In North Africa, Cedrus atlantica forests are in decline, following decades of anthropogenic pressure and repeated drought events. Mixed C . atlantica-Quercus ilex stands located in the Theniet El Had National Park (northwestern Algeria) were considered in the present study. Based on a stratified sampling, six temporary plots were established to describe structure (species composition, density, total height, diameter and basal area) and radial growth in relation to type of stand (pure and mixed), using a dendroecological approach. The diameter distribution of C . atlantica indicated the presence of few young individuals and a poor regeneration status in all the stands. The density and basal area of C . atlantica were significantly higher in pure stands, which necessitate silvicultural operations. In contrast, Q . ilex showed a progressive dynamic, at moderate altitude. Radial growth showed a significant decline from 1980 to the 2000s. The absence of an appropriate management plan and the increase in anthropogenic effects, without ruling out a role for the current climate conditions marked by drought, seem to be the causes of C . atlantica decline.
The study of the dynamics of forest systems has recently undergone a resurgence of interest, especially in the Mediterranean region where these systems show a tumultuous history of human action and climate change [
The Atlas cedar (Cedrus atlantica Manetti) is an endemic species of the highest mountains in North Africa (Algeria and Morocco), where it occupies a very fragmented area [
The Atlas cedar has an important place in the Mediterranean landscape in Algeria and Morocco [
Here, we describe and analyze the forest structure, regeneration and recruitment history of C. atlantica stands in the “Theniet El Had” Atlas cedar forest (northwestern Algeria), in pure and mixed forests located along an altitudinal gradient, as a necessary step prior to the development of forest management alternatives to promote forest conservation at the regional scale. The results will improve our understanding of the structure and dynamics of C. atlantica forests and allow a management plan to be proposed based on a comprehensive understanding of the structures and regeneration processes.
The study was conducted in the National Park of Theniet El Had (hereafter TNP), located in the Eastern extension of the Ouarsenis massif (northwestern Algeria) (coordinates 35˚47'N and 35˚54'N, 01˚54'E and 02˚02'E) (
varies between 854 ma.s.l. and 1786 ma.s.l. The geology is characterized by the presence of Oligocene sediments composed of a thick series of sandstones superimposed on thin layers of clay and marl [
In the TNP, C. atlantica forests occur naturally between 1200 and 1786 ma.s.l. but are rare below 1300 m due to land-use pressure. The stands included in this study are dominated by C. atlantica, often mixed with Quercus ilex L. subsp. ballota (Desf.) Samp and Q. faginea Lam., and, to a lesser extent, Pinus halepensis Mill., Q. suber L. and Juniperus oxycedrus L.
The climate is characterized by cool, wet winters and hot, dry summers. Annual precipitation varies between 580 mm (854 ma.s.l.) and 868 mm (1786 ma.s.l.), with only 5% falling between June and August, resulting in late-summer drought. The drier period lasts 3 - 4 months, from May to September, giving the study area a Mediterranean climate [
Atlas cedar forests in the TNP are spatially heterogeneous and can be classified along an altitudinal gradient. High-altitude stands are characterized by their pure cedar composition and absence of recent human intervention. In medium-altitude forests, the species composition is more complex and human impacts are evident. Although the extent and frequency of logging in these forests are unclear, it is likely that a few moderate, to large-sized trees, have been harvested for local use.
A long this environmental gradient, at each altitude, care was taken to select a plot representative of the site and forest conditions according to the following criteria: 1) at least 10 adult trees were present, 2) the forest patch in which the stand was located was dominated by cedar, 3) reliable information about when the stand was last logged was available, 4) there was no sign of recent disturbance (e.g., fire, windstorm) other than occasional livestock grazing, and 5) it was accessible [
Sites | Stand types | Coordinates | Elevation (m a.s.l.) | Exposure | Slope (%) | Soil type | Number of trees (cores) |
---|---|---|---|---|---|---|---|
Site 1 | Cedrus atlantica | 35˚53'23"N - 2˚00'02"E | 1460 | North East | 20 - 30 | Verti soil | 15 (29) |
Site 2 | Cedrus atlantica | 35˚51'44"N - 1˚59'18"E | 1520 | North East | 50 - 60 | Colluvial soil | 15 (27) |
Site 3 | Cedrus atlantica | 35˚51'08"N - 1˚57'08"E | 1700 | North | 50 - 60 | Colluvial soil | 15 (28) |
Site 4 | Cedrus atlantica Quercus ilex Pinus halepensis | 35˚52'06"N - 1˚58'01"E | 1325 | North East | 10 - 20 | Colluvial soil | 15 (30) |
Site 5 | Cedrus atlantica Quercus faginea Quercus ilex | 35˚52'35"N - 1˚56'00"E | 1420 | North West | 20 - 30 | Colluvial soil | 15 (30) |
Site 6 | Cedrus atlantica Quercus faginea Quercus suber | 35˚51'50"N - 1˚57'27"E | 1550 | South West | 50 - 60 | Colluvial soil | 15 (23) |
50 m plots were established in C. atlantica stands according to a stratified random design. Forests were classified as a) High altitude and North-facing exposure with pure cedar stands (sites 1, 2 and 3), b) Medium altitude with mixed cedar stands (sites 4, 5 and 6).
In each site, all living and dead trees (diameter at breast height, dbh > 10 cm) were identified, and dbh and height (H) were measured using, respectively, a caliper and a hypsometer (n = 423). Regeneration was classified in four size classes: seedlings (h < 50 cm), short saplings (50 cm ≤ h < 130 cm), tall saplings (h ≥ 130 cm and dbh < 5 cm) and juveniles (5 cm ≤ dbh < 10 cm) [
We grouped the C. atlantica trees into 10-cm-diameter size classes, averaged densities across sampling plots and generated diameter frequency distributions for each species at each site. To compare the population size structures quantitatively, the Weibull distribution was fitted to each of the empirical dbh probability distributions obtained from the censuses. We fitted two-parameter Weibull functions [
Prior to the statistical analysis we examined the dbh and stand density for normality and homoscedasticity using the Kolmogorov-Smirnov test and Levene’s test, respectively. The square-root transformation of dbh was used to stabilize the variances. Once the basic requirements were found to be met, the stand density of adult trees, dbh, height, stand basal area, and regeneration of C. atlantica, Q. ilex, and Q. faginea were compared between pure and mixed stands using analysis of variance (one-way ANOVA) [
From 15 dominant Atlas cedar individuals per plot, two cores at breast height were taken using a Suunto® increment borer, in a direction parallel to the contour lines, to study radial growth dynamics (
Cumulative growth curves for each series were established to study the effect of environmental conditions on the productivity of each site [
Among the adult trees, all six plots were dominated by C. atlantica (
The height of C. atlantica differed significantly (F = 21.81; P < 0.01) between forest types, where the average value in pure stands was 14 m and the average value in mixed stands was 11 m. The tree height of Q. faginea was comparable but for Q. ilex and J. oxycedrus were smaller than that of cedar; however, these differences were not significant between forest types (F = 0.04; P = 0.84; F = 1.63; P = 2.70, respectively).
The mean dbh of C. atlantica ranged from 37.5 cm (mixed stands) to 50.0 cm (pure stands) (
Forest type | Species (%) | Adult density (trees ha−1) | Juvenile density (tress ha−1) | Height (m) | Diameter (cm) | Basal area (m2・ha−1) |
---|---|---|---|---|---|---|
Pure stand | Cedrus atlantica Quercus faginea Quercus ilex Juniperus oxycedrus | 203.3 ± 15.0* 16.6 ± 3.3ns 43.2 ± 18.5ns 4.4 | 9.9 ± 8.3ns 0 9.0 ± 9.0ns 1.1 | 14.9 ± 1.2** 10.4 ± 1.6ns 4.3 ± 0.2ns 4.5 ± 1.3 | 50.0 ± 5.1** 36.5 ± 3.7ns 16.7 ± 2.0ns 18.6 ± 4.2 | 45.7 ± 7.2* 1.9 ± 0.0ns 1.2 ± 0.6ms 0.3 |
Mixed stand | Cedrus atlantica Quercus faginea Quercus ilex Juniperus oxycedrus Pinus halepensis Quercus suber | 94.4 ± 35.8 81.0 ± 19.2 21.0 ± 7.7 87.6 31.1 7.7 | 8.8 ± 4.8 3.3 ± 3.3 5.5 ± 5.5 2.2 0 0 | 11.7 ± 2.3 11.2 ± 3.0 6.7 ± 1.8 2.6 ± 0.1 9.0 ± 1.3 9.3 ± 2.1 | 37.5 ± 10.1 41.6 ± 17.3 28.6 ± 7.0 19.9 ± 3.5 34.5 ± 4.2 38.3 ± 11.2 | 14.3 ± 4.7 9.2 ± 6.4 1.6 ± 0.6 1.0 3.6 2.9 |
stands) to 36.5 (pure stands) cm and from 28.6 (mixed stands) to 16.7 (pure stands) cm, respectively without significant differences. The basal area of C. atlantica ranged from 14.3 m2・ha−1 (mixed stands) to 45.7 m2・ha−1 (pure stands, F = 9.39, P = 0.03), while those of Q. faginea and Q. ilex ranged from 9.2 m2・ha−1 (mixed stands) to 1.9 m2・ha−1 (pure stands, F = 0.77, P = 0.44) and from 1.6 m2・ha−1 (mixed stands) to 1.2 m2・ha−1 (pure stands, F = 0.22, P = 0.66), respectively. In pure stands, C. atlantica accounted for over 93% of the basal area, but in mixed stands less than 57%. For J. oxycedrus, P. halepensis and Q. suber in mixed stands, the mean basal areas were1, 3.6 and 2.9 m2・ha−1, respectively.
The distribution of the diameter classes of C. atlantica (
Seedlings, short saplings and tall saplings of C. atlantica were very low for both stand types, ranging from 8.8 trees ha−1 (mixed stands) to 9.9 trees ha−1 (pure stands, F = 0.014, P = 0.931), and they were completely absent in some of the plots (data non-included). The density of juveniles ranged from 3.3 (mixed stands) to 0 trees ha−1 (pure stands, F = 0.60, P = 0.49) for Q. faginea, and from
5.5 (mixed stands) to 9.0 trees ha−1 (pure stands, F = 0.108, P = 0.75) for Q. ilex.
The age data from all the samples (N = 90) and the recruitment history at the study site suggest that stands of Atlas cedar are old with a clear dominance three age classes: 100 - 120 years (44.7%), 120 - 140 years and 140 - 160 years with 21.1% respectively (
The curves representing the temporal variations in BAI show the existence of favorable and unfavorable phases for growth (
1910 (
The current structure of C. atlantica stands results from the combined effects of
site characteristics (altitude, aspect, slope and soil), climate and human pressure. Consequently, we can distinguish two types of Atlas cedar stand in the TNP: pure and mixed. The pure C. atlantica stands occupy the high altitudes (>1500 m a.l.s), where the climatic conditions are more favorable for the species and human activity is moderate. These stands are characterized by high density (≥186.6 trees ha−1) and low values of basal area (≥32.4 m2・ha−1) which indicate competition between trees. In moderate altitude (<1500 m a.s.l), C. atlantica grows in association with Q. ilex and, to a lesser degree, with J. oxycedrus, P. halepensis and Q. suber where human activity has been very intense (grazing, fire, opening of forest areas after illicit logging) and the climatic conditions are less favorable, and in association with Q. faginea in wet and deep soil [
The C. atlantica stands are jeopardized by the lack of regeneration and high tree senescence. Juveniles are absent in some sites and their density remains very low in others (3.3 to 26.6 trees ha−1). The reconstruction of tree recruitment in the TNP has shown peaks linked to climate. Indeed, young cedar recruitment has followed phases of adult tree mortality, especially in the 1860s, 1880s and 1890s, due to drought [
The role of the climate and the forestry practices during this period are at the origin of the current situation of cedar forest. In 1923, the cedar forest of Theniet El Had was proclaimed a National Park with a total ban on cutting and pasture. This situation would have favored regeneration at first time, but after the closure of the stands and severe drought conditions (during the 1940s) the regeneration process was deteriorated. After the independence of Algeria (1962), the situation did not improve, and the forest continued to undergo all kinds of aggression: illegal cutting, excessive grazing even after its proclamation as a national park in 1983 [
Regeneration problems have been mentioned for all stands of cedar throughout its range in Algeria [
The radial growth and the productivity of C. atlantica seem to be better in low-density sites with a Northern exposure, high altitude and little slope. This species is very sensitive to fluctuations in climate, especially rainfall [
The current state of C. atlantica stands requires the development of a management plan for the short- and medium-term, to improve natural regeneration and reduce tree mortality. This plan must consider the particularities of the environmental conditions of cedar forests and should integrate as much as possible the local populations. Algerian forest policy prescribes a regulatory framework for the management of national parks. Indeed, the TNP is sub-divided into several zones depending on the interests of conservation and the socio-economic equilibrium. The cedar forest is mainly found in the integral zone and benefits from a major conservation interest [
Natural regeneration can be improved through an integrated silvicultural plan. First of all, it will be necessary to apply a thinning program in some stands to reduce density [
Site history (fire, forest pests and overgrazing) and lack of management have significantly reduced the extent of old-growth C. atlantica forests in the Theniet El Had National Park during the past two decades. The results of this study show a clear relationship between structures and dynamics of C. atlantica with climate-related mortality events suggesting that structural characteristics of those old-growth woodlands may be threatened under current and projected future conditions. The lack of regeneration has led a reduction in cedar cover and structural complexity. Our analysis suggests that these changes in structure and dynamic together with mortality events may alter the future of C. atlantica forests. However, our results are limited by sampling limits as number of locations, recruitment data, or the inclusion of other perturbation events as fires and forest pests. Therefore, more research is still needed concerning the ecological dynamic and biotic and abiotic impacts on C. atlantica forests to improve sustainable ecosystem management. In particular, those related to pest and climate change impacts on cedar forest in Argelia may be useful in providing data for ecological studies of the relations between pest outbreaks, stand structure, species diversity, site history (fire, logging, grazing, etc.), and climatic events. We suggest silvicultural and conservative management strategies oriented to maintain and preserve not just the overstory canopies, but also the understory complexity and diversity that supports seedling recruitment, so that the woodland may be sustainable in future climates.
This work was funded through the PNR project (Algerian Ministry of Higher Education and Scientific Research). The authors thank the Algerian Ministry of Agriculture and Directorate General of Forests for the permission to work in the National Park of Theniet El Had. We are very grateful to Abdelkader Masloub (National Park of Theniet El Had, Algeria) for his valuable assistance during fieldwork. We also acknowledge the institutional support of the University of Cordoba-Campus de Excelencia CEIA3 and ESPECTRAMED (CGL2017-86161-R) project.
The authors declare no conflicts of interest regarding the publication of this paper.
Sarmoum, M., Navarro-Cerrillo, R.Mª., Guibal, F. and Abdoun, F. (2018) Structure, Tree Growth and Dynamics of Cedrus atlantica Manetti Forests in Theniet El Had National Park (N-W Algeria). Open Journal of Ecology, 8, 432-446. https://doi.org/10.4236/oje.2018.88026