Effect of the silvicultural treatments on the abundance and richness of EMF

The total number of sporomes collected in 2015 (1 177) was higher than that registered for 2016 (732). During the study period, the highest abundance was registered with the first thinning (Figure 1). For its part, the average richness was very similar for the various treatments: S = 16 for 2015 and S = 15 for 2016 (Figure 2).

CR = Regeneration felling; PCA = First thinning; SCA = Second thinning; CL = Release felling.

Figure 1 Number of EMF sporomes collected in four silvicultural treatments during two sampling years at ejido Rancho Nuevo Nanacamila, in the municipality of Zacatlán de Las Manzanas, Puebla. 

CR = Regeneration felling; PCA = First thinning; SCA = Secondthinning; CL = Release felling.

Figure 2 Richness of EMF for four silvicultural treatments during two sampling years at ejido Rancho Nuevo Nanacamila, in the municipality of Zacatlán, Puebla. 

The Factorial ANOVA Between-Subjects showed that the number of sporomes collected with the regeneration felling treatment in 2015 was significantly lower (p<0.05) than that registered for other silvicultural practices, while in 2016 differences were detected between the first and the second thinnings, as well as with the release felling (Table 2). No differences were observed when the richness of species was contrasted between the various treatments.

When the number of sporomes collected between 2015 and 2016 was compared by treatment, the test evidenced significant differences for the second thinning and the release felling (Table 3). There were no significant differences in the richness of species.

The results evidenced the existence of a differential distribution of the emergence of sporomes between the four assessed silvicultural practices that are associated with the felling intensity. Based on an ecosystemic perspective, the silvicultural treatment is considered as a disturbance factor that modifies the density of the forest, its spatial structure and the value of its stock (^{Zamora-Martínez, 2010}), as well as the biotic interactions. Therefore, the magnitude of the disturbance will be in terms of the type of felling -partial (thinning) or regeneration felling (total or of the seed trees)-, since its application alters in a different way the basal area, dominant height, tree stand cover, and other dasometric variables that have significant effects on the production of EMF (^{Velasco et al., 2010}; ^{Bonet et al., 2012}; ^{Martínez-Peña et al., 2012}). On the other hand, the actions performed during timber extraction, whether manual or mechanized, have an impact on fungal populations since they cause soil compaction and reduce evapotranspiration (^{de-Miguel et al., 2014}).

The regeneration felling was the silvicultural treatment that yielded the lowest number of sporomes in the two assessment periods (2015 and 2016), as can be seen in Figure 1. This treatment causes a high degree of disturbance affecting the associated fungal communities. Also, it implies the extraction of virtually all the trees (except for parent or seed trees); favors increased solar radiation, edaphic temperature and the temperature of the strata of residual herbal and shrub vegetation, as well as of the vegetation that develops during the years following the intervention (six years for the present study).

In this regard, ^{Luoma et al. (2004)} point out that EMF production diminishes as the felling intensity increases, although in Pinus pinaster Aiton. forests subjected to intensive management, it has been documented to favor the development of specific taxa, such as the Lactarius deliciosus group (^{Bonet et al., 2012}). However, it should be noted that other environmental factors that impact the emergence of sporomes are the distribution and intensity of precipitations (^{Martínez de Aragón et al., 2007}; ^{Velasco et al., 2010}).

Moreover, there is evidence that indicates that the basal area (BA) of the stand significantly affects EMF production (^{Martínez-Peña et al., 2012}). The BA of the plots located in the areas subjected to regeneration felling, six years after the intervention, was 11.58 m² ha^-1; this value was lower than the values estimated for other treatments (Table 1, Figure 1). For example, for the first and second thinnings, the respective estimated basal areas were 37.42 m² ha^-1 and 29.67 m² ha^-1, both of which are above the 15-20 m² ha^-1 interval documented for this variable as a value associated to maximum EMF productions in pine forests (^{Bonet et al., 2008}; ^{Bonet et al., 2010}).

As for the differences in the number of sporomes observed between the first felling and the second, they are biased by the difference in the number of plots considered, although a higher abundance was expected as the forest mass grew in age and vigor. A possible explanation lies in the length of the period between the intervention and the sampling year, which was ten years for the first felling, and six, for the second felling. After any felling, whether selective or for regeneration purposes, the physical and chemical properties of the soils are modified; this is reflected in the loss of nutrients, the alteration of the apparent density and the porosity, as well as of the capacity to retain water, among others (^{Kishchuk et al., 2014}).

The recovery period can exceed ten years, depending on the intensity of the felling (^{Zhou et al., 2015}), and the negative impacts at soil level affect the abundance of EMF. Given the nutritional dependence of these organisms on their arboreal hosts, as a result of the lower availability of nutrients, there is a reduction of their photosynthetic activity and the translocation of carbohydrates toward the EMF (^{Nehls, 2008}).

It is worth noting that we still lack the research required to accurately explain the effect of silvicultural practices on the fungal populations present in stands subjected to forest management programs, particularly on those species of mushrooms that are collected for commercial purposes. In this regard, ^{Pilz et al. (2006)} point out that the production of sporomes of Chantarellus spp. diminishes during the first year of timber harvesting, but recovers almost entirely after six years.

The release felling -which consists in eliminating the tree species that are in the process of growing, old trees, and herbaceous and shrub taxa (^{Daniel, 1983})- causes soil compaction, with the resulting loss of ectomycorrhizal inoculum at soil level and in particular “refuge plants” (^{Wiensczyk et al., 2002}; ^{Kennedy et al., 2003}; ^{Martínez, 2008}), reflected in a reduced abundance of sporomes.

Effect of the edaphic conditions on the abundance and richness of EMF

One of the challenges of the forest sector is the application of silvicultural practices for the sustainable management of the resources. Maintaining the productivity of the soil is critical for this purpose, as it has an impact not only on the conservation potential of the long-term timber production, but also in other functions of the forest ecosystem, such as the presence of the associated biota and, particularly, of the EMF. When the assessed silvicultural treatments were compared, significant differences (p>0.05) resulted for 11 of the edaphic variables considered: the percentage of organic matter, the percentage of sand, and available phosphorus (P), potassium(K), calcium (Ca), magnesium (Mg), sodium (Na), iron (Fe), zinc (Zn), copper (Cu) and pH (Tables 4 and 5).

The comparison between treatment pairs for the edaphic properties evidenced the same correlations; most relevant among these was those the correlation between the organic matter and the percentage of sand (Table 6).

The effect of the quantity and quality of organic matter on forest soils is a result of the fact that it is a quantitative, functional component of the ecosystem that plays a vital role in the physical and chemical environment. Its decay integrates elements of fertility and toxicity into the substrate, intercepts the light and regulates the thermal gradients between the soil and the atmosphere; furthermore, it increases the uptake of rainwater but reduces its evaporation. For this reason, it serves as a physical barrier for many chemical processes and elements (^{Facelli and Pickett, 1991}).

In general, organic matter has been documented to diminish according to the intensity of the felling (^{Zhou et al., 2015}). This agrees with the significant differences between the first thinning and the release (Table 5), but not with those between the release and the regeneration felling, probably because the waste material of the timber extraction (branches, stumps, bark, needles) is left at the exploitation site, while it is removed from the plots where the release was applied.

When the applied silvicultural treatments were contrasted, the significant differences in pH were registered between the regeneration and release fellings and the second thinning, which exhibited higher values, although the records turned out to be very similar (Tables 4 and 6).

The significant differences in K and P concentrations between the thinnings agreed with the values cited in the literature in the sense that these elements reduce as the canopy opens (Table 1) (^{Kishchuk et al., 2014}; ^{Zhou et al., 2015}). However, K concentration was higher with the release than with the first thinning (Table 4), even though the BA was smaller (Table 1). A possible cause is the presence of abundant shrubs and herbs that contribute dead leaves to the forest floor, which has a higher pH and a higher concentration of basic cations, such as K (^{Kishchuk et al., 2014}).

The main edaphic factors that were related to the abundance of sporomes were the percentages of organic matter, sand, and silt. The texture of the soil, determined by the percentage of silt and sand, affects the richness of species and the emergence of EMF sporomes (^{Martínez-Peña et al., 2012}; ^{Taylor et al., 2014}). Soils with high sand percentages reduce water retention, while silty soils increase it, thereby favoring sporome production. The results documented herein differ from those cited above, as there was a negative correlation between the percentage of silt and the abundance of EMF, and a positive correlation for sand and for the content of organic matter (Table 7), which serves as an evapotranspiration buffer and contributes to maintaining the soil temperature. These two actions are likely to have contributed to reducing the loss of water, favoring the emergence of sporomes.

As for the mineral nutrients, the elements that exhibited significant differences were K, Ca, Mg and Mn. Ca and Mn were negatively correlated with the richness of species (Table 7). This correlation is due to the affinity that EMF generally have for acidic environments, predominant in conifer forests. Thus, when the Ca content increases, the substrate becomes alkaline and, thus, less propitious for the development of fungi; consequently, the number of fungal species diminishes.

Pearson’s analysis showed a positive correlation between the abundance of sporomes, percentage of organic matter and the percentage of sand. Conversely, the correlation for K, Mg and the percentage of silt was negative. On the other hand, both the richness of species and the abundance of sporomes had a negative correlation with Ca and Mg (Table 7).

In particular, ^{Taylor et al. (2014)} highlight the existence of a fine-scale variation in the mushroom niches which is mainly related to the evolution of the soil, as the texture is closely linked to the content of assimilable microelements. Furthermore, the value of the quotient between the soluble and the totals decreases as the content of coarse elements in the texture of the soil increases.

The pH is an essential factor for the assimilation of macro and microelements. An increase in pH reduces the solubility and absorption of elements, such as copper, iron, zinc (Tables 4 and 7) and, notably, manganese; however, it increases the solubility and absorption of molybdenum (Mo); although the concentration of this element was not found to be significant for the abundance and richness of EMF (Table 7).

About EMF production, there are records of the significant effect of the pH on the yield of individual fungal species, e.g., Lactarius deliciosus group, which shows a marked preference for acidic soils (^{Bonet et al., 2012}). In the research carried out at ejido Rancho Nuevo Nanacamila, Lactarius indigo (Schwein.) Fr. and Boletus aestivalis (Paul) Fr. were collected only from plots subjected to release felling, which had a pH of 5.76; the presence of Tylopus sp. was registered in sites with less acidic values (5.93), in stands subjected to a second thinning.

Recent research points out that Boletus loyo Phil. ex Speg. develops in soils with high contents of organic matter, low to medium concentrations of N, P and Ca and high concentrations of K, Mg, Fe, Mn, Zn, and Cu, which favor the production of their sporomes (^{Pereira et al., 2016}). ^{Sun et al. (2017)} highlight the importance of various elements, including K and Mg in the emergence of EMF sporomes in an association of Cedrus deodara (Roxb.) G. Don, which agrees with the information documented herein. We should note that in the present study, both K and Mg generally had negative correlations with the abundance of EMF (Table 7), but positive correlations between treatments (Table 6).

The results agreed with those reported above, for there are significant differences in the decrease of the mineral elements of the soil, according to which of the assessed silvicultural practices is applied. For instance, with the first thinning, K, Ca and Mg diminish considerably, and the release felling results in the lowest concentrations of P, Na, and Zn (Table 4). We should point out that the cited nutrients had negative correlations with the abundance of sporomes, except the available phosphorus and sodium (Table 7).

Based on the above, particularly on the percentage of organic matter as an element indicating the quality of the soil, and on the statistically significant differences obtained for this variable, it is evident that any of these silvicultural practices alter the annual production rate of EMF and has an impact on the rest of the physical and chemical properties of the soil, as well as on the abundance and richness of EMF taxa (^{Martínez-Peña et al., 2012}).