Located in the southeastern coastal areas of China, Zhuhai is frequently confronted with extreme climate events as typhoons’ disturbance (Fig. 1). On 20th August 2017, Typhoon Hato generated upon the northwestern Pacific Ocean and constantly intensified within 72 h. At 12:50 on 23rd August, it landed on Zhuhai at wind scale 14 and moved forward in the northwest direction. During its route in Zhuhai lasting over 4 h, more than 25% of urban trees in quantitative terms in Zhuhai were damaged by the strong wind. After Typhoon Hato left, imperative ecological remediation was planned and launched according to the practical condition in each area. In Xiangzhou district, which suffered enormously from the typhoon disturbance, the local forestry administration department took several steps to recover and re-build urban greening, including sweeping typhoon-affected areas, restoring land and soil, and making ecological restoration plans. In June 2018, 18 indigenous tree species were selected and over 50,000 saplings were planted. To analyze their growing conditions that helped to evaluate the quality of the imperative ecological remediation, we selected four most-planted tree species as our objects within the whole planting area, i.e., Sterculia lanceolata Cav. (Sl), Ilex rotunda Thunb (Ir), Schima superba Gardn. et Champ. (Ss), and Elaeocarpus sylvestris (Lour.) Poir. (Es). The four tree species are all native tree species with flexible soil adaption. In order to select the most representative trees of the four tree species, five sampling plots with the size of 30 $\times$ 30 m were randomly established. In each sampling plot, all the trees were surveyed and their data of tree height and ground diameter was recorded, based on which four sampling trees of each tree species were determined. The measurements on all the 16 trees were conducted in June 2018, December 2018, June 2019, and December 2019, i.e., trees’ ages at 0 months (0M), six months (6M), 12 months (12M), and 18 months (18M).

Fig. 1.

Schematic of Zhuhai in China (blue area with red framework), landing location and route of Typhoon Hato (broad red arrow), and location of our study area in Zhuhai (green area).

National Meteorological Information Center of China (http://data.cma.cn) supplied climate data, based on which monthly climatic variables including wind velocity (km h${}^{-1}$), precipitation (mm), and temperature (${}^{\circ }$C) were collected and calculated from January 2017 to December 2019 in Zhuhai. As a coastal city under a subtropical and tropical maritime climate, Zhuhai had a relatively high average temperature at 22.5 ${}^{\circ }$C and plenty of precipitation over 2000 mm annually from 1995 to 2015. From 2017 to 2019, the average wind velocity slightly fluctuated around 10 km h${}^{-1}$ while the highest wind velocity would reach over 100 km h${}^{-1}$, primarily when extreme climatic events occurred. In August 2017, Typhoon Hato landed in the southeast coastal areas of China and intruded the front of Zhuhai city, resulting in the monthly highest wind velocity and precipitation reaching 217 km h${}^{-1}$ and 277 mm.

The primary soil type in Zhuhai is red acidic soil. The soil sampling campaign was launched in the study area in June 2018 and December 2019 to indicate the development of soil physical and chemical properties. For each time, five metallic-cylindrical sampling cores were used to dig out two kilos of soil in the depth of 30 cm, which were randomly located in the study area same as the sampling plots. Then all the soil samples were well kept in sampling bags and transported to our laboratory for further analysis. The soil’s physical properties were measured according to Li and Shao’s reports [20]. Soil bulk densities (BD, g cm${}^{-3}$) were determined by the mass of soil per unit volume (sum of solids and pore space) and the soil cores were oven-dried at 105 ${}^{\circ }$C to obtain soil moisture content (SMC, %). Then, the soil cores were put on a sand salver and allowed to drain for 2 h in order to calculate soil capillary water content (Scwc). Hence, soil capillary porosity (Scp, %) was calculated according to the equation as:

(1)$$Scp=0.1\times Scwc\times ds$$

where ds is the soil density (mg m${}^{-3}$). And soil non-capillary porosity (Sncp, %) and soil total porosity (Stp, %) were calculated as:

(2)$$\begin{array}{c}\hfill \text{𝑆𝑛𝑐𝑝}=0.1\times (Smc-Scwc)\times ds\hfill \\ \hfill \mathit{\text{Stp}}=Scp+Sncp\hfill \end{array}$$

For soil chemical properties, soil PH was determined in 1 : 2.5 soil-water slurry using a combination glass electrode. Soil organic matter (SOM, g kg${}^{-1}$) was determined by the oil bath-K${}_2$Cr${}_2$O${}_7$ titration method. Total nitrogen (TN, g kg${}^{-1}$) was determined by the semi-micro Kjeldahl method. Available nitrogen (AN, mg kg${}^{-1}$) was determined by a micro-diffusion technique after alkaline hydrolysis. Total phosphorus (TP, g kg${}^{-1}$) was determined colorimetrically after wet digestion with H${}_2$SO${}_4$ + HClO${}_4$. Available phosphorus (AP, mg kg${}^{-1}$) was exacted with 0.5 mol l${}^{-1}$ NaHCO${}_3$ solution (PH 8.5). Total potassium (TK, g kg${}^{-1}$) was determined by the Cornfield method. Available potassium (AK, mg kg${}^{-1}$) was determined by the CH${}_3$COONH${}_4$ extraction method [21].

In June 2018, the saplings of all the tree species were simultaneously planted in the study area, which had unified short and small sizes (height around 25 cm and DBH around 0.5 cm). Therefore, the four tree species were considered to have the completely same and tiny initial conditions. To investigate their above-ground growth, their ground diameters were measured with the help of a vernier caliper (Altraco Inc., Sausalito, California, USA) and their height was measured using a standard tape. The measurement was conducted in December 2018, June 2019, and December 2019 to explore the above-ground development patterns from 0M to 18M.

To clarify the below-ground progress, fine root coring campaigns were launched for all the selected trees at the same time as the above-ground measurement. A pre-test coring campaign showed that the range of the root system was similar to a cylinder, with a diameter of 70 cm and a height of 35 cm. Therefore, for the 16 trees, eight soil cores were collected for every individual tree: four at a distance of 15 cm from the trunk and the other four at 30 cm. The soil was sampled down to a depth of 30 cm using a soil auger with a length of 30 cm and a radius of 3 cm. Each sample was divided into three horizons: soil depths of 0–10 cm (upper layer), 10–20 cm (middle layer), and 20–30 cm (deep layer). Fine roots (2 mm) were filtered using sieves (2-mm mesh size) and separated by forceps in the laboratory. Then, the samples were washed and dried in an oven at 65 ${}^{\circ }$C for 72 h. Finally, all the samples were weighed using a balance with an accuracy of four decimal places to obtain the dry weight. The fine root biomass at different depths was calculated using the dry weight divided by the cross-sectional area of the auger.

The software SPSS 22.0 (IBM, State of New York, USA) was used for statistical analysis. To investigate the difference between means, two-sampled t-test and analysis of variance (ANOVA) with Tukey’s HSD (honestly significant difference) test were used. In all the cases, the means were reported as significant when P 0.05. Where necessary, data were log or power transformed in order to correct for data displaying heteroscedasticity.

Five physical and eight chemical variables were measured to analyze the soil revolution under the ecological remediation (Table 1). For physical properties, no significant difference was detected between June 2017 and December 2019, among which SMC and BD exhibited slight decreases (from 22.64% $\pm$ 1.90% to 19.90% $\pm$ 2.82%; from 1.28 $\pm$ 0.06 to 1.23 $\pm$ 0.06 g cm${}^{-3}$). In terms of soil porosity, all the three variables, Stp, Scp, and Sncp increased distinctly. For chemical properties, except for TN (from 1.02 $\pm$ 0.25 to 1.01 $\pm$ 0.23 g kg${}^{-1}$), the other variables displayed apparent development, especially for AN, AK, SOM showing drastic upsurge (from 47.78 $\pm$ 16.91 to 152.60 $\pm$ 21.05 mg kg${}^{-1}$; from 18.18 $\pm$ 9.56 to 85.80 $\pm$ 27.62 mg kg${}^{-1}$; from 10.36 $\pm$ 3.32 to 18.44 $\pm$ 3.67 g kg${}^{-1}$). Besides, soil acidity reduced with PH values decreasing from 4.20 $\pm$ 0.04 to 5.04 $\pm$ 0.74.

Under the same initial condition (all the four tree species were short and small saplings.), the four tree species displayed different growing patterns regarding tree height and ground diameter (Fig. 2). Es had the most rapid growth of tree height for the whole period and reached 2.98 $\pm$ 0.61 m at 18M. In addition, it also had the second-highest value of ground diameter (5.15 $\pm$ 0.78 cm) among the four tree species. Both Sl and Ir increased their tree height sharply within the first six months (0.88 $\pm$ 0.10 m; 1.3 $\pm$ 0.12 m), which turned gradual growth from 6M to 18M. However, we observed that Sl had the largest ground diameter (5.80 $\pm$ 0.39 cm), which mainly originated from its dramatic development from 12M to 18M. In comparison to others, Ss exhibited disadvantages in growing rates regarding both height and diameter, which reached 2.07 $\pm$ 0.20 m and 4.80 $\pm$ 0.56 cm at 18M.

Fig. 2.

Development of tree height (m) and ground diameter (cm) of Sl (red line), Ir (blue line), Ss (orange line), and Es (green line) from June 2018 (0 months, 0M) to December 2019 (18 months, 18M), around which the light red, blue, orange, and green shading areas are the 95% confidence intervals, respectively.

Numerous studies verified that above-ground growth patterns could reflect trees’ health condition, geographic adaptation, and supplying ecosystem services [22, 23, 24]. In our research, the growth of tree height and ground diameter for the four tree species were measured from 0M to 18M. Although all of them exhibited a general increase, different periodic patterns implied trees’ particular above-ground strategies. Es was distinguished for its height growth among the four tree species, implying a robust above-ground construction coupled with its second-highest value of ground diameter, which was in line with the previous findings [25, 26]. On the contrary, the other three tree species exhibited an inclination to develop ground diameter than height. Their height growth primarily occurred from 0M to 12M and slowed down from 12M to 18M, while the ground diameters increased sharply from 6M to 18M. Such growth pattern indicated a stable above-ground morphological profile, which could effectively provide wind-damage resistance that lower but thicker trees might now easily break down in typhoon or hurricane events [27]. This might also be verified by using convolutional neural networks to estimate the relationships between tree failures due to high winds and tree sizes [28]. As showed in Fig. 2, preferential growth of Sl was distinctly given to ground diameter to effectually make the above-ground tree profile stable, which could be expected to have higher survival rates under typhoon disturbances in the future.

As the primary pathway for water and nutrient uptake, fine roots were a prominent sink for carbon acquired in terrestrial net primary productivity, tightly related to multiple ecosystem services [29, 30, 31]. In our research, we not only measured the total fine root biomass but also the root biomass development in both horizontal and vertical levels, aiming to investigate their absorption capacity of water and nutrient and morphological patterns.

For total fine root biomass, Sl was the only tree species growing within the whole 18 months, showing a persistent below-ground investment. Distinct growth occurred within the first year since plantation, however, the other three tree species showed a biomass reduction from 12M to 18M. Combined with their height and ground diameter growth, it could be explained by their specific allocation strategy between the above- and below-ground processes. Generally, urban trees dynamically adjusted the above- and below-ground investment on the basis of their characteristics and environmental conditions [32, 33], such as enhancing above-ground growth for more photosynthesis or increasing root growth for water and nutrients uptake [34, 35, 36]. Nevertheless, Es’s remarkable decrease of fine root biomass showed a weakened below-ground growth, which could be deeply worried about its adaption in the future. This was because typhoons usually broke down or shaded tree trunks, leading to more severely damaged root systems. In addition, site conditions in typhoon-affected areas were generally fragile and negative that soil quality were highly reduced and ecosystem flow were disturbed, making the fine roots require a lengthy period for recovery [36, 37].

For fine root biomass in the vertical level, the four tree species had significant growth from 0M to 12M (Fig. 4). Similar decreasing patterns from 12M to 18M were found as the total fine root biomass, however, several positive phenomena existed in their deep root layer, i.e., 20–30 cm. As shown in Fig. 6, Zhuhai usually had more than six months with precipitation lower than 100 mm every year and its highest temperature from May to October reached over 30 ${}^{\circ }$C, revealing a hot and dry climate condition which could result in the trees being exposed to water stress. Previous research reported that deep roots could be of pivotal importance for various plants to alleviate water stress, absorb more nutrients, and enhance tree stability [29, 38], particularly play a central role in the tropical and subtropical environment [39]. Therefore, the increase of deep roots of Sl, Ir, and Ss could be practical approaches than Es in coping with dry environmental conditions, which was consistent with the results of [40].

Fig. 6.

Monthly climate variables including highest wind velocities (km h${}^{\mathrm{-}\mathrm{1}}$), average wind velocities (m s${}^{\mathrm{-}\mathrm{1}}$), precipitation (mm), number of rainy days, highest temperature (${}^{\mathrm{\circ }}$C), mean temperature (${}^{\mathrm{\circ }}$C), and lowest temperature (${}^{\mathrm{\circ }}$C) for Zhuhai from January 2017 to December 2019. The vertical-red-dotted line indicates when Typhoon Hato arrived at Zhuhai city.

Roots’ growth in horizontal level was closely relevant to its stabilization, which was essential for those trees confronting typhoon disturbance [41, 42]. In general, trees with higher proportion of outmost roots would be more stable than those with stem-centered roots from the morphological perspective [43]. Nevertheless, the outmost roots might be abandoned priorly when the whole roots system faced shrinkage, such as Es from 12M to 18M. From this point of view, it could support that Ss tended to be steadier in a typhoon event among the four tree species as it had relatively balanced investment on both nearby and outmost fine roots within the whole study period, even when fine roots died.

Vegetation was generally regarded to have substantial potential in soil restoration for cities suffering from extreme climate events [44, 45, 46]. Comparing the edaphic conditions in June 2017 and December 2018, the impact of the ecological remediation on local soils could be evaluated in terms of physical and chemical properties. For physical properties, all the variables showed no significant differences that SMC and BD slightly decreased and Stp, Scp, and Sncp increased to a small extent, implying the ecological remediation rarely influenced the edaphic physical structure. Regarding chemical properties, all the variables displayed escalation, notably for AN, AK, and SOM (P 0.05). As fine root growth would actively participate in edaphic chemical evolution and promote the release of microelement [47], it could give the explanation of the soil improvement. No significant difference was found for TP and AP, which was probably due to the inactive nature of phosphorus, especially in acidic conditions [48]. Overall, after the typhoon event, the ecological remediation had positive effects on soil chemical properties rather than physical ones within the first 18 months.

Accounting for the complicated interaction between the restricted urban space (compacted soil, complex microclimate, multiple human activities) and the fragmentized environment under the extreme climate events’ disturbance [49, 50, 51, 52], the efficiency of ecological remediation was essential and urgent from different perspectives. Urban trees could mitigate environmental degradation and provide multiple ecosystem services, specifically for cities encountering extreme climate events, which played a central role in ecological remediation [13]. Their above- and below-ground processes were closely related to their survival rates, morphological growth, geographic adaption, water absorption, and edaphic improvement, which primarily reflected the quality of the ecological remediation [53]. In this study, the selected four tree species showed no deaths but growth, together with which edaphic chemical variables exhibited several improvements. It could be inferred that the ecological remediation in Zhuhai after Typhoon Hato (2017) was effective. However, it still could be improved according to specific growth conditions of different tree species. For example, Ir, Ss, and Es reduced fine root biomass from 12M to 18M, which could be paid more attention to, such as specific fertilization to promote their root growth in this period. We observed that Sl had a relatively thicker tree trunk accompanying steady development of total and deep fine root biomass. This could be inferred that Sl had morphological and physiological resistance to typhoon events, which could be recommended for future plantation in Zhuhai or be preferentially considered for future ecological remediation to obtain promoted ecosystem services.