sdu faculty of forestry journal special edition 2009 - Orman Fakültesi

SDÜ ORMAN FAKÜLTESİ DERGİSİ
although these proportions were significantly different for a hypothesis test at 95%
level, with a greater level, they were not different. Thus, the risk of being damaged
is only slightly higher for exotic SYGs than for native SYGs.
Table 8. 2x2 contingency table of tree SYGs.
SYGs
Naturalness
229
Health status
Undamaged Damaged
Native 92 134
Exotic 266 567
This seems to be a coherent result since native species are expected to be better
adapted, although there are authors who maintain that exotic species do better than
native ones because their pests and diseases have not yet arrived from their home
country (Schimdt and Kerenyine-Nemstothy, 1999). Similarly, Harris explains that
native species sometimes do not perform as well as exotic ones (Harris et al.,
2004). Therefore, as it remains unclear what kind of species should be planted
more frequently, it should be consider that exotic flora affects native and overall
species richness throughout the globe (Alvey, 2006) while native species improve
the sustainability of urban forests (Clark et al., 1997). In any case, the
recommendation of not planting invasive exotic species should be followed (Alvey,
2006).
4.11 Ranking of less suitable species to be grown in Madrid.
An algorithm to establish which tree and shrub species are less adapted to the
urban environment of Madrid is created. On one hand, a quantitative scale is
established to asses the severity of the observed disturbances, assigning greater
coefficients to the most serious disturbances (Table 9). On the other, a qualitative
scale takes into consideration: a) the number of disturbances which appeared
within each species, and b) the proportion of damaged SYGs. The result of
multiplying each severity coefficient by the number of disturbances (a) is weighted
with the proportion of damaged SYGs (b) within each species. The outcome is a
ranking with the least suitable species, where the species with the highest grade
appear on top (Table 10 and Table 11).
Table 9. Severity classes of disturbances with coefficients.
Severity classes of disturbances Coefficients
Hazardous 0.5
Pretty serious 0.3
Less serious 0.2
Acceptable 0.1
This kind of species suitability ranking has been observed in other surveys
(Impens and Delcarte, 1979; Raupp and Noland, 1984; Nielsen et al., 1985; Ball,