川西亚高山典型森林生态系统截留水文效应

截留是水文循环的一个重要过程,水文功能是森林生态系统功能的重要方面,林冠和枯落物截留实现对大气降水的二次分配过程.为深入认识生态系统截留的水文效应,采用野外观测和人工降雨模拟试验相结合的方法,研究了2008年和2009年5-10月贡嘎山亚高山峨眉冷杉中龄林、峨眉冷杉成熟林和针阔混交林的冠层枯落物截留能力.结果表明,峨眉冷杉中龄林2008年林冠截留率为20.9％,针阔混交林2008年和2009年林冠截留率分别为23.0％和23.6％,林冠截留率的年际间变化不大,林冠截留主要受到降雨特征影响.3种林型枯落物饱和持水能力分别为5.1、5.1和5.7 mm,显著高于林冠的饱和持水能力,但由于冠层的截留蒸发速率较高,林冠截留蒸发仍是生态系统截留蒸发的主要组成部分.     

Gash模型在黄土区人工刺槐林冠降雨截留研究中的应用

为了验证Gash林冠截留解析模型在黄土高原人工林中的适用性,基于2009年黄土丘陵沟壑区吕梁市王家沟小流域刺槐林样地降雨观测数据,采用Gash模型对林冠截留进行了模拟。所选刺槐林分为人工纯林,林龄约30a,阴坡,坡度24°,密度为990株/hm2,平均树高10.8 m,平均胸径12.4 cm,郁闭度0.76。根据回归方法确定了Gash模型中的主要参数,包括饱和林冠的平均蒸发速率(E珔)、林冠枝叶部分的持水能力(S)、自由穿透降雨系数(p)、树干持水能力(St)和树干茎流系数(Pt)。结果显示,2009年5月至10月人工刺槐林样地实测降雨量为366.9 mm,穿透降雨量为317.5 mm,树干茎流为10.2 mm,林冠截留量为39.2 mm。模型模拟的林冠截留量为42.4 mm,高于实测值3.2 mm,相对误差为8.2%。敏感性分析表明,S、E珔、St和pt每增加10%,林冠截留量分别增加4.7%,3.1%,1.7%和0.5%;p增加10%,林冠截留量则减少2.6%。说明树干持水能力(St)和树干茎流系数(pt)两个参数对黄土高原人工刺槐林冠截留量的预测值影响程度较小。模拟值与实测值有较好的一致性,显示Gash模型适用于黄土高原人工刺槐林冠的截留计算。     

华北落叶松人工林蒸散及产流对叶面积指数变化的响应

定量评价林地蒸散和产流等水文过程对冠层叶面积指数(LAI)的响应,对于深入认识森林植被的生态水文过程及其发生机制,实现半干旱区林水综合管理和区域可持续发展是非常必要的。应用集总式生态水文模型BROOK90,模拟分析了不同降水年型(丰水年、平水年、枯水年)下,位于半干旱区的宁夏六盘山叠叠沟小流域内华北落叶松(Larix principis-rupprechtii)人工林的水文过程对冠层LAI变化的响应关系。结果发现:林分总蒸散量、冠层截留量、蒸腾量与LAI都呈显著的正相关关系(R~20.99,P0.01),而土壤蒸发量、产流量则与LAI均呈显著的负相关关系(R~20.99,P0.01);在不同的降水年型下,各水文过程变量与LAI的关系都可以很好地用指数函数来表达,且都存在着一个LAI阈值。当LAI低于阈值时,各水文过程变量随LAI的变化幅度较大;但高于阈值时,各变量的变化十分缓慢并趋于稳定。在不同降水年型下,各变量LAI阈值之间存在着一定的差异。一般地,丰水年各变量的LAI阈值要大于枯水年,尤其是冠层截留和土壤蒸发。在丰水年,各水文过程变量随LAI增加而变化的速率要比在平水年、枯水年更快,说明在水分充足年份中各变量的波动更多取决于LAI变化,而在水分亏缺的年份中则可能更多地受到水分条件的限制。模拟结果表明,通过减少冠层LAI(如间伐)导致的林分的降低蒸散耗水和增加产流的作用是有限的,这是由于林分蒸散降低的幅度要比LAI降低的幅度小。例如,在平水年,当LAI从4.2变为2.0(减少幅度52.4%)时,林分年蒸散仅从357.2 mm减少至333.9 mm(减少幅度6.5%)。     

北亚热带3种森林群落对大气湿沉降重金属的调控

森林群落截留消纳沉降重金属的能力影响着森林集水区溪水输出重金属的情况,决定着小流域的水质安全。为评价北亚热带地区典型森林生态系统对大气降水中主要重金属离子的截留能力和分配特征,以浙江庙山坞林场3种典型森林群落(毛竹林、杉木林、青冈阔叶林)和森林集水区-小溪为研究对象,于2018年7月—2019年6月监测了12次降水事件,分析、比较和讨论了大气降水、林内穿透雨、树干茎流、枯透水、地表径流和集水区溪水中7种重金属(铅(Pb)、锌(Zn)、镉(Cd)、铜(Cu)、砷(As)、镍(Ni)和锰(Mn))的质量浓度和通量的动态变化。结果表明,大气降水中7种重金属,Pb、Cd、As、Cu、Zn、Mn和Ni的年均浓度分别为0.974、0.124、0.512、3.42、36.7、8.48和1.94μg/L,3种森林群落林冠层截留的干沉降重金属中Zn和Mn的比例最高,林冠层降水对重金属Mn和Zn的淋溶量极高,超过大气降水沉降量的2.78倍和54.2倍,同时截留了降水中的As和Ni;枯落物层对Mn、Cd和Zn表现出截留作用,对Cu和Ni有淋溶或释放的作用;3种森林群落的地表径流中Pb、Mn、Cd和Zn浓度明显降低,As和Ni的浓度略高于枯透水,但由于地表径流量非常小,土壤表层对降水中重金属均表现出极强的截留作用。3种森林群落对大气降水中的重金属均表现出极高的截留率,森林群落之间没有显著差异,其中青冈阔叶林的截留能力最强,杉木林较弱。而地表径流及土壤渗透水等汇集到森林集水区后,溪水中重金属Mn、Ni、Cd和Zn的浓度和通量均有大幅的增加,仅对重金属As和Cu呈现截留的作用,这可能与森林土壤重金属本底值偏高以及森林长期接收的重金属沉降在降水淋溶下从酸性土壤中大量溶出有关。     

Photosynthesis and leaf morphology ofLiquidambar styraciflua L. under variable urban radiant-energy conditions

Diminished sunlight, characteristic of urban canyons, has been suggested as being potentially limiting to plant growth. This study investigated the response of sweetgum (Liquidambar styraciflua L.) to variable irradiance in a range of urban locations. Diurnal photosynthesis was measured in situ on mature trees, comparing an open site at an urban park with an urban canyon that received 4 h of midday sun in midsummer. Photosynthesis for trees growing in the canyon was lower both during shaded and sunlit periods compared with trees at the park. Photosynthesis of detached shoots in a growth chamber was greater in canyon than park foliage at low irradiance, indicating possible photosynthetic shade acclimation analogous to tree species growing in the forest understorey. Shoot and trunk growth and morphological characteristics were measured onL. styraciflua growing along boulevards at 15 additional urban sites and related to seasonal interception of solar radiation. Angular elevation and orientation of buildings and trees that defined the horizon topography at each site were used in modeling the potential irradiance of global shortwave radiation. Seasonal irradiance among sites ranged from 21% in the urban core to nearly 95% in outlying residential districts of that potentially received under an unobstructed horizon. Shade acclimation was confirmed by differences in leaf morphology, as foliage became flatter, thinner, and more horizontally oriented at sites with lower irradiance. Photosynthetic and morphological acclimation to shade did not compensate for lower available radiant energy as both shoot and trunk growth decreased at sites of lower irradiance. Unlike the forest understorey, the static light environment of urban canyons may subject shade-intolerant species such asL. styraciflua to chronic, low-radiant-energy stress.     

林冠截持降雨模型的初步研究

通过对林冠截持过程的研究,将单场降雨划分成很多个小的时段,顺序计算各小时段内降雨在林冠内的分配,建立了一个林冠降雨截持模型.模型考虑了冠层和树干干燥度对冠层雨期蒸发的影响,并在计算雨期蒸发时引入冠层叶面积指数、单位林地面积上树干表面积.运用模型对华北落叶松林的降雨截持进行模拟,结果表明,模拟穿透雨与实测穿透雨基本吻合,误差在±1 mm范围内,但在小降雨(＜6 mm)时模拟值偏低;树干茎流量模拟值偏低,茎流模拟相对误差随着降雨量增大而减小.同时模拟了穿透降雨的过程,结果与林内自动气象站实测穿透降雨的过程基本吻合,模拟效果较好.     

A forest-structure-based analysis of rain flow into soil in a dense deciduous Betula ermanii forest with understory dwarf bamboo

The relationship between rain flow into the soil and forest structure was investigated in a dense deciduous Betula ermanii forest in northern Japan. The forest floor was covered with dwarf bamboo Sasa kurilensis. Observation was conducted from mid-July to late October in 1998. Leaf fall of Betula started in early September and ended in late October. Stemflow was proportional to rainfall and tree size [diameter at breast height (DBH)], and for the same rainfall, stemflow increased with leaf fall. On the contrary, throughfall decreased with leaf fall. Throughfall was intercepted also by Sasa in proportion to its leaf area. Multiple linear regression analysis revealed that stemflow and throughfall of Betula and Sasa were predictable as functions of rainfall and forest structural characteristics, such as DBH, tree density, and stand leaf mass. The rain interception by plants tended to decrease from summer to autumn, but the difference in the interception was about 2% between July (fully expanded leaves) and late October (lack of leaves). About 96 and 87% of rainfall reached the above- and below-Sasa layers, respectively. Thus, this study showed that understory Sasa is a major component of rain interception within the stand and that rain flow into the soil can be estimated by using rainfall and the forest structural parameters, such as DBH, tree density and stand leaf mass.     

The potential for forest canopy litterfall interception by a dense fern understorey, and the consequences for litter decomposition

Most studies on litter decomposition have assumed that all falling plant litter reaches the ground where it then decomposes. In many forests a proportion of this litter may in fact be intercepted by understorey vegetation, but the ecological significance of this remains poorly understood. We performed two experiments in a temperate rainforest in southern New Zealand, in which there was a dense understorey of the crown fern Blechnum discolor. The fronds of this fern originate from a crown, and have a funnel‐like arrangement that can trap falling litter and prevent it from reaching the ground. The first experiment measured the effects of ferns on the spatial distribution of litter accumulation over one year. The ferns intercepted a substantial proportion of the total litterfall, and the fern crowns (from which the fronds originate) retained 10% of the total incoming litterfall (despite occupying only 2% of the ground area). The retained litter had a substantially higher ratio of twig to foliar litter than did the incoming litterfall. Further, much of the litter not retained on the crowns of the ferns accumulated at the base of the fern trunks. The second experiment considered litter decomposition in fern crowns versus the ground under the ferns. The litter that had accumulated in the crowns was characterized by higher microbial basal respiration and active microbial biomass than was the litter that had accumulated on the ground. The use of litterbags revealed that litter decomposition rates were significantly higher on the fern crowns than on the ground at 30 cm and 60 cm from the fern trunks. These results show that litter interception ameliorates the decomposer environment and increases the rate of litter decomposition. In total, this study provides evidence for understorey ferns greatly influencing both the spatial distribution of litterfall and the decomposition of plant litter. Although the ecological role of understorey vegetation in forested ecosystems has received little attention to date, our results point to understorey species as an important driver of forest ecosystem processes.     

Hydrologic balance in an intact temperate forest ecosystem under ambient and elevated atmospheric CO2 concentration

Abstract Increasing atmospheric CO₂ concentration decreases stomatal conductance in many species, but the savings of water from reduced transpiration may permit the forest to retain greater leaf area index (L). Therefore, the net effect on water use in forest ecosystems under a higher CO₂ atmosphere is difficult to predict. The free air CO₂ enrichment (FACE) facility (n = 3) in a 14‐m tall (in 1996) Pinus taeda L. stand was designed to reduce uncertainties in predicting such responses. Continuous measurements of precipitation, throughfall precipitation, sap flux, and soil moisture were made over 3.5 years under ambient (CO₂^a) and elevated (CO₂^e) ambient + 200 µmol mol^?1). Annual stand transpiration under ambient CO₂ conditions accounted for 84–96% of latent heat flux measured with the eddy‐covariance technique above the canopy. Under CO₂^e, P. taeda transpired less per unit of leaf area only when soil drought was severe. Liquidambar styraciflua, the other major species in the forest, used progressively less water, settling at 25% reduction in sap flux density after 3.5 years under CO₂^e. Because P. taeda dominated the stand, and severe drought periods were of relatively short duration, the direct impact of CO₂^e on water savings in the stand was undetectable. Moreover, the forest used progressively more water under CO₂^e, probably because soil moisture availability progressively increased, probably owing to a reduction in soil evaporation caused by more litter buildup in the CO₂^e plots. The results suggest that, in this forest, the effect of CO₂^e on transpiration was greater indirectly through enhanced litter production than directly through reduced stomatal conductance. In forests composed of species more similar to L. styraciflua, water savings from stomatal closure may dominate the response to CO₂^e.     

Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: Seasonal variations and ecosystem implications of elevated atmospheric CO2

The stable isotopic composition of soil water is controlled by precipitation inputs, antecedent conditions, and evaporative losses. Because transpiration does not fractionate soil water isotopes, the relative proportions of evaporation and transpiration can be estimated using a simple isotopic mass balance approach. At our site in the shortgrass steppe in semi-arid northeastern Colorado, ¹⁸O values of soil water were almost always more enriched than those of precipitation inputs, owing to evaporative losses. The proportion of water lost by evaporation (E/ET) during the growing season ranged from nil to about 40% (to >90% in the dormant season), and was related to the timing of precipitation inputs. The sum of transpiration plus evaporation losses estimated by isotopic mass balance were similar to actual evapotranspiration measured from a nearby Bowen ratio system. We also investigated the evapotranspiration response of this mixed C₃/C₄ grassland to doubled atmospheric [CO₂] using Open-Top Chambers (OTC). Elevated atmospheric [CO₂] led to increased soil-water conservation via reduced stomatal conductance, despite greater biomass growth. We used a non-invasive method to measure the ¹⁸O of soil CO₂ as a proxy for soil water, after establishing a strong relationship between ¹⁸O of soil CO₂ from non-chambered control (NC) plots and ¹⁸O of soil–water from an adjacent area of native grassland. Soil–CO₂ ¹⁸O values showed significant treatment effects, particularly during a dry summer: values in ambient chambers (AC) were more enriched than in NC and elevated chamber (EC) plots. During the dry growing season of 2000, transpiration from the EC treatment was higher than from AC and lower than from NC treatments, but during 2001, transpiration was similar on all three treatments. Slightly higher evaporation rates from AC than either EC or NC treatments in 2000 may have resulted from increased convection across the soil surface from the OTC blowers, combined with lower biomass and litter cover on the AC treatment. Transpiration-use efficiency, or the amount of above-ground biomass produced per mm water transpired, was always greatest on EC and lowest on NC treatments.     

我国口岸小条实蝇属截获情况分析

[目的] 小条实蝇属害虫原产非洲,在我国无分布且具有一定的入侵风险。分析该虫在我国口岸的截获情况,能更好地利用已知实蝇截获数据防范实蝇人为传播。[方法] 经动植物检验检疫信息资源共享服务平台查询,统计小条实蝇属的有害生物类别、截获途径及截获地区等。[结果] 全国口岸共截获8种共计524批次的小条实蝇属害虫;地中海实蝇的截获批次数最多,为415批次,为小条实蝇属截获率的79.2%;截获的种类最多的国家来自尼日利亚,5种;截获批次最多的国家为南非,71批次;截获方式最多的为旅客携带物,占总批次的68.3%;在我国口岸中,广东口岸截获批次最多,占总数的57.4%。[结论] 从今后防控的重点种类、加大查验力度、推广新技术在鉴定上的应用、应急预案等几个方面,加强针对该类有害生物的检验检疫措施。     

暖温带麻栎林凋落物调节土壤碳排放通量对降雨脉冲的响应

凋落物既是森林生态系统养分循环的重要构件,又是森林土壤环境和功能的关键调节因子。降雨脉冲导致的土壤碳排放变异是陆地生态系统碳汇能力评价的不确定性来源之一。凋落物在调节土壤碳排放对降雨脉冲的响应中的作用仍缺乏科学的评价。通过在暖温带栎类落叶阔叶林中设置不同凋落物处理(对照、去除凋落物和加倍凋落物)和降雨模拟实验以阐明凋落物数量变化对土壤呼吸脉冲的影响。结果表明：模拟降雨脉冲之前,不同凋落物处理下的土壤呼吸存在显著差异;与对照相比,加倍凋落物导致土壤呼吸速率显著增加57.6%,然而,去除凋落物则对土壤呼吸无显著影响。模拟降雨后52小时内,对照、去除凋落物和加倍凋落物样方的土壤累积碳排放量分别为251.69 gC/m~2,250.93 gC/m~2和409.01 gC/m~2,加倍凋落物处理下的土壤碳排放量显著高于对照和去除凋落物处理;然而,去除凋落物与对照之间无显著差异。此外,不同凋落物处理下土壤呼吸的脉冲持续时间存在显著差异;加倍凋落物显著提高降雨后土壤呼吸脉冲的持续时间,分别比对照和去除凋落物高出262%和158%。多元逐步回归分析表明,土壤总碳排放通量和土壤呼吸的脉冲持续时间与土壤理...     

祁连山青海云杉树干液流密度的优势度差异

以祁连山排露沟小流域青海云杉林为研究对象,选取有代表性的优势木、亚优势木、中等木和被压木各3—5株,2015年6月16日至10月14日应用热扩散技术对不同优势度青海云杉树干液流密度进行测定,并同步测定相关的林外气象因子。结果表明:(1)青海云杉液流密度呈昼高夜低趋势,晴天液流密度变化幅度较大,而阴雨天变化幅度较小。(2)晴天树木优势度越大,其液流在日内的启动越早,结束越晚,峰值也越大;优势木的平均液流密度为(0.0758±0.0475)m L cm~(-2)min~(-1),是亚优势木的1.5倍,是中等木和被压木的1.68倍。(3)青海云杉平均液流密度基本呈现6月份最大,其次是8月份,9、10月份明显减小,且优势木亚优势木中等木被压木。(4)相关性分析和逐步回归表明,青海云杉日均液流密度与太阳辐射强度、饱和水气压差和空气温度呈正相关关系,与空气相对湿度和降雨量呈负相关关系。影响优势木、亚优势木和中等木液流密度的主要气象因子是太阳辐射强度,被压木液流密度主要受空气相对湿度的影响。     

The influence of a neotropical herbivore (<Emphasis Type="Italic">Lamponius portoricensis</Emphasis>) on nutrient cycling and soil processes

The role of phytophagous insects in ecosystem nutrient cycling remains poorly understood. By altering the flow of litterfall nutrients from the canopy to the forest floor, herbivores may influence key ecosystem processes. We manipulated levels of herbivory in a lower montane tropical rainforest of Puerto Rico using the common herbivore, Lamponius portoricensis (Phasmatidea), on a prevalent understory plant, Piper glabrescens (Piperaceae), and measured the effects on nutrient input to the forest floor and on rates of litter decomposition. Four treatment levels of herbivory generated a full range of leaf area removal, from plants experiencing no herbivory to plants that were completely defoliated (>4,000 cm² leaf area removed during the 76-day study duration). A significant (P<0.05) positive regression was found between all measures of herbivory (total leaf area removed, greenfall production, and frass-related inputs) and the concentration of NO ₃ ⁻ in ion exchange resin bags located in the litter layer. No significant relationship was found between any of the herbivory components and resin bag concentrations of NH ₄ ⁺ or PO ₄ ⁻ . Rates of litter decay were significantly affected by frass-related herbivore inputs. A marginally significant negative relationship was also found between the litter mass remaining at 47 days and total leaf area removed. This study demonstrated a modest, but direct relationship between herbivory and both litter decomposition and NO ₃ ⁻ transfer to the forest floor. These results suggest that insect herbivores can influence forest floor nutrient dynamics and thus merit further consideration in discussions on ecosystem nutrient dynamics.     

Differential Controls of Water Input on Litter Decomposition and Nitrogen Dynamics in the Patagonian Steppe

Studies of the effects of precipitation on litter decomposition and nitrogen mineralization in arid and semiarid environments have demonstrated contradictory results. We conducted a manipulative experiment with rainout shelters in the semiarid Patagonian steppe, aimed at assessing the direct effects of water availability on litter decomposition and net nitrogen mineralization while isolating the indirect effects. We created four levels of precipitation input: control and three levels (30, 55 and 80%) of precipitation interception and we examined litter decomposition and nutrient release of a dominant grass species, Stipa speciosa, inorganic soil nitrogen, and in situ net nitrogen mineralization over two consecutive years. Litter decomposition rates (k, year⁻¹) varied significantly (P < 0.001) among precipitation interception treatments and were positively correlated with incoming annual precipitation (APPT, mm/year) (k = 0.0007 × APPT + 0.137). In contrast, net N mineralization was not correlated with incoming precipitation. Soil NO₃⁻ significantly decreased with increasing precipitation input, whereas soil NH₄⁺ concentration did not differ among precipitation interception treatments. Controls of water input on litter decomposition appear to be different from controls on N mineralization in the semiarid Patagonian steppe. We suggest that although water availability affects both the mineralization of C and N, it differentially affects the movement and fate of the inorganic products. A consequence of the accumulation of inorganic N during dry episodes is that periods of maximum water and soil nutrient availability may occur at different times. This asynchrony in the availability of N and water in the soil may explain the observed lags in the response of primary production to increases in water availability.     

The effects of the moss layer on the decomposition of intercepted vascular plant litter across a post-fire boreal forest chronosequence

Aims

Feather mosses form a thick ground layer in boreal forests that can intercept incoming litter fall. This interception may influence the decomposition of incoming litter but this has been little explored. We investigated how the moss layer influences decomposition of intercepted litter along a 362-year fire driven forest chronosequence in northern Sweden across which soil fertility declines.

Methods

We placed leaf litter from three plant species into plots in which mosses and dwarf shrubs were either experimentally removed or left intact, at each of ten stands across the chronosequence. After one year we measured litter mass loss, and litter nitrogen and phosphorous.

Results

Litter decomposed consistently faster, and had higher nitrogen and phosphorus, in the presence of mosses and at greater depth in the moss layer. Despite an increase in moss depth across the chronosequence we did not find consistent increases in effects of moss removal on litter decomposition or on litter N or P.

Conclusions

Our findings identify a clear role of the moss layer in boreal forests in promoting the decomposition of intercepted leaf litter, and highlight that this role is relatively consistent across chronosequence stages that vary greatly in productivity and moss depth.     

中国进境水果携带粉蚧疫情分析(2013—2016)

【目的】中国是水果生产大国和进口大国。水果是最容易携带虫害的产品之一,粉蚧是口岸水果检疫过程中常发现的害虫类群,中国口岸每年从进境水果截获大量粉蚧。通过分析中国进境水果携带粉蚧疫情,可为一线口岸检疫查验提供指导,为相关部门开展产地检疫和口岸监测提供依据,以防止危险性粉蚧的传入,保障中国水果生产安全,促进国际水果贸易健康发展。【方法】通过FAO网站、动植物检验检疫信息资源共享服务平台、中国知网等收集中国进境水果贸易数据及其携带粉蚧疫情,统计分析了中国水果进口贸易情况、粉蚧截获情况以及截获粉蚧的种类、来源地、截获口岸和寄主。【结果】2013—2016年,中国口岸从进境水果上截获的昆虫中粉蚧科昆虫截获量最大,占47.31%;其中截获量前十的粉蚧种类为杰克贝尔氏粉蚧、木槿曼粉蚧、双条拂粉蚧、大洋臀纹粉蚧、南洋臀纹粉蚧、柑橘棘粉蚧、甘蔗簇粉蚧、康氏粉蚧、菠萝灰粉蚧和李比利氏灰粉蚧;而这些有害生物的主要来源地为越南和泰国;粉蚧的主要截获口岸为广西、深圳和云南;而火龙果、榴莲、龙眼和山竹是截获粉蚧最多的进境水果。【结论】中国进境水果粉蚧疫情与水果的贸易量和贸易方式、输出国有害生物发生和检疫除害处理措施、口岸关注度和能力建设等情况相关。     

Short-Term Disappearance of Foliar Litter in Three Species Before and After a Hurricane1

Litter disappearance was examined before (1989) and after (1990) Hurricane Hugo in the Luquillo Experimental Forest, Puerto Rico using mesh litterbags containing abscised Cyrilla racemiflont or Dacryodes excelsa leaves or fresh Prestoea montana leaves. Biomass and nitrogen dynamics were compared among: (i) species; (ii) mid- and high elevation forest types; (iii) riparian and upland sites; and (iv) pre- and post-hurricane disturbed environments. Biomass disappearance was compared using multiple regression and negative exponential models in which the slopes were estimates of the decomposition rates subsequent to apparent leaching losses and the y-intercepts were indices of initial mass losses (leaching). Cyrilla racemiflora leaves with low nitrogen (0.39%) and high lignin (22.1%) content decayed at a low rate and immobilized available nitrogen. Dacryodes excelsa leaves had moderate nitrogen (0.67%) and lignin (16.6%) content, decayed at moderate rates, and maintained the initial nitrogen mass. Prestoea montana foliage had high nitrogen (1.76%) and moderate lignin (16.7%) content and rapidly lost both mass and nitrogen. There were no significant differences in litter disappearance and nitrogen dynamics among forest types and slope positions. Initial mass loss of C. racemiflora leaves was lower in 1990 but the subsequent decomposition rate did not change. Initial mass losses and the overall decomposition rates were lower in 1990 than in 1989 for Dacryodes excelsa. Dacryodes excelsa and C. racemiflora litter immobilized nitrogen in 1990 but released 10-15 percent of their initial nitrogen in 1989, whereas P. montana released nitrogen in both years (25-40%). Observed differences in litter disappearance rates between years may have been due to differences in the timing of precipitation. Foliar litter inputs during post-hurricane recovery of vegetation in Puerto Rico may serve to immobilize and conserve site nitrogen.     

Leaf litter decomposition and nutrient dynamics in a subtropical forest after typhoon disturbance

Decomposition of typhoon-generated and normal leaf litter and their release patterns for eight nutrient elements were investigated over 3 yr using the litterbag technique in a subtropical evergreen broad-leaved forest on Okinawa Island, Japan. Two common tree species, Castanopsis sieboldii and Schima wallichii, representative of the vegetation and differing in their foliar traits, were selected. The elements analyzed were N, P, K, Ca, Mg, Na, Al, Fe and Mn. Dry mass loss at the end of study varied in the order: typhoon green leaves > typhoon yellow leaves > normal leaves falling for both species. For the same litter type, Schima decomposed faster than Castanopsis. Dry mass remaining after 2 yr of decomposition was positively correlated with initial C:N and C:P ratios. There was a wide range in patterns of nutrient concentration, from a net accumulation to a rapid loss in decomposition. Leaf litter generated by typhoons decomposed more rapidly than did the normal litter, with rapid losses for N and P. Analysis of initial quality for the different litter types showed that the C:P ratios were extremely high (range 896 – 2467) but the P:N ratios were < 0.05 (range 0.02 – 0.04), indicating a likely P-limitation for this forest. On average 32% less N and 60% less P was retranslocated from the typhoon-generated green leaves than from the normal litter for the two species, Castanopsis and Schima. An estimated 2.13 g m^–2 yr^–1 more N and 0.07 g m^–2 yr^–1 more P was transferred to the soil as result of typhoon disturbances, which were as high as 52% of N and 74% of P inputted from leaf litter annually in a normal year. Typhoon-driven maintenance of rapid P cycling appears to be an important mechanism by which growth of this Okinawan subtropical forest is maintained.     

Response of litter decomposition and related soil enzyme activities to different forms of nitrogen fertilization in a subtropical forest

With the continuing increase in the impact of human activities on ecosystems, ecologists are increasingly becoming interested in understanding the effects of nitrogen deposition on litter decomposition. At present, numerous studies have investigated the effects of single form of nitrogen fertilization on litter decomposition in forest ecosystems. However, forms of N deposition vary, and changes in the relative importance of different forms of N deposition are expected in the future. Thus, identifying the effects of different forms of N deposition on litter decomposition in forest ecosystems is a pressing task. In this study, two dominant litter types were chosen from Zijin Mountain in China: Quercus acutissima leaves from a late succession broad-leaved forest and Pinus massoniana needles from an early succession coniferous forest. The litter samples were incubated in microcosms with original forest soil and treated with four different forms of nitrogen fertilization [NH₄ ⁺, NO₃ ⁻, CO(NH₂)₂, and a mix of all three]. During a 5-month incubation period, litter mass losses, soil pH values, and soil enzyme activities were determined. Results show that all four forms of nitrogen fertilization significantly accelerate litter decomposition rates in the broadleaf forest, while only two forms of nitrogen fertilization [i.e., mixed nitrogen and CO(NH₂)₂] significantly accelerate litter decomposition rates in the coniferous forest. Litter decomposition rates with the mixed nitrogen fertilization were higher than those in any single form of nitrogen fertilization. All forms of nitrogen fertilization enhanced soil enzyme activities (i.e., catalase, cellulase, invertase, polyphenol oxidase, nitrate reductase, urease, and acid phosphatase) during the litter decomposition process for the two forest types. Soil enzyme activities under the mixed nitrogen fertilization were higher than those under any single form of nitrogen fertilization. These results suggest that the type and activity of the major degradative enzymes involved in litter decomposition vary in different forest types under different forms of nitrogen fertilization. They also indicate that a long-term consequence of N deposition-induced acceleration of litter decomposition rates in subtropical forests may be the release of carbon stored belowground to the atmosphere.