Long-Term Decomposition Process of Leaf Litter From

Long-term decomposition process of leaf litter from Quercus pyrenaica forests across a rainfall gradient (Spanish central system) A Martin, Jf Gallardo, I Santa Regina

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A Martin, Jf Gallardo, I Santa Regina. Long-term decomposition process of leaf litter from Quercus pyrenaica forests across a rainfall gradient (Spanish central system). Annales des sciences forestières, INRA/EDP Sciences, 1997, 54 (2), pp.191-202. hal-00883141

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Long-term decomposition process of leaf litter from Quercus pyrenaica forests across a rainfall gradient (Spanish central system)

A Martin JF Gallardo2 I Santa Regina

1 Area de Edafología, Facultad de Farmacia, Universidad de Salamanca, 37080 Salamanca; 2 CSIC, Aptado 257, 37071 Salamanca, Spain

(Received 30 November 1995; accepted 1 April 1996)

Summary - A long-term (3 years) study has been made of the leaf-litter decomposition process in four forest ecosystems across a rainfall gradient. The soil organic contents comply with the factor of rainfall, but this does not appear to affect the complete decomposition process decisively, since the limiting factor is soil humidity. The rainfall distribution is similar for all the plots studied and the main differences are seen essentially in the amount of water received during the wet season (in each case with sufficient humidity for leaf decomposition to progress). Thus, leaf-litter decay is linked to the maintenance of soil humidity, mineralization slowing down when the soil dries out, and the effects of climate therefore predominating over the chemical characteristics of the material in the regulation of the decomposition process (at least at short term). In this sense, the summer drought is the most unfavourable environmental factor in this type of climate. In advanced stages of decay the rate of weight loss becomes relatively independent of climate. The decomposition rates obtained by the litterbag pro- cedure are lower than those calculated by litter production and permanent litter; the reason for these differences is probably the limitation of the mesofaune activity in the litterbags. Leaf decomposition constants ranged from 0.33 on the plot with the highest rainfall to 0.47 (on the driest plot), corresponding to mean residence times of 2.0 to 1.1 years, respectively. However, each bioelement has a different residence time. leaf-litter decomposition / forest ecosystems / Quercus pyrellaica forests / Mediterranean climate / litter quality / litterbag methods

Résumé - Processus de décomposition à long terme des litières de chênaies à Quercus pyrenaica suivant un transect pluviométrique. On a étudié le processus de décomposition à longue terme (3 années) des feuilles de quatre chênaies à Quercus pyrenaica qui suivent un transect plu- viométrique. Les contenus de matière organique des sols forestiers sont liés au facteur pluviométrique, mais la pluviométrie n’affecte pas de manière significative le processus de décomposition des feuilles

* Correspondence and reprints du chêne, car ce processus est plutôt affecté par l’humidité du sol. Cette distribution des pluies est simi- laire dans tous les sols forestiers, mais la différence de la quantité de pluie se manifeste en hiver, c’est- à-dire quand les sols sont humides. La décomposition est liée à l’humidité du sol, et elle est arrêtée quand le sol est sec. Ainsi l’effet du climat est plus important que la composition chimique à court terme; à long terme, l’importance du climat diminue. La constante de décomposition des feuilles obte- nue par la méthode de « litter-bag » est plus basse que celle calculée à partir des donnés de production des litières et de la teneur en litière permanente, ce qui est dû à la limitation de l’activité de la méso- faune dans les « litter-bags ». Les constantes de décomposition varient de 0,33, dans la station la plus pluvieuse, à 0,47, dans la station la plus sèche. Les temps de résidence varient de 2,0 à 1,1 années, res- pectivement. Il faut tenir compte du fait que chaque bioélément a son propre temps de résidence. décomposition des litières / écosystèmes forestiers / Quercus pyrenaica / climat méditerranéen / qualité de la litière / méthode « litter-bag »

INTRODUCTION The release of bioelements depends on the decomposition of organic matter and is to loss. In any forest ecosystem, both deciduous and generally proportional weight Many factors are involved in the release of evergreen, the litter fall is reflected each bioelements. It should be stressed that from year in the massive appearance of dead apart the intrinsic factors of the such as the organic matter that accumulates on the litter, ground (Mangenot and Toutain, 1980). base content (Eijsackers and Zehnder, 1990), Together with the contribution from the nitrogen (Berg and Staaf, 1980), polyphe- nols et decay of roots (McClaugherty et al, 1982), (Domínguez al, 1988), lignin (Melillo or tannins the litter on the surface of the soil represents et al, 1989), (Davies et al, 1960), there are external factors that also affect the the main source of energy, carbon, nitrogen and phosphorus for the soil microflora and decomposition rate; among these, of note are climate aera- mesofauna (Ranger et al, 1995) and also (temperature, humidity, tion, actual provides an amount of nutrients that can evapotranspiration (AET); Berg become readily available and be reused by et al, 1990; Dyer et al, 1990), soil charac- the plant covering (Rapp and Leonardi, teristics (pH, base content; Kononova, 1966; 1988). Toutain, 1981) and soil organisms (Aranda et al, 1990; Kögel-Knabner et al, 1990; Joer- It is customary to define two or three gensen, 1991). phases in the transfer of nutrients to the soil: In an ecosystem in the total in the first, soluble compounds are released equilibrium, mass of the contributions is compensated by leaching; this is a rapid phase that by the loss of an mass of litter; if this depends on the initial nutrient content (Berg equal were not so, elements would be accumu- and Staaf, 1977). In the second phase, ener- lated and retained the rise to getic compounds like cellulose and hemi- by litter, giving a progressive decrease in productivity (Jenny cellulose are decomposed after immediately et and that phase, thus making evident the degra- al, 1949; Mangenot Toutain, 1980). an essential in forest dation of the biological material. The third Accordingly, problem is to information about the which is much slower, affects ecology gain phase, laws the transformation of molecules with bonds more resistant to governing organic matter so that an excessive down degradation; this phase is carried out by soil slowing can be avoided. organisms, essentially saprophytes (bacteria of biogeochemical cycles and fungi), and depends on the lignin content Losses of organic matter due to the loss (Berg and Staaf, 1987). of carbon dioxide and leachates derived from decomposition are also important to Genista falcata) and spiny rosaceae (Rubus consider, especially in managed woodlands ulmifolius, Crataegus monogyna). In the (Howard and Howard, 1974). herbaceous stratum there is a predominance of (Festuca sp, Dactilys The aim of the present work was thus to gramineae glom- erata), at determine, in a long-term (3 years) study, leguminosae only appearing the factors governing the decay of leaf litter Fuenteguinaldo (Trifolium sp, Ornithopus As rainfall increases, the of in the different phases of the process and to sp). presence ferns (Pteridium and. in forest compare different methods and calculations aquilinum) of heathers (Erica Calluna to estimate decomposition rates. clearings, sp, vulgaris) increases. In all cases, the soils are Cambisols (gen- DESCRIPTION OF STUDY AREA erally humic) developed over slates and greywackes at Navasfrías and Villasrubias and over Ca-alkaline at El and Four experimental plots in Quercus pyre- granite Payo (Gallardo et al, naica forests across a rainfall gradient were Fuenteguinaldo 1980). chosen. The plots are situated in the munic- ipalities of Navasfrías, El Payo, Villasru- bias and Fuenteguinaldo in the region known METHODS as El Rebollar on the northern face of the To follow the of leaf-litter Sierra de Gata Mountains (southwest of the dynamics decomposition on the soil, 54 nylon litterbags with a sur- of Salamanca). province face area of2 4 dmwere placed on each plot. The a mesh of 1 mm were dis- The climate of the zone features rainy bags had pore and tributed in three of 18 distributed winters and warm summers and may be groups bags according to the topography (Martin, 1992). Each classified as Mediterranean temperate (Elías litterbag contained 10 g of recently fallen leaves. and The summer deficit in rain- Ruiz, 1977). previously dried at room temperature, from the fall (189 mm yr-1) is lower at Navasfrías, same stand. The bags were placed on the soil due to higher rainfall (1580 mm yr-1) and surface so that the conditions would be as close lower temperatures (annual mean, 10.4 °C). as possible to natural ones (Bocock, 1964). At Fuenteguinaldo the summer rainfall The experiment started in February 1990 (the deficit (259 mm yr-1) is significantly more leaves were taken on November 1989). One bag was from marked owing to higher temperatures withdrawn each group at approximately two monthly intervals until a sampling period of (12.9 °C) and lower rainfall (720 mm yr-1). 3 years had been completed. At the laboratory, Thus, the potential evapotranspiration (PET) the samples were cleaned, dried at 80 °C and the calculated is 670 mm yr-1 at Navasfrías and variations in dry matter calculated. Following 730 mm yr-1 at Fuenteguinaldo (Elías and this, the contents were ground and homogenized Ruíz, 1977). for the corresponding analytic determinations. In all samples the following were determined: The tree stratum comprises Q pyrenaica organic carbon by the dry method using a (Martin, 1995), the density varying between Carmhograph 12 Wösthoff; total nitrogen by a 1040 trees ha-1 at Villasrubias and Heraeus Macro-N analyzer; calcium and mag- 406 trees ha-1 at El Payo. The least dense nesium by atomic absorption spectroscopy; potas- sium flame and plot has the highest mean trunk diameter by photometry phosphorus by spectrophotometry (Martín, 1992). (25.4 cm) and the greatest height (17 m), the lowest values corresponding to the plot In order to compare the results obtained in the experimentation with the true decay of the at Villasrubias (11 cm and 8.5 m, respec- organic matter, different decomposition indices The shrub stratum is at the tively). present were determined (Martín, 1995); to do so, all the Fuenteguinaldo plot, with spiny or aphyl- material of the holoorganic horizon contained in lous leguminosae (Cytisus multiflorus, a 0.5 m x 0.5 m frame was collected. Fifteen samples per plot were taken. Likewise, to deter- rainfall gradient since the rainfall values mine the indices it was necessary to know leaf- recorded are 1580, 1245, 872 and litter which was achieved production, by plac- 720 mm year-1 for the plots according to ing three series of ten boxes of 0.24 m2 surface the Navasfrías to Fuenteguinaldo transect. area on each plot. The amount of litter falling to into each box was collected at time intervals However, it is more advisable use data depending on the amount fallen, November being relating to soil humidity instead of rainfall the month of major litterfall (Martin, 1995). All (Moreno, 1994) because the processes of the material was dried at 80 °C for 24 h and sep- leaf decomposition and soil humification arated into different fractions (leaves, twigs, etc). are carried out more intensely when there In order to establish possible significant dif- is an appropriate degree of humidity in the ferences in mass loss for the different plots stud- soil, this being more related to rainfall dis- ied, a one-factor analysis of variance (ANOVA) tribution than to annual rainfall (Berg et al, was applied with repeated mesures for times; 1990). obviously, a Hartley’s test had been previously made in order to verify the identity of the vari- Figure I B shows the data on soil humid- ances. When significant results were obtained ity (between 0 and 15 cm) determined in with ANOVA, Tukey’s multiple comparison situ (Moreno, 1994). dry method was to determine the reason for Very periods applied and of 1990 and the significance found. between July September between July and October 1991 and 1992 Some were lost because of various acci- bags can be noted. The mean temperatures fol- dents; to avoid using a different number of sam- low wave so that no further ples, the lower number of bags taken from the a regular pattern, reference is made to them in Jan- plots was taken into account in the ANOVA test (minimum (n = 52). uary, maximum in July). Additionally, the lowest soil humidity is observed at Fuente- guinaldo owing to lower rainfall, coarser RESULTS AND DISCUSSION texture of the soil and higher prevailing temperatures. Previous work (Gallardo et al, 1980) has Detailed scrutiny of the decay curves shown that there is a relationship between (fig 1A) during the same period reveals that the mean content of organic carbon, total there are periods of continuous mineraliza- nitrogen and the C/N ratio of the Ah surface tion together with others when decomposi- horizons of soils and annual rainfall. Using tion ceases. On comparing figures 1A and only the data from these plots (table I), I B it may be deduced that the halt in decay Martín et al (1993) observed that the occurs nearly during the dry summer periods sequence of soil contents followed the (taking into account that the litter dries before the soil, and wets also before the soil Accordingly, the winter temperature does because of the dew effect), with mineral- not appear to be a first-order limiting fac- ization continuing when humidity is high tor, as corresponds to a temperate climate despite the lower temperatures; in this case, subject to Mediterranean influence; rather, a temperature increase of a few degrees in the seasonality of the humidity (necessary the wet period has significant effects for biological activity) and of the rainfall (Shanks and Olson, 1961). The effect of the (necessary for washing and the removal of dry period on leaf decay has been addressed solutes, micelles and microparticles towards in depth by Martín et al ( 1993). During the the mineral horizons of the soil) would be late decay phases the effect of dry periods is responsible for this. Climate has a strong not detectable. As a result, in these forest influence in the first phases of decay and, ecosystems, leaf-litter decay is linked above as mentioned earlier, an increase in tem- all to humidity itself (Beyer and Irmler, perature of just a few degrees during the 1991), with mineralization slowing down wet period may have significant effects when the leaf litter is dry (the soil may con- (Shanks and Olson, 1961). Thus, the higher tinue to be moist to a depth of more than decomposition constants observed in the 40 cm). Toutain (1981) stressed, however, first 2 years of the study are seen at that there are physical and physicochemi- Fuenteguinaldo (warmer) and at Navasfrías cal processes of decay in summer (losses of (rainier). Thereafter, decomposition dry matter by animals, water or winds, could decreases, influenced by molecules with be limited). more resistant bonds (Martin et al, 1993). On considering their decomposition rates, In any case, the asymptotic fits show the the four plots may be grouped by two’s. On same determination coefficients and almost the one hand, there is Villasrubias-El Payo, the same residual squares as the double with accumulated mass loss over 1, 2 and exponential equations. Accordingly, the fol- 3 years of 32, 43 and 53%, respectively; on lowing equations would be obtained (sig- the other hand, there is Navasfrías- nificant, P < 0.0001): Fuenteguinaldo, with a higher decay rate Navasfrías (37, 52 and 58%). The ANOVA results show that there are no significant differences for Villasrubias-El Payo and Navas- frías-Fuenteguinaldo, although there are El Payo important differences between both groups (table II). Despite the seasonal fluctuations, the Villasrubias decay process can be represented by different types of regressions, with the best fits given by the following equations (significance P < 0.0001): Fuenteguinaldo Navasfrías

where R and t are the same as in the previous El Payo case. In this case, the model points to the existence of one stable residue (more abun- dant at El Payo and Villasrubias) and one unstable one, which faster at Villasrubias decomposes Fuenteguinaldo (constant 0.0024). In view of this, the above-mentioned double exponential and asymptotic equations Fuenteguinaldo applicable to the decomposition curves imply, in all cases and as has been mentioned, that the detritic material is composed of at least two fractions that decay at dif- where R is the residual dry matter expressed ferent rates. The asymptotic model shows in percentage form and t is the time in days that prolongation of the process elicits a elapsed since the start of the process. decrease in the decay rate per unit of time. The double exponential model confirms that seen at Navasfrías (the other plot with a high there would only be one material formed of initial rate) because, according to the asymp- two components that decay at very different totic equations, the resistant fraction would rates, although progressively over time (neg- only represent 27% of the organic matter ative signs in the exponents) at the plot at while at Fuenteguinaldo it would represent Navasfrías. The fact that in the other three 39%. plots the double exponential equations are The effect cannot be attributed to the of the sum of one and composed negative lithology in the sense that decaying leaves another to the positive exponential points are hardly in contact with the soil and the existence of two types of component: one differences in the chemical composition of that is released more or less and rapidly the leaves (table III) are not determinant another that is accumulated progressively (Martín, 1995), with the exception of N and and that would therefore slow down the P, which are slightly higher on the plots overall After a certain time process. (4.4, developed over granite. However, more 4.1 and 2.6 years for El Payo, Villasrubias importance could possibly be given to the and Fuenteguinaldo, respectively), there content in bases of the surface horizon of would be no decay but rather an accumula- the soil on which decay occurs since this tion of decomposing material (the positive parameter affects microbial activity function would give higher values than the (Dommergues and Mangenot, 1970); in this negative one and the double exponential sense, it can be noted (table IV) that function would pass through a minimum). Fuenteguinaldo is the most favourable which is difficult to understand in conceptual medium for decay (lower acidity, greater terms. This increase of dry matter could be degree of saturation, high contents in assim- explained by both a humification of the rapid ilable Ca; table IV) while Villasrubias is the decomposing fraction, and by an ingrowth of most unfavourable (higher acidity), although material coming from outside the bags (from this could perhaps be accounted for, as the canopy or from the underlying layers by already mentioned, in terms of leaf compo- fungi). In this sense, Fuenteguinaldo would sition and could affect this more than the be the first plot to reach the point at which actual decay process. Indeed, the C/N ratio decay is arrested; that is, the point at which of the leaves, sometimes used as an index of decay of the recalcitrant fraction should pre- the facility of leaf decay (Duchaufour, dominate because in this plot the initial rate 1984), is close to 35 in the plots on granite is the most rapid. Likewise, it can be seen and to 44 in the plots on slates, in view of the that this accumulation phenomenon is not more dystrophic nature of these soils (table III). However, in table II the differ- third years. In theory, in a natural untouched entiation according to the parent rock is not leaf litter, the first layer (L) of leaves would established. decay according to the first index, the second one to the second index, and If one wishes to compare the fit of decay (F) according using the litterbag technique with the real the third one (H) according to the third index, the leaf litter here almost humi- process, it is possible to compare the result- being ing decay indices with the previous tech- fied according to the estimated mean resi- nique and those calculated from the dence time (tr = F/A; subhorizon H) and (ko) into the soil. In our annual production of leaf litter and accu- becoming incorporated difficult to mulated soil leaf litter, according to the for- forest plots, it is very distinguish mula: the three organic subhorizons (which is very common in Mediterranean forest ecosystems). where k’ is the decay index of Jenny (Jenny Higher decay indices are seen on the et al, 1949); A is the annual leaf litter pro- granite plots than on those located on slates, duction and F represents the leaves accu- both for leaf litter (table V) and for total lit- mulated before the autumn fall. The results ter (calculated in the same way; table VI). are shown in table V. This table also shows Additionally, the sequence of the indices the decay indices calculated from the in situ does not evolve in a parallel fashion to the decay experiment (litterbags) both for the rainfall gradient, indicating that it is not the first year of decay and for the second and total amount of rain that governs the greater rate, and perhaps the greatest effect comes, of decay, the different rates undergo certain at least in the first few stages of decay, as has changes (the factors controlling the decay been mentioned, from the distribution of the rate are different), making the residence rainfall throughout the year together with times of the leaves in the soil (tr) similar in air temperature, aeration and soil texture, the two plots located on granite, on the one humidity and the temperature of the surface hand, and in the two located on slates, on horizon of the soil. Moreover, both the spe- the other (table V), in an inverse ratio to the cific characteristics of the site and the chem- productions found. ical composition of the litter govern the What does seem clear is that on using the nature of the heterotrophic community litterbag technique, one obtains decay (Kögel-Knaber et al, 1990) and its presence indices for the first year that are and are linked to the (kol) year activity physicochem- below the true values since the ical characteristics of the substrate (Toutain, technique prevents the important role of breakage 1981); that is, the coarser textures of the 1964) and that soils lead to lower water retention (Bocock, of microbiological granite insemination and and a certain increase in soil (Dommergues Mangenot, temperature 1970) the mesofauna (that fact that will alter microbial activity. by probably explains why, compared with other pub- From a comparison of the results (table lished data, the asymptotic values given here V), it may be deduced that in their natural are relatively high. Accordingly, it would medium (k’) the leaves decay at a faster rate be more rigorous to consider the leaf decay than in the nylon litterbags, which are inac- constant k’; or perhaps perform an integra- cessible for the mesofauna (Bocock, 1964; tion of the different ko for the first 3 years Joergensen, 1991), which have a very impor- (not the mean) since, as noted earlier, it is tant physical effect on the litter products. evident that layers of leaves of different ages Martin (1995) reported that whereas at in the leaf-litter decay at the same time - Navasfrías and Fuenteguinaldo the per- which cannot be longer than 3 years because centage of highly broken up leaves in the the mean residence time is not greater than litter is greater than 90%, with respect to 1.3 years; this value is lower than that found the total of leaves, this percentage is only by Rapp and Leornadi (1988) in Q ilex 74% at El Payo and 87% at Villasrubias. (5.3). With this theoretical approach, the Thus, the initial rate of leaf breakage at those values of the constants k’ and ko would con- plots seems to be greater than at Villasru- verge. Such a calculation of the true ko bias and El Payo, as happens in the lit- would be somewhat imprecise owing to the terbags. However, in more advanced stages difficulty of separating the decaying leaves from the first, second and third years in the in the drier plots N is enriched whereas it leaf litter. In any case, the true value of the is impoverished in the more humid ones. constant would lie between k’ and ko. It is also to set indices that possible up CONCLUSION indicate the residence time for the litter on the basis of the expression (Richter, 1990): The use of litterbags is a valid method for comparing the initial decomposition rates among ecosystems and for monitoring the Thus, it can be seen (table VI) that the evolution of the process although the method residence times for the litter are 2 years for is not valid for estimating the true rate of the plots located on slates and about 1.2 the cycles since litterbags slow down the years for those located on granite. This index process. This slowing down occurs because could also be applied for the leaf fraction the bags prevent access to part of the meso- of the litter and for nutrients (table VII). In fauna and hence prevent its important role in the case of the leaf fraction, the values of tr breaking up the leaf litter. Thus, the decay are close to 1 year for the oak stands on achieved in the litterbags responds mainly to granite and slightly more than I year for the climatological factors, affording asymptotic other two. The highest values of tr logically or double exponential relationships that indi- obtained for total litter can be explained in cate the presence of components that do not terms of the notion that it contains more lig- decay readily. nous such as and bark fragments, twigs The plots with the lowest fertility (those (Meentemeyer, 1978). located on slates) have the lowest production With respect to the tr of nutrients, the of litter and the lowest decay rate. Their lowest residence time corresponds to potas- cycles are therefore slowed down with sium (table VII). On comparing these resi- respect to those of plots located on granites dence times with those of the organic mat- (more fertile). However, the decay rate of ter, it may be deduced that the residual leaf the leaf fraction is initially seen to be altered litter becomes impoverished in K and Mg by abiotic factors (temperature and the since these bioelements are released faster amount of rainfall). Additionally, the total than the loss of weight of organic matter; amount of rainfall does not seem to deci- in contrast, the concentrations of P, Ca and sively affect the complete decay process C remain almost constant. The behaviour since this is influenced by soil humidity, of N depends on the type of plot considered; and the annual excess of rainfall occurs when the soil is humid, and hence this water Duchaufour Ph (1984) Edafología. I. Edafogénesis y clasificación. Masson, Barcelona, Spain, 493 p only leads to an increase in soil acidity. Dyer ML, Mcentemeyer V, Berg B ( 1990) Apparent controls of maas-loss rate of leaf litter on a regional scale. Scand J For Res 5. 311-323 ACKNOWLEDGMENTS Eijsackers H. 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