Background
This measure relates to indicator 1.5.1 – Species composition and diversity.
Before human occupation, Aotearoa New Zealand was 80% forest, and forest remains the dominant landcover on public conservation land (PCL). Forest structure and processes are largely determined by common and widespread tree species. The demographic rates (mortality and recruitment) of these species show how forests are changing. Change may be natural or from anthropogenic pressures such as climate change and introduced mammals like possums, goats and deer. Comparing demographic rates for common trees grouped by browsing mammals’ food preferences can help direct DOC’s wild animal management and is a baseline to compare with future trends and/or management outcomes.
The national population of trees on public conservation land has grown, but forest composition is changing.
What did we measure?
DOC has a national monitoring programme to assess status and trends of biodiversity. This includes repeated measurements at nearly 1,400 sites evenly spaced on an 8km grid across public conservation land (Figure 1). Approximately 280 randomly selected sites are measured each field season (September-May) so that each site is measured on a 5-year rotation.
At each site, DOC staff measure all tree stems rooted in a 20x20m plot. Stems are permanently tagged so they can be tracked over time. In addition, saplings and seedlings are counted, and every plant species in the plot is identified and recorded. Bats, birds, possums and ungulates are also monitored.
The vegetation monitoring in DOC’s programme builds on the Land Use and Carbon Analysis System to monitor carbon stored in natural forests, which was first measured from 2002-2007. The second measurement was from 2009-2014. The third measurement began in 2015. This factsheet uses data collected up to the end of the 2019/20 season, so there have been two measurements of nearly all sites and three measurements of most (Table 1).
Tagged stem data were analysed with a Bayesian individual-based response model to estimate annual rates of recruitment and mortality for species over three time intervals. The probability of recruitment for individual stems was estimated in relation to their species’ palatability to browsing mammals and animal abundance at the site, while allowing for underlying differences between species and sites. Because mammals prefer to eat some plants and not others, they can alter recruitment or mortality rates for particular species. Similar rates indicate a stable population, higher recruitment indicates population growth, and higher mortality indicates population decline.
What did we find?
The population of indigenous trees on PCL has grown. Over all sites and species, stem mortality was 1.64 - 1.7% p.a. (95% credible interval, CI) and recruitment was 2.04 - 2.1% p.a. between the first and second measurements. Between the second and third measurements, higher mortality (2.01 - 2.09% p.a.) was compensated by higher recruitment (2.16 - 2.24% p.a.). The national rate of increase from the first measurement to the most recent was estimated as 0.26 - 0.33% p.a.
There may be a shift in forest composition as numbers of ungulate-avoided species increase while the species they prefer to each decline (Figure 2). Ungulate-preferred trees have lower rates of recruitment (1.8 - 2.3% p.a., 95% CrI) than mortality (2.6 - 3.3% p.a.). However, trees of species ungulates avoid eating have similar rates of recruitment (1.4 - 1.7% p.a.) and mortality (1.1 - 1.5% p.a.). This is of concern because ungulates have increased from occupying 63% of PCL in 2012/13 to occupying 82% of PCL in 2019/20.
Populations of possum-avoided tree species are also increasing relative to those they prefer. Possum-preferred trees have lower rates of recruitment (1.8 - 2.6% p.a., 95% CI) than mortality (3.1 - 4.2% p.a.). However, for trees possums avoid eating, mortality (1 - 1.4% p.a.) is similar to recruitment (1.2 - 1.5% p.a.).
Demographic rates are highly variable across the sites sampled, as expected given underlying range in environmental factors and forest histories. This means local patterns may differ from national summaries reported here (Figure 1).
Within overall palatability group responses, rates of mortality and recruitment for individual species are variable. Species which are preferred by both possums and ungulates have the lowest rates of recruitment and highest rates of mortality (Figure 3).
Figure 1: Location of forest plots on public conservation land (PCL) with tree data showing annual rates of stem turnover, mortality and recruitment. Click on a site to see the number of live stems recorded at each measurement.
Figure 2: Estimated mean annual rates of mortality and recruitment for stems grouped by their palatability to browsing mammals. Rates are shown for the interval between the first and second measurements (1-2), between the second and third measurements (2-3) and over the whole time (w) from the first to the most recent measurement at a site
Figure 3: Balance between stem mortality and recruitment for common or widespread tree species, accounting for the impact of ungulates and possums. Click on the key to show different information and hover over a point to read its species code and sample size.
| Measurement interval | Years between measures (median, range) | Number of plots | Stems that survived | Stems that died | Stems that recruited | Total stems recorded |
|---|---|---|---|---|---|---|
| period 1 to 2 | 8 ( 6 - 12 ) | 768 | 93,088 | 12,843 | 16,730 | 122,662 |
| period 2 to 3 | 5 ( 3 - 10 ) | 828 | 103,730 | 10,662 | 11,850 | 143,783 |
| overall | 12 ( 3 - 18 ) | 910 | 97,016 | 23,850 | 29,896 | 156,683 |
Data quality
This measure complies with data quality guidelines used in the Environmental Reporting series. The data are reliable and accurate. They come from a national-scale monitoring programme that has been repeated three times. Field teams are audited, and data was checked prior to analysis.
Stem demographic rates are key metrics of forest ecosystem function and census of marked individuals in a defined area is a standard technique for measuring this. The analysis method applied here was developed from methods published in an international review and should not unduly bias results.
Results presented here are different to those reported in 2021 because:
- Two more years of data are included.
- Models were improved, to account for the combined effects of two types of browsing animals and to include an interaction of animal abundance (posssums measured by chew card index, and ungulates measured by faecal pellet index) with the relevant palatability class.
- Stems that were ‘not found’ on repeated measurements were treated as dead, rather than missing. Checking field notes indicated most ‘not found’ stems were dead but could not be relocated, for example because of land slips. The previous method of removing ‘not found’ stems from the dataset is likely to under estimate mortality, because it is probable that ‘not found’ stems have died.
Glossary of terms
95% credible interval (CI) indicates that the true mean lies inside the interval with 95% probability given the posterior probability distribution.
Annual recruitment is the rate at which trees in a population surpass 2.5cm diameter at breast height (dbh). Rates reported here use eq.7 from Kohyama et al. (2018), that is the proportion of stems in a population that recruit relative to the final size of the population, estimated with the Bayesian model described above.
Annual mortality is the rate at which a population of trees die. Rates reported here use eq. 6 from Kohyama et al. (2018), that is the proportion of stems in a population that died relative to the initial size of the population, estimated with the Bayesian model described above.
Annual turnover is the rate at which a population of trees changes. Rates reported here compare the proportion of the population that die or are recruited relative to the whole population using a metric described by Mason and Bellingham (2018) calculated for each plot.
Palatability reflects the dietary preference of a target animal species or group. Ungulate preferences used here are from Forsyth et al. (2002). Possum preferences were compiled from published research about which plants the animals prefer and actively select to eat, and those that they deliberately avoid eating. Methods to determine this include direct observations of feeding animals and examination of faecal and gut samples.
Stems are trunks of trees or treeferns over 2.5cm diameter at breast height (1.35m). If trees fork below breast height, each stem is individually tagged and monitored.
Ungulate is the collective term for a group of herbivorous mammals that walk on the tips of their hoofed toes (e.g. deer, goats, tahr and chamois). Faecal pellets from these mammals cannot be easily differentiated and so are aggregated into the group ‘ungulates’. Pigs are also ungulates but are excluded because their faeces are easily differentiated.
Additional resources
Forsyth, D., Coomes, D., Nugent, G., Hall, G., 2002. Diet and diet preferences of introduced ungulates (order: Artiodactyla) in New Zealand. New Zealand Journal of Zoology 29, 323–343.
Kohyama, T.S., Kohyama, T.I., Sheil, D., 2018. Definition and estimation of vital rates from repeated censuses: Choices, comparisons and bias corrections focusing on trees. Methods in Ecology and Evolution 9, 809–821.
Mason, N.W., Bellingham, P.J., 2018. Evaluating optimum measurement of biodiversity indicators, Contract report. Manaaki Whenua Landcare Research, Lincoln.
McGlone, M.S., McNutt, K., Richardson, S.J., Bellingham, P.J., Wright, E.F., 2020. Biodiversity monitoring, ecological integrity, and the design of the New Zealand biodiversity assessment framework. New Zealand Journal of Ecology 44, 3411.
Peltzer, D.A., Allen, R.B., Bellingham, P.J., Richardson, S.J., Wright, E.F., Knightbridge, P.I., Mason, N.W., 2014. Disentangling drivers of tree population size distributions. Forest Ecology and Management 331, 165–179.