Multiple Factors Influence Seasonal and Interannual Litterfall Production in a Tropical Dry Forest in Mexico

Litterfall production plays a fundamental role in the dynamics and function of tropical forest ecosystems, as it supplies 70–80% of nutrients entering the soil. This process varies annually and seasonally, depending on multiple environmental factors. However, few studies spanning several years have addressed the combined effect of climate variables, successional age, topography, and vegetation structure in tropical dry forests. In this study, we evaluated monthly, seasonal, and annual litterfall production over a five-year period in semideciduous dry forests of different successional ages growing on contrasting topographic conditions (sloping or flat terrain) in Yucatan, Mexico. Its relationship with climate and vegetation structural variables were also analyzed using multiple linear regression and generalized linear models. Litterfall was measured monthly in 12 litterfall traps of 0.5 m2 in three sampling clusters (sets of four 400 m2 sampling plots) established in forests of five successional age classes, 3–5, 10–17, 18–25, 60–79, and >80 years (in the latter two classes either on slopping or on flat terrain), for a total of 15 sampling clusters and 180 litterfall traps. Litterfall production varied between years (negatively correlated with precipitation), seasons (positively correlated with wind speed and maximum temperature), and months (negatively correlated with relative humidity) and was higher in flat than in sloping sites. Litterfall production also increased with successional age until 18–25 years after abandonment, when it attained values similar to those of mature forests. It was positively correlated with the aboveground biomass of deciduous species but negatively correlated with the basal area of evergreen species. Our results show a rapid recovery of litterfall production with successional age of these forests, which may increase with climate changes such as less precipitation, higher temperatures, and higher incidence of hurricanes.


Introduction
Litterfall production (amount of plant material that falls to the ground per unit area and time) is a key process of the carbon cycle in terrestrial ecosystems [1][2][3][4][5]. It is the main source of reincorporation within each plot. Red dots denote the sampling clusters that were used in this study. The inset square is a 3 × 3 km area where the late successional sampling clusters were established following a stratified systematic design; early successional sampling clusters were established outside the reserve following a chronosequence design (see Appendix A).
The local climate is warm subhumid (Aw), with mean annual temperature of 26 °C and mean annual precipitation of 1000-1200 mm [61]. The rainy season lasts from May to October and accounts for over three quarters of the total annual precipitation (763-916 mm), whereas the dry season lasts from November to April and contributes only 237-284 mm. The geomorphology of the region consists of karstic limestone, with flat areas alternating with gently sloping hills (10-25%). Cambisol and luvisol soils predominate in flat areas, and leptosol soils on hills and sites with rocky outcrops within each plot. Red dots denote the sampling clusters that were used in this study. The inset square is a 3 × 3 km area where the late successional sampling clusters were established following a stratified systematic design; early successional sampling clusters were established outside the reserve following a chronosequence design (see Appendix A).
The local climate is warm subhumid (Aw), with mean annual temperature of 26 • C and mean annual precipitation of 1000-1200 mm [61]. The rainy season lasts from May to October and accounts for over three quarters of the total annual precipitation (763-916 mm), whereas the dry season lasts from November to April and contributes only 237-284 mm. The geomorphology of the region consists of karstic limestone, with flat areas alternating with gently sloping hills (10-25%). Cambisol and luvisol soils predominate in flat areas, and leptosol soils on hills and sites with rocky outcrops [62]. Secondary vegetation of different ages since abandonment predominates in the region as a result of the long-standing use of slash-and-burn subsistence agriculture [63,64]. The dominant vegetation is a medium-stature semideciduous tropical forest, in which 50-75% of the trees shed their leaves during the dry season and canopy height is 13-18 m in late successional forests. The most abundant tree species include Neomillspaughia emarginata (H. Gross) S.F. Blake and Gymnopodium floribundum Rolfe. (Polygonaceae), Lonchocarpus xuul Lundell, Mimosa bahamensis Benth. and Caesalpinia gaumeri Greenm (Fabaceae), and Bursera simaruba (L.) Sarg. (Burseraceae), among others.

Selection of Sampling Sites
The sampling design was based on the one used by the National Forest and Soil Inventory of Mexico [65], which consists of 1-ha sampling clusters of four 0.04-ha (400 m 2 ) plots arranged in an inverted Y shape (Figure 1b,c). Three sampling clusters were established in forests of five successional age classes (years since abandonment from cultivation at the beginning of this study), I: 3-5, II: 10-17, III: 18-25, IV: 60-79, and V: ≥80 years, for a total of 15 sampling clusters and 60 plots (Appendix A). Sampling clusters in age classes I-III were selected to form a chronosequence of stands of different successional age on predominantly flat terrain (the prime land for farming activities). Sampling clusters in age classes IV and V were established following a stratified systematic design irrespective of topographic position, which varied among plots in each cluster. Thus, the effect of topography was assessed only in clusters in age classes IV and V. From these clusters, we used only those plots located on the predominant topographic position (flat (0-9% slope) or sloping (10-20% slope) terrain), including three plots from each of the three clusters for each topographic position (Appendix A).

Estimation of Litterfall Production
Litterfall production was measured over five years in a total of 180 litterfall traps (12 per sampling cluster). Three litterfall traps were established in each sampling plot, 6 m from the center, along three compass directions (north, east, and west) (Figure 1c). We used 80-cm-diameter (0.503 m 2 ) circular traps made of fine, 1-mm mesh plastic net to allow rainwater to escape while retaining fine plant material; the traps were placed 80 cm above the ground. Litterfall (leaves, twigs, bark, flowers, fruits, seeds, and frass) was collected monthly, as recommended by Aceñolaza et al. [66], from October 2013 to September 2018. The litter samples were dried at 70 • C to constant weight and weighed using an analytical balance. The monthly litterfall production rate (P; Mg/ha/month) was estimated using the following equation (modified from Honorio and Baker, [67]): P = Total dry weight in each litterfall trap(g) × 10 8 cm 2 × 30 days × 1 Mg Area of the litterfall trap(503.56 cm 2 ) × collection duration (days) × 1 ha × 1 month × 10 6 g

Climatic Variables
Climatic data were obtained from the weather station of the nearest town, Oxkutzcab, located 27.4 km from the study site and operated by the Comisión Nacional del Agua (National Water Commission). Data on monthly precipitation (mm), mean, maximum, and minimum air temperature ( • C), relative air humidity (%), wind speed (km/h), and wind gusts (km/h) were recorded every 10 min over the entire study period by the automatic weather station. Cumulative temperature values were calculated from daily maximum and minimum temperature values recorded by a conventional weather station at the same site; the daily values were added up to obtain monthly cumulative maximum and minimum temperatures (ACTmax and ACTmin, respectively). Vapor-pressure deficit (δe) was estimated after the equations proposed by Jones [68].

Measurement and Estimation of Vegetation Variables
All woody plants (trees, shrubs, lianas, and palms) with DBH (diameter at breast height, measured at 1.3 m above the ground) ≥ 7.5 cm present in all the plots where litterfall production was monitored were censused from 2013 to 2016 and in 2018 (2465 plants in total). Plants with DBH ≥ 2.5 cm were Forests 2020, 11, 1241 5 of 23 sampled in 80 m 2 subplots nested in the center of each 400 m 2 plot (2660 plants in total). Each plant was identified to species (see the complete species list in Appendix B), and the diameter and height of each stem were measured. Sample specimens of those plants that could not be identified in the field were collected and taken to the herbarium of the Centro de Investigación Científica de Yucatán for identification.
Aboveground biomass was estimated using allometric equations either developed or used in previous studies on forests of the Yucatán Peninsula. We used the equation of Ramírez-Ramírez et al. [69] for trees with DBH < 10 cm, and the equation of Chave et al. [70] for larger trees. All plant species recorded in each plot were classified according to their leaf phenology (deciduous vs. evergreen) based on specialized literature [71,72] as well as on the knowledge of local inhabitants and one of the co-authors (F May-Pat). Basal area, aboveground biomass, and stem density in each plot were calculated for all species as well as for evergreen and deciduous species separately.

Data Analysis
Each cluster was regarded as a sampling unit; the litterfall production values of the 12 litterfall traps in each sampling cluster (three traps × four 400 m 2 plots) were averaged. As indicated above, the effect of topographic position was evaluated based on the mean value of the three plots located on the predominant topographic position in the cluster. All sampling clusters were separated by at least 250 m; the spatial independence of the data was tested separately for each study year using Moran's Index (I) [73] implemented in the spatial autocorrelation tool of ArcMap 10.2 or Qgis 3.0 [74, 75]. No spatial autocorrelation was found in any year (I ≥ −0.216, Z ≥ −0.725, p ≥ 0.468).
Generalized linear mixed models (GLMM) were used, followed by Bonferroni post-hoc multiple comparison tests, to test for significant differences in the temporal (annual, seasonal, and monthly) litterfall production patterns between successional age classes or topographic positions. These analyses were carried out using the software SPSS v.17.0 [76].
The relationships between litterfall production and climatic or vegetation variables were examined by fitting multiple linear regression models using the regsubsets procedure in the "leaps" package in R (3.5.0) [77]. Annual and monthly averages of the climatic variables (over the entire study period) were used for these analyses, except for precipitation, for which the cumulative value was used. As vegetation variables were recorded only annually (except for 2017), climatic and vegetation variables (including successional age) were analyzed separately.
For each multiple regression analysis, the three best models identified by the regsubset procedure were considered and the best model was selected based on the Akaike information criterion (AIC). Akaike delta scores (∆ AIC, relative difference between the AIC of the best model and that of each other model) and Akaike weights (ωi) [78] were calculated for the three selected models. We checked for multicollinearity among the explanatory variables based on the variance inflation factor (VIF). The best model for each case was the one yielding the lowest ∆ AIC and the highest ωi [79,80], provided it included neither non-significant explanatory variables nor high multicollinearity (VIF ≥ 2).

Total Annual Litterfall Production
Average annual litterfall production ± 95% confidence interval was 5.651 ± 0.266 Mg/ha/year; annual production varied among the study years (F = 43.047; p < 0.001), with the highest value recorded in 2016 (6.173 ± 0.654 Mg/ha/year) and the lowest in 2017 (4.941 ± 0.502 Mg/ha/year) ( Table 1). The highest annual production (2016) coincided with the lowest annual precipitation of the study period; however, the lowest annual production (observed in 2017) coincided with a similarly low annual precipitation (the second lowest of the study period).

Influence of Successional Age and Topography on Inter-Annual Litterfall Production
Litterfall production varied significantly among successional age classes over the entire study period (F = 49.863, p < 0.001) ( Table 2), as well as within years (F ≥ 12.595, p < 0.001 in all study years) ( Figure 2). Although litterfall production varied widely among years, the lowest values were consistently recorded in forest age class I (3-5 years), whereas the highest values were observed in forest age class III (18-25 years) and/or V (≥80 years) ( Figure 2). Table 2. Total litterfall production (±95% confidence interval) over the entire study period in forests of different successional age classes. Different superscript letters denote significant differences among successional age classes.

Influence of Successional Age and Topography on Inter-Annual Litterfall Production
Litterfall production varied significantly among successional age classes over the entire study period (F = 49.863, p < 0.001) ( Table 2), as well as within years (F ≥ 12.595, p < 0.001 in all study years) ( Figure 2). Although litterfall production varied widely among years, the lowest values were consistently recorded in forest age class I (3-5 years), whereas the highest values were observed in forest age class III (18-25 years) and/or V (≥80 years) ( Figure 2). Table 2. Total litterfall production (±95% confidence interval) over the entire study period in forests of different successional age classes. Different superscript letters denote significant differences among successional age classes.
Litterfall production also differed between topographic positions over the entire study period (F  Annual litterfall production in forests of different successional age classes over the study period. The X-axis numbers indicate the successional age classes (see Table 2); vertical lines are ± 95% confidence intervals. Different letters denote significant differences among successional age classes in each year.
Litterfall production also differed between topographic positions over the entire study period (F

Relationships between Litterfall Production and Vegetation Variables
The best multiple linear regression model over the study period explained 60.8% of the variation in total litterfall production. Litterfall production over the study period was positively correlated with aboveground biomass of deciduous species and negatively correlated with basal area of evergreen species (F = 11.87, p = 0.0014) ( Table 3). The vegetation variables most closely related to annual litterfall production varied among years and included average tree height (positive correlation), biomass of evergreen species (negative correlation), basal area of deciduous species (positive), successional age (positive), and mean tree diameter (positive) ( Table 3).

Seasonal Dynamics of Litterfall Production
Litterfall production was significantly higher (mean ± 95% CI) in the dry (3.036 ± 0.387 Mg/ha/year) than in the rainy season (1.794 ± 0.209 Mg/ha/year) over the entire study period (F = 28.55, p < 0.001), as well as in most years (F ≥ 10.209, p ≤ 0.004), except for 2014 (F = 3.769, p = 0.06). Litterfall production in the dry season accounted for between a low of 57.5% (in 2014) and a high of 77.5% (in 2018) of total annual litterfall production.
Litterfall production was consistently higher in the dry than in the rainy season in the five years across the different successional age classes ( Figure 3a) and in the two topographic positions considered, except for sloping sites in 2015 (Figure 3b). There were significant interaction effects between seasons and age classes (F ≥ 5.906; p < 0.001 in all study years) as successional patterns differed between seasons

Temporal Variation in Litterfall Production and Its Relationship with Climatic Variables
Average monthly litterfall production varied significantly among years (F = 13.64; p < 0.001 in all cases) (Figure 4). The highest average monthly production (mean ± 95% CI) was recorded in 2018

Temporal Variation in Litterfall Production and Its Relationship with Climatic Variables
Average monthly litterfall production varied significantly among years (F = 13.64; p < 0.001 in all cases) (Figure 4). The highest average monthly production (mean ± 95% CI) was recorded in 2018   Over the whole study period, annual litterfall production was negatively related to annual precipitation (which accounted for 94.4% of the total variation), whereas seasonal litterfall production was positively related to wind speed and the cumulative maximum temperature, which jointly accounted for 87.4% of the total variation (Table 4).  Over the whole study period, annual litterfall production was negatively related to annual precipitation (which accounted for 94.4% of the total variation), whereas seasonal litterfall production was positively related to wind speed and the cumulative maximum temperature, which jointly accounted for 87.4% of the total variation (Table 4). The model that best described monthly litterfall production over the entire study period included relative humidity as the only explanatory variable, which was negatively related to the response variable ( Table 5). The climatic variables most closely related to monthly litterfall production varied between years but often included maximum, minimum, or mean temperature. No significant relationships were found between monthly litterfall production in 2015 and any of the climatic variables analyzed.

Discussion
The goal of this study was to assess how forest successional age, topography, vegetation structure, and climatic variables influence seasonal and interannual variations in litterfall production over a five-year period in a tropical dry forest (TDF). We expected litterfall production (1) to increase with successional age associated with basal area or aboveground biomass of deciduous species; (2) to be higher on flat than on sloping sites; and (3) to be negatively associated with precipitation and positively with maximum temperature and vapor-pressure deficit across months, seasons, and years. Our results showed large annual, seasonal, and monthly variations in litterfall production, which were related to various environmental factors including precipitation (negative correlation), wind speed (positive correlation), and maximum temperature (mostly positive correlation). As expected, litter production was higher in the dry versus rainy season and on flat versus sloping sites (overall, but not in most years); also, litter production increased with successional age and was positively correlated with aboveground biomass of deciduous species but negatively correlated with basal area of evergreen species. Below, we discuss our results in detail in light of the proposed hypotheses and relate them to previous findings as well as to predicted trends of climate change.

Annual Litterfall Production
Annual litterfall production ranged between 4.94 and 6.17 Mg/ha/year, with an average value of 5.65 Mg/ha/year. These values are well within the range (3.8-7.70 Mg/ha/year) reported by previous studies in TDF [16,21,25,27,28,81] (Appendix C). Souza et al. [81] studied TDFs of different successional ages in Brazil and reported litterfall production values lower (4.0-4.5 Mg/ha/year) than those found in our study. In contrast, Martínez-Yrízar and Sarukhan [27] reported higher values (6.5 Mg/ha/year) for a mature dry forest in Jalisco, Mexico.

Influence of Successional Age and Vegetation Structure on Litterfall Production
Our results partially supported hypothesis 1. As expected, litterfall production increased with successional age, being lower in the youngest age class, where vegetation structure is still poorly developed. However, and contrary to our expectations, litterfall production in age class IV (60-79 years) was lower than in age classes III (18-25 years) and V (≥80 years), which generally did not differ from each other. Average values of total aboveground biomass (14.5 Mg/ha), tree height (4.59 m), stem diameter (5.85 cm), and total basal area (6.21 m 2 /ha) were also lower in age class I (3-5 years) than in the older classes (F ≥ 6.816; p ≤ 0.006 in all cases), and most of these variables did not differ between age classes III to V (≥18 years)-see also [82]. Lawrence [21] found no significant differences in litterfall production between 12 to 25-year-old and mature tropical forests in the Yucatan Peninsula.
These results evidence a rapid increase in structural variables and litterfall production during the first 25 years of succession. This is consistent with findings from previous studies [46,83,84] and suggests that the structure of TDF can recover rapidly, likely due to the low structural complexity of these forests [46]. These results also indicate that successional age has a marked effect on litterfall production, as reported in other studies on tropical forests [16,21,81,85]. Other factors that might also explain the rapid recovery of litterfall production in our study site include its high soil fertility [47,63] and low intensity of land use associated with traditional slash-and-burn agriculture.
Annual litterfall production was positively related to the aboveground biomass of deciduous species (which account for 57.3-83.8% of the total basal area) and negatively related to the basal area of evergreen species. This indicates that the phenological strategy of plants in this seasonally dry tropical forest had a major and differential effect on litterfall production. Deciduous species characteristically exhibit a water-stress avoidance strategy consisting of shedding their leaves during the dry season (and, in some cases, storing water in stems and roots) to reduce water loss by transpiration and to avoid cavitation [86][87][88][89]. The leaves of these species generally show photosynthetic rates and nitrogen contents higher than those of evergreen species [51,[87][88][89]. Thus, our results suggest that the rapid recovery of litterfall production during secondary succession in these forests might have a synergistic effect on the recovery of soil fertility through the supply of litter from deciduous species (especially legume species), which have high foliar nitrogen contents and low C/N ratios that favor rapid litter decomposition [26,90,91].
On the other hand, the low litterfall production values recorded in successional age class IV (60-79 years) compared to those in age classes III (18-25 years) and V (≥80 years) were unexpected, especially since vegetation structure did not differ substantially among age classes III-V, as mentioned above. This unexpected result suggests a lower net primary productivity in age class IV compared to that in classes III and V, likely related to a negative net balance in the demographic processes underlying the gain (recruitment and growth) and loss (mortality) of biomass. Estimating demographic rates is beyond the scope of our study; however, we obtained preliminary estimates of the net balance between the number of plants that recruited and those that died over the entire study period in age classes III, IV, and V. Although the net balance was negative (deaths > recruits) in the three age classes, there were significant differences between them (F = 3.572; p = 0.037), being more negative in age class IV (−236 plants) than in age classes III and V (−178 and −112 plants, respectively). Future studies should address the demographic processes as well as the plant life-history strategies and functional traits underpinning these patterns.

Influence of Topography and Vegetation Structure on Litterfall Production
Our results partially supported hypothesis 2. As expected, litterfall production was significantly higher on flat than on sloping sites over the entire study period. However, significant differences between topographic positions were found only in one of the five years of the study. These results demonstrate the importance of conducting multi-year studies to identify overall patterns beyond the interannual variations that are common in seasonally dry tropical forests. In another multi-year study of a TDF on the Pacific coast of Mexico, Martínez-Yrízar and Sarukhán [27] also found litterfall production to be higher on flat than on sloping sites.
Previous studies on TDFs in the Yucatán Peninsula [63,82,92,93] reported higher tree height, aboveground biomass, and basal area, but lower tree density, on flat versus sloping sites. Exploratory analyses carried out as part of our study showed that biomass, basal area, and stem density of deciduous species were higher on flat than on sloping sites (F ≥ 9.509, p ≤ 0.037). Flat sites provide more favorable conditions for vegetation development since they have deeper soils than sloping sites [62]. Besides the above, runoff and erosion take away water, mineral nutrients, and soil particles from sloping sites and deposit them on flat sites at the bottom. More favorable microenvironmental conditions on flat sites would favor the establishment of deciduous species, which often show acquisitive strategies that require a high availability of soil resources. Sanaphre et al. [50] found a higher proportion of deciduous species on flat areas and of evergreen species on sloping sites, within the same study area. A higher proportion of evergreen species may entail a lower litterfall production on sloping sites. However, Nafarrate-Hecht et al. [94] and Huechacona-Ruíz [15] found no significant effects of topography on leaf area index or litterfall production, respectively, in the same study area. More detailed studies would be necessary to elucidate the causes underlying these contrasting litterfall production patterns in flat vs. sloping sites. Such studies should examine the factors that co-vary with slope, such as water availability and soil properties, as well as other key factors such as slope aspect and the proportion of deciduous/evergreen species.

Temporal Variation in Litterfall Production and Its Relationship with Climatic Variables
Our results partially supported hypothesis 3, namely that temporal variations in litterfall production would be negatively correlated with precipitation and positively with maximum temperature and vapor-pressure deficit (VPD). Annual litterfall production was indeed negatively correlated with precipitation, while seasonal production was positively related to maximum temperature (Table 4). However, seasonal and monthly litterfall production were also related to other climatic variables. For instance, seasonal litterfall production was positively correlated with wind speed (Table 4), while monthly litterfall production was related overall to relative humidity (negative correlation) and, in some years, to minimum or mean temperature-negative correlations ( Table 5).
As expected, litterfall production was higher in the dry season and was related to low values of precipitation and high values of VPD and maximum temperature, i.e., to limiting conditions for photosynthesis and other metabolic processes. Under these circumstances, most plants in this type of forest respond by shedding their leaves to avoid water loss by transpiration and cope with water stress [18,53,95,96]. Moreover, wind causes additional shedding of leaves and branches, reduces relative humidity, and causes the soil to dry out (especially at high temperatures), leading to a high VPD [97,98]. Previous studies have documented that litterfall production during the dry season may account for 25 to 100% of total annual production [20,85,[99][100][101]. In our study, litterfall production during the dry season accounted for 67.5% of the total production (over the entire study period), with annual values ranging between 57.5 and 77.5%. These values are similar to those reported by Aryal et al. [16], who documented that dry season production accounted for 70% of the total annual litterfall production in Calakmul, south of the Yucatán Peninsula, Mexico.
Average monthly litterfall production over the study period was 0.460 Mg/ha/month and was negatively related to relative humidity. However, the influence of climatic factors varied among years, with maximum (positive or negative correlation) and minimum temperature (negative correlation) being the variables most closely related to average monthly litterfall production. These results show that monthly litterfall production is influenced by small variations in temperature, precipitation, and other climatic variables including VPD (and possibly solar radiation), as reported in previous studies [16,19,25,27,29,30].
Climate change models for the period 2010 to 2039 project a >2 • C increase in mean annual temperature, more intense and longer droughts, a slight decrease in annual precipitation, and more frequent and intense tropical storms and hurricanes for Mexico under the A2 emissions scenario, which assumes large regional differences in economic and population growth [102]. Based on the relationships between litterfall production and climatic variables considered in our study, under this climate change scenario, an increase in both the proportion of deciduous species and litterfall production would be expected in the TDF that we examined, but not necessarily in the rate of litter decomposition, due to the likely adverse effects of the altered environmental conditions on decomposers.
On the other hand, increased rates of forest disturbance due to logging, land use change, extractive activities, and forest fires are also projected [103][104][105][106]. This would lead to forest cover loss, especially of older successional forests (which are commonly targeted by human activities), and an increase in the proportion of young secondary or disturbed forests, all of which would lead to lower litterfall production and decomposition-likely overriding potential climate-change-driven increases in litterfall production. Given these scenarios, there is an urgent need to document the temporal changes in the various components of primary productivity and in the biogeochemical cycles related to litterfall production and decomposition, as well as their relationships with climatic variables and environmental conditions.

Conclusions
As we expected, successional age, seasonality, and topography (slope) are drivers of litterfall production. This production was higher in the dry season, increased with successional age (recovering in just 18-25 years), and was positively correlated with the aboveground biomass of deciduous species and negatively with the basal area of evergreen species. This suggests that the nutrients contained in litter are recycled rapidly in this landscape, which consists of a matrix of TDF interspersed with low-intensity land uses.
Litterfall production over the entire study period was higher on flat versus sloping sites, as environmental conditions in the latter are less favorable for vegetation development. Protecting and conserving forests on sloping areas should be prioritized in order to reduce erosion and soil degradation.
The temporal patterns of litterfall production were related to several climatic variables including precipitation (negative correlation), maximum temperature, wind speed, and VPD (positive correlations). This suggests that litterfall production (but not necessarily its decomposition rate) might increase with climate change. However, land use changes would reduce litterfall production and decomposition, impairing the capability of TDF to sequester and store carbon and compromising the biogeochemical cycles that regulate the long-term sustainability of these ecosystems and the services they provide to human societies. Further multi-year studies such as the one reported here are necessary to broaden our understanding of the dynamics and functioning of tropical dry forests and to inform the design of more effective strategies for their conservation, restoration, and sustainable management. Funding: This research was funded by Norwegian government, CONAFOR, UNPD, FAO; Project: "Fortalecimiento de la preparación REDD+ en México y fomento de la cooperación Sur-Sur". y Tecnología-Mexico for the scholarship to pursue his doctoral studies. We thank the local community of Xkobenhaltun for kindly granting access to their forested land, Fernando Tun Dzul for his help with Figure 1, and Antonio Pool Chan and Cristina Moreno for their assistance with field work. María Elena Sánchez-Salazar translated a first draft of the manuscript into English.

Conflicts of Interest:
The authors declare no conflict of interest. The funders participated in the design of the study; but not in the collection, analysis, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
Appendix A Table A1. Characteristics of the sampling clusters in which litterfall production was monitored. The effect of slope was evaluated only in age classes IV and V, using the average slope of the three plots of each cluster that fell in the predominant topographic position in the cluster (*).

Cluster
Age Appendix B Table A2. List of woody plant species sampled in fifteen 1-ha sampling clusters of four 0.04-ha (400 m 2 ) plots.