Soil loss, vegetation recovery, and sediment yield following plantation harvesting, Coromandel – interim results
Michael Marden1; Chris Phillips2; Donna Rowan1
1 Landcare Research, Box 445, Gisborne, New Zealand, ph: 06 863 1345; fax: 06 863 1346; e-mail:
2 Landcare Research, Box 69, Lincoln, New Zealand, ph: 03 325 6700; fax: 03 325 2418,; e-mail:
Introduction
On-site forest-related activities, such as roading and harvesting, together with off-site impacts of localised flooding, and sediment input into estuaries downstream have been issues of concern to local residents, regional authorities, recreationists and, the forest owners since before harvesting began of forests on the Coromandel in the late 1980s and early 1990s.
A unique combination of weathered volcanic soils, steep slopes, together with a climate characterised by a history of frequent and, damaging high intensity rainfall events pre-disposes slopes erosion. Streams have a high incidence of flood flows and the potential to deliver significant volumes of sediment to coastal estuaries. The return period for a rainfall event of 133 mm in 24 hrs is estimated to be only 2 years (New Zealand Meteorological Service 1980). During any single storm event the frequency of landslides initiated in exotic cutover is greater than for areas of standing mature forest (indigenous and exotic). Exotic forest cutover, therefore, is clearly recognised as being at-risk= to landslide failure and erosion, as documented following storms in 1995 (Marden and Rowan 1995) and in 1999 (Phillips & Marden 1999).
On a harvested setting, sediment is generated both as a consequence of harvesting practice (e.g., slope scalping= during hauler-logging) and, by post-harvest erosion processes including rain-drop impact, sheetwash erosion, rilling and by storm initiated= landslides. Process-based sediment generation rates and total sediment volumes have not previously been quantified for exotic forest cutover in a weathered volcanic terrain in New Zealand. In addition, few studies in New Zealand have attempted to quantify the relative contribution of sediment generated by these different processes to stream channels and, hence to stream sediment yield.
The objectives of this research were to assess vegetation recovery and quantify sediment generation rates from hauler logging at Whangapoua Forest, and to assess the relative significance of sediment delivery from the various sources to the stream channels. Finally, the sediment delivery ratio, i.e. the ratio between what is generated and what is exported as sediment yield, will be calculated.
This paper reports interim results on sediment generation and vegetation recovery. Catchment sediment yield and sediment delivery ratios are not discussed.
Methods
A ground-based, site-disturbance survey of the harvested setting in Compartment 49 was used to identify the proportion of the logged setting occupied by different site disturbance classes (Mc Mahon 1995). Site disturbance was recorded for one thousand points along six 200m long transects spaced-out across the harvested setting and undertaken within a month of the completion of harvesting. Three disturbance classes were recognised: undisturbed, shallow disturbance and deep disturbance. Field-based bounded plots (1m2 (n= 22) and 9m2 (n= 9)) stratified into these disturbance classes were used to measure rates of sediment generation and post-harvest vegetation recovery in Compartments 43 and 49 at Whangapoua Forest.
Analysis of vegetation recovery from the 9m2 plots was by an abundance/sociability technique developed by Blanquet (1974) where a visual assessment of the percentage of groundcover vegetation is given to each of the four corner subplots and, the central subplot. The percentage of groundcover assigned to each plot is then the mean of the 5 sub plots. Species type and relative abundance was also recorded.
Plot-based rates of sediment generation were applied to the cumulative area occupied by shallow-and deep-disturbance sites across harvested area to quantify the sediment volume generated from Compartment 49. The areal extent of sites of deep-disturbance was measured directly from aerial photography (1:5000 scale) flown approximately one month after the completion of harvesting. Areas of deep disturbance together with landslide scars were marked on the photographs. If these features had direct connection to streams, this was also assessed. Areal measurement of sites of deep-disturbance and of landslide scars was by dot grid. The dimensions of landslide ‘source-zones’ and their depth were verified by field measurement. Sites of ‘shallow-disturbance’ and ‘undisturbed’ sites were indistinguishable on the aerial photography. We therefore used the percentage of points classed as ‘shallow-disturbance’ and as ‘undisturbed’ by the ‘site disturbance survey’ to apportion an ‘area’(hectares) to these two site disturbance classes.
Results
Analysis of post-harvest vegetation recovery and corresponding sediment generation during immediate post-harvest periods throughout Whangapoua Forest helped us to understand the relative significance and persistence of disturbed sites as a source of sediment and, in particular, enabled us to assess the significance of sheetwash erosion as a sediment-generating process. Post-harvest vegetation recovery peaked 9-months after the completion of harvesting. It was quickest on sites of shallow-disturbance increasing from 1% to 68% of plot area, but only covered 37% on sites of deep-disturbance (Fig. 1).
Fig. 1Vegetation recovery vs months since harvesting in Cpt 43, Whangapoua Forest
Following harvesting and before planting of the new crop, it is normal forest practice to apply dessicant to ensure reduced competition for the young trees. Within four months of the application of desiccant, groundcover vegetation had declined to 46% and 19% on the respective sites then increased during the next 3-months to 59% of plot area on sites of shallow-disturbance and, 27% on sites of deep-disturbance. Within 2 years of harvesting, vegetation recovered to 80% cover on sites of shallow disturbance and 30% cover on deeply disturbed sites.
In Compartment 43, deep-disturbance sites generated 92% of the first-year sediment total, while 8% was generated from shallow-disturbance sites. Sediment generation rates during year 2 declined on both disturbance classes by about 50%. Both disturbance classes generated 72.6 tonnes of sediment by surface erosion processes over the 16-month monitoring period.
Comparisons of sediment generated by sheetwash erosion post-harvesting with that derived by slope scalping= during harvesting and, periodically by storm-induced landsliding during the post-harvest period, provided a process-based understanding of the relative importance of these sources for Compartment 49 (Table 1). Sediment generated from deeply disturbed sites occurred in two phases-initially by ‘scalping’ and then by surface wash. A single heavy rainfall event in April 2001, with an estimated return period of 2 years, initiated shallow landslides and accounted for the greatest proportion of the sediment total derived from the logged setting.
Table 1 Process-based rates of sediment generation and surface lowering following harvesting in Compartment 49 at Whangapoua Forest, Coromandel.
Sediment generating area / Area(ha) / Total sediment
(t) / Sediment generation
(t/ha) / Surface lowering (mm)
Undisturbed / 14.5 (40%) / 0 / Na / Na
Roads & landings / 2 (6%) / Na / Na / Na
Shallow Disturbance / 16 (43%) / 16 / 1 / 0.08
Deep Disturbance / 3.6 (10%) / 57 / 16 / 1.3
Landslides n=36 / 0.4 (1%) / 600 / 1500 / 125
Scalped areas / 3.6 (10%) / 1200 / 333 / 28
TOTAL
/ 36 (100%) / 1873Mean value / 51 / 4.2
Figures in brackets are the % of total area
Na – not assessed.
Note: Scalped area is classed as deeply disturbed
Surface lowering by surface erosion processes on sites of deep-disturbance was 1.3 mm of which 82% (0.87 mm) occurred within the first 12-months of the completion of harvesting. In contrast, sites of shallow disturbance had a rate of surface lowering of 0.08mm, almost all of which occurred within 12-months of the completion of harvesting. There is therefore, at least an order of magnitude difference in the rate of surface lowering between the two site disturbance classes and an order of magnitude difference in the rate of surface lowering between years -1 and -2 for both site disturbance classes. Rates of surface lowering for landslides and areas scalped during the actual harvest operation were 125mm and 27.8mm, respectively. Sediment from all sources combined resulted in a rate of surface lowering across the harvested catchment of 4.2mm. Had harvesting coincided with a ‘landslide free period’ the rate of surface lowering would have been just 0.8mm assuming the same amount of surface erosion occurred.
The rainfall event in April 2001 that initiated shallow landslides not only accounted for the greatest proportion of the sediment total derived from the logged setting but also that delivered to stream channels during the study period to date. The 9 landslides that reached a permanent stream channel collectively delivered 72% of the sediment total to streams. Areas that were ‘scalped’ as a consequence of the harvest operations, and directly connected to the streams, delivered 26% of the total sediment. By contrast, rain-drop and sheetwash processes on these same sites delivered less than 2% of the sediment total delivered to streams during the 16-month post-harvest period (Table 2).
Table 2 Sediment generation and delivery to streams
Sediment generating area / Area connected to stream(ha) / Sediment generated
(t) / Sediment delivered to stream
(t)
Roads & Landings / n/a / n/a / n/a
Undisturbed / n/a / n/a / n/a
Shallow disturbace / n/a / n/a / n/a
Deep Disturbance / 0.18 / 2.9 / 2.9 (1.3)
Scalped sites / 0.18 / 60.0 / 60.0 (26.3)
Landslides / 0.07 (n=9) / 330.0 / 165.0*(72.4)
Totals
/ 0.40 / 392.9 / 228.0 (100)N/a not assessed
Figures in parentheses are % of total sediment delivered to permanent stream channels
* 50% of sediment generated by 9 landslides remained on slope as debris tail and levee deposit
Discussion
Our investigations have shown that while there is a significant amount of bare ground, not all of it generates sediment, nor does it all leave the forest. The data are remakably similar to other national and international studies of harvesting impacts on sediment generation. Most sediment is generated from bare areas in the first 12 months. However, in general, even with oversowing, vegetation recovery isn’t fast enough to affect sediment generation from deeply disturbed bare areas nor mitigate against storm-initiated landslides.
For environments such as the Coromandel, there is little that can be done to reduce the incidence of operational scalping. Without excessively high towers on haulers, the terrain makes it difficult to get enough lift to keep logs clear of the ground. In general, there is a tradeoff between accepting a certain amount of soil disturbance and having to add extra roads and landings. A number of studies from both New Zealand and overseas that have found that roads and landings are primary sources of sediment and any increases in the number of roads or landings would tend to increase the overall sediment being generated (Fransen et al. 2001).
Many of the effects of harvesting are relatively short-lived. Once vegetation recovers, many of the bare areas are no longer visible and, if they had been contributors of sediment early on, they would no longer be supplying sediment.
Conclusions
Comparisons of rates of surface lowering following harvesting indicate that on weathered andesitic terrain in the Coromandel area soil depletion from sites of shallow-disturbance is an order of magnitude less than for deep-disturbance sites. In addition, rates of surface lowering determined for deep-disturbance sites are 3 times greater than for similar sites on pumiceous terrain and, between 6-10 times less than that on fractured sedimentary sandstone/mudstone lithologies (Marden & Rowan 1997; in prep). Across all terrains and lithologies, the rate of surface lowering on sites of deep disturbance on forest cutover is likely to be between one and three orders of magnitude less than that for compacted tracks, unsealed roads and haul paths.
Landslides generated 1500 t of sediment per hectare while areas scalped during the harvest operation generated 333 t of sediment per hectare resulting in rates of surface lowering of 125mm and 27.8mm, respectively. Sediment generation rate from all sources was 51 t/ha, a surface lowering rate across the catchment of 4.2mm. Had harvesting coincided with a storm-free and landslide-free period the rate of surface lowering would have been 0.8mm.
Finally, the take home messages are:
- not all bare ground is bad, nor is it all eroding and contributing sediment;
- landslides are the biggest contributors of sediment and they are difficult to manage for both in time and in space;
- operational “erosion” such as scalping needs to be traded off against increased roads and landings in steep terrain.
References
Fransen, P.J.B.; Phillips, C.J.; Fahey, B.D. 2001: Forest road erosion in New Zealand: overview. Earth Surface Processes and Landforms 26: 165-174.
Marden, M.; Rowan, D. 1995: Assessment of storm damage to Whangapoua Forest and its immediate environs following the storm of March 1995. Landcare Research Contract Report LC9495/172 for Ernslaw One Ltd.
Marden, M.; Rowan, D. 1995: Relationship between storm damage and forest management practices, Whangapoua Forest. Landcare Research Contract Report LC9495/173 for Ernslaw One Ltd.
New Zealand Meterological service 1980: Depth-Duration-Frequency Tables based on daily rainfalls. New Zealand Meterological Service miscellaneous publication 162: (Supplement [1]).
Phillips, C.J.; Marden, M. 1999. Review of vegetation-slope stability in plantation forests and risk assessment of Ohui Forest to landsliding. Landcare Research Contract Report for Carter holt Harvey Forests Ltd. LC9899/66.
Quinn, J.M.; Stroud, M.; Parkyn, S.; Burrell, G. 1995: Impacts of an intense rainstorm on 3-4 March 1995 on Whangapoua Forest landscape and streams. NIWA contract report (unpubl.) 14 p.
Williams, M. 1995: Whitianga Storm Event - 4th March 1995. Environment Waikato memorandum file 410003 (unpubl). 4p.