Tree Crown Defoliation Research Articles

Extreme climatic events, biotic interactions and species-specific responses drive tree crown defoliation and mortality in Italian forests

Extreme climatic events, biotic interactions and species-specific responses drive tree crown defoliation and mortality in Italian forests

Tree crown defoliation in forest monitoring: concepts, findings, and new perspectives for a physiological approach in the face of climate change

Abstract Recurrent climate-driven disturbances impact on the health of European forests that reacted with increased tree dieback and mortality over the course of the last four decades. There is therefore large interest in predicting and understanding the fate and survival of forests under climate change. Forest conditions are monitored within the pan-European ICP Forests programme (UN-ECE International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests) since the 1980s, with tree crown defoliation being the most widely used parameter. Defoliation is not a cause-specific indicator of tree health and vitality, and there is a need to connect defoliation levels with the physiological functioning of trees. The physiological responses connected to tree crown defoliation are species-specific and concern, among others, water relations, photosynthesis and carbon metabolism, growth, and mineral nutrients of leaves. The indicators to measure physiological variables in forest monitoring programs must be easy to apply in the field with current state-of-the-art technologies, be replicable, inexpensive, time efficient and regulated by ad hoc protocols. The ultimate purpose is to provide data to feed process-based models to predict mortality and threats in forests due to climate change. This study reviews the problems and perspectives connected to the realization of a systematic assessment of physiological variables and proposes a set of indicators suitable for future application in forest monitoring programs.

Stem Diameter Decrement in Holm Oak (Quercus rotundifolia Lam.): Insights into Tree Decline Pathways in Endangered Woodlands of Southern Portugal

Stem diameter growth in living trees refers to the invariably increase in dimension over a given period. However, reversible stem diameter decrease could occur, related to water movement in the vessels within the wood (on a daily basis) and to tree water deficit and depletion of stored water (on a seasonal basis). Recently, the perennial decrement in stem diameter size has been related to a tree decline pathway, and eventually resulting in tree death. In this study, we analysed stem diameter intra-annual growth dynamics of holm oak (Quercus rotundifolia Lam.) trees for two full growing seasons in distinct woodlands, Laborela and Aldeia dos Fernandes, in southern Portugal (Ourique district). Our focus was to assess stem diameter growth patterns and trends in holm oak trees in endangered woodlands with high tree mortality and to question if perennial decrement. Holm oaks in Laborela were much more sensitive in their stem diameter variations than in Aldeia dos Fernandes and, on average, their stem diameter decreased continuously by 1.0 mm along the 2-year study period, with a slightly higher annual decrease in the first study year (0.6 mm.yr−1). In addition, on average, trees had a higher decrease in stem sectional area of 5.8 cm2 in Laborela against a decrease of 3.7 cm2 in Aldeia dos Fernandes, where the stem diameter of holm oaks only decreased in the first study year (0.1 mm.yr−1). In each study area, the repeated- measures ANOVA showed that tree size effect influenced the stem diameter variations in contrast to tree crown defoliation. Trees were, on average, relatively smaller in Laborela, with DBH = 35.5 cm against DBH = 40.6 cm in Aldeia dos Fernandes, and were highly sensitive in their stem diameter decrement along two consecutives full growing seasons. This is the first exploratory study on assessing the stem diameter fluctuations on holm oaks to address a decline pathway. Our results revealed that holm oaks can survive during two consecutive growing seasons, with a continuous decrease in their stem diameters, which might indicate one tree’s decline pathway.

A methodological approach for the assessment of basic crown parameters in Scots pine stands

Among other measurement techniques applied for the assessment of leaf area index, direct methods are still valued as the most accurate measures and often implemented as calibration tools. Even though more attention has been given to indirect measurements of tree crown properties in forest ecosystems over the last decades, the present study was designed to discuss the direct (destructive) and indirect (non-destructive) methods used for the assessment of crown measures in the stands defoliated from 20 to 90%. The stands with similar stand characteristics and representing relatively wide range of defoliation served as an appropriate target for the assessment of foliage mass variations.
 Overall, this study showed that the foliage mass or its surface area and defoliation at the stand level can be determined by the conventional methods used for the assessment of defoliation in forest monitoring programme as well as the PAR transmission methods. The findings showed that needle surface area decreased with the increase of tree defoliation; however, the changes of branch and stem surface areas were insignificant. Otherwise, the branch and shoot area contribute significantly to the total vegetation surface area at least in Scots pine stands. This study also strengthened the idea that the indirect measurement of vegetation area index underestimated vegetation area index at least in Scots pine stands defoliated less than 60%. The multivariate regression models were developed using tree diameter at breast height and tree crown defoliation ranges to estimate needle surface area. Keywords: Pinus sylvestris, crown defoliation, needle area index, regression model

Tree-crown Defoliation caused by Outbreak of Forest Insect Pests in Korea during 2020

This report describes the damage caused by gypsy moths (Lymantria dispar) and stick bugs (Ramulus mikado) to forests and host plants in 2020. Severe tree-crown defoliation was found in central parts of Korea, especially Gangwon (1,638 ha), Gyeonggi (1,134 ha), Chungbuk (726 ha) and Seoul (476 ha). Stick bug outbreaks occurred in hilly areas, such as Mt. Bongsan, located between Seoul and Goyang.

A multi-proxy approach reveals common and species-specific features associated with tree defoliation in broadleaved species

Tree crown defoliation is the most widespread indicator of forest health and vitality in Europe. It is part of the ICP Forests Pan-European survey and it is adopted for reporting under Forest Europe. It is readily understandable and can count on fairly harmonized, long-term, large-scale data series across Europe. On the other hand, it is unspecific with respect to possible causes of damage, and its relation with tree functioning remains unclear. This study focused on European beech (Fagus sylvatica L.), Turkey oak (Quercus cerris L.), and holm oak (Quercus ilex L.), three important broadleaved forest species in southern Europe. We investigated whether and to what extent morpho-physiological (functional) leaf traits and other indicators of foliar, branch and stem health condition are associated with tree defoliation. We tested the relationship between defoliation and leaf-, branch- and stem attributes, and whether indicators of damage and functional leaf traits significantly differ (Mann-Whitney U Test) between defoliated (defoliation > 25%) and undefoliated trees (defoliation ≤ 25%). For each species, we considered one site (three to five plots each) and n = 11–19 randomly selected trees. For each tree, the following indicators were measured: crown condition (defoliation; leaf-, branch- and stem damage, in terms of extent and intensity of damage), leaf morphology (leaf thickness, leaf area, lamina length, fluctuating asymmetry, specific leaf area, damaged leaf surface), leaf physiology and chemistry (chlorophyll a fluorescence, chlorophyll content, carbon and nitrogen stable isotopes composition δ13C, δ15N, carbon/nitrogen ratio). Results show that, for the selected trees of all the three species, defoliation was positively related to the extent of damage on branches. While increasing defoliation in European beech was also accompanied by several significant differences at leaf level (i.e., leaf damage, leaf volume, dry weight, carbon/nitrogen ratio and photosynthetic efficiency), for Turkey oak and holm oak the significant differences between defoliated and undefoliated trees were limited to damage on branches (both species).

Linking forest diversity and tree health: preliminary insights from a large-scale survey in Italy

Forest health is currently assessed in Europe (ICP Forests monitoring program). Crown defoliation and dieback, tree mortality, and pathogenic damage are the main aspects considered in tree health assessment. The worsening of environmental conditions (i.e., increase of temperature and drought events) may cause large-spatial scale tree mortality and forest decline. However, the role of stand features, including tree species assemblage and diversity as factors that modify environmental impacts, is poorly considered. The present contribution reanalyses the historical dataset of crown conditions in Italian forests from 1997 to 2014 to identify ecological and structural factors that influence tree crown defoliation, highlighting in a special manner the role of tree diversity. The effects of tree diversity were explored using the entire data set through multivariate cluster analyses and on individual trees, analysing the influence of the neighbouring tree diversity and identity at the local (neighbour) level. Preliminary results suggest that each tree species shows a specific behaviour in relation to crown defoliation, and the distribution of crown defoliation across Italian forests reflects the distribution of the main forest types and their ecological equilibrium with the environment. The potentiality and the problems connected to the possible extension of this analysis at a more general level (European and North American) were discussed.

Influence of climate on tree health evaluated by defoliation in the ICP level I network (Romania)

Abstract: Defoliation is the main parameter for assessing tree crown conditions, and is the result of cumulative interactions among different types of stressors, including climate, air pollution, pests and diseases, and management systems. Here, we evaluated a long-term data series (1992-2013) provided by the ICP-Forests Level I monitoring network (16 × 16 km) in Romania. Specifically, we investigated how climate influences defoliation at different spatial and temporal levels using statistical analyses. Using periodic climatic data (mean temperature and precipitation) derived from a daily grid dataset (ROCADA) with a resolution of 0.1 × 0.1° (10 × 10 km), we quantified how climatic parameters were correlated with defoliation, which was expressed as the mean tree defoliation per plot (DEF), and the proportion of damaged trees (crown defoliation > 25% - fDEF). The cross-correlation (Spearman r) between defoliation indicators and temperature was positive and relatively constant over time for all broadleaves and conifers, combined and separately, except for Fagus sylvatica (European beech), which had a negative cross-correlation coefficient. The correlation obtained for precipitation was similar to that obtained for temperature; however, this relationship was negative (except, again, for beech). The temporal influence of temperature on defoliation was much lower than that of precipitation, which had the greatest influence in dry regions (south and southeast Romania), especially for Quercus species. Furthermore, precipitation had a positive influence in moderate climate regions for conifers that were situated outside their natural distribution ranges. For beech and conifers situated at the upper altitudinal limits, temperature was negatively correlated with defoliation, i.e., temperature had a positive influence on health status.

HEALTH CONDITIONS OF THE UKRAINIAN CARPATHIANS FORESTS IN 2016 AND THEIR DYNAMICS ACCORDING TO FOREST MONITORING

ICP-Forest monitoring program provides European forests health estimation from the late 1980 s. Forty two percent of estimated trees had damages and their main cause was insects. Therefore, our colleagues have been providing Ukrainian Carpathians forests monitoring since 1989 according to the ICP-Forest methods. The results of 13 objects estimation in 2016 and the analysis of regional forests health from 1991 to 2016 are an object of the paper. Our investigation has obtained the following results. Firstly, tree crown defoliation in 2016 fluctuated from 7.1 % – for Silver fir to 12.3 % – for hornbeam. Tree crown damages in 2016 fluctuated from 9.4 % – for Silver fir to 21.4 % – for Pedunculate oak. Dry branches for coniferous species and defoliators (insects) for deciduous species were main types of crown damages this year. The proportion of trees without damages was 40.3 % in 2016. The dynamics of the forests health indicators from 1991 to 2016 was estimated on the example of main species. Their damage (defoliation) class dynamics was similar to the sinewave during last 26 years, but it substantially differs for some species. Norway spruce defoliation class fluctuated from 0.3 to 1.8 and its dynamics approximation had higher assurance (0.72) than other species. Common beech defoliation class fluctuated from 0.4 to 1.2 and its dynamics approximation was the best (0.77). Average forest damages dynamics also was similar to the sinewave during last 26 years in the Ukrainian Carpathians, but its correlation with defoliation dynamics was strong only for Norway spruce (r=0,747), but not for Common beech (r=0.411) and for Pedunculate oak (r=0.466). Norway spruce entomological damages fluctuated from 6 to 22 (mean – 13.6 %) and were maximal within main species. Its dynamics approximation was the best (0.600). Common beech entomological damages fluctuated from 4 to 12 % (mean – 7.2 %) and its dynamics approximation was one of the worst (0.311). Thus we have made certain conclusions. Average health conditions of Ukrainian Carpathians forests were good in 2016 as damages were small. The best health conditions were in regional common beech forests and their intensive improving occurred during last 6 years after their intensive deterioration from 2000 to 2011. Common regional beech forests also had the smallest entomological damages within main forest species, it was insignificant. Health conditions of Ukrainian Carpathians forests were also good during last 26 years (1991-2016). Maximal defoliation and entomological damages were in Norway spruce regional forests and in regional Pedunculate oak forests, but even they were small.

Ozone exposure affects tree defoliation in a continental climate

Ground-level ozone (O3) affects trees through visible leaf injury, accelerating leaf senescence, declining foliar chlorophyll content, photosynthetic activity, growth, carbon sequestration, predisposing to pests attack and a variety of other physiological effects. Tree crown defoliation is one of the most important parameters that is representative of forest health and vitality. Effects of air pollution on forests have been investigated through manipulative experiments that are not representative of the real environmental conditions observed in the field. In this work we investigated the role of O3 concentration and other metrics (AOT40 and POD0) in affecting crown defoliation in temperate Romanian forests. The impacts of O3 were estimated in combination with nitrogen pollutants, climatic factors and orographic conditions, by applying a non-linear modelling approach (Random Forest and Generalised Regression Models). Ozone concentration and AOT40 under Romanian conditions were more important than meteorological parameters in affecting crown defoliation. In these particular conditions, POD0 never exceeded the critical level suggested by previous literature for forest protection, and thus was not important in affecting crown defoliation.

Analysing Atmospheric Processes and Climatic Drivers of Tree Defoliation to Determine Forest Vulnerability to Climate Warming

Crown defoliation is extensively monitored across European forests within the International Co-operative Programme (ICP) as a proxy of forest health. Climate warming and drought are assumed to be the major drivers of tree growth and crown defoliation, particularly in seasonally dry areas such as the Mediterranean Basin. Here we analyse how climate, drought, and atmospheric processes are related to defoliation time series of five oak and five pine species that are dominant across Spanish ICP monitoring forest plots. We found that warmer and drier conditions during April were linked to enhanced defoliation. Warm April conditions were also related to high values of the Atlantic Multi-decadal Oscillation (AMO), thereby indicating large-scale links between atmospheric processes, temperature, and defoliation patterns. The temperature-defoliation association was species-specific since some tree species from wet sites showed a weak association (e.g., Quercus robur L.) whereas others from dry sites (e.g., Quercus ilex L.) presented the strongest associations. The latter tree species could be considered vulnerable to heat stress in terms of leaf shedding. We also explored if defoliation was related to radial growth and found negative associations in relatively dry areas. Warmer and drier conditions linked to increasing AMO values are connected to the post-1990s rise of defoliation in Spanish ICP forest plots. Combined incorporation of defoliation and growth into mortality models can provide insights into assessments of forest vulnerability.

Physiological significance of forest tree defoliation: Results from a survey in a mixed forest in Tuscany (central Italy)

A survey of tree crown defoliation and leaf physiological traits (chlorophyll a fluorescence, nitrogen content, and stable carbon isotope composition) was carried out in the thermophilous deciduous forests in Tuscany (central Italy). In contrast to large scale surveys, where variation in defoliation can be associated with the change in environmental conditions, in a limited homogenous area the defoliation of co-existing tree species may have different significance and depends on the interaction between the characteristics of each individual species with biotic stress and environmental conditions. The survey included measurements of structural and vegetational characteristics of the forest stands, such as Leaf Area Index (LAI), basal area and tree diversity, which is expressed as the Shannon diversity index. The five tree species studied (Castanea sativa, Quercus cerris, Quercus ilex, Quercus petraea and Ostrya carpinifolia) showed species-specific crown conditions and physiological features relative to stand structure and diversity. The shape of the crowns and their area (LAI) affected forest defoliation. Tree diversity reduced defoliation in C. sativa, which was the tree species most affected by defoliation, and likewise for Q. ilex. Chlorophyll a fluorescence parameters showed lower photosynthetic efficiency in defoliated C. sativa, O. carpinifolia and Q. petraea trees. Similarly, foliar nitrogen content decreased in defoliated C. sativa and O. carpinifolia trees, whereas δ13C was higher in defoliated C. sativa. These responses may be related to the health status of C. sativa, since it was subjected to pathogen damages and insect attacks. In contrast, the mast year in O. carpinifolia may have diverted the nutrient resources from leaves to fruits, and consequently explaining the physiological effects on the tree crown. These results suggest that the combined analysis of defoliation with foliar features and stand characteristics can provide insights into tree health and vitality.

Crown defoliation of Tilia cordata Mill. as an indicator of air pollution levels in landscape garden areas within city environment

The article highlights aspects of dendroindication for nitrogen dioxide and sulfur pollution of surface air layers in the gardening landscape layout of the M.M. Gryshko National Botanic Garden, NAS Ukraine. There is a number of reasons to involve Tilia cordata Mill. as a dendroindicator of air pollution for the research in such conditions. Tree crown defoliation of Tilia cordata is an important sign to indicate the presence of atmospheric pollutants (sulfur dioxide and nitrogen) in concentration above 2.5 MPC. The results presented in the article explain the dependence of the pollutants gradation of spreading and the specifics of chosen dendroindicator defoliation within the studied gardening landscape area during 2007–2010. The study used Tilia cordata species within Boyarsky forestry as control subjects. The heterogenity of air pollution influence on the gardening landscape is explained. The data on fluctuating levels of dendroindicator defoliation are presented with reference to the levels of technogenic pollution stated during the period of research. Keywords: Tilia cordata , dendroindicator, defoliation, sulfur dioxide, nitrogen dioxide.

The combined effects of a long-term experimental drought and an extreme drought on the use of plant-water sources in a Mediterranean forest.

Vegetation in water-limited ecosystems relies strongly on access to deep water reserves to withstand dry periods. Most of these ecosystems have shallow soils over deep groundwater reserves. Understanding the functioning and functional plasticity of species-specific root systems and the patterns of or differences in the use of water sources under more frequent or intense droughts is therefore necessary to properly predict the responses of seasonally dry ecosystems to future climate. We used stable isotopes to investigate the seasonal patterns of water uptake by a sclerophyll forest on sloped terrain with shallow soils. We assessed the effect of a long-term experimental drought (12years) and the added impact of an extreme natural drought that produced widespread tree mortality and crown defoliation. The dominant species, Quercus ilex, Arbutus unedo and Phillyrea latifolia, all have dimorphic root systems enabling them to access different water sources in space and time. The plants extracted water mainly from the soil in the cold and wet seasons but increased their use of groundwater during the summer drought. Interestingly, the plants subjected to the long-term experimental drought shifted water uptake toward deeper (10-35cm) soil layers during the wet season and reduced groundwater uptake in summer, indicating plasticity in the functional distribution of fine roots that dampened the effect of our experimental drought over the long term. An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, which resulted in greater crown defoliation in the drought-affected plants. This study suggests that extreme droughts aggravate moderate but persistent drier conditions (simulated by our manipulation) and may lead to the depletion of water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.

Random Forests Analysis: a Useful Tool for Defining the Relative Importance of Environmental Conditions on Crown Defoliation

Defoliation is one of the most important parameters monitored in the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). Defoliation is an indicator for forest health and vitality. Conventional statistical analysis shows weak or not significant correlations between tree crown defoliation and climatic conditions or air pollution parameters, because of its high variability. The study aims to evaluate the most important factors among climatic, pollutants (Nox and NHy) and stand parameters affecting crown defoliation of the main European tree species (Fagus sylvatica, Picea abies, Quercus ilex, Pinus sylvestris and Quercus petraea) through application of a new and powerful statistical classifier, the random forests analysis (RFA). RFA highlighted that tree crown defoliation was mainly related to age in P. abies, to geographic location in F. sylvatica and to air pollution predictors in Q. ilex, while it was similarly linked to meteorological and air pollution predictors in P. sylvestris and Q. petraea. In this study, RFA has proven to be, for the first time, a useful tool to discern the most important predictors affecting tree crown defoliation, and consequently, it can be used for an appropriate forest management.

Le condizioni dei boschi in Italia. Risultati dalle indagini estensive di Livello I (1997-2010)

The conditions of forests in Italy. Results from the extensive surveys of Level I (1997-2010). Monitoring of tree crown defoliation is carried out in Italy since the ’80, and in 1996 a comprehensive program of quality assurance has been implemented. Currently the monitoring network includes about 260 permanent plots and 7000 sample trees. The most representative tree species are: Fagus sylvatica (European beech), Quercus pubescens (downy oak), Quercus cerris (Turkey oak), Quercus ilex (holm oah), Castanea sativa (chestnut), Ostrya carpinifolia (hop hornbeam), Picea abies (spruce), Larix decidua (larch), Pinus nigra (black pine) e Pinus sylvestris (Scots pine). These species represent the 80% of the whole sample, but only European beech is evenly distributed in Italy. The most important trends at national level concern the critical sanitary conditions of Castanea sativa and Quercus pubescens, as well as the high defoliation levels of Pinus sylvestris. The importance of this program in the context of climate change studies is discussed.

Influence of meteorological conditions and crown defoliation on tree phenology in intensive forest monitoring plots in Slovenia

Data from the forest monitoring programme in Slovenia were used to assess the relationship between tree phenology, crown defoliation and meteorological conditions in Fagus sylvatica, Quercus robur and Picea abies forests in the 2004-2013 period. We hypothesized a species-specific response of first leaf unfolding, general leaf colouring, the length of the growing season to crown defoliation, air temperature, precipitation and soil water. In accordance with the hypothesis, we found a high sensitivity of first leaf unfolding to air temperature and precipitation for all species, exhibiting contrasting responses. We observed strong sensitivity of beech defoliation to precipitation and soil water conditions. Oak crown defoliation and next-year phenology were correlated, with higher crown defoliation contributing to earlier leaf unfolding, later autumn leaf colouring and longer growing season of oak in next year. Correlation between crown defoliation and phenology was found neither for beech nor spruce.

Forest Monitoring - Assessment, Analysis and Warning System for Forest Ecosystem Status

Forests provide essential benefits and services as an important component of terrestrial ecosystems. Their functionality and health result from multiple and cumulative interactions of biotic and abiotic stress factors such as air pollution, climate change, changes in land use, and poor management practices. A forest monitoring system was established to identify, analyse and assess the degradation of European forests. Two levels of forest monitoring were developed: I) large-scale forest condition surveys, based on an European grid system starting in 1986 and II) an intensive non-systematic survey network placed in representative forest ecosystems starting in 1994. Romania implemented both level I (1990-1991) and level II (1991-1992) forest monitoring surveys with the results showing the effects of increased air temperatures and a drastic decrease of precipitation since the decade of 1971-1980. Thus, the highest values of damaged trees (crown defoliation >25%) percent were recorded in 1993, 1994, 2000 and 2003 both in the national and European networks. Also, in southern and South-Eastern Romania the forests are more frequently damaged as a response to worsening of climatic factors in this region in recent decades, with temperatures rising 0.7-0.8°C. In general, in Romania, ozone concentrations remained below the critical threshold (40-50 ppb) for affecting growth or health of trees. The levels of S-SO4 and N-NO3 declined in the atmosphere but the accumulation continued to increase in the soil, leading to soil acidification, mainly at depths of 10-40 cm). In general, during the last decade, Romanian forests were affected at low to medium intensities with damage rate up to 11% of the trees and the status of general forest health improved slightly.

Assessment of sustainable forest management criteria using indicators of the international programme ICP forests

This article gives an assessment of the international criteria for the sustainable management of forests on the basis of indicators according to the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). The given program allows us to assess Criterion 2: Maintenance of Forest Health and Vitality and Criterion 4: Conservation and Improvement of the Biological Diversity of the Forest Ecosystem. The informative indicators of Criterion 2 are the degrees of the tree crown defoliation and the composition parameters of atmospheric deposition, soils, and dominant tree plant photosynthetic organs. The assessment of Criterion 4 is carried out on the basis of biodiversity items, including forest distribution by age and species composition, sets of forest community major groups (groups of forest types) and their quantitative representation in the area, number of tree and shrub species found in the forests, and species richness and species saturation. An attempt has been made to show whether the network of the permanent observation posts reflects in full measure the forest typological and age structure in Karelia and Karelian Isthmus.

Plot-scale modelling to detect size, extent, and correlates of changes in tree defoliation in French high forests

Tree crown defoliation data collected on 102 managed forest plots of the RENECOFOR programme in France were investigated to identify (i) short-term (annual) changes and medium term (1994–2009) trends, and (ii) possible correlates of such changes and trends. Methodological aspects (trees assessed, changes in methods and reporting units, observers, assessment dates) were considered. To account for the specificity of individual plots in terms of tree provenance, age, site condition and management regime, an individual plot approach was adopted. Results showed highly frequent, statistically significant and methodologically meaningful (>5% of the expected measurement error) annual defoliation changes, with pulses of increasing defoliation occurring in 1994–1997 (with a possible methodological bias), in 2002–2004 and 2008–2009. A meta-analysis of individual plot results revealed a significant overall increase, in defoliation over the examination period; when the potentially biased 1994–1996 data were excluded from the analysis, the increase in defoliation was also significant. Within this overall increasing trend, cases of stability (11–24% of the plots) or even decreasing defoliation (11–18%) were frequent. We used a Partial Least Square (PLS) regression to model defoliation on 87 plots where sufficient data was available for a standard set of predictors, including meteorology, nutrition, phenology, reported health problems, management regime and assessment methodology. The most frequent correlates of defoliation were precipitation-related variables (of the current and previous years), tree density and frequency of trees with reported health problems. Foliar nutrients, air temperature, assessment method and observers were never found to be important predictors. Within this general pattern, interactions among predictors varied on a plot basis, leading to divergent estimated effects for the same predictor. The adopted plot-based approach avoids the bias that affects traditional cross-sectional, correlative studies and makes it possible to estimate correlates of change at the scale of individual plots; it is therefore a powerful tool to identify response patterns that can be of value when considering (or re-considering) management options.