7.0 Beech Tree Harvesting Rates and Mitigating Effects on Wildlife
7.3 Problems with TWCL harvesting rates
On first analysis the harvesting rates proposed by the TWCL forest growth model appear to be well developed (if the 15 year rest between rotation is adhered to). However, from the wildlife point of view there are still some significant concerns. In reality, harvesting rates or harvesting impacts may be much higher than prescribed. Concerns are listed below:
7.3.1. The sum of cavity bearing trees which we predict would be required by long-tailed bats, kaka and parakeets amounts to a minimum of 14.2/ha with the majority being > 80 cm DBH (Table 1). Thus there is a high probability that any trees > 80 cm DBH selected for harvesting would be trees
(See comment 35) required by
these threatened species. The logging regimes
(See comment 36) proposed would have a disproportionate impact on tree of this size because of higher mortality rates (MSMP, Table 5.1). The forest growth models indicate these trees will go through a bottleneck at least over the first 3 - 4 rotations,
reducing the available pool of suitable trees below the predicted minimum requirements. Therefore, TWCL's assertion that retention of trees > 110 DBH will minimise likely impacts on birds (BSM, p. 70; & note they did no assessment for bats)
(See comment 37) seems likely to be inadequate.
Specific research undertaken in North Westland red beech forests would improve predictions.
(See comment 38) Setting the maximum harvesting limit of beech at 80 cm DBH (rather than 110 cm) would mean that the majority (ca 80%) of cavity breeding and roosting sites for threatened species would be retained. This would reduce the harvest by a maximum of 10% - but would probably have a lesser impact on wood volumes recovered because a high proportion of wood from trees > 80 cm DBH will be "defective".
7.3.2. The above calculations do not provide any estimates for other cavity-using species in the Maruia forest (Table 1. viz. short-tailed bat, robin, morepork, rifleman, yellow-breasted tit, paradise shelduck; see Heather & Robertson, 1996, O'Donnell et al . in press). Wildlife ..
(See comment 39) may
compete for cavities because none of the species in New Zealand actively excavate new cavities. Therefore, the requirements for cavity bearing trees to be exempt from felling will be greater than the 14.2/ha estimates for threatened species.
7.3.3. The above calculations do not provide any estimates for other cavity-using pest species in the Maruia forest
(See comment 40) (e.g. rats, wasps, starlings)
which may out-compete indigenous species seeking cavities for breeding. Starlings have been recorded displacing bats in studies in other countries (Rieger, 1996a; Maeder, 1974), and occupying roosts in New Zealand (Sedgeley & O'Donnell unpubl. ms.). This will further increase the required cavity rate/ha and increase the number of trees/ha which should be exempt from harvesting.
7.3.4. The MSMP estimates that 40% of trees in the working circle will be "defective" in terms of merchantibility (sic), only 67.3 trees/ha will be merchantibile (sic) (Table 3.8, p.33), and ca. 50% of trees >80 cm DBH will be unmerchantible (sic) (MOP, p. 55).
(See comment 41) It is the so-called "defective" trees which eventually provide the cavities which so many wildlife species require.
(See comment 42) unless there is a fullproof (sic) technique for determining if a tree is suitable for processing before it is felled
then excess trees may still be felled and discarded. The MARVL quality criteria (MSMP, Appendix 1) would not identify a high proportion of suitable cavity-bearing wildlife trees. The prescribed harvest rate
(See comment 43) could double as a result,
again increasing the probability of felling a critical tree. The MSMP states that "where a tree is marked as merchantible (sic) and is upon felling found internally defective, its status will remain and the stem deducted from the merchantible (sic) proportion of the permissible cut" (p.52). However, this regime is still likely to have a disproportionate impact on trees >80 cm DBH, because this cohort has a much higher frequency of cavities and 50% of trees are "defective" in terms of merchantibility(sic). If
(See comment 44) 50% of these trees
were inadvertently felled as part of the permissible cut, then this would have a huge impact on wildlife habitat.
7.3.5. In addition 2.36 immature defective stems per ha per yr will be removed for "improvement felling" (MSMP, Table 5.5. pp.52, 80).
(See comment 45) "Over time, there should be a slow improvement in the quality of trees with a gradual reduction in the number of malformed trees" (p. 43). While this is sensible in terms of silvicultural management, it means that the numbers of trees which will develop into breeding and foraging trees in the future (with well developed cavities in them) will become progressively fewer over each felling cycle.
Younger trees with the beginnings of rot or broken branches are assets in terms of the development of wildlife habitat. Research on the ontogeny of cavity development in forests needs to be undertaken to determine whether "improvement felling" would sustain enough wildlife cavities.
For example, the impact of improvement felling on red beech is that 17.4 trees (19% of the pool of 90 young trees and biassed towards "deformed" stems), will be felled per rotation. This would carry through a significant impact on the occurrence of cavity bearing trees to future generations.
(See comment 46) 7.3.6 The prescribed harvest rate per ha may be further increased by the removal of trees for roads (MSMP, p.64). landings (p.63), and firewood (p.84). Trees damaged during felling of other trees will be felled irrespective of quality (p.56) but will hopefully be included as part of the permissible cut. There is a higher probability of felling standing dead trees during normal operations, especially if additional trees are being felled for health and safety reasons (BSM, p.82). The plans do not indicate what this harvest rate would be, nor why removal of firewood is necessary.
(See comment 47) 7.3.7. The MSMP acknowledges that research on recruitment of new trees in these beech systems is required ("At the present time TWC does not have the data to determine (recruitment) functions for its beech forests", pp 73-73). Without this research, no-one will be able to properly assess if the annual rate of felling (especially in the larger tree size classes) will in fact mimic natural processes and be sustainable.
The MSMP estimates survival rates at 3%. However, if mortality rates mimic the changes in the number of trees/ha between each tree size class present (Table 5.1, p.73) then mortality rates between different tree size classes would vary markedly. For example, tree densities were reduced by 97% between the seedling and sapling classes; by 12.2% between the 30-40 cm and 40-50 cm DBH classes, and by 42.9% between the 80-90 and 90-100 cm classes. In reality there is no average mortality rate per year. Sustainability models must take into account very variable mortality rates depending on the age of the beech tree.
(See comment 48) 7.3.8. While there is currently no specific management for pinhole borer being proposed (MSMP, p.88), if future research indicates a need to treat live or standing dead trees then this has implications for wildlife. Wood borer is one mechanism leading to the formation of cavities, as well as providing food for forest parrots.
7.3.9. Group felling of trees has been prescribed in the plans, where 1 - 10 trees in a patch are felled at once (MSMP, p. 41, 43).
(See comment 49) Natural gaps will continue to be formed as well.
(See comment 50) In a worst-case scenario >10 trees per ha could be felled in one patch; the local impacts of harvesting would be much higher than the 1 tree/ha prescribed.
Felling will also be concentrated in a few close-together compartments each year, again localising the impacts (MOP, Map 2B).
7.3.10. Unlike birds, bats roost together in tight clusters within trees hollows. Unless long-tailed bat roosting areas in the managed forests are identified prior to harvesting, then patchwork felling would increase the probability of
(See comment 51) devastating bat populations.
Long-tailed bats roost in relatively small patched of forest, usually in cluster of trees. Bats move trees almost everyday, particularly lactating females with young. They move in closed social groups (O'Donnell 1995).
(See comment 52) Thus removing relatively small clusters of trees at one site could fell trees containing the whole social group.
7.3.11. The above predictions have been based on impacts of harvesting on cavity bearing trees used for breeding and roosting. There have been no studies in the North Westland area which have determined which size classes of trees are
(See comment 53) required for foraging by forest birds.
It is not known if the harvesting regimes proposed will leave sufficient habitat for foraging.
. . .