Aims

The Cedrus dataset was set up to assess the climatic niche breadth of Cedrus libani – a relict mountain species – by comparing field observations and transplant experiment measurements within and beyond the elevational range limits of Cedrus under natural conditions.

Location

Lebanon - Near East - Mediterranean region

Methods

The transplant experiment included 8 common gardens at altitudes ranging from 110 to 2330 m, within and far beyond the warm and cold limits of Cedar distribution under natural conditions. It was set up in 2018, and monitored for three years until fall 2021 to study survival and growth of the Cedar of Lebanon (Cedrus libani) and 3 potentially competing species (Quercus calliprinos, Quercus infectoria and Pinus brutia). Field observations of Cedar presence/absence were carried out on 1023 sites.

Results

We observed surprisingly high survival and growth rates of Cedar at elevations well below its natural range in Lebanon. Below its elevational limit of 1300 m under natural conditions, water stress limited the survival of juvenile Cedars at elevations below 500 m, and its low competitiveness below 900 m explained its absence between 500 and 900 m elevation. On the other hand, cold temperature and water stress limited its survival at elevations slightly above the observed natural upper elevational limit of 1830 m.

Main conclusions

The experimental setup demonstrated that the elevational range suitable for the growth and survival of the Cedar of Lebanon is twice as large as the range within which Cedar is observed today under natural conditions. The high survival rate beyond the lower limit of its natural range raises hope for its resilience to ongoing climate warming. If this pattern were common to other mountain species, it would challenge predictions of massive extinction linked to climate change, and pave the way for promoting adaptive actions such as competition management to improve their survival.

Distribution of Cedar in natural conditions

To assess the altitudinal distribution of the Cedar of Lebanon in the natural conditions of northern Lebanon, we recorded its presence/absence in 1023 sites located outside urban areas and plantations, i.e., in 750 field vegetation surveys and 273 high-altitude sites with a low tree cover visually checked for its presence using satellite imagery. This dataset was used to define the elevational range of the sites harboring Cedar. The lower and upper range limits of Cedar under natural conditions were defined by the 5^th and 95^th quantiles of the elevational distribution of Cedar occurrences (Stahl et al., 2014).

Survival and growth in the transplant experiment

A transplant experiment was set up in northern Lebanon along Kadisha Valley. This region is at the heart of the geographical range of Lebanese Cedar populations, where a 0-3000 m elevational gradient can be covered within a few tens of km. Given that the altitudinal distribution of Cedar provided by the literature extends between altitudes of approximately 1100 and 1950 m (Khuri et al., 2000), eight experimental sites were established at regular intervals between 110 and 2320 m elevation to test for Cedar fitness, starting from the middle of the range and then up and down to the cold and warm limits of the range and far beyond. Table ‘B2_ExpSites_Clim.csv’ provides the key climatic parameters of each experimental site. 

Common gardens were established on sites with optimal soil conditions, chosen to highlight the effects of climate in the absence of edaphic constraints. Since the Cedar of Lebanon grows essentially on calcareous or calcaro-dolomitic substrates (Boydak, 2003), the common gardens were located in a calcareous environment (except the site located at 970 m altitude) on deep (> 40 cm) plowed soils cleared of large stones and with a predominantly clayey texture. The effects of exposure and slope were minimized by selecting sites in topographic situations with a slope of less than 3% (except for the two sites located at high altitudes). As the Cedar of Lebanon is a heliophilous species (Boydak, 2003), the sites were located in full light. They were also covered with canvas and gravel and regularly weeded manually to avoid competition from herbaceous species. A water tank was installed at each site for plant watering. Finally, all the sites were protected from grazing by an enclosure, except the two high-altitude sites located in ungrazed, supervised areas.

Cedar seeds were collected in April-May 2018 from natural stands at the low (1232 m a.s.l), middle (1431 m a.s.l.) and high (2011 m a.s.l) elevational limits of its distribution range in northern Lebanon. Seeds of the three competing species of Cedar under natural conditions –- P. brutia, Q. calliprinos and Q. infectoria – were also collected at the low and high altitudinal limits of each species’ range in natural forests in northern Lebanon and in mid-range, at the same period as Cedar for pine, and in November-December 2017 for the two oak species. Seeds of cedar and the other species were planted in pots immediately after harvesting. All seedlings were transplanted to the experimental sites in fall 2018. Each site was composed of two batches: a non-watered batch representative of natural conditions, and a control batch watered during the summer period to study seedling survival and growth in the absence of water stress.

Seedlings of the four species were planted in the batches with a constant spacing of 50 cm to allow them to grow freely until the juvenile stage. Seedlings from low, medium, and high altitudes were distributed evenly between the sites and treatments: 12 trees per provenance and species were planted in each ‘watered’ treatment per site (72 seedlings per species in each common garden and 576 seedlings per species altogether).

The experiment lasted 3 years – from fall 2018 (planting) to fall 2021 (last measurements). To ensure seedling recovery after transplanting, both the watered and non-watered batches were watered in 2019 during the first growing season after transplanting. Watering was discontinued for the non-watered seedlings after the first rains in the fall of 2019, whereas the ‘watered’ seedlings were watered with 13.9 liters of water twice a month during spring and summer in 2020 and 2021.

Tree height, diameter at the base of the stem, and survival were measured in October of each year, after the growing season. These measurements were used to calculate survival rates and average growth in diameter and height at each site and for each species and watering treatment.

As dimensions of competition were not all measurable, we focused on early growth indicators as proxies of inter-species competition. The competitiveness of Cedar along the altitudinal gradient was analyzed by comparing its growth in each common garden with that of its competitors (Q. calliprinos, Q. infectoria and P. brutia) planted at the same elevation and in the same conditions (Gaudet & Keddy, 1988; Westoby et al., 2002; Tanner et al., 2005).

References

Boydak, M. (2003) Regeneration of Lebanon cedar (Cedrus libani A. Rich.) on karstic lands in Turkey. Forest Ecology and Management, 178, 231-243.

Gaudet, C.L. & Keddy, P.A. (1988) A comparative approach to predicting competitive ability from plant traits. Nature, 334, 242-243.

Khuri, S., Shmoury, M.R., Baalbaki, R., Maunder, M. & Talhouk, S.N. (2000) Conservation of the Cedrus libani populations in Lebanon: history, current status and experimental application of somatic embryogenesis. Biodiversity and Conservation, 9, 1261-1273.

Stahl, U., Reu, B. & Wirth, C. (2014) Predicting species' range limits from functional traits for the tree flora of North America. Proceedings of the National Academy of Sciences of the United States of America, 111, 13739-13744.

Tanner, E.V.J., Teo, V.K., Coomes, D.A. & Midgley, J.J. (2005) Pair-wise competition-trials amongst seedlings of ten dipterocarp species; the role of initial height, growth rate and leaf attributes. Journal of Tropical Ecology, 21, 317-328.

Westoby, M., Falster, D.S., Moles, A.T., Vesk, P.A. & Wright, I.J. (2002) Plant ecological strategies: Some leading dimensions of variation between species. Annual Review of Ecology and Systematics, 33, 125-159.

Data files can be opened with Excel or R.