Trees in the ecosystem pt I: Trees & fish

The extent of attention as to exactly how critical trees are for fish populations is unfortunately not all that significant (in comparison to the study or trees and birds, for example), though this is not necessarily surprising – this is perhaps because fish spend their lives largely under water, and thus their presence is not necessarily recognised to the degree it would be if fish were land-based organisms. However, there is certainly a healthy array of research that has been undertaken into this relationship of trees and fish within the forest ecosystem, as is demonstrated below.

Many undisturbed pools (areas of slow-moving or still water within in rivers and streams) in forests are either created or enhanced by the presence of deadwood (as either driftwood or sunken wood). Such deadwood presence can also raise water levels locally and create a diverse range of aquatic habitats (Hodge & Peterken, 1998) by damming up rivers and streams, and reducing flow velocity (Barbour et al., 2001; Gippel et al., 1996). Large woody debris (including fallen stems and large branches) is particularly critical in this regard, and research has shown that nearly 30% of pools within a stream or river may be created by such woody debris (Mossop & Bradford, 2004).

A gathering of many fallen branches significantly obstructs the flow of this stream through the New Forest, UK. Such obstruction creates niche habitats on both sides of the log jam. Source: Author (2016).

Other research has, whilst not focussing on large woody debris exclusively, identified that as much as 75% of all pools may be created from submerged woody debris (Robison & Beschta, 1990a). Through the creation of these habitats, fish populations can increase, as their range of viable habitat increases – notably for feeding and spawning (Harvey, 1998). However, because even the largest of woody debris will likely not persist for over 50 years, there is a need for a continuous replenishment if streams and rivers are to retain the presence of deadwood-induced pools (Hyatt & Naiman, 2001). When pools are instead created by wood jams, which are made of small (and sometimes also large) branches and stems clustered together, their average viable retention time may only be between 2-3 years (Lisle, 1986). Again, a need for a constant supply of such deadwood is necessary, and this should obviously mean management practices retain trees that can constantly provide for such woody material (Robison & Beschta, 1990b).

Driftwood may be particularly beneficial for fish populations, as not only will its presence control flow velocity, but also protect its banks from erosion, create waterfalls and pools, and thus provide protection for fish spawning as well as increasing habitat diversity (Gurnell et al., 2002). Additionally, driftwood can provide hiding places for species of fish, assisting either in their predatory pursuits or in evading predation (Crook & Robertson, 1999; Werneyer & Kramer, 2005).

Sunken (or partially submerged) deadwood, for those fish species which are insectivorous, can also be highly valuable (Barbour et al., 2001). The wood’s provision of habitat for invertebrates means there is a potential abundance of prey for such insectivorous fish (O’Connor, 1992). A study into the effects of deforestation on wood input levels into woodland stream environments there unsurprisingly showed how reduced amounts of sunken deadwood led to reduced fish diversity and abundance (Wright & Flecker, 2004). In such wood-void streams, wood-eating fish (such as certain species of catfish, whilst not ‘true’ xylivores) may also suffer (German & Bittong, 2009; Lujan et al., 2011), though the loss of diversity in a stream (or river) environment, both because of reduced wood presence and the faster flow associated with such a lack of wood, may also have wider implications for fish species overall (Lancaster et al., 2001; Shields & Smith, 2002; Tsui et al., 2000); particularly when it is understood that a lack of (large) sunken wood is indicative of a degraded stream (Shields et al., 2006). It is also suggested that sunken wood may aid with orientation for fish (Crook & Robertson, 1999).

Some significantly-decayed deadwood from a fallen willow (Salix sp.) will offer aquatic organisms – including fish – the opportunity to forage and seek shelter. Source: Author (2016).

Deadwood that has fallen and become (partially) submerged is also beneficial, as previously ascertained, because it creates pools within a stream or river ecosystem. These pools are areas of a stream or river where the flow is potentially very slow, and in the redwood forests of California downed trunks and branches of trees are considered to be crucial for constituent salmon populations (Barbour et al., 2001). Notably, in areas of steeper ground, this fallen deadwood can create tiers of pools, which actually enable salmon (that travel upstream to breed) to ascend up the river with more ease, as the salmon can ‘leap’ from one pool to another, and swim against a current with reduced velocity (which is critical for the enabling of salmon to conserve vital energy). These pools also reduce bankside erosion and catch up to 85% of sediment (which may amass behind a large branch or stem, though perhaps even more significantly amongst larger wood jams comprised of deadwood of varying sizes), ensuring the rate of sedimentation of the stream or river is slow and sustainable (Berg et al., 1998; Smith et al., 1993; Thevenet et al., 1998). This is important for the salmon, as females nest within the clean gravel beds in the riverbed, and any marked rate of sedimentation would prohibit this (Madej & Ozaki, 2009). These nesting sites may also, in fact, be located within close proximity to large pieces of woody debris (Senter & Pasternack, 2011). The very same deadwood can also support plant life, particularly when a large stem has fallen across a river, and therefore the plants growing atop the log can shade the river and keep the water cooler – this is also critical for the salmon, which prefer cooler waters (Welsh et al., 2001).

This willow has fallen but remains alive, offering a further and somewhat different aspect to the aquatic environment. Source: Author (2016).

Across the United States, in the Appalachian Mountains, research by Jones et al. (1999) has also revealed that the reduction in sedimentation created by fallen woody debris is critical for other species of fish (including the rainbow trout Oncorhynchus mykiss), that spawn in sediment-free riffles within the forest areas of the mountains. Furthermore, their research highlighted that deforestation along riparian zones as little as 1km in length can have massive adverse effects upon the quality of habitat for fish, due to the removal of the source of such critical deadwood. The associated re-growth after the felling, whilst still injecting debris into the water courses, cannot match the size of the debris from older-growth stands, and therefore rainbow trout occur less frequently and at lesser densities (Flebbe & Dolloff, 1995). Deforestation also increases the risk of severe flooding and high flow velocity within the Appalachian Mountains, which can both extensively decimate viable habitat for rainbow trout within the ecosystem. In part, this is because such factors eliminate the fauna that occupy the river bed, which the trout almost exclusively predate upon.

Beyond the realm of deadwood, the beneficial impacts of shading by large trees adjacent to such aquatic environments can also improve the suitability of the habitat for fish (Beschta, 1997; Larson & Larson, 1996). Using the redwood forests as an example once again, it has been recognised that large conifers that reside by a water course cast shade and thus reduce maximum temperatures and the risk of thermal pollution (Madej et al., 2006). Such cooler temperatures, much like how deadwood can support plants that shade and cool waters, protects critical nesting locations for female salmon, reduces the subsequent mortality of juvenile salmon, and improves their growth rate.

The shade this single hornbeam (Carpinus betulus) provides the river beneath, whilst not necessarily significant, will be of measurable benefit. Source: Author (2016).

Beyond California, the cooler waters created through significant (50-80%) canopy shading are equally as important for fish, for similar reasons (Broadmeadow & Nisbet, 2004; Broadmeadow et al., 2011; Swift Jr & Messer, 1971). Such canopy shade may also enable for rivers and streams to support macrophytes (plants growing in or near water), which can act as a food source for some fish species both directly and indirectly. Similarly, they can provide refuge for fish seeking shelter from predators (Pusey & Arthington, 2003). Therefore, retaining riparian trees is mandatory, if viable habitats for fish are to be protected (Young, 2000).

A line of willow and ash (Fraxinus excelsior) dresses the southern side of this river, meaning the water remains continually shaded throughout the day. Source: Author (2016).


Barbour, M., Lydon, S., Brochert, M., Popper, M., Whitworth, V., & Evarts, J. (2001) Coast Redwood: A Natural and Cultural History. USA: Cachuma Press.

Berg, N., Carlson, A., & Azuma, D. (1998) Function and dynamics of woody debris in stream reaches in the central Sierra Nevada, California. Canadian Journal of Fisheries and Aquatic Sciences. 55 (8). p1807-1820.

Beschta, R. (1997) Riparian shade and stream temperature: an alternative perspective. Rangelands. 19 (2). p25-28.

Broadmeadow, S., Jones, J., Langford, T., Shaw, P., & Nisbet, T. (2011) The influence of riparian shade on lowland stream water temperatures in southern England and their viability for brown trout. River Research and Applications. 27 (2). p226-237.

Broadmeadow, S. & Nisbet, T. (2004) The effects of riparian forest management on the freshwater environment: a literature review of best management practice. Hydrology and Earth System Sciences Discussions. 8 (3). p286-305.

Crook, D. & Robertson, A. (1999) Relationships between riverine fish and woody debris: implications for lowland rivers. Marine and Freshwater Research. 50 (8). p941-953.

Flebbe, P. & Dolloff, C. (1995) Trout use of woody debris and habitat in Appalachian wilderness streams of North Carolina. North American Journal of Fisheries Management. 15 (3). p579-590.

German, D. & Bittong, R. (2009) Digestive enzyme activities and gastrointestinal fermentation in wood-eating catfishes. Journal of Comparative Physiology B. 179 (8). p1025-1042.

Gippel, C., Finlayson, B., & O’Neill, I. (1996) Distribution and hydraulic significance of large woody debris in a lowland Australian river. Hydrobiologia. 318 (3). p179-194.

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Harvey, B. (1998) Influence of large woody debris on retention, immigration, and growth of coastal cutthroat trout (Oncorhynchus clarki clarki) in stream pools. Canadian Journal of Fisheries and Aquatic Sciences. 55 (8). p1902-1908.

Hodge, S. & Peterken, G. (1998) Deadwood in British forests: priorities and a strategy. Forestry. 71 (2). p99-112.

Hyatt, T. & Naiman, R. (2001) The residence time of large woody debris in the Queets River, Washington, USA. Ecological Applications. 11 (1). p191-202.

Jones, E., Helfman, G., Harper, J., & Bolstad, P. (1999) Effects of riparian forest removal on fish assemblages in southern Appalachian streams. Conservation Biology. 13 (6). p1454-1465.

Lancaster, S., Hayes, S., & Grant, G. (2001) Modeling sediment and wood storage and dynamics in small mountainous watersheds. Geomorphic Processes and Riverine Habitat. 4 (1). p85-102.

Larson, L. & Larson, S. (1996) Riparian shade and stream temperature: a perspective. Rangelands. 18 (4). p149-152.

Lisle, T. (1986) Effects of woody debris on anadromous salmonid habitat, Prince of Wales Island, southeast Alaska. North American Journal of Fisheries Management. 6 (4). p538-550.

Lujan, N., German, D., & Winemiller, K. (2011) Do wood‐grazing fishes partition their niche?: morphological and isotopic evidence for trophic segregation in Neotropical Loricariidae. Functional Ecology. 25 (6). p1327-1338.

Madej, M., Currens, C., Ozaki, V., Yee, J., & Anderson, D. (2006) Assessing possible thermal rearing restrictions for juvenile coho salmon (Oncorhynchus kisutch) through thermal infrared imaging and in-stream monitoring, Redwood Creek, California. Canadian Journal of Fisheries and Aquatic Sciences. 63 (6). p1384-1396.

Madej, M. & Ozaki, V. (2009) Persistence of effects of high sediment loading in a salmon-bearing river, northern California. Geological Society of America Special Papers. 451 (1). p43-55.

Mossop, B. & Bradford, M. (2004) Importance of large woody debris for juvenile chinook salmon habitat in small boreal forest streams in the upper Yukon River basin, Canada. Canadian Journal of Forest Research. 34 (9). p1955-1966.

O’Connor, N. (1992) Quantification of submerged wood in a lowland Australian stream system. Freshwater Biology. 27 (3). p387-395.

Pusey, B. & Arthington, A. (2003) Importance of the riparian zone to the conservation and management of freshwater fish: a review. Marine and Freshwater Research. 54 (1). p1-16.

Robison, E. & Beschta, R. (1990a) Coarse woody debris and channel morphology interactions for undisturbed streams in southeast Alaska, USA. Earth Surface Processes and Landforms. 15 (2). p149-156.

Robison, E. & Beschta, R. (1990b) Identifying trees in riparian areas that can provide coarse woody debris to streams. Forest Science. 36 (3). p790-801.

Senter, A. & Pasternack, G. (2011) Large wood aids spawning Chinook salmon (Oncorhynchus tshawytscha) in marginal habitat on a regulated river in California. River Research and Applications. 27 (5). p550-565.

Shields, F., Knight, S., & Stofleth, J. (2006) Large Wood Addition for Aquatic Habitat Rehabilitation in An Incised, Sand-Bed Stream, Little Topashaw Creek, Mississippi. River Research and Applications. 22 (7). p803-817.

Shields, F. & Smith, R. (1992) Effects of large woody debris removal on physical characteristics of a sand‐bed river. Aquatic Conservation: Marine and Freshwater Ecosystems. 2 (2). p145-163.

Smith, R., Sidle, R., Porter, P., & Noel, J. (1993) Effects of experimental removal of woody debris on the channel morphology of a forest, gravel-bed stream. Journal of Hydrology. 152 (1). p153-178.

Swift Jr, L. & Messer, J. (1971) Forest cuttings raise temperatures of small streams in the southern Appalachians. Journal of Soil and Water Conservation. 26 (3). p111-116.

Thevenet, A., Citterio, A., & Piegay, H. (1998) A new methodology for the assessment of large woody debris accumulations on highly modified rivers (example of two French piedmont rivers). Regulated Rivers: Research & Management. 14 (6). p467-483.

Tsui, K., Hyde, K., & Hodgkiss, I. (2000) Biodiversity of fungi on submerged wood in Hong Kong. Aquatic Microbial Ecology. 21 (3). p289-298.

Welsh H., Hodgson, G., Harvey, B., & Roche, M. (2001) Distribution of juvenile coho salmon in relation to water temperatures in tributaries of the Mattole River, California. North American Journal of Fisheries Management. 21 (3). p464-470.

Werneyer, M. & Kramer, B. (2005) Electric signalling and reproductive behaviour in a mormyrid fish, the bulldog Marcusenius macrolepidotus (South African form). Journal of Ethology. 23 (2). p113-125.

Wright, J. & Flecker, A. (2004) Deforesting the riverscape: the effects of wood on fish diversity in a Venezuelan piedmont stream. Biological Conservation. 120 (3). p439-447.

Young, K. (2000) Riparian zone management in the Pacific Northwest: who’s cutting what?. Environmental Management. 26 (2). p131-144.

Trees in the ecosystem pt I: Trees & fish

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