Primary forests continue to be transformed into secondary forests due to the large-scale expansion of agricultural land, high-intensity commercial logging, and continuously increasing energy demands [1, 2]. In the early 21st century, secondary forests accounted for approximately 59.5% of the global forest area [3]. Similar to primary forests, secondary forests also play important roles in habitat provision, biodiversity conservation, and carbon sink expansion [4–6]. However, the majority of secondary forests tend to have simple vegetation structures and show a loss of reproductive capacity and are vulnerable to natural disasters [7]. Accordingly, maintaining secondary forest ecosystem stability, improving productivity and stimulating ecosystem C sequestration are urgently needed to mitigate the greenhouse effect [8, 9].
Thinning operation, a key forest management practice, is widely employed to maintain secondary forest ecosystem stability and promote stand productivity by increasing stand structural heterogeneity, improving resistance and enhancing ground flora diversity [10, 11]. Nevertheless, little is known about the impact of thinning practices on belowground productivity, although the belowground parts of vegetation make critical contributions to terrestrial productivity and carbon stock [12]. This is especially true for fine roots (∅ ≤ 2 mm). Fine roots account for less than 5% of all forest biomass but for 30-50% of total net primary production [13] and for approximately 50%-70% of the soil C flux in forest ecosystems [14]. Unlike sampling other aboveground organic parts, sampling plant roots at the stand level is destructive, laborious and technically challenging, leading to incomplete estimations and understanding of fine root processes [15].
Abiotic and biotic factors are the main drivers of fine root dynamics (biomass, production, necromass, mortality and the turnover rate) [16, 17]. Thinning treatments can directly modify the physical structure of trees and understory vegetation and indirectly influence belowground environmental conditions (e.g., soil fertility and microclimate), potentially impacting fine root dynamics [18, 19]. However, a few studies have reported contrasting results regarding thinning. For instance, some studies have shown that the biomass and production of fine roots generally increase following thinning due to improved soil nutrient ability and soil moisture levels [20, 21]. In contrast, fine root biomass and production responses to thinning have been reported in other studies, including negative and negligible responses [22, 23]. Undoubtedly, exploring the underlying mechanisms of fine root dynamic responses to thinning practices will provide a theoretical basis for further understanding productivity and nutrient cycling in terrestrial ecosystems.
Fine roots were traditionally defined as roots with a diameter of < 2 mm, and this definition has recently been debated [24]. Even in this small-diameter category (∅≤ 2 mm), fine roots of different sizes exhibit functional heterogeneity due to their different physiological activities [25, 26]. Fine roots of < 0.5 mm (very fine) are mainly responsible for the acquisition and absorption of resources in the soil, and 0.5-2 mm fine roots (thicker) are responsible for resource transfer and storage [27, 28]. Very fine roots are typically more dynamic than thicker roots and are extremely sensitive to environmental change since the ratio of nonstructural to structural mass is much higher in smaller roots [20, 29]. For instance, the responses of fine roots to soil nutrient and water changes are diameter dependent [30, 31]. Furthermore, Ma et al. (2013) reported that very fine roots have a higher turnover rate and productivity levels than thicker roots under thinning measures. Therefore, we expect the dynamic characteristics of very fine roots to be more sensitive to thinning practices. Moreover, since thinning practices remove a large volume of aboveground organic components, leading to the death of belowground roots [32, 33], we hypothesized that the necromass and mortality of fine roots would tend to increase with increasing thinning intensity.
Plants may also adjust their root systems to use resources at different soil depths [34]. When stimulated by more resource competition in surface soil, plants can adjust their roots at deep soil depths to identify more soil resources [15]. When competition for surface soil resources decreases, the exploration of deep layer soil resources is also reduced, and the plant root system will again shift to utilizing surface resources [35]. Thinning practices alter the original vegetation configuration, resulting in a change in resource competition among species that in turn leads to varied resource use strategies employed by fine roots at different soil depths [36]. Light and high-intensity thinning measures slightly or significantly reduce resource competition, leading deep soil resource exploration to decrease [21]. Under a suitable thinning treatment, understory species diversity increases and resource competition intensifies, and fine roots gradually explore deep soil resources, resulting in increased fine root biomass and production at deeper soil depths [19]. Thus, collecting samples at deeper soil depths is needed to fully quantify fine root dynamic characteristics and spatial distribution variability [37], and to clarify the resource acquisition strategies of fine roots under different thinning treatments.
In the present study, we determined the biomass, necromass, production, mortality, and turnover rate dynamics of <0.5 mm, 0.5–1 mm, and 1–2 mm fine roots at 0-20 cm, 20-40 cm and 40-60 cm soil depths under five thinning intensities (0%, 15%, 30%, 45%, and 60% of the stand volume removed) in secondary forests. Our hypotheses are as follows: (a) very fine root dynamic characteristics are more sensitive to thinning practices than thicker fine roots due to their greater responsiveness to changing environments; (b) fine root necromass and mortality increase when thinning intensities increase; and (c) fine root dynamics in deep soils are more sensitive to thinning measures than those in shallow soils and exhibit fluctuations. Furthermore, to understand the driving effects of abiotic and biotic conditions on the biomass, production, necromass, mortality and turnover rate of fine roots, we explored the linkages between fine root dynamics, soil properties and stand characteristics.