Understory bamboo flowering provides a very narrow light window of opportunity for canopy-tree recruitment in a neotropical forest of Misiones, Argentina

Chusquea ramosissima is a native monocarpic bamboo species growing in subtropical forests of northeastern Argentina, which can dominate gaps and open forests in the region, particularly after human disturbance. This bamboo species started to flower in different areas of northeastern Argentina in 2001, with the flowering peak during 2002 and 2003 and small isolated flowering events still occurring until 2010. We studied the effects of C. ramosissima flowering and die-back on microclimate, litter decomposition, nutrient availability, sapling growth, abundance and regeneration of tree canopy species. We wanted to know how environmental conditions and ecosystem processes change through time after bamboo flowering and if bamboo die-back would favor regeneration of canopy trees. Twenty 50 × 50 m plots of flowering and non-flowering bamboo were permanently marked and vegetation dynamics as well as nutrient cycling and microclimate studies were performed. C. ramosissima die-back enhanced growth and reduced mortality rate of tree saplings during the first year after flowering. Only growth of tree saplings previously established was enhanced by the flowering event and tree-species richness and saplings abundance of canopy trees did not change as expected due to bamboo flowering. The short-term effect of tree saplings growth was likely due to incident solar radiation at the forest floor which doubled in the first year after the bamboo flowering event. Increased light availability at the forest floor simultaneously promoted the growth of other understory plants such as ferns, lianas and Piper spp. that rapidly colonized gaps and intercepted a percentage of the incident solar radiation after the first year, which together with an increased litter layer due to the senescence of the bamboo, may have inhibited establishment of new tree individuals and affected tree growth. Contrary to predictions, soil water, litter decomposition and soil nutrients were not significantly affected by bamboo flowering. Thus, successful tree regeneration in gaps following bamboo flowering appears to be restricted to a very narrow window of increased light availability (i.e., 1 year) before growth of other understory plants and rapid re-colonization of bamboo. Changes in resource availability, and the opportunity for overstory regeneration after bamboo flowering events appears to depend on climatic and community characteristics of the ecosystem where the flowering event occurs and also, on the flowering patterns and their synchronicity.     

Forest gap size can efficiently promote litter decomposition and nutrient release in south-western China

Forest gaps are important in forest dynamics and management; however, the gap size that is most conducive to the decomposition of litter and promotion of nutrient cycling in forests remains poorly understood. The mass loss and nutrient release from Pinus massoniana and Toona ciliata litter in response to gap size classes were determined in south-western China during a 1.5-year litter decomposition experiment. One site with a closed canopy (CK) and seven sites with forest gaps of 100, 225, 400, 625, 900, 1 225 and 1 600 m² were established in a P. massoniana plantation in the Sichuan Basin of China; the CK site (fully shaded) was treated as the control. After 540 d, the mass and carbon (C), nitrogen (N) and phosphorus (P) contents in the litter of the control treatments decreased by 58.23%, 60.81%, 65.62% and 57.82% for P. massoniana litter and by 91.17%, 80.76%, 73.66% and 64.55% for T. ciliata litter, respectively, compared with the initial amounts. Most of the C, N and P were released from both tree species during the first 90 d of decomposition, although the temperature and moisture conditions were very low. The mass loss and C and N release rates for the two tree species and the P release rate from T. ciliata litter were higher in the 400–900 m² gap sites than in the other gap sites and the CK site, whereas the P release rate from P. massoniana litter was greater under large and medium-sized gaps (400–1 600 m²). The mass loss and C, N and P release rates were positively correlated with the soil moisture content in the seven different gap size treatments, with the soil moisture content representing the best predictor of litter decomposition. Therefore, our results indicate that medium-sized gaps (400–900 m²) can promote decomposition by changing the environmental conditions and may accelerate nutrient cycling in forest ecosystems.     

林窗效应研究综述

森林群落常发生一些小规模的内源干扰,从而形成林窗。林窗的形成对推动森林群落的演替更新和生态系统发展至关重要。林窗面积的大小与树木的倒伏方式和林冠冠幅及大小有关。林窗面积及林窗内位置的不同,导致其小气候和土壤理化性质等环境因子发生改变,进而影响到林窗内树种更新和物种组成、林窗植被的物种多样性及其微生物和土壤动物的种类、数量等方面。未来林窗研究重点应该放在次生林和人工林的林窗效应,林窗对森林生态系统碳储量影响机制,林窗凋落物分解因子间的相互关系、作用机理和养分循环,不同树种的林窗与最适更新面积的关系,林窗的边缘效应,林窗的土壤动物和微生物动态及过程。     

Litterfall, litter decomposition and nutrient dynamics in a subtropical natural oak forest and managed plantation in northeastern India

In northeastern India, subtropical forests are over-exploited for timber, fuel wood and common agricultural practice like shifting cultivation, which are responsible for the degradation of natural forest. In degraded areas, large-scale plantations of different species of Quercus have been raised since 1980 for the production of economic Tasar silk. Conversion of natural forest into plantation affects the process of nutrient cycling due to management practices. Thus, it would be of importance to study the litterfall, litter decomposition process and the factors regulating the rate of litter decay in these ecosystems to improve recommendations for their management and conservation. We recorded litterfall by using litter traps and decomposition of leaf litter by nylon net bag technique to understand the amount of organic matter and nutrient return and their release in soils of forest and plantation in Manipur, northeast India. Total litterfall was 419.9 g m⁻² year⁻¹ in plantation and 547.7 g m⁻² year⁻¹ in forest. Litter decomposition rate was faster at plantation site than the forest in the early stage of litter decomposition whereas the reverse was observed at later stages of decomposition. Stepwise regression analysis showed the significant role of relative humidity and mean temperature on mass loss rates in the forest. Relative humidity, maximum temperature, population of fungi and actinomycetes were the best predictor variables for mass loss rates in plantation. Nutrient retranslocation efficiency and the immobilization of N and P in forest litter were higher than plantation. This suggests that Q. serrata growing in natural ecosystem in oligotrophic condition adapted strong nutrient conservation mechanisms to compete with the other plant species for the meager soil nutrients. The same species in plantation loses these adaptive capabilities because of exogenous supply of nutrients and in the absence of intense competition with other plant species. Thus, the optimization of organic and chemical fertilizer input in plantation is recommended for maintaining the soil fertility level to produce quality leaf for silkworm by conserving essential nutrients in the system.     

Nitrogen dynamics across silvicultural canopy gaps in young forests of western Oregon

Silvicultural canopy gaps are emerging as an alternative management tool to accelerate development of complex forest structure in young, even-aged forests of the Pacific Northwest. The effect of gap creation on available nitrogen (N) is of concern to managers because N is often a limiting nutrient in Pacific Northwest forests. We investigated patterns of N availability in the forest floor and upper mineral soil (0–10 cm) across 6–8-year-old silvicultural canopy gaps in three 50–70-year-old Douglas-fir forests spanning a wide range of soil N capital in the Coast Range and Cascade Mountains of western Oregon. We used extractable ammonium (NH₄⁺) and nitrate (NO₃⁻) pools, net N mineralization and nitrification rates, and NH₄⁺ and NO₃⁻ ion exchange resin (IER) concentrations to quantify N availability along north-south transects run through the centers of 0.4 and 0.1 ha gaps. In addition, we measured several factors known to influence N availability, including litterfall, moisture, temperature, and decomposition rates. In general, gap-forest differences in N availability were more pronounced in the mineral soil than in the forest floor. Mineral soil extractable NH₄⁺ and NO₃⁻ pools, net N mineralization and nitrification rates, and NH₄⁺ and NO₃⁻ IER concentrations were all significantly elevated in gaps relative to adjacent forest, and in several cases exhibited significantly greater spatial variability in gaps than forest. Nitrogen availability along the edges of gaps more often resembled levels in the adjacent forest than in gap centers. For the majority of response variables, there were no significant differences between northern and southern transect positions, nor between 0.4 and 0.1 ha gaps. Forest floor and mineral soil gravimetric percent moisture and temperature showed few differences along transects, while litterfall carbon (C) inputs and litterfall C:N ratios in gaps were significantly lower than in the adjacent forest. Reciprocal transfer incubations of mineral soil samples between gap and forest positions revealed that soil originating from gaps had greater net nitrification rates than forest samples, regardless of incubation environment. Overall, our results suggest that increased N availability in 6–8-year-old silvicultural gaps in young western Oregon forests may be due more to the quality and quantity of litterfall inputs resulting from early-seral species colonizing gaps than by changes in temperature and moisture conditions caused by gap creation.     

马尾松两种林型土壤养分特征及其与凋落物质量的关系

【目的】研究土壤养分和地被层凋落物养分含量的差异,为马尾松人工林营林措施及地力维持提供科学依据。【方法】以鼎湖山两种典型林型(马尾松纯林和马尾松-黧蒴混交林)为研究对象,对比分析0~60 cm土层的土壤养分含量及地被层凋落物养分含量的差异,探索凋落物质量如何影响土壤养分。【结果】1)林型对土壤有机质、全氮和硫酸根含量有显著影响(P<0.05),对土壤全磷、交换性K+、Ca2+和Mg2+有极显著影响(P<0.01),混交林土壤养分含量(除硝态氮含量和交换性H+含量以外)均高于纯林。2)相同林型不同土层间土壤养分含量差异极显著(P<0.01),其中,土壤有机质和全氮含量随土层的加深而递减,且主要聚集在0~10 cm土层,表聚效应十分明显。3)纯林凋落物有机碳、全氮、C/N和全磷等含量高于混交林;相同林型不同分解层凋落物有机碳、全钙和全镁含量有显著差异(P<0.05),均表现为未分解层>半分解层>腐殖质层。4)土壤养分与地被层凋落物质量的RDA分析表明,0~10 cm土层土壤养分与腐殖质层有机碳呈极显著负相关(P<0.01),与腐殖质层C/N呈显著负相关(P<0.05);在10~20 cm土层,土壤养分与腐殖质层有机碳呈极显著负相关(P<0.05)。【结论】纯林的土壤养分低于混交林的主要原因是纯林凋落物具有较高的C/N和有机碳含量。     

Impacts of mixed litter decomposition from Robinia pseudoacacia and other tree species on C loss and nutrient release in the Loess Plateau of China

The productivity of Robinia pseudoacacia(R.p.) pure forest usually declines at the late growth stage,and reforming it into mixed forests could be a promising way to resolve this problem. When choosing a suitable tree species that can be mixed with R.p., the interspecific relationship is an important issue. Therefore, we gathered the autumn litter fall from R.p. and 10 other species from the Loess Plateau of China were mixed in dual species litterbags(R.p.+each other species) and buried them in soil for a 345 days lab decay incubation. We measured the litter mass loss and nutrient contents to determine whether the nutrient release was affected by mixed species litter decomposition. The impacts of mixed litter decomposition on macro-elements release were more obvious than on micro-elements. The litters with similar substrate quality might show variable impacts on nutrients release in mixed decomposition. The C loss and release of nutrient was improved by descending order when R.p. litter was mixed with Hippophae rhamnoides, Ulmus pumila, Populus simonii, Larix principis-rupprechtii and Quercus liaotungensis(Q.l.). But, except for Q.l., only the other species were recommended as suitable mix-plants for R.p. since promoting a high turnover of the nutrient in the litter compartment and a rapid availability for tree.     

Nutrient cycling and distribution in different-aged plantations of Chinese fir in southern China

The distribution in tree biomass and understorey vegetation and annual biological and geochemical cycling of total nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) were measured in young, middle-aged and mature plantations (8-, 14- and 24-years old) of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) in southern China. Although >98% of nutrients occurred in the soil, soil nutrient content decreased with plantation age. Nutrient outputs from the soil exceeded inputs in stands of all ages but the net soil nutrient loss increased significantly for N, P and Ca with plantation age. Comparison of nutrient fluxes showed that the smallest (and hence limiting for nutrient cycling) fluxes were litter decomposition in the young plantation in contrast to canopy fluxes (apart from for Mg) in the middle-aged and mature plantations. Nutrient use efficiency, release of nutrients from litter decomposition and nutrient return, particularly in litterfall, increased significantly with plantation age. These results suggest that, as stand age increases, nutrient cycling in Chinese fir plantations is increasingly dominated by biological processes and becomes less dependent on external nutrient sources in rainfall and the soil. It therefore appears that prolonging the rotation length of Chinese fir plantations by approximately 5 years could be beneficial for maintaining the soil nutrient status for successive plantings.     

Nutrient status of managed and natural forest fragments of Fagus sylvatica in southern Europe

Beech forests located in the southwestern limit of Europe have been affected by severe deforestation and long-term fragmentation. Some of these forests have been subjected to partial cutting, whereas others have been maintained with little or no active management. It has previously been shown that past management has led to substantial changes in tree structure, diversity and plant species. These perturbations, through their influence on the litterfall and forest floor, may affect nutrient cycling and the nutritional status of such fragile ecosystems. Mineral nutrition was investigated in 53 forest fragments by analysis of data corresponding to nutrient concentrations in forest floor, mineral soils and foliage. In comparison with other beech forests in Central Europe subjected to higher levels of air pollution, the stands showed fewer incidences of nutrient deficiencies and lower foliar concentrations of S and heavy metals. Partial cuts carried out in recent decades have reduced the forest floor mass proportional to the intensity of the harvesting. The effect was probably due to the lower litter input and the increased decomposition of litter as a consequence of the environmental changes in forest gaps. The partially cut stands displayed higher foliar levels of K and Mg, which could be attributed to the greater release of these elements as a consequence of the increased decomposition of litter. However, past management has brought about lower foliar concentrations of P and N. Both effects were found to be proportional to basal area and the forest floor mass, which suggests that they are related to the intensity of harvesting. Although the causes are uncertain, this negative effect may be due to a reduction in forest floor thickness, which implies the loss of preferred rooting space for trees.     

Tree recruitment in gaps of various size,clearcuts and undisturbed mixed forest of interior British Columbia,Canada

Tree seedling recruitment was monitored after various types of logging in mixed conifer and deciduous forests of northern British Columbia, Canada. Predicting tree seedling recruitment after disturbance is fundamental to understanding forest dynamics and succession and is vital for forest management purposes. Seedling recruitment success in multi-species northern latitude forests varied as a function of mature tree canopy cover, gap size and position in a gap. Recruitment was abundant within canopy gaps across a wide range of gap sizes (20–5000 m²), but recruit numbers dropped off rapidly under the closed forest canopy and in the open conditions of clearcuts. Inside canopy gaps, recruitment was similar by gap position in small gaps (<300 m²) but, in these northern latitude forests, exhibited a trend of increasing density from the sunny north to shady south end of larger gaps. This was true for all tree species regardless of their shade tolerance ranking. There was no evidence of gap partitioning by any of the tree species during the regeneration phase suggesting that adaptation to the subtleties of gap size during early recruitment are not well developed in these tree species. Favorable locations for emergence and early establishment of germinants were less favorable for growth and survival of established seedlings, i.e. the regeneration niches in these forests were discordant. Tree abundance and species diversity appears to be controlled more by differentiation among growth and survival niches than by the regeneration niches. From the perspective of forest management, abundant natural regeneration of all the dominant tree species of these mixed-species forests can be obtained after partial cutting.     

Soil improvement by trees in sub-Saharan Africa

Trees can influence both the supply and availability of nutrients in the soil. Trees increase the supply of nutrients within the rooting zone of crops through (1) input of N by biological N₂ fixation, (2) retrieval of nutrients from below the rooting zone of crops and (3) reduction in nutrient losses from processes such as leaching and erosion. Trees can increase the availability of nutrients through increased release of nutrients from soil organic matter (SOM) and recycled organic residues. Roots of trees frequently extend beyond the rooting depth of crops. Research on a Kandiudalfic Eutrudox in western Kenya showed that fast-growing trees with high N demand (Calliandra calothyrsus, Sesbania sesban and Eucalyptus grandis) took up subsoil nitrate that had accumulated below the rooting depth of annual crops. Sesbania sesban was also more effective than a natural grass fallow in extracting subsoil water, suggesting less leaching loss of nutrients under S. sesban than under natural uncultivated fallows. Nutrient release from SOM is normally more dependent on the portion of the SOM in biologically active fractions than on total quantity of SOM. Trees can increase inorganic soil N, N mineralization and amount of N in light fraction SOM. Among six tree fallows of 2- and 3-year duration on an Ustic Rhodustalf in Zambia, inorganic N and N mineralization were higher for the two tree species with lowest (lignin + polyphenol)-to-N ratio (mean = 11) in leaf litter than for the two tree species with highest ratio (mean = 20) in leaf litter. Trees can also restore soil fauna, which are important for SOM and plant residue decomposition. Some agroforestry trees have potential to provide N in quantities sufficient to support moderate crop yields through (i) N inputs from biological N₂ fixation and retrieval of nitrate from deep soil layers and (ii) cycling of N from plant residues and manures. The cycling of P from organic materials is normally insufficient to meet the P requirements of crops. Sustained crop production with agroforestry on P-deficient soils will typically require external P inputs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Single-tree influences on soil properties in agroforestry: lessons from natural forest and savanna ecosystems

Climate, organisms, topographic relief, and parent material interacting through time are the dominant factors that control processes of soil formation and determine soil properties. In both forest and savanna ecosystems, trees affect soil properties through several pathways. Trees alter inputs to the soil system by increasing capture of wetfall and dryfall and by adding to soil N via N₂-fixation. They affect the morphology and chemical conditions of the soil as a result of the characteristics of above- and below-ground litter inputs. The chemical and physical nature of leaf, bark, branch, and roots alter decomposition and nutrient availability via controls on soil water and the soil fauna involved in litter breakdown. Extensive lateral root systems scavenge soil nutrients and redistribute them beneath tree canopies. In general, trees represent both conduits through which nutrients cycle and sites for the accumulation of nutrients within a landscape. From an ecological perspective, the soil patches found beneath tree canopies are important local and regional nutrient reserves that influence community structure and ecosystem function. Understanding species-specific differences in tree-soil interactions has important and immediate interest to farmers and agroforesters concerned with maintaining or increasing site productivity. Lessons from natural plant-soil systems provide a guide for predicting the direction and magnitude of tree influences on soil in agroforestry settings. The challenge for agroforesters is to determine under what conditions positive tree effects will accumulate simultaneously within active farming systems and which require rotation of cropping and forest fallows.     

马尾松、桉树及其混交林土壤养分变化特征及评价

研究马尾松纯林、桉树纯林和3种不同模式混交林土壤养分含量的变化规律,并对其土壤养分状况进行主成分综合排序。结果表明：土壤有机质、全 N、全 P、水解 N、速效 P 和速效 K 皆表现为随土壤深度增加而减少,全 K 则表现相反;在各林分中,桉树纯林的土壤有机质、速效 P 含量最高,混交林次之。混交林则土壤全 N、全P 和速效 N 含量总体上大于纯林。马尾松纯林除在浅层土中速效 P 含量最高外,其他养分指标都低于桉树纯林和混交林。土壤养分综合评价为桉树纯林＞混交林 A＞混交林 C＞混交林 B＞马尾松纯林,表现为桉树纯林养分状况最好,马尾松纯林最差,与针阔树种的生长特性和凋落物养分归还能力有关。混交林中种植密度较大的混交林A 养分状况相对较好,表明其养分储存能力较好,消耗较少。而相同种植密度下,混交林 C 的林分土壤养分状况优于混交林 B。     

A review of the roles of forest canopy gaps

Treefall gap, canopy opening caused by the death of one or more trees, is the dominant form of disturbance in many forest systems worldwide. Gaps play an important role in forest ecology helping to pre- serve bio- and pedo-diversity, influencing nutrient cycles, and maintain- ing the complex structure of the late-successional forests. Over the last 30 years, numerous reviews have been written describing gap dynamics. Here we synthesize current understanding on gap dynamics relating to tree regeneration with particular emphasis on gap characteristics consid- ered critical to develop ecologically sustainable forest management sys- tems and to conserve native biodiversity. Specifically, we addressed the question： how do gaps influence forest structure？ From the literature re- viewed, the size of gaps induces important changes in factors such as light intensity, soil humidity and soil biological properties that influence tree species regeneration and differ in gaps of different sizes. Shade- tolerant species can colonize small gaps; shade-intolerant species need large gaps for successful regeneration. Additionally, gap dynamics differ between temperate, boreal, and tropical forests, showing the importance of climate differences in driving forest regeneration. This review summa- rizes information of use to forest managers who design cutting regimes that mimic natural disturbances and who must consider forest structure, forest climate, and the role of natural disturbance in their designs.     

Quantification of organic carbon and primary nutrients in litter and soil in a foothill forest plantation of eastern Himalaya

The present study was an effort to understand the amount of litter fall and its subsequent decomposition and quantify the release of available nutrients and soil physicochemical characteristics in plantations of four forest tree species(Lagerstroemia parviflora, Tectona grandis, Shorea robusta and Michelia champaca) in the Chilapatta Reserve Forest of the Cooch Behar Wildlife Division in the Terai zone of West Bengal, India. The most litter(5.61 Mg ha~(-1))was produced by T. grandis plantation and the least(4.72 Mg ha~(-1)) by L. parviflora. The material turnover rate to the soil through decomposition from total litter was fastest during the first quarter of the year and subsequently decreased during the next two quarters. The material turnover rate was only 1 year, which indicates that more than90% of the total litter produced decomposed within a year.The available primary nutrient content in litter varied across the four plantations over the year. The plantations generally did not significantly influence the soil physical characteristics but did significantly influence the availability of primary nutrients and organic carbon at two depths(1–15 and16–30 cm) over the year. The availability of soil primary nutrients in the four plantations also increased gradually from the first quarter of the year to the third quarter and then decreased during the last quarter to the same level as in the first quarter of the year at both depths. The availability for soil organic carbon in the plantations followed a similar trend. The amount of litter produced and the material turnover in the soil in the different plantations differed, influencing the nutrient availability and organic carbon at the plantations. The amount of soil organic carbon was highest for T. grandis(2.52 Mg ha~(-1)) and lowest for L. parviflora(2.12 Mg ha~(-1)). Litter is the source of soil organic matter,and more the litter that is produced by the plantations, the higher will be the content and amount of soil organic carbon in the plantation.     

Influence of forest growth on former heathland on nutrient input and its consequences for nutrition and management of heath and forest

The low nutrient supply of heathland soils is often insufficient for the nutrient demand of growing forests and woodlands, and additional atmospheric input of nutrients is beneficial for the tree growth. On old heathland soils tree species influencing nutrient input with regard to higher amounts have competitive benefits on the early stages of succession and/or as first planted trees with consequences for both the successional development and the nutrition and management of heathland and forests. In three stages of heathland forest succession on highly acidified and nutrient poor soil, the influence of the canopies of a Calluna heathland, a pioneering birch-pine woodland, and a terminal oak-beech forest on nutrient input was investigated. Of all investigated species Scots pine has the highest interception of water and nutrients (N, K, Ca, Mg). As a consequence, the nutrient input into the pioneering birch-pine forest is the highest of the three types of ecosystems. This ability to meliorate the nutrient supply by increasing the nutrient input favours pine in the early stages of the succession. The enhanced nutrient input and accumulation within the young successional forest ecosystems involves two different succession and/or management considerations depending upon the further ecosystem development.

• 1.The increasing nutrient availability mitigates the negative influence of the highly acidified nutrient poor soil on the growth of oak and beech and facilitates the conversion of pine dominated woodlands and forests into forests dominated by broadleaved species.
• 2.For regeneration of heathland from naturally established pine woodlands and forests, deforestation have to be combined with techniques of nutrient impoverishment of the soil.

     

Inorganic and organic soil phosphorus and sulfur pools in an Amazonian multistrata agroforestry system

In the central Amazon basin, the effects of secondary vegetation and primary forest on inorganic and organic P and S pools were compared with those of different fruit and timber tree species in a multistrata agroforestry system. The soils (Xanthic Ferralsols) were low in readily available P and S. Fertilizer applications increased the less accessible nutrient pools more than the plant available pools. For example, dilute-acid extractable P increased substantially (from 2 to 76 mg P kg⁻¹), whereas Mehlich P (plant available) increased less (from 3 to 19 mg P kg⁻¹). In contrast, the recalcitrant soil P pools, such as the residual P, did not increase on the short term, but only after more than six years following application. The proportion of less available ester-sulfate S was significantly higher in fertilized sites than in unfertilized sites, in contrast to soluble inorganic sulfate S or carbon-bonded S. The marked increase of successively available soil P and S pools through fertilization was advantageous with respect to the long-term effect of nutrient applications. Soil nutrient availability was not only related to the amount of nutrients applied but was also influenced by tree species. Nutrient return by litterfall and litter quality played an important role in soil P and S dynamics. Incorporation of applied nutrients into successively available organic nutrient pools will decrease potential P fixation and S losses by leaching and increase long-term nutrient availability. Therefore, tree species with rapid above-ground nutrient cycling and high quality litter (such as annato [Bixa orellana] and peach palm [Bactris gasipaes]) should constitute the majority of crops in multistrata agroforestry systems on infertile soils to ensure adequate medium to long term availability of P and S. This revised version was published online in June 2006 with corrections to the Cover Date.     

Litter decomposition and nutrients dynamics in <Emphasis Type="Italic">Nothofagus antarctica</Emphasis> forests under silvopastoral use in Southern Patagonia

The role of environmental variables on litter decomposition and its nutrient release in Nothofagus antarctica forest in Patagonia is poorly understood. Moreover, in these forests under silvopastoral use there are few antecedents. Litter decomposition and nutrient release of grasses and tree leaves were evaluated under different crown cover and two site quality stands during 480 days. Organic matter decomposition varied with crown cover for both types of litter, achieving mean values of 23 and 34% for maximal and minimal crown cover, respectively. Total transmitted radiation was the main environmental factor explaining 61 and 49% of the variation of grass and tree leaves decay rates, respectively. N, P, and Ca were mineralized during first 60 days in decomposing tree leaves and then immobilized without differences between crown cover. The K was immobilized during the evaluated period. In decomposing grass leaves the results varied according to site quality and time. There was a tendency of nutrient mineralization at the first 120 days and then immobilization. The removal of trees for silvopastoral use of N. antarctica may increase litter decomposition by changing the microclimate, but nutrients release or immobilization was mainly affected for their concentration in decomposing material.     

Leaf-litter decomposition of 15 tree species in a lowland tropical rain forest in Sarawak: dynamics of carbon,nutrients, and organic constituents

The effects of tree species on the dynamics of nutrient transformations during leaf-litter decomposition are not well understood in tropical rain forests. To examine differences in the dynamics of C, nutrients, and organic constituents during decomposition among tree species, the leaf-litter decomposition of 15 trees was assessed using a litter-bag method in a lowland tropical rain forest in Sarawak. The dynamics of C was parallel to that of weight loss. The dynamics of nutrients were grouped into three patterns. The dynamics of K was characterized by a high leaching loss in the initial stages, and that of Ca and Mg generally showed a gradual decrease over the course of decomposition. The dynamics of N and P showed highly different patterns with the weight loss, and was characterized by relatively higher remaining mass at the end of the experiment. The variations or exceptions of nutrient dynamics among tree species were considered to be related to the initial concentration of each nutrient. For the dynamics of organic constituents, water-soluble carbohydrates disappeared quickly at the initial stages, and acid-soluble carbohydrates were the second fastest decomposing fraction; the decomposition of acid-insoluble residue (AIS) was the slowest. The release of limiting nutrients (N and P) generally followed the disappearance of AIS, but was independent of the disappearance of AIS when the initial concentrations of these nutrients were very low.     

Contribution of agroforestry trees to nutrient requirements of intercropped plants

A major tenet of agroforestry, that trees maintain soil fertility, is based primarily on observations of higher crop yields near trees or where trees were previously grown. Recently objective analyses and controlled experiments have addressed this topic. This paper examines the issues of tree prunings containing sufficient nutrients to meet crop demands, the timing of nutrient transfer from decomposition to intercrops, the percent of nutrients released that are taken up by the crop, and the fate of nutrients not taken up by the crop.The amount of nutrients provided by prunings are determined by the production rate and nutrient concentrations, both depending on climate, soil type, tree species, plant part, tree density and tree pruning regime. A large number of screening and alley cropping trials in different climate-soil environments indicate that prunings of several tree species contain sufficient nutrients to meet crop demand, with the notable exception of phosphorus. Specific recommendations for the appropriate trees in a given environment await synthesis of existing data, currently only general guidelines can be provided.Tree biomass containing sufficient nutrients to meet crop demand is not enough, the nutrients must be supplied in synchrony to crop needs. Nutrient release patterns from organic materials are, in part, determined by their chemical composition, or quality. Leguminous materials release nitrogen immediately, unless they contain high levels of lignin or polyphenols. Nonlegumes and litter of both legumes and nonlegumes generally immobilize N initially. There is little data on release patterns of other nutrients. Indices that predict nutrient release patterns will assist in the selection of species for synchronizing with crop demand and improve nutrient use-efficiency.Field trials with agroforestry species ranging in quality show that as much as 80% of the nutrients are released during the course of annual crop growth but less than 20% is captured by the crop, a low nutrient-use efficiency. There are insufficient data to determine how much of the N not captured by the crop is captured by the trees or is in the soil organic matter, the availability of that N to subsequent crops, or how much of that N is lost through leaching, volatilization or denitrification. Longer term trials are needed.