Tree survival, growth, sequestration and natural regeneration
of a Wairarapa swamp forest restoration project.
Stephen Hartley
1
, Benoit Magat
2
, Aurore Fanal
3
, Manon Hulley
2
1. Centre for Biodiversity and Restoration Ecology, Victoria University of Wellington, NZ. 2. AgroSUP, Dijon, France. 3. Gembloux Agro-Bio Tech, Belgium
RESULTS
After eight years, mānuka (LS) and cabbage trees (CA) had the highest survival rates (52-56%), the podocarps, kahikatea (DD) and tōtara (PT), had survival rates of 41-48%. Cabbage trees grew tallest, Pittosporum tenuifolium (KO) and Olearia virgata (OV) sequestered the most carbon over the eight years (7.4 and 5.1 kg per tree, respectively) and kahikatea and tōtara the least (0.1 to 0.8 kg per tree).
METHODS
Over 2000 trees of 8 species were planted in experimental plots in a retired paddock at Wairio wetland (near Lake Wairarapa). Survival and height (H) were measured at annual intervals, and from 2017, diameter at breast height (DBH) at 1.35m and root collar diameter (RCD) at 10cm were recorded from a subsample of each species. Linear models (LM) were fitted to the relationship between height and RCD, and compared to relationships recorded by Marden et al. 2018 (M18) for tōtara and kahikatea. The ratio of DBH to RCD was examined by species. Total carbon content was estimated following the methods of Bergin et al. (2011) using the equationCarbon(kg)= 0.0186(RCD(cm)2H(m))0.976, except for cabbage trees for which we developed our own volumetric equation.
Species % Survival after 8 years Average carbon (kg) per tree
Leptospermum scoparium (LS) 55.9 4.67
Pittosporum tenuifolium (KO) 48.1 7.38
Coprosma propinqua (CP) 34.1 1.62
Coprosma robusta (CR) 15.4 3.32
Olearia virgata (OV) 42.9 5.10
Cordyline australis (CA) 52.3 4.64
Podocarpus totara (PT) 41.1 0.81
Dacrycarpus dacrydioides (DD) 47.7 0.12
INTRODUCTION
Natural, restored and reconstructed wetlands can enhance biodiversity and many ecosystem services, including carbon sequestration (Tomscha et al. 2019). However, there is a lack of documented data on native tree species’ survival, growth rates and sequestration trajectories in such projects. Here we report the results of eight years of monitoring at the Wairio Wetland Restoration site.
ACKNOWLEDGEMENTS
The Wairio Wetland Restoration Trust, Ducks Unlimited, Greater Wellington Regional Council, Department of Conservation, Fish & Game, Ngati Kahungunu, Bridget Johnson, Aprille Gillon, Tapuwa Mapara and Florence Wellington.
REFERENCES
Bergin et al. (2011) Technical Bulletin Tane’s Tree Trust Sect. 10.5 & pers. comm. Marden et al. (2018) NZJ Forestry Science 48:9.
Tomscha et al. (2019) Ecosphere 10:10.
See also, https://www.wgtn.ac.nz/sbs/…/wetlands-for-people-and-place
CONCLUSION
At initial planting densities of 4440 trees per hectare (1.5m spacing), similar planting projects bordering ephemeral wetlands in the lower North Island using the same species mix may expect to sequester 33 tonnes of CO2per hectare
in the first eight years.
More importantly, the eight year old mānuka and P. tenuifolium have shaded out tall fescue grass and created conditions in which natural regeneration of kahikatea and tōtara can begin. Podocarps have slow growth initially, but ultimately these will be the most massive trees of a restored swamp forest. Jan 2014 (2 yr 7 mo.) Apr 2016 (4 yr 10 mo.) KO LS Nov 2018 (7 yr 5 mo.) KO LS Dec 2018 IMAGES ↑ Growth of kohuhu (KO) and mānuka (LS) trees. RIGHT Tōtara seedling under kohuhu and manuka Sep 2019 (8 yr 3 mo.) CA CR Sep 2019
remnant kahikatea trees in background (left)
OV 1 4 6 8 0 100 200 300 400 years post-planting H ei gh t ( cm ) manuka 0 50 100 150 0 100 200 300 400 RCD (mm) H ei gh t ( cm ) LM 6yr 8yr 0 50 100 150 0 100 200 300 400 500 RCD (mm) D B H ( m m ) 1 4 6 8 0 100 200 300 400 years post-planting H ei gh t ( cm ) kohuhu 0 50 150 0 100 200 300 400 RCD (mm) H ei gh t ( cm ) 0 50 150 0 100 200 300 400 500 RCD (mm) D B H ( m m ) 1 4 6 8 0 100 200 300 400 500 years post-planting H ei gh t ( cm ) cabbage tree 0 100 200 300 0 100 200 300 400 500 RCD (mm) H ei gh t ( cm ) 0 100 200 300 0 50 100 150 200 RCD (mm) D B H ( m m ) 0 4 6 8 0 50 100 150 200 250 300 350 years post-planting H ei gh t ( cm ) totara 10 30 50 70 0 50 100 150 200 250 300 350 RCD (mm) H ei gh t ( cm ) 10 30 50 70 0 20 40 60 80 100 RCD (mm) D B H ( m m ) 0 4 6 8 0 50 100 150 200 250 300 350 years post-planting H ei gh t ( cm ) kahikatea years post-planting 10 30 50 70 0 50 100 150 200 250 300 350 RCD (mm) H ei gh t ( cm ) LM M18 6yr 8yr RCD (mm) 10 30 50 70 0 20 40 60 80 100 RCD (mm) D B H ( m m ) 1:1 RCD (mm) RCD = Root collar diameter
