Modeling the effect of physiological responses to green pruning on net biomass production of Eucalyptus nitens

Green pruning of Eucalyptus nitens (Deane and Maiden) Maiden increases instantaneous rates of light-saturated CO(2) assimilation (A), and changes patterns of total leaf area and foliage distribution. We investigated the importance of such changes on the rate of recovery of growth following pruning. A simple process-based model was developed to estimate daily net biomass production (G(d)) of three-year-old plantation-grown trees over a 20-month period. The trees had been pruned by removal of 0, 50 or 70% of the length of green crown, equivalent to removal of 0, 55 or 88% of leaf area, respectively, when the plantation verged on canopy closure. Total G(d) was reduced by only 20% immediately following the 50%-pruning treatment, as a result of both the high leaf dark respiration and low A in the portion of the crown removed compared to the top of the crown. Pruning at the time of canopy closure preempted a natural and rapid decline in G(d) of the lower crown. Although leaf area index (L) was approximately 6.0 at the time of pruning, high light interception (95%) occurred with an L of 4.0. The 50%-pruning treatment reduced L to 3.5, but the physiological responses to pruning were sufficient to compensate fully for the reduction in intercepted radiation within 110 days of pruning. The 70%-pruning treatment reduced L to 1.9, and reduced G(d) by 77%, reflecting the removal of branches with high A in the mid and upper crown. Physiological responses to the 70%-pruning treatment were insufficient to increase G(d) to the value of unpruned trees during the study. Model sensitivity analysis showed that increases in A following pruning increased G(d) by 20 and 25% in the 50- and 70%-pruned trees, respectively, 20 months after pruning. Changes in leaf area/foliage distribution had a greater effect on G(d) of 50%-pruned trees (47% increase) than did changes in A. However, the reduction in photosynthetic potential associated with the 70%-pruning treatment resulted in only small changes in leaf area/foliage distribution, which consequently had little effect on G(d). The effects of physiological processes occurring within the crown and in response to green pruning on G(d) are discussed with respect to pruning of plantations.     

Growth responses, physiology and decay associated with pruning plantation-grown Eucalyptus globulus Labill. and E. nitens (Deane and Maiden) Maiden

Detailed studies have been undertaken to define pruning regimes for Eucalyptus nitens, but little is known of E. globulus responses to pruning although this is a more commonly planted species. This paper describes experiments that aimed to identify (a) levels of pruning that reduce E. globulus growth, (b) physiological processes contributing to those growth responses, (c) the incidence of decay and factors influencing decay incidence following pruning of E. globulus, and (d) comparative responses of E. nitens and E. globulus to live branch pruning. Results of a field experiment indicated that removal of between 30 and 50% of the crown length was appropriate for E. globulus plantations verging on canopy closure. The significant reduction in height growth associated with removal of 50 or 70% of crown length suggested pruning should remain below 50% of crown length if reduced stem growth of pruned trees was to be avoided. Stem volume was only significantly reduced over the period of the experiment by 70% pruning, but it was estimated that standing volume following removal of 50% of crown length would be reduced by 82 m³ ha⁻¹ over a 20-year rotation if there were no other silvicultural interventions. The growth responses observed were probably related to large reductions in leaf area following 50 or 70% removal of crown length. Trees responded to pruning by changing patterns of biomass partitioning to favour stem growth at the expense of branch growth. A glasshouse study determined that light-saturated net CO₂ uptake (A_max) increased following pruning. E. nitens seedlings had both a higher baseline A_max and higher A_max following pruning than did E. globulus, which could partially explain the greater effect of pruning on E. globulus growth than has been observed for E. nitens in other studies. This result, as well as apparently different patterns of foliage distribution through the crowns of E. globulus and E. nitens, suggested that models of pruning responses need to be parameterised for both species. In addition, a more conservative pruning regime may be appropriate for E. globulus than E. nitens. Pruning increased the frequency of branch traces with decay infection, and there was a trend towards increasing decay outbreaks with increasing pruning severity. Decay outbreaks were more likely to occur following pruning of high angle or larger diameter branches.     

Anatomical and histochemical defence responses induced in juvenile leaves of Eucalyptus globulus and Eucalyptus nitens by Mycosphaerella infection

The responses of juvenile leaves of two Eucalyptus species, with contrasting susceptibility to infection by Mycosphaerella leaf disease, were compared. The anatomical changes, accumulation of phenolics, suberin, lignin and anthocyanin and the retention of chlorophyll were studied in leaf lesions of varying developmental stages caused by species of Mycosphaerella. Enhanced resistance of Eucalyptus nitens in southern Australia was attributed to the formation of an effective lignified and suberized necrophylactic periderm, to restrict pathogen spread. Leaves of E. nitens contained isobilateral palisade which resulted in both abaxial and adaxial cell division and the initiation of a strong reinforced cellular zone from an early lesion stage. Eucalyptus globulus formed a slower, distorted necrophylactic periderm through hypertrophic changes to existing mesophyll and limited cell divisions of the single adaxial palisade layer. Deposits of lignin and suberin did not occur until later in lesion development, which were not effective in preventing further disease development. From this study it is hypothesized that tolerance of eucalypts to Mycosphaerella pathogens may be associated with constitutive mesophyll density.     

Effects of green pruning on growth and stem shape of Eucalyptus nitens (Deane and Maiden) Maiden

An experiment was established in two high quality Eucalyptus nitens Deane and Maiden (Maiden) plantations in Tasmania. At the start of the experiment the trees were three years old and the plantations were on the point of canopy closure. Selected trees were pruned to remove 0, 50% or 70% of the lower green crown length, and each was surrounded by eight unpruned trees. The 50% treatment had no impact on height or diameter increment in the two years following treatment, but removal of 70% of the lower crown length resulted in significant decreases in both height and diameter increment. There were no changes in the height of 50%-pruned trees relative to the height of surrounding unpruned trees (relative height), and it was concluded that dominance would not be affected by this treatment. The relative height of 70%-pruned trees was less at one site, and this treatment may result in loss of dominance. Stem taper was generally unaffected by either pruning treatment. Changes in stem form were restricted to trees in the 70% pruning treatment and were only transient. It was concluded that removal of 50% of the lower green crown length is an appropriate level of pruning for the species provided that growth rates are rapid and pruning is timed to coincide with canopy closure. Since dominance was unaffected by this level of pruning, thinning at the time of pruning is unnecessary. It may be possible to minimise the impact on growth of higher levels of pruning by thinning at the time of pruning.     

Do artificial and natural defoliation have similar effects on physiology of Eucalyptus globulus Labill. seedlings?

? Artificial defoliation is often used to simulate defoliation by herbivory and is usually considered a good indication of a plant’s response to a given type of damage. However, the findings of studies directly comparing the two defoliation types are inconsistent.

? Here, the short term effects of artificial and insect defoliation by larvae of Paropsisterna agricola on growth, biomass allocation and photosynthetic capacity of Eucalyptus globulus seedlings were compared in a glasshouse experiment. The artificial defoliation was carried out to closely resemble the spatial patterns observed for insect defoliation.

? Height and diameter increments were reduced as a result of insect defoliation, whereas artificial defoliation had no significant effect on height. Increased photosynthetic capacity was observed in response to both treatments, but the magnitude of this increase was larger in insect-than in artificially-defoliated seedlings. Significant reductions in foliar carbohydrate content and total biomass were noticeable in artificially-defoliated seedlings. Although the foliar carbohydrate levels also decreased across the crown zones following insect defoliation treatment, seedlings allocated a large amount of their biomass in the branches of the damaged zone.

? Despite our best endeavours to simulate insect defoliation in the artificial treatment, the latter may not reflect accurately the full strength of the effects. However, artificial and insect defoliation were similar in their direction of the responses they caused in E. globulus seedlings.

     

Responses of transpiration and canopy conductance to partial defoliation of Eucalyptus globulus trees

Partial defoliation has been shown to affect the water relations and transpiration (gas exchange) of plants. Over one growing season, the water relations in response to partial (∼45%) defoliation were examined in four-year-old Eucalyptus globulus trees in southern Australia. Daily maximum transpiration rates (E_max), maximum canopy conductance (GCmax), and diurnal patterns of tree water-use were measured over a period of 215 days using the heat-pulse technique in adjacent control (non-defoliated) and defoliated trees. Sap-flux measurements were used to estimate canopy conductance and soil-to-leaf hydraulic conductance (K_P); leaf water potential (Ψ) and climate data were also collected. Following the removal of the upper canopy layer, defoliated trees exhibited compensatory responses in transpiration rate and canopy conductance of the remaining foliage. Defoliated E. globulus had similar predawn but higher midday Ψ_l, transpiration rates (E), canopy conductance (G_C) and K_P compared to the non-defoliated controls, possibly in response to increased water supply per unit leaf area demonstrated by higher midday Ψ_l. Higher E in defoliated E. globulus trees was the result of higher G_C in the morning and early afternoon. This paper also incorporates the cumulative effect of defoliation, in a phenomenological model of maximum canopy conductance of E. globulus. These results contribute to a mechanistic understanding of plant responses to defoliation, in particular the often observed up-regulation of photosynthesis that also occurs in response to defoliation.     

Process-based modelling of the severity and impact of foliar pest attack on eucalypt plantation productivity under current and future climates

We examined the impacts of a defoliating pest, Mycosphaerella leaf disease (MLD), on rotation-length Eucalyptus globulus plantation productivity under current and future climates by using the ecoclimatic species niche model CLIMEX to generate severity, frequency and seasonality scenarios for MLD for specific E. globulus sites. These scenarios were used as inputs to the process-based forest productivity model CABALA. Climate projections from two global climate models were used to drive CABALA with either no or full acclimation of photosynthesis to elevated atmospheric CO₂ assumed. In addition we varied water and nitrogen availability to examine the impacts of different severities of MLD on plantation productivity across environmental gradients. We predicted that, under current climatic conditions, rotation-length reductions in V associated with MLD damage would be no greater than 12%, with an across-site average of 6%. There was considerable between-site variation in predictions that reflected variation in site productivity. Under future climates, we predicted that MLD may reduce rotation length V by as much as 42%, although the reduction averaged across all sites was 11%. The predicted impact of MLD on V was greatest at lower productivity sites. The importance of N and water availability in recovery following MLD attack was highlighted. Uncertainty in model predictions revolved around the climate models used and assumptions of degree of photosynthetic acclimation to elevated CO₂. Large differences in predicted impact of MLD were associated with this uncertainty. Our results suggest that the effects of defoliation due to pests on plantation productivity should not be ignored when considering future management of forest plantations. The approach developed here provides managers with a tool to appraise risk and examine possible impacts of management interventions designed to reduce or manage risk.     

Effects of partial defoliation on closed canopy Eucalyptus globulus Labilladière: Growth, biomass allocation and carbohydrates

Herbivory caused by leaf-eating insects continues to be a severe risk to forest trees and forest stands. Besides quantifying the extent of defoliation, the quantification of the trees’ response to the loss of biomass is a challenge to plant ecologists and foresters alike, and an important precondition for the application of appropriate silvicultural measures. While many defoliation studies target small trees as model systems, little is known about the effect of defoliation on larger trees. In the present study, we investigated the effects of 45% removal of leaf area on growth, biomass allocation and carbohydrates of 13 m tall, four-year-old, plantation Eucalyptus globulus Labill. in southern Tasmania. Responses were measured in three crown zones (lower, middle, upper) over a period of 11 months. Height increment was unaffected by defoliation, but diameter increment was significantly reduced 155 days after treatment. Defoliation treatment had no effect on stem volume and biomass partitioning compared with the control treatment. Trees responded to defoliation by decreased branch senescence in the lower crown, greater leaf area development in the mid crown and increased specific leaf area. Defoliation reduced concentration of soluble sugars (SS) in foliage by 22% and the pools of SS in the coarse roots by 34%. Decrease in root SS was only observed in 10-15 mm diameter class and the rootball. We concluded that this four-year-old E. globulus stands with a closed canopy was able to tolerate a single, partial artificial defoliation event, which is similarly observed with younger trees.     

Impacts of Teratosphaeria leaf disease on plantation Eucalyptus globulus productivity

The effects of Teratosphaeria leaf disease (TLD) on Eucalyptus globulus are varied, and it is currently poorly understood whether infection by TLD can cause long‐term growth effects. Such information would greatly assist disease management and pruning regimes on E. globulus plantation sites, resulting in both financial and ecological benefits. Two trials were established to quantify the effects of TLD on long‐term growth. The first was a 2‐year fungicide exclusion trial that aimed to determine initial growth losses between trees treated with fungicide and untreated trees. It was found that tree growth was not affected until a threshold value of 20% damage was reached. Volume was reduced by 17% between treated and untreated trees over the course of the 2‐year trial. The second trial, a 5‐year growth study, used differentially affected adjacent stands (one infected and the other unaffected) to look at the longer term effects of more severe defoliation (44–60%) caused by an epidemic of TLD. Results recorded 5 years after the epidemic showed that trees recovered to regain normal growth trajectories after the epidemic, but growth was retarded by ca. 1.2 years for both height and diameter compared with that of the adjoining unaffected stand. As the growth of trees was not permanently reduced by the epidemic, it is concluded that the financial impacts of TLD are more likely to be associated with the loss of income resulting from extensive branch death in the lower crown after leaf and stem infection, which makes the affected stands not suitable for pruning and hence prevents them from being managed as a higher value solid wood crop.     

Estimating forest net primary production under changing climate: adding pests into the equation

The current approach to modelling pest impacts on forest net primary production (NPP) is to apply a constant modifier. This does not capture the large spatial and temporal variability in pest abundance and activity that can occur, meaning that overestimates or underestimates of pest impacts on forest NPP are likely. Taking a more mechanistic approach that incorporates an understanding of how physiology is influenced by pest attack, enables us to better capture system feedbacks and dynamics, thereby improving the capacity to predict into novel situations such as changing climate, and to account for both changes in pest activity and host responses to the growing environment now and into the future. We reviewed the effects of pests on forest NPP and found a range of responses and physiological mechanisms underlying those responses. Pest outbreaks can clearly be a major perturbation to forest NPP, and it seems likely that the frequency and intensity of pest outbreaks, and the ways in which host species respond to pest damage, will change in the future. We summarized these impacts in the form of a conceptual model at leaf, tree and stand scales, and compared the physiological processes embedded within that framework with the capacity of a representative range of NPP models to capture those processes. We found that some models can encapsulate some of the processes, but no model can comprehensively account for the range of physiological responses to pest attack experienced by trees. This is not surprising, given the paucity of empirical data for most of the world's forests, and that the models were developed primarily for other purposes. We conclude with a list of the key physiological processes and pathways that need to be included in forest growth models in order to adequately capture pest impacts on forest NPP under current and future climate scenarios, the equations that might enable this and the empirical data required to support them.