Báo cáo khoa học: "Organic matter dynamics in beech and pine stands of mountainous Mediterranean climate"

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Original article Organic matter dynamics in beech and pine stands of mountainous Mediterranean climate area Teresa Tarazona b Santa Ignacio a Regina a IRNA-C.S.I.C., Cordel de Merinas 40, Apdo. 257, 37071 Salamanca, Spain b C.L. Villar y Macías no. 1, Salamanca, Spain J. (Received 4 March 1999; accepted 16 July 1999) Abstract - litter loss of litter due to monitored in two forest and Aboveground biomass, production weight decomposition were ecosystems in the Sierra de la Demanda, Spain, a Mediterranean climatic zone, over a 3-years period. The two ecosystems were a mature beech forest (Fagus sylvatica L.) and a Scots pine plantation (Pinus sylvestris L.). The aboveground biomass was estimated by cutting and weighing seven trees from each site according to diameter classes, recording the categories of trunk, branches and leaves. The results indicate a total biomass of 152.1 Mg·ha in the pine stand and 132.7 Mg·ha in the beech stand. The percentage distrib- -1 -1 ution of biomass weight of trunks, branches and leaves was similar in both forests. The higher biomass in relation to DBH was esti- mated in the beech forest, which seems to indicate that it would not be very suitable to reforest land that is appropriate for beech with pine. The litter fall was 5 791 kg·ha in the pine forest and 4 682 kg·ha in the beech forest, although variations from ·year -1 ·year -1 year to year were observed, mostly due to water stress in summer. Weight loss due to decomposition of litter was similar in the two forest ecosystems, apparently due to the similarity in rainfall distribution at the sites. Jenny’s litter decomposition index (K) and Olson’s litter decomposition index (K were higher for the Scots pine stand than for the beech stand, K: 0.46 and 0.37, K 0.82 and 0: ) o 0.59, respectively, and Jenny’s leaves and Olson’s decomposition indices were similar. © 1999 Inra/Éditions scientifiques et médi- cales Elsevier SAS. aboveground biomass / litter fall / weight loss / forest ecosystems / Fagus sylvatica / Pinus sylvestris Résumé - Dynamique de la matière organique d’une hêtraie et d’une pinède en zone climatique méditerranéenne. On a estimé pendant trois années la biomasse aérienne, la production de litière et la perte de poids à partir des litières de feuilles en décomposition dans une hêtraie (Fagus sylvatica L.) et une pinède (Pinus sylvestris L.) de la Sierra de la Demanda, (Espagne). La biomasse a été esti- mée par coupe et pesée de sept arbres dans chaque peuplement selon la distribution des diamètres. Le poids des troncs, branches et feuilles a été mesuré. Les résultats indiquent une biomasse totale de 152,1 Mg ha dans la pinède et 132,7 Mg ha dans la hêtraie, -1 -1 Les pourcentages de poids du tronc, branches et feuilles son similaires dans les deux forêts. En comparant les biomasses en relation avec les classes de diamètres qui sont les plus importantes dans la hêtraie, on peut penser qu’il n’est pas opportun de reboiser en pin sylvestre dans l’aire potentielle de la hêtraie, La chute de litière est de 5 791 kg ha y dans la pinède et 4 682 kg ha y dans l’hêtraie, cependant on a observé variations chaque -1 -1 -1 -1 année, principalement dues au stress hydrique estival. La perte de poids due a la décomposition de la litière est similaire dans les deux écosystèmes, en relation avec le fait que la distribu- tion de la pluie est la même dans les deux stations. Les index de décomposition de Jenny (K) et Olson (K son plus élevés dans la ) 0 pinède: K 0,46 et 0,37, K0,82 et 0,59 respectivement, et ces index sont similaires pour les feuilles. © 1999 Inra/Éditions scien- 0 = = tifiques et médicales Elsevier SAS. biomasse aérienne / chute de litière / perte de pois / écosystème forestier / Fagus sylvatica / Pinus sylvestris * Correspondence and reprints ignac@gugu.usal.es
1. Introduction 2. Materials and methods 2.1. Site description Quality of organic matter is of prime importance for the majority of the functional processes occurring in the The experimental site is located in the Sierra de la soil of forest ecosystems. The most important contribu- Demanda mountains in the province of Burgos and tion to the soil humus occurs through plant aboveground Logroño in northern Spain. Its mountainous topography and root litter [16]. Aboveground litter plays a funda- is located on the north-west flank of the Central Iberian mental role in the nutrient turnover and in the transfer of Range. Its co-ordinates are: 42° 20’ N, 4° 10’ E. energy between plants and soil, being the source of the nutrients accumulated in the uppermost layers of the soil. The climate in the study area is attenuated meso- It is particularly important in the nutrient budgets of for- Mediterranean and becomes sub-Mediterranean with est ecosystems on nutrient-poor soils, where the vegeta- increasing altitude (1 000 m). Figure 1 shows the tion depends to a large extent on the recycling of the ombrothermic diagrams of the site and the studied plots, nutrients contained in the plant detritus [46]. the summer drought typical of the Mediterranean cli- mates is readily seen. The primary net productivity of forest vegetation is subject to external environmental factors such as soil and The weather station at Pradoluengo, near the experi- climate, and by inherent factors such as age and the type mental plots, at an altitude of 960 m, has an annual mean of tree [43]. Plants retain a substantial part of their temperature of 12.4 °C, the average of the minima and cover in perennial structures (trunks, branches, maxima of the monthly absolute being 6.5 and 35.1 °C, production whose nutritive elements form the mineralo- respectively. The mean annual rainfall recorded during etc.) roots, of the phytocenosis the study period was 895 mm (data from 1961 to 1980). [9]. mass Mean annual evapotranspiration was 705 mm (345 mm in Whittaker and Likens [51]established a general rela- June, July and August). The mean duration of the dry tionship between the aboveground biomass of the wood and its primary net productivity, enabling a comparison among the different productivities of different popula- tions of plants [48]. It is also important to study carbon and nitrogen, both as regards the distribution of these ele- ments within (i.e. structural) and among (i.e. composi- tional) community types since they affect the develop- ment processes and pathways of the ecosystem [32]. In any kind of forest, the highest litter fall occurs year- ly during certain periods, depending on the phenology of the dominant species. The production of litter is intimate- ly related to the edaphoclimatic factors of the zones in such a way that the total mass due to shedding is directly proportional to the fertility of the soil [11]. Root biomass and turnover are difficult to estimate owing to the diffi- culty in measuring them [50]. In a forest ecosystem, litter production is mainly expressed as a massive contribution of dead organic mat- ter that accumulates on the ground [26]. This accumulat- ed leaf litter on the soil surface, together with the contri- bution made by root decomposition [28], represents the basic source of energy, C, N, P, and other bioelements for the participating microflora and mesofauna of the soil, as well as a quantity of easily available nutrients [38]. The aim of the present work within to encompass was organic matter dynamics in a climax general study a on beech forest a comparison to that occurring in pine a stand planted on terrain suitable for beech 3-year over a period of experimentation.
period in the area is 2 months per year (summer), and the duration of the cold period is 6 months per year (+ 7 °C) [44]. is 3.1 The Mediterranean index of the [40]. The area siderably in depth, clay contents increasing with depth thermicity index is 195, corresponding to the lower supra- and is classified as Humic Acrisol [12]. These and other Mediterranean bioclimatic horizon. soil characteristics are indicated in table III. In the Sierra de la Demanda, the beech forest is dis- tributed in small islets, each occupying some 5 000 ha at pine trees (Pinus sylvestris L.) at La Rasada The Scots the bottom of valleys and on northern slopes from 900 to in a reforestation project initiated 50 years planted were 1 600-1 700 m in altitude. on land suitable for beech. Mean tree density at this ago plot is 581 trees·ha with a predominance of trees with -1 During the cold season, the beech forest displays a diameters between 30 and 40 cm (292 trees) (figure 3). lower thermal fluctuation (+ 3 °C) than the Scots pine for- Their mean height is approximately 15 m. The soil of this est and a higher maximum temperature (+ 1 °C). Table I plot varies in depth and has a low clay content, an acid shows the values obtained at the studied sites and those (pH 5.2) and desaturated character and is classified as a obtained from the National Weather Station at Humic Cambisol [12] (table III). Pradoluengo. Relative humidity in the beech forest is always from 1 On comparing the distribution of the trees according to 1.5 % lower than in the pine forest. Accordingly, evap- to their diameter classes, the Scots pine forest displays a otranspiration is higher in the pine forest (table II). typical Gaussian bell-shaped curve in which most trees are concentrated around the intermediate diameter class Tres Aguas is a mature beech (Fagus sylvatica L.) for- est, with a density of 523 trees·hacomprised of 300 , -1 (32.5-37.5 cm). The altitude is 1 250 m.a.s.l. (table III). The beech forest trees are distributed in such a way that young trees (4-20 cm DBH, 30 years old), the rest being the smallest trees are the most representative, and their adults (70 years old approximately). The altitude is 1 100 distribution is closer to a negative exponential. This dif- m a.s.l. This stand is a coppice with standard (figure 2), with mean height ranging from 20 to 22 m. The estimat- ferent behaviour reflects structural differences, such as ed mean age of the plot is 50 years. The soil varies con- degree of maturity and management [45].
2.2. Sampling Seven Fagus sylvatica trees and seven Pinus sylvestris trees, representative of different classes, were felled to establish their aboveground biomass. Each tree thus har- vested was divided into trunk, branch and leaves. The trunks were separated into sections, according to their height (0-1.30 m, 1.30-3 m, 3-5 m, 5-7 m.) and weighed. The wood was separated from the leaves. where A is the annual leaf or litter fall to the soil and F is Fifteen litter traps were randomly distributed on the the leaf or litter accumulation on the surface soil before two experimental sites during a 3-year period. The litter the period of massive litter shedding. was removed monthly and the material collected subdi- production of leaf or litter can The losses in the annual vided into different plant organs (branches, leaves, fruits be established from and flowers). The leaf decomposition dynamics was assessed in lit- bags, made of nylon with a pore diameter of 1 mm and ter where P is the annual loss of leaf litter produced. a surface area of 400 cm Each litter bag contained 5 g of . 2 beech leaves or pine leaves (’needles’) recently fallen Calculation of the decomposition coefficient K [33] is o from their own tree canopy. The bags were placed over defined by the holorganic horizon in three different locations at each plot. Forty-five litter bags were placed in each ecosystem, distributed in three groups. The experiment was begun in The parameter K (coefficient of accumulation of leaf or d December 1990 and ended in January 1994. After also determined litter) was December 1990, every 2 months, three bags per plot, one from each of the three locations, were collected during the study period. Additionally, from each site, litter sam- ples were collected from a 50 x 50 cm area of the ground Data were subjected to a one-factor statistical analysis of to determine the indices of natural decomposition in the variance algorithm (ANOVA). The regression curves two forests [44]. were also established according to the best r. 2 All subsamples were taken to the laboratory for further analysis. The leaves and the litter were cleaned and dried at 80 °C for 24 h to constant weight to determine the 3. Results moisture [45]. content 3.1. Aboveground biomass For the evaluation of litter dynamics, we used the coef- ficient K by Jenny et al. [ 19], which relates the humus and the aboveground litter. K is a constant for any given Tables IV and V summarize the overall set of dendro- ecosystem and is defined by metric and weight characteristics of the seven trees from
each plot studied, representative of each population according to diameter classes. Figure 4 shows the DBH/height ratio. Correlation coefficients of r 0.84 for the beech forest and r 0.90 2 2 = = for the Scots pine forest were obtained. These predictions give a maximum of approximately18.2 m for the beech- es and 15.3 m for the pines. The following regression equations for the total aboveground biomass (kg), expressed in terms of DBH (cm), were calculated for each plot (table VI). On comparing the values of total aboveground biomass obtained from the felled trees from both sites according to diameter classes (figure 5), a clear divergence may be seen especially in the mature phases. On relating DBH to biomass, the following regression equations are obtained (table VI) The trunk is the part of the tree that most contributes to the total biomass. This has a value of 74.4 % in the beech forest (table IV) and 75.7 % in the pine forest (table V).
Figures of 98.6 Mg·ha -1 are obtained for deciduous forest -1 Mg·ha and 115.1 for evergreen forest. On estimating trunk biomass in relation to the DBH (figure 6) greater productivity is seen for beech, with cor- relation coefficients of r = 0.99 in both cases. 2 The regression equations for the DBH/trunk biomass ratio are as follows (table VI): The branch fractions behave in a manner similar to the trunks (tables IV and V); mean percentages of 23.1 and 19.7 % were obtained for the beech and pine forests, respectively, obtaining 30.7 Mg·hafor the deciduous -1 species and 30.0 Mg·ha for the evergreen species. -1 On exploring the biomass of branches with respect to DBH index (figure 7), the productivity of the beech trees Mg·ha(table -1 leaves to total biomass is 2.3 % with 4.5 was seen to be greater than that of the pines. However, IV); in the pine forest these figures have values of 4.6 % some of the r correlation coefficients are poorer than 2 and 7.0 Mg·hawith r correlation coefficients 0.97 for 2 , -1 those found for the previous fraction (trunks) r 0.98 for 2 the beech and 0.88 for the pine (table V). = the beech forest and r 0.93 for the pine forest. 2 = However, on establishing leaf biomass with respect to The regression equations obtained from the the DBH parameter (figure 8), the greatest productivity is DBH/branch biomass ratio are as follows (table V): also obtained for the beech forest. The for the leaf biomass/DBH regression equations ratio follows (table VI): are as A divergence can be seen in the determination of the bio- of leaves. In the beech forest, the contribution of the mass
3.3. Litter decomposition 3.2. Litter fall The decomposition indices were determined for total The amounts of yearly litter fall for leaf litter and total litter in each forest ecosystem and for leaves only of both litter (leaves + wood + reproductive organs + indetermi- stands (table VIII). Considering both total litter and nate organs) are indicated in table VII. leaves separately, higher K and K decomposition indices o were observed in the pine forest than in the beech forest. Leaf litter production was very similar in both forests However, the K index in the beech forest was higher for while litter production was more important in the pine total litter than for leaves alone. The greatest losses were forest. from the pine litter and the beech leaves. The differences appearing between the estimated leaf The decomposition indices of leaves when confined to biomass and the leaf litter are mostly in relation to the litter bags were lower than those obtained under natural date of biomass sampling. Canopy leaf mass varies dur- conditions (0.29 and 0.31 versus to 0.37 and 0.46 (table ing the season. If the biomass estimate occurs in summer, VIII). at the peak of leaf growth, this could explain the differ- ences between leaf litter amounts. In addition, leaf litter was only sampled from September to December, under- 4. Discussion estimating some possible earlier leaf and litter fall. most commonly used to estimate the bio- The procedure in forest ecosystems involves destructive techniques mass in combination with the application or regression equa- tions to manage the data. The best fitted model is the allo- , b X where Y is biomass and X tree metric model Y = diameter at a height of 1.30 m. It should be stressed that this model is quite complex; indeed some authors [2, 3, 47] have proposed corrections with a view to avoiding under estimations of the true values. This method has been used by several authors [37, 45]. diameter classes, comparing biomass according On to seen that it in the beech forest, it may be much higher
that these represent in the whole set of litter. The impor- would not be very suitable to reforest land appropriate for tance of having knowledge of the amounts of each of beech with pine, as confirmed by the contents in C and N these fractions is evident since the return of elements to in the different tree fractions [45]. Thus, if the total num- the soil will follow different recycling patterns, which ber of trees in each ecosystem is known, figures of132.7 and 152.1 Mg·hafor the beech and pine stands, respec- -1 may overlap in space and time. tively, are obtained; this is because the distribution in the As in the case of most forest systems, the leaves com- latter sites follows the Gaussian bell-shaped curve, with the most important fraction, representing 61.9 and prise few trees belonging to the extreme classes, while in the 50.4 % of the total contribution in the beech wood and first site many trees were found in the lower classes and pine forest, respectively. This shows that the forest sys- no sampling in the upper classes. tems in question are immature, since according to Kira The references found in the literature report conflicting and Shidei [21], especially the beech stand, maturity is data, depending on the forest species studied, the age of reached when leaf shedding tends towards 50 % of the the stand, the kind of soil and the environmental condi- total. tions. In Fagus sylvatica forest Calamini et al. [8] estab- Leaf abscission displays a seasonal behaviour, which lished an aboveground biomass of 319 Mg·ha , -1 coincides with that described for the overall production. Ovington [34] at 50 years old, reported 164 Mg·ha and -1 The formation of tissues triggers a mobilisation of nutri- Reiners [39] 124 Mg·hain gymnosperms of 50-year- ; -1 ents towards those from older organs, which in turn leads old communities Green and Grigal [17] described a range to the abscission of older leaves and twigs [22]. of 92-169 Mg·hawhile Tappeiner and John [49] report- -1 ed 102-136 Mg·hain stands of 50-90 years old. -1 In other resinous species, maximum leaf litter fall For trunk biomass Calamini et al. [8] obtained 89.1 % later, as in the case of Pinus sylvestris: in October occurs and November [1, 7] and in P. elliotti [15]. The early with respect to total aboveground biomass, whereas for branch biomass they obtained values of 29 Mg·ha or -1 senescence observed in the forest studied in the present work is probably a direct consequence of the summer 9.1 % with respect to total biomass, and Grier et al. [18] reported 65 % in Pinus edulis. For leaf biomass the liter- drought in Mediterranean regions, which according to Rapp [36] triggers the early senescence of plant organs. ature reports different values: in Fagus sylvatica Calamini et al. [8] calculated 2.7 Mg·haor 0.8 % of -1 Branches occupy the second most important place in leaves; Lemée [23] reported 3.5 Mg·haand Lemée and -1 the amount of aboveground biomass, within the whole set Bichaut [24] 3.1 Mg·haIn Juniperus occidentalis, . -1 of litter components (823 kg·ha in the beech plot ·year -1 Gholz and Fisher [15] indicated 20 % of needles; in Pinus and 1 766 kg·ha in the pine plot, representing ·year -1 sylvestris, Rodin and Bazilevich [41] established values 17.6 and 30.5 %, respectively (table VII). of 9.6 and 5.5 % of needle biomass with respect to the total forest aboveground biomass. The fall or bark contributes to the formation of humus which conserves the humidity of the soil; the late maxi- mum can be related to meteorological factors, rain and 4.1. Litter fall wind that are typical of this season. These findings sug- gest that there could be two alternative possibilities at the Table VII shows the annual production values obtained moment of the retranslocation of nutrients towards struc- for the different fractions together with the percentages tures in formation.
needles alone than the leaves lignin (twigs, branches) The fraction corresponding to the fruits displays a peri- or od of maximum return. The fraction represents the same [29, 30]. proportion in the two stand (12.3 % in beech and 13.4 % in pine). One explanation of this difference can be sought in the different distribution of auxins in apical meristems 5. Conclusions from one year to another [35]. On comparing biomass according to diameter classes, small pro- The flowers and other fractions represent a much higher in the beech forest, it may be noted that it portion with respect to total litter fall. would not be very suitable to reforest land appropriate for beech with pine. of trunks and branches with exploring the biomass On 4.1. Litter decomposition index, the productivity of the beech to the DBH respect forest is seen to be greater than that of the pine stand. In both forest ecosystems, greater K and K indices o However, some of the r correlation coefficients are sim- 2 obtained for total litter than for leaves alone. It is were ilar in both cases for the trunks r 0.99 and the correla- 2= possible that the mean soil humidity was not a limiting tion coefficients are r2 0.89 for the beech forest and = factor in the decomposition process and this effect would 2 r 0.93 for the pine forest. = be due to the distribution of rainfall rather than to the total A divergence can be seen in the determination of the amount of precipitation together with elevated tempera- biomass of the leaves; 2.3 % with respect to total biomass ture and airing of the holorganic soil horizon. Similar val- in the beech forest and 4.6 % in the pine forest with r 2 ues have been reported [6, 10, 31]. The values reported by correlation coefficients of 0.92 and 0.88 for the beech and Maheswaran and Attiwill [25] were higher and those of pine, respectively. Gallardo and Merino [13] lower. of most forest ecosystems, the leaves As in the case The litter bags may have hindered free access to the comprise the most important fraction of the total litter mesofauna [20] and may have created microclimatic con- fall, representing 61.9 and 50.4 % in the beech forest and ditions that delayed the decomposition rate. Also, the F pine forest, respectively. values may be underestimated, since it is often difficult to During the decomposition cycle, the loss of dry matter distinguish decomposing leaves from other plant remains, was 40 % in the beech forest and 42 % in the pine forest. especially when small sizes are involved. F had fairly low It is likely that the effect of precipitation during the peri- values that cannot be entirely explained by the presence od of decomposition was not decisive, since its distribu- of twigs and barks rich in lignin substances [29] and low tion over the time period was similar for both forests. in N [4, 27]. The decomposition indices of leaves when confined to A similar type of behaviour was observed in both litter bags were lower than those obtained under natural ecosystems, but with occasional divergences. During the conditions. first 3 months of the 2 year cycle, a noteworthy loss of weight was observed. The precipitation recorded created Acknowledgements: We thank the ground staff who conditions conducive to the leaching of water-soluble have collaborated with us. Field assistance was provided substances from the decomposing material. During the by C. Relaño. The English translation was supervised by ensuing summer period, the process ceased, and a second, N. Skiner. slower stage of degradation occurred that affected mole- cules with stronger bonds. During this phase, soil micro- organisms play a more active role. Finally, a new accel- References eration of decomposition was observed in weight loss during the autumn/winter period. This was more pro- [1] Aussenac G., Production de litiére dans divers peuple- nounced in the beech forest. France, Acta Oecologica, Oecol. Plant. 4 ments de l’Est de la (1969) 225-235. Lemée and Bichaut [24] reported an annual weight [2] Baskerville G.L., Use of logarithmic regression in the loss between 15 and 40 % in Fagus sylvatica and Pinus estimation of plant biomass, Can. J. For. 2 (1972) 49-53. sylvestris. Berg and Lundmark [5] reported values of [3] Beauchamp J.J., Correction for bias in regression esti- 31 % and Santa Regina [42] a value of 27 %. mates after logarithmic transformation, Ecology 54 (1973) 1403-1407. to see that the leaf litter decomposition It is possible lower than the total litter decomposition [4] Berg B., Dynamics of nitrogen (15 N) in decomposing constants are constants; nevertheless the total litter includes more wood pine (Pinus sylvestris) needle litter: Long-term decompo- Scots
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