Freshwater ecosystems may be more resilient than we think, according to the findings of a recently completed five-year study.

The storm that led to a study

Dr Brenda Baillie was about to begin a study monitoring freshwater quality in three streams in a recently harvested planted forest when a large rainstorm swept across the East Coast of the North Island, leaving a trail of destruction in its wake. The combination of harvesting and a one-in-100 year storm resulted in ‘debris flows’ that transformed the freshwater ecosystems, from cool, shady environments with indigenous riparian vegetation and pine forest to open stream channels scoured down to bedrock in places.

Brenda had recently completed her PhD studying the presence of wood in freshwater ecosystems at the same sites in Houpoto Forest. She had collected a wealth of preharvest and pre-storm data and the storm gave her the opportunity to transform the study into a new five-year project monitoring the recovery of recently harvested freshwater streams after debris flow events.

The results might surprise you

After five years of tracking the presence of aquatic invertebrates, fish and riparian margin vegetation, Brenda found that different parts of the stream are on different recovery trajectories. Brenda explains, “The stream environment was completely transformed - from a cool, diverse habitat for a range of species, to sunny open channels with simplified stream and riparian environments, but that changed over five years.”

The increased sunlight exposure warmed the water and increased algal growth conditions which seems to have suited some native fish and invertebrate species.

Brenda says, “Our data showed that fish biodiversity and numbers dropped initially after the storm, but numbers have now exceeded what we recorded prior to the storm, especially bullies and long and short-finned eels.”

Other fish species, including banded kōkopu and smelt, that are known to have a narrower range of living conditions, were found in the streams prior to harvesting and the storm but have not been found since. The data showed a similar pattern of response for the aquatic invertebrates, although the recovery of the aquatic invertebrate communities is more
advanced than the fish.

Brenda says the debris flows have simplified aquatic community biodiversity, particularly for fish. “We’ve had a huge increase in some species that favour the new conditions, but the stream environment lacks the riparian and in-stream cover and more complex pool habitats that were present prior to this event, limiting the re-establishment of some species for now.”

How long it will take for these communities to fully recover and for the fish and aquatic invertebrate densities to return to pre-harvesting and storm levels isn’t known but is likely to be linked to the recovery of the riparian vegetation and re-establishment of woody debris in the stream channel.

The debris flows removed the vegetation, soil and seed sources in the riparian margins, causing plant re-growth to be slower than what would occur after harvesting alone when most of this material remains intact. The re-growth vegetation shading the stream has begun to cool water temperatures toward pre-harvest and storm temperatures.

Brenda says that it is important to remember that ecosystems aren’t static, and that natural events like this storm, will alter ecosystems as a matter of course. In this case, the storm occurred in conjunction with harvesting, allowing Brenda to identify the species that were resilient and thrived in these conditions, and it pinpointed the species with limited resilience. She says, “It’s very encouraging to see some of these species bouncing back, and we hope that over time some of the other species do too.”

Recommendations for foresters

The study has led to a number of recommendations for forest managers to encourage riparian and stream recovery after storms that generate debris flows. These include assisting riparian vegetation regeneration and natural processes through practices such as proactively planting or seeding riparian margins.

For further information on the results of this study, contact

Dean Meason