Can native trees grow through scotch broom in dry environments?
Published: 10 October 2013 - by Larry Burrows
Nitrogen-fixing plants such as gorse (Ulex europeaus) have been advocated as ‘nurse-crops’ to aid indigenous vegetation restoration, especially in moist environments.
Experiences in a North Canterbury Conservation Area.
Nitrogen-fixing plants such as gorse (Ulex europeaus) have been advocated as ‘nurse-crops’ to aid indigenous vegetation restoration, especially in moist environments. Very little work has focused on scotch broom (Cytisus scoparius) as a nurse crop in dry locations, but re-measurement of a Department of Conservation (DOC) trial by Larry Burrows and Debra Wotton (Landcare Research), and Anita Spencer and Nicholas Head (DOC) indicates broom can provide significant benefits.
In 2000 DOC developed a restoration management plan for the Balmoral Fire Lookout Conservation Area, Amuri Basin, North Canterbury (42 52.469S, 172 46.593E) (Figure 1). The site is arid or semi-arid – NW facing, and with rainfall of ~600 mm/year. It is hot and dry in the summer and cold and dry in the winter. Botanists would classify the site as a cold dry broadleaf-podocarp forest biome. The area has had a long history of human disturbance including burning, grazing (ceased in 1987) and weed invasion. When the management plan was drawn up broom was already common and spreading on the lower slopes, as can be seen in aerial images from 2003.
Weed control was planned where broom was still scattered or sparse. In those open areas, characteristic dryland shrub species (e.g. shrubby pohuehue (Muehlenbeckia astonii), Coprosma intertexta, prostrate kowhai (Sophora prostrata)) would be established and natural values would be enhanced. However, where broom was already abundant and cover continuous, the costs of weed control were thought to exceed the benefits. Instead it was proposed to ‘kick-start’ woody restoration while simultaneously controlling broom through re-colonization by native tree species that would, over time, overtop and smother it, and a trial was instigated to find out whether this was realistic.
Seeds or cuttings for planting into the broom were sourced from a limited range of native tree and shrub species present in the Conservation Area, and from the closest known sources beyond, which were grown on in root-trainers for 1-2 years at the DOC nursery. Species included kōwhai (Sophora microphylla), kānuka (Kunzea erecioides), karamū (Coprosma robusta), kōhūhū (black matipo, Pittosporum tenuifolium) and lancewood (Pseudopanax crassifolium) – although kānuka, karamū and kōhūhū were the most abundant.
Five areas of ~0.2 ha within blocks of broom of various ages were selected for trial plantings, and fenced to exclude rabbits, hares and stray domestic stock (Fig 2). In some blocks, the broom was young (~1-3 years old), waist-high and still somewhat grassy underneath at the time of planting, while in other blocks the broom was already head-high and provided an almost continuous canopy with a ground-cover of broom litter (~4-7 years old). A small handful of remnant kānuka and cabbage trees (Cordyline australis) up to 6 m tall remained in the vicinity so we know trees will overtop broom.
In spring 2001 and 2002, Nick, Geoff Speirs and others planted the tree and shrub seedlings. Plants were distributed throughout the blocks spaced approximately 2.5 – 3 m apart (equivalent to 1100-1600 stems/ha). There was no weed control or watering at any stage.
In January 2003, Anita and Nick ran three transects through two of the blocks (Face Block and Gully Block – Fig 2) and the height and health was recorded for each surviving seedling encountered – these individuals were permanently marked. About 25 seedlings were assessed on each transect (~75 per block). In June 2006, Anita and others re-assessed some transects for plant mortality and condition. In May 2012, Nick, Anita, Debra and Larry relocated all the transects and marked plants and measured them again.
Despite the very harsh conditions at the site and despite seedlings getting no help to aid establishment or survival, they have done remarkably well (see Table 1 under “More details for the scientifically-minded” below). More than 30% of all trees planted have survived in Face Block, and >70% in Gully Block, although some individual species have done much better. Surviving seedlings that were 20 or 30 cm tall when planted, by 2012 were >2.5 m tall in both blocks and beginning to overtop the broom. Some were seeding (Fig 3) and self-sown seedlings were beginning to appear under the broom.
Some species did better in one block than the other. In the drier Face Block 91% of kānuka survived while 52% of kōhūhū survived. That contrasted with the Gully Block where 72% of kānuka and 96% of kōhūhū survived. Karamū was the other species planted in useful numbers and 13% of them survived in the Face Block, while 86% survived in the Gully Block.
We are not quite sure why there were such clear differences between the Blocks, with kānuka surviving better and growing faster at the Face Block, while the opposite was true for kōhūhū and karamū. It may be explained by preferred habitat of the species and differences in soil depth and possibly soil moisture between the Blocks. There were also differences in broom stand structure at the time of planting but we don’t know how much that has affected results. At the time of planting the age of broom in the Face Block was younger and about 1 m tall. In the Gully Block the broom was about 2 m tall.
Spectacular survival in a dry environment was facilitated by broom
Considering the harsh, semi-arid local environment, the survival and growth of planted trees and shrubs in the scotch broom stands at Balmoral Lookout has been spectacularly successful. Although the trial didn’t include seedlings planted into the grass sward outside the broom as a comparison, it appears that broom had facilitated survival of planted seedlings, acting as a ‘nurse crop’. It is possible that a number of mechanisms are involved. For example, the cover of broom reduced grass competition, provided shelter (reduced sunlight and heat, increased humidity, reduced evapo-transpiration), altered soil conditions (cooler in summer, warmer in winter, possibly wetter in both), as well as probably adding nitrogen (from decomposing broom litter or roots) and providing some protection from herbivores. Such facilitation by nitrogen-fixing nurse shrubs that outweighs competition and climate has been described overseas from arid lands.
In sum, the trial results suggest broom can facilitate native tree establishment and growth, even in a dry environment such as the Balmoral Lookout. Furthermore, if reversion to native trees is your goal, an implication of the trial is that planting seedlings directly into scotch broom may be more successful and cost-effective than attempting to remove the broom first and planting later. Spraying, burning or mowing are techniques commonly used to attempt to control broom, and most have limited effectiveness due to the huge broom seed-bank. Attempted control also removes any natural regeneration of desirable species and costs a considerable amount of money. The most common outcome is an initial flush of exotic grasses which compete with native plantings, and, eventually, reversion to broom. The Balmoral Lookout trial hints that dollars spent on weed control could be better spent on direct planting.
More details for the scientifically-minded:
Age and size of broom stands
Mean stand height and age of broom was different between Face Block and Gully Block. We cut discs from the bases of 10 well-grown individual canopy broom bushes from beside each Block in 2012 to ascertain mean stand age. Mean canopy broom in the Face Block was ca. 2.5 m tall and 11.4 (SD 1.5) years of age. Broom in the Gully Block was ca. 3.5 m tall and 17.3 (SD 2.9) years of age. These ring-counts indicate that Face block broom was ~1-3 years old at planting and Gully block was ~4-7 years old. Mean broom ring-widths in the Gully Block and Face Block were 2.4 mm and 2.0 mm respectively, although there was no statistically significant difference between them suggesting they have grown at about the same rate in the two blocks.
Height and growth
Mean annual height growth rate of plants are given in Table 1. Kānuka grew equally fast in both Blocks while kōhūhū and all species on average grew fastest in the Gully Block.
Mean height of kānuka in the Face Block by 2012 was 2.1 m, while it was 1.4 m in the Gully Block. Mean heights for kōhūhū were 1.7 m and 1.9 m respectively.
Table 1: Survival of plants marked in Jan 2003 and remeasured in May 2012, and mean annual height growth of plants in the Face and Gully Blocks.
|Face Block||Gully Block|
|Survival (%)||Height growth (cm per year ± 1 SD)||Survival (%)||Height growth (cm per year ± 1 SD)|
|Kānuka||91||13.3 ± 5.6||72||13.1 ± 6.1|
|Kōhūhū||52||12.2 ± 5.7||96||17.3 ± 6.6|
|All species||33||12.9 ± 5.4||75||15.6 ± 6.2|
Reproductive state of the planted trees and new seedlings
Many kānuka >2 m tall and with their top branches in full sunlight have flowered and set seed (Fig 3.). No new kānuka seedlings were observed but kōhūhū seedlings were noted under broom in one of the fenced blocks. Seedlings of Coprosma propinqua were observed within the broom, although this bird-dispersed species was not planted.