Spatial variability and temporal stability of throughfall deposition under beech (Fagus sylvatica L.) in relationship to canopy structure

Nitrogen addition changes the canopy biological characteristics of dominant tree species in an evergreen broad-leaved forest

Many studies have focused on the impact of nitrogen deposition on plants, but due to technical limitations, research on the responses of forest canopy to manipulated nitrogen deposition is relatively scarce. Based on a canopy nitrogen addition (CN) platform, this study used laboratory analysis and unmanned aerial vehicle (UAV) observations to assess the impact of CN on the canopy traits of dominant tree species (Engelhardia roxburghiana, Schima superba, and Castanea henryi) in an evergreen broad-leaved forest in China. The results showed that nitrogen application at 25 kg N ha-1 y-1 (CN25) and 50 kg N ha-1 y-1 (CN50) significantly increased the actual net photosynthetic rate (An) of all the three tree species. CN25 significantly increased superoxide dismutase (SOD), catalase (CAT), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities in C. henryi. CN50 significantly increased the leaf area of all the three tree species and significantly reduced the leaf thickness of C. henryi, and significantly increased the POD and Rubisco activities in S. superba and C. henryi. CN significantly changed the number of forest gaps, but did not significantly change the area of forest gaps within the sample plots. CN25 significantly decreased the vertical projection area but increased the canopy flowering coverage of S. superba in dominant directions. CN25 and CN50 significantly increased the flowering coverage of C. henryi in favorable directions. It is found that under long-term (10-year) nitrogen addition, the balance between carbon fixation and antioxidant defense functions of E. roxburghiana may be broken down, but the carbon assimilation, antioxidant capacity and reproduction potential of S. superba and C. henryi may be well coordinated, which will have a potential impact on the species composition and ecological functions of the evergreen broad-leaved forest. This study may also provide scientific basis for forest management in the context of enhanced atmospheric nitrogen deposition.

The capacity of ion adsorption and purification for coniferous forests is stronger than that of broad-leaved forests

In the past few decades, industrialization has caused a large number of pollutants to be released into the atmosphere. Forest ecosystems play an important function in regulating the biogeochemistry and the circulation of metal ions pollutants. Forest ecosystems affect the absorption of pollutants and dissolution of nutrients from the atmosphere and vegetation canopy, thereby influencing the content and composition of forest floor leachate and soil solution. This study examined changes in acid anions (NO₃^-, SO₄^2-, Cl^-) and metal cations (K⁺, Ca²⁺, Na²⁺, Mg²⁺, Fe³⁺, Pb²⁺, Cu²⁺, Cd²⁺) in rainfall, throughfall, stemflow, and forest floor leachate for five different forests (Larix principis-rupprechtii, Picea wilsonii, Picea crassifolia, Betula platyphylla and Rhododendron communities). The results showed that the enrichment capacity of acid anions and metal cations in the vegetation canopy of the coniferous forests (L. principis-rupprechtii, P. wilsonii, P. crassifolia) was stronger than that of the broad-leaved forests (B. platyphylla and Rhododendron communities). The content of acid anions and metal cations in stemflow of coniferous forests were 3.7-5.6 times and 0-9.3 times higher than those of broad-leaved forests, respectively. Corresponding values in throughfall were 1-1.4 times and 0.3-2.4 times, respectively^. The contents of NO₃^-, Cl^-, K⁺, Mg²⁺, Fe³⁺, Pb²⁺, Cu²⁺, and Cd²⁺ in leachate filtered from the soil layers that are deepening gradually showed consistent decreasing trend for all the forest stands. In addition, NO₃^-, Cl^-, K⁺, Mg²⁺, Fe³⁺, and Pb²⁺ were also concentrated in the topsoil_, except for Cu²⁺ and Cd²⁺. Nevertheless, SO₄^2- and Na⁺ were concentrated in the subsoil, whereas Ca²⁺ was concentrated in the upper soil layers. Soil organic carbon (SOC) and total nitrogen (TN) contents in coniferous forest stands were 20-37% and 34-63% higher than those in broad-leaved forest stands, respectively. This results also shown that the contents of OC and TN has a strong correlation with the content of partial metal cations in soil and litter, indicating that coniferous forest stands had stronger ion scavenging and adsorption capacity in soil layer and litter layer than broad-leaved forest stands. Therefore, L. principis-rupprechtii, P. wilsonii, P. crassifolia had higher air pollutant adsorption and soil pollution remediation capacities than the other two forests. Thus, we recommend planting coniferous tree species (L. principis-rupprechtii, P. wilsonii and P. crassifolia) for eco-rehabilitation and water purification to improve the ecological service function of forest ecosystems.

Leaf Phenology Drives Spatio-Temporal Patterns of Throughfall under a Single Quercus castaneifolia C.A.Mey

Throughfall (TF) makes up the majority of understory rainfall and thereby plays an important role in controlling the amount of water reaching the forest floor. TF under a single Quercus castaneifolia (C.A.Mey, chestnut-leaved oak) tree in Northern Iran was measured during the leafed and leafless periods. TF quantity under the Q. castaneifolia canopy made up 69.3% and 88.0% of gross rainfall during leafed and leafless periods, respectively. Phenoseason influenced TF distribution patterns as TF temporal patterns during the leafed period were slightly more stable than during the leafless periods. The minimum number of TF collectors needed to yield a representative mean TF with accepted errors of 10% at 95% confidence level was twenty-six and twelve TF collectors for leafed and leafless periods, respectively. We conclude that phenoseasonality significantly affects TF spatiotemporal variability and presented the required number of collectors necessary for sampling TF under an individual Q. castaneifolia tree.

Interspecific Differences in Canopy-Derived Water, Carbon, and Nitrogen in Upland Oak-Hickory Forest

Oaks (Quercus) are a dominant forest species throughout much of the eastern United States. However, oak regeneration failure due to a myriad of issues (e.g., suppression of natural fire, excess nitrogen deposition, pressure from herbivore activity) is leading to a decline in oak dominance. This change may alter forest hydrology and nutrients through variation in species characteristics. Throughfall (TF) and stemflow (SF) quantity and chemistry were sampled during storm events under oak and non-oak (hickory, Carya) species to quantify differences in canopy-derived water and nutrients from an upland oak-hickory forest in Mississippi. Stemflow partitioning was 86% higher in hickory species compared to oak species (394.50 L m−2; p < 0.001). Across all species, dissolved organic carbon (DOC) was 1.5 times greater in throughfall (p = 0.024) and 8.7 times greater in stemflow (p < 0.001) compared to rainfall. White oak DOC concentrations (TF: 22.8 ± 5.5 mg L−1; SF: 75.1 ± 9.5 mg L−1) were greater compared to hickory species (TF: 21.0 ± 18.3 mg L−1; SF: 34.5 ± 21.0 mg L−1) (p = 0.048). Results show that while smoother-barked hickory species generate more stemflow volume, rougher-barked oak species generate stemflow that is more enriched in nutrients, which is a function of the canopy characteristics of each species. Within a single stand, this study demonstrates how variable water and nutrient fluxes may be and provide insights into species-level variability in oak-hickory forest types that may be undergoing compositional changes.

Spatial and Temporal Variability of Throughfall among Oak and Co-occurring Non-oak Tree Species in an Upland Hardwood Forest

Canopy throughfall comprises the largest portion of net precipitation that is delivered to the forest floor. This water flux is highly variable across space and time and is influenced by species composition, canopy foliage, stand structure, and storm meteorological characteristics. In upland forests throughout the central hardwoods region of the Eastern United States, a compositional shift is occurring from oak-hickory to more mesic, shade-tolerant species such as red maple, sweetgum, and winged elm. To better understand the impacts of this shift on throughfall flux and the hydrologic budget, we monitored throughfall for one year in Northern Mississippi under the crowns of midstory and overstory oak (post oak and southern red oak) and non-oak species (hickory, red maple, and winged elm). In general, oak had more throughfall than co-occurring non-oak species in both canopy levels. In the overstory during the leaf-off canopy phase, white oak had relatively higher throughfall partitioning (standardized z-score = 0.54) compared to all other species (z-score = −0.02) (p = 0.004), while in the leaf-on canopy phase, red maple had relatively lower throughfall (z-score = −0.36) partitioning compared to all other species (z-score = 0.11). In the midstory, red maple was the only species to exhibit a difference in throughfall between canopy phases, with much lower throughfall in the leaf-off compared to the leaf-on canopy phase (z-score = −0.30 vs. 0.202, p = 0.039). Additionally, throughfall under oak crowns was less variable than under non-oak crowns. These results provide evidence that the spatial and temporal distribution of throughfall inputs under oak crowns are different than non-oak species, likely due to differences in crown architecture (i.e., depth and density). As oak dominance diminishes in these forests, it is possible that the portion of rainfall diverted to throughfall may decrease as well. The net impacts to watershed hydrology are still unknown, but these results provide one mechanism by which the distribution of water resources may be affected.

Effect of water stage and tree stand composition on spatiotemporal differentiation of spring water chemistry draining Carpathian flysch slopes (Gorce Mts)

The purpose of this study was to identify the factors affecting spring water chemistry in different tree stands and to measure the influence of water stage on the physicochemical parameters of spring waters in a small Carpathian catchment. Water samples were collected three times per year at various stages of the water: after the spring thaw, after a period of heavy rain and after a dry period in 2011 and 2012. Water samples were left in the laboratory to reach room temperature (19-20°C) and analyzed for EC (reference T=25°C) and pH. After filtration through 0.45μm PTFE syringe filters, the water samples were analyzed by means of ion chromatography using a DIONEX ICS 5000 unit. The following ions were analyzed: Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^-, SO₄^2-, Cl^-, and NO₃^-. Multivariate analysis (PCA) allowed the identification of two factors of spring water chemistry: factor 1, water stage and factor 2 tree stand composition. Seasonal variation of spring water chemistry showed that, higher pH values and mineralization as well as higher concentrations of Ca²⁺ and Mg²⁺ were measured during low water stage periods while lower EC and pH values were noted after spring snowmelt and rainfall, when higher concentrations of NO₃^- and SO₄^2- were also found. Higher concentrations of Ca²⁺ and Mg²⁺ and higher pH of spring waters located in beech-fir stands and in those mixed with a large proportion of beech as well as a lower concentration of Ca²⁺, Mg²⁺ and HCO₃^-, pH, conductivity and mineralization of these spring waters, in which the alimentation areas were covered by upper subalpine spruce stands were noted.

Factors controlling throughfall in a Pinus tabulaeformis forest in North China

The factors that control throughfall in Pinus tabulaeformis plantations were investigated using linear and curve analyses based on direct measurements of rainfall, throughfall and stemflow from 36 rainfall events. The results showed the following: (1) there was significant spatial heterogeneity in throughfall rates in P. tabulaeformis plots; (2) the throughfall rate increased with increasing rainfall; and (3) the rate of increase gradually decreased. When rainfall reached approximately 25 mm, the throughfall rate stabilized. The coefficient of variation of the throughfall rate decreased with increasing rainfall, with a peak at approximately 10 mm of rainfall. The coefficient of variation of throughfall stabilized at 20%, and the coefficient of variation of the throughfall rate stabilized at 17%. A linear regression equation (R2 = 0.76) was derived by fitting the P. tabulaeformis average diameter at breast height (DBH), average tree height, average branch height, stand density, canopy thickness, canopy density, and the rainfall and throughfall rate. A highly positive correlation was found between the throughfall rate, canopy density, rainfall class and tree height (P < 0.01). By establishing a quadratic response surface model of the stand structure indicators and the throughfall rate, R2 was increased to 0.85 (P < 0.01). The quadratic regression analysis demonstrated a highly positive correlation between throughfall rate, canopy density and rainfall class.

Equation to predict the (137)Cs leaching dynamic from evergreen canopies after a radio-cesium deposit

The Fukushima Daiishi nuclear power plant (FDNPP) accident led to a massive radionuclide deposition mainly onto Japanese forest canopies. In our previous study, an improved double exponential (IDE) equation including rainfall intensity was proposed to estimate the (137)Cs hydrological transport from evergreen canopies to the ground. This equation used two types of parameters, kinetic (k1 and k2) and leachable stock (A1 and A2). Those parameters have been estimated by adjusting them in the IDE equation in order to accurately describe the measured cumulative leached (137)Cs from canopies (k1 = 4.2E-04-5.0E-04 d(-1), k2 = 1.2E-02-1.7E-02 d(-1), A1 = 62-99 kBq/m(2), A2 = 25-61 kBq/m(2)). In this study, we linked the total leachable stock (Aleachable, a parameter of the IDE equation corresponding to A1 + A2) to a physiological criteria (the canopy closure CC, which can be measured with a simple camera equipped with a fish-eye objective). Furthermore, the kinetic parameters measured for Japanese cedar (k1 = 5.0E-04 d(-1), k2 = 1.2E-02 d(-1), and r12 = 0.22 (r12 = A1/A2) could also be used for two other coniferous species: Japanese cypress and spruce. This suggests that these parameters could be constants for coniferous forests.

Base flow and stormwater net fluxes of carbon and trace metals to the Mediterranean sea by an urbanized small river

Base flow and storm flow events from a small, urbanized Mediterranean river located in the South of France were studied to evaluate net fluxes from the continent to the coastal sea water. Considered variables were: pH, conductivity, redox potential, temperature, dissolved O(2), SPM (Suspended Particulate Matter), Cl(-), NO(3)(-), SO(4)(2-), Na(+), K(+), Mg(2+), Ca(2+), DOC (Dissolved Organic Carbon), Cu, Zn, Cd and Pb in the dissolved fraction and POC (Particulate Organic Carbon), Cr, Mn, Fe, Ni, Cu, Zn, Cd and Pb in the particulate fraction. If rainfall intensity and antecedent rainfall history were sufficient to explain many observed variations, the patterns of particles and solutes transport greatly varied from one storm event to another. SPM, POC, particulate Cu, Zn, Cd and Pb had similar behavior characterized by an immediate increase at the beginning of the storm flow and the highest values at the first high discharge peak. Among dissolved species, Cl(-), SO(4)(2-), Na(+), Ca(2+) and Mg(2+) had a behavior very similar one to the other. They exhibited high concentrations and enrichment factors at the beginning of the storm flow, due to fast leaching of highly labile species. Their concentrations decreased during posterior discharge peaks but positive enrichment factors indicated permanent sources for these ions. DOC, K(+) and NO(3)(-) had different behaviors which indicated sources positively correlated with rainfall intensity. A 3D-fluorescence study showed that the humified organic matter contribution to DOC increased during storm flow. Relationships between DOC and dissolved Cu and Pb indicated differences in organic-matter binding properties between dry and humid periods. Storm flow were responsible for more than 90% of the annual output to the sea of SPM, POC and particulate Cu, Zn, Cd and Pb and more than 70% for dissolved Pb, Cd, NO(3)(-) and DOC. For the other dissolved species, outputs were balanced between base flow and storm flow. Contrary to what was observed in large rivers, organic carbon was mainly transported in the POC fraction. The average specific fluxes of Cu and Pb to the sea were 3.8 and 3.4 kg km(-2) y(-1), respectively, of the same order of magnitude than specific fluxes of other North-Mediterranean rivers, but outputs were more intense during shorter durations. The extrapolation of the Eygoutier River data to the Mediterranean non-desert coastline showed that the order of magnitude of Cu and Pb annually brought to the sea by the whole of small anthropized coastal rivers can be similar to the annual input by the Rhône or the Po River.

Throughfall deposition and canopy exchange processes along a vertical gradient within the canopy of beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst)

To assess the impact of air pollution on forest ecosystems, the canopy is usually considered as a constant single layer in interaction with the atmosphere and incident rain, which could influence the measurement accuracy. In this study the variation of througfall deposition and derived dry deposition and canopy exchange were studied along a vertical gradient in the canopy of one European beech (Fagus sylvatica L.) tree and two Norway spruce (Picea abies (L.) Karst) trees. Throughfall and net throughfall deposition of all ions other than H(+) increased significantly with canopy depth in the middle and lower canopy of the beech tree and in the whole canopy of the spruce trees. Moreover, throughfall and net throughfall of all ions in the spruce canopy decreased with increasing distance to the trunk. Dry deposition occurred mainly in the upper canopy and was highest during the growing season for H(+), NH(4)(+), NO(3)(-) and highest during the dormant season for Na(+), Cl(-), SO(4)(2-) (beech and spruce) and K(+), Ca(2+) and Mg(2+) (spruce only). Canopy leaching of K(+), Ca(2+) and Mg(2+) was observed at all canopy levels and was higher for the beech tree compared to the spruce trees. Canopy uptake of inorganic nitrogen and H(+) occurred mainly in the upper canopy, although significant canopy uptake was found in the middle canopy as well. Canopy exchange was always higher during the growing season compared to the dormant season. This spatial and temporal variation indicates that biogeochemical deposition models would benefit from a multilayer approach for shade-tolerant tree species such as beech and spruce.

Tropical land-cover change alters biogeochemical inputs to ecosystems in a Mexican montane landscape

In tropical regions, the effects of land-cover change on nutrient and pollutant inputs to ecosystems remain poorly documented and may be pronounced, especially in montane areas exposed to elevated atmospheric deposition. We examined atmospheric deposition and canopy interactions of sulfate-sulfur (SO4(2-)-S), chloride (Cl-), and nitrate-nitrogen (NO(3-)-N) in three extensive tropical montane land-cover types: clearings, forest, and coffee agroforest. Bulk and fog deposition to clearings was measured as well as throughfall (water that falls through plant canopies) ion fluxes in seven forest and five coffee sites. Sampling was conducted from 2005 to 2008 across two regions in the Sierra Madre Oriental, Veracruz, Mexico. Annual throughfall fluxes to forest and coffee sites ranged over 6-27 kg SO4(2-)-S/ha, 12-69 kg Cl-/ha, and 2-6 kg NO(3-)-N/ha. Sulfate-S in forest and coffee throughfall was higher or similar to bulk S deposition measured in clearings. Throughfall Cl- inputs, however, were consistently higher than Cl- amounts deposited to cleared areas, with net Cl- fluxes enhanced in evergreen coffee relative to semi-deciduous forest plots. Compared to bulk nitrate-N deposition, forest and coffee canopies retained 1-4 kg NO(3-)-N/ha annually, reducing NO(3-)-N inputs to soils. Overall, throughfall fluxes were similar to values reported for Neotropical sites influenced by anthropogenic emissions, while bulk S and N deposition were nine- and eightfold greater, respectively, than background wet deposition rates for remote tropical areas. Our results demonstrate that land-cover type significantly alters the magnitude and spatial distribution of atmospheric inputs to tropical ecosystems, primarily through canopy-induced changes in fog and dry deposition. However, we found that land cover interacts with topography and climate in significant ways to produce spatially heterogeneous patterns of anion fluxes, and that these factors can converge to create deposition hotspots. For land managers, this finding suggests that there is potential to identify species and ecosystems at risk of excess and increasing deposition in montane watersheds undergoing rapid transformation. Our data further indicate that montane ecosystems are vulnerable to air pollution impacts in this and similar tropical regions downwind of urban, industrial, and agricultural emission sources.