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Article

Effects of Tree Functional Traits on Soil Respiration in Tropical Forest Plantations

by
Natthapong Ontong
1,
Roongreang Poolsiri
2,*,
Sapit Diloksumpun
2,
Duriya Staporn
3 and
Michael Jenke
2
1
Graduate School, Kasetsart University, Bangkok 10900, Thailand
2
Department of Silviculture, Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand
3
Forest Research and Development Bureau, Royal Forest Department, Chatuchak, Bangkok 10900, Thailand
*
Author to whom correspondence should be addressed.
Forests 2023, 14(4), 715; https://doi.org/10.3390/f14040715
Submission received: 12 March 2023 / Revised: 28 March 2023 / Accepted: 29 March 2023 / Published: 31 March 2023
(This article belongs to the Special Issue Management and Restoration of Post-disturbance Forests)
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Abstract

:
Fast-growing tree species, including Eucalyptus sp. and Acacia sp., are widely used to rehabilitate degraded tropical forestland quickly, while mitigating climate change. However, the extent of carbon losses through soil respiration (RS) often remains unknown. Moreover, the promotion of these non-native species has raised concerns over their impact on other ecosystem services, including N2-fixation-induced soil acidification and nutrient cycling. This study compared two non-native and native species, with one of each being N2-fixing, growing in 11-year-old monospecific plantations in NE Thailand. Hourly RS was measured monthly over one year and combined with stand characteristics, as well as soil microclimatic and chemical properties. Mixed-effects models were used to capture this hierarchical, diurnal, and seasonal dataset. RS rates were influenced by soil temperature and moisture following a parabolic relation, and negatively affected by acidity. Overall, RS varied significantly according to species-specific microclimates and productivity. Despite the high input of organic matter, non-native species failed to ameliorate extreme soil moisture or temperature; limiting microbial decomposition and reducing RS. Hopea odorata produced moderate levels of carbon sequestration, but maintained soil fertility. The choice of tree species can significantly affect carbon sequestration and storage, as well as nutrient cycling, and careful species selection could optimize these ecosystem services.

1. Introduction

The prevalence of forest degradation and deforestation in many tropical areas has led to the establishment of monospecific forest plantations [1]. More recently, the establishment of plantations has also been promoted in an effort to remove carbon dioxide from the atmosphere [2,3]. However, these forests have the potential to act both as a carbon sink, by sequestering carbon through photosynthesis, and as a carbon source, by releasing carbon through respiration and disturbance [4]. The selection of tree species and their corresponding functional traits has a significant impact on the potential of a forest to function as either a carbon sink or source [5]. Non-native tree species are often favored for their fast growth, canopy closure, and suppression of herbaceous weeds, but they may unintentionally alter soil nutrient cycling and water availability [6]. Similarly, leguminous tree species may increase soil fertility and productivity, but could also increase soil acidification and alter soil microbial communities [7,8]. Moreover, seasonal variations in soil temperature and moisture can have a distinct impact on soil respiration [9,10], particularly in monsoon climates [11,12,13,14,15,16,17]. Therefore, understanding the impact of both species selection and seasonal variation on soil respiration in tropical plantations is necessary to predict the impact on the carbon cycle and to develop effective strategies to mitigate the effects of climate change. However, the effects of non-native and leguminous tree species on soil respiration in tropical forest plantations are not yet well understood.
Species-specific growth rates, biomass partitioning, and tissue chemistry can significantly impact carbon balances and carbon sequestration in temperate [18] and moist tropical forests [19]. Specifically, soil respiration rates can correlate positively with litterfall rates and the associated input of organic matter as the activity of soil microorganisms increases, with more energy and nutrients [20]. Moreover, younger stands tend to exhibit higher growth rates, resulting in higher soil respiration compared to that of older stands [21]. In addition to an addition of organic matter at the soil surface, tree roots can also release exudates belowground, stimulating microbial activity [22]. However, the amount and effect of root exudates is strongly species-dependent [23].
In addition to the amount of litter added to soil, its quality also affects soil respiration. Litter with a higher nutrient content and a lower C/N ratio is more easily decomposed, leading to higher soil respiration rates [24]. Thus, soil respiration rates may be higher under leguminous tree species, which can increase soil nitrogen availability through their symbiotic relationship with N2-fixing bacteria [25]. However, N2-fixation can also drive soil acidification by taking up more cations than anions, causing the release of hydrogen ions and thus, a decrease in soil pH levels [26,27]. As many degraded forest areas in the tropics are already acidic, a further reduction could induce nutrient deficiencies and Al toxicity [28]. Acidification would not only reduce tree growth but also reduce microbial activity, decomposition rates, and consequently, soil respiration [29]. Thus, the afforestation of degraded land in the tropics using leguminous trees, particularly in monoculture, should be well-considered, given these potential downsides [30].
Besides biotic and chemical factors, microclimatic conditions affect soil respiration rates as well, with soil temperature and moisture being the key variables [31]. Respiration tends to increase with rising temperatures and moisture [20]. However, both relationships are non-linear, with respiration rates increasing rapidly alongside increasing temperature and moisture up to a certain threshold, beyond which respiration may decrease [32]. High moisture can limit the availability of oxygen in the soil, which can inhibit the activity of aerobic soil microorganisms and reduce the rate of soil carbon respiration. Both microclimatic conditions can be moderated by forest cover, depending on the characteristics of the tree species. Tree species with a denser crown, i.e., a higher LAI, provide more shade, reducing the amount of solar radiation reaching the forest floor and thereby, reducing evaporation. However, denser canopies might also intercept more rainfall. Depending on their root systems, tree species can either facilitate water retention or promote drainage [33]. Tree species with deep-reaching tap-roots may access deeper soil layers and reduce the reduction of soil moisture during the dry season [34]. Consequently, different tree species can create unique microclimates, depending on their canopy and root structure.
Forest restoration efforts in Southeast Asia, particularly in Thailand, have been underway for several decades. These efforts have focused on restoring degraded forestlands. Forest restoration in Thailand has been guided by the National Forest Policy, which emphasizes the importance of maintaining the country’s forest resources to support sustainable development. Consequently, the Thai government has established various programs and initiatives, including reforestation and afforestation programs, as well as forest protection measures. In Thailand, forest restoration efforts have typically involved the planting of both native and non-native tree species [35]. Some of the most commonly planted native species include Tectona grandis, Dalbergia cochinchinensis, Hopea odorata, and Xylia xylocarpa [36]. These species are typically slow-growing, but can help to restore degraded forest ecosystems. Non-native species, including Eucalyptus spp., Acacia spp., Leucaena leucocephala, and Casuarina equisetifolia, have also been widely used in restoration efforts due to their fast growth rates and economic value [37]. However, the use of non-native species has been a subject of debate, as some have argued that these species can have negative impacts on biodiversity and ecosystem function.
The aim of this study was to investigate the impact of species selection (native/non-native and leguminous/non-leguminous) on soil respiration rates in tropical plantations. The first objective was to assess the seasonal variation of soil respiration rates in four monospecific plantations established 11 years prior. The second objective was to compare physical and chemical soil properties, as well as stand characteristics. Four different tree species with contrasting functional traits were studied, namely Acacia auriculiformis A. Cunn. ex Benth, Eucalyptus urophylla Dehnh., Hopea odorata Roxb., and X. xylocarpa (Roxb.) W.Theob. var. kerrii (Craib & Hutch.) I.C.Nielsen. A. auriculiformis is a fast-growing and N2-fixing tree not native to Thailand, which forms a dense canopy and a shallow but dense root system [38]. E. urophylla is, similarly, fast-growing and exotic, but not nitrogen-fixing and with a deeper-reaching root system [39]. H. odorata is commonly found in the dry evergreen forest ecosystems of Thailand, where they develop a wide-spreading crown [40]. In contrast, the N2-fixing tree species X. xylocarpa is found in dry deciduous forest ecosystems, where it develops a deep tap-root [41]. Among the four tree species studied, X. xylocarpa is the only deciduous species shedding its leaves in response to the onset of the dry NE-monsoon season (pers. observation). Both non-native species are commonly used for pulpwood and paper production, while the native examples are valuable hardwood trees used for construction and furniture-making.

2. Materials and Methods

2.1. Study Site

This study was conducted at the Silvicultural Research Center 6, operated by the Thai Royal Forest Department, in Nakhon Ratchasima Province, northeastern Thailand (14°30′22″ N, 101°54′16″ E). The mean annual air temperature was 25.62 °C, and the mean annual rainfall was 1365.34 mm, according to meteorological data collected at the station over the past decade (2003–2012). This area has a monsoon climate, with highly seasonal rainfall and a 6-month-long dry period, lasting from November until April (Figure 1), culminating in a severe vapor pressure deficit (Figure A1). The soil is deep loamy Acrisol (A-E-Bt-C), formed on sandstone laid down in the Triassic to Cretaceous periods [42], and it generally contains only small amounts of organic matter [43]. The area around the study site had been covered with dry evergreen forest until the 1960s. The forest was then converted to farmland. A reforestation project by the Japan International Cooperation Agency (JICA) and the Royal Forest Department (RFD) was launched in 1982, and by 1994, over 2300 ha had been planted with exotic fast-growing tree species [44,45]. The experimental plantations investigated in this study were established in 2004, with an initial spacing of 2 m × 4 m in adjacent areas of 0.5–0.8 ha.

2.2. Data Collection

2.2.1. Forest Inventory

In June 2015, one sample plot (50 m × 50 m) was set up in each plantation stand. Position, diameter at breast height, and height of all trees within each sample plot were subsequently measured. Based on these inventories, basal area, quadratic mean DBH, the mean annual DBH increment, and the top height (mean height of 5% trees with largest DBH) were determined.

2.2.2. Soil Respiration

Soil respiration was sampled at 16 locations within each forest plot between tree rows to capture the spatial heterogeneity within the plot (Figure 2). At each location, soil respiration was measured once every month from July 2015 until June 2016. These measurements were taken hourly from 07.00–17.00 at the soil surface using a commercial respiration chamber (SRC-1 Soil Respiration Chamber, PP systems; Amesbury, MA, USA) and an infrared gas analyzer system (EGM-4 Environmental Gas Analyzer for CO2, PP systems). The respiration chamber was mounted on a PVC collar (10 cm in diameter, 5 cm high), which was carefully inserted 3 cm into the soil 1 month before the first measurement [46].

2.2.3. Climate Data

Soil temperature was measured around the collar at a 5 cm depth using a drip-proof type digital thermometer (Model STP-1, PP systems) attached to the EGM-4. Soil moisture was measured simultaneously at a 5 cm soil depth using a moisture sensor (Theta proof type MLL; Delta T Devices, series HH 150, Cambridge, UK). Air temperature was measured using a 4-in-1 multifunctional environmental meter, series LM-8000, REED, at the same time as the soil respiration measurements were obtained. Daily rainfall data for the study period were obtained from the Silvicultural Research Center 6 (Nakhon Ratchasima) from July 2015 to June 2016.

2.2.4. Soil Sampling and Analysis

Using a split tube soil sampler, five undisturbed soil samples were collected at random locations within each sample plot from the topsoil layer (0–5 cm) monthly after removing the litter layer. Samples were collected from the topsoil layer, as it is the most biologically active and dynamic layer in the soil profile. Bulk density (BD) was determined using the core method [47]. Samples were subsequently dried at room temperature for approximately 48 to 72 h. Roots and contaminants were removed from the sample and sieved through a 0.5 and 2 mm mesh screen. Soil pH was measured using a pH meter at a ratio of water and soil of 1:1 [48]. Soil organic matter (SOM) was analyzed using Walkley and Black’s rapid titration method [49]. Total carbon (C) and total nitrogen (N) were measured using the Dumas method [50] with a CHNS analyzer (Perkin Elmer 2400, series II CHNS/O Elemental Analyzer). Soil organic carbon stock was calculated from SOC = %C × BD × Depth, where SOC was soil organic carbon storage (Mg ha−1), %C was carbon content in the soil (%), BD was soil bulk density (g cm−3), and depth was soil depth (cm), here 5 cm.

2.3. Data Analyses

The Kruskal–Wallis test was used to test for differences in the median values of six soil chemical properties, specifically soil organic carbon stock (SOC), soil organic matter (SOM), pH, bulk density (BD), C/N ratio, and total nitrogen, among different tree species in the forest plantations. Dunn’s test for pairwise multiple comparisons was used as a post-hoc analysis to compare all possible pairwise differences between medians. Non-parametric tests were used, as soil properties were not normally distributed, according to the Shapiro–Wilk normality test.
The species-specific differences in soil moisture and temperature were estimated using a mixed-effects model. The measurement point within each plantation (n = 12), month (n = 12), and hour (n = 11) were included as random effects. The model was fitted using maximum likelihood estimation, and the significance of the fixed effects was assessed using the likelihood ratio test or the Wald test. The model also included a variance-covariance matrix to account for the correlation between repeated measurements within the same plantation.
Soil respiration is often interactively affected by multiple environmental factors, making it difficult to understand their individual relevance [51]. Consequently, a second mixed-effects model was used to analyze the effects of tree species, soil temperature, soil moisture, pH, and basal area on hourly soil respiration rates, while accounting for within-plot variation. For each of the 16 soil respiration measurement locations, the combined basal area (m2 m−2) of all trees within a 5 m radius around the measurement point was included as an indicator of forest productivity [5]. In addition to these fixed factors, measurement location (n = 12), month (n = 12), and hour (n = 11) were considered as random factors in the mixed-effects model. These random factors account for the within-plot variability in soil respiration rates, as well as the temporal variability due to seasonal changes and diurnal cycles. This more comprehensive approach helps to ensure that the analysis captures the complex and dynamic nature of soil respiration rates.
All analyses were performed in R [52] using the packages rstatix [53], lme4 [54], and ggeffects [55].

3. Results

3.1. Comparison of Stand Characteristics, Soil Chemical, and Microclimatic Conditions

The non-native A. auriculiformis and E. urophylla grew faster than the native species, as indicated by the larger basal area and the higher mean annual diameter increment. In contrast, the growth rates and stem sizes of X. xylocarpa were smaller, indicating lower productivity. The stem maps show empty rows or patches in all stands except for A. auriculiformis, indicating that the reduction in stand density from the initial 1250 seedlings ha−1 was mainly driven by seedling mortality and less by self-thinning processes (Table 1).
Kruskal–Wallis tests and Dunn’s tests were used to compare the median soil chemical conditions between tree species. A distinct difference between the soil properties of native and non-native species could only be found when looking at the soil organic carbon stocks in the upper soil layer (p < 0.05; Figure 3A). It was significantly higher in non-native A. auriculiformis and E. urophylla stands (9.39 and 9.38 MgC ha−1, respectively) compared to H. odorata and X. xylocarpa (8.46 and 7.42 MgC ha−1, respectively). Similarly, soil organic matter was significantly higher in A. auriculiformis (2.22%) compared to the native species (p < 0.05; Figure 3B). In contrast, the median pH values differed significantly between leguminous and non-leguminous species (p < 0.05; Figure 3E). It was significantly lower in the leguminous A. auriculiformis and X. xylocarpa stands (3.92 and 3.99, respectively) compared to H. odorata and E. urophylla (4.1 and 4.06, respectively), which had a higher soil pH. Although total nitrogen content was the highest in A. auriculiformis (0.16%), it was, unexpectedly, significantly lower in X. xylocarpa stands (0.14%) compared to all other species (Figure 3F). Nevertheless, the C/N ratio was still the lowest in the X. xylocarpa stands (8.69) and significantly higher in A. auriculiformis (10.29), despite its N2-fixation (Figure 3C). Soil texture in the X. xylocarpa stand differed significantly from that of other species, containing approximately 5%–10% more sand and 2%–4% less silt than the soil samples from the other stands (p < 0.05, Table A1 and Figure A2). Despite the lower amounts of clay, the bulk density (Figure 3D) was significantly higher in the X. xylocarpa stand (1.28 g cm−3). However, the bulk density was the highest in the E. urophylla stand (1.31 g cm−3).
Soil respiration varied significantly between, but also within, the three seasons (Figure 4A). All species showed reduced soil temperatures relative to air temperature, especially during the pre-monsoon season, which is the hottest time of the year (Figure 4B). Cooling effects were significantly higher in A. auriculiformis and H. odorata stands (p < 0.05). In contrast, soil moisture levels were higher in E. urophylla and H. odorata (p < 0.05) stands, revealing a unique soil microclimate in each stand, depending on the dominant tree species (Figure 4C).

3.2. Effects of Tree Species, Soil Conditions, and Basal Area on Soil Respiration

Compared to the non-native tree species, soil respiration rates were significantly higher in the H. odorata stand, but significantly lower in the X. xylocarpa stand (p < 0.05; Figure 5A). However, respiration rates did not differ significantly between the non-native tree species, as indicated by a pairwise comparison of the estimated marginal means (p > 0.05). Soil respiration rates followed a parabolic relationship with both soil temperature and moisture, with the maxima at 29 °C (Figure 5B) and 9.5% (Figure 5C), respectively. It could be estimated that soil respiration ceased at a soil moisture of more than 27%. The increase in soil acidity was negatively associated with soil respiration (Figure 5D). The relationship of basal area and soil respiration varied in its direction across the different tree species, being significantly positive among the non-leguminous tree species (H. odorata and E. urophylla, p < 0.001), but negative in in the A. auriculiformis stand (Figure 5E, p < 0.001). However, the relation remained insignificant in the X. xylocarpa stand (p > 0.05).

4. Discussion

Tropical forest plantations are often established on degraded lands with the aim of improving soil fertility. However, tree species selection can have strong impacts on the biogeochemical cycling of carbon, nitrogen, and other elements. So far, most studies of soil respiration in Thailand’s forest ecosystems have focused on natural forests [11,12,14]. Thus, this study of four monospecific plantations may support forest managers in selecting tree species for increasing carbon sequestration and storage. Furthermore, it enhances our general understanding of how non-native species could be used to promote sustainable forest management in the region.
The species-specific comparative analysis shows that carbon cycling was strongly influenced by tree species selection. Non-native species, here A. auriculiformis and E. urophylla, showed higher productivity and soil organic carbon stock than did native species. This suggests that fast-growing non-native species have a higher soil carbon sequestration potential than the studied native species, confirming the results of earlier studies [56]. Among other factors, these species retained soil organic matter more effectively due to lower respiration losses. In contrast, the native tree species H. odorata is a more active carbon source in which the high respiration rates result in a slower buildup of soil organic carbon stock in the topsoil. The second native and N2-fixing species, X. xylocarpa, appeared to be ineffective in sequestering and storing carbon due to its low productivity. These differences can be explained by the species-specific effects on soil microclimate and chemical properties, in addition to related but unobserved factors, such as microbial activities.
These findings show that soil respiration rates varied significantly between and within the seasons of this monsoon climate, confirming the results of previous studies [57,58]. More importantly, our results indicate that the species-specific impact on soil microclimate moderated soil respiration rates. Both non-native species altered the soil microclimate in a manner that was detrimental to decomposition processes. Although A. auriculiformis effectively ameliorated extremely high temperatures through shading, it promoted drainage, resulting in lower soil moisture levels. E. urophylla stands, in contrast, showed higher soil moisture levels, but also higher soil temperatures. Higher bulk density might have been one factor driving water retention in this stand, reducing soil aeration as well. In contrast, H. odorata is the only species that ameliorated both soil temperature and moisture through its extensive canopy cover and root system, providing optimal conditions for microbial decomposition. X. xylocarpa was the least effective in providing shade due to its low canopy cover and leaf shedding.
Both leguminous stands showed signs of soil acidification, but did not enhance soil nitrogen compared to the non-leguminous H. odorata. The acidification is likely caused by the release of hydrogen ions during N2-fixation [26] and could limit nutrient availability for other plants in the future. This acidification also reduced soil respiration, contributing to the accumulation of soil carbon. The positive relationship between basal area and soil respiration among non-leguminous tree species suggests that these species may contribute to a more rapid soil carbon cycling than do leguminous species. Particularly, soils in the H. odorata stands had a higher substrate quality (lower C/N ratio) and soil alkalinity. These microclimatic and biochemical conditions promoted microbial decomposition, resulting in higher soil respiration rates, but also in potentially higher levels of nutrient cycling and availability. Thus, H. odorata offers a compromise of carbon sequestration potential, soil fertility improvement, and microclimatic amelioration.
Lastly, low litter quality can reduce decomposition and thus, lower respiration rates. However, these properties were not measured in this study, but are reported in the literature. Litter of Eucalyptus sp. tends to be decomposed slowly [59]. Moreover, Acacia sp. has been found to release allelopathic compounds, inhibiting soil microorganisms and limiting the availability of nutrients in the soil [60]. In the studied region, the litter decay constant of H. odorata was determined to be 1.14 and only 0.57 for X. xylocarpa, indicating higher decomposition rates for the former species [61]. One aspect not investigated is the potential for priming, which occurs when the addition of fresh organic matter to soil leads to an increase in microbial activity and thereby, soil respiration. It could be hypothesized that priming might be found particularly in X. xylocarpa stands, where the shedding of leaves with a high N-content could stimulate microbial activity and prevent the accumulation of organic matter, similar to the activities of other deciduous tree species [62]. However, further research would be necessary to confirm the presence of priming in X. xylocarpa stands.
This study has several limitations that could potentially impact the research findings. First, soil respiration was only measured over a single year, preventing an analysis of inter-annual variability, which can be substantial, as shown in a separate four-year long experiment [63]. Second, soil respiration was only measured during the day, and not at night, potentially leading to an underestimation of total soil respiration rates. Third, the relative importance of autotrophic and heterotrophic soil respiration could not be determined based on our results. Fourth, the full range of soil conditions could not be captured due to the lack of plot replications. Thus, these limitations suggest that the findings may not be generalizable to other forest ecosystems.
Mixed-species plantations should be further investigated as alternatives to monocultures in order to balance the potential benefits of carbon sequestration with the potential risks to soil nutrient cycling and other ecosystem services. Combining native and non-native species with varying functional traits could be a potential path to develop a sustainable management system that offers effective climate change mitigation while preventing soil degradation. Specifically, the introduction of native species, including H. odorata, into gaps within non-native monocultures offers a promising approach [64]. Furthermore, the selective admixture of leguminous species into stands of non-leguminous species might capture their potential beneficial effects on soil nitrogen, while limiting their acceleration of soil acidification, particularly on less productive sites [65,66,67]. Similar studies could be conducted at our study site as well. However, more information is needed about the effects of species combinations and planting densities on soil respiration, as well as other ecosystem functions, such as nutrient cycling and water use. Such studies could help inform the development of more sustainable and resilient tropical forest plantations, which are essential for achieving global climate and biodiversity goals.

5. Conclusions

This study investigated the effects of tree species selection on soil organic carbon storage, soil respiration rates, and the microclimatic conditions that influence biogeochemical cycling in tropical forest plantations. The comparative analysis revealed that non-native tree species exhibited higher productivity, and micro-climatically induced reduction of soil respiration resulted in a greater soil organic carbon stock compared to that of the native species. However, the native species H. odorata was particularly effective in creating an optimal microbial microclimate, enhancing nutrient availability. While non-native fast-growing species might be promoted for their ability to sequester more carbon and restore degraded forestland, it is important to consider the potential impacts on biodiversity, soil pH, and fertility, as well as the overall long-term sustainability of these plantations. Nevertheless, these findings should be considered with caution, as they are based only on a single year of soil respiration measurements, made during the daytime, and with a limited number of sample plots. Further research is needed to better understand the effects of leguminous tree species on soil acidification and nutrient cycling. The combination of native and non-native species in mixed stands should be investigated to achieve both carbon sequestration goals, while maintaining soil fertility.

Author Contributions

Conceptualization, R.P., S.D. and M.J.; methodology, R.P. and M.J.; formal analysis, M.J.; investigation, N.O. and D.S.; resources, S.D. and D.S.; data curation, N.O. and M.J.; writing—original draft preparation, N.O. and M.J.; writing—review and editing, M.J.; visualization, M.J.; supervision, R.P. and S.D.; project administration, R.P.; funding acquisition, R.P. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by a graduate research grant from the National Research Council of Thailand, 2016. M.J. was financially supported by the Office of the Ministry of Higher Education, Science, Research, and Innovation; as well as the Thailand Science Research and Innovation Center, through the Kasetsart University Reinventing University Program, 2022.

Data Availability Statement

The data presented in this study are openly available in the Zenodo repository at https://doi.org/10.5281/zenodo.7709997 (accessed on 9 March 2023).

Acknowledgments

We would like to thank the staff of the Silvicultural Research Center 6 (Nakhon Ratchasima) of the Royal Forest Department for permitting us to access the study site and for their tireless assistance during the data collection.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Appendix A

Forests 14 00715 g0a1 550
Figure A1. Monthly vapor pressure deficit between July 2015 and June 2016, calculated from measurements of air temperature and relative humidity above the soil surface as the difference between saturation vapor pressure and actual vapor pressure. The line indicates average values; the ribbon represents 95% CI.
Figure A1. Monthly vapor pressure deficit between July 2015 and June 2016, calculated from measurements of air temperature and relative humidity above the soil surface as the difference between saturation vapor pressure and actual vapor pressure. The line indicates average values; the ribbon represents 95% CI.
Forests 14 00715 g0a1
Forests 14 00715 g0a2 550
Figure A2. Distribution of soil texture in 11-year-old monospecific tree plantations in tropical forest soils. The graph displays the mean and standard deviation of soil texture components—sand, silt, and clay—across each species.
Figure A2. Distribution of soil texture in 11-year-old monospecific tree plantations in tropical forest soils. The graph displays the mean and standard deviation of soil texture components—sand, silt, and clay—across each species.
Forests 14 00715 g0a2
Table
Table A1. Mean and standard deviation of soil properties by type and depth in four plantation plots. Soil properties include physical properties (sand (%), silt (%), and clay (%)), chemical properties (carbon (%), nitrogen (%), pH, and soil organic matter (%)), and nutrient properties (exchangeable cations of calcium (mg kg−1), magnesium (mg kg−1), potassium (mg kg−1), and phosphorus (mg kg−1), which were determined following the methods described by [68,69,70]).
Table A1. Mean and standard deviation of soil properties by type and depth in four plantation plots. Soil properties include physical properties (sand (%), silt (%), and clay (%)), chemical properties (carbon (%), nitrogen (%), pH, and soil organic matter (%)), and nutrient properties (exchangeable cations of calcium (mg kg−1), magnesium (mg kg−1), potassium (mg kg−1), and phosphorus (mg kg−1), which were determined following the methods described by [68,69,70]).
Soil
Characteristic		Depth (cm)A. auriculiformisE. urophyllaH. odorataX. xylocarpa
Sand (%)0–1558.79 (1.15)59.63 (0.76)62.04 (1.5)68.54 (3.46)
15–3056.96 (0.5)57.63 (1.26)59.51 (2.58)64.04 (0.5)
Silt (%)0–1515.81 (0.58)16.31 (0.29)15.64 (0.87)12.14 (2.00)
15–3014.64 (0.5)14.31 (1.04)15.14 (0.87)12.31 (1.04)
Clay (%)0–1525.40 (1.01)24.07 (0.58)22.32 (1.73)19.32 (2.00)
15–3028.40 (0.01)28.07 (1.53)25.35 (3.44)23.65 (0.58)
Carbon (%)0–151.25 (0.12)1.14 (0.12)1.15 (0.22)0.86 (0.05)
15–300.68 (0.09)0.71 (0.14)0.8 (0.24)0.59 (0.06)
Nitrogen (%)0–150.16 (0.03)0.14 (0.03)0.14 (0.01)0.13 (0.01)
15–300.10 (0.03)0.10 (0.02)0.11 (0.02)0.09 (0.01)
pH0–153.62 (0.03)3.87 (0.34)3.85 (0.14)3.73 (0.07)
15–303.90 (0.14)4.06 (0.19)3.97 (0.11)3.89 (0.07)
SOM (%)0–151.73 (0.39)1.61 (0.14)1.83 (0.49)1.37 (0.35)
15–301.12 (0.05)1.14 (0.24)1.13 (0.35)0.85 (0.28)
Exch. Ca2+
(mg kg−1)		0–1559.39 (12.7)177.67 (137.64)131.49 (82.58)90.46 (41.21)
15–3017.46 (0.72)139.16 (104.98)87.74 (10.8)23.12 (4.52)
Exch. Mg2+
(mg kg−1)		0–1524.34 (10.03)39.15 (27.75)35.48 (14.17)21.39 (10.02)
15–3014.56 (2.13)29.03 (21.46)22.19 (6.27)7.94 (3.58)
Exch. K+
(mg kg−1)		0–1542.55 (9.62)60.66 (35.25)38.47 (6.02)46.45 (20.89)
15–3020.67 (6.04)29.27 (7.27)18.87 (4.01)16.62 (3.26)
P (mg kg−1)0–154.81 (1.97)5.22 (1.37)5.09 (0.56)6.56 (1.80)
15–302.29 (0.46)2.65 (0.96)2.85 (1.13)2.95 (0.57)
Table
Table A2. Estimates, 95% confidence intervals (CI), and p-values of fixed-effects predictors for soil respiration, including tree species, soil temperature (°C), soil moisture (%), soil pH, and basal area (BA; cm2 m−2), based on a mixed-effects model. The overall variance (σ2) and intraclass correlation coefficient (ICC) are reported for the random effects, including hour, month, and measurement location.
Table A2. Estimates, 95% confidence intervals (CI), and p-values of fixed-effects predictors for soil respiration, including tree species, soil temperature (°C), soil moisture (%), soil pH, and basal area (BA; cm2 m−2), based on a mixed-effects model. The overall variance (σ2) and intraclass correlation coefficient (ICC) are reported for the random effects, including hour, month, and measurement location.
PredictorsEstimatesCIp
Intercept0.780.62–0.94<0.001
Species [E. urophylla]−0.10−0.12–0.09<0.001
Species [H. odorata]−0.12−0.15–0.10<0.001
Species [X. xylocarpa]−0.07−0.09–0.06<0.001
Temperature [1st degree]0.950.43–1.46<0.001
Temperature [2nd degree]−0.65−0.97–0.32<0.001
Moisture [1st degree]−3.14−3.63–2.64<0.001
Moisture [2nd degree]−4.40−4.71–4.09<0.001
pH−0.10−0.15–0.06<0.001
Species [A. auriculiformis] × BA−0.00−0.00–0.00<0.001
Species [E. urophylla] × BA0.010.01–0.01<0.001
Species [H. odorata] × BA0.020.02–0.02<0.001
Species [X. xylocarpa] × BA−0.00−0.00–0.000.081
Random Effects
σ20.01τ00 point0.00
τ00 h:month0.01ICC0.38
Marginal R2/Conditional R20.296/0.565

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Forests 14 00715 g001 550
Figure 1. Climate chart of the study site based on measurements between January 2015 and December 2016. Darker bars and points correspond to times of soil respiration measurement.
Figure 1. Climate chart of the study site based on measurements between January 2015 and December 2016. Darker bars and points correspond to times of soil respiration measurement.
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Figure 2. Distribution of soil respiration measurements within each sample plot. The tree stem position is shown with green dots, which are scaled according to their DBH. Black squares indicate the location of soil respiration measurements. Grey circles with a 5 m radius highlight trees considered in the subsequent analysis.
Figure 2. Distribution of soil respiration measurements within each sample plot. The tree stem position is shown with green dots, which are scaled according to their DBH. Black squares indicate the location of soil respiration measurements. Grey circles with a 5 m radius highlight trees considered in the subsequent analysis.
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Figure 3. Comparison of soil conditions across the four tree species: (A) carbon stock, (B) soil organic matter (SOM), (C) C/N-ratio, (D) bulk density, (E) pH, and (F) total nitrogen. The dot represents the median value, the vertical lines represent the interquartile range (IQR), and the widths of the violin plots indicate the density of observations at each level. Letters represent significant differences between medians, as determined by Dunn’s test (p < 0.05).
Figure 3. Comparison of soil conditions across the four tree species: (A) carbon stock, (B) soil organic matter (SOM), (C) C/N-ratio, (D) bulk density, (E) pH, and (F) total nitrogen. The dot represents the median value, the vertical lines represent the interquartile range (IQR), and the widths of the violin plots indicate the density of observations at each level. Letters represent significant differences between medians, as determined by Dunn’s test (p < 0.05).
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Figure 4. (A) Seasonal variation of hourly soil respiration in four monospecific plantation stands measured during the daytime (mean ± 95% CI). The rug plot shows the distribution of days with rainfall over the study period indicating the frequency of precipitation events in the study area. Seasons are indicated above. Seasonal differences between tree species and cooling (B) as well as soil moisture (C) based on mixed-effects models covering the 1 year sampling period. Letters represent significant differences between means, as determined by Tukey’s HSD test (p < 0.05).
Figure 4. (A) Seasonal variation of hourly soil respiration in four monospecific plantation stands measured during the daytime (mean ± 95% CI). The rug plot shows the distribution of days with rainfall over the study period indicating the frequency of precipitation events in the study area. Seasons are indicated above. Seasonal differences between tree species and cooling (B) as well as soil moisture (C) based on mixed-effects models covering the 1 year sampling period. Letters represent significant differences between means, as determined by Tukey’s HSD test (p < 0.05).
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Figure 5. Estimated relationship between hourly rates of soil respiration (RS) during daytime and the marginal effects of the mixed-effects model, specifically tree species (A), soil climatic conditions (B,C), soil pH (D), and local basal area (E). The solid line represents the fitted curve, while the shaded area represents the 95% confidence interval. Significance stars represent p-value (*** p < 0.001; n.s. p > 0.05). See Table A2 for a tabular description of the model estimates.
Figure 5. Estimated relationship between hourly rates of soil respiration (RS) during daytime and the marginal effects of the mixed-effects model, specifically tree species (A), soil climatic conditions (B,C), soil pH (D), and local basal area (E). The solid line represents the fitted curve, while the shaded area represents the 95% confidence interval. Significance stars represent p-value (*** p < 0.001; n.s. p > 0.05). See Table A2 for a tabular description of the model estimates.
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Table
Table 1. Stand characteristics of each 11-year-old plantation, including stand density, basal area (BA), quadratic mean diameter at breast height (QMD), mean annual increment of stem diameter (MAIDBH), and top height (HDom).
Table 1. Stand characteristics of each 11-year-old plantation, including stand density, basal area (BA), quadratic mean diameter at breast height (QMD), mean annual increment of stem diameter (MAIDBH), and top height (HDom).
Tree SpeciesDensity (Trees ha−1)BA (m2 ha−1)QMD (cm)MAIDBH (cm yr−1)HDom (m)
A. auriculiformis101213.813.21.218.6
E. urophylla81211.613.51.2321.8
H. odorata8126.810.30.9411.2
X. xylocarpa9647.19.70.8812.4
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Ontong, N.; Poolsiri, R.; Diloksumpun, S.; Staporn, D.; Jenke, M. Effects of Tree Functional Traits on Soil Respiration in Tropical Forest Plantations. Forests 2023, 14, 715. https://doi.org/10.3390/f14040715

AMA Style

Ontong N, Poolsiri R, Diloksumpun S, Staporn D, Jenke M. Effects of Tree Functional Traits on Soil Respiration in Tropical Forest Plantations. Forests. 2023; 14(4):715. https://doi.org/10.3390/f14040715

Chicago/Turabian Style

Ontong, Natthapong, Roongreang Poolsiri, Sapit Diloksumpun, Duriya Staporn, and Michael Jenke. 2023. "Effects of Tree Functional Traits on Soil Respiration in Tropical Forest Plantations" Forests 14, no. 4: 715. https://doi.org/10.3390/f14040715

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