Proliferation and spread of an alien species – Tetraclinis articulata (Vahl) Mast., in Mediterranean woodland in Israel

Ken Karu

Tetraclinis articulata (Vahl) Masters is a Mediterranean coniferous species. The tree, which reaches an average height of 10-15 meters, has a straight pyramid shape and usually grows on a single trunk. The natural distribution of the tree is restricted to the south west Mediterranean basin – mostly in the region of the Atlas Mountains, but small fragmented populations of the species can be found in South Europe. Of all the conifers brought to Israel for afforestation efforts, T. articulata was selected as one of the most promising species, mainly because of its high drought resistance. The propagation and spread of the tree outside plantation sites have been observed recently in various locations in Israel, such as the Ramat Hanadiv Nature Park. Despite these observations, the extent of the spread of this alien species has not been examined yet in these places and is now not declared invasive.

in Ramat Hanadiv Nature Park. The data regarding specific date and location of Tetraclinis plantations is not available, however, most of the planting was probably carried out in the late 1970s and 1980s. Tetraclinis articulata trees were, presumably, mainly planted in two central locations, but the widespread expansion of trees throughout the park in recent years has raised concerns about the potential of this species’ invasiveness in the Mediterranean woodland. This research examined the chronology and ecology of T. articulata colonization in Ramat Hanadiv while trying to identify the biological characteristics of the tree and the environmental factors that contribute to this phenomenon. The main objective of this study was to characterize the invasiveness of T. articulata and to predict future trends of this colonization within Ramat Hanadiv Nature Park. The outcome

of this study will allow an informed choice of management tools to deal with the phenomenon in Ramat Hanadiv, and can be used as a case study of the colonization and proliferation of T. articulata in other areas in Israel. The basic hypotheses for this study were: (1) The biological properties of T. articulata increase its invasion potential; (2) the invasion intensity is influenced by proximity to seed sources; (3) environmental factors affect the invasion intensity; and (4) the increase in frequency and severity of drought events accelerate the invasion process. The study was based on a preliminary field survey of invasive species, conducted by Ramat Hanadiv workers and volunteers in the years 2010-2015. During my research I surveyed and mapped all individual trees taller than 100 cm in the park. For each individual tree I measured: (1) maximum height; (2) trunk diameter at

ground level; (3) canopy diameter; and (4) presence of shrub or rock in the tree’s micro-site. In addition, a reproductive survey was conducted to evaluate annual seed production by the species. The survey lasted more than a year (2016-2017) and included phenological observations. To evaluate the age of the trees, several tree of different heights and diameters, were felled and annual rings at the base of the trunk were counted. The surveyed population of T. articulata was divided into two groups. The first is assumed to be parent trees – seed dispersers, above 40 years of age. The second includes all the rest of the population, under the age of 40, which, by the same assumption, includes all the saplings that colonized the park as a result of seed distribution from parent trees. I investigated the influence of environmental factors and proximity to seed sources on the colonization of T. articulata, using the Maximum Likelihood Estimation. I found 1248 trees > 100 cm height in the park, with the minimum age of trees at that height being 8 years. An analysis of the age of T. articulata trees in the park showed that since the beginning of the 2000’s the number of colonizers in the park has increased strongly each year. Examination of the three growth characteristics of the trees (height, age, and trunk diameter), showed that the average annual growth rate had a significant positive association with changes in tree size. Results from this study have shown that T. articulata is, in fact, an invasive species in Ramat Hanadiv Nature Park. Seed production potential in 2017 reached almost 75 million seeds throughout the park, while the survival rate of seeds was approximately 1:350,000. Nevertheless, the level of inter-annual variation in seed production is not yet known. I also found that the seed production rate is increasing with the number and age of colonizing trees, which means that invasion pressure will rise over time. The minimal reproductive age found for T. articulata is 12 years. The phenological survey showed that maturation and dispersal of seeds begin in late summer and continue in autumn.

Based on all the biological parameters examined in this study, I concluded that the trees in the park are still in positive growth phase, i.e., an exponential rise in tree density with an increase in the average age of the population. Consequently, there is also an increase in colonization pressure, which is expressed by seed production. This conclusion also applies to the oldest trees in the park, as I found no moderation or decrease in their current growth rate. The colonization process of T. articulata in the Nature Park reflects a typical colonization process of an alien species in a new habitat. Until the early 1990’s T. articulata colonization lagged and was affected by plantation of new individuals. Since the early 1990sthere has been an exponential increase in the rate of spread of the species. The main factor that dictated this colonization increment is the constant increase in the colonization pressure, i.e. seed dispersal intensity, derived from the natural dynamics of the population. Cattle grazing was identified as a strong resistance factor to T. articulata recruitment. In addition, dense and high, as well as low and sparse vegetation structures strongly reduce the chances of recruitment from seeds in the park area. These findings might indicate that under shading conditions T. articulata exhibit a slower growth rate and as a result, was more susceptible to disturbances compared to sapling in low and dense vegetation structure, which facilitates seedlings recruitment, as neighboring shrubs might provide shelter against predation or grazing. According to our estimations, in the coming years thousands of trees will colonize throughout the park, and within 80 years the advancement of T. articulata could cover almost the entire park ground, and even spread outside of the park area. In order to restrain this phenomenon, we have shown that thinning only a few hundred individual trees will result in a reduction of the invasion pressure by about 90%.

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