Litterfall plays a key role in the dynamic of forest ecosystems, ultimately determining forest productivity and carbon and nutrient cycling. Increasing our understanding on the role of structural and environmental factors controlling litterfall amount and seasonality is of paramount importance for modelling and estimating soil carbon sequestration and nutrient cycling under climate change scenarios. However, the effect of climatic conditions on litterfall has been scarcely studied, especially in Mediterranean ecosystems. Here, we used nine years of seasonally collected litterfall data in two contrasting Mediterranean cork oak forests to evaluate the effect of climatic variables on leaf fall and litterfall. First, we isolated the litterfall seasonal trend and the between-sites differences in production by using linear mixed models. Then, we evaluated the effect of climatic variables and whether this effect was site-specific. We found a consistent litterfall seasonal pattern, mainly determined by leaf shedding (70% of litterfall). Leaf fall mainly occurs in spring with a second but much smaller peak in autumn some years. Mean temperature, precipitation and mean wind speed strongly influenced litterfall, but this effect was site-specific. In the forest site located at higher latitude and altitude, leaf fall increased linearly with temperature and showed a positive quadratic response to precipitation. In the water-limited site, leaf fall was reduced as temperature increased and did not respond to precipitation. These results have implications for modelling and predicting soil carbon sequestration, nutrient cycling, and the forest ecosystem productivity. Specifically, carbon and nutrient cycling models can be improved by incorporating idiosyncratic forest sites responses to climatic variability.
In forest ecosystems, litterfall is the largest source of aboveground organic material input to soil (
The amount of litterfall is related to environmental and structural properties, such as stand age, basal area, or canopy cover (
The Mediterranean region has been identified as a primary hotspot particularly vulnerable to the impacts of climate change (
Cork oak is a sclerophyllous evergreen oak and one of the most important forest tree species of the Mediterranean basin. Cork oak woodlands support high levels of biodiversity and ecosystem services, and they represent a model of sustainable ecosystem management with co-existing human activities, such as cork extraction and natural resource conservation. This has led to its inclusion in the Annex I of the European Union Habitats Directive (92/43/CEE). In this study, we used nine years of seasonally collected litterfall data in two contrasting cork oak (
The study sites were located in two contrasting forest areas, Hinojos and Montseny. These differ mainly in forest structure (tree density and basal area) and climate (
The Montseny site is located in a Mediterranean cork oak forest in northeastern Spain (Barcelona, Catalonia), in a flat area with an altitude of 780 m a.s.l. (41° 44′ N, 02° 23′ W) and with a tree density of 1082 trees ha-1. Cork oak is the dominant tree species, with a density of 700 trees ha-1 and a basal area of 49.5 m2 ha-1, while holm oak (
The Hinojos site is located in a Mediterranean cork oak woodland in southwestern Spain (Huelva, Andalusia), in a flat area with an altitude of 100 m a.s.l. (37° 19′ N, 06° 25′ W) and with a tree density of 100 trees ha-1. Cork oak is the dominant tree species, with a density of 84 trees ha-1 and a basal area of 7.2 m2 ha-1, while holm oak appears as secondary tree species (16 trees ha-1 and 0.9 m2 ha-1). The understory community is mainly composed of
Litterfall was collected using the trapping method (
Samples were collected monthly for three years (2004-2006), and seasonally (winter, spring, summer and autumn) for six more years (2007-2012). Once collected, litterfall samples were oven-dried at 65 °C for 48 h, separated into cork oak leaves, cork oak twigs, acorns (if present) and others (mainly flowers and other plant remains), and finally weighed (± 0.01 g). Litterfall and leaf fall data were expressed as g m-2, by dividing the dry weight of litter collected from each sampling point by the surface area of the litter traps. Acorns were not included as a part of litterfall. For seasonal analysis, monthly samples were pooled by season in order to analyze the whole sampled period (2004-2012).
Climatic covariates were obtained from an automatic weather station at each site. Air temperature, precipitation, relative humidity, PAR radiation, and wind speed were registered every 15 minutes. Climatic covariates, including mean temperature, mean minimum temperature, mean maximum temperature, mean relative humidity, minimum relative humidity, precipitation, maximum precipitation in 24 hours, mean solar PAR radiation, mean wind speed and absolute wind speed, were calculated per season for each site.
To analyze the seasonality of litterfall (not including acorns) and leaf fall (main litterfall fraction) we used linear mixed models (LMM). Models were fitted to log-transformed leaf fall and litterfall data in a season (9 years, from 2004 to 2012) and in a monthly basis (3 years, from 2004 to 2006), respectively. Leaf fall and litterfall data was log-transformed to achieve normality and homocedasticity. In both cases, our experimental design resulted in temporary autocorrelation due to the fact that litterfall series results in non-independency among observations within the same sampling point (
Once the best random structure was selected, we identified the best-supported fixed effect structure by following a backward model selection procedure. First, we fitted the full model including site, year, season or month (depend on the set of data), and the interaction between them, and we compared it with a reduced model in which the triple interaction was dropt. Then, if necessary, we compared the selected model with models that ignored each pairwise interaction and all the main effects, respectively. If the difference in AICc between two models was ≤ 4, then the simpler model was selected (
To analyze the effect of climatic variables on leaf fall and litterfall, we first fitted LMMs with the selected random structure (see above) but considering site and season as main fixed effect, respectively. Then, we extracted the Pearson’s residuals from the selected models. These residuals are values of leaf fall and litterfall, without the effect of seasonality and site. This procedure allowed to estimate the relative effect of climatic variables avoiding likely confounding effects due to: (i) the seasonality of both, litterfall production and climatic variables; and (ii) differences in the amount of litterfall due to site characteristics. Before fitting the model, we performed a Pearson’s correlation analysis for the initial set of 10 meteorological covariates (Tab. S1 in Supplementary material). In order to avoid multicollinearity, we included in the model those variables that showed a correlation coefficient (
Mean yearly litterfall (± SD) for the study period was 391.1 ± 147.2 g m-2, with overall greater production in Montseny than in Hinojos (429.3 ± 183.8 g m-2 and 353.0 ± 110.5 g m-2, respectively). Litterfall was only greater in Hinojos in three out of the nine study years (2005, 2007 and 2010 -
The first-order autoregressive structure was selected as the random term structure for the LMMs used to analyze leaf fall and litterfall in a seasonally and monthly basis (Tab. S2 in Supplementary material). This means that a given observation within the same sampling point shows a temporary autocorrelation with the previous one. In all cases, the best supported model for the structure of the fixed effects included the triple interaction between site, year, and season or month (depending on the set of data). When models including the triple interaction were compared with models without this but considering pairwise interactions between fixed effects, the increase in AICc was greater than at least 100 units (Tab. S3). In the selected models, VIF values were lower than 2 units indicating an acceptable degree of collinearity.
Leaf fall and litterfall showed a similar pattern both, at seasonal and monthly basis (
The pairwise interactions between site and the climatic covariates (mT, P and mWS) were included in the selected models for seasonal leaf fall and litterfall (
Our results show similar patterns in the seasonality of litterfall and leaf fall between contrasting sites in terms of forest structure and climate. Once seasonality was removed, mean temperature, precipitation and mean wind speed were identified as the main determinants of litterfall and leaf fall. Interestingly, the effect of climatic variables differed between study sites, indicating an idiosyncratic forest response to climatic variability.
The amount of litterfall in our study (3.9 Mg ha-1 year-1) was similar to those values reported in other studies in Mediterranean cork oak (3.9-5.1 Mg ha-1 year-1 -
Whereas leaf fall peak always occurs in spring in Hinojos, we found high summer leaf fall values during some years in Montseny (years 2004, 2009, 2011 -
After accounting for seasonality, our results identify mean temperature, precipitation and mean wind speed as the most important climatic factors affecting litterfall and leaf fall. Curiously, the direction of this effect was site-specific suggesting an idiosyncratic control of climatic conditions over litterfall. This differential effect might be mediated by endogenous factors, such as forest structure or age, but also by contrasting environmental severity between sites (
Our results have practical implications for forest carbon cycle and productivity models. Most of these models use simplified algorithms to simulate litterfall process (
Litterfall in Mediterranean cork oak forests shows a consistent seasonal pattern regardless of differences in forest structure. This pattern is mainly determined by leaf fall seasonality, which contributes to 70% of the total litterfall. Leaf fall is concentrated in spring, matching with the renewal of the foliar cover after the bud flush. In addition, a second but much smaller peak was observed some years in autumn. Differences in seasonality between sites were confined to an earlier leaf fall in spring in the study site located at lower latitude.
Mean temperature, precipitation and mean wind speed were the most important climatic factors affecting litterfall and leaf fall. Curiously, the effect of climatic variables differed between study sites indicating an idiosyncratic forest site response that might be mediated by differences in forest characteristics and environmental severity. Specifically, temperature increased leaf fall in the forest site located at a higher latitude and altitude, while leaf fall response to precipitation followed a positive quadratic relationship. Contrastingly, under water scarcity conditions leaf fall was reduced with increasing temperatures and did not respond to precipitation.
This study was supported by Ministry of Science and Innovation of Spain, the National Agriculture Research Institute (INIA; ref: SUM2006-00026-00-00), and the
All authors contributed to the study design and data collection. EA performed statistical analyses with inputs from JVP. EA wrote the manuscript with contributions from the rest of authors.
The authors declare no conflict of interest.
Locations of the study sites (Hinojos and Montseny) in the Iberian Peninsula and their climate diagram during the study period. The Mediterranean ecoregion is depicted in dark red.
Seasonal pattern of litterfall (above) and leaf fall (below) for each study site over the study period. Error bars represent the standard error. Different years are separated by vertical lines. (Wi): winter; (Sp): spring; (Su): summer; (Au): autumn.
Monthly pattern of litterfall (above) and leaf fall (below) for each study site for the period 2004-2006. Error bars represent the standard error. Different years are separated by vertical lines. Months are numerically indicated in the horizontal axis.
Model predictions and 95% CI for the effect of climatic covariates on seasonal leaf fall in the study sites (Montseny and Hinojos). Leaf fall for each covariate was predicted using a fixing mean value for the other two covariates.
Yearly mean amounts (± standard deviation) for each litterfall fraction and location (g m-2).
Location | Year | Leaves | Twigs | Others | Total |
---|---|---|---|---|---|
Hinojos | 2004 | 275.1 ± 63.4 | 72.9 ± 48.4 | 68.3 ± 16.3 | 416.3 ± 71.6 |
2005 | 201.4 ± 66.8 | 17.3 ± 28.5 | 34.3 ± 18.6 | 252.9 ± 83.4 | |
2006 | 149.5 ± 46.8 | 48.1 ± 57.3 | 41.0 ± 23.4 | 238.7 ± 93.1 | |
2007 | 225.8 ± 69.0 | 175.3 ± 185.8 | 133.0 ± 49.7 | 534.0 ± 113.7 | |
2008 | 215.0 ± 53.4 | 21.2 ± 22.0 | 65.4 ± 20.3 | 301.6 ± 85.4 | |
2009 | 244.7 ± 61.3 | 0.1 ± 0.2 | 71.6 ± 18.2 | 316.4 ± 105.2 | |
2010 | 236.0 ± 43.4 | 62.1 ± 56.0 | 157.3 ± 82.7 | 455.5 ± 71.9 | |
2011 | 241.4 ± 49.9 | 37.4 ± 40.1 | 95.2 ± 70.8 | 374.0 ± 90.5 | |
2012 | 175.6 ± 49.0 | 50.9 ± 120.5 | 60.9 ± 25.9 | 287.3 ± 88.5 | |
Montseny | 2004 | 392.0 ± 79.5 | 206.9 ± 79.6 | 79.6 ± 20.2 | 678.7 ± 110.8 |
2005 | 210.2 ± 46.1 | 29.6 ± 34.5 | 41.0 ± 35.6 | 280.7 ± 72.5 | |
2006 | 340.7 ± 54.5 | 44.2 ± 36.4 | 72.8 ± 24.0 | 457.7 ± 66.5 | |
2007 | 147.5 ± 43.8 | 3.8 ± 4.0 | 23.6 ± 12.2 | 174.9 ± 67.6 | |
2008 | 354.0 ± 52.8 | 77.3 ± 109.6 | 43.5 ± 6.8 | 474.8 ± 73.2 | |
2009 | 176.3 ± 17.2 | 54.5 ± 33.9 | 28.6 ± 12.6 | 259.5 ± 75.4 | |
2010 | 331.6 ± 84.5 | 18.1 ± 11.9 | 104.1 ± 51.1 | 453.8 ± 110.6 | |
2011 | 480.0 ± 182.3 | 16.4 ± 16.3 | 63.2 ± 22.2 | 559.5 ± 299.3 | |
2012 | 458.8 ± 47.0 | 19.2 ± 21.5 | 46.3 ± 11.0 | 524.3 ± 74.1 |
Mean values (± SE) for each estimated parameter in the selected models for evaluating the effect of climate covariates on seasonal leaf fall and litterfall. For all models, “
Parameter | Leaf fall | Litterfall |
---|---|---|
Intercept [“ |
0.12 ± 0.04** | 0.13 ± 0.04** |
Site [“ |
0.72 ± 0.11*** | 0.62 ± 0.11*** |
mT | -0.31 ± 0.06*** | -0.21 ± 0.06** |
mT 2 | - | - |
P | 1.55 ± 1.38 | 2.25 ± 1.43 |
P2 | -0.74 ± 1.22 | -2.19 ± 1.26 |
mWS | -0.06 ± 0.04 | -0.17 ± 0.04*** |
mWS 2 | - | - |
Site [“ |
1.28 ± 0.10*** | 1.03 ± 0.10*** |
Site [“ |
- | - |
Site [“ |
-4.79 ± 2.25* | -6.82 ± 2.34** |
Site [“ |
8.46 ± 2.10*** | 9.56 ± 2.18*** |
Site [“ |
0.26 ± 0.09** | 0.32 ± 0.09*** |
Site [“ |
- | - |
Tab. S1 - Correlation among climatic variables.
Tab. S2 - Model comparison between autoregressive random structures.
Tab. S3 - Model comparison for fixed effect.
Tab. S4 - Value of parameters for the model used to assess climate effect on leaf fall and litterfall.
Fig. S1 - Climatic variables effect on litterfall.