The Impact of Canopy on Nutrient Fluxes through Rainfall Partitioning in a Mixed Broadleaf and Coniferous Forest
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1. Introduction
Nutrient inputs have a significant impact on plant growth and the nutrient cycle in forest ecosystems, and rainfall is an important hydrological highway for nutrient transport [1,2], thus playing a crucial role in the stability of forest ecosystems. The forest canopy can significantly modulate nutrient fluxes by partitioning rainfall and selectively absorbing or adding nutrients [3]. Therefore, exploring the rainfall redistribution and nutrient input characteristics as influenced by the forest canopy is of great significance for a better understanding of the nutrient cycling processes and stability mechanisms of forest ecosystems.
The forest canopy affects rainfall redistribution by partitioning it into interception, stemflow, and throughfall [4,5]. The combination of throughfall and stemflow is called net precipitation, in which throughfall generally accounts for 60%–80% of the rainfall, while stemflow accounts for only 2%–6% [6]. This partition is affected by many factors, including vegetation characteristics (e.g., canopy structure, tree height, and bark roughness) [7,8,9], rainfall characteristics (e.g., amount, intensity, duration, and interval time) [10,11], and meteorological conditions (e.g., wind speed and direction and air temperature) [12]. Specifically, stemflow is primarily influenced by vegetation traits such as bark texture, diameter at breast height (DBH), and the ratio of canopy height to width [13], whereas throughfall and interception are primarily influenced by vegetation traits and meteorological conditions [14].
Rainfall redistribution in the forest canopy subsequently affects the chemical properties of rainwater through the accumulation of dry deposition materials and secondary metabolites secreted by plants. These substances are washed away by rain and enter the forest via throughfall and stemflow, increasing the complexity of rainfall composition to a large extent [15,16]. For example, previous studies found that a large number of metal ions initially present in rainfall are leached out and a portion of ammonium ions are absorbed after passing through the canopy [17,18]. The nutrient concentrations of throughfall and stemflow are reported to be higher than the initial rainfall; the nutrient concentrations of stemflow especially can be up to 20 times higher than throughfall and rainfall [19,20]. This is mainly because the longer contact time with bark during stemflow allows soluble nutrients to be leached out more efficiently.
Stemflow also acts as a link between the forest canopy and ground soil through the transport of animal remains, plant tissue, and other organic matter to the soil [21,22]. Stemflow has a considerable impact on the moisture conditions, physical and chemical properties, nutritional status, and microbial composition of the soil around the tree stem [23,24]. For example, stemflow leads to higher nutrient and water contents in the areas covered by a vegetation canopy compared to bare land, which is called the fertile island effect [25,26,27,28]. Most studies regarding stemflow’s effect on soil water and nutrients focused on the whole forest stand [29,30,31,32], ignoring the areas near the stem under the canopy. However, differences in stemflow characterizations may result in spatial variations in soil nutrients, especially in mixed forests [33].
The impact of the canopy on rainfall varies largely with the type of forest, due to differences in tree species composition and age structure. Previous studies found that evergreen coniferous forests are more likely to acidify rainwater and precipitate a large amount of acid anions, whereas deciduous broad-leaved forests can increase the pH of rainwater and tend to precipitate metal cations [3]. Trees at different growth stages influence nutrient composition mainly through phenological changes in seasonal canopy (e.g., leaf emergence, flowering, and leaf falling) [34]. In mixed forests, both tree species and age structure vary largely, making the canopy structure and bark morphology more complex. As a result, the impact of different tree species on the hydrochemistry process is still unclear in mixed forests.
To answer these questions, this study investigated the variations in the amounts and the nutrient contents between the rainfall, throughfall, and stemflow of different tree species in broadleaf and coniferous mixed forests on Changbai Mountain in Northeast China using a canopy budget model and chemical analysis. We tested three hypotheses: (1) different tree species have distinct patterns of stemflow generation; (2) the nutrients in throughfall and stemflow are different from rainfall; and (3) the nutrients in stemflow vary greatly between different tree species. The aim of the study is to (1) reveal the rainfall redistribution process in mixed forests and clarify the difference in hydrological processes among different species; (2) evaluate the effect of the forest canopy of forest hydrochemistry; and (3) quantify the nutrient fluxes from the canopy to the forest floor. Thus, this study will contribute to the understanding of the pattern of nutrient inputs through rainfall events in mixed temperate forests.
2. Materials and Methods
2.1. Research Site
This study was conducted in a mixed broadleaf and coniferous forest in the Changbai Mountain National Natural Reserve in Northeast China (42°24′ N, 128°05′ E, altitude 768 m). The area is a basalt platform with a north slope and flat terrain with a slope of 3.28°. The region has a temperate continental mountain climate affected by monsoon, with an annual precipitation of 700–800 mm and an annual mean temperature of 3.6 °C. The rainy season is mainly from June to August. The forest type in the research site is a mature primary mixed broadleaf and coniferous forest, with high homogeneity in stand landscape, adequate vegetation representation, and little human interference. The forest floor is covered by albic dark brown forest soil with a thickness of around 40 cm. The dominant tree species are Acer mono (AM), Tilia amurensis (TA), Pinus koraiensis (PK), Quercus mongolica (QM), and Fraxinus mandshurica (FM). The structural attributes of the forest stand and the five dominant tree species are listed in Table 1.
2.2. Field Measurement and Sample Collection
We studied the effect of the canopy on rainfall distribution and nutrient concentration by comparing the water content and nutrient fluxes in throughfall, stemflow, and rainfall samples during the 2021 growing season. The study was carried out within a 60 m × 100 m plot. We measured the amount of rainfall, throughfall, and stemflow of different tree species for each rain event using automatic rain gauges. A rain event was considered independent when there was no rainfall for an interval of 4 h or more. Meanwhile, we collected rainwater samples at least three times a month for chemical analysis, and a total of 10 precipitation events were sampled. Additionally, to investigate the impact of stemflow from different tree species on soil properties, we collected soil and measured the soil pH near the stem. The specific methods for rainfall measurement and sample collection are described below.
Rainfall: We measured rainfall in an open field outside the experimental plot using three automatic rain gauges (HOBO RG3-M, Onset, MA, USA; measuring range, 0–127 cm h−1; accuracy, ±1.0%; resolution, 0.2 mm). Meanwhile, we placed three self-made buckets, with a diameter of 20 cm and a depth of 30 cm, near the rain gauges to collect rainwater samples. In order to avoid disturbances from splashing dust, the buckets were placed 50 cm above the ground. The water from each collector was stored in polypropylene bottles (20 mL each) previously washed with deionized water to ensure that the subsamples were not contaminated. All collectors were cleaned before each rainfall event. The samples were promptly brought back to the laboratory and stored at −20 °C.
Throughfall: We measured the throughfall and collected samples using the same instruments and methods as for rainfall. Twelve additional collectors were placed across the plot randomly. The pretreatment of the samples was as same as for rainfall.
Stemflow: We measured the stemflow from five dominant tree species. According to the average DBH, we selected three trees for each tree species and fifteen trees in total. Stemflow was collected using plastic tubing with a 30 mm inner diameter (Figure 1a). The top part of the tube was cut in half and fixed around the trunk at a height of 1.2 m above the ground with stainless steel thumbtacks and then sealed with a neutral silicone sealant [35]. The bottom part of the plastic tubing was put into a collection box with a rain gauge in it, and the stemflow in the collection box was drawn into a sampling bag with a pipe. The pretreatment of the samples was as same as for the rainfall and throughfall.
Soil samples: We collected soil samples from beneath 15 trees similar to the sample trees used for collecting stemflow. Soil samples were collected using a drill at four depths of 0–10 cm, 10–20 cm, 20–30 cm, and 30–40 cm underground and at locations of 0 cm, 50 cm, 100 cm, 150 cm, and 200 cm from the stem in four orthogonal directions. Four soil samples, each taken at the same distance and depth in four directions, were mixed to create one composite sample. And a total of 20 composite soil samples were collected under each sample tree. The soil samples were tabbed and brought back to the laboratory for air-drying and later use.
Bark measurement: We collected 15 cm × 4 cm bark samples (the edge was deep to the xylic part) on a sunny day after a week without rain. In the laboratory, we measured the water retention capacity and leachability of the nutrient elements of the bark. To determine the bark’s water retention capacity, the bark was fixed at an angle of 45 degrees, and water was uniformly dropped vertically at a rate of 1 mL min−1. The time taken for the first drop of water to flow out of the bark was recorded (Figure 1b). To determine the leachability of nutrient elements, different bark samples were soaked with fresh rainwater, and the total dissolved solids (TDSs), which reflects the amounts of dissolved matter in water in ppm, were measured every hour until the measured values were stable (Figure 1c).
2.3. pH and Nutrient Concentration Analysis of Collected Samples
Three raw rainfall samples and fifteen stemflow samples were collected after each selected rainfall event. Considering both workload and sample representativeness, we randomly collected 4 samples from the 12 throughfall collectors each time. In total, 220 water samples were used for chemical analysis.
We measured the pH and nutrient concentrations of the collected water samples. pH was measured on site using a pH meter (PB-10, Sartorius, Göttingen, Germany). Water samples were filtered through cellulose acetate filter (pore size 0.45 μm) and divided into two parts. One part was used to directly measure the concentrations of NO3−, SO42−, Cl−, and F− using an ion chromatograph (ICS-5000, Thermo Corp., Waltham, MA, USA). The other part was first acidified with nitric acid, and then used to measure the concentrations of K+, Ca2+, Na+, and Mg2+ using inductively coupled plasma optical emission spectroscopy (ICP-OES 5100, Agilent, Santa Clara, CA, USA).
We determined the soil pH using the following method. After air-drying, the soil samples were first passed through a mesh sieve with a 2 mm aperture, then 10 g (accurate to 0.01 g) of the sample was put into a 50 mL beaker, to which 25 mL of pure water without CO2 was added. The solution was stirred with a mixer for 1 min to make sure the soil particles were fully dispersed. The solution was allowed to stand for 30 min, and then a pH meter (the same model as above) was used to measure pH.
2.4. Data Analysis
The average values shown on the rain gauges for rainfall or throughfall observation were taken as the measure of stand rainfall or throughfall depth. In order to reveal the rainfall redistribution pattern, we first upscaled the tree-level stemflow to the stand level using the following Equation (1) [36,37]:
where SF is the stand stemflow depth (mm), m is the number of trees belonging to a certain tree species, Vi is the average stemflow volume (mL) collected from a certain species, A is the area of the study plot (m2), and n is the number of tree species (n = 5).
To quantify the nutrient inputs from stemflow, throughfall, and rainfall, we determined the volume-weighted means per event E, calculated using Equation (2) [3,37]:
where C represents the mean concentration of a certain nutrient (mg L−1), Ci,E represents the nutrient concentration of the water sample in the collector i (mg L−1), and Vi,E represents the amount of water in collector i after rainfall event E (mm). Then, the input of a certain nutrient was computed using the following Equation (3) [37]:
where I is the nutrient input from a rainfall event (kg ha−1), C is the mean nutrient concentration in water samples (mg L−1), and V is the total amount of water sampled (mm).
The total wet input was measured as the sum of the solutes in the stemflow and throughfall. The canopy exchange effect was measured as the difference between total wet input and precipitation deposition, which was estimated by Equation (4) [3,38]:
where IN is the canopy exchange effect (kg ha−1), IT is the throughfall deposition (kg ha−1), Is is the stemflow deposition (kg ha−1), and IP is the precipitation deposition (kg ha−1).
2.5. Statistical Analysis
We used one-way analysis of variance (ANOVA) to compare the differences in nutrient concentrations between the rainfall, throughfall, and stemflow of different tree species. Since the nutrient concentration data did not meet the normal distribution, we transformed the data using the lg10 function to make them roughly meet this requirement. Additionally, we used linear regression analysis to test the correlations between throughfall, rainfall, stemflow, and rainfall. All statistical analyses were performed using IBM SPSS Statistics 26 (IBM, Armonk, NY, USA).
5. Conclusions
This study investigated the impact of the canopy on nutrient fluxes through rainfall partitioning in a mixed broadleaf and coniferous forest by measuring the variations in the amounts and concentrations of nutrients in the rainfall, throughfall, and stemflow of different tree species. Our results illustrated that the forest canopy significantly affected rainfall redistribution and nutrient content, and this impact varied largely between tree species due to the differences in their canopy structures and bark morphology. In general, throughfall and stemflow had more enriched nutrient and chemical elements than rainfall, especially the stemflow, which had the greatest enrichment. Specially, FM yielded less stemflow, but the nutrient contents were much higher in this species than in others because of its thicker and rougher bark. QM generated more stemflow because of its funnel shape, with its wide leaves intercepting more rainfall.
After rainfall passed through the canopy, most nutrient fluxes increased, except for F− and Na+, and the stemflow deposition of nutrients was different between species. The acidic secretion produced by PK acidified the stemflow, leading to a higher input of acid anions in its stemflow, whereas QM contributed the largest flux of cations. Such variation may change the soil microenvironment (including, but not limited to, soil pH) around the tree. Based on this study, coniferous tree species should be carefully selected for afforestation in areas heavily affected by soil acidification.
This study only briefly explores the differences in stemflow among tree species and their impact on the soil beneath the canopy. More in-depth research will be needed in the future to reveal the underlying mechanisms. And there is a lack of relevant research in cold climate zones currently. Conducting more related work in these areas may help us to better understand and protect the forest ecosystems in these regions.
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