3.4.1 Seedling growth
Curtis (1 967) showed that the relationship between Douglas fir (Pseudotsuga
menziesii) height and diameter measures can be generally described using linear functions. Foil owing this, West ( 1979) demonstrated that linear functions work well for describing the same relationship for established eucalypt trees of20-100
years of age. In this study strong, positive, linear relationships were observed
between the diameter and height of E. nit ens seedlings up to one year of age.
Work by Donald ( 1976) indicated that such relationships may remain constant regardless of site variation. However, a more recent study by Snowdon ( 1 981 ), which showed that the application of fertiliser can affect the relationship between
height and diameter for Pinus radiata, has cast serious doubt on Donald's ( 1 976)
conclusions.
Generally, the mean daily height growth values recorded at plantations were similar to the range of growth rates which Turnbull et al. ( 1993) reported for apparently unbrowsed first year growth of E. nit ens and E. globulus in Tasmania However, in my study there was a large variation in the growth rates of seedlings from site to site, and at some plantations mean daily height growth values were
considerably lower than those reported by Turnbull et a/. (1993). The variation observed both within and between studies is likely to reflect differences in genetic stock, silvicultural practices and environmental conditions such as soil type, aspect, altitude, rainfall, nutrient levels and the level of attack from insects and mammals.
3.4.2 Browsing damage
The occurrence of browsing damage is widespread. Of the 35 plantations surveyed there was not one at which no browsing damage was recorded. However,
'browsing damage severities were not uniform across sites, ranging from a very low value of 0.24 to a severe damage score of 4.69 (mean browse scores recorded at 12 months). Such spatial variation in damage is consistent with anecdotal evidence for Tasmanian forestry plantations (Wilkinson and Nielsen 1995; Coleman et al. 1997) and results reported from studies of a wide range of crops and their pest species. Montague (1996), for example, observed browsing damage to vary amongst eucalypt plantations in Victoria. Hone (1995b) observed gre'at spatial variation in the amount of damage caused by feral pigs (Sus scrofa),
Buckle et al. (1984) found variation in rat damage to rice crops, and Hothem et al.
(1988) found that sunflowers were highly damaged at some sites but not damaged at many others.
The factors which are thought to influence such spatial variation in damage are varied. For example, Conover (1989) observed wide variation between nursery sites in the percentage of yew shoots (0-94%) which were damaged by white tailed deer (Odocoileus virginianus). He successfully related this variation t_o a measure of deer abundance and the size of woodlots adjacent to nurseries. Kay (1993) observed spatial variation in levels of deer damage to coppiced woodlands. The variation observed in her study was best explained by the presence/absence of fences, the length of perimeter of the woodlot and the abundance of deer species. Anecdotal evidence would suggest that the damage caused to eucalypt seedlings in Tasmania may �so vary in time (Cremer 1969; Wilkinson and Nielsen 1995). Coleman et al. (1997) suggest that damage may be greatest in the June to August period when alternative foods are in shortest supply. Montague (1996) observed browsing damage to occur to eucalypts in Victorian plantations throughout the year, but damage peaked between September and November. In my study the damage levels recorded at 12 months were significantly higher than the damage levels recorded at six months. Unfortunately, little can be inferred from this result in relation to the timing of damage. It was impossible to distinguish between the
6-:1·2 month period. It is possible that the higher values observed at 12 months reflect a cumulative effect rather than any real temporal difference. In other words, the higher damage levels recorded at 12 months may be a result of recording the 'old' damage from the six months assessment plus any 'new' damage which occurred since then.
A very strong relationship was observed between the two measures of browsing damage used in this study : the percentage of seedlings observed with damage at a plantation and the mean browsing score for that plantation. This held true for both the mean values calculated for browse scores for all seedlings (includes zero
browsed seedlings) and the mean values calculated for just those seedlings with signs of browsing damage. For any given plantation then, the average severity of damage to individual seedlings appears to be related to how widespread the damage is at the plantation. The more widespread the damage, the more severe the damage to individual seedlings is likely to be. One possible explanation for
this trend is that, whilst feeding in plantations, Tasmanian herbivores do not expend a large proportion of total feeding effort on actively seeking out seedlings to consume. Instead they may consume small portions of seedlings as they are encountered during foraging bouts. However, this data is not conclusive and further experimentation will be required to confirm or invalidate this suggestion.
3.4.3 Browsing damage and seedling growth
Much of the literature concerning the effects of browsing on plant growth comes from the northern hemisphere. The results of these studies are conflicting and the reported effects of browsing or simulated browsing range from deleterious (Grant
et al. 1982; Elliot et aL 1993) through neutral (Strauss 199 1 ) to beneficial (see review by Belsky 1986). For woody tree species, researchers have generally
found that if defoliation is extensive enough then growth will be affected (e.g.
Bergstrom and Danell 1987, 1995), although this may also depend on the timing and persistence of the browsing attack.
of this study indicate that browsing damage has a strong negative effect on seedling growth. At 1 8 of the surveyed plantations, seedlings recorded with browsing damage grew significantly slower than seedlings which were not recorded with damage. This result is consistent with other Tasmanian and mainland Australian studies. Both Gilbert ( 1961 ) and Cremer ( 1969) found that fenced E. regnans seedlings grew better than unfenced seedlings exposed to browsing damage. Various simulated browsing experiments (Cremer 1969, 1 972,
1973; Nielsen and Pataczek 1 991; Wilkinson and Nielsen 1995,) have also shown that extreme levels of defoliation reduce the growth rates of eucalypt seedlings,
although it should be noted that the magnitude of these effects might have been inflated by the effects of competing weed species.
Nielsen and Pataczek ( 1991) and Wilkinson and Nielsen (1 995) observed that the effect of simulated browsing damage on seedling growth was dependent on the amount of foliage removed. Low levels of foliage removal did not result in significantly reduced growth rates. However, when foliage removal was severe, growth was significantly reduced, an effect which lasted for several years. The results of this study also show that the effect of browsing damage on growth is highly dependent on damage severity. The amount by which a browsed E. nit ens seedling's growth was reduced, as compared to unbrowsed seedlings, was found to increase linearly with increasing browsing damage score. As browse score approached the maximwn value of five, reductions in seedling growth of up to
100% could be expected. Indeed where browsing damage was extreme, some seedlings actually decreased in height.
3.4.4 Seedling survival
Although there was some variation between plantations, there was generally a high rate of seedling survival. The average survival of seedlings to 12 months was 88%. This result is remarkably similar to that reported by Montague ( 1 996)
concluded from the weak evidence presented here that browsing damage had any effect on seedling survival. It appears that, while browsing damage may exercise a subtle influence on seedling survival, other factors are more likely to be
responsible for early losses. In the short term this may include the stresses associated with transferring seedlings from a nursery environment to the forestry coupe. At one plantation (Brfl ), for example, an exceptionally low survival rate to six months ( 46%) was attributed to seedlings being allowed to dry out before planting (P. Naughton pers. comm. 1994). In the longer term, contributing factors to mortality may include water stress, disease, insect attack, competition from weed species, stochastic weather events like heavy frosts, as well as the damage caused by browsers.
Other authors have also found plant mortality to be unaffected by removal of biomass (e.g. Bergstrom and Danell 1 987). However, it generally appears that the mortality of seedlings increases when browsing is severe. In a North American study, Myster and McCarthy (1989) found that only 1 0% of Carya seedlings which were unprotected from herbivores survived to an age of2 years. Cremer (1969, 1972, 1973) and Wilkinson and Nielsen (1995) observed that heavy levels of simulated browsing (e.g. total crown removal) significantly reduced the
survival of eucalypt seedlings. This effect could extend for up to seven years (Wilkinson and Nielsen 1 995). In separate field studies, Gilbert ( 1 961) and Cremer (1969) demonstrated that E. regnans seedlings exposed to browsing suffered greater mortality than those protected by fencing. It should be noted that mortality may have been more the result of a reduced ability to out�compete weed species than a direct effect of biomass removal.
It is possible that, if the seedlings in this study continued to suffer browsing pressure, long term seedling survival may be adversely affected. Persistent browsing may, for example, reduce a plant's ability to compete with its
neighbours. The literature is replete with studies showing that plant species in a community can attain dominance by virtue of low susceptibility to browsing ( eg.
Neave and Tanton 1989; Anderson and Katz 1993). Wilkinson and Nielsen (1995) observed that some heavily defoliated eucalypt seedlings in their study were totally suppressed by competing species.
3.4.5 Management implications
The results presented in this chapter confirm that the browsing problem is a real one. Damage to seedlings was recorded at all plantations and in some instances it was severe. The main effect of this damage was to reduce the growth rates of seedlings. By quantifying the relationship between damage and seedling growth a framework now exists whereby managers are able to determine what constitutes an acceptable level of damage and what levels of damage should be aimed for if eucalypt plantations are to be successfully established The implications of these findings for management will be explored in detail in Chapter 5.
3.4.6 Limitations and confounding effects
This study has been largely one of observation, rather than experimentation. There are a number o flimitations associated with such an approach. Several comparisons were made between the growth of browsed and 'unbrowsed'
seedlings. Effectively those seedlings which were not scored with damage were treated as a control group. One assumption was that seedlings which were scored as having no damage were not attacked by browsers and their growth was
consequently not affected by browsing. However, this assumption does not necessarily hold. It is possible that seedlings which were not scored as being browsed could have actually been damaged at some stage. Damage was recorded at six monthly intervals. Conceivably, if a seedling was damaged sufficiently early, it could recover from browsing damage and still be scored as a zero browsed seedling This is despite the fact that its growth rate may have been reduced by the browsing attack. As a result, the data presented in this chapter may actually underestimate the negative effects of browsing damage on seedling
In order to use seedlings which were not observed to be browsed as a control group, it must also be assumed that they are not, in some way, inherently different from those seedlings which were observed to be browsed. If this assumption is incorrect the conclusions drawn in this chapter would be compromised Consider, for example, the comparisons made between the mean daily growth in height of browsed seedlings to that ofunbrowsed seedlings (Table 3.4). The results
strongly suggest that browsing damage exerts a strong, negative influence on daily height growth. However, this cannot be stated unequivocally because the same pattern in results would be observed if herbivores deliberately chose to feed on seedlings with a slower growth rate.
These limitations will be addressed in Chapter 4.
CHAPTER 4
AN EXPERIMENTAL STUDY OF THE EFFECTS OF BROWSING DAMAGE ON SEEDLINGS
4.1 INTRODUCTION
In Chapter 3, a study was outlined which aimed to quantify the relationships
between severity of browsing damage and the growth and survival of eucalypt seedlings during their first year. The study was carried out at a large number of sites, encompassing a wide range of environmental conditions. The results suggested that the primary effect of browsing damage was to exert a strong negative influence on the rate at which seedlings increased their height and diameter. The degree to which daily height/diameter growth was reduced was linearly related to the severity of browsing damage, as measured by mean browse score, which varied greatly between plantations.
The study was largely observational rather than experimental. As such, the findings are weakened by the lack of control seedlings (free from browsing) for use as a comparison. Instead, comparisons of daily height growth, diameter growth and survival were made between seedlings observed to be damaged by browsers, and seedlings which showed no signs of browsing at either six or 12 months after planting. These comparisons strongly implicated browsing in
reducing the rate at which seedlings increased their height and diameter, however, the causal effects could not be totally isolated. For example, an alternative
explanation for the results may be that, for some reason, browsers chose to feed on slower growing seedlings.
It is also uncertain whether the seedlings which were never scored with damage were, in fact, totally free from damage themselves. The intervals between damage
. �
r
1--t:-have
been severely browsed before being sampled and, provided browsing events' t
...
; '\
' did not persist, may still have regrown sufficiently that they were assessed as
having zero damage. Gilbert ( 1961) noted that
E.
regnans seedlings regeneratedrapidly following release from browsing attack. In effect, the results presented in Chapter 3 may underestimate the effects of browsing damage on the growth of seedlings.
In this chapter, an experimental field study is described which aimed to determine more accurately the effect of browsing damage on seedling growth. The data were used to test some of the assumptions made in Chapter 3. Additionally, the effects of browsing damage on seedling form were studied.