A classification framework for woody vegetation on farms
Retention and restoration of on-farm forest is a critical tool for sustaining native biodiversity in our agricultural landscape.
While the area and distribution of remaining native vegetation can be quantified using GIS data, it is also important to understand the function of the different types of native vegetation present.
Within NZ farms, some native vegetation occurs as remnants of the original forests, some has regenerated following earlier clearance and subsequent abandonment of agriculture, while restoration continues to establish new areas of native vegetation.
Remnant native forest (left) creates a different functional ecosystem to forest that has been grazed and is now regenerating (right).
The degree of intactness of native woody vegetation is important. In many agroecosystems the presence of grazing and other agricultural practices mean that native woody species might occur as scattered trees, or groups of trees, within a predominantly pasture-dominated matrix.
Farms also contain significant areas of non-native woody species that can be important for biodiversity conservation, erosion control and carbon sequestration.
Different areas of Aotearoa support different species, but we wanted to come up with a framework that can compare the type and function of the vegetation, regardless of species composition.
This would enable us to compare woody vegetation patches on farms across the country.
A functional framework like this is important for planning how to implement new restoration initiatives, as well as for deciding which vegetation to retain in the landscape.
What we did
We mapped all of the woody vegetation on our three study farms onto satellite imagery of the area. We then used this, and remote sensing data, to develop farm-scale maps of woody vegetation.
Based on these surveys and more general knowledge of New Zealand woody vegetation, we identified key functional attributes for the classification:
1) The history of the vegetation (remnant/regenerating/planted)
2) The origins of the dominant woody species (native/exotic)
3) The amount of canopy cover (low-100%)
Each class can be further refined by how modified the vegetation has been by anthropogenic factors (timber extraction, grazing etc).
An excerpt of the vegetation map for our Ruapehu farm
Continuous (>70% canopy cover)
Diffuse (<70% canopy cover)
Patches that have always been present
Containing old-growth canopy trees
Could have been modified (eg. historical logging)
More open version of continuous
Includes grassy clearings
Old growth canopy trees still present
Established on sites that were previously pasture
Fast-growing canopy trees (eg. tōtara, kahikatea) or seral species (eg. kānuka, mānuka) are dominant Widespread in rural NZ
Same as continuous but with lower cover and lots of grassy clearings
Unplanted weedy forest
Dominated by wild conifers, willows, sycamore, etc
Same as continuous, but with grassy clearings
Native forest restoration sites
Forest trees (as opposed to shrubs) have been planted
Discontinuous native forest restoration sites
Usually a result of being at an early stage of growth or having partially successful establishment
Plantation forests and farm woodlots
Pine, Douglas fir, eucalypts etc.
Erosion plantings (eg. poplars, willows) and shelterbelts Classical agroforestry