Among the naturalized species, 16 legumes (10.96% of the total naturalized flora) in north Taiwan were observed in the field (Table 2-1).
The major compositions of life form of the 16 naturalized legumes were herb and shrub; the major origins were America and Asia; and the major climate region of their origin geographic distribution was tropical.
According to the IVI value, the most dominant species was Pueraria
lobata (Willd.) Ohwi ssp. thomsonii (Benth.) Ohashi & Tateishi (1.874),
followed by Calopogonium musunoides Desv. (0.616), Mimosa pudica L.(0.422), Leucaena leucocephala (Lam.) de Wit. (0.392), and Sesbania
sesban (L.) Merr (0.255).
Among the twelve habitats, the top three habitat types with the total proportion of native species were trail in grass (95%; 38/40), forest (92.2%; 141/153), and trail in wood (89.8%; 167/186); and that of naturalized species were in cemetery (46.8%; 65/139), orchard (41.4%;
24/58), and abandoned field (39.8%; 82/206) (Table 2-2). Moreover, only nine habitat types accommodated naturalized legumes. Abandoned field and riparian are the most important habits for naturalized legumes according to species number and IVI value respectively, followed by arable land, cemetery, and roadside.
Significant differences of biomass and biodiversity were found among three groups (Table 2-3). Firstly, for whole flora, total biodiversity and biomass in quadrats with naturalized legumes (QWt treatment) were significantly higher than other two groups, CHN and QWo; however, Berger-Parker Dominance index of QWt quadrats was significantly lower than those of CHN and QWo groups in overall and top five habitat types besides cemetery (Table 2-3a & Table 2-4a). Secondly, for native flora, total biodiversity and biomass in quadrats entirely without naturalized legumes (CHN treatment) were significantly higher than QWo quadrats;
however, Berger-Parker Dominance index of CHN quadrats was significantly lower than those of QWo group in overall and arable land.
Besides, biomass of QWt treatment in riparian and abandoned fields was significantly lower than the other two groups, QWo and CHN. On the other hand, there were no significant differences in the remaining index between QWt treatment and the other two groups in all habitats (Table 2-3a & Table 2-4b). Finally, for naturalized flora, the tendency was the same with whole flora that total biodiversity and biomass in quadrats with naturalized legumes (QWt treatment) were significantly higher than other two groups, CHN and QWo; however, Berger-Parker Dominance index of QWt quadrats is significantly lower than those of CHN and QWo groups in
overall and top five habitat types besides cemetery (Table 2-3a & Table 2-4c).
When naturalized legumes were deducted from QWt treatment (QWt –NL, Table 2-3b & Table 2-5), the tendencies in whole and naturalized flora were different from Table 2-3a & Table 2-4. Biomass of QWt treatment became significantly lower than those CHN and QWo groups in overall and abandoned field of whole and naturalized flora, arable land and riparian of whole flora. Besides, there were no significant differences in the remaining indices between QWt after naturalized legumes were deducted and those two groups, QWo and CHN.
Soil data
No significant differences of average soil nitrogen are shown between the quadrats with and without naturalized legumes. However, the soil nitrogen of samples collected in the centers of the quadrats with and without naturalized legumes is marginally significantly different (Table 2-6). On the other hand, nearly significantly difference of the soils collected in the center and in the corners is represented (Table 2-7)
DISSCUSSIONS
According to our results (Table 2-3a), the first hypothesis that presence of naturalized legumes is associated with relatively higher biomass and biodiversity is supported. This difference, as we anticipate, can be a result of elevating soil nutrients in terms of soil nitrogen (Table 2-6). Symbiosis of naturalized legumes and nitrogen fixing bacteria
enriches soil nitrogen as well as local carrying capacity; therefore, additional biomass can be supported (Aronson, et al., 1992; Pan, et al., 1993; Maron, et al., 1996; Parveen, et al., 1996). Complicated mechanisms, inter-specific and intra-specific interactions may be involved in elevating local biodiversity as we have seen in our study (Throop, 2005; Saidi, et al., 2007). However, the beneficiaries, in terms of native or naturalized species, of the enriched soil nutrients require further studies.
The second hypothesis is partially supported, since the biomass and biodiversity of native flora did not increase in the quadrats with naturalized legumes. The biomass and biodiversity of naturalized flora, excluding naturalized legumes, are not elevated on the enriched soils, either. Additional biomass and biodiversity of total flora are; therefore, most from naturalized legumes alone (Table 2-3) (Vitousek, 1990; Maron
& Connors, 1996). Dominant naturalized legumes with top IVI value in our case are either shrubs or lianas, and their advantages of exploiting available resources and nutrients have been confirmed (Aronson, et al., 1992); however, competition exclusion of native and naturalized species does not occur. Although enriched soil may provide additional niche and equal opportunities for native and naturalized species to arrive and establish (Herben, et al. 2004), the advantages of being exposed to available resources at the first minute may facilitate the inhabited naturalized legumes to fill the niche immediately (Tilman 2004; Kreyling,
et al., 2008).
The impacts of naturalized legumes on local communities are varied
along habitat types, and this result supports our third hypothesis. In legume-preferred habitats, e.g. arable land, riparian, abandoned field, and roadside, legumes do not suppress the biomass and biodiversity of other naturalized species (Table 2-4 & Table 2-5). Base on the previous studies, the leguminous seeds were always with thick and impermeable coats, and made them with dormancy (Rolston, 1978). To break the dormancy, daily alternating temperature and mechanical damage was most commonly used in natural habitat (Souza & Marcos-Filho, 2001; Baskin, 2003; Van Assche, et al., 2003). High frequency and large-scale disturbance such like cultivator working in arable land, water erosion in riparian and weeding behavior in roadside, would all supply a force for leguminous seed coat easily being broken (Paynter, et al., 2003; Spooner, 2005).
Once the seed coat was broken, leguminous cotyledons could rapidly absorb water and expand, and then germinate for later individual establishment. The advantages of well expressions in their favorite habitats may facilitate rapid exploitation of additional nitrogen and fill the niche (Jonathan, et al., 2008; Tilman 2004; Kreyling, et al., 2008);
however, the impacts of local communities by naturalized legumes through symbiosis with nitrogen-fixing bacteria were not evident.
Perhaps, soil nitrogen is not a limiting factor in these habitats and species do not experience intensive competition of available resources. Another explanation might be unfulfilled carrying capacity. Niches are always available in these habitats; therefore, competition exclusion did not exist.
Cemetery showed another special pattern of co-existence of naturalized legumes and other species. It seems to provide equal
opportunities for native and naturalized species, since no significant difference of native and naturalized flora was found among the treatments, CHN, QWo, and QWt (Table 2-4 & Table 2-5). Chinese worship culture implemented in this kind of habitat seems to create empty niches frequently. Periodical weeding and burning behaviors were cyclically occurred, and this may accelerate nutrients returning to the earth (Døckersmith, et al., 1999). Relatively higher average soil nitrogen content (2.38mg/g soil) then others habitats (0.45 in roadside to 1.14 in arable land) and resulted in relatively larger biomass of naturalized flora in cemetery (74.7% ± 30.7%) than others habitats (45.2% in riparian to 56.6% in roadside, data not shown). Even though naturalized legumes were dominant in cemetery according to the IVI value, the effect of biomass and biodiversity might be diluted by amounts of others naturalized flora.
The ratio of naturalized legumes to total naturalized flora in north Taiwan was much lower than in whole Taiwan Island (13.34%; Wu, et al., submitted), China (15.13%; Wu, et al., submitted), Singapore (21.32%;
Corlett, 1988), and India (16.76%; Reddy, 2008). Comparing with temperate countries, the ratio of naturalized legumes was much higher than Czech (5.59%; Pyšek, et al., 2002). Besides, we could see that most of the 16 naturalized legumes in our field investigation were from tropical or subtropical areas (Table 2-1). It seemed to show that naturalized legumes in Taiwan preferred to the tropical habitat in south Taiwan (Mabberley, 1997).
The presence of naturalized legumes elevates soil nitrogen, and this
result supports our fourth hypothesis (Table 2-6). However, the increased soil nitrogen caused by naturalized legumes did not impact local biomass and biodiversity as our expected (Table 2-3). According to Table 2-1
&2-7, total soil nitrogen was only increased restricted and localized in the standing site of each individual, and produced nitrogen by naturalized legumes cannot diffuse and transport to distance even within a meter. The transformation function of naturalized legumes seems to be amplified, and did not exist in our case, and the distribution pattern of naturalized legumes may be the key point. Naturalized legumes usually distribute individually in the field without forming monoculture and large coverage in north Taiwan; therefore, synthesized nitrogen by root nodules may simply accumulate on site and utilized by naturalized legumes alone.
Although neighbors of native and naturalized plant are very close to the naturalized legumes, stationary soil nitrogen cannot be shared. In consideration of the total IVI of naturalized legumes, the impacts of soil nutrient caused by naturalized legumes, the “transformers”, do not seem to be serious in the north Taiwan.
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