Bedrock Type Mediates the Response of Vegetation Activity to Seasonal Precipitation in the Karst Forest

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Bedrock Type Mediates the Response of Vegetation Activity to Seasonal Precipitation in the Karst Forest


1. Introduction

The southwest of China is anticipated to undergo a series of changes due to global warming in this century, including temperature rising (3.0–4.0 °C) and altered pre-capitation patterns characterized by increased rainfall intervals, a shift from summer to spring distribution, and a decrease in the annual amount of rain [1]. Such changes might increase the intensity and frequency of droughts, resulting in a large reduction in vegetation growth [2]. Particularly, southwestern China is widely recognized as the most extensive Karst region globally, which is highly susceptible to water deficit [3,4,5]; therefore, understanding the vegetation growth response to the precipitation is crucial important to predict the climate–carbon storage feedback. However, several studies indicate that the precipitation–vegetation activity relationship commonly exhibits weak characteristics in the karst region: despite receiving sufficient precipitation, plants still suffer water stress, which suggests that the ecohydrological process is more complex in this region.
The spatial and temporal variation in water availability is strongly dependent on the function and structure of the earth’s critical zone, which means it is not only influenced by land surface climate and soil conditions [6,7,8], but also by deeper components of the ecosystem [9,10]. In the karst region, the significance of bedrock in regulating water availability cannot be disregarded. Karst developed from carbonate rocks, which are highly soluble in water. Carbonate rocks develop large numbers of crevices on their surface due to rainfall-induced water erosion, which significantly increases the bedrock permeability [11]. Moreover, because only small amounts of residues are left after dissolution, the regolith formation rate is shallow, and the thin regolith further limits its water-holding capacity. Consequently, drought events are more likely to occur in carbonate rocks regions (CRR), and it has been observed that for rainfall severe enough to surpass the soil’s field capacity, the water availability it supplies is only adequate to meet the transpiration requirements of plants for a duration of 7 to 14 days [12] (Figure 1).
In CRR, the temporal variation in water availability can be very strong, and drought events frequently occur in early spring and autumn [13,14], depending on precipitation patterns such as amount, intermittency, and magnitude. For example, Nerantzaki and Nikolaidis [15] show that spring drought events are more frequent in the Mediterranean regions because of the decrease in precipitation, whereas in the karst area of southern Italy, long time intervals are the main reason for drought events. Therefore, the frequent transient water scarcity resulting from soil water fluctuations is regarded as one of the most significant factors restraining vegetation growth [16,17]. Vegetation in CRR might be more vulnerable in the period with little or no precipitation. In contrast, vegetation in NCRR can use water contained in deep soil layers during drought episodes, thus showing greater resilience to drought episodes [18]. During a period with sufficient rainfall, the difference in vegetation productivity between CRR and NCRR might be reduced. In other words, vegetation growth is slower in CRR during periods with lower rainfall than in NCRR, but such differences may not exist as the rainfall increases.
Figure 1.
Conceptual models of critical regions in carbonate (left) and noncarbonate (right) rock regions in southwest China (cited by Green [19]). Compared to noncarbonate rock, carbonate rock can more easily dissolve and develop several fractures and pavements, and surface water can quickly move through the rock underground. Moreover, carbonate rock usually develops thin regolith, which further influences the water-holding capacity of regolith. Thus, we assume that vegetation in the carbonate rock region is more sensitive to drought than in the noncarbonate region.

Figure 1.
Conceptual models of critical regions in carbonate (left) and noncarbonate (right) rock regions in southwest China (cited by Green [19]). Compared to noncarbonate rock, carbonate rock can more easily dissolve and develop several fractures and pavements, and surface water can quickly move through the rock underground. Moreover, carbonate rock usually develops thin regolith, which further influences the water-holding capacity of regolith. Thus, we assume that vegetation in the carbonate rock region is more sensitive to drought than in the noncarbonate region.
Sustainability 16 01281 g001

As a main ecosystem of southwestern China, forests provide crucial ecosystem services, including watershed protection, erosion prevention, and carbon storage, and play a critical role in sustainable development. Moreover, forest ecosystems provide a habitat for diverse plants and animals. However, the response of forests to precipitation changes is still not clear. In this study, we assume the bedrock has the potential to obscure or distort the relationship between vegetation activity and environmental factors, and we investigate the indirect effects of lithology-associated features that affect vegetation growth in the Guizhou Province (one of the provinces in southwest China) based on a climate, soil, and bedrock database. We compare the vegetation dynamic in CRR with that in NCRR. Second, we test the vegetation growth response to precipitation in both regions. Third, we analyze the role of the bedrock type by considering how it changes the relationship between precipitation and vegetation activity. Specifically, we seek answers to the following two questions: (1) Does the relationship between precipitation and vegetation activity differ between CRR and NCRR? (2) Is vegetation growth more sensitive to droughts in CRR than in NCRR?

4. Discussion

Vegetation development strongly depends on the function and structure of the earth’s critical zone, which means it is not only influenced by land surface climate and topsoil conditions but also by deeper components of the ecosystem, such as bedrock (Figure 2). However, until now, the role of the belowground system in vegetation growth has been relatively poorly understood. We found that the interaction of vegetation with precipitation is mediated by bedrock type. The intermittent drought might be more frequent in the karst zone due to its low water capacity, which plays an essential role in moisture subtropic regimes. In forests characterized by carbonate rock, vegetation growth is more sensitive during the dry spell. In contrast, during periods with adequate precipitation, the difference in forest growth between lithological regions disappears. Notably, Qiao [31] found the NDVI of karst regions related to precipitation to be stronger than in no-karst regions during the drought period, which is consistent with our results. Our findings might encourage further research to clarify the importance of bedrock on vegetation.
Our findings are consistent with past research, proving the role of bedrock cannot be ignored. The bedrock is the foundation of soil formation and its properties can significantly impact the soil’s ability to store water. Zhong et al. [32] found that vegetation growth and soil water content strongly related to lithologies in the karst region of southwest China; by using meta-analysis, Zhu et al. [33] showed that bedrock has a significant relationship with regolith water capacity globally, which is crucial to vegetation growth. Moreover, in the karst area, the bedrock is composed of more permeable materials, such as limestone or sandstone; so, the water can easily pass through the bedrock [34]. In summary, the karst bedrock plays a crucial role in determining the water storage capacity of soil and subsequently affects the growth of vegetation by providing an essential resource for plant survival and growth.
It is imperative that maximum rainfall and sum precipitation are performed as an essential predictor of variation in NDVI in the CRR, which might suggest that the precipitation has a more crucial role in regulating the growth of karst forest. One plausible reason for this phenomenon is the high permeability and thin soil layer. CRR commonly exhibits a low water storage capacity [31,32]. Another possible reason is that the soil developed from carbonate rock is rich in clay, which has a low water-holding capacity because it lacks porosity. Notably, Nachtergaele et al. [34] found that limestone and dolomite usually cover clay soil, which is essential supporting evidence for this inference. It is also likely that because of the adaption strategy of plant species, excessive precipitation might create temporarily stable water sources, such as water in the temporarily saturated zone, and groundwater from lower depths; in addition, some species such as Bursera simaruba, can rapidly absorb and store the amount of water necessary to sustain a high water potential throughout periods of drought, and therefore, their growth can be maintained [2].
The AGE value is minimal for both karst and non-karst zones, suggesting that drought should be an essential limiting factor to the growth of forests in Guizhou Province [35,36,37]. However, the separate univariate analyses conducted for karst and non-karst forest in this study provide evidence that the drought in Guizhou may have been caused by different reasons (Table 2). Actual evapotranspiration was the best predictor for forest growth in the non-karst zone, indicating that the drought event may result from high temperatures. In contrast, the karst forest might be limited by the length of the period with rainfall since the water factor shows the best explanatory power for predicting NDVI in the karst zone. However, we cannot rule out the influence of temperature on karst forest since high temperatures can intensify drought stress during the lack of rainfall and increase the fire frequency in the karst zone [38,39]. Further research might benefit from exploring the interaction of precipitation and temperature in karst forests.
Although the forest in CRR suffers more drought stress, the vegetation activity only exhibits slightly lower values than those in NCRR (Figure 3). One possible reason for that is that the trees in the karst environment develop several special adaptabilities against drought stress [40]; for example, the trees in the karst zone often have thick and sponge leaves to reduce transpiration [41]; some plants can reduce the damage of a drought event by improving the activity of their antioxidant enzymes [42,43]. Such adaptive mechanisms can effectively reduce water loss and improve water use efficiency. However, it is noteworthy that the NDVI of karst forests is significantly lower than that of non-karst forests during spring and autumn when the drought is most severe. This result shows that karst forests might be highly vulnerable when drought occurs and reaches sufficient intensity.

5. Conclusions

The bedrock type can decouple vegetation responses from the climate, demonstrating that substrate prosperity can contribute to the vegetation dynamics in a complex way, suggesting the importance of expanding our focus beyond surface climate and soil, and exploring the connections of each critical component. Our findings can be used to simulate the potential consequences of future climate changes for the different bedrock types. For vegetation located in CRR, a wetter climate can benefit vegetation greenness. In past decades, land management (such as the afforestation project) in Guizhou Province has shown a significant positive impact on vegetation growth; however, the increase in the frequency and intensity of drought events needs to be considered. As regions with carbonate bedrock cover 15% of the Earth’s surface, this threshold should be considered when studying vegetation growth responses to climate change in a karst landscape. To further improve our understanding of bedrock impacts, it is necessary to carry out field observation and control experiments to clarify the water-holding capacity of different bedrock zones. Meanwhile, we also need more details on species-specific drought adaptation mechanisms in karst forests, which can be crucial to enhance our accuracy regarding the prediction of how karst forests respond to climate change.


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