South West

Establishing Native Trees For Timber Using Swales And Mulch

This trial is now complete

Key Messages

  • Swales improved tree survival on light, sandy, water repellent soils and slopes greater than 5%
  • Using swales increases the risk of erosion, especially as swale length and catchment size above the swale increases. Lack of benefit on heavier loamy soils suggests the risk of erosion outweighs potential benefit
  • Mulching proved too intensive to apply broadly, but may be an option if spring turns hot and dry when plants are still establishing
  • Mixing species within and across rows can result in less need for replanting
  • Trial Rationale
  • Steps Taken
  • Lessons Learnt
  • Looking Forward

Trial Rationale

The project aimed to demonstrate the potential of local tree species grown in woodlot/plantation configurations to sustainably produce high value timber as part of an integrated farm production system. In particular:

  • To compare growth and survival of local species during the first 1-2 years of establishment across four key soil types under seven different establishment treatments.
  • Identification of local species adapted to local soil types, site conditions and a drying climate, and with the least short-term impact to soil health, with the potential to produce high value timber (specifically targeting the furniture, veneer and flooring markets).
  • The use of planting design/methodology and mixed species as key strategies in the development of a robust production system – in particular in the event of the failure/unsuitability of individual species and/or changes in the anticipated market.

 

Small landholders David Singe and Natalee Kuser love native landscapes. So when they purchased 45 hectares of mostly cleared land on the Blackwood River, traditional farming was not on their minds.

“We’re not farmers. This is a lifestyle block,” David said. “We could have simply leased it all out but we wanted to give something back to the environment. It was also an opportunity to create a long term asset for the family.”

Both landholders have a background in native revegetation, so it was a logical step for them to protect and connect the remnant vegetation on the property, and bolster it with areas of further revegetation in the cleared areas.

Their revegetation skills were complemented in 2010 when David completed a Master Tree Growers course. Using their combined experience, the pair developed their first woodlot in 2012. They used a mix of primarily local species, which were matched to their soil types and other site characteristics.

In 2013, they entered into an agreement with the South West Catchments Council (SWCC) to demonstrate the establishment of eight south west timber species in two woodlots over a range of soil types, while also trialling methods aimed at maximising tree survival and growth.

Steps Taken

Woodlot 1—planting in 2015

The first woodlot tested the effect of four treatments on survival and growth; swales (furrow lining), pine mulch, soil wetter and summer watering. Swales were cut on alternating rows across the woodlot. Three replicate plots for the remaining treatments and two control plots, each consisting of about 100 seedlings (50 on swales, 50 without swales), were positioned on sandy soils with slopes ranging between 2-10%.

The woodlot was planted in July 2015 with 2,600 seedlings. Species included jarrah (Eucalyptus marginata), river banksia (Banksia seminuda) and western sheoak (Allocasuarina fraseriana). Blackbutt (E. patens) was also planted, but not within the trial plots.   

 Rainfall over the following months proved to be close to the lowest on record.

“The season was a shocker, both for lack of rainfall and the hot start to spring. We had never lost so many plants so early in summer,” David said. “With sandy soils and small seedlings, it was a losing combination.”

The benefits of using swales were already becoming apparent by December 2015. Average survival of seedlings on swales was 80%, compared to 47% without swales. From visual inspection, David thought the effect of swales increased with increasing slope, and became obvious on slopes greater than 5%, and also increased with increasing soil-sand content.

Other treatments appeared to have less impact on survival, although David thought mulching also improved growth and assisted with watering.

“When we were watering we were able to get about twice as much water on the mulched plants before it began to run away.”

The results encouraged David and Natalee to mulch surviving seedlings outside the trial plots.

Twelve-month survival results recorded in June 2016 were similar to the December results. Swales improved survival for all species, with the significance varying between species (Fig. 1).

Final counts could not detect a significant effect on survival from mulching, summer watering or using a soil wetter (Fig. 2).

The results were not surprising to Richard Moore, Executive Officer of South West Agroforestry Network (SWAN).

“Swaling, or furrow-lining, is most commonly used in agroforestry on sandy soils in low rainfall areas. It enables greater access to soil moisture and can also aid weed control by scraping topsoil and associated weed seeds to the side.”

SWAN committee member Bob Hingston says that swaling can also be beneficial for non-wetting or water repellent soils, which are most common in sandy soils with less than 5% clay.

“It is used with ripping on sandy soils in preference to mounding because the furrow lining will get below the non-wetting layer.

“The problem with this method is that the removal of the topsoil takes out most nutrients, so spot-fertiliser will need to be applied.”

David spot-fertilised all seedlings with native tree tablets and also cultivated the no-swale lines to a depth of 200mm. This may have reduced non-wetting and the effect of the wetter treatment.

Another issue with swaling is increased erosion risk from water flowing along swales. David and Natalee managed this risk by swaling on the contour and by having the first swale close to the top of slope so the catchment above it was small.

There was little sign of erosion using this method, even after 100 mm of rain fell in one week in January 2016. Mulching also had a positive effect on erosion by reducing the velocity of water flowing along the swale.

This erosion risk can also be reduced by limiting the length of the swale, and ending the swale with a level sill that spreads water evenly onto established groundcover with no wheel ruts.

Overall survival was possibly also affected after the supplying nursery was exposed to Phytophthora and couldn’t fill part of the order. Seedlings that were sourced to make up the order were largely the remains of other orders. They tended to be small and therefore more vulnerable to a poor season. This outlines the value of having strong seedlings grown early in the season.

Woodlot 2—planted in 2016

The 2016 woodlot was planted on a mix of heavier clay/gravel soils with some light yellow/white sand soils lower in the landscape. Species planted on the heavier soils were wandoo (E. wandoo), powderbark wandoo (E. accedans), and salmon gum (E. salmonophloia).

To encourage good form, these species were interplanted with sacrificial, faster growing sugar gums, (E. cladocalyx), which will be removed in the first thinning for firewood.

Species planted on the lighter soils in the 2016 woodlot were western sheoak, rock sheoak (Allocasuarina huegeliana), jarrah and blackbutt.

Alternate rows on heavier soils were swaled and mulched, with all rows on lighter soils swaled.

In contrast to 2015, the second woodlot was planted in a wetter than average year with an early break. Some areas of lower lying white sand soils became waterlogged and were only planted in late winter or not at all.

Survival was estimated to be over 95% after 12 months, with no effect of treatment. Soils showed signs of erosion, particularly at the end of a long swale in the lighter soils. Survival did not appear to vary across species, and in some cases was attributed to bird or kangaroo damage.

 

 

 

Lessons Learnt

Grazing of the established Allocasuarina seedlings by kangaroos in both woodlots was significant at times, limiting both growth and form, particularly in the western sheoak.

 

“The western sheoak isn’t one of our local species,” said David. “It’s more coastal but is better recognised as a timber species. But we’re finding that the rock sheoak is less likely to be

grazed by kangaroos and also seems better adapted to lighter sands specific to our property.”

“Although the seed for jarrah seedlings was sourced from remnant trees on our property, we found that the jarrah is probably least adapted to our soils,” David said. “In 2017 we had a late start to the winter and lost quite a lot of jarrah planted in both 2015 and 2016 woodlots, primarily the ones that were just that little bit smaller.”

As well as demonstrating the value of planting with early potted and strong seedlings, the project also confirmed the benefits of alternating two species within each row and varying the species mix across rows, something also implemented in their 2012 woodlot.

“By using this strategy each species is more regularly dispersed, so if one fails in one area, others better suited to that area will survive. Over time as thinning occurs, more trees will be retained overall. If we’d planted a single species we would have whole areas with little survival.”

Looking Forward

For most of the species planted, there are no recognised silvicultural techniques best suited to timber production. Pruning of the 2012 woodlot to date indicates that what works for one species doesn’t necessarily work for another, and different growth rates within a species requires each individual tree to be assessed.

The nature of consistently and successfully establishing local tree species is arguably time and resource inefficient when compared with a plantation of more robust eastern states species. However, David and Natalee consider the environmental benefits and long-term value of their craft and furniture timbers should outweigh these challenges.

Boyup Brook

Project Snapshot

Land Manager
David Singe and Natalee Kuser
Shire:
Boyup Brook
NRM Region:
South West
Property Size
45 ha
Average Rainfall:
High - 450-750mm
Enterprise Mix:
Agroforestry
System Constraints:
Low water holding capacity, Water Repellence
Partners:
NLP, SWCC
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