Forest Leaf Litter Nutrient Dynamics
Forest Leaf Litter Nutrient Dynamics
Forest Leaf Litter Nutrient Dynamics
Funded by: USDA-NRCS Researcher: Thomas Marler
When residues of plants are added to the forest floor litter pool, their constituents are released over time through decomposition. Decomposition of plant litter is a fundamental source of nutrients and energy for forested ecosystems. Numerous factors influence decomposition including the environment, the initial quality of the litter as defined by its physical traits and chemical constituents, and the decomposer community of microorganisms and soil animals. The influential environmental factors include the chemical traits of the soil and climatic factors like temperature and rainfall. The litter quality factors are defined by various characteristics such as plant species, plant organ, and age of the organ when it was released from the plant.
Chemistry of plant residues has been extensively studied in relation to decomposition. A high relative proportion of nutrients such as calcium, manganese, nitrogen, phosphorus, and potassium enable speedy decomposition by the decomposer community. In some cases, their relative proportion in relation to residue carbon is a better predictor of decomposition speed than their absolute amounts. Some of the larger plant compounds such as lignin are difficult to break down, so higher contents of these compounds can delay the decomposition process. Other plant compounds such as phenolics directly inhibit microbial colonization or function, and their presence also slows down decomposition.
In addition to the direct effect that a plant species exerts on litter decomposition through chemistry of its residues that are added to the litter layer, the chronic additions of litter from a tree also influence future decomposition indirectly through the changes in the soil over time. This is sometimes called the “footprint” that a tree imposes on the soil. These influences on soil properties are one way that invasive tree species bring about substantial changes in ecosystem properties.
Understanding litter decomposition dynamics greatly contributes to understanding how terrestrial ecosystems function and factors that control storage of soil carbon. Although the literature on litter decomposition is robust, there is a conspicuous bias toward temperate ecosystems.
Native Trees
Cocos nucifera |
The coconut tree is found throughout Guam and the rest of the Micronesia.
Coconut leaf litter decomposed more slowly than most of the other tree species in this study. 45% of the litter had decomposed by 4 months. Most farm sites exhibited complete decomposition by 24 months. The farm site in Tinian exhibited slower coconut leaf litter decomposition than the other farm sites.
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Cycas micronesica
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This Cycas species occurs in Guam, Rota, Yap, and Palau. This species is of importance for many reasons, and is called fadang in the Mariana Islands.
A robust 2002 forestry survey of Guam revealed fadang was the most abundant tree on the island at that time.
The historical value of fadang trees to forests of the Mariana Islands may never be known, because several alien insect invasions that began in 2003 have killed more than 90% of the population in less than a decade. This widespread mortality has occurred before adequate research was conducted to assess the ecosystem services provided by this unique species.
Fadang trees do not produce true wood. The stem from this and other cycad species is mostly parenchyma tissue that decomposes very rapidly after the tree dies. The vascular tissue in fadang stems is arranged in cylinders interspersed throughout the parenchyma tissue. The leaves live many years, indicating decomposition of fadang leaf litter may be slower than that of most species.
The Western Pacific Tropical Research Center has been studying pollination biology of fadang for several years. The male cones emit a strong aroma that attracts the insects that can carry pollen from males to females.
Fadang leaf litter decomposed more slowly than most of the other tree species in this study. About half of the litter had decomposed by 4 months. The Rota farm site exhibited slower fadang leaf litter decomposition than the two Guam farm sites.
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Hibiscus tiliaceus |
This Hibiscus species is native in the Mariana Islands and throughout many islands in Oceania. One of the common names for the tree is Sea-hibiscus because of its ability to thrive in coastal forests. It is called pago In the Mariana Islands. Various parts of the sea-hibiscus have been used on Guam. The inner bark tissue was used for cordage and nets. The wood was used for furniture and frames for canoe construction. The flowers were used to make a poultice that was applied to wounds. The value of the species for the landscape trade has increased through selection of cultivars that produce atypical flower colors and variegation in leaves.
Sea-hibiscus leaf litter decomposed at a moderate rate compared to the other tree species. About one-fourth of the litter had decomposed by 4 months. Decomposition was almost complete by 1 year, but then it slowed down such that a full two years was required to complete leaf litter decomposition for this species.
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Morinda citrifoloia |
The noni tree is indigenous in the Mariana Islands and is found in most soil types. Additionally, the tree is grown in home gardens. This species is called lada on Guam.
Noni seeds are constructed with an air chamber that enables flotation in sea water. This ability to float and disperse on oceanic currents may be one of the reasons the species is so widespread. Scientists call this form of seed dispersal thalassochory.
One of the reasons that noni is so widespread in the Mariana islands is its adaptive ability to grow in all of the habitat types. Coastal sites impose salt spray and salinity intrusion, volcanic soils impose calcium deficiency and aluminum toxicity, and the calcareous soils impose acute deficiencies of several micronutrients. Noni is on a short list of native plants that seem to tolerate all of these stressors.
The Western Pacific Tropical Research Center has studied the ability of noni tissues to accumulate aluminum. The recent results revealed a possibility of excessive aluminum accumulation in leaves for noni trees growing in Guam’s acid volcanic soils. The research illuminated the possibility of excessive exposure to aluminum while ingesting leaf infusions from these trees.
Noni leaf litter was one of the most rapid to decompose. More than 90% of the litter had decomposed by four months. The litter of this species also exhibited minimal variation in decomposition speed among the farm sites.
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Pandanas tectorius |
This widespread Pandanus species Grows throughout the Mariana Islands, where it is called kafu.
Pandanus tectorius is ubiquitous throughout the Mariana Islands. It is one of two species, with Pandanus dubius being less common and widespread. This tree can be used as a living hedge that is in-penetrable. Leaves have been used for mats and baskets. Ripe fruits are eaten by Guam’s fruit bats.
The sword-shaped leaves give Pandanus plants a distinct and aesthetic appearance. They are ideal for container culture.
The fruits resemble pineapple fruits for some, and they can reach 2 kilograms in size. Some decomposition studies have shown that mixtures of leaf litter from closely related plants generate more synergistic response where the decomposition is much more rapid than predicted by single species decomposition studies. Pandanus tectorius and Pandanus dubius litter mixtures would be excellent species to study this phenomenon on Guam because they grow side-by-side in many locations.
Pandanus leaf litter decomposed more rapidly than expected, considering the leaves of this genus are used for making many utilitarian products and the leaves are built to withstand the mechanical forces imposed by typhoons. About 70% of the litter had decomposed by four months. All of the farm sites exhibited complete decomposition by 24 months.
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Premna serratifolia |
Premna is a widespread genus found throughout Africa, Asia, and the western Pacific. Guam’s species grows to become a handsome tree that is a common member of the limestone or coastal forests. It is called ahgao in the Mariana Islands.
Premna wood is hard and was once used for construction. The tree does not produce a straight trunk, so sometimes the logs were not suitable for use due to imperfections.
The galls that appear on Premna leaves are quite distinctive, and can be used as one of the diagnostics for locating Premna trees in the forest. Premna leaves have been used for medicinal purposes, consumed as a tea for relief of muscle pains. Insect damage such as these galls often leads to chemical changes in the leaf tissue, which may affect decomposition speed. Premna is one Guam species that would be ideal to study this phenomenon in the Mariana Islands.
Premna bark has also been used for medicinal purposes, specifically to treat nerve problems. Small white flowers lead to many small green fruits that turn purple-black upon maturity.
Premna leaf litter decomposed rapidly in comparison to all of the species in this study. More than 90% of the litter had decomposed by four months. Most of the farm sites exhibited complete decomposition by 18 months.
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Common Agroforestry Trees
Artocarpis atilis |
The breadfruit is one of the most common fruit trees for agroforestry settings in
Guam and the rest of the Micronesia. It is called lemai on Guam. The species produces fruit without seeds. Propagation must be accomplished by humans.
This is often accomplished by removing root suckers. This fruit tree is an outstanding multi-purpose tree. It is an excellent shade tree,
especially for agroforestry settings. It is rarely toppled in typhoons because the
medium-sized branches break off in the early stages of the storm and reduce wind drag.
These falling branches may cause damage if breadfruit trees are planted in residential
areas. Breadfruit wood is used for furniture, paneling, and canoe-building. The fruit is an excellent source of carbohydrates, potassium and vitamin C. The interesting breadfruit leaf is the subject of stunning artwork, pottery, and carvings.
Breadfruit leaf litter decomposed at moderately rapid rate compared to the other tree species. About 80% of the litter had decomposed by 4 months. Most of the farm sites exhibited complete decomposition by 18 months. The farm site in Tinian exhibited slower breadfruit leaf litter decomposition than the other farm sites.
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Mangifera indica |
The mango is one of the most sought after dooryard fruit trees for home gardens and agroforestry settings in Guam and the rest of the Mariana Islands. The local name for this important fruit tree is manga.
This fruit tree is not a true tropical tree, and this leads to inadequate flowering when the trees are grown in the Mariana Islands. Vegetative growth is so plentiful and unchecked that the trees rarely slow down long enough to for the stems to transition into reproductive growth.
The major disease problem is caused by the fungus Anthracnose.
The Western Pacific Tropical Research Center introduced numerous mango cultivars to Guam and spent years evaluating their productivity. Several cultivars like ‘Edward’ and ‘Dot’ responded moderately well to forcing of flower production and produced high quality fruits, but production levels were not sufficient for commercial production. Continued search for locally adapted cultivars is urgently needed.
Mango leaf litter decomposed at a relatively slow rate compared to the other tree species. About half of the litter had decomposed by 4 months. Most of the farm sites exhibited complete decomposition by 18 months. The farm site in northern Guam exhibited slower avocado leaf litter decomposition, reaching 100% decomposition at 32 months.
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Persea americana |
The avocado is one of the most common dooryard fruit trees for home gardens and agroforestry settings in Guam and the rest of the Mariana Islands.
The avocado is native to Central America. Any attempts of the early Spanish galleons to introduce the tree to the Mariana Islands were unsuccessful. William Safford is credited with the first successful introduction of the tree to Guam during his 1899-1900 tenure as the Assistant Governor of Guam. It is called alageta on Guam.
This fruit tree has two major limitations in the Mariana Islands. First, almost all trees are grown from seeds, so fruit quality traits vary from tree to tree. A commercial fruit industry cannot be built around this extreme variability. Second, almost all of the avocado trees produce fruit at the same time each year. A single tree can be highly productive, and sometimes the fruits are so plentiful they cannot be marketed. Cultivars that produce fruits out of season are needed.
The Western Pacific Tropical Research Center introduced numerous avocado cultivars to Guam and spent years evaluating their productivity. ‘Malama’ passed all of the tests indicating the tree would flower consistently in Guam’s climate and produce fruits several months later than the bulk of Guam’s avocado trees.
Avocado leaf litter decomposed at a relatively slow rate compared to the other tree species. About half of the litter had decomposed by four months. Most of the farm sites exhibited complete decomposition by 18 months. The farm site in northern Guam exhibited slower avocado leaf litter decomposition, reaching 100% decomposition at 32 months.
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Common Invasive Trees
Leucaena lucocephala |
This small tree species is one of the aggressive invasive species in the Mariana Islands, where it can rapidly occupy any cleared areas on limestone soils. The local name for this problematic weedy tree is tangantangan.
When Leucaena trees invade a site and out-compete the other plant species, a homogeneous canopy of the species develops.
Leucaena is a member of the bean family of plants, and it is able to associate with microorganisms in its roots that deliver nitrogen for the plant’s use. This adds to the concerns about how the nitrogen cycling in the invaded sites is being altered and affecting ecosystem services.
Leucaena leaf litter decomposed at a rapid rate compared to the other tree species in this study. About85% of the litter had decomposed by 4 months. One of the concerns with invasive alien species that can so aggressively displace native tree species is what the changes in species composition do to nutrient and carbon cycling in the habitats.
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Vitex
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The Vitex tree is one of the most troublesome invasive alien trees for Guam. It is native in Indonesia, Malaysia, and Philippines. This is an unusual species in that it is considered vulnerable to threats in its native range, but a detrimental weed in other locations like Guam.
The attractive purples flowers and tree canopy are traits that make the species popular in the landscape trade, which is one way the tree gets introduced to regions like Guam where it escapes cultivation and becomes a threat to native forests.
Vitex leaf litter decomposed rapidly compared to the other tree species. About 85% of the litter had decomposed by four months. Both Guam farm sites exhibited complete decomposition by 12 months.
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The native species that exhibited the most sluggish leaf decomposition were Cocos nucifera and Cycas micronesica.
The native species that exhibited the most rapid leaf decomposition were Hibiscus tiliaceus, Morinda citrifolia, and Premna serratifolia.
Pandanus tectorius leaf litter decomposed at moderate speed in relation to the other species.
The agroforestry species as a group exhibited leaf litter decomposition that was slower than the native species as a group. Litter decomposition for Artocarpus altilis was more rapid than for Mangifera indica and Persea americana.
The invasive species exhibited accelerated leaf litter decomposition compared to the other species in this study.
This raises concerns about how Leucaena leucocephala, Vitex parviflora, and other alien trees are changing carbon and nutrient cycling processes in the Mariana Islands.
- Greater amounts of nutrients in the litter such as nitrogen and phosphorus generally increase decomposition speed.
- Sometimes the concentration of the nutrient in relation to carbon concentration is more important than the absolute amount of the nutrient.
- Higher concentrations of biochemicals produced by the plant usually leads to slower decomposition.
- Thinner leaves generally decompose more rapidly than thicker leaves.
We determined the influence of many of these litter traits on decomposition speed among the 11 species included in this study. We focused on the initial rapid decomposition period of the first four months after deploying the experiments.
The best predictor of decomposition speed was leaf lignin content, with a significant increase in amount of litter remaining (slower decomposition) with increased lignin.
Initial leaf potassium content was also a good predictor of decomposition speed. Greater potassium significantly increased decomposition speed. The relationship of carbon to potassium also significantly influenced decomposition speed. A significant increase in amount of litter remaining (slower decomposition) occurred with increased C:K.
The leaf nitrogen content did not significantly influence initial decomposition speed.
The leaf phosphorus content did not significantly influence initial decomposition speed.
The relationship of carbon to nitrogen did not significantly influence initial decomposition speed.
The relationship of carbon to phosphorus did not significantly influence initial decomposition speed.
The leaf cellulose content did not significantly influence initial decomposition speed.
The specific leaf area (mm2 / mg) did not significantly influence initial decomposition speed.
The total phenolics content of leaves did not significantly influence initial decomposition speed.
The soil characteristics of a farm or forest habitat may influence speed of decomposition in a manner that is distinct from the influence of litter traits. For example: Nutrient status of the soil directly influences decomposition. If leaf litter is deficient in nitrogen or other nutrients, a soil with high relative levels of nutrients may make up for the deficiency in the litter and speed up the decomposition process.
Sites that are deficient also indirectly influence decomposition by increasing the nutrient resorption efficiency of the trees. As leaves begin to age and die, the plant can retrieve the nutrients prior to ultimate leaf death. This occurs to a greater extent in poor soils, and the result is slower decomposition of the resulting leaf litter.
A phenomenon called “home field advantage” often exerts strong control over litter decomposition traits. The concept is founded in the fact that the decomposer community that is highly efficient at decomposing litter from a particular species tends to increase in population nearby trees of that species. Therefore, if litter from that species falls in these microsites where the species-specific decomposers have increased in population, the decomposition speed is increased.
The weather patterns of a farm or forest site also influence speed of decomposition.
Temperature exerts a strong controlling effect on litter decomposition, with the decomposer microorganisms performing best in moderate temperatures. The temperatures in the Mariana Islands are ideal for growth and performance of these microorganisms.
Rainfall abundance and seasonal distribution also control litter decomposition speed. As with all organisms, the community of microorganisms that decompose litter require abundant but not excessive available water to function.
About 75% of the litter had decomposed by 4 months. The variation among the experimental sites was not substantial, especially in relation to the variation among the species. Litter decomposition in Tinian was slowest and litter decomposition in Saipan was most rapid.
Considering the influence of rainfall on litter decomposition and the distinct rainy versus dry seasons of the Mariana Islands, we conducted paired experiments to determine litter decomposition during the first 8 months of deployment. The first experiment was initiated at the beginning of the rainy season, and the second experiment was initiated at the beginning of the dry season. The two Guam sites were included and we used all 11 tree species for this study.
Litter decomposition during the initial 4 months was more rapid during the rainy season as predicted. However, the season in which leaf litter fall occurs will influence initial speed of litter decomposition, which is under the control of rainfall abundance. However, this divergence of decomposition speed as influenced by season disappears during the 4 to 8 month period when rainfall is limited for the litter that began decomposing in the rainy season but rainfall is abundant for litter that began decomposing in the dry season.