Short-Term Changes in the Soil Respiration of Casuarina equisetifolia L. Plantations After Severe Typhoon Disturbance

Abstract

Typhoon disturbances significantly influence forest carbon cycling by altering both physical structures and biogeochemical processes. Typhoon-induced fluctuations in soil respiration can substantially affect the carbon balance in forest ecosystems. In this study, we conducted a comparative investigation of soil respiration in plantations of Casuarina equisetifolia L. that were either affected or unaffected by the severe Typhoon Yagi, which ravaged Hainan Island, China, in 2024. The soil respiration and its components in Casuarina equisetifolia L. plantations in the coastal areas of Hainan, China, as well as their responses to environmental factors before and after typhoon disturbance, were investigated based on total soil respiration rate (R_s), heterotrophic respiration rate (R_h), 5 cm soil temperature (T₅), and 10 cm soil moisture (W₁₀) to support the carbon emission estimation in coastal sandy land plantations. The mean R_s and R_h in the typhoon-disturbed plots were (1.82 ± 0.16) and (1.19 ± 0.26) μmol·m⁻²·s⁻¹, respectively, while those in the control plots were (2.62 ± 1.08) and (1.41 ± 0.23) μmol·m⁻²·s⁻¹, respectively, with statistically significant differences (p < 0.05). In both plots, R_s exhibited a significant positive correlation with T₅ (p < 0.01). The T₅ correlation and Q₁₀ values for soil respiration were significantly higher in the typhoon-disturbed plots than in the control plots (p < 0.05). W₁₀ of the soil exhibited significant negative correlations with R_s and R_h in typhoon disturbance plots (p < 0.05). Consequently, typhoon disturbance markedly inhibited soil respiration and its components in the Casuarina equisetifolia L. plantations, indicating substantial impacts of typhoons on soil respiration processes and carbon cycling within the forest ecosystem. This study provides key parameters and empirical evidence to improve the accuracy of soil carbon emission estimates in Casuarina equisetifolia L. plantations on coastal sandy soils affected by typhoon events.

1. Introduction

In recent years, global climate change has markedly influenced the carbon cycling within ecosystems. The dynamics of soil carbon sinks have received increasing attention as key sensitivity indicators of terrestrial ecosystem responses to climate change [1,2,3]. Since the Industrial Revolution, fossil fuel combustion has elevated atmospheric CO₂ concentrations to over 415 ppm [4], and climate models project a global temperature rise of 1.5–4.4 °C by the end of the 21st century [5]. Consequently, the stability of soil carbon sinks, comprising approximately two-thirds of the total terrestrial carbon sinks, is facing growing threats. Soil respiration, which represents the flux of CO₂ released from soil organic carbon decomposition, is the second-largest carbon flux in terrestrial ecosystems following plant photosynthesis [6]. Globally, soil respiration accounts for approximately 80–98 Pg·C·yr⁻¹, nearly tenfold higher than fossil fuel emissions, indicating that even minor fluctuations could substantially affect the atmospheric carbon balance [7].

Forest soil respiration is a key process governing material exchange among the atmosphere, the biosphere, and the lithosphere. Its dynamic fluctuations affect the carbon cycle balance, energy flow, and chemical element migration within the Earth’s surface system, making it an important entry point for understanding inter-sphere interactions in the Earth system. As the largest carbon sink in terrestrial ecosystems, forest ecosystems play a pivotal role in global carbon balance. Soil respiration in forests comprises autotrophic respiration (R_a), originating from root metabolic activity, and heterotrophic respiration (R_h), generated by the microbial decomposition of organic matter. The dynamic equilibrium between these components directly governs the forest’s carbon sink capacity [8]. Low-latitude forests store approximately one-third to one-half of the global vegetation carbon (~200–500 Gt·C) and exhibit 30%–50% greater sensitivity in soil respiration to climate change than temperate forests [9]. However, previous research has primarily concentrated on subtropical forests, while the mechanisms regulating soil respiration in tropical forests, particularly those frequently affected by extreme disturbances, such as typhoons, remain inadequately understood.

Typhoon disturbances exert substantial effects on forest carbon cycling through the combined effects of physical disruption and biogeochemical processes. First, the mechanical damage directly caused by typhoons can increase the forest canopy openness by 50%–80%, resulting in a 2- to 3-fold increase in the surface solar radiation and expanding diurnal soil temperature fluctuations by 1.5–2 °C [10]. Second, post-typhoon litterfall inputs may surge to 3–5 times the normal annual level, significantly altering soil substrate availability and enhancing soil respiration by 30%–60% within 3–6 months [11]. Third, typhoons can modify local microclimatic conditions by bringing precipitation, which decreases soil temperature while increasing soil moisture, thereby affecting organic matter decomposition rates [12]. Although the short-term soil CO₂ flux may rise by 20%–30% following typhoon events [12,13], soil respiration responses remain highly heterogeneous, and the influence of forest type and typhoon intensity on CO₂ flux remains uncertain. Moreover, the differential responses of R_a and R_h to typhoon disturbances remain inadequately explored.

This study focused on the Casuarina equisetifolia L. coastal shelter forest in northern Hainan, a region experiencing an annual average of 2.6 typhoons between 1995 and 2020. Notably, Super Typhoon Rammasun (central wind speed: 60 m/s) in 2014 and Typhoon Yagi (58 m/s) in September 2024 severely damaged approximately 3.5 × 10⁶ hm² of forest. Because of the limited availability of dynamic soil respiration data following typhoon disturbances in this region, we hypothesized that typhoon disturbance would affect soil respiration and its components and alter the regulatory roles of environmental factors, ultimately increasing the temperature sensitivity of carbon release. To test this hypothesis, this study aimed to address three key issues: (1) accurate quantification of the temporal variation in soil respiration during the growing season after typhoon disturbance, (2) analysis of the effects of typhoon disturbance on the distinct components of soil respiration, and (3) identification of the principal environmental factors regulating soil respiration under typhoon influence. These findings are expected to clarify the role of typhoon disturbance in soil carbon cycling within Casuarina equisetifolia L. plantations and provide a mechanistic basis for model-based carbon sink accounting for coastal ecosystems.

2. Experiment

2.1. Study Area

The study area is situated in Wenchang City, northeastern Hainan Island (N: 19°52′–20°03′; E: 110°57′–111°01′), within a tropical monsoon climate zone. The region has an average annual temperature of approximately 23.9 °C, with minimum temperatures rarely falling below 10 °C and extreme lows ranging from 0.3 to 6.6 °C. The annual precipitation averaged 1721.6 mm, with a pronounced seasonal distribution. The rainy season spans from May to November, accounting for 80% of the total annual precipitation, and includes the typhoon season (August–November). The dry season extends from December to April, contributing only 20% of the annual precipitation. This area is frequently affected by typhoons, averaging 2.6 occurrences per year. The average elevation is 30 m, and the soil is classified as coastal sandy soil. The dominant tree species are Casuarina equisetifolia L. and Cocos nucifera L.

2.2. Plot Arrangement

This study examined Casuarina equisetifolia L. plantations following Typhoon Yagi in September 2024. Three fixed plots were randomly established in typhoon-disturbed areas with uniform soil conditions, each separated by over 30 m. In addition, three undisturbed control plots were established in the adjacent areas (Figure 1). Both the disturbed and control plots measured 20 m × 30 m, resulting in a total of six plots. The vegetation characteristics of the control and disturbance plots are summarized in

Table 1. Vegetation in the control plot and typhoon disturbance plot.

Treatment	Altitude (m)	Soil Type	Stand Age (Year)	Diameter at Breast Height (cm)	Tree Height (m)	Crown Density	Planting Density (Plant/ha)	Stand Density (Plant/ha)	Understory Vegetation
Control plot	33	Coastal sandy soil	10	13.46 ± 0.76	10.40 ± 0.88	0.87	1475	1401	Bidens pilosa L. Waltheria indica L. Cyperus rotundus L. Spermacoce pusilla Wallich.
Typhoon disturbance plot	30	Coastal sandy soil	10	14.17 ± 0.85	10.85 ± 0.71	0.90	1370	135	Bidens pilosa L. Cyperusrotundus L.

.

2.3. Methods

2.3.1. Soil Respiration Measurement

The soil CO₂ flux was measured using an automated soil respiration flux system (LI-8100A, LI-COR Inc., Lincoln, NE, USA) via the dynamic closed chamber method. The built-in infrared gas analyzer (IRGA) had a precision of ±1.5% of the reading, with a 30 s sampling interval. The measurements were recorded continuously until the CO₂ concentration rate stabilized, typically within 3–5 min. In December 2023, three 20 m × 10 m monitoring zones were established within each plot. Each zone was randomly equipped with three PVC soil respiration rings (inner diameter: 20 cm; height: 12 cm), resulting in nine rings per plot. Total soil respiration rate (R_s) was measured directly under natural conditions. R_h was determined using the trenching method [14,15,16], wherein four 1 m × 1 m isolation quadrats were established 1–2 m outside each plot in the four cardinal directions. The trenches were dug to a depth of 60–80 cm (until no fine roots were visible), and a double-layer polyethylene film (0.2 mm thick) was installed along the trench walls before backfilling with the original soil to restore the bulk density. All live vegetation within the quadrats was removed, and the PVC rings were installed using the same procedure as for the R_s measurements. R_a was calculated as R_a = R_s − R_h, and the root contribution rate to soil respiration (R_c) was expressed as R_c = R_a/R_s × 100%. Soil respiration was measured between 9:00 and 11:00 on clear days during the early and late parts of each month from September 2024 to March 2025 using trenched quadrats that were maintained free of live plants. Moreover, the built-in temperature probe and moisture sensor were used to monitor 5 cm soil temperature (T₅) and soil moisture of the 0–10 cm soil layer (W₁₀).

2.3.2. Statistical Analysis

Data processing and analysis were conducted using Excel 2010 and SPSS 18.0. One-way ANOVA was applied to verify the differences in the group means, with Origin 2021 employed for graphical visualization. The regression model describing the relationship between soil respiration and T₅ was established as follows [17,18,19]:

R_s = α × e ^β×T,

(1)

where R_s represents the soil respiration rate (μmol·CO₂·m⁻²·s⁻¹); T represents the T₅ (°C); and α and β are the regression coefficients.

The temperature sensitivity (Q₁₀) of soil respiration can be calculated as [20,21]

Q₁₀ = e ^10×β,

(2)

where β is the regression coefficient derived from Equation (1).

3. Results

3.1. Changes in Soil Respiration, Soil Temperature, and Soil Moisture in Casuarina equisetifolia L. Plantations After Severe Typhoon Disturbance

As shown in Figure 2a, R_s in Casuarina equisetifolia L. plots affected by the super typhoon was significantly lower than in the control plots (p < 0.05). Specifically, R_a in the control plots was markedly higher than in the typhoon-disturbed plots (p < 0.01), whereas no significant difference in R_h was observed between the two groups (p > 0.05). Compared with the control, T₅ in the typhoon-disturbed plots was significantly higher (p < 0.05; Figure 2b). However, no significant differences in T₅ were found between R_s and R_h in either the control or disturbed plots (p > 0.05). Following the super typhoon disturbance, W₁₀ associated with R_s in the disturbed plots was significantly lower than that in the control plots (p < 0.01; Figure 2c). Similarly, W₁₀ corresponding to R_h in the disturbed plots was significantly reduced compared with that of the control (p < 0.05). Moreover, a significant difference in W₁₀ was observed between R_s and R_h in the control plots (p < 0.05), whereas no such difference was detected in typhoon-disturbed plots.

3.2. Dynamic Trends of Soil Respiration in Casuarina equisetifolia L. Plantations After Typhoon Disturbance

As shown in Figure 3a, R_s exhibited significant temporal variation within seven months following typhoon disturbance (p < 0.05), displaying an inverted single-peak trend. In both the control and disturbance plots, R_s initially decreased and subsequently increased. In the control plot, the maximum and minimum R_s values were recorded in March (3.69 ± 0.27 μmol·m⁻²·s⁻¹) and December (1.48 ± 0.11 μmol·m⁻²·s⁻¹), respectively. In the typhoon-disturbed plot, the corresponding values occurred in October (2.60 ± 0.25 μmol·m⁻²·s⁻¹) and December (0.87 ± 0.05 μmol·m⁻²·s⁻¹).

In both typhoon-disturbed and non-disturbed (control) plots, the maximum and minimum T₅ were recorded in September 2024 and January 2025, respectively. The monthly T₅ showed significant temporal variation (p < 0.05) (Figure 3b). In the control plot, the maximum and minimum T₅ were 26.03 ± 0.39 °C and 18.87 ± 0.22 °C, respectively. In contrast, in the typhoon-disturbed plot, the corresponding values were 28.79 ± 0.25 °C and 19.86 ± 0.35 °C.

In both the control and typhoon-disturbed plots, W₁₀ exhibited significant temporal variation (p < 0.05, Figure 3c). The maximum values were recorded in September and October, whereas the minimum values occurred in March.

The temporal patterns of R_h and R_a closely mirrored those of R_s (Figure 3a and Figure 4), with all exhibiting an inverted single-peak trend. Significant temporal variability in R_h was observed between typhoon-disturbed and control plots across all months, except December (p < 0.05; Figure 3a). In both plots, R_h consistently decreased initially and then increased, with the control plot reaching its maximum in March (2.33 ± 0.29 μmol·m⁻²·s⁻¹) and minimum in December (0.75 ± 0.09 μmol·m⁻²·s⁻¹), while the disturbed plot peaked in October (2.13 ± 0.28 μmol·m⁻²·s⁻¹) and reached its lowest value in December (0.64 ± 0.12 μmol·m⁻²·s⁻¹). Similarly, R_a also showed significant temporal variability (p < 0.05; Figure 3b), following the same pattern of an initial decline followed by an increase. The control plot recorded the highest R_a in March (1.61 ± 0.28 μmol·m⁻²·s⁻¹) and the lowest in December (0.74 ± 0.17 μmol·m⁻²·s⁻¹), whereas the disturbed plot reached its maximum R_a in September (0.96 ± 0.21 μmol·m⁻²·s⁻¹) and minimum in December (0.24 ± 0.06 μmol·m⁻²·s⁻¹). Additionally, R_c showed significant temporal variability between typhoon-disturbed and control plots across all months (p < 0.05; Figure 3c). During the observation period, the mean R_c was 46.86% ± 1.49% in the control plot and 33.38% ± 0.79% in the disturbed plot, indicating a significant reduction of 13.48% following typhoon disturbance.

Following typhoon disturbance, soil respiration and its components in both the control and disturbed plots exhibited distinct seasonal variations. During the dry season (December–March), R_s in the control plot was significantly higher than that in the disturbed plot (p < 0.05, Figure 5a), whereas no significant difference was observed during the rainy season (September–November). In the dry season, R_h in the control plot also exceeded that in the disturbed plot (p < 0.05, Figure 5b). Conversely, during the rainy season, R_h in the control plot was significantly lower than that in the disturbed plot (p < 0.01). For R_a, significantly higher values were observed in the control plot during the dry season (p < 0.001), whereas no notable differences were detected between the two plots during the rainy season.

3.3. Correlations Between Soil Respiration and Soil Temperature and Soil Moisture of the Soil

In both the control and typhoon-disturbed plots, T₅ was significantly positively correlated with R_s (p < 0.01) and showed a significant positive correlation with R_h (p < 0.05). In the typhoon-disturbed plot, W₁₀ was significantly negatively correlated with both R_s and R_h (p < 0.05), whereas no such correlations were observed in the control plot (

Table 2. Correlation analysis of R_s, R_h with T₅ (°C) and W₁₀.

	T5/°C	W10/%
Rs of Control plot	0.524 **	−0.36
Rs of Typhoon disturbance	0.889 **	−0.58 *
Rh of Control plot	0.517 *	−0.27
Rh of Typhoon disturbance	0.786 *	−0.61 **

Note: * p < 0.05; ** p < 0.01.

). The Q₁₀ value of R_s in the control plot (1.90 ± 1.22) was significantly lower than that in the disturbed plot (2.29 ± 1.15) (p < 0.05), while that of R_h was significantly lower in the control plot (1.73 ± 1.20) than in the disturbed plot (2.01 ± 1.16) (p < 0.05) (

Table 3. Exponential regression model of T₅ with R_s, R_h, and Q₁₀.

	N	Fitting Equation Parameter α	Fitting Equation Parameter β	Q10	R2	F	p
Rs of Control plot	126	0.50 ± 0.26	0.064 ± 0.020	1.90 ± 1.22	0.62	227.49	<0.001
Rs of Typhoon disturbance	126	0.25 ± 0.09	0.083 ± 0.014	2.29 ± 1.15	0.47	247.04	<0.001
Rh of Control plot	84	0.35 ± 0.16	0.055 ± 0.018	1.73 ± 1.20	0.20	283.63	<0.001
Rh of Typhoon disturbance	84	0.23 ± 0.08	0.070 ± 0.015	2.01 ± 1.16	0.34	208.54	<0.001

).

4. Discussion

4.1. Short-Term Effects of Typhoon Disturbance on Soil Respiration

As a major natural disturbance, typhoons profoundly influence the structure and function of forest ecosystems and alter soil carbon cycling [22,23]. Previous studies have suggested that typhoons can enhance soil respiration. For instance, Chiang et al. [24] reported that the mean annual soil respiration of four saplings increased from 7.65 to 9.13 t C·ha⁻¹ following a typhoon, indicating a 19.3% increase in soil carbon emissions. Similar findings were reported by Maneke-Fiegenbaum et al. [25]. In contrast, this study suggested that typhoon disturbance suppressed soil respiration in the Casuarina equisetifolia L. plantations. Specifically, Typhoon Yagi reduced the mean R_s by 22.9% in the short term, which may be attributed to the storm’s high intensity. As a Category 17 typhoon, Yagi caused severe structural damage, with approximately 90% of the trees snapped or uprooted, resulting in extensive root destruction and reduced soil respiration. The analysis of respiration components demonstrated no significant difference in mean R_h between the disturbed and control plots (p > 0.05), whereas the mean R_a in the control plot was significantly higher than that in the disturbed plot (p < 0.01), confirming that reduced autotrophic respiration due to root damage contributed to the observed decline in R_s.

R_h has been shown to account for 50%–68% of R_s in various forest ecosystems [26,27,28]. In this study, the mean R_h contributed 56.84% of R_s in the control plot and 67.35% in the typhoon-disturbed plot, indicating a marked increase in the relative contribution of R_h following typhoon disturbance. This increase may be attributed to the substantial litter input caused by extensive treefall and branch breakage in Casuarina equisetifolia L. plantations, which introduced abundant labile carbon substrates into the surface soil and stimulated microbial respiration [29,30]. This pattern is consistent with observations in older fallow fields, where greater soil organic carbon accumulation highlights how significant organic inputs enhance carbon availability and microbial activity [31]. Additionally, typhoon-induced root mortality likely increases root exudation and the availability of decomposable organic matter, thereby further promoting heterotrophic respiration [32].

4.2. Seasonal Effects of Typhoon Disturbance on Soil Respiration

Following typhoon disturbance, distinct seasonal patterns were observed in R_s, R_h, and R_a within Casuarina equisetifolia L. plantations. During the dry season, R_s and R_h were significantly higher in the control plot than in the disturbed plot. This difference was primarily attributed to the intact vegetation in the control plot, which maintained greater root activity through enhanced root metabolism and rhizodeposition, thereby sustaining higher R_a [33]. In contrast, vegetation damage in the disturbed plot (including root fractures and canopy loss) reduced photosynthetic capacity and suppressed root metabolic activity, thereby leading to significantly lower R_a [33]. Meanwhile, the relatively stable microbial community in the control plot facilitated continuous organic matter decomposition, whereas typhoon-induced alterations in soil temperature, moisture, and aeration disrupted the microbial environment in the disturbed plot, likely reducing microbial activity and suppressing R_h [34,35,36].

However, during the rainy season, increased precipitation elevated W₁₀ in both plots. Excess water saturated soil pores, which limited oxygen diffusion and inhibited R_s to a similar extent in both plots; hence, no significant difference in R_s was observed [37,38]. Conversely, R_h was significantly higher in the disturbed plot during this season, likely due to the accumulation of readily decomposable plant debris and organic fragments following the typhoon. These substrates, combined with high soil moisture, stimulate microbial activity, thereby enhancing R_h [39].

4.3. Relationships Between Soil Respiration and Soil Temperature and Soil Moisture After Typhoon Disturbance

Soil temperature and moisture are the two primary factors that regulate soil respiration [40,41,42]. In this study, their effects on R_s and R_h differed significantly, and typhoon disturbances modified their regulatory roles. Specifically, T₅ demonstrated a significant positive correlation with R_s in both control and disturbed plots (p < 0.01), indicating that elevated temperatures promoted soil respiration in Casuarina equisetifolia L. plantations. This finding was consistent with the known temperature sensitivity of microbial and root metabolism, where increased soil temperature enhances microbial activity and root exudation to increase CO₂ emissions [43]. Stronger correlations between R_s, R_h, and T₅ were observed in the typhoon-disturbed plot than in the control plot (Table 1). This could be due to the canopy damage that increased the solar radiation at the soil surface, thereby amplifying the temperature response of soil respiration components [44]. Furthermore, Q₁₀ of the disturbed plot was significantly higher than that of the control plot. This difference may have resulted from the increased availability of labile organic matter following the influx of litterfall, which decomposes more rapidly under elevated temperatures [45] and contributes to a higher Q₁₀ value.

Soil moisture has been widely recognized as a key factor influencing soil respiration, although the findings remain inconsistent. Some studies have reported significant negative correlations between soil moisture and respiration [40,46], while others have observed positive correlations within specific soil moisture ranges [47,48] or no clear relationship at all [49,50]. In this study, W₁₀ was significantly negatively correlated with R_s and R_h in the typhoon-disturbed plot (p < 0.05) compared to the control plot. This may be attributed to typhoon-induced structural damage to the soil, including reduced porosity and disrupted aggregates, which altered pore distribution and water movement. Elevated W₁₀ during the rainy season could exacerbate the oxygen limitations of microorganisms, thereby inhibiting aerobic microbial respiration. Additionally, the affected gas exchange and diffusion due to structural changes may further suppress respiration [51]. It is worth noting that the sharp increase in W₁₀ in the disturbed plot in October likely resulted from the combined effects of the rainy season and the substantial accumulation of plant debris and litterfall following the typhoon, which collectively enhanced soil water retention capacity (Figure 2). Although typhoon disturbance significantly influenced the temperature sensitivity of soil respiration, the effects of soil moisture appeared to be more dependent on site-specific vegetation and physical soil conditions. Future research should incorporate long-term observations and mechanistic modeling to quantify the sustained impacts of extreme climate events on soil carbon cycling.

5. Conclusions

This study, based on observations of Casuarina equisetifolia L. plantations on northern Hainan Island following the disturbance of Typhoon Yagi, found that the typhoon significantly reduced R_s, primarily due to a notable decrease in R_a caused by root damage. Meanwhile, the disturbance altered the regulatory effects of environmental factors on R_s. Although T₅ remained a strong positive predictor of R_s in typhoon-disturbed and control plots, W₁₀ showed a significant negative correlation with R_s and R_h only in the disturbed stand. Furthermore, the increase in Q₁₀ after disturbance suggests enhanced vulnerability of carbon release processes to climate change. These findings indicate that typhoons not only suppress carbon release in the short term but may also affect the long-term carbon balance of Casuarina equisetifolia L. plantations by altering environmental regulatory mechanisms. The results provide new data and theoretical insights into the effects of frequent strong typhoons on soil respiration and carbon cycling in coastal protective forests under global climate change. However, this study was limited to 7 months of monitoring, which may not fully capture the long-term recovery dynamics of R_s components. Future studies should extend the observation period and incorporate dynamic monitoring of soil physicochemical properties and microbial community structure to further elucidate the mechanisms underlying soil respiration responses to typhoon disturbances.