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Microsuccession of Cryptogams and Phanerogams in the Dead Sea Area, Israel
r Flora (1982) 172: 173-179
Microsuccession of Cryptogams and Phanerogams in the Dead Sea Area, Israel
Summary A cyclic microsuccession on arid, loessal slopes facing the Dead Sea is described. Concentric bands of vegetation dominated in turn by blue-green algae, lichens, mosses, and succulent annual phanerogams are correlated with microtopography and salinity in the top 10 cm of soil. The entire sere may take place within an area of 2 m 2 , and along an elevational gradient of 10 cm, yet some environmental factors change more than one order of magnitude. Soil conductance, surface roughness, species diversity, and biomass were measured; most taxa were identified to species level.
Introduction In a previous study (DANIN 1976), species diversity of annual plant communities of xerohalophytes and glycophytes in the Dead Sea area was related to soil salinity a.nd to a. biological crust made of cryptogams (algae, lichens, mosses). The effect of cryptogams on trapping air-borne silt (loess) has been discussed by DANIN & YA,ALON (1981). This paper, the third in a series, discussed the formation of the cryptogamic crust and its place in a cyclic micro succession which leads from barren, saline soil to a nearly closed community of phanerophytes on non-saline soil. The study was carried out in the Dead Sea Valley, 7 km southeast of Jericho, 35° 29' E longitude by 31 ° 48' N latitude, at an elevation of 320 m below sea level. Yearly precipitation averages 100 mm, mostly falling as rain between December and March. The 1979-1980 rainfall was about 200 mm, far above average. Mean annual temperature is 25°C, and January/August means are 15/33°C (Atlas of Israel, 1970). The study area is on a gentle (1_2°) east-facing slope of the Lisan formation, which has been described by LARTET (1899), PICARD (1938), BEGIN et 301. (1974), and NEEV & HALL (1977). When Pleistocene Lisan Lake withdrew 11,000-12,000 year BP, the Lisan marl was exposed. Being a soft-grained substrate, it is suitable for colonization by poikilohydric, cryptogamic plants. Such plants protected the ground from further erosion and caused trapping and accumulation of loess over time to a present depth of 20 cm above the Lisan marl (DANIN & YAA,LON 1981). The study area exhibits a pattern of microtopography which consists of vegetated plateaus 1-3 m in diameter, and eroded sloping edges bare of plant cover which lead to small basins and runoff channels about 5-10 cm below the vegetated plateaus. Plateau vegetation is zoned into bands parallel to the eroding slopes, dominated in turn by algae, lichens, mosses, and herbaceous (often succulent) annual seed plants. 1) Botany Department, Hebrew University, Jerusalem 2) Botany Department, University of California, Davis
:: A. DAXIX and
174
~I.
G. BARBOUR
Cryptogamic and cryptogamic-phanerogamic zones have their own microtopography of plates and cracks or of hills and valleys in the soil surface, over an elevational range of as much as 15 mm. We hypothesized that cycles of soil erosion, local salinization, and leaching and accumulation of soil, all accompanied by changes in the plant cover, were responsible for the microtopographic patterns. Our objective in this paper was to test this hypothesis. Methods We established two replicate transects, 10 cm wide and about 70 cm long, such that each crossed the patches of microtopographic and vegetational diversity described above. Within each discrete and homogeneous zone, 10 X 10cm sample areas were marked and utilized along the transects. Sampling was done at the end of March, 1980, at the close of the growth season of most species, when plant cover was maximum. When phanerogam zones were sampled, all above-ground biomass was harvested and later weighed dry, and the number of individuals of each species was counted. Species diversity was calculted by the Shannon-Wiener formula (SHANNON & WEAVER 1949, with the modification that proportional values were in terms of weight), and also by HILL'S (1973) method. In all zones, we estimated the relative cover by algae, lichens, and mosses. On one transect only we measured the degree of surface roughness, expressing it as the fraction of sample surface that was approximately level and the range in elevation (in millimeters) between highs and lows. In all zones, soil was collected at depths of 0 -0.5 cm, 0.5 - 2 cm, and 8 -10 cm, which defined the extent of the root zone. Soils were later mixed 1: 1 with distilled water, and the filtrates were analyzed for total salt content by conductivity bridge. Soil samples were also washed through sieves to isolate the remains of seeds, as an indication of the past flora.
Results 1. Soil salinity
Soil conductivity values are shown in Table 1, and their relationship to topography is shown in Fig. 1. There were two orders of magnitude variation in surface conductivity, with the highest values along the eroded, barren slopes (Transect I position 4, Transect II position 2), and the lowest values on plateau sites dominated by mosses and phanerogams (II, Ill, II5). Except for slope sites, salinity increased with depth. Soil structure also changed with depth, from blocky to single grain, possibly because increasing sodium concentration dispersed the soil particles. Soil structure was absent on the barren slopes, and the powdery soil was relatively unstable and erodeable. Salinity at the surface averaged more than 20 mmhos cm-1 (sites 14 and II2), but less than 9 mmhos cm-1 at 8-10 em (Table 1), indicating that water tends to move up to the surface and evaporate. No living soil crust was found on the slopes, but bits of dead crust, possibly eroded from the plateau above, were present. If we assume that conductivity results from several mixed salts, then we can use approximate relationships developed by RICHARDS (1954) to convert conductivity to parts per million or percent salt. Our maximum value of 22.5 mmhos m-1 thus corresponds to 8,500 ppm, and our lninimum value of 0.1 mmhos cm-1 corresponds f'"'o.I
to '" 40 ppm.
175
Microsuccession near the Dead Sea
Table 1. Some traits of sample sites within Transects I and II. Data include: soil conductivity (mmhos cm-1 at 25 °0) at the surface (01), at 0.5-2 cm depth (02), and at 8-10 cm depth (03); species diversity by Shannon-Wiener (H) and Hill (eH ) indices; and surface roughness (percent of sample that was level and range in millimeters between highs and lows). For diagrammatic summary of each transect see Fig. 1 Transect Oonductivity position 01 02 03 Il 12 13 14
15
III 112 113 114 115
0_2 1.4 3.6 18.5 2.1 0.2 22.5 1.9 0.5 0.2
0.1 1.7 7.3 1.4 0.1 0.9 1.3 0.2
Diversity
1.4 4_5 6.7 9.2 1.5 1.2 9.5 2.9 3.5 0.8
Roughness
H
eH
0.68 0.26 0.00 0.00 0.00 0.95 0.00 0.00 0.21 0.57
1.97 1.30 1.00 1.00 1.00 2.59 1.00 1.00 1.23 1.77
Level
Site description
Range
10
10-15
60 30 15
1- 4 2- 5 4-11
Phanerogam plateau Phanerogam· cryptogam Algal zone above slope Barren slope Algal flat behind slope Phanerogam plateau. Barren slope Algal flat ·behind slope Phanerogam-cryptogam Phanerogam plateau
30
45
75
0
100
L
20
40
25
0
0
M
50
15
0
0
0
A
(A)
Site
.::- ::.. :~.'~~', I': :-.,: ... :
11
"
12
......
..~ .
13
'
IS
.':..
~ , ":. ! .-:-:. :.::: ;-.
S
2
W
4.04
A L
5 10
M
8S
',-: :': ...·.··1:··... ·. ,,::. (8)
Site
111
2 1.01
..
1
0
1
0.003
0
0.79
0
90
82
40
0 0
10 0
15 3
40
113
114
lIS
• '.'. I :~ ~
':',
..
.~
3
0
W
3.16
0
"'.:-;',:",,:.--; ~~-..: ... :;~··~.l·' : ~
1 0.03
level
20
-:.
S
Ground
~;.:::-: f'..
3
2
1.85
4.02
• Ground '.' level
Fig. 1. Topographic and vegetational traits of sample sites within Transects I(A) and II(B). Data shown include percent relative cover by algae (A), lichens (L), and mosses (M), total number of phanerogam species (S), and above-ground biomass (W, g dm-2 ). Ohange in elevation along the slopes (14 and 112) was 5 -10 cm.
AA
176
A.
DA~IN
and M. G.
BARBOUR
2. Phanerogam distribution Only annuals were found. The principal halophytes were M esembryanthemum nodiflorum L. in Transect I and Aizoon hispanicum L. in Transect II, with Gymnarrhena micrantha DESF. accompanying both. Glycophytes were found mainly in Transect II and they included Astragalus tribuloides DEL., Torularia torulosa (DESF.) O. E. SCHULZ, and Trigonella stellata FORSSK. The most diverse site (III) had only three species. present, apart from a cryptogamic understory. Species diversity, consequently, was low (H can vary from 0, for a stand with one species only, to well over 7; our values were < 1), as was biomass (Fig. 1). Phanerogam diversity and biomass were inversely correlated with soil salinity, confirming a pattern already reported for this area by DANIN (1976). All phanerogams were restricted from sites with more than 9 mmhos cm-I conductivity in the zone (Table 1), and our field notes revealed that all glycophytes were restricted from sites with more than 1.4 mmhos cm-I . Only seeds of the halophytes A. hispanicum and M. nodiflorum were found in the soil profiles. In soil samples 12,13,14, and 15, M. nodiflorum was the dominant fossil seed, whereas in sample 11 and all of Transect II, A. hispanicum was the dominant fossil seed. The frequency of occurrence did not change with depth. An additional two samples, taken throughout the entire loessal deposit down to Lisan marl, showed seed coats of A. hispanicum throughout. The seeds of both species are globular and have no structure for soil penetration, such as the wedge shape or awns of many grass diaspores. Their existence in the soil profile, then, indicates burial in place by loess deposition. The rate of this deposition in Israel is estimated to be 15-20 pm yr- I (DANIN & YAALON 1981). We can suggest, then, that the recovery of seeds down to a 10 cm depth means that Transect I was dominated by M. nodiflorum and Transect II by A. hispanicum for at least the last 5,000 year. 3. Cryptogam distribution Cryptogams, which build a biological soil crust, were found in all but the most saline sites, on eroded slopes. Algae appeared to prevail at the more saline extreme, and they produced a smooth crust separated by cracks into asymmetric polygons. Such algal crusts have been called "epedephic algae" by FRIEDMANN & GALUN (1974). The most prominent alga appears to be a filamentous blue-green with a mucilaginous sheath. Some filaments are solitary and 5-15 pm in diameter, while others are a cable-like collection of filaments within one sheath, 30-120 pm in diameter. The algal crust begins to develop just beyond the barren slope (15 and II3 in Fig. 1). Here it is 1-2 mm thick and is como sed of the following layers, from top to bottom: (1) a fine-grained soil cover 60-120 pm thick; (2) a 300-600 pm thick layer of densely packed filamentous algae, horizontally arranged; (3) a 500-800 pm thick layer of fewer algal filaments, both horizontal and vertical, and with empty channels 20 to 40pm in diameter which are probably the remnants of dead algal filaments; and (4) a transition zone, also with empty channels, leading to a homogeneous subsoil. The most common algae are Microcoleus spec. and Scytonema spec.
-
Microsuccession near the Dead Sea
177
Just above the eroding slope (eg, 13 in Fig. 1) is an algal crust with a different profile. It is 5-6 mm thick and consists of two layers: (1) an overburden of fine-grained soil, 60-120p.n thick, as already described; and (2) a 5 mm thick layer of algal filaments which are mainly vertically arranged and which follow the vertical channels made by dead mosses, the remains of which are still visible . The cracks in the algal crust are likely to be more leached of salt than the crusted pans between. As salinity overall declines (Table 1 and Fig. 1), lichens invade, first on the margins about the cracks, then throughout the crust. The most common lichens in the area include Lecidea decipiens (EHRH.) ACH., Oollema crispum (HUDS.) G. H. WEB., Dermatocarpon lachneum (AcH.) A. L. SM., Pterygiopsis spec. and Peccania spec. As lichen cover increases to 20 %, the crust takes on a hill-and-valley topography, with a range of elevation to 15 mm (Table 1). Apparently, this roughness contributes to further leaching, for our da.ta showed a strong correlation between roughness and topsoil salinity: increasing roughness from II3 to II4 to II5 to III was accompanied by decreasing topsoil conductivity from 1.9 to 0.2 mmhos cm-1 • Lichens dominate the hills, mosses come to dominate the valleys. Within the phanerogam zone, mosses tend to dominate the cryptogamic understory, and it is here tha.t they achieve their highest cover. The most common moss species are Orossidium crassinerve (DE NOT.) JUR., Aloina bilrons (DE NOT.) DELGADILLO, Pottia spec., and Bryum spec.
Discussion: Putative successional pathway An hypothetical pathway of succession in the study area is summarized in Fig. 2 . We can begin with aeolian silt deposition on exposed Lisan marl (Fig. 2A and 2B). Small drainage courses develop in the silt. Water at first percolates through the silt downward, but then moves latera.lly because of Lisan marl impermea.bility until it reaches the walls of the drainage ways, carrying salts to the point of evaporation. Soil samples at wall sites 14 and II2 showed that subsoil salinity was only half that of the surface, indicating water movement up/sideways to the wall. Once salinity has increased, soil structure is negatively affected, the plant cover dies, and the silty soil becomes highly erodable. Below the drainage way wall, vegetation is absent, probably because of residual high salinity; but within a few centimeters an algal crust begins to develop. The algae protect the soil from further erosion and cracks in the crust begin to become leached. The edges of the cracks become colonized by lichens, either from fungi innoculating the resident algae or from the arrival of lichen propagules (Fig. 20, 2D). The increasing growth of lichens results in the soil surface becoming rough, which promotes water retention in the valleys and ultimately its infiltration into the soil. Leaching is thus promoted and mosses become established in the valleys. In winter, when the crust is wet, the hilly microtopography is even more pronounced due to swelling of lichen thalli. In time, mosses increase their cover, trap incresing amounts of loess, and the topography becomes less rough and is invaded by halophytic and even glycophytic pha-
I
I
!
• A. DA:NIN and M. G. BARBOUR
178
KEY
(Al
~
I:
:: :::::: :: :i: ::1 M+L
L ~
T
A
phanerogams M dJI. Mosses
110 cm
(0)
(B)
water movement
M+L (p
.I
1/. ... .11..
•• A
A + L Jsalty soil
Lichens Algae dead crusts
A+L
__
(E)
(e)
I
A
A+L
M+L
A
Fig. 2. Diagrammatic scheme of succession in the study area. (A) The exposed Lisan marl is first (B) covered evenly with non-saline loess and is colonized by algae, lichens, mosses, and phanerogams. Small drainage ways become established (0) and lateral movement of soil water to their walls results in a saline, barren, erodeable surface. As the wall advances (to the left), increasing salinity above it has led to degeneration of the cryptogam-phanerogam community: first into a moss-lichen zone, then (closer to the wall) into an algae-lichen zone. Bits of once-stable craptogamic crust are found on the saline, eriding slope. As the eroded material beyond the wall (to the right) is leached by rain, it can be colonized by algae, then by algae and lichens (D), finally by phanerogams (E).
nerogams. This is the phanerogam plateau we have described earlier (see also Fig. 2D, 2E), which has the lowest soil salinity and the highest species diversity. The succession is cyclic, because the drainage way walls are not static. Above the wall are zones of vegetation that are degenerate stages of the phanerogam plateau which has been affected by increasing salinity and erosion near the wall. Pieces of lichen-algae crust can be found on the eroding wall; at one time these were part of a continuous crust above the advancing wall. In time, all such detached pieces of crust die and disappear. At site 13, which was adjacent to the advancing slope (14), there were remnants of dead mosses covered by a more recent, living algal crust, indicating retrogressive successicn. On site 12, further from the saline slope, algae and lichens contributed nearly equal cover, mosses were alive, and more species of phanerogams were present (Fig. 1 A).
Microsuccession near the Dead Sea
179
Thus, the advancing wall induces retrogressive succession above it (in front of it) and leaves a sere of progressive succession behind it. We may assume that the same patch of ground may proceed through this cycle, over and over. The driving forces are fluctuations in salinity and the reaction of cryptogams on their habitat, in particular the formation of a topographically diverse crust that leads to leaching and a relatively non-saline substrate. It is likely that a more detailed examination of algal, lichen, and moss zones would reveal a succession of species within each life form, but those details are beyond the scope of tl:is paper.
Acknowledgements Thanks are due to Dr. ILANA HERNSSTADT for determining the mosses, and to Dr. J. GARTY and to Dr. KELLA MARTON for determining the lichens. Illustrations were prepared by Mrs. TAMAR SOFFER.
References Atlas of Israel, 2nd ed. (1970): Department of Surveys, Ministry of Labour, Jerusalem. BEGIN, Z. B., EHRLICH, A., & NATHAN, Y. (1974): Lake Lisan and the Pleistocene precursor of the Dead Sea. Geol. Survey of Israel Bull. 63: 1- 30. DANIN, A. (1976): Plant species diversity under desert conditions. 1. Annual species diversity in the Dead Sea Valley. Oecologia 22: 251- 259. - & YAALON, D. H. (1981): Trapping of silt by lichens and mosses in the desert environment of the Dead Sea region, Israel. Int. Conf. Aridic Soils Jerusalem, Israel, p. 29. FRIEDMANN, E. 1., & GALU:''f, M. (1974): Desert algae, lichens, and fungi. In: G. W. BROWN, Desert biology ed., vol. II, pp. 165- 221, Academic Press, New York. HILL, M. O. (1973): Diversity and evenness: a unifying rotation of its consequences. Ecology 54: 427- 432. LARTET, L. (1869): Essai sur II", geologie de la Palestine. Ann. Sci. Geol. 1: 1- 292. NEEv, D., & HALL, J. K. (1977): Climatic fluctuations during the Holocene as reflected by Dead Sea levels, Int. Conf. on Terminal Lakes, Ogden, Utah, 8 pp. PICARD, L. (1938): Synopsis of stratigraphic terms in Palestinian geology. Bull. Geol. Dep. Hebrew Univ., Jerusalem 2: 1- 22. RICHARDS, L. A. (ed.) (1954): Diagnosis and improvement of saline and alkali soils. U.S.D.A. Handbook No. 60, Washington, D.C. SHANNON, C. E., & VVEAYER, 'V. (1949): The mp,thematicel theory of communication. Univ. of Illinois Press, Urbana. Received June 18, 1981 Authors' addresms: AVINOAM D.-\.~Gs, The Hebrew Univer3ity, ,Terusa.lem, Israel; MICHAEL G. BARBOUR, Botany Department, University of Californi~, Davis, CA, U.S.A. 95616.
Microsuccession of Cryptogams and Phanerogams in the Dead Sea Area, Israel
Summary A cyclic microsuccession on arid, loessal slopes facing the Dead Sea is described. Concentric bands of vegetation dominated in turn by blue-green algae, lichens, mosses, and succulent annual phanerogams are correlated with microtopography and salinity in the top 10 cm of soil. The entire sere may take place within an area of 2 m 2 , and along an elevational gradient of 10 cm, yet some environmental factors change more than one order of magnitude. Soil conductance, surface roughness, species diversity, and biomass were measured; most taxa were identified to species level.
Introduction In a previous study (DANIN 1976), species diversity of annual plant communities of xerohalophytes and glycophytes in the Dead Sea area was related to soil salinity a.nd to a. biological crust made of cryptogams (algae, lichens, mosses). The effect of cryptogams on trapping air-borne silt (loess) has been discussed by DANIN & YA,ALON (1981). This paper, the third in a series, discussed the formation of the cryptogamic crust and its place in a cyclic micro succession which leads from barren, saline soil to a nearly closed community of phanerophytes on non-saline soil. The study was carried out in the Dead Sea Valley, 7 km southeast of Jericho, 35° 29' E longitude by 31 ° 48' N latitude, at an elevation of 320 m below sea level. Yearly precipitation averages 100 mm, mostly falling as rain between December and March. The 1979-1980 rainfall was about 200 mm, far above average. Mean annual temperature is 25°C, and January/August means are 15/33°C (Atlas of Israel, 1970). The study area is on a gentle (1_2°) east-facing slope of the Lisan formation, which has been described by LARTET (1899), PICARD (1938), BEGIN et 301. (1974), and NEEV & HALL (1977). When Pleistocene Lisan Lake withdrew 11,000-12,000 year BP, the Lisan marl was exposed. Being a soft-grained substrate, it is suitable for colonization by poikilohydric, cryptogamic plants. Such plants protected the ground from further erosion and caused trapping and accumulation of loess over time to a present depth of 20 cm above the Lisan marl (DANIN & YAA,LON 1981). The study area exhibits a pattern of microtopography which consists of vegetated plateaus 1-3 m in diameter, and eroded sloping edges bare of plant cover which lead to small basins and runoff channels about 5-10 cm below the vegetated plateaus. Plateau vegetation is zoned into bands parallel to the eroding slopes, dominated in turn by algae, lichens, mosses, and herbaceous (often succulent) annual seed plants. 1) Botany Department, Hebrew University, Jerusalem 2) Botany Department, University of California, Davis
:: A. DAXIX and
174
~I.
G. BARBOUR
Cryptogamic and cryptogamic-phanerogamic zones have their own microtopography of plates and cracks or of hills and valleys in the soil surface, over an elevational range of as much as 15 mm. We hypothesized that cycles of soil erosion, local salinization, and leaching and accumulation of soil, all accompanied by changes in the plant cover, were responsible for the microtopographic patterns. Our objective in this paper was to test this hypothesis. Methods We established two replicate transects, 10 cm wide and about 70 cm long, such that each crossed the patches of microtopographic and vegetational diversity described above. Within each discrete and homogeneous zone, 10 X 10cm sample areas were marked and utilized along the transects. Sampling was done at the end of March, 1980, at the close of the growth season of most species, when plant cover was maximum. When phanerogam zones were sampled, all above-ground biomass was harvested and later weighed dry, and the number of individuals of each species was counted. Species diversity was calculted by the Shannon-Wiener formula (SHANNON & WEAVER 1949, with the modification that proportional values were in terms of weight), and also by HILL'S (1973) method. In all zones, we estimated the relative cover by algae, lichens, and mosses. On one transect only we measured the degree of surface roughness, expressing it as the fraction of sample surface that was approximately level and the range in elevation (in millimeters) between highs and lows. In all zones, soil was collected at depths of 0 -0.5 cm, 0.5 - 2 cm, and 8 -10 cm, which defined the extent of the root zone. Soils were later mixed 1: 1 with distilled water, and the filtrates were analyzed for total salt content by conductivity bridge. Soil samples were also washed through sieves to isolate the remains of seeds, as an indication of the past flora.
Results 1. Soil salinity
Soil conductivity values are shown in Table 1, and their relationship to topography is shown in Fig. 1. There were two orders of magnitude variation in surface conductivity, with the highest values along the eroded, barren slopes (Transect I position 4, Transect II position 2), and the lowest values on plateau sites dominated by mosses and phanerogams (II, Ill, II5). Except for slope sites, salinity increased with depth. Soil structure also changed with depth, from blocky to single grain, possibly because increasing sodium concentration dispersed the soil particles. Soil structure was absent on the barren slopes, and the powdery soil was relatively unstable and erodeable. Salinity at the surface averaged more than 20 mmhos cm-1 (sites 14 and II2), but less than 9 mmhos cm-1 at 8-10 em (Table 1), indicating that water tends to move up to the surface and evaporate. No living soil crust was found on the slopes, but bits of dead crust, possibly eroded from the plateau above, were present. If we assume that conductivity results from several mixed salts, then we can use approximate relationships developed by RICHARDS (1954) to convert conductivity to parts per million or percent salt. Our maximum value of 22.5 mmhos m-1 thus corresponds to 8,500 ppm, and our lninimum value of 0.1 mmhos cm-1 corresponds f'"'o.I
to '" 40 ppm.
175
Microsuccession near the Dead Sea
Table 1. Some traits of sample sites within Transects I and II. Data include: soil conductivity (mmhos cm-1 at 25 °0) at the surface (01), at 0.5-2 cm depth (02), and at 8-10 cm depth (03); species diversity by Shannon-Wiener (H) and Hill (eH ) indices; and surface roughness (percent of sample that was level and range in millimeters between highs and lows). For diagrammatic summary of each transect see Fig. 1 Transect Oonductivity position 01 02 03 Il 12 13 14
15
III 112 113 114 115
0_2 1.4 3.6 18.5 2.1 0.2 22.5 1.9 0.5 0.2
0.1 1.7 7.3 1.4 0.1 0.9 1.3 0.2
Diversity
1.4 4_5 6.7 9.2 1.5 1.2 9.5 2.9 3.5 0.8
Roughness
H
eH
0.68 0.26 0.00 0.00 0.00 0.95 0.00 0.00 0.21 0.57
1.97 1.30 1.00 1.00 1.00 2.59 1.00 1.00 1.23 1.77
Level
Site description
Range
10
10-15
60 30 15
1- 4 2- 5 4-11
Phanerogam plateau Phanerogam· cryptogam Algal zone above slope Barren slope Algal flat behind slope Phanerogam plateau. Barren slope Algal flat ·behind slope Phanerogam-cryptogam Phanerogam plateau
30
45
75
0
100
L
20
40
25
0
0
M
50
15
0
0
0
A
(A)
Site
.::- ::.. :~.'~~', I': :-.,: ... :
11
"
12
......
..~ .
13
'
IS
.':..
~ , ":. ! .-:-:. :.::: ;-.
S
2
W
4.04
A L
5 10
M
8S
',-: :': ...·.··1:··... ·. ,,::. (8)
Site
111
2 1.01
..
1
0
1
0.003
0
0.79
0
90
82
40
0 0
10 0
15 3
40
113
114
lIS
• '.'. I :~ ~
':',
..
.~
3
0
W
3.16
0
"'.:-;',:",,:.--; ~~-..: ... :;~··~.l·' : ~
1 0.03
level
20
-:.
S
Ground
~;.:::-: f'..
3
2
1.85
4.02
• Ground '.' level
Fig. 1. Topographic and vegetational traits of sample sites within Transects I(A) and II(B). Data shown include percent relative cover by algae (A), lichens (L), and mosses (M), total number of phanerogam species (S), and above-ground biomass (W, g dm-2 ). Ohange in elevation along the slopes (14 and 112) was 5 -10 cm.
AA
176
A.
DA~IN
and M. G.
BARBOUR
2. Phanerogam distribution Only annuals were found. The principal halophytes were M esembryanthemum nodiflorum L. in Transect I and Aizoon hispanicum L. in Transect II, with Gymnarrhena micrantha DESF. accompanying both. Glycophytes were found mainly in Transect II and they included Astragalus tribuloides DEL., Torularia torulosa (DESF.) O. E. SCHULZ, and Trigonella stellata FORSSK. The most diverse site (III) had only three species. present, apart from a cryptogamic understory. Species diversity, consequently, was low (H can vary from 0, for a stand with one species only, to well over 7; our values were < 1), as was biomass (Fig. 1). Phanerogam diversity and biomass were inversely correlated with soil salinity, confirming a pattern already reported for this area by DANIN (1976). All phanerogams were restricted from sites with more than 9 mmhos cm-I conductivity in the zone (Table 1), and our field notes revealed that all glycophytes were restricted from sites with more than 1.4 mmhos cm-I . Only seeds of the halophytes A. hispanicum and M. nodiflorum were found in the soil profiles. In soil samples 12,13,14, and 15, M. nodiflorum was the dominant fossil seed, whereas in sample 11 and all of Transect II, A. hispanicum was the dominant fossil seed. The frequency of occurrence did not change with depth. An additional two samples, taken throughout the entire loessal deposit down to Lisan marl, showed seed coats of A. hispanicum throughout. The seeds of both species are globular and have no structure for soil penetration, such as the wedge shape or awns of many grass diaspores. Their existence in the soil profile, then, indicates burial in place by loess deposition. The rate of this deposition in Israel is estimated to be 15-20 pm yr- I (DANIN & YAALON 1981). We can suggest, then, that the recovery of seeds down to a 10 cm depth means that Transect I was dominated by M. nodiflorum and Transect II by A. hispanicum for at least the last 5,000 year. 3. Cryptogam distribution Cryptogams, which build a biological soil crust, were found in all but the most saline sites, on eroded slopes. Algae appeared to prevail at the more saline extreme, and they produced a smooth crust separated by cracks into asymmetric polygons. Such algal crusts have been called "epedephic algae" by FRIEDMANN & GALUN (1974). The most prominent alga appears to be a filamentous blue-green with a mucilaginous sheath. Some filaments are solitary and 5-15 pm in diameter, while others are a cable-like collection of filaments within one sheath, 30-120 pm in diameter. The algal crust begins to develop just beyond the barren slope (15 and II3 in Fig. 1). Here it is 1-2 mm thick and is como sed of the following layers, from top to bottom: (1) a fine-grained soil cover 60-120 pm thick; (2) a 300-600 pm thick layer of densely packed filamentous algae, horizontally arranged; (3) a 500-800 pm thick layer of fewer algal filaments, both horizontal and vertical, and with empty channels 20 to 40pm in diameter which are probably the remnants of dead algal filaments; and (4) a transition zone, also with empty channels, leading to a homogeneous subsoil. The most common algae are Microcoleus spec. and Scytonema spec.
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Just above the eroding slope (eg, 13 in Fig. 1) is an algal crust with a different profile. It is 5-6 mm thick and consists of two layers: (1) an overburden of fine-grained soil, 60-120p.n thick, as already described; and (2) a 5 mm thick layer of algal filaments which are mainly vertically arranged and which follow the vertical channels made by dead mosses, the remains of which are still visible . The cracks in the algal crust are likely to be more leached of salt than the crusted pans between. As salinity overall declines (Table 1 and Fig. 1), lichens invade, first on the margins about the cracks, then throughout the crust. The most common lichens in the area include Lecidea decipiens (EHRH.) ACH., Oollema crispum (HUDS.) G. H. WEB., Dermatocarpon lachneum (AcH.) A. L. SM., Pterygiopsis spec. and Peccania spec. As lichen cover increases to 20 %, the crust takes on a hill-and-valley topography, with a range of elevation to 15 mm (Table 1). Apparently, this roughness contributes to further leaching, for our da.ta showed a strong correlation between roughness and topsoil salinity: increasing roughness from II3 to II4 to II5 to III was accompanied by decreasing topsoil conductivity from 1.9 to 0.2 mmhos cm-1 • Lichens dominate the hills, mosses come to dominate the valleys. Within the phanerogam zone, mosses tend to dominate the cryptogamic understory, and it is here tha.t they achieve their highest cover. The most common moss species are Orossidium crassinerve (DE NOT.) JUR., Aloina bilrons (DE NOT.) DELGADILLO, Pottia spec., and Bryum spec.
Discussion: Putative successional pathway An hypothetical pathway of succession in the study area is summarized in Fig. 2 . We can begin with aeolian silt deposition on exposed Lisan marl (Fig. 2A and 2B). Small drainage courses develop in the silt. Water at first percolates through the silt downward, but then moves latera.lly because of Lisan marl impermea.bility until it reaches the walls of the drainage ways, carrying salts to the point of evaporation. Soil samples at wall sites 14 and II2 showed that subsoil salinity was only half that of the surface, indicating water movement up/sideways to the wall. Once salinity has increased, soil structure is negatively affected, the plant cover dies, and the silty soil becomes highly erodable. Below the drainage way wall, vegetation is absent, probably because of residual high salinity; but within a few centimeters an algal crust begins to develop. The algae protect the soil from further erosion and cracks in the crust begin to become leached. The edges of the cracks become colonized by lichens, either from fungi innoculating the resident algae or from the arrival of lichen propagules (Fig. 20, 2D). The increasing growth of lichens results in the soil surface becoming rough, which promotes water retention in the valleys and ultimately its infiltration into the soil. Leaching is thus promoted and mosses become established in the valleys. In winter, when the crust is wet, the hilly microtopography is even more pronounced due to swelling of lichen thalli. In time, mosses increase their cover, trap incresing amounts of loess, and the topography becomes less rough and is invaded by halophytic and even glycophytic pha-
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• A. DA:NIN and M. G. BARBOUR
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Fig. 2. Diagrammatic scheme of succession in the study area. (A) The exposed Lisan marl is first (B) covered evenly with non-saline loess and is colonized by algae, lichens, mosses, and phanerogams. Small drainage ways become established (0) and lateral movement of soil water to their walls results in a saline, barren, erodeable surface. As the wall advances (to the left), increasing salinity above it has led to degeneration of the cryptogam-phanerogam community: first into a moss-lichen zone, then (closer to the wall) into an algae-lichen zone. Bits of once-stable craptogamic crust are found on the saline, eriding slope. As the eroded material beyond the wall (to the right) is leached by rain, it can be colonized by algae, then by algae and lichens (D), finally by phanerogams (E).
nerogams. This is the phanerogam plateau we have described earlier (see also Fig. 2D, 2E), which has the lowest soil salinity and the highest species diversity. The succession is cyclic, because the drainage way walls are not static. Above the wall are zones of vegetation that are degenerate stages of the phanerogam plateau which has been affected by increasing salinity and erosion near the wall. Pieces of lichen-algae crust can be found on the eroding wall; at one time these were part of a continuous crust above the advancing wall. In time, all such detached pieces of crust die and disappear. At site 13, which was adjacent to the advancing slope (14), there were remnants of dead mosses covered by a more recent, living algal crust, indicating retrogressive successicn. On site 12, further from the saline slope, algae and lichens contributed nearly equal cover, mosses were alive, and more species of phanerogams were present (Fig. 1 A).
Microsuccession near the Dead Sea
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Thus, the advancing wall induces retrogressive succession above it (in front of it) and leaves a sere of progressive succession behind it. We may assume that the same patch of ground may proceed through this cycle, over and over. The driving forces are fluctuations in salinity and the reaction of cryptogams on their habitat, in particular the formation of a topographically diverse crust that leads to leaching and a relatively non-saline substrate. It is likely that a more detailed examination of algal, lichen, and moss zones would reveal a succession of species within each life form, but those details are beyond the scope of tl:is paper.
Acknowledgements Thanks are due to Dr. ILANA HERNSSTADT for determining the mosses, and to Dr. J. GARTY and to Dr. KELLA MARTON for determining the lichens. Illustrations were prepared by Mrs. TAMAR SOFFER.
References Atlas of Israel, 2nd ed. (1970): Department of Surveys, Ministry of Labour, Jerusalem. BEGIN, Z. B., EHRLICH, A., & NATHAN, Y. (1974): Lake Lisan and the Pleistocene precursor of the Dead Sea. Geol. Survey of Israel Bull. 63: 1- 30. DANIN, A. (1976): Plant species diversity under desert conditions. 1. Annual species diversity in the Dead Sea Valley. Oecologia 22: 251- 259. - & YAALON, D. H. (1981): Trapping of silt by lichens and mosses in the desert environment of the Dead Sea region, Israel. Int. Conf. Aridic Soils Jerusalem, Israel, p. 29. FRIEDMANN, E. 1., & GALU:''f, M. (1974): Desert algae, lichens, and fungi. In: G. W. BROWN, Desert biology ed., vol. II, pp. 165- 221, Academic Press, New York. HILL, M. O. (1973): Diversity and evenness: a unifying rotation of its consequences. Ecology 54: 427- 432. LARTET, L. (1869): Essai sur II", geologie de la Palestine. Ann. Sci. Geol. 1: 1- 292. NEEv, D., & HALL, J. K. (1977): Climatic fluctuations during the Holocene as reflected by Dead Sea levels, Int. Conf. on Terminal Lakes, Ogden, Utah, 8 pp. PICARD, L. (1938): Synopsis of stratigraphic terms in Palestinian geology. Bull. Geol. Dep. Hebrew Univ., Jerusalem 2: 1- 22. RICHARDS, L. A. (ed.) (1954): Diagnosis and improvement of saline and alkali soils. U.S.D.A. Handbook No. 60, Washington, D.C. SHANNON, C. E., & VVEAYER, 'V. (1949): The mp,thematicel theory of communication. Univ. of Illinois Press, Urbana. Received June 18, 1981 Authors' addresms: AVINOAM D.-\.~Gs, The Hebrew Univer3ity, ,Terusa.lem, Israel; MICHAEL G. BARBOUR, Botany Department, University of Californi~, Davis, CA, U.S.A. 95616.