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).

References

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