Transport, retention, and geomorphic role of wood in tropical headwater streams, Costa Rica

The role of wood in shaping stream channels is an often overlooked component of the fluvial system. In forested catchments, wood is an integral part of the geomorphic and ecological function of streams. Geomorphically, stationary wood pieces and jams may do any or all of the following: 1) increase resistance to flow (Curran and Wohl, 2003), 2) deflect flow toward channel margins (Daniels and Rhoads, 2003), 3) shield channel margins (Brooks et al., 2003), 4) form steps (Gomi et al., 2003), 5) induce scour (Baillie and Davies, 2002), 6) trap sediment or nutrients (Faustini and Jones, 2003), 7) force avulsions (Maser and Sedell, 1994), and 8) increase overbank flow (Jeffries et al., 2003). Ecologically, the presence of wood usually leads to increased stream habitat complexity (Bisson et al., 1987; Kail, 2003), higher rates of coarse particulate organic matter (CPOM) retention which is a major source of energy and nutrients in many streams (Bilby and Likens, 1980), and greater surface area for colonization by algae, fungi, and microbes which contribute to the basal layer of aquatic food webs (Maser and Sedell, 1994).

Wood is delivered to streams by natural mortality of adjacent trees, by bank erosion and channel migration that undercuts standing trees and exhumes trees buried in floodplain sediment, and by land sliding and snow avalances. Although anthropogenic impacts have reduced global forest cover to half of the maximum Holocene extent, nearly one third of the Earth's land surface remains forested and subject to the wood-channel interactions described above. Yet even in these forest areas human efforts tend to reduce the amount of wood in channels. The primary motivations for wood removal are facilitation of transportation, flood conveyance, fish passage (although wood actually aids fish migration), and fuel consumption (Maser and Sedell, 1994; Montgomery and Wohl, 2004; Wohl, 2004).

Several features of tropical forests have the potential to change the nature of wood-channel interactions relative to the temperate zone. First, primary biological productivity (i.e., plant growth) in the tropics is higher than in less humid and warm climates, creating a more abundant source of wood. Second, the microbial diversity and decay fungus abundance is higher, causing wood to break down more quickly. Third, many tropical hardwoods contain compounds that resist decay and in some species the wood is more dense than water. Finally, the rainfall is abundant, flooding is common, and the hydrograph is generally 'flashy', meaning that stream stage rises and falls very quickly during flood events. Individually, the first factor would increase wood volume in the stream, the second would decrease the wood residence time, the third would increase wood residence time, and the fourth would decrease wood residence time. To find the cumulative effect of these factors, we spent 3 years surveying wood in the rainforests of eastern Costa Rica.

By measuring all wood pieces larger that 10 cm in diameter and 1 m long in 30 50-m-long stream segments in the Organization for Tropical Research's La Selva rainforest reserve in Costa Rica (Figures 1 & 2) we are able to compare wood load in this set of tropical streams with values reported from the temperate zone (Figure 3). The instantaneous load at La Selva is in the lower range of temperate wood loads. At 10 of the study sites we flagged and numbered each wood piece and tracked the gain and loss of logs over 3 years. The range of mean residence times from our study overlapped with some temperate zone studies, but in general wood passes through the streams at La Selva more quickly and jams are less frequent than at study sites along the Pacific Coast of North America (Hyatt and Naiman, 2001; Keller and Swanson, 1979; Lienkaemper and Swanson, 1987; Murphy and Koski, 1989).

La Selva, Costa Rica, is not necessarily representative of all tropical forests. It is located at the transition from volcanic mountains to coastal plains (Figure 4) and no landslides have been observed at La Selva in 40 years of scientific work. In contrast, the Rio Chagres drainage in Panama, which is the primary source of water used to operate the Panama Canal, has frequent landslides associated with tropical storms. In the Chagres, large wood jams form after major tropical storms, but are typically broken apart by non-landslide-inducing floods within 2-3 years (Wohl et al., 2009). This results in dramatic swings between periods of high wood load and nearly zero wood load, as opposed to La Selva where wood load is fairly constant.

• Baillie, B.R. and Davies, T.R., 2002. Influence of large woody debris on channel morphology in native forest and pine plantation streams in the Nelson region, New Zealand. New Zealand Journal of Marine and Freshwater Research, 36: 763-774.
• Bilby, R.E. and Likens, G.E., 1980. Importance of Organic Debris Dams in the Structure and Function of Stream Ecosystems. Ecology, 61(5): 1107-1113.
• Bisson, P.A. et al., 1987. Large woody debris in forested streams in the Pacific Northwest: past, present, and future. In: E.O. Salo and T.W. Cundy (Editors), Streamside management: forestry and fishery interactions. University of Washington, Seattle, pp. 143-190.
• Brooks, A.P., Brierley, G.J. and Millar, R.G., 2003. The long-term control of vegetation and woody debris on channel and flood-plain evolution: insights from a paired catchment study in southeastern Australia. Geomorphology, 51(1-3): 7-29.
• Curran, J.H. and Wohl, E.E., 2003. Large woody debris and flow resistance in step-pool channels, Cascade Range, Washington. Geomorphology, 51(1-3): 141-157.
• Daniels, M.D. and Rhoads, B.L., 2003. Influence of a large woody debris obstruction on three-dimensional flow structure in a meander bend. Geomorphology, 51(1-3): 159-173.
• Faustini, J.M. and Jones, J.A., 2003. Influence of large woody debris on channel morphology and dynamics in steep, boulder-rich mountain streams, western Cascades, Oregon. Geomorphology, 51(1-3): 187-205.
• Gomi, T., Sidle, R.C., Woodsmith, R.D. and Bryant, M.D., 2003. Characteristics of channel steps and reach morphology in headwater streams, southeast Alaska. Geomorphology, 51(1-3): 225-242.
• Hyatt, T.L. and Naiman, R.J., 2001. The residence time of large woody debris in the Queets River, Washington, USA. Ecological Applications, 11(1): 191-202.
• Jeffries, R., Darby, S.E. and Sear, D.A., 2003. The influence of vegetation and organic debris on flood-plain sediment dynamics: case study of a low-order stream in the New Forest, England. Geomorphology, 51(1-3): 61-80.
• Kail, J., 2003. Influence of large woody debris on the morphology of six central European streams. Geomorphology, 51(1-3): 207-223.
• Keller, E.A. and Swanson, F.J., 1979. Effects of large organic material on channel form and fluvial process. Earth Surface Processes, 4: 361-380.
• Lienkaemper, G.W. and Swanson, F.J., 1987. Dynamics of large woody debris in streams in old-growth Douglas-fir forests. Canadian Journal of Forest Research, 17(2): 150-156.
• Maser, C. and Sedell, J.R., 1994. From the forest to the sea: the ecology of wood in streams, rivers, estuaries, and oceans. St. Lucie Press, Delray Beach, FL, 200 pp.
• Montgomery, D.R. and Wohl, E.E., 2004. Rivers and riverine landscapes. In: A.R. Gillespie, S.C. Porter and B.F. Atwater (Editors), The Quaternary period in the United States. Elsevier, Amsterdam, pp. 221-246.
• Murphy, M.L. and Koski, K.V., 1989. Input and depletion of woody debris in Alaska streams and implications for streamside management. North American Journal of Fisheries Management, 9(4): 427-436.
• Wohl, E., 2004. Disconnected rivers: linking rivers to landscapes. Yale University Press, New Haven, Connecticut.
• Wohl, E., Ogden, F.L. and Goode, J., 2009. Episodic wood loading in a mountainous neotropical watershed. Geomorphology, 111(3-4): 149-159.