Tree species effect on litter decomposition and nutrient 2

45 Pages·2012·1.01 MB·English

Tree species effect on litter decomposition and nutrient
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release in Mediterranean oak forests change over time
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3 Authors: Cristina Aponte*, Luis V. García and Teodoro Marañón
4 Affiliation: Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS), CSIC
5 PO Box 1052, E-41080 Sevilla, Spain
6 *Correspondence and present address:
7 Department of Forest and Ecosystem Science; University of Melbourne
8 500 Yarra Boulevard, Richmond, Victoria 3121, Australia.
9 E-mail: [email protected]
10 Author contributions: CA, LVG, and TM conceived of the idea and wrote the article; CA conducted
11 chemical analyses and analyzed the data.
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13 Running headline: Leaf litter decomposition in Mediterranean oak forests
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15 ABSTRACT
16 Tree species can affect the decomposition process through the quality of their leaf fall and
17 through the species-specific conditions that they generate in their environment. We compared the
18 relative importance of these effects in a two-year experiment. Litterbags containing leaf litter of
19 the winter-deciduous Quercus canariensis, the evergreen Q. suber and mixed litter were
20 incubated beneath distinct plant covers. We measured litter carbon loss, 9 macro- and
21 micronutrients and 18 soil chemical, physical and biological parameters of the incubation
22 environment.
23 Tree species affected decay dynamics through their litter quality and, to a lesser extent,
24 through the induced environmental conditions. The deciduous litter showed a faster initial
25 decomposition but left a larger fraction of slow decomposable biomass compared to the
26 perennial litter; in contrast the deciduous environment impeded early decomposition while
27 promoted further carbon loss in the latter decay stages. The interaction of these effects led to a
28 negative litter-environment interaction contradicting the “home-field advantage” hypothesis.
29 Leaf litter N, Ca and Mn as well as soil N, P and soil moisture were the best predictors for
30 decomposition rates. Litter N and Ca exerted counteractive effects in early versus late decay
31 stages; Mn was the best predictor for the decomposition limit value, i.e. the fraction of slowly
32 decomposable biomass at the later stage of decomposition; P and soil moisture showed a
33 constant and positive relation with carbon loss. The deciduous oak litter had a higher initial
34 nutrient content and released its nutrients faster and in higher proportion than the perennial oak,
35 significantly increasing soil fertility beneath its canopy.
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36 Our findings provide further insights into the factors that control the early and late stages of
37 the decomposition process and reveal potential mechanisms underlying tree species influence on
38 litter decay rate, carbon accumulation and nutrient cycling.
39 Keywords: decomposition limit value, lignin, litterbag, litter chemistry, Quercus, soil fertility,
40 plant-soil interactions,
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42 INTRODUCTION
43 Differences between tree species litter decomposition have commonly been related to distinct
44 substrate quality with litter C:N and N:P ratios, lignin content, Ca and Mn concentration
45 emerging as the main rate-controlling factors (Melillo et al., 1982; Cornelissen et al., 2006;
46 Hobbie et al., 2006; Cornwell et al., 2008; Güsewell and Gessner, 2009; Berg et al., 2010). But
47 tree species can also alter decomposition rates indirectly through their effects on environmental
48 conditions. For example, tree species can induce changes in soil fertility, microclimate and
49 faunal and microbial communities in the forest floor (Mitchell et al., 2007; Aponte et al., 2010a;
50 Aponte et al., 2011), all of which influence the decomposition process (Hobbie, 1996; Sariyildiz
51 and Anderson, 2003; Austin and Vivanco, 2006). The simultaneous effects of trees on
52 decomposition both through their litter quality and by modifying the environmental conditions
53 might cause positive litter-environment interactions and further increase decomposition. This
54 interaction, termed “home-field advantage”, implies litter decomposes faster beneath the tree
55 species from which it is derived than beneath other plant covers and could be explained as an
56 adaptation of the local soil communities to the litter produced by the plant species above them
57 (Negrete-Yankelevich et al., 2008; Ayres et al., 2009). Despite the implication for ecosystem
58 functioning and carbon cycling, the environment effect of tree species on litter decomposition
59 has barely been explored and the relative importance of the litter vs. environment tree species
60 effect on decomposition process still remain unclear (but see (Hansen, 1999; Hobbie et al., 2006;
61 Vivanco and Austin, 2008).
62 The litter decomposition process is ultimately driven by specific controlling factors related to
63 the requirement of the decomposer community and whose availability is partly determined by
64 tree species. As litter decomposition progresses through time litter quality varies and the factors
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65 controlling litter mass loss might change (Berg and McClaugherty, 2008). Early decomposition
66 is often determined by the availability of limiting elements such as N and P whereas in late
67 stages the carbon loss has been related to elements required to decompose recalcitrant
68 components such as lignin that accumulate in remaining litter (Güsewell and Gessner, 2009;
69 Berg et al., 2010). Thus variables controlling the early decomposition stage and nutrient release
70 could differ from those influencing the proportion of slow decomposing litter and therefore the
71 build up of soil organic matter and carbon sequestration. Occasionally, the same variable could
72 have counteractive effects on the early and late stages of decomposition (Berg and
73 McClaugherty, 2008; Hobbie et al., 2012). For instance litter N is positively related to initial
74 decomposition rates (Melillo et al., 1982), but negatively related to late stages decay (Berg and
75 Ekbohm, 1991). Whereas the factors controlling decomposition have commonly been identified
76 in studies addressing either the early or late decay stages, few studies have followed the changes
77 in rate-regulating factors over the same long-term experiment.
78 The decay patterns of chemical elements in decomposing litter dynamics are highly diverse,
79 even for litters of a similar type and often reflect the requirements and availability of nutrients to
80 the decomposer community (Swift et al., 1979; Staaf and Berg, 1982). Limiting nutrients
81 occurring in suboptimal amounts would be accumulated by the decomposers whereas nutrients
82 exceeding the needs of decomposers would be released (Laskowski et al., 1995). The analysis of
83 the amounts and concentrations of nutrients along the decomposition process of different species
84 litter can reveal changes in the limiting elements over time, reflecting changes in the
85 decomposition stages and processes and showing the differences in species nutrient cycling.
86 We aimed to compare the effects that tree species exert on litter decomposition via litter
87 quality and via environmental conditions and to evaluate whether the factors mediating these
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88 effects change over time by studying the leaf litter decomposition and nutrient release of two co-
89 occurring oak species: the evergreen Quercus suber and the winter deciduous Q. canariensis. We
90 previously demonstrated that these species generate significantly different biotic and abiotic
91 environments beneath their canopy though their distinct leaf litter nutrient return (Aponte et al.,
92 2010a; Aponte et al., 2010b; Aponte et al., 2011). We studied litter decay using litterbags with
93 single and mixed species litter since the effects of individual species may differ in mixed forest
94 conditions as a result of positive, negative or neutral interactions between litter types (Gartner
95 and Cardon, 2004; Hättenschwiler and Gasser, 2005). Litterbags were incubated in four
96 microsites: beneath the two oak species, under shrubs and in open areas.
97 Our specific objectives were four: 1) To investigate the tree species effect on decomposition
98 via litter quality both in single and mixed species conditions. 2) To evaluate tree species effect
99 on decomposition via the distinct environment they generate beneath their canopy. We also
100 tested for a positive litter-environment interaction supporting the home-field advantage
101 hypothesis. 3) To identify the litter and soil chemical properties that best predicted the decay
102 parameters associated with different stages of the decomposition process. 4) To analyse the
103 patterns of liberation and immobilization of chemical elements from the decomposing litter of
104 the two oak species.
105 METHODS
106 Study area
107 This study was conducted in the Aljibe Mountains, near the Strait of Gibraltar, southern Spain.
108 The bedrock is dominated by Oligo-Miocene sandstone that produces acidic, nutrient-poor soils
109 (Palexeralfs), which are frequently interspersed with layers of marl sediments that yield soils
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110 richer in clay (Haploxererts; nomenclature follows Soil Survey Staff 2010). The climate is sub-
111 humid Mediterranean, with a dry and warm summer period of 3-4 months and most rainfall
112 (95%) occurring from October to May (Anonymous, 2005). The dominant vegetation is a mixed
113 forest of evergreen cork oak (Quercus suber L.) and winter-deciduous Algerian oak (Q.
114 canariensis Willd.). These oak species differ in their leaf fall and litter quality. Leaf fall from Q.
115 canariensis has a higher nutrient content (Ca, K, Mg and S) than Q. suber, and this difference
116 induces distinct soil conditions via nutrient return (Aponte et al., 2011). The arborescent shrubs
117 Erica arborea L., Phillyrea latifolia L. and Pistacia lentiscus L. are abundant in the understorey
118 (Ojeda et al., 2000). The area has been protected since 1989 as "Los Alcornocales" (meaning
119 "the cork oak forests") Natural Park.
120 Two structurally different mixed forest sites, 40 km apart, were selected within the study
121 area. The site at San Carlos del Tiradero (hereafter called Tiradero) (36º 9’ 46’’ N; 5º 35’ 39’’
122 W) is located in the southern area of the Park, near the coast, at 335–360 m a.s.l. on a NE-facing
123 slope. The mean annual rainfall is 964 mm, and the mean annual air temperature is 16.6 ºC, with
124 a minimum of 4.1 ºC. This stand has a high density of trees (769 stems ha-1), with a basal area of
125 47 m2 ha-1. The other site, at Sauceda (36º31’54’’N; 5º34’29’’W), is located inland, in the
126 northern area of the Park, at 530–560 m a.s.l. on a NW-facing slope. It has a mean annual
127 temperature of 15.5 ºC, with a minimum of 1.8 ºC, and a mean annual rainfall of 1470 mm. The
128 tree density at Sauceda is relatively low, with 219 stems ha-1 and a basal area of 22 m2 ha-1. The
129 two oak species, Q. canariensis and Q. suber, co-occurred at both forest sites (Pérez-Ramos et
130 al., 2008).
131 Litter decomposition experiment
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132 Freshly senesced leaves of the two oak species were collected from a large forest tract near one
133 of the sites (Sauceda) to minimize within species litter chemistry heterogeneity. The leaves were
134 obtained by gently shaking the tree branches. The collections were made at the end of March (for
135 Q. canariensis) and June (for Q. suber) 2007, during the respective leaf-fall periods of the two
136 tree species. Litter was air-dried and stored at room temperature. We prepared 11 x 11 cm
137 litterbags (2 mm fibreglass mesh) with approximately 2.00 g of air-dried leaf litter of a given
138 species or an equivalent mixture of the two species. The exact litter weight of each bag was
139 recorded in grams with an accuracy of two-decimal places. Six litter bags of each species were
140 dried at 65°C for 48h and weighed to determine the dry mass conversion that was used to
141 calculate the initial dry mass of each sample. The bag size was consistent with the average size
142 of Q. canariensis (7.4 x 3.7 cm) and Q. suber (4.1 x 2.4 cm) leaf litter. The mesh size was
143 chosen to optimise access by organisms to the litter while minimising particle loss (Karberg et
144 al., 2008). We placed the litterbags beneath the canopy of six adult individuals of Q. suber and
145 six of Q. canariensis at the two forest sites (i.e. 4 types of microsite). The footprint of a tree
146 species on the soil is expected to be more intense within the vertical projection of the canopy
147 (Finzi et al., 1998a; Bennett et al., 2009), particularly if canopies are segregated, as is the case in
148 Sauceda. The trees selected had their closest heterospecific neighbour at a distance of 8 m in
149 Sauceda and at 3 m in Tiradero. In addition, at Sauceda, we located litterbags in two other types
150 of microsites (with 6 replicates each): under shrubby cover and in forest gaps with herbaceous
151 vegetation. Litterbags were placed on the surface of the standing litter layer and fastened to the
152 soil with 15cm long wooden sticks. In all, 432 litterbags (3 litter types x 6 types of microsites x 6
153 replicates x 4 harvests) were placed in the field in November 2007 and harvested every 6 months
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154 for 2 years. On each occasion, six replicate litterbags of each litter and microsite type were
155 collected.
156 Upon harvest, the litter was removed from the bags, separated from roots and large soil
157 aggregates, dried (65 º C, 48 h) and weighed. The weight of the remaining biomass was corrected
158 for the water content of the initial air-dried samples. The leaves from the two species in the
159 mixed litterbags were carefully separated and were treated independently thereafter. Subsamples
160 of the initial leaf litter from each species and the harvested litter samples were ground and
161 analysed for C and N content (using a Leco TruSpec analyser) and for the total concentration of
162 several nutrients (Ca, K, Mg, P, S, Mn, Cu and Zn) by acid digestion followed by ICP-OES
163 (Varian 720-ES) determination to asses changes in nutrient content over time. The proportion of
164 remaining carbon (RC) was calculated by dividing the amount of carbon at any harvest date (C
165 concentration per g of remaining litter at that time) by the initial amount of carbon (initial
166 concentration per g of initial litter).
167 Microsite soil characterisation
168 Several inorganic and biological properties of the soils beneath the selected trees (Table 1) had
169 been previously determined in our parallel studies of element cycling (Aponte et al., 2011) and
170 soil microbial biomass (Aponte et al., 2010b). Briefly, the methods used were as follows. In
171 November 2006, soil cores 25 cm deep were extracted with a cylindrical auger at each microsite
172 (6 replicates per type of microsite). We determined soil pH in a 1:2.5 soil:H O solution. The
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173 available soil P was estimated using the Bray-Kurtz method. The soil NH + was extracted with
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174 KCl (2 M) and determined by steam distillation. The total concentrations of several nutrients
175 (Ca, K, Mg, P, S, Mn, Cu and Zn) were determined by acid digestion followed by ICP-OES
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176 analysis (Sparks, 1996). In addition, in May, September and December 2007 we sampled 8-cm-
177 deep soil cores at the same microsites to estimate gravimetrical water content and to determine
178 microbial C, N and P using a chloroform fumigation-extraction procedure (Brookes et al., 1985;
179 Vance et al., 1987). For simplicity we use here the values of May 2007, which showed the
180 largest variability between microsites. These measurements were used to characterize the
181 incubation sites and determine the best predictors of litter decomposition.
182 Data analysis
183 We fitted litter change over time with two alternative decay models proposed by Wieder and
184 Lang (1982): a single-exponential decomposition model, M=e-ket, where M is the proportion of
t t
185 remaining biomass at time t and k is the decay rate, and an asymptotic model, M=m + (1 – m)e-
e t
186 kt where M is the proportion of remaining mass at time t, m is the fraction of the initial mass with
t
187 a decomposition rate of zero (i.e., the asymptote) and k is the decomposition rate of the
188 remaining fraction (1–m). The asymptotic model implies that there is a limit value (m) for mass
189 loss. This value corresponds to a very stable fraction of the litter that decomposes extremely
190 slowly over the time span of the experiment (Berg et al., 2003). In this study we have used
191 carbon instead of biomass data to analyse decay rates, and thus avoid the confounding effects of
192 the interactions between litter and mineral soil. All models were fitted using nls (nonlinear least
193 squares) function in R freeware (http://www.r-project.org/) and they all constrained the
194 proportion of initial mass (carbon) remaining at time zero to be 1. Model selection was
195 performed using Akaike´s Information Criterion (AIC). Models whose AIC values differed by
196 less than 2 were considered to have an equivalent ability to describe the data.
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