Apr 3, 1998 - these lands (Cocks and Osman; Russi et al., I992a,. 1992b). .... reported by others (Cocks and Osman, '996; ..... John Wiley and Sons.
Seed Bank Assessment on Heavily Grazed Mediterranean
Grasslands in Jordan
M.M. Abu-Zanat, A.E. Osman, and MJ. Tabba'a*
ABSTRACT Soils
from
four
sites
on
(Osman et al., 1991). The total area of such land in Jordan is estimated to be 0.15 million hectares (Jaradat, 1988). The majority of these lands have limitations for cultivation and cropping because of steepness, rocky, shallow or infertile soils. The misuse of these grasslands has resulted in the disappearance of desirable and palatable species and poor soil conditions.
Mediterranean
grasslands in Jordan were sampled in June 1992 and 1994, to determine the size and composttiun of seed banks following a long period of uncontrolled grazing. Seed-bank species composition and seed densities varied greatly from site to site. The total number of plant species present on the experimental sites was 35, compared to 33 species represented in the
The grasslands of Jordan are dominated by annual species such as Hordeum bulbosum L., Poa bulbosa L., Heteranthelium piliferum (Banks & Sol.) Hochst., Trifolium campestre Schreb. and T. tomentosum L. (AI-Eisawi, 1982). Sheep and goats graze the herbage of these lands in ~pring (April - May) when most of the plants species are in flowering or producing seed. This disturbance affects the soil seed banks, which represent the mechanism of survival for species growing on these lands (Cocks and Osman; Russi et al., I992a, 1992b). The presence or absence of the preferred species in a seed bank gives an indication of the rehabilitation techniques that might be employed such as reseeding or transplanting or just relying on natural dispersal (Roberts, 1981).
soil seed banks. Leguminous species constituted about 60% of the population of growing plants and represented a similar value of species In the associated seed banks. However, the seed reserves of leguminous species were low at all sites except for one. Total seed number ranged from 6338 ± 839 01- 2 to 35781 ± 4336 01- 2. The relative contributions of grasses, legumes and weeds to the seed banks averaged 66%, 23% and 13 %, respecti vely. Th ree legumes iTr i]o li UIIl t om en t o s u m , T. c am p e str e an d tr i g o n e ll a m e s o p o t am ic ay and five grasses (Poa o ulb osa, Lo p o c h l o a p u m ila, Het e ranth elium p iliferu m, Br omus lanc e ol atus and Nardur us m aritimuss comprised more than 90% of the total seeds buried in the soils. We concluded that the diversity of flora at the study sites was poor, due to uncontrolled grazing and the seeds of annual species, mostly grasses, comprised most of the soil seed banks.
Key words: seed bank; grassland; Jordan. INTRODUCTION Natural grasslands in Jordan used to. be an important forage resource for small rurmnunts. The term grasslands here refers to non-arable lands located within or adjacent to the arable land'
* Facult y of Agriculture, University of Jordan, Amman, Jordan (Res. I and 3); ICARDA, Aleppo, Syria (Res. 2). Received on 7/4/1997 4/3/1998.
and Accepted
for Puhlicutiou
011
There are no published data for soil seed densities in the Mediterranean grasslands of Jordan and this lack of knowledge has hampered efforts to predict the response of natural vegetation to disturbance. Seed bank estimates can be informative tools in developing management systems for the grasslands in Jordan. Thus, this study was conducted in order to characterize the size and composition of seed banks 111 the grasslands of Jordan.
MA TERIALS AND METHODS four sites representing the Mediterranean grasslands were selected in northern (Om Rumunch village near Irbid city), central (Hisban and Zarqa'a Mai'n villages near Madaba city) and southern (Mreigah village near Qatraneh) Jordan. The area of the selected sites ranged between 3 to 5 hectares. All sites are located within cropping
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Dirasat , Agricultural Sciences, Volume 25, No.2, 1998
areas, and were no! cropped because of either steep slope or rock outcrops. Details of the sites are given in (Table 1). In 1992, 10 soil cores from each site were collected for the analysis of organic matter and a vai lab le phosphorus (Watanabe and Olsen, 19(15). The vegetation of the study sites was surveyed in April of J 992, using a O. J m? loop. Plant cover of all species was estimated visually, and species of grasses and legumes present at each of 10 randomly selected plots at each site were recorded. Frequency was calculated as the percentage of total plots which contains at least one individual of a given plant species. The size of seed hank was assessed in June of 1992 and 1994 when seed set had been completed. The soil samples, extracted with a metal cylinder 1() Col diameter and 15 cm deep, were located at random points 30-50 III apart in a "W" configuration. At each sampling point, a J.O 2 III quadrat was placed on the ground and four soil cores, one from each corner, were collected, hulked into a single sample and placed in a pol yet helene hag and stored at 5"C until examination. Twelve composite soil samples were collected from each site. This sampling technique yielded a soil sample of approximately 4g00 crn', which is g times larger than the minimum sample volume (500 -- 600 crrr') recommended by Nurnata et al. (1964) for assessment of seed hanks in grassland. Sheep and goats feces were isolated from the soil samples and processed separately. The seeds present in the feces were identified and counted. The extraction of seeds present in the feces separately from those of soil was performed only in 1992. In 1994, feces were macerated with the soil samples, so that the total number of seeds included those present in the feces. Soil samples were macerated by hand and washed through a 250 urn sieve, and seeds were separated from the retained organic matter and small stones. The seed residue was left on a filter paper until dry, and then transferred to vials for storage unti 1 identi fication. Seeds were ident ified and counted by species by placing under a lOx magnification dissecting scope and comparing them to reference seed samples previously
collected from plants existing at the experimental sites. Seed counts were converted to numbers per m ' for each 15 - cm depth. Transformation of the data to natural logarithms was performed before analysis of variance. The analysis of variance was performed on the data using General Linear Models (SAS. 1995). The statistical model included the effect of site, year and their interaction. However, the results are presented without trnnsf'ormatiou. Means were separated using the Duncan Multiple Range Test.
RESULTS Species Present The results of the vegetation survey are summarized in (Table 2). The total number of plant species present at the experimental sites was 35. Species diversity was greater at site I (10 grasses and 20 legumes) and site 2 (8 grasses and 12 legumes) compared with site 3 (8 grasses and 5 legumes) and site 4 (6 grasses and 9 legumes). The most frequent grass species encountered at the four sites were Loporhloa pumila (Desf.) Bor, roo hullrosa L.. Hct cranthelium piliferutn (Banks & Sol.) l lochst., and Nardurus maritimus (L.) Murh. The dominant legumes were Trigonella II/CSOI1O!o1l1i('a, Trifolium tomentosum L. and T. campcstrc Schreh. Weed species were numerous at site 3. The dominant weeds included Capsella bursa-pastoris (L.) Medik., Achillea aleppica, Coullllls fJynlOccfJlwllls L. and Adonis aestivalis
L. Seed-hank Densities The size of the seed bank differed significantly (P < 0.0(01) among the four sites (Table 3). Highest seed density was found at site
3. where 35781 ± 4336 seeds per m 2 were estimated. Seed density was lowest (6338 ± 839 Ill~) at site 4. The average number of seeds m- 2 was 19438 ± 2093 in 1992 cornapred to 17322 ± 2713 in 1994. However, this yearly variation was not statistically significant (P > O. J 8).
The buried seeds in the soil were grouped into three major categories: grasses, legumes and weeds. The total number of grass seeds was significantly (P < 0.(5) different among the experimental sites and between the two years of
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Seed Bank Assessment on Heavily Grazed Mediterranean...
sampling. A significant interaction in the total number of grass seeds existed between sites and years. The highest number of grass seeds was measured at site 2 (15025 ± 1532). Sites 2 and 4 contained the highest number of seeds from bulbous grasses (8978 ± 1651 and 5395 ± 856, respecti vely). The total number of legume seeds was significantly (P < 0.05) different among sites. The highest number of legume seeds were present at site I (4114 ± 1166) and site 3 (19476 ± 4926). As with the grasses, the total number of weed seeds were significantly different among sites and between years, but there was no significunt interaction between sites and years. The soils of site 3 contained the highest number ( I 1231 ± 2848) of weed seeds. The relative contributions of grasses, legumes and weeds to the seed banks, pooled for the two years, were 62, 29 and 9 for site I; 87, 4 and 9 for site 2; 14, 55 and 31 for site 3, and 95, 2 and 3 for site 4. The overall proportion of seeds, pooled for the four sites, in the soil seed banks averaged 52% and 43% for grasses, 13% and 56% for legumes, 35% and 1% for weeds in 1992 and 1994, respectively.
Seed-bank Species The majority of species represented in the seed banks were annuals (Table 4). The total number of species recorded in the seed banks was 29 (11 grasses and 18 legumes) in 1992 and 27 (10 grasses and 17 legumes) in 1994. The overall percentage of legume species represented in the seed banks in both years were similar (61 % vs. 63%). The most abundant legume seeds were those of Trifolium tomentosum, T. campestre and Trigonella mesopotamica which contributed about 88% and 97% of the total seed banks of legumes in 1992 and 1994, respectively. The grass species which had high numbers of buried seeds in the soil included Heteranthelium
piliferum, Lopochloa pumila, Poa bulbosa, Bromus lanceolatus and Nardurus maritimus. The proportion of seeds of these grass species wus 94% in 1992 and 91 % in 1994. Although there were differences in the composition of seed banks between sites, the most frequent species were Poa
bulbosa,
Trifolium
toment osum,
Lopochloa
pumila, Nurdurus maritimus and Heteranthelium piliferusn, Number of plant species represented in soil seed banks per site were higher in 1992 compared to 1994 (Table 5). The decline over two years varied from 8% of species at site I to 67% 01" species at site 4. The variation in number of species was more in grasses than legumes. The feces of sheep and goats collected from sites I and 3 contained the highest number of seeds (73 and 85 seeds m- 2) compared to /2 and 10 seeds III 2 at sites 2 and 4. The seeds present in the feces that were collected from sites 2 and 4 were solely of grasses. On the other hand, seeds of legume species comprised 77% and 84% of the total number of seeds present in the feces at sites I and 3.
DISCUSSION Seed-bank Densities There were large differences between sites in the density of seeds in the soil. Our estimate of seed bank density (800-36000 m- 2) falls within the range of values (400 to 70000 m- 2) reported in a review by Roberts, (1981). These values reflect the current seed reserves of overgrazed grasslands in Jordan. However, because of differences in the depth of sampling and technique of estimation, comparisons between the results of different workers must be made with cure, The wide variation in the size of seed banks
of the four sites indicates a high degree of spatial heterogenity. This inter-site variation of soil seed banks could be attributed to several factors such as pattern of seed dispersal, local eduphic conditions and degree of disturbance (Kellman,
1978). Compared to legumes, the size of seed banks for grasses was larger on all sites except site 4. This could be attributed to the fact that in habitats subject to summer drought, such as the grasslands ill Jordan, seeds of many grasses are present only during the dry season (Grime, 1979). The seeds of grasses are capable of germination under a wide range of conditions and moisture supply is the main factor determining the time of germination. This means that the seeds of grasses
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Dirasat, Agricultural Sciences, Volume 25, No.2, 199R
often form an appreciable contribution to seed banks during the summer period.
cantp e st re, Po a bulbosa, Heteranthelium piliferum, Bromus lanceolatus and Lopochloa pumila which logether comprised about 90% of the seed hank. The legume seed population was dominated hy small-seeded Trifolium species. Since small seeds are more likely to pass unharmed through the digestive tract of ruminants, small seeded species should be chosen when degraded grasslands need to be re-sown (Russi et aI., I992b). Among grass species, Poa bulbosa persistently maintained large numbers of buried seeds in hoth years and at all sites. Vegetative besides sexual reproduction probably allowed this bulhous species to compete well with associated species.
Leguminous species are of particular interest as components of grassland seed banks because of their forage value. In our study and in most grassland types, the numher of seeds of legumes tend to he low compared with those of other groups of species. Heavy grazing of legume plants during the seed set followed by severe predation of seeds could he responsible for reducing the reserves of leguminous species in the soi Is of the study sites.
Seed-hank Species The species composition of grassland seed hanks is of greater significance than the total number of seeds present. Seed-hank species composition varied greatly from site to site. The seed hank composition was similar to that reported by others (Cocks and Osman, '996; Porcella and Gill, 1986) in that the majority of the seeds belonged to only a few species. Seed hanks developed under prolonged time of overgrazing were dominated hy Trifolium tomentos um, T.
Table 1.
ACKNOWLEDGMENTS We wish to express our sincere gratitude to Mr. Fahim Ghassaly, Ameen Al-Khateeb and Adel Nassar for assistance with collection of soil cores. The financial support of the International Center for Agricultural Research in the Dry Areas nCARDA), Aleppo, Syria is acknowledged.
Major characteristics of the experimental sites sampled for seed bank assessment. Sites* ----------- ---------4 3
---------- ----------1
2
3
3
5
4
380
900
750
950
Aspect
east
flat
south
south
Slope
> 20 %
Area (ha)
Elevation ( m)
Rainfall (mm)
Plant Cover (% )
%
3 %
3-8 %
450
350
350
250
90
SS
75
70
Organic Matter (% )
2.4 ± 0.7
Available P (ppm)
9.6 + 2.7
*Sites: 1. 2. 3. 4.
0-3
::1
3.2
-/
1.1
~~
I
8.7
.3
..
1 . 8 ± 0.9
1 . 6 + 0.8
31 .2 ± 9.9
18.9 ± 3 . 1
Om Rumaneh, YubJa village, Irbid Governorate. Hmoudiyah, Hishan village, Amman Governorate. Mazfour, Zarqa'a Main village, Madaba Governorate. Mreigah, near QaLraneh village, Karak Governorate. - 19R
n Tn
n
$
Seed Bank Assessment on Heavily Grazed Mediterranean...
Table 2.
Frequency (%)* of grasses and legume species found at the experimental sites. Sites 1
2
3
4
Grasses
Aeqi.lops sp. Avella sLerilis L. Bromus lanceolatus Roth Heteranthelium piliferum Hochst. Hordeum bulbosum L. Hordeum glaucum Steudel Lolium multiflorum Lam. Lopochloa pumila (Desf.) Bor Nardurus maritimus (L.) Murb Poa bulbosa L. Poa sinaica Steudel
30 20
100 20 10 30 70 90 70 50 30
30 90 20 40 30 80 70 90
60 90 10 30 10 90 80 70
Astragalus asterias SteVe ex Ledeb. Astragalus guttatus Banks & Sol. Astragalus tribuloides Delile Astragalus corrugatus Betrol. 30 Hippocrepis unisiliquosa L. 20 Hymenocarpus circinnatus L. Medicago coronata (L. ) Bartal. 50 Medicago rotata Boiss 30 40 Medicago rugosa Desv. 20 Medicago turbinata (L. ) All. Medicago polymorpha L. 40 Onobrychis crista-galli (L. ) Lam. 10 Ononis antiquorum L. 10 Trifolium bullatum Boiss. et. Hausskn.l0 Trifolium campestre Schreber 40 20 Trifolium clusii Gren. & Godr. Trifolium pilulare Boiss 30 'J'rifolium stellatum L. 20 30 Trifolium scabrum L. 10 Trifolium resupinatum L. Trifolium tomentosum L. 60 Trigonella mesopotamica L. 50 Trigonella spinosa L. 30 v i c i e sp. 10
20
10
50 20 70 10 100
Legumes
* Each
value represents a mean of 10 samples.
- 199
20 10 30 10
10 20 20 30
30 10 20
10
100 30
10 10
30 10 10 20 20 10 10
10
Dirosat, Agricultural Sciences, Volume 25. No, 2, 1998
Table 3.
of soil seed densities m- 2 for the expe~ime~al sites in 1992 and 1994.
Mean + SE
Site 2
Site 1
Site 3
Site 4
Grasses 199~
1994
11828 -I 2917a 5612 ± 2938b
l\verage8720 ± 2126b
15559 + 7.24Sa 14491 ± 2173a
7680 ± 1710a 2469 ± 924c
4978 ± 816b 7084 ±1459a
-l
J53la
5074 ± 1095c
6031 ± 846b
I 7.'1'7 .L
IOStle
157 ±
SOc
6187 ± 1247ab 32764 ± 8235a
198 ± 64 ±
78c 14c
717 ±
530b
19476 ± 4926a
1 31 ±
41b
1~025
Legumes 1992 1994
7.152. -I 896b 60/7 ± 2045ab
l\verage4114 ± 1166a Weeds I () q 7. 19':)11
;U110 157
-'-I
-
l\verage1198 ±
6'l8b
32c
397b
3 I ;W -' 1 2. J --l
1626 ±
1 () lH) 2lk
586b
7.27.01
± 3471a
267- ±
27c
1 1 231 ± 2848a
324 ± 131c 28 ± 18d 176 ±
71c
Total 1992 '1994
16220 + 3450bc 1 1846 ± 3097cd
19964 1: 25WJau 14771 + 211\~bc
Average14033 ± 231 21:.
17368 ± 1729b
36067 ± 3433a 35495 ± 8174a 35781 ± 4336a
5500 ± 838d 7176 ±1455d 6338 ± 839c
s t anda rd error of Ul0. moa n . Nurnhers followed by similar le~ters are not significantly (P < 0.05) different.
SP: lll :
- 200
, I
Seed Bank Assessment on Heavily Grazed Mediterranean... °l'i\Ldo
SCid-LolIll apeci0.s alld ."eallf SD* of soil s e e d densities m" encountered [01. each s p e c Le s , pooled for the four sites, in 1992 and 1994.
4.
0
j
1992
species
i
I
mean ±
so ---------- 1994
Grasses
;.
/\"'lI i lop.'; ,cop. Avella steril.is BI01ll1lS lanceolatus fleterallthe.l.iulII p i l i [eruIII flordeulII vu 11.>OSUlII Hsn. " ~
~y.l U-l 1 .:.. 1 r l..:i:u......1 ~J .~y.ll ,...... 1iJ~y.1
CtJ.:Pl1 ~ ;'J~".Ll JJ~l ~ wK.1 - c>~1 ~J ;'~J
-
~,pJ1 .::.ot~1
JJl.,.1.l
,.,KIt
JJ,i,o.:,' ~ .y4J1
UJJ'i1 ~ ~yll crt).1 ~jiJ
