An invasive species, according to the U.S. government, can be defined as an alien species (nonindigenous or exotic) whose introduction does or is likely to cause economic or environmental harm or harm to human health. The process of invasion can be characterized in stages. The first stage is “introduction” which means that the plant has been transported, either intentionally or inadvertently, by humans across a major geographical barrier or obstacle (mountains, oceans). The second stage of establishment or naturalization occurs when abiotic and biotic barriers to survival and regular reproduction are overcome, allowing a population of nonnative plants to sustain itself over several life cycles without the help of humans. Once an established nonnative or alien species produce reproductive offspring, undergoes widspread dispersal and becomes incorporated into the resident flora it is considered invasive. However, unlike invasive plants, naturalized or established plants are not always a severe threat. Only a small percentage of naturalized plants become invasive. According to Williamson and Fitter (2006), approximately 10% of the exotic plants that are introduced into a new region will actually escape into the wild. Of that 10% of plants that escape, only 10% will become naturalized, and only 10% of the naturalized plants will become invasive. Also, the invasion process does happen immediately. Rather, many alien species experience a lag phase between introduction and rapid expansion.
Population Growth Pattern of a Newly Introduced Species
Source: (FFTC) http://www.agnet.org/library/eb/539/#picf2
“Wilding Conifers” is the expression New Zealanders use for conifers that have escaped from lumber plantations and have become locally established and invasive. It is not surprising that the problem of alien conifers was first noticed in New Zealand, considering the forestry industry there almost exclusively uses northern hemisphere conifers which quite easily adapt to the gentle New Zealand climate and favorable soils. New Zealand is exemplar of oceanic island vulnerabilty to alien plants, but this problem has also occured throughout the southern hemisphere on continents as well. There are also numerous examples in the northern hemisphere of alien conifer establishment and invasion.
According to Richardson and Rejmanek, (Diversity and Distributions vol. 10: pp. 321-331, Sept. 2004), world wide there are 80 conifer taxa known to be naturalized and 36 of these are invasive (6% of all conifer species). 28 of these known invasive conifers belong in the family Pinaceae and 21 are in the genus Pinus. Cupressaceae has 6 invasive species in four genera. Besides the marked bias towards Pinaceae and Pinus in particular, invasivness of conifers is associated with 3 characterisitcs: (a) small seed mass, (b) short juvenile period and (c) short intervals between seed crops.
Rejmanek has developed a Z Score to quantify the invasive potential of pine species: Z=19.77 – 0.51 x square root of M – 3.14 x square root of J – 1.21 x S
M=weight of 1000 seeds (in grams), J=time between germination and reproductive maturity (in years), S=difference between mass seed crops (in years).
Some examples of pines with high Z scores are: P. contorta (11.41), P. virginiana (10.02), P. radiata (9.27), P. banksiana (8.85), P. halepensis (8.21), P. muricata (8.14), P. pinaster (7.46), P. patula (7.30) and P. sylvestris (7.12).
Some pines with very low Z scores include: P. pinea (-13.58), P. cembra (-10.31), P. lambertiana (-12.35), P. sabineana (-7.97), P. torreyana (-7.62) and P. palustris (-7.36). However those low Z score pines which have very large seeds (contributing to the low score) have another avenue of dissemination and invasivness. This is because these seeds are food and are subject to spread by vertebrates (birds in the family Corvididae and rodents such squirrels). Examples of this are Pinus pinea, Pinus cembra and Pinus sabineana. This mechanism also explanes the invasive propensity of some of the Cupressaceae family members, in particular Juniperus and Taxus species.
Pinus pinea-Angelina unit
Our pinetum is constantly aware of the potential for inadvertently introducing invasive species in the noble interest of furthering horticulture. We endeavor to avoid this by considering the invasive potential of all taxa we plant in each of our three units. In some instances this means avoidance of such introductions and in other instances continual surveillence of suspect trees.