi had a significant negative effect on the growth and development of woodwasps, which could be useful in the monitoring and effective management of woodwasps. © 2018 Society of Chemical Industry.