Variability and dynamic response of the cedar to climate change in the Eastern Middle Atlas Mountains, Morocco

512

Variability and dynamic response of the cedar to climate change in the Eastern Middle Atlas Mountains, Morocco

R. Ilmen^(a)*, A. Sabir^(b), M. Benzyane^(c) etM.S. Karrouk^(d)

(a)Laboratory of Dendrochronology, Center of Forest Research, P.O .Box: 763, Agdal, Rabat, Morocco.

(b)Department of Physics, Faculty of Sciences, University Mohamed V Agdal, P.O. Box: 1014, 4 Avenue Ibn Battouta, Rabat, Morocco.

(c)High Commissary of Water, Forest and Combating Desertification, P.O. Box : 605-Chellah,Rabat, Morocco.

(d)Research Center of Climatology, University Hassan II, BP: 8220 Oasis, Casablanca, Morocco.

*Corresponding author. E-mail: ilmenrachid@gmail.com

Received 3 Sept 2014, Revised 20 Oct 2014, Accepted 24 Oct 2014

Abstract

Climate change represents more and more a reality to master and whose impacts are important and it’s a necessity to limit them. On a global scale and particularly in Morocco, we expect an increase of temperatures as well as level of sea. The Moroccan cedar forest is subjected to a multitude of challenges and multi-form pressures leading to a regressive evolution of its ecosystems and to an imbalance of rural societies which live there, what makes it an interesting model for studying the climate change effect susceptible to affect the cedar forest ecosystem productivity in Morocco. The impacts of climate change on the Moroccan cedar forest and the evaluation of their vulnerability have not been the object of a specific study yet. Thus, the dendroecology which has for support the measure of trees growth has data well adapted to study climatic factors impacts on forest stands productivity. The present work was conducted in a forest site named

"Taoalt" located in the Eastern Middle Atlas. The sampling consists in taking 2 cores/tree from 20 trees of Cedrus atlantica M., wether 40 cores are extracted from the whole site. The obtained results show that tree ring chronologies going from 1498 till 2011 AD are very sensitive to climatic variations with a high mean sensitivity of 0,267 and important fluctuations noticed in the ring width evolution due to a specific natural disorder affecting the cedar forest. The Pearson's correlation coefficients indicated a negative and significant response (p< 0,05) of radial growth to monthly mean temperatures of June and October stemming from the meteorological station of Midelt. While precipitations of September necessary for the production of final wood influence positively the cedar growth. The low temperatures in winter (January) and especially frosts can lead to a strong reduction of the growth.

Keywords: Climate change; Cedrus atlantica; Tree ring; radial growth; Morocco.

1.

Introduction

The dendrochronology is a method of dating which allows to determine the period during which a tree has lived and, for example, to specify its year of cutting down (possibly the season). This discipline is based on

513 analysis of wood growth. Every year, the tree produces a growth ring; their number indicates the lifetime of a tree [1]. The properties of tree rings contain mainly the cells size, the wood density, the stable isotope ratios and the width. The Ring Width (RW) is modulated by three climatic parameters: rainfall, temperature and illumination, therefore (RW) is going to register all annual variations of those parameters and to establish an excellent climate change signature [2]. Given that the climate is the factor limiting the forest species growth in arid and semi-arid regions, the sequence of narrow and wide rings in the trees chronologies is an indicator of annual climatic fluctuations [3]. The Goals of the present study is to:

 Build a local chronology of cedar trees rings in the Eastern Middle Atlas,

 Highlight the relationship between the climate (temperatures and precipitations) and the cedar tree ring width.

2.

Materials and Methods

The Taoalt forest situated in the Esatern Middle Atlas, Morocco extends over a surface of 19910 ha of which contains 8020 ha of cedar. The trees mean growth is 0,2 m3 / ha / year, the basal area is 6 m² / ha. The cedar wood production is estimated at 59 m 3 / ha. The dominant trees (D) have an average diameter [Diameter Breast Height (DHB)] of 297 ± 4,02 cms, while dominated trees (d) had a DBH of 195,36 ± 2,83 cms. The mean height of trees is 14,4 ± 0,40 m and the substratum is shale- marl-calcareous.

2.2. Sampling and development of chronology of the cedar

The sampled site is situated at a height of 2154 m and in approximately 200 km from the Atlantic Ocean. A total of 40 cores were taken from 20 trees situated on a south exposure and in a slope of 35°C by using the Pressler increment borer (figure1). Then, the cores are coded and their transverse surfaces are smoothed. A machine called "Lintab" is used for measurement of ring width. An individual chronology is elaborated for every tree and correlations between trees are calculated to build a master chronology for the studied site. The mean sensibility and the first-order autocorrelation coefficient were calculated [4]. The programs Cofecha and Arstan were used to develop the tree ring chronology [5]. The standard chronology will highlight the relation climate-ring.

Figure 1. Cedar tree cores sampling.

514 2.3. Growth-climate correlation

The Pearson correlation analysis is used to explore the relation between climatic conditions such as the monthly mean temperature and the annual total precipitation in one hand and the thicknesses of trees rings in the other hand. In this calculation, the meteorological station of Midelt near the sampled site supplied climatic data from 1973 till 2011. During the aforementioned period, the lowest monthly temperatures in December, January and February took place on 2006 (4,2 °C), 2006 (3,2 °C) and on 2005 (4,3 °C), respectively. The precipitations are plentiful between October and January (total of 3584,97 mm) and they are minimal from June until August (1183,77 mm) over the period 1973-2011.

3.

Results and Discussions

3.1. Dynamics of the radial growth

In order to reduce the statistical noise in tree ring chronologies and to improve the climatic signal and the long-term tendencies, all sequences containing some compression wood were excluded from the

analysis. Therefore, two cores were eliminated. The statistical parameters shows that the Taoalt cedar forest is very sensitive to climatic variations (0,267) > 0,2 (table1). The important fluctuations noticed in the tree ring width evolution can be due to a natural phenomenon affecting the cedar forest of the aforementioned site (figure 2).

Table 1. Dendrochronologiqes characteristics of cedar Taoalt.

Master chronology length 14986-2011 (514 years)

Mean width measurement (mm) 1,052

Mean sensitivity 0,267

Standard deviation 0,239

First order autocorrelation 0,632

Period with a common interval 1781 à 2011 (231 ans)

Mean correlation among all series 0,312

Mean correlation between trees 0,243

Mean correlation within trees 0,225

Signal to Noise Ratio (SNR) 12,55

Expressed population signal (EPS) 0,940

Variance explained 13,15%

The mean values of correlations between trees and the mean sensibility give to think that the cedar of Taoalt is suitable for a dendrochronological analysis. The cedar forest of this site has a great value of correlation among all series (0,312) which suggests that this species responds to a climatic signal. The high value of the standard deviation also indicates that the trees ring width is subjected to important intra- and inter-annual fluctuations.

The cedar tree ring chronology shows random fluctuations during the whole period (1498-2011) (figure 2).

The lowest radial growth is recorded during the year 1499 (0, 07 mm) and in 1525 (0, 22 mm), whereas the maximal radial growth is noticed in 1958 (2,04 mm), 1738 (1,95 mm) and in 1810 (1,94 mm). The most important changes which represent the long-term oscillations are registered during the period 1975-2003

515 when the trees growth tendency is decreasing and during the period 1723-1738 when the radial growth has an upward trend which peaks in the year 1738.

Figure 2. Master Chronology for Taoalt cedar population.

3.2. Relation ring- climate

The correlation results between cedar chronology and mean monthly temperatures and monthly total precipitations recorded in the meteorological station of Midelt are represented in the figure 3. Indeed, the significant influence of climatic conditions is proved; in a great majority of cases, extreme growths coincide with particularly dry or wet years. The Taoalt cedar forests reacted massively and negatively during 1980, 1981 and 1983, years known for their hot and dry summers with high summer temperatures from 0,2 to 0,4°C and lower summer precipitations from 5 % to 46 % compared with the average calculated over 1973- 2011. During these three years, the growth reduction was on average -16 %, -17 % and -23 %, respectively.

Figure 3. Response Function of cedar radial growth to climatic conditions.

However, cedars responded positively in years with cool and/or wet summers, such as in 1975, 1977, 1978 and 2010 during which the growth was superior respectively of 11 %, 10,6 %, 9,9 % and 9,3 % to the average. The Pearson correlation coefficients between the local climatic data and the cedar standard chronology showed a negative and a significant response (p< 0,05) with the monthly mean temperatures of

0 0.5 1 1.5 2 2.5

1498 1548 1598 1648 1698 1748 1798 1848 1898 1948 1998

Tree ring width (mm)

Years

-0.4 -0.2 0.0 0.2 0.4

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Pearson correlation

Months

Precipitations (mm) Temperature(°C)

516 June and October. The relation climate- cedar ring showed a positive growth response to September. On the other hand, the spring and autumnal precipitation (in May, October) influence negatively the annual growth.

The rainfall during the end of the growth season second part (in September) is decisive for the production of final wood (more and more at the season end especially in October), the positive relation between the November-December precipitations and the tree ring width favor the hydric refill of soil reserves. While low temperatures in winter (January) and especially frosts can lead to a strong growth reduction, which delays implementation of rings from the spring. The cedar response analysis to climate in the Eastern Middle Atlas Mountains highlighted the importance of June temperatures and the September precipitations in agreement with other studies [6-7]. Years characteristics- thin rings seem connected to colder or drier climatic conditions compared to the normal, this property can affect either the year in general, or the season in particular.

In Taoalt, the temperatures of June are negatively correlated to the initial wood width. Indeed, the soil drought caused by the evapotranspiration can lead to a building of smaller needles that can reduce not only the water loss, but also the quantity of available photosynthates for the growth restart. Increasing temperatures in late winter lets the dormancy by stimulating hormone secretion causing a cambium reaction [8]. The mean radial growth during the period 1597-1608 is 0,691 mm / year which is widely lower than the mean tree rings width of 36,1 % over the period 1498-2011. This result coincides with a period of drought that our planet knew between 1597-1608 [9].

4.

Conclusion

The dendroclimatological study conducted in the Taoalt site allowed to explore the relations growth-climate in cedars rings. The cedar chronology goes back up until 1498, whether an age of 514 years. The Pearson correlation results indicate that the temperature of June and October have a negative and significant impact on the ring width. However, there is a positive relation between the precipitation of September and the cedar growth. Thus, the climatic conditions are important factors influencing tree ring variations in the study area.

References

[1] F. Lebourgeois, P. Merian. LERFOB, AgroPariTech., 85 (2012).

[2] R.N. Jones, Clim. Change., 67 (2004) 13-26.

[3] R. Touchan, M.K. Hughes, J. Arid. Environ., 42 (1999) 291-303.

[4] H.C. Fritts, Academic Press., 567 (1976).

[5] E.R. Cook, R.L. Holmes University of Arizona, 65 (1986).

[6] M. Carrer, P. Nola, J.L. Eduard, R. Motta, C. Urbinati, J. Ecol., 95 (2007) 1072-1083.

[7] W. Oberhuber, W. Kofler, K. Pfeifer, A. Seeber, A. Gruber, G. Wieser, Trees., 22 (2008) 31-40.

[8] G.T. Creber, W.G. Chaloner, The vascular cambium (1990).

[9] H. Safi, UMV, 190 (1990).