Canopy throughfall links canopy epiphytes to terrestrial vegetation in pristine conifer forests

To what extent does epiphytic community composition in boreal forests reflect soil nutrient status? To answer this question, we investigated potential links between terrestrial plants and lichen-dominated communities in Picea glauca x engelmannii canopies in south-central British Columbia. We combined original data on vegetational composition in the ground and tree layers with published data on elemental uptake in standardized transplants of the lichen Lobaria pumonaria affixed to the lower branches of studied trees. Procrustean association metrics revealed a significant concordance between community composition among the epiphytic and terrestrial vegetation, which was stronger in Mn-enriched conifer canopy settings, and weaker within the cation-enriched dripzones of adjacent Populus trees. We also found that the NMDS1-gradient for each vegetation layer was strongly correlated with base cations (and inversely with Mn) in canopy throughfall, as well as with soil and bark pH. We conclude that elemental composition in canopy throughfall forms a functional link between terrestrial and epiphytic vegetation.     

鼎湖山季风常绿阔叶林大气降雨、穿透雨和树干流的养分特征研究

对鼎湖山季风常绿阔叶林大气降雨、穿透雨和树干茎流中的5种养分元素K、Ca、Mg、N、P进行了测定,结合水量分配规律,研究了森林降雨过程中养分在水相中的含量变化特征和输入规律。结果表明：(1)所有离子浓度均为大气降水<穿透雨<树干流,且增幅较大,而平均浓度以K+和总氮(TN)含量最高;总磷(TP)、磷酸盐(HPO42-)、总有机磷(TOP)含量均最低。(2) 大气降雨中的离子平均浓度中以总有机氮(TON)的变异系数最大,为1.282;最小的是NO3-(0.502);穿透雨中变异系数最大的是TOP(2.357);最小的是TN(0.621)。树干流中各养分元素浓度与树种的相关性不显著(P>0.05)。(3) 季风常绿阔叶林树干流和穿透雨各养分对森林土壤的年输入量为TN>K+>Ca2+>Mg2+>TP,树干流和穿透雨对森林土壤层Ca2+的输入大于凋落物分解输入。因此,大气降雨是养分从林冠层转移到土壤层的重要因素。     

Hydrologic flowpaths influence inorganic and organic nutrient leaching in a forest soil

Hydrologic pathways through soil affect element leaching by determining the relative importance of biogeochemical processes such as sorption and decomposition. We used stable hydrogen isotopes of water (δD) to examine the influence of flowpaths on soil solution chemistry in a mature spruce–hemlock forest in coastal Oregon, USA. Soil solutions (50 cm depth, n = 13) were collected monthly for 1 year and analyzed for δD, major ions and dissolved organic carbon (DOC) and nitrogen (DON). We propose that the variability of δD can be used as an index of flowpath length and contact time. Throughfall variability in δD was much greater than soil solution variability, illustrating that soil solution integrates the variation in inputs. Lysimeters with greater variation in δD presumably have a greater proportion of flow through rapid flowpaths such as macropores. The variation in soil solution δD for individual lysimeters explained up to 53% of the variation in soil solution chemistry, and suggests that flowpaths influence leaching of some constituents. Soil solutions from lysimeters with greater δD variation had higher DOC and DON (r ² = 0.51 and 0.37, respectively), perhaps because transport via macropores reduces interaction of DOM with the soil matrix. In contrast, nitrate concentrations were highest in lysimeters with a small variation in δD, where long contact time and low DOC concentrations may yield higher net nitrification. Our results demonstrate the utility of stable isotopes to link flowpaths and soil solution chemistry, and illustrate how the spatial complexity of soils can influence ecosystem-level nutrient losses.     

Nitrogen Cycling and Mass Balance for a Forested Catchment in the Italian Alps. Assessment of Nitrogen Status

During 1999–2001 the chemical composition and fluxes were measured in rainfall, throughfall, soil solution and stream water in a remote forested site in the Italian Alps. The analysis of temporal patterns revealed the differential behaviour of nitrogen and sulphur and suggested that different mechanisms controlled their flux. No important changes in sulphate concentration and fluxes emerged as the solution passed through the various components of the forest ecosystem, and temporal variations of SO₄ in the soil solution and stream were likely driven by the physical process of dilution. The availability of nitrate and ammonia, by contrast, was drastically reduced as throughfall water entered the soil and passed through the mineral layers, irrespective of season. The calculated hydrochemical budget based on throughfall and soil solution N fluxes revealed that ~80% N retention in the forest soil, corresponding to 12 kg ha⁻¹ yr⁻¹, despite a relatively high N deposition loading (15 kg ha⁻¹ yr⁻¹). Most of the leached nitrogen (90%) was in the organic form. Indicators of the N status of this ecosystem, such as C/N ratio in solid and solution phase of the soil and N foliage content as well as land use history were examined. Despite the strong N retention in the forested part of the catchment, the stream water N–NO₃ levels were consistently above 10 μg l⁻¹ suggesting that the Val Masino catchment as a whole was less efficient in processing atmospheric N inputs. This contrasting N behaviour illustrates the role of landscape features, such as the soil cover and vegetation type, that is characteristic of an alpine catchment.     

Increased mercury in forest soils under elevated carbon dioxide

Fossil fuel combustion is the primary anthropogenic source of both CO₂ and Hg to the atmosphere. On a global scale, most Hg that enters ecosystems is derived from atmospheric Hg that deposits onto the land surface. Increasing concentrations of atmospheric CO₂ may affect Hg deposition to terrestrial systems and storage in soils through CO₂-mediated changes in plant and soil properties. We show, using free-air CO₂ enrichment (FACE) experiments, that soil Hg concentrations are almost 30% greater under elevated atmospheric CO₂ in two temperate forests. There were no direct CO₂ effects, however, on litterfall, throughfall or stemflow Hg inputs. Soil Hg was positively correlated with percent soil organic matter (SOM), suggesting that CO₂-mediated changes in SOM have influenced soil Hg concentrations. Through its impacts on SOM, elevated atmospheric CO₂ may increase the Hg storage capacity of soils and modulate the movement of Hg through the biosphere. Such effects of rising CO₂, ones that transcend the typically studied effects on C and nutrient cycling, are an important next phase for research on global environmental change.     

暖温带两种针叶林生态系统中茎流和穿透雨的养分特征研究

通过对暖温带地区两种典型人工针叶林生态系统内树干茎流和穿透雨的收集,对其中Ｎ、Ｐ、Ｋ、Ｃａ、Ｍｇ、Ｓ和Ａｌ等７种元素进行了测定。研究发现这些养分元素在树干茎流和穿透雨中的含量在年内有明显的时间动态变化,这些变化主要受元素在水分中的含量和它们在植物生命活动中的活跃程度等影响,甚至在不同的针叶林生态系统之间都有一定的差异,其中表现为明显的是Ｎ、Ｋ和Ｓ。     

几种常用造林树种冠层对降水动能分配及其生态效应分析

本文从理论上系统地论述了林冠如何调节到达林地的降水动能,推导了计算大气降水和穿透水动能的理论和方法。经由理论推导指出并非所有的森林都适合作为水土保持林,即由穿透水的水滴直径较大,而且不受大气降水强度的影响及穿透水常常不是以最大的稳定速度到达林地这两个规律来讨论森林冠层在水土保持功效方面所具有的地位,可以做为营造水土保持林的理论基础。     

Trace elements in the hydrologic cycle of a forest ecosystem

Summary Concentrations of Cu, Fe, Mn, and Zn were measured in bulk atmospheric precipitation, throughfall, stemflow, and soil solutions at 10−, 15−, 25−, and 30-cm depths, in aEucalyptus globulus forest in the Berkeley hills, California, during the 1974–75 wet season after each main storm event. Litter and plant samples were analyzed. There was some similarity in the behavior of Cu, Fe, and Zn, but Mn behaved differently. Mn and Zn are largely deposited on the forest canopy by impaction during dry-deposition episodes, whereas most of the Cu and Fe input occurs in rain. For the hydrologic components measured, concentrations of Cu and Fe increase in the order: precipitation<throughfall<stemflow <soil solutions. For Zn the order is: precipitation<stemflow<throughfall<soil solutions. Concentrations of Cu, Zn, and Fe in the soil solution fluctuate with downward movement of wetting fronts and are negatively correlated with pH. Concentrations of Fe in soil solution are about 10 times greater than those of throughfall and stemflow; the corresponding relative differences for Cu and Zn were much less. Plant uptake of Mn exceeds that of Cu, Zn, and Fe. The increases in Mn concentrations from precipitation to throughfall and stemflow are much greater than those for Cu, Zn, and Fe because precipitation has very low Mn concentrations. The concentration series for Mn is: precipitation<soil solutions<throughfall<stemflow. Concentrations of Mn in the soil solution are negatively correlated with pH. During the dry summer Mn accumulates in the soil, but is quickly flushed by early rains of the wet season.     

Effects of land-use change on solute fluxes to floodplain lakes of the central Amazon

A time-series analysis of airborne photographs and Landsat thematic mapper (TM and ETM+) images and hydrochemical data were used to examine the effects of land-use change from 1930 to 2001 on solute inputs to Lake Calado, a floodplain lake in the central Amazon. Deforestation from slash-and-burn agricultural activities has dramatically decreased the amount of primary growth upland and flooded forests in the basin. The increasing area that is converted to agricultural plots and pasture in the Lake Calado basin has increased solute loading to the lake from upland tributaries (storm and base flow), bank seepage and overland flow, and decreased throughfall inputs. Whereas solute concentrations in stream water were generally higher in 1992 than 1930, Na⁺ and Cl^– concentrations were also considerably higher in 2001 than 1992, likely because of an increase in the number of humans and cattle in the watershed. Estimates of solute inputs to Lake Calado via throughfall indicate that the mass transfer of some major solutes in the throughfall of undisturbed flooded forests can be larger than that from a combination of all other sources in areas that do not have a strong influence from the Solimões River. Chemical gains in rain as it passed through the forest canopy occurred for most major ions and relatively large gains were observed for and Ca²⁺. Although often neglected in studies of tropical forest ecosystems, throughfall can be an important source of solutes to relatively undisturbed lake environments in the central Amazon.     

Solute fluxes in throughfall and stemflow in four forest ecosystems in northwest Amazonia

The contribution of throughfall and stemflow as pathways for solute inputs into the forest floor in four mature forests in northwest Amazonia was investigated. Total solute inputs, resulting from the changes of atmospheric deposition after rainfall passes through the forest canopy, are presented in the form of throughfall and stemflow nutrient inputs and their possible sources are discussed. Throughfall is by far the most important solute input into the forest floor of the forests studied. On average, it represents about 98% of the total solute inputs. Although trends in solute enrichment varied among the forests, there is a general tendency in all ecosystems towards a distinct enrichment of SO₄, K, Cl, NO₃ and NH₄ in throughfall and stemflow and a small increase of protons, Mn, orthoP and Fe. When comparing the net enrichment between the forests, the relative increase of solutes in throughfall and stemflow was higher in the flood plain and low terrace than in the high terrace and sedimentary plain forests. While highest values for total cation inputs were observed in the flood plain, the low terrace showed the highest value for total inorganic anions. The length of the antecedent dry period was the main factor affecting throughfall and stemflow composition, concentrations increasing with increasing length. A second, less important factor was the amount of throughfall and stemflow, which showed a poor and negative correlation with solute concentrations. The increased activity of frugivores in the canopy during fruiting periods seemed to lead to temporary increased solute concentrations in throughfall and stemflow as a result of the wash off of deposited faecal materials and detritus in the canopy. Leaching from leaves and wash off of exudes, of solutes deposited on the foliage after evaporation of intercepted rainfall and of dry deposited materials were all found to contribute to the concentration of solutes in the throughfall and stemflow. Gross rainfall enrichment after passing the forest canopy, mainly by nutrient leaching, is considerably lower than the amounts of nutrients released in litterfall implying a tight nutrient cycling and nutrient conserving mechanisms by forests studied.