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Abstract Understanding the influence of climatic variations on forest decline is a major challenge for scientists investigating global changes. Although reductions in tree growth have previously been associated with forest decline, comprehensive efforts to understand these relationships are rare. Based on ring‐width variations, we determine the influence of climatic fluctuations on the onset and temporal evolution of Nothofagus pumilio forest decline in the Patagonian Andes. Basal area increment (BAI) data from 294 Nothofagus trees at 11 stands in a 500‐km latitudinal transect along the forest–steppe ecotone were used to identify the dominant patterns of regional growth. Three Regional dominant patterns, showing common variations in BAI, were derived. Two BAI patterns show high rates of growth from early to mid‐20th century, followed by sustained negative trends over the last 3–6 decades, whereas the third pattern is characterized by a positive trend since the 1960s. Tipping points in growth trends of the first two patterns are associated with two extreme dry–warm climate events in spring–summer of 1942–1943/1943–1944/1944–1945 and 1978–1979. Both severe droughts were preceded by up to 10 yr of wet periods that promoted above‐average tree growth. We concluded that severe droughts occurring after wet periods trigger the decline of large, dominant N. pumilio trees with high rates of growth. The coincidence between major changes in regional growth with two of the most severe droughts in the instrumental records shows that climatic variations over northern Patagonia synchronize the beginning of forest decline at a regional scale. As these dry–mesic N. pumilio sites will face more severe droughts in the 21st century, as suggested by future climate scenarios, the areas affected by forest decline would increase substantially.

Biogeographical and ecological theory suggests that species distributions should be driven to higher altitudes and latitudes as global temperatures rise. Such changes occur as growth improves at the poleward edge of a species distribution and declines at the range edge in the opposite or equatorial direction, mirrored by changes in the establishment of new individuals. A substantial body of evidence demonstrates that such processes are underway for a wide variety of species. Case studies from populations at the equatorial range edge of a variety of woody species have led us to understand that widespread growth decline and distributional shifts are underway. However, in apparent contrast, other studies report high productivity and reproduction in some range edge populations. We sought to assess temporal trends in the growth of the widespread European beech tree ( Fagus sylvatica) across its latitudinal range. We explored the stability of populations to major drought events and the implications for predicted widespread growth decline at its equatorial range edge. In contrast to expectations, we found greatest sensitivity and low resistance to drought in the core of the species range, whilst dry range edge populations showed particularly high resistance to drought and little evidence of drought-linked growth decline. We hypothesize that this high range edge resistance to drought is driven primarily by local environmental factors that allow relict populations to persist despite regionally unfavourable climate. The persistence of such populations demonstrates that range-edge decline is not ubiquitous and is likely to be driven by declining population density at the landscape scale rather than sudden and widespread range retraction. [ABSTRACT FROM AUTHOR]

Understanding the influence of climatic variations on forest decline is a major challenge for scientists investigating global changes. Although reductions in tree growth have previously been associated with forest decline, comprehensive efforts to understand these relationships are rare. Based on ring-width variations, we determine the influence of climatic fluctuations on the onset and temporal evolution of Nothofagus pumilio forest decline in the Patagonian Andes. Basal area increment (BAI) data from 294 Nothofagus trees at 11 stands in a 500-km latitudinal transect along the forest-steppe ecotone were used to identify the dominant patterns of regional growth. Three Regional dominant patterns, showing common variations in BAI, were derived. Two BAI patterns show high rates of growth from early to mid-20th century, followed by sustained negative trends over the last 3-6 decades, whereas the third pattern is characterized by a positive trend since the 1960s. Tipping points in growth trends of the first two patterns are associated with two extreme dry-warm climate events in spring-summer of 1942-1943/1943-1944/1944-1945 and 1978-1979. Both severe droughts were preceded by up to 10 yr of wet periods that promoted above-average tree growth. We concluded that severe droughts occurring after wet periods trigger the decline of large, dominant N. pumilio trees with high rates of growth. The coincidence between major changes in regional growth with two of the most severe droughts in the instrumental records shows that climatic variations over northern Patagonia synchronize the beginning of forest decline at a regional scale. As these dry-mesic N. pumilio sites will face more severe droughts in the 21st century, as suggested by future climate scenarios, the areas affected by forest decline would increase substantially. [ABSTRACT FROM AUTHOR]

The long lifespan of trees makes them sensitive to climate warming, particularly when abrupt changes in climatic conditions occur and when trees are already growing near to their climatic tolerance thresholds. In many Mediterranean pine forests, drought stress induced by warmer temperatures and increased aridification compromise tree survival, causing growth decline and, eventually, triggering tree mortality. We evaluated the effect of climate on radial growth and mortality comparing dead and living trees of two pine species, Pinus sylvestris and Pinus nigra subsp. salzmannii , in a continental Mediterranean area over a 50-year long period. We used linear mixed models and correlations to evaluate the effect of climate on basal area increment (BAI) and on ring-width indices, respectively. In P. nigra , growth was enhanced by wet and cold conditions, whilst growth increased with temperatures in P. sylvestris . A gradual BAI reduction was detected in P. nigra trees since the late 1970s, becoming more pronounced in recently dead trees. Contrarily, P. sylvestris growth did not show such decline, whilst death events in this species were linked to severe and punctual droughts. Our findings show that warming and drought stress trigger contrasting responses in functionally and phylogenetically similar tree species, and suggest climate-mediated important changes on competitive dominance in Mediterranean forests. [ABSTRACT FROM AUTHOR]

• Floodplain and riparian forests are among the most vulnerable ecosystems. • We compared growth of coexisting trees in a temperate and Mediterranean conditions. • Climate and tree-ring data were combined with growth models to detect growth decline. • Warmer springs and drier summers reduce oak and ash growth. • Alder was the most drought resistant species. Floodplain forests are sensitive to climate warming and increased drought, as showed by recent oak (Quercus robur) dieback and mortality episodes. However, a comprehensive comparison of coexisting tree species under different climate settings or biomes are lacking. Herein, we compared growth rates, growth responses to climate and drought severity, and modeled climate mediated growth of oak and three coexisting tree species (ash, Fraxinus angustifolia ; alder, Alnus glutinosa ; elm, Ulmus minor). Two floodplain forests subjected to cooler (temperate climate, Ticino) and warmer (Mediterranean climate, Bosco Pantano) conditions in northern and southern Italy, respectively, were analyzed. Ash seemed to be the most sensitive to drought, particularly at the Mediterranean site where oak and elm growth were also negatively affected by water shortages. Alder appeared to be the least sensitive species in terms of growth variability to drought under both temperate and Mediterranean climate conditions. Furthermore, the growth model revealed the influence of soil moisture in spring and summer on the constrained growth of ash and oak and illustrated how oak growth could be severely reduced during drastic hotter droughts. Alder seemed to be the most drought-resistant species under both environmental conditions. These results could represent the first attempts in documenting the ecological consequences of drought in terms of projected climate trends in less investigated Mediterranean floodplain forests. Furthermore, these results highlight how climate and tree-ring data combined with growth models could be useful tools to detect early warning signals of growth decline and impending dieback in floodplain forests in response to dry spells. [ABSTRACT FROM AUTHOR]

Drought-induced dieback is a matter of global concern. It has been widely reported and could compromise the climate warming mitigation potential of forests. That is why we need reliable early-warning proxies to achieve better forecasts of forest dieback and tree death. Tree-ring data can provide some of these needed proxies. Here I propose considering minimum (MND) and maximum (MXD) wood density values as proxies of tree vulnerability against drought. This hypothesis is evaluated in silver fir (Abies alba) forests from the western Spanish Pyrenees showing ongoing dieback processes after the severe 1985 drought. MXD increased in response to warm summer conditions in 1985 whereas MND increased in response to growing-season water deficit in 1986. The average between prior MXD (MXD t-1) and subsequent MND (MND t) could be used to detect severe drought impacts and forecast dieback. [ABSTRACT FROM AUTHOR]

Two tree-ring width chronologies of Picea koraiensis at two altitudes in Fenglin National Nature Reserve of Xiaoxing'an Mountains, China were developed by using dendrochronological methods. To identify main limiting factors of P. koraiensis radial growth at the two altitudes, the relationships between the chronologies and local temperature, precipitation, Palmer drought severity index (PDSI), and large-scale climatic factors were investigated. Meanwhile, the reasons of P. koraiensis growth decline in recent years were also explored. Results showed that radial growth of P. koraiensis in Xiaoxing'an Mountains was mainly limited by temperatures, especially by the minimum temperature in growing season, while the limiting effect of precipitation was relatively weak. Climate responses of P. koraiensis growth at the different altitudes showed significant differences. Radial growths of P. koraiensis at the low altitude were positively correlated with precipitation in the current growth season (June-September) and whole year, and negatively correlated with soil temperatures at different depths, especially at 80 cm depth in growing season. Meanwhile, it was significantly positively correlated with PDSI in growing season. However, the relationships between radial growth of P. koraiensis at the high altitude and precipitation, air and soil temperatures, and PDSI were not significant as that at the low altitude. Growth decline of P. koraiensis in Xiaoxing'an Mountains could be related to the phase changes in Atlantic multidecadal oscillation (AMO) and Pacific decadal oscillation (PDO) and the significant global warming since 1980. The coupling effects of the above changes might result in increased soil evaporation and exacerbated warming and drying phenomena, consequently causing the growth decline of P. koraiensis at the low altitude. [ABSTRACT FROM AUTHOR]

Within-range effects of climatic change on tree growth at the sub-regional scale remain poorly understood. The aim of this research was to use climate and radial-growth data to explain how long-term climatic trends affect tree growth patterns along the southern limit of the range of Pinus nigra ssp. salzmannii (Eastern Baetic Range, southern Spain). We used regional temperature and precipitation data and measured sub-regional radial growth variation in P. nigra forests over the past two centuries. A dynamic factor analysis was applied to test the hypothesis that trees subjected to different climates have experienced contrasting long-term growth variability. We defined four representative stand types based on average temperature and precipitation to evaluate climate-growth relationships using linear mixed-effect models and multi-model selection criteria. All four stand types experienced warming and declining precipitation throughout the twentieth century. From the onset of the twentieth century, synchronised basal-area increment decline was accounted for by dynamic factor analysis and was related to drought by climate-growth models; declining basal-area increment trends proved stronger at lower elevations, whereas temperature was positively related to growth in areas with high rainfall inputs. Given the contrasting sub-regional tree-growth responses to climate change, the role of drought becomes even more complex in shaping communities and affecting selection pressure in the Mediterranean mountain forests. Potential vegetation shifts will likely occur over the dry edge of species distributions, with major impacts on ecosystem structure and function. [ABSTRACT FROM AUTHOR]

Concern exists as to the status of red spruce (Picea rubens Sarg.) in the Great Smoky Mountains, with evidence both for and against an unprecedented decline in radial growth during the past century. On the basis of a dendrological record from 1850 to 1998, our analyses support a decline in radial growth starting as early as the 1940s through to the 1970s; in the 1970s there was a reversal of this decline. In comparing trees near ridges (2000 m) with those in draws (1500 m), we found differences in the (a) timing of the decline, (b) rate of decline, and (c) homogeneity of the decline, with trees near ridges showing earlier, faster, and more homogeneous declines than trees in draws. We hypothesized that changes in climatic conditions and/or atmospheric pollutants, both of which changed beyond ranges of natural variability, were related to the observed decline in radial growth. In trees near ridges, up to 67.1% of changes in radial growth could be explained by a combination of climatic conditions (7.6%) and annual emissions of nitric oxides (NOx) and sulfur dioxide (SO2) (an additional 59.5%). In trees from draws, up to 38.3% of the changes in radial growth could be explained by climatic conditions only. A conceptual model is presented, where trees in naturally acidic soils with low base saturation provide a sensitive signal for the changing nature of acidic pollutants, but trees in anthropogenically acidifying soils with an initially higher baser saturation provide a signal that is confounded by a transient increase of calcium (Ca) and magnesium (Mg) in the soil that results in a transient increase in radial growth. [ABSTRACT FROM AUTHOR]

Forest growth is important both economically (yielding billions of dollars of annual revenues) and ecologically (with respect to ecosystem health and global carbon budgets). The growth of all forests follows a predictable general trend with age. In young forests, it accelerates as canopies develop; it then declines substantially soon after full canopy leaf area is reached. The classic explanation for the decline in growth invoked the increasing respiration costs required to sustain the larger masses of wood characteristic of older forests. Direct measurements of respiration have largely refuted this hypothesis, and recent work has focused on stand-level rates of resource supply, resource use, and growth. We developed and tested a hypothesis at the scale of individual trees (in relation to stand structure) to explain this declining stand-level rate of stem growth. According to our hypothesis, changes in stand structure allow dominant trees to sustain high rates of growth by increasing their acquisition of resources and using these resources efficiently (defined as stem growth per unit of resource used); smaller, nondominant trees grow more slowly as a result of their more limited acquisition of resources and a reduced rate of growth per unit of resource acquired. In combination, these two trends reduce overall stand growth. We tested this hypothesis by comparing growth, growth per unit of leaf area, and variation among trees within plots in two series of plantations of Eucalyptus in Brazil and by estimating individual-tree rates of growth and use of light, water, and nutrients in a plantation of Eucalyptus saligna in Hawaii. Our results supported the individual-tree hypothesis. We conclude that part of the universal age-related decline in forest growth derives from competition-related changes in stand structure and the resource-use efficiencies of individual trees. [ABSTRACT FROM AUTHOR]

[Display omitted] • Growth trend analysis in dry-marginal populations of 5 tree species in SE Europe. • Declining growth in Fagus and Tilia due to a deteriorating climate water balance. • Stable growth of the 3 Quercus species despite increasing climatic aridity. • Results suggest future species shifts from beech to oak in a warming climate. • Admixing oaks will make silviculture less susceptible to climate warming. Global warming and increasing drought severity are exposing temperate forests to increasing stress, challenging silvicultural decision making. Growth analyses in marginal tree populations at drought-induced range limits may provide valuable information on tree species' adaptive potentials and species-specific climate turning points. We studied the climate sensitivity and resilience to drought of radial growth, and long-term growth trends of mesic and rear-edge populations of Fagus sylvatica in comparison to three oak species (Quercus petraea, Q. frainetto, Q. cerris) and Tilia tomentosa in natural ecotones from mesic beech to xeric oak forests along three elevation transects in western Romania. Radial growth of all species was positively influenced by summer precipitation and low drought intensity, and negatively by high summer temperatures. The basal area increment (BAI) of F. sylvatica and T. tomentosa has declined in the last 10–20 years with warming and a deterioration of the summer water balance, while the three Quercus species maintained stable growth rates, though at lower BAI levels, suggesting a negative relationship between mean BAI and drought resistance among the five species. Growth reductions during three severe drought events (2000, 2003, 2012) were stronger, and growth resilience lower, in F. sylvatica and T. tomentosa than in the Quercus species, pointing at a thermal limit of beech at June–August temperatures of 20–21 °C. As the climate of the studied ecotones is similar to the predicted climate at colline/submontane elevation in Central Europe in about 50 years, a decline in beech growth and vitality is likely also in drought-affected regions in the distribution centre with future warming. Our results demonstrate that choosing stress-tolerant Q. petraea (as well as Q. frainetto and Q. cerris) instead of more productive timber species is a relatively safe option for Central European forestry in a warmer climate. [ABSTRACT FROM AUTHOR]

The United States Forest Service inventory specialists discovered that since 1972 a significant decline has occurred in growth of natural pine stands in the southeastern United States. This decline was puzzling because neither internal nor external changes in stand conditions were a sufficient explanation. Nor did related observations help solve the puzzle: pine plantations in the same region showed no growth reduction; the decline was not associated with an increased level of pollution; no excessive loss of needles or change in needle coloration were found. Reexamination of the original inventory data that included the latest, third, growth period revealed that high basal area increments during 1961–72 were due to abnormally low basal areas at the 1961 inventory (third inventory). These data suggest forest decline prior to 1961, rather than after this date. A more plausible explanation is the deficiency of the data especially that collected during the third inventory when, for the first time, point sampling was used instead of fixed area plots. An analysis of increments from remeasured plots, numerous outliers, and inconsistencies in relationships between other variables raise further questions about the validity of the data. It seems that the reported forest decline is an artifact of shifting inventory protocols, the complexities of point sampling, and faulty treatment of the data. We need to know the true state of the nation's forests—both over-estimates and under-estimates of decline ultimately defeat sound management. My analysis concludes with the following suggestions for a revision of the Forest Service inventory system: incorporate forest inventory into the wider framework of Geographic Information Systems (GIS); make forest inventory useful to practicing foresters and not just to regional or national planners; use fixed area methods instead of point sampling; establish external quality control and quality assurance procedure, and develop... [ABSTRACT FROM AUTHOR]

The increased frequency and intensity of warming-induced droughts have triggered dieback episodes affecting many forest types and tree species worldwide. Tree plantations are not exempt as they can be more vulnerable to drought than natural forests because of their lower structural and genetic diversity. Therefore, disentangling the physiological mechanisms leading to growth decline and tree mortality can provide tools to adapt forest management to climate change. In this study, we investigated a Pinus nigra Arn. plantation situated in northern Spain, in which some trees showed canopy dieback and radial-growth decline. We analyzed how radial growth and its responses to drought events differed between non-declining (ND) and declining (D) trees showing low and high canopy defoliation, respectively, in combination with carbon (δ13C) and oxygen (δ18O) isotope ratios in tree rings. The radial growth of P. nigra was constrained by water availability during the growing season and the previous autumn. The radial growth of D trees showed higher sensitivity to drought than ND trees. This fact is in accordance with the lower drought resilience and negative growth trends observed in D trees. Both tree classes differed in their growth from 2012 onwards, with D trees showing a reduced growth compared to ND trees. The positive δ13C-δ18O relationship together with the uncoupling between growth and intrinsic water-use efficiency suggest that D trees have less tight stomatal regulation than ND trees, which could involve a high risk of xylem embolism in the former class. Our results suggest that different water use strategies between coexisting ND and D trees were behind the differences in growth patterns and point to hydraulic failure as a possible mechanism triggering dieback and growth decline. [ABSTRACT FROM AUTHOR]

• Junipers shrub dieback and mortality triggered by drought are understudied. • We compared growth and isotopes of living vs. dead junipers in two different sites. • Growth and isotopes showed site-specific patterns in living and dead junipers. • Growth was constrained by warm-dry summer conditions enhancing evapotranspiration. • Growth statistics were not robust early-warning signals of impending shrub death. Drought-induced dieback episodes are globally reported among forest ecosystems but they have been understudied in scrublands. Chronically-stressed individuals are supposed to be more vulnerable prior to drought which triggers death. We analyzed drought-triggered dieback and mortality events affecting Mediterranean Juniperus phoenicea scrublands in two sites with contrasting climate and soil conditions located in Spain. We characterized the radial growth patterns of coexisting living and dead junipers, including the calculation of growth statistics used as early-warning signals, quantified growth response to climate, and analyzed the wood C and O isotope discrimination. In the inland, continental site with rocky substrates (Yaso), dead junipers grew less than living junipers about three decades prior to the dieback started in 2016. However, in the coastal, mild site with sandy soils (Doñana), dead junipers were smaller but grew more than living junipers about two decades before the dieback onset in 2005. The only common patterns between sites were the higher growth coherence in both living and dead junipers prior to the dieback, and the decrease in growth persistence of dead junipers. Cool and wet conditions in the prior winter and current spring, and cool summer conditions enhanced juniper growth. In Doñana, growth of living individuals was more reduced by warm July conditions than in the case of dead individuals. Higher δ13C values in Yaso indicate also more pronounced drought stress. In Yaso, dead junipers presented lower δ18O values, but the opposite occurred in Doñana suggesting different changes in stomatal conductance prior to death. Warm summer conditions enhance evapotranspiration rates and trigger dieback in this shallow-rooted species, particularly in sites with a poor water-holding capacity. Chronic, slow growth is not always a reliable predictor of drought-triggered mortality. [ABSTRACT FROM AUTHOR]

Water deficiency is the primary limiting factor for tree growth in arid and semi-arid areas. Droughts associated with rising temperatures have increased in severity and frequency globally over the past few decades, making the trees in the drought-prone sites first be affected by water shortages. However, our understanding of tree growth status in these areas, and of their response to drought, is currently insufficient; especially in the context of global warming. Here, we studied 94 Chinese pine (Pinus tabulaeformis) and 86 spruce (Picea crassifolia) trees from different altitudes [2,100–2400 m above sea level (a.s.l.)] distributed at the desert margins of Northwestern China to explore tree growth and drought response from multiple perspectives using dendroecological approaches. Significant growth decline, across all tree species and altitudes, was detected in response to an interdecadal trend towards a drier climate. Moreover, the extent of tree growth decline, the proportion of affected trees, and the degree of moisture dependence have all tended to increase in each sample site, most likely due to enhanced drought severity and duration in recent decades. The more sensitive and susceptible trees were found at lower elevations (drier sites) and may signify a higher vulnerability to heating-induced drought stress. Tree resistance to drought showed strong negative correlation with drought severity across all sample sites. However, the connection between post-drought tree resilience and drought intensity is weak, perhaps because the samples were all collected from living trees, while those that had died were not sampled. The priority for future work should be to combine surviving and dead trees simultaneously, thus achieving a more representative view of tree resilience to drought; this will improve our knowledge of forest dynamics and even ecosystem succession in these vulnerable and sensitive environments. [ABSTRACT FROM AUTHOR]

Climate change particularly threatens the xeric limits of temperate-continental forests. In Hungary, annual temperatures have increased by 1.2 °C–1.8 °C in the last 30 years and the frequency of extreme droughts has grown. With the aim to gain stand-level prospects of sustainability, we have used local forest site variables to identify and project effects of recent and expected changes of climate. We have used a climatic descriptor (FAI index) to compare trends estimated from forest datasets with climatological projections; this is likely for the first time such a comparison has been made. Four independent approaches confirmed the near-linear decline of growth and vitality with increasing hot droughts in summer, using sessile oak as model species. The correlation between droughts and the expansion of pest and disease damages was also found to be significant. Projections of expected changes of main site factors predict a dramatic rise of future drought frequency and, consequently, a substantial shift of forest climate classes, especially at low elevation. Excess water-dependent lowland forests may lose supply from groundwater, which may change vegetation cover and soil development processes. The overall change of site conditions not only causes economic losses, but also challenges long-term sustainability of forest cover at the xeric limits. [ABSTRACT FROM AUTHOR]

More than a third of Romania’s surface has low-productive soils, at the same time exposed to risks of climatic phenomena and generating high economic loss. Afforestation with pine has been the most common solution for the recovery of sheet erosion. Many of the pines grown on such land have run down. This paper presents the results of the first dendroecological investigation of degraded lands in Romania, 80 years after the first ecological reconstruction. In this way, the effects of reconstruction were assessed, supporting the adoption of future solutions for the improvement and efficiency of recovered ecosystems. Reconstructed radial growth was set against rainfall, air temperature, and management history. A total of 330 black pine and Scots pine trees (Pinus sylvestris L. and Pinus nigra Arn.) of different ages and social positions from 11 stands of different densities were cored for retrospective tree-ring analysis. Scots pine has made better use of these sites, with a better growth rate than black pine especially in plantations with lower survival and on dominant trees. The dynamics of radial growth distinguish the two pine species, with Scots pine showing an accentuated juvenile growth spurt and bigger growth range. The growth decline is predominantly a maturation effect that begins when the tree is around 40 years old and seems to be irreversible. After this age, weak or moderated removal is not enough to revive growth. The contribution of climate (air temperature and rainfall) to the last radial increments in decline is 3–57% and is higher than in the previous decades. On moderately degraded land by farming and grazing, the mixture of Scots pine and black pine, rather than monocultures, proved to be a sustainable solution. Dendrochronological surveying of restored ecosystems allows development of management strategies, which becomes critically important in the circumstances of climate warming. [ABSTRACT FROM AUTHOR]

Robinia pseudoacacia has been widely planted in the Loess Plateau of China for soil and water conservation. The growth decline of R. pseudoacacia plantations has become a recently emerging challenge for the revegetation program and sustainable forest management in this region. As to the scientific definition, identified criteria and quantitative indices have not yet been comprehensively quantified, our current understanding of the ecological and physiological mechanisms for growth decline of R. pseudoacacia plantations is limited. The knowledge could enrich the basic theories of vegetation restoration and benefit the sustainable development of the afforestation project in the Loess Plateau. Through the comprehensive compilation of literatures on forest decline and tree mortality in the Loess Plateau and other regions across the world, this review summarized the mechanisms and recent research progress on growth decline for R. pseudoacacia plantations in the Loess Plateau, primarily demonstrated from ecological (e.g., climatic change, soil desiccation, the imbalance of community structure and the misconduct of forest management) and physiological (e.g., hydraulic failure, carbon starvation, genetic and molecular regulation) perspectives. Finally, we highlighted the research gap with regard to growth decline of R. pseudoacacia plantations in the Loess Plateau.