Which Invasive Plant?
In a famous song, Ehud Manor writes about a cypress tree he saw, but it’s possible that this was actually a different tree, a relative of the cypress. The sandarac tree (Tetraclinis articulata) came from the Atlas Mountains and in the words of the song, “stands in the field facing the sun”; but it is actually an invasive species that has settled in Israel. The ecologist, Ken Karu, conducted a study on the invasive cousin of the cypress at Ramat Hanadiv and we present the main results here
The penetration and establishment of alien species in habitats that they did not inhabit in the past is known by its more common name – biological invasion. Invasion by species that aren’t part of the local ecosystem poses a growing threat to biodiversity, ecosystem services, local economies and even public health. Invasion, which is increasing around the world due to expansion of transportation systems, technological developments, climate change, and geopolitical events, is now a central issue of interest for ecologists and open landscape managers. There are many ways to deal with invasive species; however, understanding the biology and ecology of these alien species is a fundamental stage necessary for dealing with their expansion and containing them.
One alien species, which has been planted in Israel in recent decades, and raised concerns about its invasibility in the Nature Park at Ramat Hanadiv, is Tetraclinis articulata – a tree from the cypress family. To inexperienced eyes this alien species could be easily mistaken for a cypress, but Tetraclinis is actually a relative of the Mediterranean cypress. Tetraclinis is a pyramid-shaped conifer (similar in shape to a Christmas tree) that reaches an average height of about 10-15 m. The tree has good dispersal ability, due to its small, winged seeds, which float on the wind; in addition, it is one of the few conifers that are able to regenerate from the trunk or root (rather than from seed) after pruning, grazing or fire. The tree has many uses in its native habitat in the Atlas Mountains, including grazing, provision of wood products – for burning, furniture production, construction, and even a range of medical and anti-bacterial products. Due to fire, intensive grazing and increased use of the tree for human needs, Tetraclinis has almost completely disappeared from its native habitat, and today, extensive efforts are being invested to rehabilitate and preserve Tetraclinis forests in North America and Spain. Moreover, the tree is even considered to be endangered in certain parts of the world.
Tetraclinis was brought to Israel as part of a project to acclimatize trees for forestry, mainly due to its high resistance to drought. Since Tetraclinis has not been considered invasive on a global scale, no previous effort has been made to examine its invasiveness in Israel. However, in recent years, massive establishment of Tetraclinis has been observed in the natural woodland areas of the Nature Park at Ramat Hanadiv. According to our estimates the tree was planted in the late 1970s to early 1980s at a number of locations around the Park.
This study was based on the work of Ramat Hanadiv’s volunteer staff, working in the Park with Dr. Racheli Schwartz-Tzachor, who thoroughly and diligently identified and mapped all the invasive species growing in the Park including about 1,500 trees of the invasive species Tetraclinis articulata. Within the framework of the study, we examined whether Tetraclinis is an invasive species at Ramat Hanadiv and the factors influencing its outbreak in the Park. The basic assumption of the study was that a combination of the tree’s biological traits and its resistance to drought, together with the environmental factors characteristic of Ramat Hanadiv and trends of climate change (resulting from global warming), are what brought about the outbreak. The study included a survey of the trees in the Park, mapping of Tetraclinis trees that established within it, biological measurements (height, stem circumference, number of pinecones and more) and characterization of environmental factors (vegetation formation, cattle grazing, rainfall and more).
The main conclusion from the analysis of the research data is that Tetraclinis is an invasive species at Ramat Hanadiv. The many trees, which are scattered among a number of locations around the Park, provide a source of seeds for its continued expansion. Tetraclinis has a very high ability for seed production, even higher than that of the Aleppo pine that has established itself in the Nature Park at Ramat Hanadiv over a number of decades. The establishment of Tetraclinis in the Park reflects the natural dynamics of an alien species establishing in a new habitat; as the number of individuals of this species increases, total seed production increases – and as a result the number of individuals establishing in the Park increases. Based on the results of the study we estimate that Tetraclinis has reached a stage of intensive establishment at the Park. At this stage, establishment is localized in certain areas, but according to the present expansion rate, we can estimate that by the end of the current century we will see these trees throughout the entire Park.
Furthermore, we found that in places with grazing or tall, dense woodland, establishment of the tree’s seedlings is minimal, while in areas with dense cover of low shrubs, seedling establishment is maximal. To date we have not found a clear effect of climate change or low rainfall, as observed in recent years in the Ramat Hanadiv region, on the outbreak of Tetraclinis in the Park. Nevertheless, it is not unreasonable that this effect will be expressed more significantly in the near future. We must remember that the fact that this species is recognized as an invasive species does not immediately make it a dangerous species that will eventually harm the local ecosystem, and we cannot know whether it will affect it at all. However, we must consider all possibilities and act with precaution.
Today, we know much more about Tetraclinis, and about future trends related to its establishment at Ramat Hanadiv, but there is still room for research. Meanwhile,’ all’ that is left is to decide whether and how to act, in order to contain the spread of this species beyond acceptable levels.
Ken Karu, a Master's student at the Agriculture Faculty of the Hebrew University, conducted the study under the supervision of Dr. Yagil Osem and Dr. Efrat Sheffer.
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