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Impact of burning and fertilization on dry Mediterranean grassland productivity and diversity
Acta Oecologica 40 (2012) 19e26
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Original article
Impact of burning and fertilization on dry Mediterranean grassland productivity and diversity Mohammad N. Alhamad*, Mohammad A. Alrababah, Mamoun A. Gharaibeh Department of Natural Resources and Environment, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2011 Accepted 21 February 2012 Available online 14 March 2012
Fire alters plant community structure by changing dominant plant species and shifting resource mobilization, while fertilizer application alters plant community structure through its impact on productivity. Here we studied prescribed burning and fertilizer treatments on the productivity and diversity of a semiarid Mediterranean grassland ecosystem. Variables measured were aboveground plant productivity, vegetation, litter and soil cover, species richness and plant density of an Avena/Hordeum (A-H) dominated plant community using a split-plot experimental design over a three-year period. Burning increased vegetative cover by 21.2% and reduced litter cover by 22.6%, while fertilization increased vegetative cover by 31.6%. The combined effect of burning and fertilization increased vegetative cover by 35.5% and reduced litter cover by 13.9%. Aboveground biomass was also higher in burned treatments (52.41 g 0.25 m2) by 39.5% over unburned treatments. Burning reduced the proportion of the dominant Avena/ Hordeum complex by 43.1%, thus significantly enhancing overall species richness by 52.5% over the unburned plots. On the other hand, fertilizer application increased biomass production and species richness by 37.4% and 13.5%, respectively, over the unfertilized plots. The combined effect of burning and fertilization enhanced biomass production and species richness by 90.8% and 69.8%, respectively, and reduced the dominant A-H by 45.5%. Burning generally resulted in lowering the dominance of the A-H complex compared to the control while fertilizer application had variable effect, reducing the proportion of A-H with burning but increasing it without burning. Reduction in the A-H complex increased the presence of other species, therefore enhancing diversity. Our findings showed that burning and fertilization treatments have a synergistic effect on both productivity and diversity of an arid grassland. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Fire Competition Biodiversity Light Grassland Avena
1. Introduction Disturbance conserves and maintains species diversity (Huston, 1979; Meffe and Carroll, 1997; Prober and Smith, 2009) and restores some degraded grassland plant communities (Higgins, 1986; Hatch et al., 1999; Howe, 1999; Roques et al., 2001). Disturbances in general trigger change in plant community (Paine and Levin, 1981; Collins, 1987; Petraitis et al., 1989) through altering resource availability (Huston, 1979; Collins et al., 1998) and modifying competition among plant species (Wilson and Tilman, 1993; NoyMeir, 1995; Collins et al., 1998). Such change is expected to affect biomass production (Redmann, 1978; Knapp and Seastedt, 1986) and species diversity (Al-Mufti et al., 1977; Moore and Keddy, 1989;
* Corresponding author. Tel.: þ962 2 7201000x22073; fax: þ962 2 7201078. E-mail addresses: [email protected] (M.N. Alhamad), [email protected] (M.A. Alrababah), [email protected] (M.A. Gharaibeh). 1146-609X/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2012.02.005
Wilson and Tilman, 2002; Alrababah et al., 2007; Alhamad and Alrababah, 2008). Among all disturbances, fire is considered a major driving ecological process (Bond et al., 2005) that maintains grasslands through recycling immobilized nutrients of accumulated litter (DeBano et al., 1998) and regulating successional trends (Pyne, 1982; Miller and Chamberlain, 2008). Periodic fire for grassland communities reduces the presence of woody species (McPherson, 1995), suppresses the growth of less fire-tolerant species (Gatewood, 1992), and enhances species diversity (Brockway et al., 2002). Therefore, prescribed burning was used as a management tool to increase native species richness (Parsons and Stohlgren, 1989; Dyer and Rice1997; Meyer and Schiffman, 1999) and to suppress certain plant species (diTomaso et al., 1999). For example, fire has been found to decrease grass cover (Gillespie and Allen, 2004) which in turn enhanced the growth of other species. Uys et al. (2004) observed that grass species were strongly reduced by burning in mesic, montane and semi-arid grasslands, whereas forbs showed higher degree of tolerance in the three grasslands. In
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M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
another experiment, Köhler et al. (2005) found that burning effectively reduced the coverage of Bromus erectus, a dominant species of dry limestone grasslands in Northern Switzerland, from 40 to 70% to 5e30%. Moreover, Cleary et al. (2010) found that the resilience of sagebrush rangeland following burning was enhanced due to rapid and vigorous growth of perennial bunchgrasses and increased soil organic carbon and live fine roots. However, the effect of fire on plant communities varies from one region to another. Fire may or may not interact with other disturbances such as grazing depending on prevailing environmental conditions. For example, studies in mesic grasslands in North America (Collins et al., 1998) and tall grasses in Africa (Belsky, 1992) have shown that plant communities subjected to fire and grazing were significantly more diverse than those subjected to either fire or grazing, alone. However, grassland studies in arid communities of the Mediterranean (Noy-Meir, 1995) and North America (Valone and Kelt, 1999; Drewa and Havstad, 2001) have shown insignificant interactive effects of fire and grazing on community structure. Fire plays an important role in maintaining diversity of rangelands through reducing competitiveness of dominant species and creating open space for the emergence of new species (Collins, 1987). Experimental work on elevated litter accumulation caused a shift from small-seeded grasses into large-seeded grasses, leading to a reduction of species diversity (Amatangelo et al., 2008). Therefore, litter removal can enhance species diversity and alter community structure. Moreover, litter was found to affect both grass and non-grass species (Xiong and Nilsson, 1999), leading to inconsistent change in a grassland plant community. Literature that addresses how fire structures plant biodiversity of semi-arid grassland, especially in the Mediterranean areas, is rather limited. Recent studies conducted on the semi-arid east Mediterranean grassland ecosystems in Jordan proposed that competition for light may explain the relationship between productivity and diversity (Alhamad, 2006; Alrababah et al., 2007; Alhamad and Alrababah, 2008; Alhamad et al., 2008,2010). The findings of these studies were in agreement with Newman’s theory (1973) that stresses the importance of light rather than nutrients in shaping diversity. However, those studies did not include any experimental quantitative measure of nutrient impact on diversity. Litter was found to be a major player in such arid ecosystems although water and nutrients are expected to be more limiting than light. Heavy litter accumulation was mainly contributed by the fast growing and drought tolerant large-seeded Avena and Hordeum. These grass species are opportunistic and can germinate and establish quickly due to the large reserves in their large seeds. As soon as they establish, they outcompete other species through light first. Under grazing pressure, however, these grass species will have
no chance to deprive other species of light and therefore other species will germinate, enhancing diversity. It is expected that fire will have a similar effect on the ecosystem since fire eliminates heavy litter and improves light penetration. But fire, simulating the addition of fertilizers, is also expected to play a major ecological role through recycling and mobilizing nutrients from litter back to the soil. Hobbs et al. (1988) found that fertilizer application to an annual grassland community caused significant increases in aboveground biomass and changes in species relative abundances. Semi-arid Mediterranean grasslands in Jordan cover a narrow strip from north to south, separating the semi-humid Mediterranean ecosystem to the west from the arid Saharo-Arabian ecosystem to the east. These ecosystems are known as marginal land ecosystems and are considered the first line of defense against desertification encroaching from the east. The conservation of such ecosystems is important to the overall conservation of grassland ecosystems and biodiversity in general. In this study, an attempt was made to study the simultaneous effect of prescribed burning and fertilizer application on the productivity and diversity of semiarid Mediterranean ecosystems. Therefore, our hypothesis is that reintroducing fire to semi-arid Mediterranean Avena grasslands as a management tool will suppress the competitiveness of dominant annual grass species (Avena and Hordeum species) and maintain higher species diversity and improve aboveground annual grassland productivity. Specifically, the present study was conducted to investigate the response of a Mediterranean grassland to prescribed burning with and without fertilizer application examining: a) above ground biomass production and vegetative cover b) species richness c) changes in species composition (Avena-Hordeum A-H complex vs. other species) The outcome of this research will enable us to better understand the role of fire in constructing grassland communities in general and specifically Mediterranean annual grasslands. The usefulness of prescribed burning in maintaining and conserving semi-arid grasslands cannot be appropriately evaluated without understanding the response of dominant species to treatments. 2. Materials and methods 2.1. Site description The experimental site was selected to represent a relatively homogenous Avena grassland plant community at Jordan University of Science and Technology (JUST) campus (520 m above sea
Table 1 Rainfall, minimum and maximum temperatures ( C) during 2006/7e2007/8 and 2008/9 growing seasons and the long term average rainfall (1990e2009) at JUST research station. Season Rainfall (mm) 2006/7 2007/8 2008/9 Long term average Min temp ( C) 2006/7 2007/8 2008/9 Long term average Max temp ( C) 2006/7 2007/8 2008/9 Long term average
Sept
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
June
Total
0.0 0.0 0.0 0.3
36.5 0.0 4.4 10.7
9.0 24.7 8.5 27.2
17.8 8.6 25.2 43.7
38.1 67.8 8.0 46.5
51.2 18.1 94.6 47.4
29.4 5.0 38.7 38.0
22.5 0.0 6.4 8.0
4.4 5.8 3.1 2.2
0.0 0.0 0.0 0.0
208.9 130.0 188.9 224.1
18.3 18.8 19.0 16.6
17.3 17.3 17.6 14.3
13.8 14.6 13.9 10.0
7.5 9.4 9.9 6.3
4.0 0.9 5.0 5.0
5.6 3.7 6.0 4.8
6.0 9.9 6.4 6.7
9.0 11.3 9.4 9.8
15.1 11.9 13.1 13.1
16.0 16.0 17.9 15.4
32.7 31.3 31.6 30.3
26.4 29.0 26.5 27.0
18.9 21.8 22.5 21.9
14.7 16.0 17.0 15.8
10.0 11.0 15.3 13.7
15.2 14.3 15.8 14.4
18.2 23.0 16.8 18.2
22.3 26.6 23.7 23.6
29.6 27.8 27.7 28.8
32.2 32.4 32.3 31.3
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
level), in the northeastern semi-arid grasslands of Jordan (32 340 N, 36 010 E). The campus was established in 1976, and thereafter grassland communities on campus were protected from grazing. The location is characterized by semi-arid Mediterranean conditions with mild rainy winters and dry hot summers. The long-term average precipitation is 225 mm for the growing season (i.e. the rainy season), usually from October/November to April/May. December, January and February are the wettest months of the year. Precipitation, minimum and maximum temperature for the 2006e07, 2007e08 and 2008e09 growing seasons are presented in Table 1. The soils are generally characterized as deep silty clay soils over a gently rolling topography. 2.2. Plant community and species description The eastern Mediterranean grassland communities evolved under natural and human- induced disturbances (Naveh, 1999). When protected from disturbances (e.g. grazing and fire), the plant community tends to be dominated by few tall annual grasses including Avena and Hordeum species (Noy-Meir, 1995). The present plant community is a typical semi-arid annual grassland community dominated by wild oat (Avena sterilis L.) and wild barley (Hordeum marinum Huds.). Both species are considered tall annual therophytes (Zohary and Feinbrum, 1966e1986). 2.3. Experimental design The experiment was conducted in a split-plot design with five replicates of whole plots over three growing seasons (2006e07, 2007e08 and 2008e09). Two levels of burning (burned vs. unburned) were applied to whole plots randomly, and two levels of fertilizers (fertilized vs. unfertilized) were applied to each subplot. The whole plot is 0.1 ha, while subplots are 0.05 ha each. Burning was conducted during September 2006, and granular fertilizer DAP (diammonium phosphate 18% N and 46% P2O5) was applied at a rate of 100 kg ha1 in early November 2006. Plant communities of each treatment were sampled using three 0.25 m2 quadrats. Measured parameters were: percentage of vegetative and litter coverage, fraction of bare soil (%), and species richness and evenness. All plants in each quadrat were identified to the species level following Zohary and Fienbrum (1966e1986) and the number of plants per species in each quadrat was recorded during peak vegetation growth (approximately between MarcheApril) of each season. Vegetative, litter and fraction of bare soil (%) in each quadrat was visually estimated, then all plant material was cut to ground level at maturity at the end of the growing season, then oven dried for 72 h at 70 C to determine the aboveground plant biomass. 2.4. Statistical analysis Measured plant community variables affected by burning, fertilizer application and their interaction was tested by constructing an analysis of variance of split-plot design, using a JMP fitted model and SAS statistical analysis software (Version 7, SAS Institute Inc. Cary NC USA, 1989e2007). Model parameters included the main effect of year (3) and block (5). Whole plots included the burning treatment (burned and unburned), while the fertilizer treatment (fertilized and unfertilized) was the subplot treatment. The model also included the interaction between burning and block as a random effect and used the whole plot error term. The interactive effect of burning and fertilizer was tested against the model error term. Means separation was run using a least squares difference test for measured variables that expressed a significant F value.
21
Table 2 Proportions of plant species (number of individuals per total number of plants per plot) and families’ names (bold names) found in Avena grassland under burned (B) and unburned (UB) plots during the 2006/7, 2007/8 and 2008/9 growing seasons. Family/species Asteraceae Leontodon arabicus Boiss. Caryophyllaceae Gymnocarpos decander Forssk. Silene palaestina Boiss. Silene rubella L. Vaccaria pyramidata Medikus Cistaceae Helianthemum ledifolium (L.) Mill. Compositae Achillea santolina L. Anthemis palestina Reuter Calendula arvensis L. Carduus australis L. fil. Carduus getulus Pomel Centaurea iberica Spreng. Crepis aspera L. Crepis sancta (L.) Bornm. Leontodon laciniatus (Bertol.) Widder ex Bornm. Scorzonera papposa DC. Senecio leucanthemifolius subsp. vernalis Poir Convolvulaceae Convolvulus althaeoides L. Cruciferae Chorispora purpurascens (Banks and Sol.) Eig Diplotaxis erucoides (L.) DC. Erucaria hispanica (L.) Druce Euphorbiaceae Euphorbia arguta Banks and Sol. Fumaria densiflora DC. fumaria parviflora Lam. Geraniaceae Erodium cicutarium (L.) L’Her. Erodium gruinum (L.) L’Her. Erodium malacoides (L.) L’Her Gramineae Avena sterilis L. Bromus tectorum L. Cynosurus callitrichus Barbey Hordeum marinum Huds. Phalaris minor Retz. Piptatherum miliaceum (L.) Coss. Poa annua L. Poa eigii Feinbrun Setaria viridis (L.) P.Beauv. Hypericaceae Hypericum triquetrifolium Turra Labiatae Lamium amplexicaule L. Malvaceae Malva sylvestris L. Papaveraceae Hypecoum aegyptiacum (Forssk.) Asch. and Schweinf. Roemeria hybrida (L.) DC. Papilionaceae Astragalus spinosus (Forssk.) Muschl. Trigonella arabica Delile Vicia narbonensis L. Vicia peregrina L. Plantaginaceae Plantago notata Lag. Primulaceae Anagallis arvensis L. androsace maxima L. Ranunculaceae Adonis dentata Delile Resedaceae Reseda lutea L. Rutaceae Haplophyllum buxbaumii (Poir.) G.Don f. Umbelliferae Anethum graveolens L. Malabaila secacul (Banks and Sol.) Boiss.
B
UB
0.21
0.00
0.00 0.70 0.26 0.26
0.08 0.30 0.00 0.12
0.36
0.00
0.07 9.19 5.61 2.46 4.62 9.25 0.31 0.64 0.48 0.03 4.28
1.63 4.94 3.36 2.60 0.37 2.91 0.27 0.41 1.53 0.00 0.66
0.42
0.00
0.04 8.91 7.37
0.19 0.35 5.05
0.18 0.69 0.08
0.00 3.07 0.23
0.37 1.39 0.38
1.18 0.91 0.30
9.52 0.72 1.27 9.35 4.12 0.00 0.99 5.44 0.61
21.44 0.00 0.14 12.28 8.30 8.63 6.67 5.68 0.39
0.19
0.11
0.12
0.00
0.27
0.25
0.19 0.39
0.00 0.00
0.48 3.80 0.10 1.62
1.54 1.99 0.07 0.64
0.00
0.34
0.21 1.10
0.00 1.01
0.32
0.00
0.23
0.00
0.04
0.00
0.02 0.37
0.02 0.00
22
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
Table 3 Analysis of variance (ANOVA) for vegetative, litter and fraction of bare soil (%) in grassland communities as affected by burning and fertilizer application during the 2006/7, 2007/8 and 2008/9 growing seasons (N ¼ 180). SoV
DF
MS
F-value
P-value
Vegetative cover (%) Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square Fraction of bare soil (%) Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square
MS
F-value
P-value
16.04 2.81 8.71 1.51 0.75 0.96
<0.0001 0.170 0.042 0.202 0.387 0.329
Litter cover (%)
2 4 1 4 1 1 166
3926.29 216.78 4185.69 53.49 8487.2 381.36 178.34 0.72
22.01 4.05 78.24 0.30 47.59 2.14
<0.001 0.102 <0.001 0.877 <0.001 0.146
2 4 1 4 1 1 166
4453.8 51.77 1717.42 122.19 6408.20 77.36 179.63 0.68
24.79 0.42 14.06 0.68 35.67 0.43
<0.001 0.787 0.02 0.607 <0.001 0.512
3. Results 3.1. Weather conditions Average precipitation and recorded temperatures of the study area are presented in Table 1. During the first growing season (2006e07), rainfall was lower (208 mm) than the long-term average (225 mm). The second year (2007e08) was much drier than average with 130 mm of precipitation. Moreover, the temporal distribution of rainfall was unfavourable for plant growth in the second year, as more than 50% of the total rainfall occurred during January (Table 1). In the 2008e09 season, rain occurred in February and March (Table 1) which significantly promoted plant growth. 3.2. Impact of burning and fertilizer on species abundance A total of 53 vascular plant species belonging to 19 families was found in the study plots (Table 2). As expected, grass species dominated the unburned plots. The dominant species were A. sterilis (21.44% individuals per total number of plants per plot), H. marinum (12.28%), Piptatherum miliaceum (8.63%), and Poa annua (6.67%). Growth of grass species was suppressed in the burning treatments whereas the growth of non-grass species was promoted. Moreover, burning enhanced Centaurea iberica, which increased from 2.91% to 9.25%, and increased Diplotaxis erucoides from 0.35% to 8.91%. Anthemis palestina increased from 4.95% in unburned plots to 9.19% in burning treatments (Table 2). 3.3. Impact of burning and fertilizer on cover parameters Burning significantly affected vegetative cover (p < 0.001) and litter cover (p ¼ 0.042) (Table 3), while fertilizer application significantly affected vegetative cover (p < 0.001), but not litter cover (p ¼ 0.387). The interaction between burning and fertilizer was not significant for both vegetative cover (p ¼ 0.146) and litter cover (p ¼ 0.329). Burning the grassland community significantly increased the vegetative cover over the unburned community by 21.2%, whereas litter cover was significantly reduced by 22.6% (Table 4). Under fertilization, plots had higher vegetative cover by 31.6% over the unfertilized treatment, while litter cover was reduced by 5.9%, though this reduction was not statistically significant (Table 4). The combined effect of burning and
721.57 190.61 590.42 67.756 33.8 43.02 45.00 0.75
fertilization increased vegetative cover by 35.5% and reduced litter cover by 13.9% (Table 4). Vegetative cover was the highest in plots that received both burning and fertilizers (63.53%) compared to all other treatment combinations. The highest litter cover was observed in the unburned plots regardless of whether fertilizer was applied. Burning resulted in the lowest litter cover only if fertilizers were applied. Burning without fertilizer application produced intermediate litter cover, which was not significantly different from the unburned plots. 3.4. Impact of burning and fertilizer on productivity and diversity parameters Split-plot ANOVA revealed significant effects of year on biomass (p < 0.001), species richness (p < 0.001) and the dominant grass species percentage (p < 0.001). Burning significantly affected Table 4 Mean and standard error (SE) values of vegetative, litter and fraction of bare soil (%) in grasslands community as affected by burning, fertilizer application and burning fertilization interaction during the 2006/7, 2007/9 and 2008/9 growing seasons. Treatment level Burning Burned Unburned SE % change due to burning Fertilization Fertilized Unfertilized SE % change due to fertilizer Burning Fertilization Burned þ Fertilized Burned þ Unfertilized Unburned þ Fertilized Unburned þ Unfertilized SE % change due to burning þ fertilizer as compared to Unburned þ Unfertilized
Vegetative cover (%)
Litter cover (%)
Fraction of bare soil (%)
55.2a 45.6b 0.8 21.2
12.5b 16.0a 0.9 22.6
32.1b 38.3a 1.2 16.1
57.3a 43.5b 1.4 31.6
13.7a 14.6a 0.7 5.9
29.2b 41.2a 1.4 29.0
63.5a 46. 9b 51.0b 40.2c 2.0 35.5
11.4b 13.3ab 16.0a 15.9a 1.0 13.9
25.4c 38.7a 33.0b 43.6a 2.0 34.5
Numbers along column with different letters are significantly different at the 0.05 significance level for each treatment.
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
23
Table 5 Analysis of variance (ANOVA) for aboveground plant biomass, species richness, proportion of Avena-Hordum and other species in grassland communities as affected by burning and fertilizer application and burning fertilization interaction during the 2006/7, 2007/8 and 2008/9 growing seasons (N ¼ 180). SoV
DF
MS
F-value
P-value
Biomass (g 0.25 m2) Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square
2 4 1 4 1 1 166
Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square
2 4 1 4 1 1 166
31451.8 221.5 9890.6 186.8 9028.4 366.8 174.4 0.67
180.4 1.2 53.0 1.1 51.8 2.1
A-H % 7216.7 10297.0 9213.5 988.6 117.4 3584.1 570.4 0.69
12.6 10.4 9.3 1.7 0.2 6.3
biomass (p ¼ 0.019), species richness (p ¼ 0.015), and A-H% (p ¼ 0.038) (Table 5). Fertilizer application affected biomass production (p < 0.001) and species richness (p ¼ 0.011), but had no effect on A-H% (p ¼ 0.651). The interaction between burning and fertilizer application was not significant for biomass (p ¼ 0.149) but was significant for species richness (p ¼ 0.048) and for A-H% (p ¼ 0.013). Above ground biomass was significantly higher in the burning treatment (52.41 g 0.25 m2) than in the unburned treatment by 39.5% (Table 6). Burning increased species richness by 52.5% relative to the unburned treatment, while the dominant A-H complex was reduced by 43.1%. On the other hand, fertilizer application significantly increased biomass production and species richness by 37.4% and 13.5%, respectively (Table 6) over the unfertilized plots. Although the dominant A-H complex increased by 6.4%, the
Table 6 Mean values and standard error (SE) of aboveground plant biomass, species richness, Avena-Hordum % and other species% in semi-arid Mediterranean grasslands community as affected by burning, fertilizer and burning fertilizer interaction during the 2006/7, 2007/8 and 2008/9 growing seasons. Treatment level Burning Burned Unburned SE % change due to burning Fertilization Fertilized Unfertilized SE % change due to fertilizer Burning Fertilization Burned þ Fertilized Burned þ Unfertilized Unburned þ Fertilized Unburned þ Unfertilized SE % Change due to burning þ fertilizer as compared to unburned þ unfertilized
Biomass (g 0.25m2)
Species rich. (no. 0.25m2)
A-H %
Other species %
52.4a 37.6b 1.4 39.5
6.6a 4.3b 0.4 52.5
18.9b 33.2a 3.3 43.1
81.1a 66.8b 3.3 21.4
52.1a 37.9b 1.4 37.4
5.8a 5.1b 0.2 13.5
26.8a 25.2a 2.5 6.4
73.2a 74. 8a 2.5 2.1
60.9a 43.9b 43.2b 31.9c 2.0 90.8
7.2a 6.0b 4.4c 4.2c 0.3 69.8
15.2b 22.5b 38.5a 27.9ab 3. 6 45.5
84. 8a 77.5a 61.6b 72.1ab 3.56 17.6
MS
F-value
P-value
Species rich. (no. 0.25 m2)
Numbers along column with different letters are significantly different at the 0.05 significance level for each treatment.
<0.001 0.436 0.019 0.373 <0.001 0.149
<0.0001 0.022 0.038 0.145 0.651 0.013
91.4 12.9 231.2 13.7 21.4 12.8 3.22 0.72 Other spp % 7216.7 10297.0 9213.5 988.6 117.5 3584.1 570.4 0.69
28.4 0.9 16.9 4.3 6.6 4.0
<0.001 0.522 0.015 0.003 0.011 0.048
12.6 10.4 9.3 1.7 0.2 6.3
<0.001 0.022 0.038 0.145 0.651 0.013
increase was not statistically significant. The combined effect of burning and fertilizer application significantly increased plant biomass by 90.8% and species richness by 69.8%, and reduced A-H by 45.5% (Table 6). The highest biomass was observed on burned and fertilized plots (60.92 g 0.25 m2) which was significantly higher than in all other combinations. The lowest biomass was observed for the unburned unfertilized plots, while the fertilized unburned or burned unfertilized plots were intermediate in biomass production. The highest species richness was observed in burned and fertilized plots while the lowest species richness was observed in unburned plots regardless of the fertilizer application. In other words, fertilizer application had different impacts on species richness depending on burning whereas fertilizer application increased species richness under burning but had no impact under the no burning regime. Burning generally resulted in lower A-H% compared to the control, while fertilizer application had variable effect on A-H%, reducing A-H% with burning but increasing it without burning. 3.5. Impact of burning and fertilizer application on plant community in response to weather conditions Burning increased vegetative cover and biomass production but reduced A-H% in the first growing season (2006e07) following the burning treatment, with profound effects in the third growing season, in 2008e09 (Fig. 1). Both of these years had average growing seasons. While burning showed no effect on the vegetative cover, biomass production and A-H% decreased in the dry 2007-08 growing season (Fig. 1). Burning increased species richness in all seasons (Fig. 1) compared to the unburned plots; however, the difference was highest in the favourable growing seasons and lower in the dry growing season. Reduction in litter cover was noted only in the first growing season, although the difference between burned and unburned plots in litter cover diminished in the second and third growing seasons. Fertilization increased vegetative cover and biomass in all growing seasons with a more pronounced increase in the first growing season (Fig. 2) following fertilizer application, while fertilization showed no effect on litter cover and the dominant A-H % in all growing seasons (Fig. 2). The fertilizer effect on species richness was insignificant for the first two growing seasons and showed a positive effect only in the third growing season.
a Spp. rich.
6 3 2007
2008
2009
Veg. cover %
d
9
55 30 2007
Year
Litter cover %
Biomass
45 20 2008
28 16 4
2009
2007
2008
Year
Fraction of bare soil %
A+H %
f 55
40 20 0 2007
2009
Year
c
60
2009
e
70
2007
2008 Year
b
95
Burned Unburned
80
2008
35 15 2007
2009
2008
2009
Year
Year
Fig. 1. Response of (a) species richness, (b) aboveground plant biomass (g 0.25m-2), (c) proportion of Avena and Hordeum complex (A-H), (d) vegetative cover %, (e) litter cover %, (f) fraction of bare soil (%) % to burning treatment grasslands community in the 2006/7, 2007/8 and 2008/9 growing seasons.
a
6
2008
70 50 30 2007
3 2007
2009
b Litter cover %
2008 Year
2009
14 8 2007
25
5 2008 Year
2008
2009
Year
c
2007
20
Fraction of bare Soil %
Biomass A+H %
45
2009
e
50 20 2007
2008 Year
Year
80
Fertilized Unfertilized
d Veg. cover %
Spp. rich.
9
2009
55
f
35 15 2007
2008 Year
2009
Fig. 2. Response of (a) species richness, (b) aboveground plant biomass (g 0.25m-2), (c) proportion of Avena and Hordeum complex (A-H), (d) vegetative cover %, (e) litter cover %, (f) fraction of bare soil %, to fertilizer treatment grasslands community in the 2006/7, 2007/8 and 2008/9 growing seasons.
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
4. Discussion and conclusions Burning and fertilizer application had diverse effects on litter cover and species composition. In addition, the effect of burning and fertilization on species diversity showed a significant interaction, while no interaction was found with respect to cover parameters and biomass. The significant interaction of some parameters and the insignificant interaction of other parameters lead us to believe that these two management options exert different impacts on plant communities. Therefore, fire impact on the community is not only a matter of nutrient recycling, and fertilizer applications do not compensate for fire. On the one hand, fire reduced the Avena/Hordeum complex and increased diversity by increasing the presence of other species and improving vegetative cover and biomass. On the other hand, fertilizer application did not affect litter cover and the presence of Avena/Hordeum complex but had positive impact on vegetative cover and biomass productivity as expected. Unexpectedly, fertilizer application improved species diversity but did not affect litter cover. Though the interaction of burning and fertilization is significant, we cannot draw conclusions regarding each treatment separately. Burning and fertilizer application had a synergistic effect on vegetative cover. Whereas fertilization improved vegetative cover by approximately 10.8% and burning improved vegetative cover by approximately 6.7%, their combined effect (burning and fertilization) increased vegetative cover by 23.4%, exceeding their sum (17.5%). There was also a synergistic effect of burning and fertilization on litter cover, with burning and fertilization combined significantly reducing the litter cover compared to the individual treatments. Improved biomass production for the burned plots over the unburned plots indicates that these semi-arid grasslands are not at their potential production capability and that burning improves their biomass production. This confirms previous studies on the same area in which light to moderate defoliation (surrogate for grazing) increased the biomass production. These grasslands have evolved under grazing pressure, thus protection seems to increase competition and reduce production potential (Alhamad and Alrababah, 2008). Eastern Mediterranean grasslands have evolved under livestock grazing and other anthropogenic pressures over the last 10,000 years (Harlan and Zohary, 1966; Perevolotsky and Seligman, 1998), resulting in the high resilience of these plant communities and facilitating their continued existence under harsh environmental pressure (Lavorel et al., 1999). Although the study area has been protected from grazing for about 35 years, the plant community is still dominated by large-seeded annual grasses such as Avena and Hordeum. These grass species have developed adaptive mechanisms including early seed shedding, seed dormancy, polymorphism (Naveh, 1999) and efficient dispersal and burial mechanisms that allow them to successfully invade and persist in many grasslands (Naveh, 1967), thereby impacting plant biodiversity. Burning caused significant increase in species richness, plant biomass, and vegetative cover. These responses to burning are in agreement with previous studies (Parsons and Stohlgren, 1989; Dyer and Rice, 1997; Meyer and Schiffman, 1999). Results clearly show that prescribed burning caused significant decrease in litter cover and in the proportion of Avena-Hordeum complex in the plant community. The suppression of certain plant species due to burning was also reported by diTomaso et al. (1999). Grassland community response to burning, including increased species richness and enhanced overall productivity, most likely resulted from removing litter, thus altering the availability of light and soil nutrients for different plant species (diTomaso et al., 1999; Gimeno-Garcia et al., 2000; Harrison et al., 2003; Fynn et al., 2004).
25
Litter has direct and indirect effects on the physical and chemical environment. The decomposition of litter may release certain phytotoxic materials in the environment. Moreover, the accumulated litter reduces light reaching the soil surface and reduces maximum soil temperature, affecting seed germination and seedling growth of companion species (see Facelli and Pickett, 1991 for complete review). The direct and indirect effects of the litter of one species on another may alter the competitive interaction and affect the structure of plant community (Grime, 1979; Facelli, 1994). We hypothesized that under unburned treatments, accumulated litter decreased light reaching the soil surface, providing an opportunity for large-seeded grasses (including Avena and Hordeum species) to germinate earlier and deplete resources (Facelli and Pickett, 1991). The increase in species richness and decrease in Avena-Hordeum complex by litter removal under burning treatment are consistent with the findings of Amatangelo et al. (2008) in which large-seeded grasses (e.g. Avena fatua) performed poorly due to increased heat and decreased water availability in plots where litter was removed from an annual grassland community in California. High light environments under litter removal by burning may have induced the early germination of companion small-seeded species, enhancing their competitive potential for resources such as nutrients, water, and light over large-seeded grass species (Fowler, 1988; Dyer et al., 2000). These findings illustrate that AvenaeHordeum complexes benefited from litter accumulation, so were the most impacted by burning. Therefore, we conclude that fire can be used as a management tool to improve the productivity and enhance diversity of these ecosystems when employing appropriate fire intervals since frequent fire in low productivity ecosystems may reduce productivity.
Acknowledgements The authors would like to thank the Deanship of Scientific Research at the Jordan University of Science and Technology (JUST) for their financial support. We also thank Rika Muhl for editing the language of our Manuscript.
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Contents lists available at SciVerse ScienceDirect
Acta Oecologica journal homepage: www.elsevier.com/locate/actoec
Original article
Impact of burning and fertilization on dry Mediterranean grassland productivity and diversity Mohammad N. Alhamad*, Mohammad A. Alrababah, Mamoun A. Gharaibeh Department of Natural Resources and Environment, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2011 Accepted 21 February 2012 Available online 14 March 2012
Fire alters plant community structure by changing dominant plant species and shifting resource mobilization, while fertilizer application alters plant community structure through its impact on productivity. Here we studied prescribed burning and fertilizer treatments on the productivity and diversity of a semiarid Mediterranean grassland ecosystem. Variables measured were aboveground plant productivity, vegetation, litter and soil cover, species richness and plant density of an Avena/Hordeum (A-H) dominated plant community using a split-plot experimental design over a three-year period. Burning increased vegetative cover by 21.2% and reduced litter cover by 22.6%, while fertilization increased vegetative cover by 31.6%. The combined effect of burning and fertilization increased vegetative cover by 35.5% and reduced litter cover by 13.9%. Aboveground biomass was also higher in burned treatments (52.41 g 0.25 m2) by 39.5% over unburned treatments. Burning reduced the proportion of the dominant Avena/ Hordeum complex by 43.1%, thus significantly enhancing overall species richness by 52.5% over the unburned plots. On the other hand, fertilizer application increased biomass production and species richness by 37.4% and 13.5%, respectively, over the unfertilized plots. The combined effect of burning and fertilization enhanced biomass production and species richness by 90.8% and 69.8%, respectively, and reduced the dominant A-H by 45.5%. Burning generally resulted in lowering the dominance of the A-H complex compared to the control while fertilizer application had variable effect, reducing the proportion of A-H with burning but increasing it without burning. Reduction in the A-H complex increased the presence of other species, therefore enhancing diversity. Our findings showed that burning and fertilization treatments have a synergistic effect on both productivity and diversity of an arid grassland. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Fire Competition Biodiversity Light Grassland Avena
1. Introduction Disturbance conserves and maintains species diversity (Huston, 1979; Meffe and Carroll, 1997; Prober and Smith, 2009) and restores some degraded grassland plant communities (Higgins, 1986; Hatch et al., 1999; Howe, 1999; Roques et al., 2001). Disturbances in general trigger change in plant community (Paine and Levin, 1981; Collins, 1987; Petraitis et al., 1989) through altering resource availability (Huston, 1979; Collins et al., 1998) and modifying competition among plant species (Wilson and Tilman, 1993; NoyMeir, 1995; Collins et al., 1998). Such change is expected to affect biomass production (Redmann, 1978; Knapp and Seastedt, 1986) and species diversity (Al-Mufti et al., 1977; Moore and Keddy, 1989;
* Corresponding author. Tel.: þ962 2 7201000x22073; fax: þ962 2 7201078. E-mail addresses: [email protected] (M.N. Alhamad), [email protected] (M.A. Alrababah), [email protected] (M.A. Gharaibeh). 1146-609X/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2012.02.005
Wilson and Tilman, 2002; Alrababah et al., 2007; Alhamad and Alrababah, 2008). Among all disturbances, fire is considered a major driving ecological process (Bond et al., 2005) that maintains grasslands through recycling immobilized nutrients of accumulated litter (DeBano et al., 1998) and regulating successional trends (Pyne, 1982; Miller and Chamberlain, 2008). Periodic fire for grassland communities reduces the presence of woody species (McPherson, 1995), suppresses the growth of less fire-tolerant species (Gatewood, 1992), and enhances species diversity (Brockway et al., 2002). Therefore, prescribed burning was used as a management tool to increase native species richness (Parsons and Stohlgren, 1989; Dyer and Rice1997; Meyer and Schiffman, 1999) and to suppress certain plant species (diTomaso et al., 1999). For example, fire has been found to decrease grass cover (Gillespie and Allen, 2004) which in turn enhanced the growth of other species. Uys et al. (2004) observed that grass species were strongly reduced by burning in mesic, montane and semi-arid grasslands, whereas forbs showed higher degree of tolerance in the three grasslands. In
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M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
another experiment, Köhler et al. (2005) found that burning effectively reduced the coverage of Bromus erectus, a dominant species of dry limestone grasslands in Northern Switzerland, from 40 to 70% to 5e30%. Moreover, Cleary et al. (2010) found that the resilience of sagebrush rangeland following burning was enhanced due to rapid and vigorous growth of perennial bunchgrasses and increased soil organic carbon and live fine roots. However, the effect of fire on plant communities varies from one region to another. Fire may or may not interact with other disturbances such as grazing depending on prevailing environmental conditions. For example, studies in mesic grasslands in North America (Collins et al., 1998) and tall grasses in Africa (Belsky, 1992) have shown that plant communities subjected to fire and grazing were significantly more diverse than those subjected to either fire or grazing, alone. However, grassland studies in arid communities of the Mediterranean (Noy-Meir, 1995) and North America (Valone and Kelt, 1999; Drewa and Havstad, 2001) have shown insignificant interactive effects of fire and grazing on community structure. Fire plays an important role in maintaining diversity of rangelands through reducing competitiveness of dominant species and creating open space for the emergence of new species (Collins, 1987). Experimental work on elevated litter accumulation caused a shift from small-seeded grasses into large-seeded grasses, leading to a reduction of species diversity (Amatangelo et al., 2008). Therefore, litter removal can enhance species diversity and alter community structure. Moreover, litter was found to affect both grass and non-grass species (Xiong and Nilsson, 1999), leading to inconsistent change in a grassland plant community. Literature that addresses how fire structures plant biodiversity of semi-arid grassland, especially in the Mediterranean areas, is rather limited. Recent studies conducted on the semi-arid east Mediterranean grassland ecosystems in Jordan proposed that competition for light may explain the relationship between productivity and diversity (Alhamad, 2006; Alrababah et al., 2007; Alhamad and Alrababah, 2008; Alhamad et al., 2008,2010). The findings of these studies were in agreement with Newman’s theory (1973) that stresses the importance of light rather than nutrients in shaping diversity. However, those studies did not include any experimental quantitative measure of nutrient impact on diversity. Litter was found to be a major player in such arid ecosystems although water and nutrients are expected to be more limiting than light. Heavy litter accumulation was mainly contributed by the fast growing and drought tolerant large-seeded Avena and Hordeum. These grass species are opportunistic and can germinate and establish quickly due to the large reserves in their large seeds. As soon as they establish, they outcompete other species through light first. Under grazing pressure, however, these grass species will have
no chance to deprive other species of light and therefore other species will germinate, enhancing diversity. It is expected that fire will have a similar effect on the ecosystem since fire eliminates heavy litter and improves light penetration. But fire, simulating the addition of fertilizers, is also expected to play a major ecological role through recycling and mobilizing nutrients from litter back to the soil. Hobbs et al. (1988) found that fertilizer application to an annual grassland community caused significant increases in aboveground biomass and changes in species relative abundances. Semi-arid Mediterranean grasslands in Jordan cover a narrow strip from north to south, separating the semi-humid Mediterranean ecosystem to the west from the arid Saharo-Arabian ecosystem to the east. These ecosystems are known as marginal land ecosystems and are considered the first line of defense against desertification encroaching from the east. The conservation of such ecosystems is important to the overall conservation of grassland ecosystems and biodiversity in general. In this study, an attempt was made to study the simultaneous effect of prescribed burning and fertilizer application on the productivity and diversity of semiarid Mediterranean ecosystems. Therefore, our hypothesis is that reintroducing fire to semi-arid Mediterranean Avena grasslands as a management tool will suppress the competitiveness of dominant annual grass species (Avena and Hordeum species) and maintain higher species diversity and improve aboveground annual grassland productivity. Specifically, the present study was conducted to investigate the response of a Mediterranean grassland to prescribed burning with and without fertilizer application examining: a) above ground biomass production and vegetative cover b) species richness c) changes in species composition (Avena-Hordeum A-H complex vs. other species) The outcome of this research will enable us to better understand the role of fire in constructing grassland communities in general and specifically Mediterranean annual grasslands. The usefulness of prescribed burning in maintaining and conserving semi-arid grasslands cannot be appropriately evaluated without understanding the response of dominant species to treatments. 2. Materials and methods 2.1. Site description The experimental site was selected to represent a relatively homogenous Avena grassland plant community at Jordan University of Science and Technology (JUST) campus (520 m above sea
Table 1 Rainfall, minimum and maximum temperatures ( C) during 2006/7e2007/8 and 2008/9 growing seasons and the long term average rainfall (1990e2009) at JUST research station. Season Rainfall (mm) 2006/7 2007/8 2008/9 Long term average Min temp ( C) 2006/7 2007/8 2008/9 Long term average Max temp ( C) 2006/7 2007/8 2008/9 Long term average
Sept
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
June
Total
0.0 0.0 0.0 0.3
36.5 0.0 4.4 10.7
9.0 24.7 8.5 27.2
17.8 8.6 25.2 43.7
38.1 67.8 8.0 46.5
51.2 18.1 94.6 47.4
29.4 5.0 38.7 38.0
22.5 0.0 6.4 8.0
4.4 5.8 3.1 2.2
0.0 0.0 0.0 0.0
208.9 130.0 188.9 224.1
18.3 18.8 19.0 16.6
17.3 17.3 17.6 14.3
13.8 14.6 13.9 10.0
7.5 9.4 9.9 6.3
4.0 0.9 5.0 5.0
5.6 3.7 6.0 4.8
6.0 9.9 6.4 6.7
9.0 11.3 9.4 9.8
15.1 11.9 13.1 13.1
16.0 16.0 17.9 15.4
32.7 31.3 31.6 30.3
26.4 29.0 26.5 27.0
18.9 21.8 22.5 21.9
14.7 16.0 17.0 15.8
10.0 11.0 15.3 13.7
15.2 14.3 15.8 14.4
18.2 23.0 16.8 18.2
22.3 26.6 23.7 23.6
29.6 27.8 27.7 28.8
32.2 32.4 32.3 31.3
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
level), in the northeastern semi-arid grasslands of Jordan (32 340 N, 36 010 E). The campus was established in 1976, and thereafter grassland communities on campus were protected from grazing. The location is characterized by semi-arid Mediterranean conditions with mild rainy winters and dry hot summers. The long-term average precipitation is 225 mm for the growing season (i.e. the rainy season), usually from October/November to April/May. December, January and February are the wettest months of the year. Precipitation, minimum and maximum temperature for the 2006e07, 2007e08 and 2008e09 growing seasons are presented in Table 1. The soils are generally characterized as deep silty clay soils over a gently rolling topography. 2.2. Plant community and species description The eastern Mediterranean grassland communities evolved under natural and human- induced disturbances (Naveh, 1999). When protected from disturbances (e.g. grazing and fire), the plant community tends to be dominated by few tall annual grasses including Avena and Hordeum species (Noy-Meir, 1995). The present plant community is a typical semi-arid annual grassland community dominated by wild oat (Avena sterilis L.) and wild barley (Hordeum marinum Huds.). Both species are considered tall annual therophytes (Zohary and Feinbrum, 1966e1986). 2.3. Experimental design The experiment was conducted in a split-plot design with five replicates of whole plots over three growing seasons (2006e07, 2007e08 and 2008e09). Two levels of burning (burned vs. unburned) were applied to whole plots randomly, and two levels of fertilizers (fertilized vs. unfertilized) were applied to each subplot. The whole plot is 0.1 ha, while subplots are 0.05 ha each. Burning was conducted during September 2006, and granular fertilizer DAP (diammonium phosphate 18% N and 46% P2O5) was applied at a rate of 100 kg ha1 in early November 2006. Plant communities of each treatment were sampled using three 0.25 m2 quadrats. Measured parameters were: percentage of vegetative and litter coverage, fraction of bare soil (%), and species richness and evenness. All plants in each quadrat were identified to the species level following Zohary and Fienbrum (1966e1986) and the number of plants per species in each quadrat was recorded during peak vegetation growth (approximately between MarcheApril) of each season. Vegetative, litter and fraction of bare soil (%) in each quadrat was visually estimated, then all plant material was cut to ground level at maturity at the end of the growing season, then oven dried for 72 h at 70 C to determine the aboveground plant biomass. 2.4. Statistical analysis Measured plant community variables affected by burning, fertilizer application and their interaction was tested by constructing an analysis of variance of split-plot design, using a JMP fitted model and SAS statistical analysis software (Version 7, SAS Institute Inc. Cary NC USA, 1989e2007). Model parameters included the main effect of year (3) and block (5). Whole plots included the burning treatment (burned and unburned), while the fertilizer treatment (fertilized and unfertilized) was the subplot treatment. The model also included the interaction between burning and block as a random effect and used the whole plot error term. The interactive effect of burning and fertilizer was tested against the model error term. Means separation was run using a least squares difference test for measured variables that expressed a significant F value.
21
Table 2 Proportions of plant species (number of individuals per total number of plants per plot) and families’ names (bold names) found in Avena grassland under burned (B) and unburned (UB) plots during the 2006/7, 2007/8 and 2008/9 growing seasons. Family/species Asteraceae Leontodon arabicus Boiss. Caryophyllaceae Gymnocarpos decander Forssk. Silene palaestina Boiss. Silene rubella L. Vaccaria pyramidata Medikus Cistaceae Helianthemum ledifolium (L.) Mill. Compositae Achillea santolina L. Anthemis palestina Reuter Calendula arvensis L. Carduus australis L. fil. Carduus getulus Pomel Centaurea iberica Spreng. Crepis aspera L. Crepis sancta (L.) Bornm. Leontodon laciniatus (Bertol.) Widder ex Bornm. Scorzonera papposa DC. Senecio leucanthemifolius subsp. vernalis Poir Convolvulaceae Convolvulus althaeoides L. Cruciferae Chorispora purpurascens (Banks and Sol.) Eig Diplotaxis erucoides (L.) DC. Erucaria hispanica (L.) Druce Euphorbiaceae Euphorbia arguta Banks and Sol. Fumaria densiflora DC. fumaria parviflora Lam. Geraniaceae Erodium cicutarium (L.) L’Her. Erodium gruinum (L.) L’Her. Erodium malacoides (L.) L’Her Gramineae Avena sterilis L. Bromus tectorum L. Cynosurus callitrichus Barbey Hordeum marinum Huds. Phalaris minor Retz. Piptatherum miliaceum (L.) Coss. Poa annua L. Poa eigii Feinbrun Setaria viridis (L.) P.Beauv. Hypericaceae Hypericum triquetrifolium Turra Labiatae Lamium amplexicaule L. Malvaceae Malva sylvestris L. Papaveraceae Hypecoum aegyptiacum (Forssk.) Asch. and Schweinf. Roemeria hybrida (L.) DC. Papilionaceae Astragalus spinosus (Forssk.) Muschl. Trigonella arabica Delile Vicia narbonensis L. Vicia peregrina L. Plantaginaceae Plantago notata Lag. Primulaceae Anagallis arvensis L. androsace maxima L. Ranunculaceae Adonis dentata Delile Resedaceae Reseda lutea L. Rutaceae Haplophyllum buxbaumii (Poir.) G.Don f. Umbelliferae Anethum graveolens L. Malabaila secacul (Banks and Sol.) Boiss.
B
UB
0.21
0.00
0.00 0.70 0.26 0.26
0.08 0.30 0.00 0.12
0.36
0.00
0.07 9.19 5.61 2.46 4.62 9.25 0.31 0.64 0.48 0.03 4.28
1.63 4.94 3.36 2.60 0.37 2.91 0.27 0.41 1.53 0.00 0.66
0.42
0.00
0.04 8.91 7.37
0.19 0.35 5.05
0.18 0.69 0.08
0.00 3.07 0.23
0.37 1.39 0.38
1.18 0.91 0.30
9.52 0.72 1.27 9.35 4.12 0.00 0.99 5.44 0.61
21.44 0.00 0.14 12.28 8.30 8.63 6.67 5.68 0.39
0.19
0.11
0.12
0.00
0.27
0.25
0.19 0.39
0.00 0.00
0.48 3.80 0.10 1.62
1.54 1.99 0.07 0.64
0.00
0.34
0.21 1.10
0.00 1.01
0.32
0.00
0.23
0.00
0.04
0.00
0.02 0.37
0.02 0.00
22
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
Table 3 Analysis of variance (ANOVA) for vegetative, litter and fraction of bare soil (%) in grassland communities as affected by burning and fertilizer application during the 2006/7, 2007/8 and 2008/9 growing seasons (N ¼ 180). SoV
DF
MS
F-value
P-value
Vegetative cover (%) Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square Fraction of bare soil (%) Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square
MS
F-value
P-value
16.04 2.81 8.71 1.51 0.75 0.96
<0.0001 0.170 0.042 0.202 0.387 0.329
Litter cover (%)
2 4 1 4 1 1 166
3926.29 216.78 4185.69 53.49 8487.2 381.36 178.34 0.72
22.01 4.05 78.24 0.30 47.59 2.14
<0.001 0.102 <0.001 0.877 <0.001 0.146
2 4 1 4 1 1 166
4453.8 51.77 1717.42 122.19 6408.20 77.36 179.63 0.68
24.79 0.42 14.06 0.68 35.67 0.43
<0.001 0.787 0.02 0.607 <0.001 0.512
3. Results 3.1. Weather conditions Average precipitation and recorded temperatures of the study area are presented in Table 1. During the first growing season (2006e07), rainfall was lower (208 mm) than the long-term average (225 mm). The second year (2007e08) was much drier than average with 130 mm of precipitation. Moreover, the temporal distribution of rainfall was unfavourable for plant growth in the second year, as more than 50% of the total rainfall occurred during January (Table 1). In the 2008e09 season, rain occurred in February and March (Table 1) which significantly promoted plant growth. 3.2. Impact of burning and fertilizer on species abundance A total of 53 vascular plant species belonging to 19 families was found in the study plots (Table 2). As expected, grass species dominated the unburned plots. The dominant species were A. sterilis (21.44% individuals per total number of plants per plot), H. marinum (12.28%), Piptatherum miliaceum (8.63%), and Poa annua (6.67%). Growth of grass species was suppressed in the burning treatments whereas the growth of non-grass species was promoted. Moreover, burning enhanced Centaurea iberica, which increased from 2.91% to 9.25%, and increased Diplotaxis erucoides from 0.35% to 8.91%. Anthemis palestina increased from 4.95% in unburned plots to 9.19% in burning treatments (Table 2). 3.3. Impact of burning and fertilizer on cover parameters Burning significantly affected vegetative cover (p < 0.001) and litter cover (p ¼ 0.042) (Table 3), while fertilizer application significantly affected vegetative cover (p < 0.001), but not litter cover (p ¼ 0.387). The interaction between burning and fertilizer was not significant for both vegetative cover (p ¼ 0.146) and litter cover (p ¼ 0.329). Burning the grassland community significantly increased the vegetative cover over the unburned community by 21.2%, whereas litter cover was significantly reduced by 22.6% (Table 4). Under fertilization, plots had higher vegetative cover by 31.6% over the unfertilized treatment, while litter cover was reduced by 5.9%, though this reduction was not statistically significant (Table 4). The combined effect of burning and
721.57 190.61 590.42 67.756 33.8 43.02 45.00 0.75
fertilization increased vegetative cover by 35.5% and reduced litter cover by 13.9% (Table 4). Vegetative cover was the highest in plots that received both burning and fertilizers (63.53%) compared to all other treatment combinations. The highest litter cover was observed in the unburned plots regardless of whether fertilizer was applied. Burning resulted in the lowest litter cover only if fertilizers were applied. Burning without fertilizer application produced intermediate litter cover, which was not significantly different from the unburned plots. 3.4. Impact of burning and fertilizer on productivity and diversity parameters Split-plot ANOVA revealed significant effects of year on biomass (p < 0.001), species richness (p < 0.001) and the dominant grass species percentage (p < 0.001). Burning significantly affected Table 4 Mean and standard error (SE) values of vegetative, litter and fraction of bare soil (%) in grasslands community as affected by burning, fertilizer application and burning fertilization interaction during the 2006/7, 2007/9 and 2008/9 growing seasons. Treatment level Burning Burned Unburned SE % change due to burning Fertilization Fertilized Unfertilized SE % change due to fertilizer Burning Fertilization Burned þ Fertilized Burned þ Unfertilized Unburned þ Fertilized Unburned þ Unfertilized SE % change due to burning þ fertilizer as compared to Unburned þ Unfertilized
Vegetative cover (%)
Litter cover (%)
Fraction of bare soil (%)
55.2a 45.6b 0.8 21.2
12.5b 16.0a 0.9 22.6
32.1b 38.3a 1.2 16.1
57.3a 43.5b 1.4 31.6
13.7a 14.6a 0.7 5.9
29.2b 41.2a 1.4 29.0
63.5a 46. 9b 51.0b 40.2c 2.0 35.5
11.4b 13.3ab 16.0a 15.9a 1.0 13.9
25.4c 38.7a 33.0b 43.6a 2.0 34.5
Numbers along column with different letters are significantly different at the 0.05 significance level for each treatment.
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
23
Table 5 Analysis of variance (ANOVA) for aboveground plant biomass, species richness, proportion of Avena-Hordum and other species in grassland communities as affected by burning and fertilizer application and burning fertilization interaction during the 2006/7, 2007/8 and 2008/9 growing seasons (N ¼ 180). SoV
DF
MS
F-value
P-value
Biomass (g 0.25 m2) Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square
2 4 1 4 1 1 166
Year Block Burning Block Burning and random Fertilizer Burning Fertilizer Error R-square
2 4 1 4 1 1 166
31451.8 221.5 9890.6 186.8 9028.4 366.8 174.4 0.67
180.4 1.2 53.0 1.1 51.8 2.1
A-H % 7216.7 10297.0 9213.5 988.6 117.4 3584.1 570.4 0.69
12.6 10.4 9.3 1.7 0.2 6.3
biomass (p ¼ 0.019), species richness (p ¼ 0.015), and A-H% (p ¼ 0.038) (Table 5). Fertilizer application affected biomass production (p < 0.001) and species richness (p ¼ 0.011), but had no effect on A-H% (p ¼ 0.651). The interaction between burning and fertilizer application was not significant for biomass (p ¼ 0.149) but was significant for species richness (p ¼ 0.048) and for A-H% (p ¼ 0.013). Above ground biomass was significantly higher in the burning treatment (52.41 g 0.25 m2) than in the unburned treatment by 39.5% (Table 6). Burning increased species richness by 52.5% relative to the unburned treatment, while the dominant A-H complex was reduced by 43.1%. On the other hand, fertilizer application significantly increased biomass production and species richness by 37.4% and 13.5%, respectively (Table 6) over the unfertilized plots. Although the dominant A-H complex increased by 6.4%, the
Table 6 Mean values and standard error (SE) of aboveground plant biomass, species richness, Avena-Hordum % and other species% in semi-arid Mediterranean grasslands community as affected by burning, fertilizer and burning fertilizer interaction during the 2006/7, 2007/8 and 2008/9 growing seasons. Treatment level Burning Burned Unburned SE % change due to burning Fertilization Fertilized Unfertilized SE % change due to fertilizer Burning Fertilization Burned þ Fertilized Burned þ Unfertilized Unburned þ Fertilized Unburned þ Unfertilized SE % Change due to burning þ fertilizer as compared to unburned þ unfertilized
Biomass (g 0.25m2)
Species rich. (no. 0.25m2)
A-H %
Other species %
52.4a 37.6b 1.4 39.5
6.6a 4.3b 0.4 52.5
18.9b 33.2a 3.3 43.1
81.1a 66.8b 3.3 21.4
52.1a 37.9b 1.4 37.4
5.8a 5.1b 0.2 13.5
26.8a 25.2a 2.5 6.4
73.2a 74. 8a 2.5 2.1
60.9a 43.9b 43.2b 31.9c 2.0 90.8
7.2a 6.0b 4.4c 4.2c 0.3 69.8
15.2b 22.5b 38.5a 27.9ab 3. 6 45.5
84. 8a 77.5a 61.6b 72.1ab 3.56 17.6
MS
F-value
P-value
Species rich. (no. 0.25 m2)
Numbers along column with different letters are significantly different at the 0.05 significance level for each treatment.
<0.001 0.436 0.019 0.373 <0.001 0.149
<0.0001 0.022 0.038 0.145 0.651 0.013
91.4 12.9 231.2 13.7 21.4 12.8 3.22 0.72 Other spp % 7216.7 10297.0 9213.5 988.6 117.5 3584.1 570.4 0.69
28.4 0.9 16.9 4.3 6.6 4.0
<0.001 0.522 0.015 0.003 0.011 0.048
12.6 10.4 9.3 1.7 0.2 6.3
<0.001 0.022 0.038 0.145 0.651 0.013
increase was not statistically significant. The combined effect of burning and fertilizer application significantly increased plant biomass by 90.8% and species richness by 69.8%, and reduced A-H by 45.5% (Table 6). The highest biomass was observed on burned and fertilized plots (60.92 g 0.25 m2) which was significantly higher than in all other combinations. The lowest biomass was observed for the unburned unfertilized plots, while the fertilized unburned or burned unfertilized plots were intermediate in biomass production. The highest species richness was observed in burned and fertilized plots while the lowest species richness was observed in unburned plots regardless of the fertilizer application. In other words, fertilizer application had different impacts on species richness depending on burning whereas fertilizer application increased species richness under burning but had no impact under the no burning regime. Burning generally resulted in lower A-H% compared to the control, while fertilizer application had variable effect on A-H%, reducing A-H% with burning but increasing it without burning. 3.5. Impact of burning and fertilizer application on plant community in response to weather conditions Burning increased vegetative cover and biomass production but reduced A-H% in the first growing season (2006e07) following the burning treatment, with profound effects in the third growing season, in 2008e09 (Fig. 1). Both of these years had average growing seasons. While burning showed no effect on the vegetative cover, biomass production and A-H% decreased in the dry 2007-08 growing season (Fig. 1). Burning increased species richness in all seasons (Fig. 1) compared to the unburned plots; however, the difference was highest in the favourable growing seasons and lower in the dry growing season. Reduction in litter cover was noted only in the first growing season, although the difference between burned and unburned plots in litter cover diminished in the second and third growing seasons. Fertilization increased vegetative cover and biomass in all growing seasons with a more pronounced increase in the first growing season (Fig. 2) following fertilizer application, while fertilization showed no effect on litter cover and the dominant A-H % in all growing seasons (Fig. 2). The fertilizer effect on species richness was insignificant for the first two growing seasons and showed a positive effect only in the third growing season.
a Spp. rich.
6 3 2007
2008
2009
Veg. cover %
d
9
55 30 2007
Year
Litter cover %
Biomass
45 20 2008
28 16 4
2009
2007
2008
Year
Fraction of bare soil %
A+H %
f 55
40 20 0 2007
2009
Year
c
60
2009
e
70
2007
2008 Year
b
95
Burned Unburned
80
2008
35 15 2007
2009
2008
2009
Year
Year
Fig. 1. Response of (a) species richness, (b) aboveground plant biomass (g 0.25m-2), (c) proportion of Avena and Hordeum complex (A-H), (d) vegetative cover %, (e) litter cover %, (f) fraction of bare soil (%) % to burning treatment grasslands community in the 2006/7, 2007/8 and 2008/9 growing seasons.
a
6
2008
70 50 30 2007
3 2007
2009
b Litter cover %
2008 Year
2009
14 8 2007
25
5 2008 Year
2008
2009
Year
c
2007
20
Fraction of bare Soil %
Biomass A+H %
45
2009
e
50 20 2007
2008 Year
Year
80
Fertilized Unfertilized
d Veg. cover %
Spp. rich.
9
2009
55
f
35 15 2007
2008 Year
2009
Fig. 2. Response of (a) species richness, (b) aboveground plant biomass (g 0.25m-2), (c) proportion of Avena and Hordeum complex (A-H), (d) vegetative cover %, (e) litter cover %, (f) fraction of bare soil %, to fertilizer treatment grasslands community in the 2006/7, 2007/8 and 2008/9 growing seasons.
M.N. Alhamad et al. / Acta Oecologica 40 (2012) 19e26
4. Discussion and conclusions Burning and fertilizer application had diverse effects on litter cover and species composition. In addition, the effect of burning and fertilization on species diversity showed a significant interaction, while no interaction was found with respect to cover parameters and biomass. The significant interaction of some parameters and the insignificant interaction of other parameters lead us to believe that these two management options exert different impacts on plant communities. Therefore, fire impact on the community is not only a matter of nutrient recycling, and fertilizer applications do not compensate for fire. On the one hand, fire reduced the Avena/Hordeum complex and increased diversity by increasing the presence of other species and improving vegetative cover and biomass. On the other hand, fertilizer application did not affect litter cover and the presence of Avena/Hordeum complex but had positive impact on vegetative cover and biomass productivity as expected. Unexpectedly, fertilizer application improved species diversity but did not affect litter cover. Though the interaction of burning and fertilization is significant, we cannot draw conclusions regarding each treatment separately. Burning and fertilizer application had a synergistic effect on vegetative cover. Whereas fertilization improved vegetative cover by approximately 10.8% and burning improved vegetative cover by approximately 6.7%, their combined effect (burning and fertilization) increased vegetative cover by 23.4%, exceeding their sum (17.5%). There was also a synergistic effect of burning and fertilization on litter cover, with burning and fertilization combined significantly reducing the litter cover compared to the individual treatments. Improved biomass production for the burned plots over the unburned plots indicates that these semi-arid grasslands are not at their potential production capability and that burning improves their biomass production. This confirms previous studies on the same area in which light to moderate defoliation (surrogate for grazing) increased the biomass production. These grasslands have evolved under grazing pressure, thus protection seems to increase competition and reduce production potential (Alhamad and Alrababah, 2008). Eastern Mediterranean grasslands have evolved under livestock grazing and other anthropogenic pressures over the last 10,000 years (Harlan and Zohary, 1966; Perevolotsky and Seligman, 1998), resulting in the high resilience of these plant communities and facilitating their continued existence under harsh environmental pressure (Lavorel et al., 1999). Although the study area has been protected from grazing for about 35 years, the plant community is still dominated by large-seeded annual grasses such as Avena and Hordeum. These grass species have developed adaptive mechanisms including early seed shedding, seed dormancy, polymorphism (Naveh, 1999) and efficient dispersal and burial mechanisms that allow them to successfully invade and persist in many grasslands (Naveh, 1967), thereby impacting plant biodiversity. Burning caused significant increase in species richness, plant biomass, and vegetative cover. These responses to burning are in agreement with previous studies (Parsons and Stohlgren, 1989; Dyer and Rice, 1997; Meyer and Schiffman, 1999). Results clearly show that prescribed burning caused significant decrease in litter cover and in the proportion of Avena-Hordeum complex in the plant community. The suppression of certain plant species due to burning was also reported by diTomaso et al. (1999). Grassland community response to burning, including increased species richness and enhanced overall productivity, most likely resulted from removing litter, thus altering the availability of light and soil nutrients for different plant species (diTomaso et al., 1999; Gimeno-Garcia et al., 2000; Harrison et al., 2003; Fynn et al., 2004).
25
Litter has direct and indirect effects on the physical and chemical environment. The decomposition of litter may release certain phytotoxic materials in the environment. Moreover, the accumulated litter reduces light reaching the soil surface and reduces maximum soil temperature, affecting seed germination and seedling growth of companion species (see Facelli and Pickett, 1991 for complete review). The direct and indirect effects of the litter of one species on another may alter the competitive interaction and affect the structure of plant community (Grime, 1979; Facelli, 1994). We hypothesized that under unburned treatments, accumulated litter decreased light reaching the soil surface, providing an opportunity for large-seeded grasses (including Avena and Hordeum species) to germinate earlier and deplete resources (Facelli and Pickett, 1991). The increase in species richness and decrease in Avena-Hordeum complex by litter removal under burning treatment are consistent with the findings of Amatangelo et al. (2008) in which large-seeded grasses (e.g. Avena fatua) performed poorly due to increased heat and decreased water availability in plots where litter was removed from an annual grassland community in California. High light environments under litter removal by burning may have induced the early germination of companion small-seeded species, enhancing their competitive potential for resources such as nutrients, water, and light over large-seeded grass species (Fowler, 1988; Dyer et al., 2000). These findings illustrate that AvenaeHordeum complexes benefited from litter accumulation, so were the most impacted by burning. Therefore, we conclude that fire can be used as a management tool to improve the productivity and enhance diversity of these ecosystems when employing appropriate fire intervals since frequent fire in low productivity ecosystems may reduce productivity.
Acknowledgements The authors would like to thank the Deanship of Scientific Research at the Jordan University of Science and Technology (JUST) for their financial support. We also thank Rika Muhl for editing the language of our Manuscript.
References Alhamad, M.N., 2006. Ecological and species diversity of arid mediterranean grazing land vegetation. Journal of Arid Environment 66, 698e715. Alhamad, M.N., Alrababah, M.A., 2008. Defoliation and competition effects in a productivity gradient for a semiarid Mediterranean annual grassland community. Basic and Applied Ecology 9, 224e232. Alhamad, M.N., Alrababah, M.A., Bataineh, M.M., Al-Horani, A.S., 2008. Environmental gradients and community attributes underlying biodiversity patterns of semi-arid Mediterranean grasslands. Plant Ecology 196, 289e299. Alhamad, M.N., Oswald, B.B., Bataineh, M.M., Alrababah, M.A., Al-Gharaibeh, M.M., 2010. Relationships between herbaceous diversity and biomass in two habitats in arid Mediterranean rangeland. Journal of Arid Environments 74, 277e283. Al-Mufti, M.M., Sydes, C.L., Furness, S.B., Grime, J.P., Band, S.R.J., 1977. A quantitative analysis of shoot phenology and dominance in herbaceous vegetation. Journal of Ecology 65, 759e791. Alrababah, M.A., Alhamad, M.N., Suwaileh, A., Al-Gharaibeh, M., 2007. Biodiversity of semi-arid Mediterranean grasslands, impact of grazing and afforestation. Applied Vegetation Science 10, 257e264. Amatangelo, K.L., Dukes, J.S., Field, C.B., 2008. Responses of a California annual grassland to litter manipulation. Journal of Vegetation Science 19, 605e612. Belsky, A.J., 1992. Effects of grazing, competition, disturbance and fire on species composition and diversity in grassland communities. Journal of Vegetation Science 3, 187e200. Bond, W.J., Woodward, F.I., Midgley, G.F., 2005. The global distribution of ecosystems in a world without fire. New Phytologist 165, 525e538. Brockway, D.G., Gatewood, R.G., Paris, R.B., 2002. Restoring fire as an ecological process in short grass prairie ecosystems, initial effects of prescribed burning during the dormant and growing seasons. Journal of Environmental Management 65, 135e152. Cleary, M.B., Pendall, E., Ewers, B.E., 2010. Aboveground and Belowground Carbon Pools After Fire in Mountain Big Sagebrush Steppe. Rangeland. Ecology Management 63, 187e196.
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