Factors influencing red and silver beech regeneration
Operational Research Project Code: POL 114
Programme Leader: Susan Wiser, Landcare Research Ltd
Goal
To examine seedling and sapling regeneration patterns a decade after harvesting and develop relationships between regeneration success and local environment, stand structure and the presence of advanced growth before harvesting, size of, and position within, the harvested area, and the nature of the ground layer (as a measure of the competitive environment).
Context of the project
Landcare Research (Lincoln) was contracted by MAF to determine the major influences on regeneration success of red and silver beech. We focus on four classes of influences: local environment; stand conditions before harvesting; size of, and position within, the harvested area; and competition. This information can be used to revise the Ministry’s implementation of sustainability requirements in the Forests Act. The outcomes of this work will be robust recommendations on optimal size of harvested area, how best to locate areas for harvesting, and the post-harvest site management required to assure forest replacement processes in mixed red-silver beech forests.
Approach
From 1994 to 1998, a series of silvicultural trials were established in mixed red-silver beech forest at Station Creek, Westland. One goal was to compare the impacts of three treatments: unharvested forest, harvesting with small (less than 0.2 hectares) coupes and group-selection (~ 4 large trees removed) silviculture. Permanent transects were established and periodically remeasured to assess the impacts of harvesting on a range of ecological values. In subplots along each transect, seedling and sapling density of all woody species and presence of all other plant species has been monitored since transect establishment. Site conditions including topography, relative position in the coupe, and soil nutrient and moisture conditions were collected in December 2005. Here we present summaries of these data and models predicting red and silver beech sapling density from variables reflecting coupe properties, local environment, competition, and stand conditions before harvesting.
Outcomes
Coupe and group selection subplots differed initially in that group selections tended to be on higher landform positions (more on ridges and spurs, less in gullies) and have higher potential solar radiation than coupes. This corresponded to lower soil total P on the group selection subplots than on coupe subplots.
Coupe subplots also had higher maximum water fern frequency, soil Ca, soil pH, soil mineralisable N, mean litter depth, and exotic plant species occurrence than group-selection subplots. All of these variables appear to be directly related to the size of the harvested area.
Coupes appear to provide the best conditions for regeneration of red beech, whereas forest adjacent to coupes and group selection harvests provide the best conditions for silver beech.
On coupes, red beech saplings increased in density as crown fern frequency and total P decline and where the canopy is most open and the soil has the least evidence of water logging.
On coupes, silver beech saplings were most abundant on higher topographic positions (slopes and ridges) where pH is low (<4.2) and litter is shallow.
On group selections, red beech saplings increased in density where the canopy is most open. Crown fern density and total P are less important to red beech saplings on group selections.
On group selections, silver beech saplings were most abundant on higher topographic positions, sites with lower Ca and total P, and sites with lower crown fern frequency.
There is no evidence for competition between red and silver beech saplings on coupes, and only a weak negative association between the species on group selections.
Recommendations
To get a more complete understanding of beech regeneration dynamics, it will be important to determine whether the same factors that lead to high sapling density also lead to high sapling growth and low sapling mortality. To this end, individuals in a range of positions and microenvironments in the coupes and group selections could be permanently marked and monitored.
Data from empirical studies, such as this one, need to be linked with simulation models (e.g. SORTIE, Pacala et al. 1996) to predict the nature of future forest development over longer time-spans than the relatively short time span (6–8 years after harvesting) examined here.
The findings of this report only apply directly to red-silver beech forests in the Maruia. It is important to test whether these findings are consistent across red-silver beech forests nationally and extend to other types of forests. A range of potential research projects are described that could address this, using data from a similar study in hard beech forest and national-scale data existing in the National Vegetation Survey databank.
To better understand the importance of ferns and soil P to forest regeneration in New Zealand generally, experimental manipulations of fern densities and soil P are required. Further temporal dynamics of crown fern need to be better understood. Die-off of crown fern has been observed in drought years in the Canterbury foothills (J. Wardle pers. comm.) but the effect of such an event on tree seedlings is unknown.
A better understanding of the nature of gap conditions that lead to high mortality of advanced growth is needed. Studies using tagged seedlings should be initiated in regions where regeneration is problematic and ongoing harvesting is occurring.
It is important that the results presented here, particularly the management recommendations, are made available to forest managers and researchers in forest management by being made available both in popular media (e.g. Indigena) and peer-reviewed articles.
Summary
That coupes favour red beech saplings and group-selections favour silver beech saplings follows expectations about how a shade-intolerant (red beech) versus a shade-tolerant (silver beech) tree species would be expected to respond to different sizes of canopy gap openings. Therefore, a range of sizes of harvested areas will promote a more mixed-species forest, whereas uniform sizes of harvested areas are likely to favour one species over the other. Of the two species, only red beech appears to regenerate more successfully in a particular position (towards the centre) of the harvested area.
Death of advanced-growth seedlings during and after harvesting has likely obscured any relationship between density of advanced growth before harvesting and current sapling density. Low-impact harvesting systems and a better understanding of the nature of gap conditions that lead to high mortality of advanced growth are required to ensure survival of advanced growth.
Density of red beech saplings is unrelated to topography, aspect, or slope steepness on either coupe or group-selection subplots. In contrast, silver beech sapling density is related to topography, with saplings being more abundant on higher topographic positions on both coupe and group selection subplots. Sapling density of both species is related to soil chemistry, but the nutrients that are important differ between species.
Crown fern frequency on coupes is negatively related to red beech sapling density. Crown fern increases in frequency and red beech sapling density decreases as total P increases. Crown fern frequency after harvesting is strongly correlated to crown fern frequency before harvesting. On group selections, crown fern frequency is negatively related to silver beech sapling density. Overall, the distribution of crown fern is patchy enough to allow adequate regeneration across the coupes and group selections. Still, in this type of forest, managers should be aware that sites where crown fern is abundant before harvesting will tend to have lower sapling density after harvesting.
There is no relationship between water fern or exotic plant species and red or silver beech sapling density.
Contact for Enquiries
Manager, Innovation Policy
Ministry of Agriculture and Forestry
MAF Policy
PO Box 2526
Wellington
New Zealand
Tel:+64 4 894 0618
Fax:+64 4 894 0741
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