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Tropentag 2008 University of Hohenheim, October 7-9, 2008

Conference on International Research on Food Security, Natural Resource Management and Rural Development

Litterfall Deposition Along an Altitudinal and Vegetation Gradient, Northeastern Mexico Humberto, Gonzáleza∗, Tilo G. Domíngueza, Israel Cantúa, Marco V. Gómezb, Marisela Pandoa, Roque G. Ramírezc a Universidad Autónoma de Nuevo León, Facultad de Ciencias Forestales, Apartado Postal 41,

Linares, NL 67700, Mexico. b Universidad Autónoma de Nuevo León, Facultad de Economía, Monterrey, NL, México. c Universidad Autónoma de Nuevo León, Facultad de Ciencias Biológicas. Monterrey, NL,

México. Abstract Litterfall and litter decomposition are key fundamental processes in nutrient cycling of forest ecosystems. In northeastern Mexico, however, few studies have addressed the spatial and temporal patterns of litterfall and nutrient deposition. Thus, the aim of this study was to determine the litterfall deposition through leaves litter along an altitudinal gradient in northeastern Mexico. Litter constituents (leaves, reproductive structures (flowers, fruits and seeds), twigs or branches, and miscellaneous (unidentified residues, fine plant material such as bark, insect bodies and feces, among others) were measured at 15-day intervals between December 21, 2006 and December 20, 2007 in four experimental sites: one site was located in a pine (Pinus pseudostrobus Lindl.) forest (Bosque Escuela at 1600 m of altitude), second in the ecotone of a Quercus spp. forest and the Tamaulipan thornscrub (Crucitas at 550 m), third and fourth sites were in the Tamaulipan thornscrub (Campus at 350 m and Cascajoso at 300 m). Each site had a plot (50 m x 50 m) in which ten litter traps (1.0 m2 each) were used for collections. Since for most sampling dates and litter constituents data had not equality of variances, even when data was logarithmically transformed, thus, the Kruskal-Wallis non parametric test was employed to detect significant differences among sites at each sampling date. Total annual litterfall deposition was 4407, 7397, 6304 and 6527 kg ha-1 y-1 for Bosque Escuela, Crucitas, Campus and Cascajoso, respectively. Of total annual litter production, leaves were higher varying from 74% (Bosque Escuela) to 86% (Cascajoso) followed by twigs from 4% (Cascajoso) to 14% (Crucitas), reproductive structures from 6% (Bosque Escuela) to 10% (Crucitas), and miscellaneous litterfall from <1% (Campus) to 12% (Bosque Escuela). Introduction Litterfall and litter decomposition are key processes in nutrient cycling of forest ecosystems (Baker III et al., 2001; Isaac and Nair, 2006). In addition to these processes, throughfall and stemflow are the main sources to maintain soil fertility to the forest floor (Vasconcelos and Luizâo, 2004). Litterfall plays a fundamental role in nutrient turnover and in the transfer of energy between plants and soil, the main source of organic material and nutrients being accumulated in the uppermost

∗ Corresponding author: E-mail: humberto@fcf.uanl.mx

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layer of the soil. Nutrient release from decomposing litter is an important internal pathway for nutrient flux in forest ecosystems (Santa Regina and Tarazona, 2001).

Evaluation of litterfall production is important for understanding nutrient cycling, forest growth, successional pathways, carbon fluxes, disturbance ecology, and interactions with environmental variables in forest ecosystems (Vasconcelos and Luizão, 2004; Zhou et al., 2007). However, litterfall inputs varies widely among forest ecosystems in terms of quality (Duchesne et al., 2001; Vasconcelos and Luizão, 2004) and quantity (Rothe and Binkley, 2001; Zhou et al., 2007). The quality of soil organic matter is of great importance for the majority of the functional processes occurring in the soil of forest ecosystems (Salazar and Santa Regina, 2005). Despite of the great number and well documented floristic studies carried out at the Tamaulipan thornscrub or subtropical thornscrub woodlands, northeastern Mexico, little studies have been carried out to address the spatial and temporal patterns of litterfall and nutrient deposition, in spite of the importance of this process for nutrient cycling in this type of vegetation which is distinguished and dominated by a wide range of taxonomic groups exhibiting differences in growth patterns, leaf life spans, textures, growth dynamics, and phenological development (Reid et al., 1990). This is of key importance in forest communities where sustainable litterfall production is an essential part of the aboveground net primary production and depends upon the nutritional status of soil, such as those found in the northeastern region of Mexico, whose vegetation depends on the biogeochemical cycles of plant nutrients contained in plant detritus (Vasconcelos and Luizâo, 2004). Therefore, this region provides an opportunity to investigate litterfall production and nutrient returns of forest and native vegetation in order to gain a better understanding of how to sustain and improve productivity in response to changes in resource availability. Thus, the aim of this study was to assess the spatial and temporal litterfall deposition along an altitudinal and vegetation gradient in northeastern Mexico throughout an annual cycle. Material and Methods Description of study sites This study was carried out at four research sites located in the state of Nuevo Leon, northeastern Mexico. The first site (Bosque Escuela) was located in a pine (Pinus pseudostrobus Lindl.) forest plot located in the Experimental Forest Research Station of the Universidad Autónoma de Nuevo León in the Sierra Madre Oriental Mountain, Iturbide county, (24°43’N; 99°52’W; 1,600 m a.s.l.). Mean annual air temperature is about 13.9o C. Average annual rainfall is approximately 639 mm. According to FAO-UNESCO classification, the soil at the research site belongs to Calcic Kastañozem (Cantú and González, 2001; 2002). The second site (Crucitas) was located in the ecotone of a Quercus sp. forest and the Tamaulipan thornscrub, knows as matorral or srhubland (24º46’N; 99º41’W; elev 550 m). Average total annual rainfall is 755 mm with an annual mean air temperature of 21o C (Bravo-Garza, 1999). The third site (Campus) was located at the Experimental Research Station of the Facultad de Ciencias Forestales (Faculty of Forest Sciences), Universidad Autónoma de Nuevo León (24º47' N; 99º32' W; elev. 350 m). The climate is subtropical and semiarid with warm summer. Monthly mean air temperature ranges from 14.7o C in January to 22.3o

C in August, although daily high temperatures of 45o C are common during summer. Average total annual precipitation is about 805 mm with a bimodal distribution. The four site (Cascajoso) was located in the Hacienda Guadalupe “Ejido” (24°54’N; 99°32’W; elev. 300 m). Averaged annual temperature is 21o C with a mean maximum and mean minimum of 30o C and 12o C, respectively. Total cumulative annual rainfall is 672 mm (Cavazos, 1992). Crucitas, Campus and Cascajoso sites are located in Linares county. For sites, Campus and Cascajoso, the main type of vegetation is known as the Tamaulipan thornscrub or subtropical thornscrub woodlands (SPP-INEGI, 1986). Some physical and chemical properties of the soil at a profile depth of 0-20 cm are shown in Table 1. In this study, registered mean monthly air temperatures (oC) and cumulative monthly rainfall (mm) are shown in Table 2.

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Table 1: Some physical and chemical properties of the soil (profile depth 0-20 cm) at the studied sites in northeastern Mexico

Soil property Site Bosque Escuela Crucitas Campus Cascajoso Sand (g kg-1) 170.0 120.0 250.0 130.0 Silt (g kg-1) 430.0 420.0 500.0 610.0 Clay (g kg-1) 400.0 460.0 250.0 260.0 Bulk Density (Mg m-3) 0.9 1.2 0.8 1.2 Organic matter (%) 6.0 4.0 7.0 2.0 pH (CaCl2; 0.01 M) 6.6 6.8 6.6 7.1 EC (µS cm-1) 255.0 103.0 216.0 123.0 Ca (mg kg-1) 8,839.1 5,063.5 8,555.6 8,000.2 K (mg kg-1) 297.4 307.7 109.5 134.7 Mg (mg kg-1) 150.3 310.4 216.0 98.5 Total P (mg kg-1) 631.6 1,278.7 1,158.6 648.6 Total N (mg kg-1) 2,957.3 3,461.2 5,604.6 1,897.3

Table 2: Mean monthly air temperature (oC) and cumulative monthly rainfall (mm) at research

sites in northeastern Mexico Month-Year Site

Bosque Escuela Crucitas Campus Cascajoso oC mm oC mm oC mm oC mm

December 2006 10.8 24.2 14.7 26.4 15.1 32.2 16.0 6.6 January 2007 10.1 31.8 17.2 32.4 11.8 39.8 12.3 29.6 February 2007 12.4 10.0 19.5 64.8 16.2 64.2 17.1 22.0 March 2007 15.2 12.2 19.8 14.0 21.2 9.2 22.0 1.6 April 2007 17.0 19.6 20.6 52.6 21.8 30.8 23.8 49.8 May 2007 18.9 71.4 23.7 285.8 25.1 156.2 26.4 101.8 June 2007 20.6 109.4 25.5 127.8 26.8 86.8 28.3 24.4 July 2007 19.7 139.2 25.0 3.4 26.3 148.2 28.0 75.8 August 2007 19.4 179.2 25.6 118.2 26.7 159.2 28.3 73.2 September 2007 18.4 129.4 24.5 154.8 25.6 112.8 26.6 58.6 October 2007 16.4 31.8 22.0 35.2 23.0 19.2 23.8 15.6 November 2007 13.6 13.6 17.5 25.8 19.2 23.8 20.0 3.0 December 2007 13.6 2.4 16.2 0.8 17.3 0.6 18.3 1.6 Total Annual 774.2 942.0 883.0 463.6 Litter sampling frequency and deposition At each site, litterfall deposition was evaluated in an undisturbed experimental plot (50 m x 50 m). At each plot, ten (replications) litter traps, made with wooden sides fitted with the nylon net bottom (1 mm mesh size), were randomly scattered over the entire area. Each trap covered an area of 1.0 m2 and was placed approximately 0.30 m above the soil level to intercept litterfall. Contents of traps were collected at 15-day intervals between December 21, 2006 and December 20, 2007. At each sampling date, research site and trap, the total collected litter was manually sorted into the following categories: leaves, reproductive structures (flowers, fruits and seeds) twigs or branches

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(<2 cm in diameter), and miscellaneous residues (unidentified, fine plant tissue such as bark, pieces of insect bodies or feces). The samples were oven dried to a constant weight at 65oC for 72 h and weighed to the nearest milligram. For each site, litterfall deposition (dry weight basis) at each sampling date is reported as g m-2, whereas on an annual basis it is reported as kg ha-1 y-1. Statistical analyses Separate analyses were made for total litter production and for each of the main constituents of the litterfall. Normal distribution and homogeneity of variances for each litter constituent data at each sampling date were tested using the Kolmogorov-Smirnov test (Steel and Torrie, 1980) indicated that for many litterfall data are non-normally distributed. Since for most sampling dates and litter constituents did not show the assumption the equality of variances even tough when data sets were logarithmically transformed, thus Kruskal-Wallis non parametric test was employed (Ott, 1993) to detect significant differences among sites at each sampling date for litter deposition. Based on this statistical procedure, differences in litter deposition between sites were validated using the T-test and were considered statistically significant at P=.05 (Steel and Torrie, 1980). All applied statistical methods were according to the SPSS® (Statistical Package for the Social Sciences) software package (standard released version 13.0 for Windows, SPSS Inc., Chicago, IL). Results and Discussion Spatial and seasonal variation in litterfall deposition It appears that there was a spatial and temporal variability in litterfall deposition along the altitudinal gradient, because all litter components were significantly different among sampling dates (Table 3). In Bosque Escuela site, total litterfall deposition (Figure 1) ranged from 3.6 (Feb-18) to 42.4 g m-2 (Dec-20) in Crucitas site, from 10.6 (May-19) to 91.9 (Mar-05) in Campus site, from 12.7 (Jul-20) to 44.8 (Nov-05) and in Cascajoso site, from 8.8 (May-04) to 87.6 (Nov-20). Leaf litter deposition (Figure 2) for Bosque Escuela varied from 2.9 (Feb-18) to 34.2 g m-2 (Dec-20), for Crucitas from 7.1 (May-19) to 83.9 (Mar-05), for Campus from 9.8 (Jul-20) to 39.5 (Dec-20) and for Cascajoso from 2.7 (May-04) to 84.2 (Nov-20). The dynamics of reproductive structures litter inputs is shown in Figure 3. Mean minimum reproductive structures deposition values for Bosque Escuela, Crucitas, Campus and Cascajoso sites were 0.1 (Nov-05), 0.3 (Feb-03), 0.2 (Feb-08) and 0.2 g m-2 (Dec-20), respectively; whereas, maximum deposition was 5.4 (Feb-03), 23.1(Apr-04), 10.3 (Feb-03) and 7.3 (Apr-04), respectively. The twigs litter seasonal deposition (Figure 4), mean minimum values ranged from 0.06 (Bosque Escuela; Dec-20) to 1.37 g m-2 (Crucitas; Feb-03), while mean maximum deposition values varied from 4.08 (Cascajoso; Apr-19) to 14.23 g m-2 (Crucitas; Apr-19). Figure 5 shows the seasonal trend of miscellaneous litter deposition which illustrates that the minimum and maximum observed values ranged from 0.0 (Campus) to 7.0 g m-2 (Bosque Escuela). Annual litterfall deposition Total annual litter production and its components (leaves, reproductive structures, twigs, and miscellaneous) were significantly different (Table 3) among all sampling sites. Crucitas site, resulted with the highest values followed by Cascajoso, Campus and Bosque (Table 4). Leaves represented the main component with a deposition that ranged 74% (Bosque Escuela) to 86% (Cascajoso). Reproductive structures (flowers, fruits, and seeds) deposition ranged from 6.3% (Bosque Escuela) to 9.5% (Crucitas). Twigs annual deposition varied from 4% (Cascajoso) to 14% (Crucitas). The contribution of the miscellaneous constituents of litterfall ranged from <1% (Campus) to 12% (Bosque Escuela) of total annual litter deposition.

It appears that litter constituents (leaves, reproductive structures, twigs, and miscellaneous) differed quantitatively and qualitatively on a seasonal and annual basis among research sites. It has been reported that litter production and litter chemistry in forest ecosystems is determined by age and rainfall (Lawrence, 2005), species composition (Pavón et al., 2005; Sariyildiz and Anderson, 2005), soils and nutrient availability (Read and Lawrence, 2003; Vasconcelos and Luizão, 2004; Dent et al., 2006), stand structure (Yang et al., 2006; Zhou et al., 2007), and successional stage

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(Yankelevich et al., 2006). Besides, rates of nutrient return to the forest soil are controlled not only by the amount of litter production but also by the nutrient concentrations in litter components (Yang et al., 2006; Mlambo and Nyathi, 2007). As in this study, seasonal differences in biomass production might have been related to extreme climatic fluctuations and/or changes in phenological events such as leaf abscission and shedding, shoot initiation, flowering and fruiting (Palma et al., 2000). Previous findings in the northeastern Mexico have revealed that during wet months (August and September) when rainfall was heavy, branches and fruits were the main litter constituents; whereas, in the dry and winter months there was a greater contribution of leaves due to drought and freezing temperatures registered (González et al., 2007).

In this study, litterfall deposition occurred throughout the experimental period; however, there were significant main seasonal peaks for the different litter components in relation to input quantity, variability, and order of magnitude (Figures 2, 3, 4 and 5). This litter production varied regardless of total or individual studied litter components at each site.

Foliar nutrient movements from senescing leaves to active plant tissues or woody structures has been considered as a physiological mechanism of nutrient cycling since it plays a major role in nutrient conservation by deciduous tree species because nutrients following this pathway are not lost through litterfall (Duchesne et al., 2001). It has been proposed that tree species on fertile soils tend to produce leaves and leaf litter with high nitrogen content which in turn decompose rapidly and support high plant production through fast turnover of the nutrient pools (Sariyildiz and Anderson, 2005). Forrester et al. (2005) and Rothe and Binkley (2005) state that mixed forests stands containing N-fixing species increase and improve nutrient cycling through litterfall when compared with monoculture or stands containing fewer N-fixing species. Read and Lawrence (2003) proposed that nutrient limitation in the dry tropics is related to water deficits since dry conditions prevent nutrient absorption from soil, and consequently affect the release and mineralization of nutrients by slowing decomposition.

Native vegetation in northeastern Mexico, composed mainly by shrubs and small trees, is characterized by low biomass productivity (Villalón, 1989) because during dry seasons native plants have to deal with soil water deficits, high temperatures, high irradiance levels and low leaf water potential (González et al., 2000; 2004).

On an annual basis, the contribution of leaves, reproductive structures, twigs and miscellaneous to total litterfall deposition in Campus and Cascajoso sites are within the range of previously documented studies (González et al., 2007) in the sub-tropical woodlands of northeastern Mexico. However, observed differences in leaf fall in Crucitas, Campus and Cascajoso compared to Bosque Escuela could be related to the continuous shedding of leaves of deciduous plant species, which is a characteristic of the Tamaulipan throscrub ecosystem, as a morphological and physiological mechanism to cope with water deficits and high temperatures to avoid plant tissue dehydration and water lost through transpirational flux.

Implications Results of this study suggest the importance of litterfall production not only in terms of nutrient cycling to the forest soil in different stand communities, but also to maintain fundamental ecological and ecosystems processes such as soil formation, prevent soil erosion, maintain soil fertility and substrates for plant and microbial species, support and sustain life for invertebrate fauna, increase organic matter mineralization, improve soil physical and chemical properties such as soil water availability and infiltration to enhance nutrient absorption, plant regeneration and establishment, and root growth; all of these are interrelated and integrated to sustain and maintain ecosystem productivity and biodiversity.

In addition, further research lines could be suggested in order to explain how litter constituents decomposition processes are affected by their own chemistry composition, physicochemical environmental variables, soil properties, microbial communities, and consequently, elucidate nutritional fluxes.

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Santa Regina, J., Tarazona, T., 2001. Nutrient pools to the soil through organic matter and throughfall under a Scots pine plantation in the Sierra de la Demanda, Spain. Eur. J. Soil. Biol. 37; 125-133.

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Table 3: Kruskal-Wallis test to detect significant differences among litter constituents Sampling Date Statistic Litter constituents Leaves Reproductive Twigs Miscellaneous Total Jan-04-07 χ2 10.3 11.4 24.6 18.4 12.2 P value .016 .01 <.001 <.001 .01 Jan-19-07 χ2 15.4 6.3 8.2 20.9 6.9 P value .001 .1 .04 <.001 .1 Feb-03-07 χ2 15.4 4.1 12.5 20.1 8.7 P value .002 .2 .01 <.001 .03 Feb-18-07 χ2 26.8 11.0 23.8 5.8 27.0 P value <.001 .01 <.001 .1 <.001 Mar-05-07 χ2 24.8 21.4 18.4 10.2 20.9 P value <.001 <.001 <.001 .01 <.001 Mar-20-07 χ2 19.8 4.9 16.7 14.1 17.3 P value <.001 .1 .001 .01 .001 Apr-04-07 χ2 12.0 22.9 29.3 13.9 19.6 P value .007 <.001 <.001 .01 <.001 Apr-19-07 χ2 5.1 10.2 22.2 18.1 4.8 P value .1 .01 <.001 <.001 .1 May-04-07 χ2 21.1 4.5 24.0 20.0 12.2 P value <.001 .2 <.001 <.001 .01 May-19-07 χ2 12.4 20.3 17.6 9.8 2.6 P value .01 <.001 .001 .02 .4 Jun-04-07 χ2 14.3 15.6 9.3 24.0 16.7 P value .002 .001 .02 <.001 .001 Jun-20-07 χ2 12.1 8.8 11.9 14.2 2.5 P value .01 .03 .01 .01 .4 Jul-20-07 χ2 1.8 6.3 23.3 17.7 6.9 P value .6 .1 <.001 <.001 .1 Aug-04-07 χ2 5.3 11.5 26.0 11.5 7.7 P value .1 .01 <.001 .01 .051 Aug-20-07 χ2 16.0 2.5 20.6 8.9 15.2 P value .001 .4 <.001 .03 .01 Sep-04-07 χ2 23.454 10.4 10.2 10.9 10.3 P value <.001 .01 .01 .012 .01 Sep-20-07 χ2 12.2 9.9 19.9 16.6 10.0 P value .01 .01 <.001 .001 .01 Oct-05-07 χ2 17.5 14.7 16.4 9.5 15.5 P value .001 .01 .001 .02 .001 Oct-20-07 χ2 5.5 7.9 11.8 1.8 6.6 P value .1 .04 .01 .6 .1 Nov-05-07 χ2 14.7 11.8 20.9 0.7 14.2 P value .01 .01 <.001 .8 .01 Nov-20-07 χ2 12.1 10.9 24.5 7.1 9.1 P value .01 .01 <.001 .06 .02 Dec-05-07 χ2 14.5 14.5 21.0 20.7 15.7 P value .01 .01 <.001 <.001 .001 Dec-20-07 χ2 17.8 15.2 27.2 11.2 16.0 P value <.001 .01 <.001 .01 .001 Total χ2 13.4 9.5 25.6 24.1 13.2 P value .01 .02 <.001 <.001 .01

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Table 4: Mean annual litter deposition (kg ha-1 y-1) at research sites in northeastern Mexico. Total literfall deposition was calculated as the arithmetic sum of leaves, reproductive structures, twigs, and miscellaneous

Deposition Site

Bosque Escuela Crucitas Campus Cascajoso

Total 4,407.3 7,397.1 6,304.1 6,527.2

Leaves 3,254.0 5,560.5 4,892.0 5,612.6

Reproductive 277.8 700.9 504.2 545.5

Twigs 356.3 1,041.1 855.5 255.5

Miscellaneous 519.2 94.5 52.3 113.5

Figure 1: Mean total litterfall deposition pattern at research sites, northeastern Mexico. Values are means ± standard errors (n=10).

Figure 2: Mean leaf litter deposition pattern at research sites, northeastern Mexico. Values are means ± standard errors (n=10).

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Figure 3: Mean reproductive structures litter deposition pattern at research sites, northeastern Mexico. Values are means ± standard errors (n=10).

Figure 4: Mean twigs litter deposition pattern at research sites, northeastern Mexico. Values are means ± standard errors (n=10).

Figure 5: Mean miscellaneous litter deposition pattern at research sites, northeastern Mexico. Values are means ± standard errors (n=10).