Litter decomposition is an important process occurring in forest ecosystems, where it affects the carbon balance as a whole. In Mediterranean area, seasonal changes and climate variations associated to latitude and structural characteristics of forest stands have a real effect on decomposition rates. Current leaf litter decomposition models are frequently too general to represent local climate variations in Mediterranean forests. We developed a new dynamic semi-empirical-based model, which simulated the early stage of decomposition of leaf litter based on local climate conditions and few operational parameters. Leaf litter was divided in two components, settled on different carbon compound concentrations. The effects of temperature and moisture were characterized by specific equations and the decomposition rates were time-depending functions. Equations were calibrated by the best fitting procedure performed on field data obtained by the litterbag method followed in mixed deciduous forests in central Apennines (Italy). Model validation showed an excellent correlation between observed and predicted values (R2 between 0.89 and 0.95), predicting thus differences in decomposition rates among different local climates. The simple structure of the model and the satisfactory reliability of outputs are important features for a practical alternative to other CO2 release evaluation methods applied to forest ecosystems.
Litter decomposition is a central feature of the ecosystem dynamics, being involved in the carbon and nutrient cycling through the active transformation of organic to inorganic matter (
The decomposition dynamics have frequently been modeled for temperate ecosystems (
In this study, we developed a semi-empirical model for assessing the early-stage litter degradation dynamics over two years (2011-2012) in three sites located along a north-to-south transect in central Italy (Umbria, Lazio and Campania) and covered by sub-Mediterranean mixed woods. Two litter compartments showing different litter qualities and decomposing at different rates were considered. The role of different local climates has been taken into consideration by applying multiplicative functions that influenced the simulated decomposition process. The decomposition model has been parametrized using field data collected in the Lazio test site, and then applied to the other test sites (Umbria and Campania) for cross-validation. The use of leaf litter collected from a single site (Monti Lucretili) also in the other sites avoided possible influences resulting from using different leaf material and allowed to better test the temperature-based heterotrophic respiration for the carbon balance
The study has been carried out in three sites from central Italy located along a north-south transect (
The leaf litter decomposition experiment conducted in Monte la Sauceda, southwest Spain (36° 30′ N, 5° 35′ W, 432 m a.s.l. -
The litterbag approach (
During October 2010, leaves from branches of Italian maple, field maple and European hornbeam have been collected from trees growing in the Regional Park of Monti Lucretili. Such unique sampling site has been chosen in order to exclude differences in the physical and chemical properties of leaves due to differences at growing sites. In this way, the decomposition rates were only dependent on local climate, local micro- and meso-fauna and soil properties of the test sites. Sampled leaves were pooled and air-dried at room temperature until reaching a constant weight. Three grams of leaf biomass were enclosed in each bag, according to the percentage of presence of species in the forest stand: 40% Italian maple, 40% field maple and 20% European hornbeam. Sixty fine mesh litterbags and sixty coarse mesh litterbags have been placed at the soil surface in each site in January 2011, and periodically collected (six items for each bag type) until June 2012. Moreover, six items of each bag-type have been used to determine the weight losses due to manipulation,
Leaves from litter bags were carefully cleaned with ultra-pure distilled water to eliminate soil particles and meso-fauna at each sampling. After cleaning, leaves were oven-dried at 50 °C for 72 hours and then weighted. The litter mass loss was calculated as percentage of variation from the initial weight. Since small amounts of soil mineral elements could contaminate leaf samples in the bags, the remaining mass of each sample was calculated by detracting its ash content from the dry mass. The ash content was determined by burning 0.3 g of each sample at 550 °C for 2 hours in a high temperature electric oven (GenLab, UK). Leaves were pulverized in a planetary ball mill (PM400, Retsch - Germany). Lignin content of the mixed species litter was determined using the method by
Time-dependent simulation of leaf litter decay has been performed on a dynamic semi-empirical model based on the Olson’s model (
The semi-empirical model of litter decomposition (
Sub models and their equations to simulate the parameters are described below (see also
Data concerning latitude, annual average air temperature (
where
This module has been developed to simulate the soil water content (
where
The time period considered for the
Litter decomposition followed two exponential dynamics made as function of time and climatic conditions (
where
where
Then, the time-based equations of
where
The decomposition rates have been corrected by incorporating the climatic decomposition index (
The effect of temperature has been simulated taking into account three functions from the literature, suited for Mediterranean conditions (
The decomposition rates were strongly affected by temperature functions by slowing down of decomposition process of about 85-75% at low winter temperature. The three functions were affecting decomposition rates in different ways during summer period. The arctan function (
The relative water content [
where
Further information about decomposition model are reported in
Multivariate analysis of variance (MANOVA) and principal component analysis (PCA) were performed to test for differences among test sites in the climatic parameters (precipitation, average, minimum and maximum temperature), and to classify all the variables considered in order of importance.
The MANOVA analysis of climatic variables (precipitation, average, minimum and maximum temperature) carried out over the whole experimental period (January 2011 to June 2012) revealed significant differences among test sites (F = 106.76, p < 0.01). However, the
According to the Kaiser’s criterion (
The litterbag method has been widely utilized to quantify the litter mass decay over time. However, we observed that decomposition rates were overestimated both in early and late phases of the process when the coarse-mesh (10 mm) litterbags were utilized. This was likely due to losses of organic matter not related to the decomposers’ activity. Although the use of fine-mesh (3 mm) bags could reduce the decay rates by excluding some invertebrates, errors in the estimation of decomposition rates resulted higher when coarse-mesh bags were used. Therefore, the decomposition model has been parametrized using only data from fine-mesh litterbags. The litter mass decay over time is displayed in
Equations describing the decomposition rates [
The climatic dependence indexes (
Model validation was carried out using the dataset obtained from the leaf litter experiment at Monte la Sauceda, southwest Spain. Simulations carried out adequately reflect the litter decay dynamics of two litter species at the test site (
Our results showed a pronounced dependence of leaf litter decomposition upon local climatic conditions. Decomposition was affected by low temperature that strongly reduced the decay rate of labile compounds of the litter during winter months (
The model proposed in this study proved to simulated quite accurately the early stage decomposition dynamic for Mediterranean mixed woods. Although the model was based on a consolidated approach, the time-based decomposition rates are a novel mathematical implementation. Notably, these equations were not site-specific and were applied indiscriminately to the all study sites.
Different local climates required different approaches for modeling the litter decomposition process, which is highly dependent on temperature in Mediterranean ecosystems. Moreover, by comparing the experimental litter mass loss to its model predictions (data not shown), we noticed that labile and recalcitrant litter components did respond differently to changes in the climatic conditions. Furthermore, the application of the decomposition model optimized for mixed woods of central Italy to the hot and dry site in Monte la Sauceda (Spain), allowed us to better focus on the most important aspects of the model in order to improve its performances under different environmental conditions. Despite the model overestimation of mass losses in the
The decomposition model developed in this study is able to describe the early stages of leaf litter decomposition along a climatic gradient in the Mediterranean mixed woods using few important geo-climatic parameters. Despite some systematic deviations of predictions from observed data were detected, the model may represent an operational compromise between formal simplicity and satisfactory reliability of simulations, as demonstrated by the strong correlations between observed and predicted values. Although different local microclimates required different approaches for modeling the litter decomposition process, the model achieved can take into account the heterogeneity of the Mediterranean environment. Notably, the general time-based equations for modeling the decomposition rates resulted not site-specific, though derived from experimental data.
The litter decomposition model presented here will be next embedded in a process-based model for the quantification of carbon assimilation (photosynthesis) and autotrophic respiration, thereby allowing an assessment of the carbon balance for the considered forest ecosystem under different microclimates. The final aim is to provide forest managers with an important tool for assessing the effect of different managing activities on the carbon balance of forest ecosystems.
The authors wish to thank the Regional Agency for Development and Innovation of Agriculture of Lazio (ARSIAL) and the Campania Regional Council (A.G.C.
Conceptual diagram of the decomposition model. Dashed boxes refer to input variables, squares are stocks, rounded square are driver variables, circles represent specific information dealing with model control, dashed arrows denote fluxes and solid arrows represent impacts.
Trends of the three soil temperature-based functions
Values of
Scatter plots (a) and decay curves (b) showing the comparison between observed and simulated data for the site located at Monti Lucretili.
(Upper panels): observed and simulated values of mass loss with time at two study sites: (a) Boschi di Pietralunga; (b) Monti Picentini. (Lower panels): comparisons between observed and simulated data obtained at two study sites: (c) Boschi di Pietralunga; (d) Monti Picentini.
(Upper panels): observed and simulated values of mass loss with time at the validation site: (a)
Main characteristics of the three test sites analyzed for model calibration. (FC): field capacity (mm/m2) of the upper 30-cm soil layer (measured value); (WP): wilting point (mm/m2) of the upper 30-cm soil layer (measured value); (Tmean): mean annual temperature (°C); (Tjan): average of January minimum temperature (°C); (SDS): Mitrakos’ index of summer drought stress; (WCS): Mitrakos’ index of winter cold stress (
Site | Coordinates | Elevation(m a.s.l.) | Aspect | FC(mm/m2) | WP(mm/m2) | Year | Tmean(°C) | TJan(°C) | Rainfall(mm) | SDS | WCS |
---|---|---|---|---|---|---|---|---|---|---|---|
Boschi di Pietralunga | 12° 25′ 27.48″ E43° 25′ 47.64″ N | 643 | N | 95.4 | 27 | 2008 | 14.2 | 3 | 969.2 | 56.3 | 64.3 |
2009 | 13.5 | 1 | 897.6 | 42.3 | 71.6 | ||||||
2010 | 12.8 | -0.1 | 1254.4 | 33.7 | 72 | ||||||
2011 | 13.9 | 1.4 | 638.4 | 32.9 | 66.1 | ||||||
average | 13.6 | 1.3 | 939.9 | 41.3 | 68.5 | ||||||
Monti Lucretili | 12° 51′ 51.12″ E42° 02′ 56.40″ N | 843 | N | 94.2 | 29.1 | 2008 | 14.5 | 2.8 | 1449.8 | 42.1 | 61.9 |
2009 | 13.8 | 2.2 | 1598.2 | 9.5 | 61.6 | ||||||
2010 | 13.1 | 0.7 | 1569.4 | 3.2 | 69.1 | ||||||
2011 | 14.4 | 2.1 | 908.8 | 22.9 | 58.9 | ||||||
average | 14 | 2 | 1381.6 | 19.4 | 62.9 | ||||||
Monti Picentini | 15° 00′ 36.72″ E 40° 42′ 03.60″ N | 612 | N | 100.5 | 36.1 | 2008 | 16.3 | 6.2 | 1322.6 | 58.9 | 33.1 |
2009 | 16.4 | 6.8 | 1119 | 48.2 | 32.5 | ||||||
2010 | 15.4 | 4.8 | 1819.2 | 33.3 | 40.3 | ||||||
2011 | 15.9 | 5.3 | 1014.4 | 55.1 | 35.7 | ||||||
average | 16 | 5.78 | 1318.8 | 48.9 | 35.4 |
Parameter and equations used for litter decomposition modeling. (
Parameter | Units | Equations and values | Source |
---|---|---|---|
°C | 0.769 + 0.826 · |
This work | |
dimensionless | 0.56 + (1.46 · arctan(π · 0.0309 · ( |
|
|
dimensionless | 0.57658 · exp(308.56 · ((1 / 56.02) - (1 / (273 - Tsoil - 227.13)))) |
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|
dimensionless | 0.198306 + |
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|
mm day-1 | 0.66 · |
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|
|
mm day-1 | ( |
This work |
|
unitless | ( |
|
|
mm day-1 | rain · (1 - exp(- KC · LAI)) |
|
|
dimensionless |
|
|
mm year-1 |
|
||
mm day-1 | 0.408 ( |
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|
mm day-1 | ((( |
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|
mm | 254 · [(100 / |
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|
dimensionless | 2.56 / [1 + 1430 · exp(-0.0471 · t)] | Calibration |
|
dimensionless | 55 |
|
dimensionless | 5 · (0.287 + (arctan( |
|
|
|
% | 38 | This work |
% day-1 | 1 / (-2.04 · 103 + 1.99 · 103 · |
This work | |
% day-1 | 9.09 · 10-4 + 2.06 · 10-5 · |
This work |
Analysis of homogeneity of temperature and precipitation among the studied stands after Tukey’s HSD test (α=0.05). (Tmean): average temperature (°C); (Tmin): minimum temperature (°C); (Tmax): maximum temperature (°C); (****): significant homogeneity between site pairs for the parameter considered.
Parameter | Boschi di Pietralunga | Monti Lucretili | Monti Picentini |
---|---|---|---|
Tmean (°C) | **** | **** | - |
Tmin (°C) | **** | **** | - |
Tmax (°C) | - | - | - |
Rainfall (mm) | - | **** | **** |
Leaf litter weight decay during the experiment at the three test-sites.