During the four years of the research, only 6 alien plant species have been found within the established plots. They are presented in Table 1 and Figure 2:

Three of them (Erigeron canadensis L., Erigeron annuus (L.) Desf., Oenothera biennis L.) are included in the Black Data Book of Central Russia (Vinogradova et al., 2010) as the most invasive alien plant species of European Russia.

Erigeron canadensis (Compositae Giseke) has the highest occurrence frequency and abundance within all established plots. This species was found in all established plots with an occurrence frequency of 100% (Fig. 2). Moreover, Erigeron canadensis is the most widely distributed plant species invaded the fire-damaged forests in the whole area of the Mordovia Reserve. Erigeron canadensis is one of the first and abundant (together with Epilobium angustifolium L.) ruderal plants (according to Grime, 1979) in fire-damaged forests of the Mordovia Reserve during the early stages of pyrogenic succession (Khapugin et al., 2012). By 2014 its projective cover had decreased at most studied plots except for Pin27, despite the fact that this species is annually observed in all plots established during the study period. Due to xerophilous conditions of habitat Pin27 and stringent conditions of fire in 2010 (Table 1), the growth rate of undergrowth of Betula pendula Roth has decreased at this plot. Under these favourable conditions Erigeron canadensis is characterized by a high abundance, annual flowering and fruiting. Dominance or presence of this species in the vegetation cover during the early stages of pyrogenic successions is in line with the results of other researchers (Dymova, 2014; Malinovskikh, 2014). Thus, we conclude that this alien species is the most invasive plant in natural ecosystems of the Mordovia State Nature Reserve.

Erigeron annuus (Compositae) is a less competitive species than the previous alien plant. It was first found only in 2012 year at most parts of the established plots (except plot Bet30) (Fig. 2). Erigeron annuus was presented by isolated individuals scattered throughout the areas of the established plots, without thicket forming. By 2014, Erigeron annuus was observed only in two studied plots (Bet30 and Que36) as few isolated plant individuals. Apparently, Erigeron annuus is able to penetrate into natural ecosystems of the Mordovia Reserve, but it cannot compete with native perennial plants (e.g. Pteridium aquilinum (L.) Kuhn, Calamagrostis epigejos (L.) Roth, undergrowth of Betula pendula and Populus tremula L.) recovering after a forest fire.

The Western European-Ancient Mediterranean species Lactuca serriola L. (Compositae) was observed in most parts of the studied plots in both coniferous and deciduous forest communities. The species was found as single individuals scattered throughout the areas of the studied plots. However, the plants are clearly visible because of their tall stems with numerous inflorescences. Last fact promotes active dissemination of Lactuca serriola. Also, high seed germination (Mikulka & Chodová, 2003) and its better effect in shade conditions (Kapeluszny et al., 2011) were shown. All these facts provide establishing of Lactuca serriola in conditions of high competition with other plants. Because of the high competitiveness of this species and resistance to shading, Lactuca serriola is able to exist in the shade of the undergrowth of pioneer deciduous tree species. Thus, this species is a sustainable forest ecosystem component which can be considered as one of the invasive species that deserve special attention.

The finding of a single generative individual of Oenothera biennis (Onagraceae Juss.) in studied plot Pin38 in 2012 suggests that the seeds of this biennial plant were introduced into the area of this plot immediately after or during the wildfire in 2010 along forest roads and narrow clearings. Apparently, this biennial plant has not been found during the study of this plot in 2011, because the plant was found with a basal rosette of leaves. In warm and humid spring an average of 30% of seeds of Oenothera biennis are able to germinate within 4–5 days (Gladunova et al., 2014); that is why we expect new findings of this plant in this established plot and adjacent areas in the future. In addition, we do not exclude the possibility of further dissemination of Oenothera biennis in the other areas of the Mordovia Reserve either, especially along forest roads and narrow clearings in areas damaged by the fire in 2010.

Among all the alien species revealed in the Mordovia Reserve fire-damaged forests, Viola arvensis Murr. (Violaceae Batsch) and Sambucus racemosa L. (Caprifoliaceae Juss.) belong to archaeophytes which have been known in the Republic of Mordovia since the sixteenth century. They do not show a high invasive behaviour. Apparently, their findings are caused by random drift of seeds along the forest narrow clearings, which are located in the immediate vicinity of the findings of these plants. We have to say that the Sambucus racemosa bushes were depressed despite annual flowering and fruiting. Viola arvensis has been observed only in the established plot Que37 since 2011, but it was not found there in 2014. Thus, these two alien species have the lowest invasive activity. Their new findings are possible in disturbed habitats: narrow clearings, forest roads, neighborhoods of forest lodges, etc.

All studied plots are located in one of these types of forest: Pinetum (Pin07, Pin26, Pin27, Pin38, Pin40), Piceetum (Pic28, Pic29), Quercetum (Que36, Que37), Betuletum (Bet30), Tilietum (Til35). Of these, the deciduous forest ecosystems are the least resistant to plant invasion (Table 2). Coniferous forests were invaded by an average of 1.75 invasive species per established plot. Nevertheless, differences between deciduous and coniferous forest types are minimal.

In addition to the abovementioned alien plants, two invasive species included in the Black Book of Central Russia Flora (Vinogradova et al., 2010) were found in the Mordovia Reserve outside the plots listed above. These are Solidago canadensis L. and Bidens frondosa L. from the Compositae family.

Despite the long history of the study of the Mordovia State Nature Reserve flora (since 1936), the first locations of Solidago canadensis in this area were found at first in 2012. These were located along a railway in the northeastern part of the Mordovia Reserve (Khapugin et al., 2013). In 2013, only one individual was recorded at the roadside nearby Pushta settlement. But in 2014 and 2015, new findings of Solidago canadensis were found relatively far away from the settlements, deep in the forests. Solidago canadensis penetrates in new localities predominantly along forest roads and narrow clearings. This is confirmed by the observation that all known localities of this invasive species are presented at roadsides and forest glades. We suggest that invasive plant diaspores penetrated into the Mordovia Reserve from the south, where abandoned arable lands are located. These abandoned areas are occupied by individuals of Solidago canadensis which not infrequently form a yellow aspect during flowering. The high level of penetration of Solidago canadensis along forest roads and narrow clearings coupled with its high invasiveness and closest location of its dispersal center (abandoned arable lands) are the main reasons to foresee a wide penetration of this invasive species in natural forest ecosystems in near future. In our opinion, the wildfire of 2010 was the main factor which provided Solidago canadensis to start its invasion due to a wide range of invasion ways as well as thinning of the forest stand, promoting the penetration of the anemophilous seeds of this plant.

Newly revealed locations of Bidens frondosa are few. It was first found in the Mordovia Reserve in 2013 (Khapugin et al., 2015b). This locality is placed between the established plots Pic29 and Que36 in alder-spruce forest damaged by the fire. Another locality of Bidens frondosa was found on the shores of Malaya Valza Lake. Here, the invasive plant was presented by single individuals scattered throughout the hygrophytic vegetation. We expect that new findings of Bidens frondosa in the Mordovia Reserve in future will be found predominantly near water bodies often used by human (e.g. ponds in the Pushta settlement and near the Steklyannyi cordon, the lakes Picherki, Inorki, the rivers Pushta, Satis).

Thus, despite the resistance of forest ecosystems to plant invasions (Pearsall, 1959; Keeley et al., 2003), eight alien plant species were registered in different types of forest damaged by a wildfire in 2010. No differences in invasibility of tested forest ecosystems were detected.

In conclusion, Erigeron canadensis, Lactuca serriola and Solidago canadensis are the most invasive plants in the Mordovia Nature Reserve and these species are the most probable invaders of studied forest ecosystems in the future. These preliminary findings point out the potential effect of fire to exacerbate the invasibility of some ruderal species in the taiga zone.