Risk Of Tree Mortality Research Articles

Pre-Emptive Detection of Mature Pine Drought Stress Using Multispectral Aerial Imagery

Drought, ozone (O3), and nitrogen deposition (N) alter foliar pigments and tree crown structure that may be remotely detectable. Remote sensing tools are needed that pre-emptively identify trees susceptible to environmental stresses could inform forest managers in advance of tree mortality risk. Jeffrey pine, a component of the economically important and widespread western yellow pine in North America was investigated in the southern Sierra Nevada. Transpiration of mature trees differed by 20% between microsites with adequate (mesic (M)) vs. limited (xeric (X)) water availability as described in a previous study. In this study, in-the-crown morphological traits (needle chlorosis, branchlet diameter, and frequency of needle defoliators and dwarf mistletoe) were significantly correlated with aerially detected, sub-crown spectral traits (upper crown NDVI, high resolution (R), near-infrared (NIR) Scalar (inverse of NDVI) and THERM Δ, and the difference between upper and mid crown temperature). A classification tree model sorted trees into X and M microsites with THERM Δ alone (20% error), which was partially validated at a second site with only mesic trees (2% error). Random forest separated M and X site trees with additional spectra (17% error). Imagery taken once, from an aerial platform with sub-crown resolution, under the challenge of drought stress, was effective in identifying droughted trees within the context of other environmental stresses.

Tree rings reveal the adverse effect of water pumping on protected riparian Platanus orientalis tree growth

The ongoing increase in human population and the subsequent freshwater demands raise conflicts regarding conservation of riparian ecosystems. Identifying anthropogenic effects on these vulnerable nature resources is crucial for preventing future damages. Here we measured tree-ring width and isotopic carbon composition (δ13C) in stem wood of protected Platanus orientalis trees at the Kziv Nature Reserve, a Mediterranean riparian ecosystem in northern Israel. In this reserve, water was pumped for human needs during 1976–2006 from the major spring feeding the stream. We show a negative exponential correlation between stem growth and pumped water amounts. Spring drawdown had a significant negative effect on growth even in years when amounts were reduced to ~2 ∙ 106 m3 year−1, leveling off at around 5 ∙ 106 m3 year−1. Precipitation and spring volume effects on growth were exposed only after pumping stopped, further indicating its role in inhibiting tree growth. During the pumping years, stem wood δ13C was up to 1.8‰ higher than after pumping cessation, indicating the drought stress imposed on trees. Our results provide an unequivocal evidence for the adverse effect of water pumping on riparian Platanus orientalis tree growth. Such effects, and their related tree mortality risk, must be considered in sustainable water management planning.

Dying on time: traits influencing the dynamics of tree mortality risk from drought.

Dying on time: traits influencing the dynamics of tree mortality risk from drought.

Managing for disturbance stabilizes forest carbon

Forest ecosystems sequester approximately 12% of anthropogenic carbon emissions, and efforts to increase forest carbon uptake are central to climate change mitigation policy (1). Managing forests to store carbon has focused on increasing forested area, decreasing area lost to logging and clearing, and increasing forest carbon density. Warming, drought, and wildfires challenge the stability of carbon stored in forests (2, 3). By contrast, natural cycles of low-intensity fires in dry forests can, over the long term, promote forest carbon storage by protecting carbon in soil and in large, old trees. The conundrum is how to balance immediate, disturbance-driven carbon loss with long-term, stable carbon storage and account for these risks in policies for forest carbon management (Fig. 1). Fig. 1. Carbon-management policies would do well to use disturbance ecology to factor in tree mortality risk. For wildfire and other impactful disturbances, researchers now have the capability to incorporate these risks into policy mechanisms that enhance forest carbon storage. Doing so would substantially improve global forest carbon policies aimed at climate change mitigation. Image credit: Shutterstock/Christian Roberts-Olsen. What has been missing is the explicit use of disturbance ecology to factor in tree mortality risk. For wildfire and other impactful disturbances, our understanding is now sufficient to incorporate these risks into policy mechanisms that enhance forest carbon storage. Doing so would substantially improve global forest carbon policies aimed at climate-change mitigation. Governments currently use our understanding of natural hazards and societal risk to inform building codes for earthquakes (4) and wildfires (5) and for national flood insurance (6). Underlying these policies are the quantification of the probability of a natural hazard occurring and an assessment of the societal impact. Current carbon policy and management need to use stability and risk accounting based on our understanding … [↵][1]1To whom correspondence should be addressed. Email: mhurteau{at}unm.edu. [1]: #xref-corresp-1-1

Consequences of clear-cutting and drought on fine root dynamics down to 17 m in coppice-managed eucalypt plantations

Improving our understanding of the spatiotemporal dynamics of fine roots in deep soil layers is of utter importance to manage tropical planted forests in a context of climate change. Our study aimed to assess the effect of clear-cutting and drought on fine-root dynamics down to the water table in Brazilian ferralsol under eucalypt plantations conducted in coppice. Fine roots (i.e. diameter <2 mm) were sampled down to 17 m deep in a throughfall exclusion experiment comparing stands with 37% of throughfall excluded by plastic sheets (−W) and stands without rain exclusion (+W). Root dynamics were studied using minirhizotron in two permanent pits down to 17 m deep, over 1 year before clear-cutting, then over 2 years in coppice, as well as down to 4 m deep in a non-harvested plot (NH) serving as a control. After harvesting, a spectacular fine root growth of trees conducted in coppice occurred in very deep soil layers (>13 m) and, surprisingly, root mortality remained extremely low whatever the depth and the treatment. Total fine-root biomass in coppice down to 17 m depth was 1266 and 1017 g m−2 in +W and −W, respectively, at 1.5 year after the clear-cut and was 1078 g m−2 in NH 7.5 years after planting. Specific root length and specific root area were about 15% higher in −W than in +W. Proliferation of fine roots at great depths could be an adaptive mechanism for tree survival, enhancing the access to water stored in the subsoil. The root system established before clear-cutting provides access to water stored in very deep layers that probably contribute to mitigate the risk of tree mortality during prolonged drought periods when the eucalypt plantations is conducted in coppice after the clear-cut.

Comparing the efficacy of various aerial spraying scenarios using Bacillus thuringiensis to protect trees from spruce budworm defoliation

Large-scale aerial spray operations against the spruce budworm (Choristoneura fumiferana (Clemens)) (SBW) with the biological insecticide Bacillus thuringiensis ssp. kurstaki (Btk) aim at maintaining trees alive during outbreaks. This objective is thought to be achieved when ≥ 50% of current-year foliage is preserved until the end of the outbreak. This protection target is associated with low balsam fir (Abies balsamea [L.] Mill.) mortality. However, it is unknown whether this approach is always needed or whether less frequent interventions could provide similar results at a lower cost. Between 2010 and 2016, we conducted field experiments in Quebec’s Côte-Nord region to determine the efficacy of five different Btk spraying scenarios for protecting balsam fir, white spruce (Picea glauca [Moench] Voss) and black spruce (P. mariana [Mill.] BSP) in mixed stands. We used the residual photosynthetic capacity (RPC) to evaluate the efficacy of the five scenarios. RPC makes it possible to take into account the impact of SBW defoliation on foliage contribution to tree photosynthetic effort over several years, and can be used as a proxy of the risk of tree mortality. We hypothesized that less frequent Btk applications could maintain the required RPC level to keep trees alive. Our results show that areas not protected resulted in great losses of RPC in balsam fir and white spruce. Btk applications every 3 years kept RPC above 50% for 2 years in balsam fir and 4 years in white spruce. RPC losses were above 62% after 4 years in both species. The strategy currently employed in Quebec (spraying every year after a first year of moderate-severe defoliation) and the intensive protection scenario (Btk applications every year) meet the protection goals for these hosts. However, their cost prevents their application at a large scale. Btk applications every 2 years seems a relevant alternative to the current strategy to protect balsam fir and white spruce stands given the adequate level of protection provided (RPC above 39%) and the reduction in the number of Btk applications required (36% fewer applications over 7 years, resulting in 36% lower cost), particularly if the objective is to maintain trees alive. Black spruce maintained at least 54% of its RPC, even without protection. Btk applications every 3 years might be a valid alternative to reduce growth losses in black spruce-dominated stands. The use of different spraying scenarios may allow us to develop cost-efficient treatment strategies to protect Quebec’s forests.

Short-term effects of drought on tropical forest do not fully predict impacts of repeated or long-term drought: gas exchange versus growth.

Are short-term responses by tropical rainforest to drought (e.g. during El Niño) sufficient to predict changes over the long-term, or from repeated drought? Using the world's only long-term (16-year) drought experiment in tropical forest we examine predictability from short-term measurements (1–2 years). Transpiration was maximized in droughted forest: it consumed all available throughfall throughout the 16 years of study. Leaf photosynthetic capacity was maintained, but only when averaged across tree size groups. Annual transpiration in droughted forest was less than in control, with initial reductions (at high biomass) imposed by foliar stomatal control. Tree mortality increased after year three, leading to an overall biomass loss of 40%; over the long-term, the main constraint on transpiration was thus imposed by the associated reduction in sapwood area. Altered tree mortality risk may prove predictable from soil and plant hydraulics, but additional monitoring is needed to test whether future biomass will stabilize or collapse. Allocation of assimilate differed over time: stem growth and reproductive output declined in the short-term, but following mortality-related changes in resource availability, both showed long-term resilience, with partial or full recovery. Understanding and simulation of these phenomena and related trade-offs in allocation will advance more effectively through greater use of optimization and probabilistic modelling approaches.This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.

Partitioning risks of tree mortality by modes of death in managed and unmanaged northern hardwoods and mixedwoods

We used a permanent sample plot network established to monitor the effects of forest management practiced in mature northern hardwood and mixedwood stands in Québec, Canada to address two important questions related to tree mortality: (1) How does partial cutting affect tree mortality? (2) Which variables best explain tree mortality by different modes of death (standing death, uprooting or stem breakage)? Decennial tree deaths were lower after partial cutting than in unharvested plots, but the annual proportion of tree death was similar (1.25%·year-1). At the tree level, a risk-product classification demonstrated the strongest evidence on influencing total mortality, followed by species, the presence of a logging injury, and tree size. Annual temperature was the only climatic variable explaining variations in total risk of tree mortality, and always among the first three variables explaining differences among the three modes of death. The risk of standing death also increased with increasing annual precipitation, while the risk of stem breakage and, to a lesser extent, of being uprooted, decreased with increasing precipitation. Additionally, we found strong evidence to support the effect of the potassium/magnesium ratio of the humus layer on the risk of being uprooted. This information is critical to better understand tree mortality processes.

A synthesis of tree functional traits related to drought‐induced mortality in forests across climatic zones

SummaryForest dieback caused by drought‐induced tree mortality has been observed world‐wide. Forecasting which trees in which locations are vulnerable to drought‐induced mortality is important to predict the consequences of drought on forest structure, biodiversity and ecosystem function.In this paper, our central aim was to compile a synthesis of tree traits and associated abiotic variables that can be used to predict drought‐induced mortality.We reviewed the literature that specifically links drought mortality to functional traits and site conditions (i.e. edaphic variables and biotic conditions), targeting studies that show clear use of tree traits in drought analysis. We separated the review into five climatic zones to determine global vs. regionally restricted relationships between traits and mortality.Our synthesis identifies a number of traits that have clear relationships with drought‐induced mortality (e.g. wood density at the species level and tree size and growth at the individual level). However, the lack of direct relationships between most traits and drought‐induced mortality highlights areas where future research should focus to broaden our understanding.Synthesis and applications. Our synthesis highlights established relationships between traits and drought‐induced mortality, presents knowledge gaps for future research focus and suggests monitoring and research avenues for improving our understanding of drought‐induced mortality. It is intended to assist ecologists and natural resource managers choose appropriate and measurable parameters for predicting local and regional scale tree mortality risk in different climatic zones within constraints of time and funding availability.

Economic consequences of altered survival of mixed or pure Norway spruce under a dryer and warmer climate

The ecological consequences of climate change on forest ecosystems have been intensively studied in forest science. In contrast, studies on the economic performance of important tree species that investigate the financial impact of growing those species as monocultures or mixed stands are scarce. We use climate-sensitive survival probabilities to account for natural risk of tree mortality, as well as risk of fluctuating timber prices, to assess the financial performance of spruce and beech stands, pure and mixed, in the context of portfolio theory. We also compare the influence of changing climate to the impact of silvicultural options that forest owners can use to influence financial consequences: pruning spruce and avoiding planting by allowing beech to regenerate naturally. Results show that stands of pure spruce are affected by climate change through reduced mean annuities and increased standard deviation of these annuities. Admixing beech into the stands lowers this shift; by analyzing the displacement vector, we found that 7% share of beech halves the shift and a share of 49% beech reduces the length of the trajectory to 20–25%. However, the changes in risk detected for stands of spruce are relatively low compared to the influence of the silvicultural actions pruning or planting. A sensitivity scenario assuming reduced timber yields for spruce, due to climate change, and slightly increased timber yields for beech resulted massive losses in economic return in stands containing spruce. However, the economic return of spruce-dominated stands was still much higher than that of beech-dominated stands. A small admixture of beech in a spruce-dominated stand led to a comparable economic return compared to a spruce monoculture under all scenarios, with a lower level of risk.

Land cover change impacts on atmospheric chemistry: simulating projected large-scale tree mortality in the United States

Abstract. Land use and land cover changes impact climate and air quality by altering the exchange of trace gases between the Earth's surface and atmosphere. Large-scale tree mortality that is projected to occur across the United States as a result of insect and disease may therefore have unexplored consequences for tropospheric chemistry. We develop a land use module for the GEOS-Chem global chemical transport model to facilitate simulations involving changes to the land surface, and to improve consistency across land–atmosphere exchange processes. The model is used to test the impact of projected national-scale tree mortality risk through 2027 estimated by the 2012 USDA Forest Service National Insect and Disease Risk Assessment. Changes in biogenic emissions alone decrease monthly mean O3 by up to 0.4 ppb, but reductions in deposition velocity compensate or exceed the effects of emissions yielding a net increase in O3 of more than 1 ppb in some areas. The O3 response to the projected change in emissions is affected by the ratio of baseline NOx : VOC concentrations, suggesting that in addition to the degree of land cover change, tree mortality impacts depend on whether a region is NOx-limited or NOx-saturated. Consequently, air quality (as diagnosed by the number of days that 8 h average O3 exceeds 70 ppb) improves in polluted environments where changes in emissions are more important than changes to dry deposition, but worsens in clean environments where changes to dry deposition are the more important term. The influence of changes in dry deposition demonstrated here underscores the need to evaluate treatments of this physical process in models. Biogenic secondary organic aerosol loadings are significantly affected across the US, decreasing by 5–10 % across many regions, and by more than 25 % locally. Tree mortality could therefore impact background aerosol loadings by between 0.5 and 2 µg m−3. Changes to reactive nitrogen oxide abundance and partitioning are also locally important. The regional effects simulated here are similar in magnitude to other scenarios that consider future biofuel cropping or natural succession, further demonstrating that biosphere–atmosphere exchange should be considered when predicting future air quality and climate. We point to important uncertainties and further development that should be addressed for a more robust understanding of land cover change feedbacks.

Do denser forests have greater risk of tree mortality: A remote sensing analysis of density-dependent forest mortality

In forests of the western United States, high levels of forest stocking attributed to fire exclusion and settlement-era logging practices have coincided with periods of extensive forest mortality associated with extreme drought and forest pest outbreaks. This has led to the widespread but largely untested generalization that increased stand density causes increased levels of forest mortality, even for uneven-aged, multi-species, mature stands that are not actively undergoing self-thinning. We used archival Landsat imagery to examine: (1) variations in forest mortality among forest types and climate periods from 1985 to 2010 in the mixed conifer forest of the Lake Tahoe Basin, (2) relationships between stand density and mortality in the subsequent year, and (3) the influence of environmental variables on forest mortality. We found that positive density dependent mortality, where increased stand density is associated with increased probability of mortality, is associated more with lower elevation forests and drier climate periods. In mid- to upper-elevation forests, increased density was more often associated with decreased probability of mortality, especially during wetter periods. Results showed highest tree mortality during a drought in the beginning of our 25-year time series, but not in a subsequent drought period of similar severity. Our study also found increased risk of tree mortality on north-facing slopes across all forests and all climate periods, even when stand density was controlled for. There is a nuanced relationship between stocking level, forest mortality and drought effects, suggesting that no single density-reduction forest management strategy will increase forest resilience under all climate periods and in all forest types.

Tree survival and maximum density of planted forests – Observations from South African spacing studies

BackgroundAmong the most important aspects of risk and hazard studies relating to forest ecosystems are maximum forest density and density-dependent tree survival.MethodsLong-term observations about the maximum density of unthinned Pinus patula and P. elliottii field plots based on the Correlated Curve Trend (CCT) spacing studies which were established almost 8 decades ago by O’Connor (Forest Research with Special Reference to Planting Distances and Thinning, 1935) in South Africa. Three specific approaches were introduced for analysing maximum density and tree survival, namely the ‘limiting line’, Nilson’s sparsity and tree survival with the Weibull function.ResultsThe main results are:a) Maximum densities differ greatly among the two species grown on the same site and within the same species grown on different sites; it is possible to relate these differences to site index in both species.b) The relationship between the quadratic mean diameter and the minimum average spacing of surviving trees (known as Nilson’s Sparsity) appears to be surprisingly similar in both species.c) An analysis of tree survival in response to different initial planting espacements shows that the Weibull survival function parameters can be estimated if the initial planting density is known. This result is presented for each of the eight large experiments used in this study.ConclusionsThis study contributes to a better understanding of tree survival and maximum density which are the key factors required for estimating risk and uncertainty. The risk of tree mortality is not constant, but varies with tree species, planting density, tree age and growing site. For estimating that risk, therefore, continuous long-term observation on different sites and with varying planting densities, as provided by the unthinned CCT series, are essential.

Level and pattern of overstory retention influence rates and forms of tree mortality in mature, coniferous forests of the Pacific Northwest, USA

Mortality of retained trees can compromise the ecological objectives of variable-retention harvest. We used a large-scale experiment replicated at six locations in western Washington and Oregon to examine the influences of retention level (40% vs. 15% of original basal area) and its spatial pattern (aggregated vs. dispersed) on the rate and form of tree mortality for 11–12years following harvest. Cumulative mortality of conifers was greater at lower levels of retention and in dispersed treatments—a result common to most seral groups and canopy strata. The greatest losses, averaging 18% of stems, occurred at low levels of dispersed retention. Mortality peaked in the first year in dispersed treatments and at low levels of aggregated retention, then rates declined to levels comparable to the controls (∼0.8% of stems/year). Harvest-related bole damage—common in the dispersed treatments—did not increase risk of mortality. Standing dead accounted for most mortality at greater levels of retention, but uprooting was nearly as common at lower retention—particularly in dispersed treatments. Forest aggregates (1ha in size) did not exhibit greater overall mortality than controls. In fact, at 40% retention, mortality rates were reduced in the aggregates due to greater survival of late-seral and suppressed stems, presumably a response to edge creation and increased resource supply. Although cumulative mortality was no higher at 15% retention, larger harvest-unit openings and greater exposure of aggregates resulted in greater uprooting among dead trees. Current standards and guidelines for retention harvests on federal lands in the Pacific Northwest require a minimum of 15% retention within each harvest unit, with 70% of this distributed in aggregates of ⩾0.2ha. Our results suggest that these minimum standards lead to greater risk of tree mortality in dispersed settings and to elevated rates of uprooting regardless of pattern, potentially compromising the objectives of live-tree retention. However, with larger (1-ha) aggregates and moderate levels of retention, managers have considerable flexibility in implementing variable retention without risk of excessive mortality.

Patterns and predictors of survival in Tsuga canadensis populations infested by the exotic pest Adelges tsugae: 20 years of monitoring

Infestations of the exotic pest, hemlock woolly adelgid (HWA; Adelges tsugae Annand), have resulted in the widespread decline and mortality of eastern hemlock (Tsuga canadensis (L.) Carrière) throughout much of the eastern United States. As HWA continues to spread across the range of eastern hemlock, forest managers need a better understanding of the projected rates of hemlock mortality and improved quantification of the site and environmental factors that influence this rate. Our main objectives in this study, encompassing 1993–2012, were: (1) to document the long-term patterns of hemlock tree mortality following HWA infestation and (2) to assess the importance of tree, site, and weather factors in influencing mortality patterns in hemlock trees infested with HWA. In addition, to provide forest managers with a means of assessing the risk of tree mortality in infested hemlock stands, we evaluated the use of crown condition rating data to predict hemlock mortality at various time scales.Our results suggest that HWA-induced mortality can be a slower process than has previously been reported. Ten-year survivorship at our study sites ranged from 70% to 94%. From 1993 to 2012, 65% of the studied hemlock trees survived, with survival by site ranging from 39% to 82%. When calculated across all sites, survivorship after ten years of HWA infestation was 73%. Our findings indicate that inaccurate dating of HWA arrival and interaction with weather patterns may contribute to reports of elevated mortality rates. Our analysis found no support for inclusion of tree and site factors in models of hemlock mortality. However, winter temperature and summer drought explained a significant proportion of the variation in reported time to mortality of HWA infested trees. In addition, our analysis suggests that the use of crown condition indices can provide a reliable means of estimating near-term risk of hemlock mortality in HWA infested stands. Models based on foliar transparency and crown dieback were able to predict hemlock mortality with excellent discrimination at one, three, and five years following measurement. These indices can be used to provide information about impending hemlock mortality on a time scale that is relevant to many management decisions.

Risk rating for mountain pine beetle infestation of lodgepole pine forests over large areas with ordinal regression modelling

The mountain pine beetle Dendroctonus ponderosae Hopkins is endemic to lodgepole pine, Pinus contorta var. latifolia Engelmann, forests in western Canada. However, the current beetle epidemic in this area highlights the challenges faced by forest managers tasked with prioritizing stands for mitigation activities such as salvage harvesting and direct control methods. In western Canada, the operational risk rating system for mountain pine beetle is based on biological knowledge gained from a rich legacy of stand-scale field studies. Due to the large spatial (millions of hectares affected) and temporal (over 10 years) extents of the current epidemic, new research into large-area mountain pine beetle processes has revealed further insights into the landscape-scale characteristics of beetle infested forests. In this paper, we evaluated the potential for this new knowledge to augment an established system for rating the short-term risk of tree mortality in a stand due to mountain pine beetle. New variables explored for utility in risk rating include direct shortwave radiation, site index, diameter at breast height, the temporal trends in local beetle populations, Biogeoclimatic Ecosystem Classification and beetle–host interaction variables. Proportional odds ordinal regression was used to develop a model for the Vanderhoof Forest District in west-central British Columbia. Prediction on independent data was assessed with the area under the receiver operator curve (AUC), indicating good discriminatory power (AUC = 0.84) for predicting levels of mountain pine beetle-caused pine mortality.

SPATIAL ELEMENTS OF MORTALITY RISK IN OLD-GROWTH FORESTS

For many species of long-lived organisms, such as trees, survival appears to be the most critical vital rate affecting population persistence. However, methods commonly used to quantify tree death, such as relating tree mortality risk solely to diameter growth, almost certainly do not account for important spatial processes. Our goal in this study was to detect and, if present, to quantify the relevance of such processes. For this purpose, we examined purely spatial aspects of mortality for four species, Abies concolor, Abies magnifica, Calocedrus decurrens, and Pinus lambertiana, in an old-growth conifer forest in the Sierra Nevada of California, USA. The analysis was performed using data from nine fully mapped long-term monitoring plots. In three cases, the results unequivocally supported the inclusion of spatial information in models used to predict mortality. For Abies concolor, our results suggested that growth rate may not always adequately capture increased mortality risk due to competition. We also found evidence of a facilitative effect for this species, with mortality risk decreasing with proximity to conspecific neighbors. For Pinus lambertiana, mortality risk increased with density of conspecific neighbors, in keeping with a mechanism of increased pathogen or insect pressure (i.e., a Janzen-Connell type effect). Finally, we found that models estimating risk of being crushed were strongly improved by the inclusion of a simple index of spatial proximity. Not only did spatial indices improve models, those improvements were relevant for mortality prediction. For P. lambertiana, spatial factors were important for estimation of mortality risk regardless of growth rate. For A. concolor, although most of the population fell within spatial conditions in which mortality risk was well described by growth, trees that died occurred outside those conditions in a disproportionate fashion. Furthermore, as stands of A. concolor become increasingly dense, such spatial factors are likely to become increasingly important. In general, models that fail to account for spatial pattern are at risk of failure as conditions change.

Defining phreatophyte response to reduced water availability: preliminary investigations on the use of xylem cavitation vulnerability in Banksia woodland species

The consideration of phreatophyte response to changes in water availability is important in identifying ecological water requirements in water-resource planning. Although much is known about water-source partitioning and intra- and interspecific variability in groundwater use by Banksia woodland species, little is known about the response of these species to groundwater draw-down. This paper describes a preliminary study into the use of xylem cavitation vulnerability as a measure of species response to reduced water availability. A response function and critical range in percentage loss of conductance is identified for four Banksia woodland overstorey species. Similarity in the vulnerability curves of B. attenuata R.Br. and B. menziesii R.Br. at low tensions supports the notion that they occupy a similar ecohydrological niche, as defined by their broad distributions relative to depth to groundwater. B. ilicifolia R.Br., however, as an obligate phreatophyte, has a range restricted to environments of higher water availability and shallower depth to groundwater and this is reflected in greater vulnerability to cavitation (relative to other Banksia) at lower tensions. The wetland tree Melaleuca preissiana Schauer generally expressed a greater vulnerability at any given xylem water potential (Ψx). This paper identifies the range in Ψx within which there is an elevated risk of tree mortality, and represents a first step towards quantifying the critical thresholds in the response of Banksia woodland species to reduced water availability.

Application of the process-based model BIOMASS to Eucalyptus globulus subsp. globulus plantations on ex-farmland in south western Australia : I. Water use by trees and assessing risk of losses due to drought

The process-based model BIOMASS (McMurtrie, R.E., Rook, D.A., Kelliher, F.M., 1990a. Modelling the yield of Pinus radiata on a site limited by water and nitrogen. For. Ecol. Manage. 30, 381–413) was applied to simulate the water balance at five plantations of Eucalyptus globulus growing in the Mediterranean climatic region in the southwest of Australia. Seasonal patterns of variation of soil water content, measured using a neutron moisture meter, were in good agreement with simulations for two of the sites. At these sites the capacity of the soils to store available water was well defined. In one case the soil water storage capacity was limited by the depth of soil to basement rock, while in the other it was limited by the depth to impenetrable siliceous hardpan. At another two sites, where the soils were deep, measurements of seasonal changes in soil water content showed that roots extracted water from at least 6 m depth. The soil water contents simulated for the surface 3 m depth agreed well with measurements for the winter/spring period but were lower than measured values for the late summer/autumn period. This suggests that the model was not completely successful in simulating partitioning of soil water extraction from different depths. There were small differences between the simulations of variation in soil water content using the earlier version of the BIOMASS model (McMurtrie et al., 1990a) and the modified version (McMurtrie, R.E., Leuning, R., Thompson, W.A., Wheeler, A.M., 1992. A model of canopy photosynthesis and water use incorporating a mechanistic formulation of leaf CO 2 exchange. For. Ecol. Manage. 52, 261–278). The differences are due to different assumptions for estimating the average maximum stomatal conductivity ( G c) of the canopy. In the earlier version of the model, a constant value of G c was assumed for all plantations. However, in the modified version G c varied with integrated C assimilation for each plantation, which in turn depended on the proportion of shaded leaves in the canopy. Simulations using the modified version agreed more consistently with measured soil water contents. At a fifth site, where roots would have access to a watertable at 3 m depth, the agreement between the simulated seasonal pattern of soil water content and values estimated from measurements was poor. Measured soil water contents were much higher than those simulated. This observation is explained by upward movement of water in the capillary fringe above the watertable. The success of the BIOMASS model in simulating soil water contents at plantations where the soil depth, and therefore soil water storage, can be defined, suggests the model has potential for assessing the risk of tree mortality due to drought on these sites. The risk is discussed in relation to effects of rainfall and its seasonal distribution, soil water storage and leaf area index.

Tree mortality risk of oak due to gypsy moth

SummaryWe present prediction models for estimating tree mortality resulting from gypsy moth, Lymantria dispar, defoliation in mixed oak, Quercus sp., forests. These models differ from previous work by including defoliation as a factor in the analysis. Defoliation intensity, initial tree crown condition (crown vigour), crown position, and species grouping classes were highly significant in categorical analysis of variance for mortality. Heavy defoliation intensity was shown to have a strong, consistent influence in increasing the probability of tree mortality. Classification and Regression Tree (CART) analysis, a binomial decision tree procedure, was used to develop prediction models of mortality risk for use by forest managers. The best decision tree had 65 groups that correctly classified 75% of the live trees and 76% of the dead trees. Models were run separately by defoliation class and provided correct classifications between 63 and 78% of the trees. Forest land managers can use these models to assign probabilities of death for moderate and heavy defoliation intensity levels and compare predicted mortality to mortality of undefoliated trees to determine how gypsy moth defoliation will affect their stands. The probabilities can be used to develop marking guides Lased on projected defoliation levels for implementing silvicultural treatments to minimize gypsy moth effects in forest stands prior to infestation.