Deadwood is an important component of forest ecosystems, and plays an irreplaceable role in serving as a habitat and food resource for animals, plants and microorganisms, storing forest nutrients, energies and water, and especially in maintaining the integrity of ecological processes. Their decomposition is crucial to the material cycle of forest ecosystem and its process is related to the microbial community in it closely. Species composition, ecological function and their dynamic changes of course woody debris (CWD) all affect their own degrading rate. Accordingly, the physical and chemical properties of CWD, including water content, pH value, and C, N, P concentrations, also influence the distribution and structure of microbial communities. Therefore, understanding the composition of wood-inhabiting fungal communities in dead wood requires a clear cognition of the physi-chemical traits of their growth substrates.
In this study, we measured the physical and chemical properties of fallen logs of seven dominant tree species with all decay levels in the temperate forest ecosystem and some interesting results were observed. Different tree species show a different changing trend. TC of P. koraiensis increased slightly as with increasing wood decay class and was higher than other angiosperm trees. And this similar increasing trend was also found in A. pictum subsp. mono and F. mandshurica. Whereas Acer pseudo-siebodianum, Maackia amurensis and Tilia amurensis showed an decreased trend and Quercus mongolica did not show apparent variety over the course of decomposition (Fig. 1a, supplementary 2). This corresponds with results found in other researches from Europe regions of the boreal forests. The CWD carbon concentration research of 60 tree species from the Northern Hemisphere reveals that angiosperms had a significantly lower TC than gymnosperms, with means of 47.8% and 50.6%, respectively[3]. Further study has found that the C content of Picea abies gradually increased as the CWD decomposition progressed, but TC of Populus tremula showed a downward trend, while Alnus glutinosa did not change significantly[38]. These differences about TC in different CWD during the decomposition process are caused by the variation in wood properties between coniferous and broadleaved tree species, since there is an increase of lignin and a decrease in cellulose with the increasing wood decay level and lignin has a higher proportion of TC than cellulose[3, 39].
Although TC content varied for all studied tree species for all decay levels, the amounts are relative closest, unlike TN and TP. The TN content of different tree species changed significantly. Contrary to TC content in gymnosperm was higher, angiosperm has a higher TN content. For example, Maackia amurensis with decay level 3 had the highest TN content (3014 µg g− 1), several times more than the lowest one, Pinus koraiensis with decay level 2 (769.33 µg g− 1). The mean TN content of pine is lower than all angiosperm (supplementary 2). And for all the assessed tree species except Acer pictum subsp. mono, TN content in the strongest decay level was the highest (supplementary 2), which coincides with the results of some researches[40–42]. However pine, spruce and birch in Russia have the increasing N concentration with increasing decay class[40]. And the mean N concentration of pine in Estonia ranges from 0.117–0.547%[42]. The reason of increasing N content on supposition is that the CWD will exposes to inorganic environments gradually, the contacting area with soil will be expand as the decomposition progress, which is more microbial corrosion of CWD. However, the more and more microorganisms’ increasing in CWD will promote the degradation. Meanwhile the N-fixing microorganism will also increase with species and abundance.
In the dead wood for all trees, there is not clear changing trend about TP content. Maackia amurensis has the lowest amount and Pinus koraiensis is slightly higher. Both of them have not showed a clear changing trend with the increasing decay class. A research about radiata pine has reported that the concentration of TP remained the same in log-wood during the period of decay[43]. And some other studies found that along with the decomposition of pine and spruce stumps, the amount of TP in stumps increased[44]. In our results, the TP amount of Acer pseudo-siebodianum, Fraxinus mandshurica, Quercus mongolica and Tilia amurensis initially decreased and then increased. Only Acer pictum subsp. mono has a constantly increasing.
The composition of wood-inhabiting fungal community is affected by many factors, including environmental elements (such as precipitation, latitude, light, temperature and humidity)[45–47], tree species and the characters of host wood (chemical composition, physical structure, diameter, volume and decay degree)[14, 19].
In a certain nature forest with small scale range, when the climate, temperature, precipitation are relatively consistent, host-tree identity is an important determinant of the species composition of wood-inhabiting fungal communities[12, 13, 48]. The fungal community composition of fallen wood differed significantly and fungal alpha diversity differed significantly across tree species[6, 28]. Wood-inhabiting fungi exhibited tree species preferences in temperate forests, especially in broad-leaf trees the saprotrophic fungal community were more specific to tree species. In our study, the NMDS analysis also showed the similar results. Every tree species has a distinct fungal community and the sharing fungal species among seven tree species were only 6. There were significant differences in fungal community structure between deadwood of different tree species, either the fungal species or their abundance. In fact, even within a same tree species, the fungal community composition of deadwood with different decaying level also changed, just as the Venn diagram showed (Fig. 2d). The sharing fungal species in three degrees were few and fungal community composition were distinct.
Further researched found that the chemical-physical traits of wood will be the key factors influencing the fungal community composition. Wood-density, water content, wood constitutes (Klason and acid-soluble lignin, organic extractives, water-soluble lignin fragments), C and N were important factors regulating fungal community composition in decaying stems[10, 14, 19, 20, 49]. Among them, C and N are major constituents of the proteins required for the synthesis and activities of degrading enzymes[50], therefore played important roles on fungal community assembly[51, 52]. Other studies in a natural, beech-dominated forest also found that N was the key factor driving fungal community composition in deadwood[6, 50]. P had been reported having relation to extracellular enzyme activity during wood decomposition[53]. But compared to the clear importance of C and N, research on the impact of P on fungal communities is relatively rare.
In our study, fungal community structure of three tree species (Pinus koraiensis, Acer pictum subsp. mono and Maackia amurensis) differed significantly from each other by the NMDS analysis (Fig. 4). This is consistent with the PCA result of the physico-chemical properties (Fig. 1b). These three tree species have distinct C, N and P contents respectively. Their fungal community structure were also dependent. Fungal community in Pinus koraiensis logs showed significantly correlation with TC content, fungal community in Acer pictum subsp. mono showed the preference with TP and fungal community in Maackia amurensis has closely related to TN content.
In general, broadleaved and coniferous dead trees hold quite distinct fungal communities, broadleaved trees have more species rich[1, 12, 54]. According to Rajala et al., the higher species richness in broadleaved trees results from a higher diversity of Ascomycota[12]. Yang et al. found Ascomycota appeared more specific to tree species than those of the Basidiomycota[28]. In fact, these preferences were a phenomenon based on fungal phylum class and the mechanism was not clear. Here we selected the dominant species of different fungal community and tried to make certain this problem.
As representative species in each community, the 50 dominant fungal species exhibit tree species specialization similar to their respective communities. Only a few species can appear simultaneously on diversified growing substrate including tree species and decaying levels, just like Eutypa spinosa. These dominant species play a major role in the assemblages of community and occur its own niche separately. PCA analyze of the dominant fungal species and important factors showed fungal species have clear nutrient tendency. And their activity characteristics may present the whole community to a certain extent for their huge number of abundance. Bjerkandera adusta, Ceriporiopsis gilvescens, Pholiota adiposa and Pholiota lubrica are the dominant ones inhabiting on deadwood of Acer pseudo-siebodianum and showed an obvious TN content tendency. And Conferticium ochraceum and Xylodon sambuci from dead wood of Quercus mongolica also showed the same nitrophilous. In addition, these dominant fungal species, including Barbatosphaeria arboricola, Clonostachys rosea and Eutypa spinosa from Tilia amurensis fallen logs, Hypoxylon cercidicola Phaeoacremonium minimum and Talaromyces infraolivaceus from Fraxinus mandshurica dead logs have a positive correlation with TP content. Interestingly enough, in our study most Basidiomycetes showed the TN preference and Acomycetes have P preference. In previous studies, saprotrophic fungi showed a higher demand for N and P than mycorrhizal fungi [55] .