Edge effects on red beech growth rates
Operational Research Project Code: POL 115
Programme Leader: Susan Wiser, Landcare Research Ltd
Goal
The goal of this research was to determine whether depressed stem growth in coupe-edge trees is related to compensatory growth by roots, and to examine how depressed rates of stem growth affect stand-level volume increment. This information may be used to revise the Ministry’s implementation of sustainability requirements in the Forests Act.
Context of the project
Sustainable management of indigenous forests for timber production rests upon reliable estimates of growth in tree stem diameter, as these are used to calculate non-diminishing harvesting levels. Currently, harvesting levels are calculated from rates of diameter growth in unharvested forest, yet in forests that are managed using coupe harvesting, a certain proportion of trees become coupe-edge trees. If stem diameter growth of coupe-edge trees differs from that of trees in adjacent forest with natural gap dynamics then there is a need to adjust estimates of diameter growth and, therefore, stand-level volume increment. Previous studies have demonstrated that eight years after harvesting, the rate of stem diameter growth of large red beech trees on coupe-edges was slower than that of similar-sized trees in unharvested forest. Depressed stem growth after harvesting may reflect a shift in resource allocation away from the stems to structural roots that stabilise large trees in coupe-edge positions. Research is needed to examine why large coupe-edge red beech trees grow more slowly and to apply these coupe-edge effects to estimates of stand-level volume increment.
Approach
This research was conducted in red–silver beech forest in north Westland where six silvicultural trial coupes were established in 1994 and where subsequent forest recovery has been monitored using permanently marked plots and compared with unharvested, experimental control plots.
To evaluate whether stem and/or root growth rates responded to harvesting, we examined annual growth rings of stems and buttress roots from increment cores. Cores were collected from 50 coupe-edge red beech trees (with a diameter at breast height (DBH) of 50 centimetres or more) and from the 24 similar-sized red beech trees that were present in the unharvested forest plots. The mean annual growth rate in each increment core was calculated for the 11-year period either side of harvesting. We examined whether the change in growth rate after harvesting was determined by tree size and tree harvesting position (coupe-edge vs. unharvested forest). The ratio of root growth to stem growth was used as an index of relative allocation. We tested whether change in the root-to-stem ratio after harvesting was determined by tree size and tree harvesting position. Finally, for coupe-edge trees only, we examined whether change in stem growth, root growth, and the root-to-stem ratio after harvesting were determined by two individual tree characteristics, damage by natural causes in the year of harvesting, and damage related to harvesting.
To demonstrate how information on growth of coupe-edge trees could be applied to a management plan, we developed a working example for red beech at Maruia illustrating how stand-level volume increment can be calculated, while accounting for the effect of harvesting on growth rates of coupe-edge trees. We used measurements of tree diameters taken from trees in unharvested forest and from coupe-edge forest from 1995 (one year after harvesting) through to 2006. These stem diameter measurements were used to estimate standing volume. We examined how standing volume (cubic metres per hectare) changed over the 12-year period since harvesting in unharvested (experimental control) forest and in coupe-edge forest. Changes in standing volume were partitioned into changes due to growth, mortality and recruitment. These data were then used in an example management plan, to illustrate how reduced growth rates in coupe-edge forest can influence estimates of stand-level volume increment.
Outcomes
Readable increment cores were only obtained from a subset of the trees sampled and there was a strong bias towards sampling smaller trees. No increment core data were obtained for trees with a DBH of greater than 95 centimetres in unharvested forest.
In contrast to tree diameter growth data, increment core data indicated that most large coupe-edge trees had greater stem growth after harvesting. Large coupe-edge trees also tended to have greater root growth rates after harvesting. For roots, change in growth rate after harvesting was not related to tree size or harvesting location. For stems, the location of the tree had a significant effect on the change in growth rate following harvesting; coupe-edge trees grew more quickly after harvesting than trees in unharvested forest.
There was no significant effect of either harvesting location, or tree size, or the interaction between these two terms, on the change in root-to-stem ratio after harvesting.
Changes in the stem growth rate, root growth rate, and root-to-stem ratio of coupe-edge trees after harvesting were not related to individual tree damage sustained during the year of harvesting, either through natural agents of damage or through harvesting activities.
The results obtained in this study from increment cores contrast with earlier results from diameter tape data. This may reflect the low replication obtained from increment cores and a bias towards sampling smaller stems. Increment core data may overestimate growth rates in a population because cores from senescent and rotten trees are more likely to be unreadable and therefore discarded from the analysis.
Standing volume of red beech over the period 1995–2006 varied in unharvested plots from 414.4 to 443.4 cubic metres per hectare, with a mean of 430.9 cubic metres per hectare. Standing volume of red beech trees over the same period in coupe-edge forest ranged from 392.8 to 420.0 cubic metres per hectare with a mean of 400.8 cubic metres per hectare. In unharvested forest, loss of standing volume over the period 1995–2006 to mortality was approximately equal to gain in standing volume through growth and recruitment. This was not the case in coupe-edge forest: standing volume declined after 1995 and loss of standing volume through mortality was greater than volume gained through growth and recruitment.
These data were applied to an example forest of 100 hectares, with a standing volume of 425 cubic metres per hectare in which standing volume was calculated over a 12-year period from 1994 to 2006.
If coupe-edge effects were ignored, standing volume of unharvested forest after 12 years in the 100-hectare forest was 38 930 cubic metres per hectare – 91.6 percent of the original standing volume. If coupe-edge effects are included, this value changes to 38 728 cubic metres per hectare – 91.1 percent of the original standing volume.
Recommendations
Our study has shown how bias in increment core data can lead to conclusions about tree responses to disturbance that contrast greatly to those obtained by time-series diameter tape measurements. This has important implications for any use of increment core data to understand stand-level processes, especially in old-growth forests with many large, senescing trees where responses within a stand may be a highly variable depending on the vigour of individual trees.
Volume increment models for indigenous forests would benefit from further knowledge of tree recruitment into harvested coupes and coupe-edge forest. In particular, research is needed to determine the timing of recruitment and the rates of sapling growth and to synthesise these data into stand-level models. Data could be acquired from circular coupes in Westland beech forests and from rectangular/square coupes in Southland silver beech forest.
Measurements of tree diameters around existing coupes should be continued and synthesised over the next 5 to 15 years to evaluate growth and volume increment of coupe-edge forest.
Recovery of standing volume in coupes could be accelerated by leaving poles and saplings in the coupe.
Managers would benefit from a more comprehensive guide to calculating standing volume and volume increment across a range of situations in beech forest and mixed rimu–beech and rimu–tawa forests. There is currently inadequate attention paid to within-forest variation in standing volume.
Summary
This study examined stem and root growth of large red beech trees in unharvested forest, in coupes, and in coupe-edge forest. Previous studies determined that stem growth of these trees was depressed after harvesting and it has been proposed that this was because tree resources were being allocated to development of structural roots for tree stabilisation. There was no evidence from root increment cores for such a response. Data from stem increment cores produced conflicting results to diameter tape data from the same study area and this is likely to reflect the poor ability of increment cores to sample annual growth in large trees. Diameter tape data were used to examine how standing volume varied among three forest compartments (unharvested forest, coupes and coupe-edge forest). These data suggest that coupe-edge forest has depressed standing volume for the 12-year period after harvesting. This depression was attributable to mortality in coupe-edge forest that was only partially compensated for by recruitment and growth. This result is presented cautiously as the monitoring approach used here is not ideal for detecting recruitment into coupe-edge forest. Continued monitoring of coupe-edge forest is necessary to determine the time frame over which depressed standing volume persists. Standing volume within 12-year-old coupes was zero as all new recruits were less than 10 centimetres diameter, and research is needed to quantify when recruitment occurs into coupes and how quickly the new recruits grow.
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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