Litterfall

Litterfall (sometimes known as leaf litter or plant litter) is the transport of leaves, bark, twigs and other forms of dead organic material and its constituent nutrients from the aerial parts of the biosphere to the top layer of soil, commonly known as the litter layer or O horizon.

Litterfall has occupied the attention of ecologists at length for the reasons that it is an instrumental piece of in ecosystem dynamics, is indicative of regional net primary productivity (NPP), and may be useful in predicting regional nutrient cycling and soil fertility.

Contents

Characteristics and variability

Litterfall is characterized as fresh, undecomposed, and easily recognizable (by species and type) plant debris. This can be anything from leaves, cones, needles, twigs, bark, seeds/nuts, logs, or reproductive organs (e.g. the stamen of flowering plants). Items larger than 2 cm diameter are referred to as coarse litter, while anything smaller is referred to as fine litter or litter. The type of litterfall is most directly affected by ecosystem type. For example, leaf tissues account for about 70 percent of litterfall in forests, but woody litter tends to increase with forest age.[2] In grasslands, there is very little aboveground perennial tissue so the annual litterfall is very low and quite nearly equal to the net primary production.[3]

The litter layer is quite variable in its thickness, decomposition rate and nutrient content and is affected in part by seasonality, plant species, climate, soil fertility, elevation, and latitude. The most extreme variability of litterfall is seen as a function of seasonality; each individual species of plant has seasonal losses of certain parts of its body, which can be determined by the collection and classification of plant litterfall throughout the year, and in turn affects the thickness of the litter layer. In tropical environments, the largest amount of debris falls in the latter part of dry seasons and early during wet season.[4] As a result of this variability due to seasons, the decomposition rate for any given area will also be variable.

Latitude also has a strong affect on litterfall rates and thickness. Specifically, litterfall declines with increasing latitude. In tropical rainforests, there is a thin litter layer due to the rapid decomposition of the litterfall, while in boreal forests, the rate of decomposition is slower and leads to the accumulation of a thick litter layer, also known as a mor.[3] Net primary production works inversely to this trend, suggesting that the accumulation of organic matter is mainly a result of decomposition rate.

Net primary productivity

Net primary production and litterfall are intimately connected. In every terrestrial ecosystem, the largest fraction of all net primary production is lost to herbivores and litterfall. Therefore these factors must be accounted for. Ecologists account for this affect by subtracting the accumulated litterfall from the net primary production, resulting in what is called the true increment of net primary production. Due to their interconnectedness, global patterns of litterfall are similar to global patterns of net primary productivity.[3]

Habitat and food

Litterfall is the principal, and sometimes only, source of energy and shelter for the saprobionts and detritivores that live on the forest floor (this includes, but is not limited to bacteria, fungi, mollusks, arthropods, amphibians, reptiles, and even some mammals). Sometimes litterfall even provides energy to much larger mammals, such as in boreal forests where lichen litterfall is one of the main constituents of wintering deer and elk diets.[5]

Most of the organisms that live in the litter layer are decomposers. Their consumption of the litterfall results in the breakdown of simple carbon compounds into carbon dioxide (CO2) and water (H2O), and releases inorganic ions (like nitrogen and phosphorus) into the soil where the surrounding plants can then reabsorb the nutrients that were shed as litterfall. In this way, litterfall becomes an important part of the nutrient cycle that sustains forest environments.

Nutrient cycle

During leaf senescence, a portion of the plant’s nutrients are reabsorbed into the leaves. The nutrient concentrations in litterfall differ from the nutrient concentrations in the mature foliage by the reabsorption of constituents during leaf senescence.[3] Plants that grow in areas with low nutrient availability tend to produce litter with low nutrient concentrations, but a larger proportion of the available nutrients is reabsorbed. After senescence, the nutrient-enriched leaves become litterfall and settle on the soil below.

Litterfall is the dominant pathway for nutrient return to the soil, especially for nitrogen (N) and phosphorus (P). The accumulation of these nutrients in the top layer of soil is known as soil immobilization. Once the litterfall has settled, decomposition of the litter layer, accomplished through the leaching of nutrients by rainfall and throughfall and by the efforts of detritivores, releases the breakdown products into the soil below and therefore contributes to the cation exchange capacity of the soil. This holds especially true for highly weathered tropical soils.[7]

Leaching is the process by which cations such as iron (Fe) and aluminum (Al), as well as organic matter are removed from the litterfall and transported downward into the soil below. This process is known as podzolization and is particularly intense in boreal and cool temperate forests that are mainly constituted by coniferous pines whose litterfall is rich in phenolic compounds and fulvic acid.[3] By the process of biological decomposition by microfauna, bacteria and fungi, CO2 and H2O, nutrient elements, and an exceedingly resistant organic compound called humus are released. Humus composes the bulk of organic matter in the lower soil profile.[3]

The decline of nutrient ratios is also a function of decomposition of litterfall (i.e. as litterfall decomposes, more nutrients enter the soil below and the litter will have a lower nutrient ratio). Litterfall containing high nutrient concentrations will decompose more rapidly and asymptote as those nutrients decrease.[8] Knowing this, ecologists have been able to use nutrient concentrations as measured by remote sensing as an index of a potential rate of decomposition for any given area.[9] Globally, data from various forest ecosystems shows an inverse relationship in the decline in nutrient ratios to the apparent nutrition availability of the forest.[3]

Once nutrients have re-entered the soil, the plants can then reabsorb them through their roots. Therefore, nutrient reabsorption during senescence presents an opportunity for a plant’s future net primary production use. A relationship between nutrient stores can also be defined as:

annual storage of nutrients in plant tissues + replacement of losses from litterfall and leaching = the amount of uptake in an ecosystem

Collection and analysis

The main objectives of litterfall sampling and analysis are to quantify litterfall production and chemical composition over time in order to assess the variation in litterfall quantities, and hence its role in nutrient cycling across an environmental gradient of climate (moisture and temperature) and soil conditions.[10]

Ecologists employ a simple approach to the collection of litterfall, most of which centers around once piece of equipment, known as a litterbag. A litterbag is simply any type of container that can be set out in any given area for a specified amount of time to collect the plant litter that falls from the canopy above.

Litterbags are generally set in random locations within a given area and marked with GPS or local coordinates, and then monitored on a specific time interval. Once the samples have been collected, they are usually classified on type, size and species (if possible) and recorded on a spreadsheet.[12] When measuring bulk litterfall for an area, ecologists will weigh the dry contents of the litterbag. By this method litterfall flux can be defined as:

litterfall (kg m−2 yr−1) = total litter mass (kg) / litterbag area (m2)[13]

The litterbag may also be used to study decomposition of the litter layer. By confining fresh litter in the mesh bags and placing them on the ground, an ecologist can monitor and collect the decay measurements of that litter. An exponential decay pattern has been produced by this type of experiment: \frac{X}{X_o} = e^{-k}, where X_o is the initial leaf litter and k is a constant fraction of detrital mass.[3]

The mass-balance approach is also utilized in these experiments and suggests that the decomposition for a given amount of time should equal the input of litterfall for that same amount of time.

litterfall = k(detrital mass)[3]

References

  1. ^ Litter Fall in the North American Baldcypress Swamp Network, Illinois to Louisiana, 2003
  2. ^ W. M. Lonsdale (1988). "Predicting the amount of litterfall in forests of the world". Annals of Botany 61 (3): 319–324. 
  3. ^ a b c d e f g h i Schlesinger, William H. Biogeochemistry: An Analysis of Global Change. 2nd Edition. Academic Press. 108, 135, 152–158, 180–183, 191–194. (1997).
  4. ^ A. V. Spain (1984). "Litterfall and the standing crop of litter in three tropical Australian rainforests". Journal of Ecology 72: 947–961. JSTOR 2259543. 
  5. ^ Richard L. Ward & C. Les Marcum (2005). "Lichen litterfall consumption by wintering deer and elk in western Montana". Journal of Wildlife Management 69 (3): 1081–1089. doi:10.2193/0022-541X(2005)069[1081:LLCBWD]2.0.CO;2. JSTOR 3803347. 
  6. ^ Breymeyer, A.I., B. Berg, S.T. Gower, & D. Johnson. “Temperate Coniferous Forests” Scientific Committee on Problems of the Environment (SCOPE). Vol. 56: Global Change: Effects on Coniferous Forests and Grasslands Carbon Budget, Ch. 3. (1996).
  7. ^ J. Chave, D. Navarrete, S. Almeida, E. Álvarez, L. E. O. C. Aragão, D. Bonal, P. Châtelet, J. E. Silva-Espejo, J.-Y. Goret, P. von Hildebrand, E. Jiménez, S. Patiño, M. C. Peñuela, O. L. Phillips, P. Stevenson & Y. Malhi (2009). "Regional and seasonal patterns of litterfall in tropical South America" (PDF). Biogeosciences 7 (1): 43–55. doi:10.5194/bg-7-43-2010. http://frutos.uniandes.edu.co/pagina/Documentos/Pdf/Chave%20Litterfall%2009.pdf. 
  8. ^ Scott D. Bridgham, John Pastor, Charles A. McClaugherty, & Curtis J. Richardson (1995). "Nutrient-use efficiency: a litterfall index, a model, and a test along a nutrient-availability gradient in North Carolina peatlands" (PDF). The American Naturalist 145 (1): 1–21. http://ceeb.uoregon.edu/Bridgham/pubpdfs/art_5.pdf. 
  9. ^ Melillo, J.M., & J.R. Gosz. “Interactions of Biogeochemical Cycles in Forest Ecosystems” Scientific Committee on Problems of the Environment (SCOPE). Vol. 21: The Major Biogeochemical Cycles and Their Interactions, Ch. 6. (1983).
  10. ^ Simmons, Jeffrey A. “Measuring Litterfall Flux.” West Virginia Wesleyan College (2003).
  11. ^ "Spatial variations of nitrogen deposition and its effect on forest biochemical processes". Forest Research. http://www.forestresearch.gov.uk/fr/INFD-75PJ9E. Retrieved March 27, 2011. 
  12. ^ Estrella, Stephanie. “Standard Operating Procedures for Litterfall Collection, Processing, and Analysis: Version 2.0.” Washington State Department of Ecology. (2008).
  13. ^ Bastrup-Birk, A., & Nathalie Bréda. “Report on Sampling and Analysis of Litterfall” United Nations Economic Commission for Europe Convention on Long-Range Transboundary Air Pollution: International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests. (2004).

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