Context

Mast seeding is the synchronous production of large quantities of seeds within a population of plants at irregular intervals. A wide range of New Zealand plant species exhibit mast seeding, including southern beech (Fuscospora spp. and Lophozoniz menziesii), four species of podocarp, notably rimu (Dacrydium cupresinum), and tussock grasses in the genus Chionochloa. Masting provides a pulse of food for seed consumers, resulting in a cascade of responses by native invertebrates and birds. Masting can also drive irruptions of predator populations (rats and stoats) with potentially catastrophic implications for vulnerable wildlife. Conservation managers use measures of seedfall intensity to identify triggers for concern about the impact of introduced predators on native species. During 2017/2018, seedfall monitoring was undertaken at 61 locations in a range of forest types across New Zealand. 40 monitoring sites occur within beech forest in the South Island and lower North Island, the remainder are in mixed-species forests across the country.

Key findings

Mast seeding is the synchronous production of large quantities of seeds within a population of plants at irregular intervals, and occurs in a number of New Zealand forest tree and tussock grass species. Masting provides a pulse of food for seed consumers, resulting in a cascade of responses by native invertebrates, birds and introduced mammals. Thus, conservation managers use measures of seedfall intensity to identify triggers for concern about the impact of introduced predators on native species.

Beech forest

Following Wardle (1984) a full mast year is defined as one during which more than 4000 seeds/m² are produced; a partial mast is one producing 500-4000 seeds/m²; a poor mast is one producing fewer than 500 seeds/m². The majority of beech seed falls during autumn.
Similar to 2017, relatively small quantities of beech seed were produced this year. 2018 is a poor beech mast year.
All monitored sites produced fewer than 100 seeds/m² (Figure 1). The most productive sites were Hauturu (Little Barrier Island) and Station Creek (Maruia catchment, North Westland) where 90 and 58 seeds/m² respectively were collected. Poteriteri (southern Fiordland) produced approximately 13 seeds/m². All other sites (94% of those monitored) produced <10 seeds/m²; 27 sites (56% of those monitored) produced <1 beech seed/m²; 18 sites (38% of those monitored) produced no seed.(Figure 1).
Seedfall monitoring confirms widespread pest control operations are not required to avert predator irruptions in southern beech forest this year.

Mixed-species forest

Rimu is a common component of lowland pododcarp/broadleaf forest. Like beech, it is a masting species which can drive irruptions of predator populations. Following Norton and Kelly (1988), a rimu mast year is defined as one producing more than 150 seeds/m². Ripe seed begins falling during summer/autumn and can continue through winter and into spring.
Of the 13 sites where rimu is monitored, the highest recorded fruiting levels occurred at Evans Clearing, a lowland podocarp/broadleaf forest in the Able Tasman National Park, where approximately 280 seeds/m² were collected. However, monitoring at this site targets individual rimu trees, making the results difficult to compare with other rimu sites where canopy-level seedfall is sampled. Concurrent tracking tunnel monitoring of small mammal abundance did not indicate a pest response to rimu fruit production at Evans Clearing during 2018.
Low levels of fruiting were recorded at all other rimu monitoring locations. Fewer than 100 seeds/m² were recorded at 12 sites (92% of those monitored); fewer than 10 seeds/m² were collected at eight sites (62% of those monitored)(Figure 1).

Overall

Low quantities of seedfall were recorded at beech and rimu monitoring sites across New Zealand during 2018. Widespread pest control was not required to avert mast-driven predator plagues.
Climate-based predictions of beech seed production suggest seedfall is very likely in all but a few areas of beech forest in 2019.

Select a place

Figure 1: Beech forest sites monitored for beech seedfall and trends in seedfall throughout the season by site for 2017/18. The map shows the sites monitored and the number of seeds/m² for the season. The graph shows the trend in seedfall in each site throughout the season. Select a site in the dropdown box to highlight the data for that site.

Figure 2: Rimu and total beech seedfall and rat tracking rates at Waitutu Forest. The dashed constant line represents the level of seedfall which defines a rimu mast year following Norton and Kelly (1988). (Seedfall data is not available for 2010).

Definitions and methodology

This measure records the seed production of selected species at particular sites throughout New Zealand.

Seedfall monitoring is conducted in a range of forest types (beech forest and mixed-species forest communities) by collecting seeds at regular intervals from permanently located traps, counting the number of seeds per species and entering the results into a national database. The majority of seeds are collected in South Island beech forest, where the data are used to predict the likelihood of mast seeding. Indices of rodent and mustelid abundance are also monitored in parallel with seed collection to help inform management decisions on the need for pest control, as well as to increase our general understanding of the relationship between seedfall and small mammal abundance in New Zealand forests(Figure 2).

Data quality

Seedfall collection follows a standardised field protocol, and all samples are sorted and counted under contract by the University of Canterbury, Christchurch. Seedfall results are entered into a national database with inbuilt data validation and quality assurance mechanisms, and are analysed using standardised statistical methods and published via web-based reporting tools.

The use of standardised systems and processes at each step, from field collection through to data analysis, allows nationally consistent measures of local seedfall intensity to be derived, which provides high-quality information to support conservation management decisions.

This measure is classified as a case study.

Relevance

This measure relates to indicator 1.1.2 - Ecosystem function.

Spatial and temporal variation in the seed production of trees is an important driver of population dynamics at the community level, with high seedfall events impacting on native invertebrates, birds and introduced mammals. New Zealand beech trees drop most of their seeds between February and May, creating an abundant food resource for rats (Rattus spp.) and/or mice (Mus musculus). Rodent populations can respond to this by growing exponentially throughout the year until early summer, by which time most of the seeds have germinated. Stoat (Mustela erminea) populations then respond to this increase in their prey by increasing to plague levels, which can have catastrophic effects on vulnerable wildlife.

Conservation managers use seedfall intensity to identify triggers for concern about the impact of introduced predators on native species. These triggers range from 500 to 2000 seeds/m² depending on the pest abundance at the time of seedfall and the vulnerability of native species that are present at the site, and form the basis for decisions about the necessity for pest control.

Accuracy

This measure complies with the data quality guidelines used in New Zealand’s Environmental Reporting framework.

Outcomes Monitoring Framework

The Department of Conservation’s (DOC’s) Outcomes Monitoring Framework provides a platform on which DOC and others can assess outcomes in a clear, structured and transparent way (Lee et al., 2005). It has been developed as a logical hierarchy that is based on broad, overarching Outcomes, beneath which are nested Outcome Objectives, Indicators, Measures and Data Elements to provide ever increasing levels of detail. The framework is scalable, as the indicators and measures remain compatible and consistent whether applied locally, regionally or nationally.

The recently updated framework provides a roadmap for gathering information to meet the specific objectives of DOC and other agencies (McGlone and Dalley, 2015). The provision of a national framework with agreed outcomes, indicators and measures supports collaboration with land management and regulatory agencies, allowing for more integrated environmental policy and ‘State of the Environment’ reporting. DOC has partially implemented a national monitoring and reporting system, whereby priority indicators and measures are routinely used to report on progress against the objectives and outcomes. This factsheet reports on a measure for the 2017/2018 year.

Glossary of terms

Population dynamics describes changes in the size of a population in time and/or space.

References

Lee, W., McGlone, M., Wright, E., 2005. Biodiversity inventory and monitoring: A review of national and international systems and a proposed framework for future biodiversity monitoring by the Department of Conservation. Landcare Research Contract Report LC0405/122 (unpublished) for the Department of Conservation, Wellington.

McGlone, M., Dalley, J., 2015. A framework for Department of Conservation inventory and monitoring: Intermediate outcomes 1-5. Landcare Research Contract Report LC2427 (unpublished) for the Department of Conservation, Wellington.

Norton, D., Kelly, D., 1988. Mast seeding over 33 years by Dacrydium cupressinum Lamb.(rimu)(Podocarpaceae) in New Zealand: the importance of economies of scale. Functional ecology 399–408.

Wardle, J., 1984. The New Zealand beeches: ecology, utilisation and management.

Flowering and fruit production

Mast seedfall events in beech forests