Branch growth of riparian cottonwoods: a hydrologically sensitive dendrochronological tool

 The conservation of riparian (river valley flood plain) forests relies on the provision of instream flows that are sufficient to sustain tree growth. In the present study, annual branch growth increments were investigated as an indicator of environmental favorability for riparian cottonwoods. Trees of three species, Populus angustifolia, P. balsamifera, and P. deltoides, and their natural interspecific hybrids, were studied at five sites along the Oldman and South Saskatchewan rivers in Alberta, Canada. Annual branch growth increments for the interval from 1983 to 1992 were positively correlated with stream flows (r ² = 0.79 at Lethbridge) and slightly negatively correlated with weather variables that contribute to water demand: evaporation, temperature, wind, and/or sunshine. The combination of January to May stream flow (water supply) and June evaporation (water demand) almost entirely accounted for the branch growth variation across years (r ² = 0.91 at Lethbridge). Tree ring increments were also investigated but were less closely correlated than branch increments across trees or with stream flow. Branch growth increments thus provide an accurate but short duration (1 or 2 decades) record of environmental favorability for growth. The close correlation between branch growth and stream flow indicates that water is the principal limitation to growth of these riparian cottonwoods and that these trees obtained their water from a source linked to the stream, the riparian water table. Analyses of branch increments should provide a management tool for (i) determining instream flow needs for riparian cottonwoods and (ii) analyzing impacts of stream flow alterations due to river damming or water diversion. Received: 8 May 1997 / Accepted: 23 September 1997     

Early season cuticular conductance and gas exchange in two oaks near the western edge of their range

Seasonal changes in minimum leaf conductance to water vapor (g_min), an estimate of cuticular conductance, and photosynthetic gas exchange in two co-occurring oak species in north-east Kansas (USA) were examined to determine if leaf gas exchange characteristics correlated with differences in tree distribution. Bur oak (Quercus macrocarpa Michx.) is more abundant in mesic gallery forest sites, whereas chinquapin oak (Quercus muehlenbergii Englm.) is more abundant in xeric sites. Early, during leaf expansion, g_min was significantly lower in chinquapin oak than in bur oak, though midday water potentials were similar. After leaves had fully expanded, g_min decreased to seasonal minimum values of 4.57 (±0.274) mmol m^-2 s^-1 in bur oak, and 2.66 (±0.156) mmol m^-2 s^-1 in chinquapin oak. Water potentials at these times were significantly higher in chinquapin oak. As leaves were expanding, photosynthesis (A_net) was significantly higher in chinquapin oak than in bur oak. Later in the growing season, A_net and gleaf increased dramatically in both species, and were significantly higher in bur oak relative to chinquapin oak. We concluded that bur and chinquapin oak have a number of leaf gas exchange characteristics that minimize seasonal water loss. These characteristics are distinct from trees from more mesic sites, and are consistent with the distribution patterns of these trees in tall-grass prairie gallery forests.     

Climate response of cork growth in the Mediterranean oak (Quercus suber L.) woodlands of southwestern Portugal

In the Mediterranean climate regions, drought events are expected to affect the growth of forests ecosystems by changing trees growth rates and eventually inducing shifts in their growth patterns. Cork oak (Quercus suber L.) is a strictly western Mediterranean tree species periodically harvested for its bark, the cork. So far, cork oak has received limited attention for dendroclimatological studies due to its typical faint and erratic tree wood rings. Moreover, its distinct cork rings chronologies have been completely neglected. In this study we introduce an approach using cork ring chronologies dated back 9–10 years for climate response. Despite enhancing interannual variability and increasing statistical response to short-term climatic variability, still poorly understood, this study will possibly allow infer long-term climate response. We analyzed the cork ring chronologies of 55 cork samples collected in mature (under exploitation) trees in three distinct locations in southwestern Portugal. Cork growth recorded a high climate signal, with highly significant and coherent responses to the yearly climate-related sources of variation. We successfully assessed trends of cork growth via correlation analysis including selected climate variables among mean monthly temperature, monthly precipitation and, on an annual basis, eight precipitation indices. The high mean sensitivities and inter-series correlations found for cork ring chronologies combined with the significant variance explained by climate variables suggest that climate is likely one dominant signal that affects cork growth, but local environmental stresses can decisively affect this (climate) signal. Assuming cork growth as a proxy for cork oak growth, it seems conceivable that despite the trees being highly resistant to drought stress, cork oak woodlands in southwestern Portugal would have to face lesser growth in a global warming scenario.     

Are climatic factors responsible for the process of oak decline in Poland?

Oak decline, a complex process leading to increased mortality of this species, has been observed in Europe for many years. Previous studies suggest that climate conditions, especially drought, may be one of the most important factors that trigger this phenomenon. The paper investigates the radial growth and wood anatomy features of pedunculate oak (Quercus robur) trees of various health status as well as their response to climate conditions. Wood samples including all annual increments were taken at two sites (western and central Poland, 15 trees each). Based on the crown defoliation level, three health groups (healthy, weakened and dead oaks) were distinguished. Cross-sections were prepared with sliding microtome and Cell P image analysis software was used for the measurements. Tree-ring width (TRW), earlywood vessels density (VDen) and non-weighted vessels diameter (VD) were determined and correlated with mean monthly values of temperature, precipitation, vapour pressure, and Palmer Drought Severity Index (PDSI). Radial increment and anatomical parameters were significantly higher for the healthy oaks than for the weakened and the dead trees. TRW showed smaller dependence on climate than analysed anatomical attributes. No obvious pattern of relationship was found between oak radial growth and climate regarding tree health status. Our results revealed that the drought has a weak impact on the process of oak decline on investigated sites in Poland.     

Size-dependent responses to summer drought in Scots pine, Norway spruce and common oak

In this study, we provide a detailed analysis of tree growth and water status in relation to climate of three major species of forest trees in lower regions of Bavaria, Southern Germany: Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and common oak (Quercus robur). Tree-ring chronologies and latewood δ¹³C were used to derive measures for drought reaction across trees of different dimensions: growth reduction associated with drought years, long-term growth/climate relations and stomatal control on photosynthesis. For Scots pine, growth/climate relations indicated a stronger limitation of radial growth by high summer temperatures and low summer precipitation in smaller trees in contrast to larger trees. This is corroborated by a stronger stomatal control on photosynthesis for smaller pine trees under average conditions. In dry years, however, larger pine trees exhibited stronger growth reductions. For Norway spruce, a significantly stronger correlation of tree-ring width with summer temperatures and summer precipitation was found for larger trees. Additionally, for Norway spruce there is evidence for a change in competition mode from size-asymmetric competition under conditions with sufficient soil water supply to a more size-symmetric competition under dry conditions. Smaller oak trees showed a weaker stomatal control on photosynthesis under both dry and average conditions, which is also reflected by a significantly faster recovery of tree-ring growth after extreme drought events in smaller oak trees. The observed patterns are discussed in the context of the limitation-caused matter partitioning hypothesis and possible species-specific ontogenetic modifications.     

芦芽山阳坡不同海拔华北落叶松径向生长对气候变化的响应

为研究树木生长对气候变化的响应状况,选取芦芽山阳坡的3个海拔高度建立了华北落叶松(Larix principis-rupprechtii)的树轮宽度年表。年表的统计参数表明,3条年表均为研究气候信息的可靠资料。结果表明,芦芽山阳坡华北落叶松的径向生长和生长与气候的关系均具有海拔差异,中海拔(2440 m)和高海拔(2540 m)的华北落叶松具有相似年际生长变化,而二者均与低海拔(2330 m)华北落叶松的年际生长不同。低海拔华北落叶松的生长与4月平均气温和上一年11月降水量显著负相关,而中海拔和高海拔的生长均与上一年10月平均气温和6月降水量显著负相关。通过年表与气候因子之间的滑动相关分析发现,3个海拔高度华北落叶松生长与气候因子的关系均不稳定,生长与气温条件之间的显著相关关系是随着气温升高而出现的。气温的升高引起了华北落叶松生长与气温因子关系的海拔差异,以及径向生长的海拔差异。这一结果对于气候变化对植被垂直梯度影响的研究具有一定参考价值。     

Comparison of foliar water use efficiency among 17 provenances of Larix gmelinii in the Mao’ershan area

 研究环境变化下的树木水分利用效率对探讨森林生态系统碳水耦联关系及其对气候变化的响应和适应对策具有重要意义。落叶松(Larix gmelinii)为我国北方森林的建群种之一。将水热条件不同的17个种源落叶松种植在帽儿山森林生态系统研究站的同质园内30年后, 测定其针叶水分利用效率(WUE)及其相关因子。结果表明: WUE、净光合速率(P_n)、蒸腾速率(T_r)、气孔导度(G_s)、比叶面积(SLA)和叶片氮含量(N_L)均存在显著的种源差异(p < 0.05)。WUE和G_s呈显著指数相关关系, 当G_s < 0.2 时WUE随G_s的增大而明显增大, 而当G_s > 0.2时WUE趋于稳定。WUE和SLA及N_L分别呈线性负相关和正相关关系, 且随种源原地的干燥度指数(AI)的增大其相关性明显增强。WUE和种源原地年平均气温、平均年降水量及AI分别呈线性负相关、负相关和正相关关系, 并且相关系数依次增大; T_r则仅和种源原地年平均气温呈线性正相关关系, 而P_n和种源原地AI呈线性正相关关系。不同种源落叶松由于对种源原地环境条件的适应而存在针叶结构和生理特征的显著差异, 并因此引起针叶水分利用效率的差异。     

Connections between climatic variables and the growth and needle dynamics of Scots pine (Pinus sylvestris L.) in Estonia and Lapland

The relationships between climatic variables and Scots pine (Pinus sylvestris L.) growth and needle dynamics were studied in three stands in Estonia and in four stands located near the northern timberline in Lapland. The trees sampled in Estonia had low correlations with the analysed climatic variables (air temperature, precipitation and indices of atmospheric circulation). Moreover, the weak cross-correlation of the time-series of the Estonian sample trees indicated that Scots pine is affected mainly by local factors in that region. In Lapland, however, height increment and needle production correlated strongly among trees within a stand (mean r=0.45 and 0.46, respectively) and between stands (r=0.32 and 0.37). Radial increment also showed a high inter-correlation among the trees within a stand in Lapland (r=0.45). Both height increment and needle production were strongly influenced by the temperature regime of the previous summer in Lapland (mean r=0.64 and 0.64, respectively). Radial increment was correlated with the mean July temperature of the current year (mean r=0.29). The correlations between the indices of atmospheric circulation and tree attributes were weak, while the strongest correlation was between the Ponta Delgada NAO index (PD–NAO) and height increment and needle production in Lapland. Height increment, needle production and radial increment have increased since the 1990s in the trees growing in Lapland. This may indicate a positive effect of climate warming on tree growth in Lapland. In Estonia, where climatic conditions do not limit tree growth, the climate warming seems not to directly influence the growth and needle dynamics of Scots pine.     

Effect of ecological factors on intra-annual stem girth increment of holm oak

Key message

The intra-annual stem girth increment of Quercus ilex is mainly driven by water availability and secondly by temperature. Tree size and competition modulate the growth response to climate.

Abstract

Holm oak (Quercus ilex ssp. ballota [Desf.] Samp.) is the most widespread species in the Iberian peninsula, being one of the most representative trees in forests and open woodlands. The analysis of stem girth increment of holm oak may provide valuable information about how Mediterranean ecosystems will respond to the forecasted climate changes. However, due to the variability of the Mediterranean climate, the knowledge of intra-annual patterns of growth is needed for a better understanding of the influence of the climatic variables at this scale. To this end, we used band dendrometers to measure monthly stem girth increments of 96 holm oak trees from 2003 to 2010, located in open woodlands and dense Mediterranean forests in southwestern Spain. We assessed the effects of climate, competition, topography, and initial stem diameter on stem girth increment. The major stem increment periods were in spring and autumn whereas increment rates were very low or even negative in winter and summer. Spring was not every year the season with the higher stem increments, but autumn when spring was very dry. Higher precipitation, soil moisture, and relative humidity had significant positive effects on stem increment, whereas higher temperature, reference evapotranspiration, and solar radiation had significant negative effects. Initial tree diameter and competition from nearby trees partly explained significant differences in stem increment of individual trees. Therefore, the forecasted climatic changes, in which decreased rainfall in spring and increased summer drought are expected in the Mediterranean region, may be a significant threat to the Q. ilex ecosystems.     

桂西地区南亚热带落叶栎林群落组成及物种多样性研究北大核心CSCD

落叶栎林是桂西地区南亚热带的典型次生林,也是该区域落叶阔叶林的重要群系组。该研究采用典型样方法对桂西地区落叶栎林群落进行调查,分析了该区域落叶栎林群落的物种组成、区系成分、物种多样性特征及其与地形因子的关系,为桂西地区南亚热带植物多样性保护与恢复提供依据。结果显示:(1)研究区落叶栎林群落维管束植物共计269种,隶属80科178属。(2)种子植物区系以热带成分为主,同时表现出一定程度的温带过渡性质。(3)聚类分析表明,调查的落叶栎林群落可分为云南波罗栎林、栓皮栎林、白栎林、麻栎林4种林分类型,其中以白栎林群落的物种多样性最高,且灌木层的物种多样性显著高于乔木层和草本层。(4)RDA分析显示,落叶栎林群落不同层次物种多样性的差异受地形因子的影响,多样性指标与经度、纬度、海拔之间具有明显的相关性(P<0.05),其中乔木层物种多样性主要与经度、纬度呈显著的相关性,灌木层物种多样性与纬度、海拔相关,草本层物种多样性与经度、纬度、海拔之间均有相关性。     

Dendrochronological analysis of white oak growth patterns across a topographic moisture gradient in southern Ohio

Moisture availability is a key factor that influences white oak (Quercus alba L.) growth and wood production. In unglaciated eastern North America, available soil moisture varies greatly along topographic and edaphic gradients. This study was aimed at determining the effects of soil moisture variability and macroclimate on white oak growth in mixed-oak forests of southern Ohio. Using accurately dated and measured tree rings, we analyzed 119 white oaks growing across an integrated moisture index (IMI), a computer-generated GIS model that simultaneously combines topographic and edaphic features into a moisture index scale. Growth trends varied considerably across the IMI, with trees in mesic sites exhibiting patterns much different from those in either xeric or intermediate sites. BAI growth and biomass increments were higher for trees growing in the intermediate and mesic sites than those from the xeric sites. Correlation and response function analyses, and redundancy analysis revealed significant relations between ring-width indices and climate, with current year May–July PDSI, precipitation and temperature as the most important correlates of white oak growth. Additionally, climatic influences on growth rate were variable across the IMI; trees in xeric sites showed much greater coefficients relative to those from the intermediate and mesic sites. Despite these differences, xeric and intermediate trees exhibited similar growth patterns. The present results provide further evidence of the usefulness of the IMI for identifying and comparing white oak growth patterns across the complex, dissected landscape of southern Ohio.     

Climate controls act at different scales on the seasonal pattern of Quercus ilex L. stem radial increments in NE Spain

Long-term data on radial increment dynamics in Mediterranean species may identify which climatic variables are the main constraints for radial growth and at which temporal scales they act. To this end, we examined stem radial fluctuations in Quercus ilex L., the dominant evergreen oak species in the Western Mediterranean Basin, over a period of 11 years (1994–2004) at a coastal site in north-eastern Spain. We used manual band dendrometers to record girth changes in trees on north- and south-facing slopes. Annual increments measured by dendrometers showed good agreement with annual tree-ring width. North-facing trees showed a lower long-term cumulative radial increment than south-facing trees. The seasonal radial increment pattern of Q. ilex was bimodal, being characterized by a greater increase in May and a lesser, more variable increase peak in September. Both phases corresponded to warm and moist climatic conditions, whereas radial increase of stems stopped in winter and occasionally in summer. Considering the whole year, mean maximum air temperature was the main factor positively affecting radial increment of Q. ilex from short- (5 days) to- long (30 days) time scales, whereas the accumulated precipitation exerted a similar effect at longer (30 days) scales, but only on south-facing trees. In summer, all trees were positively correlated with precipitation at long-time scales (30 days); however, only stem increment of south-facing trees showed a significant relation to the temperature at short-time scales (10 days). We confirmed the dominant role of temperature as the major constraint on radial increment at short time scales, despite most previous studies were mostly biased towards precipitation effects at monthly scales.     

Increased water‐use efficiency translates into contrasting growth patterns of Scots pine and sessile oak at their southern distribution limits

In forests, the increase in atmospheric CO₂ concentrations (C_a) has been related to enhanced tree growth and intrinsic water‐use efficiency (iWUE). However, in drought‐prone areas such as the Mediterranean Basin, it is not yet clear to what extent this “fertilizing” effect may compensate for drought‐induced growth reduction. We investigated tree growth and physiological responses at five Scots pine (Pinus sylvestris L.) and five sessile oak (Quercus petraea (Matt.) Liebl.) sites located at their southernmost distribution limits in Europe for the period 1960–2012 using annually resolved tree‐ring width and δ¹³C data to track ecophysiological processes. Results indicated that all 10 natural stands significantly increased their leaf intercellular CO₂ concentration (C_i), and consequently iWUE. Different trends in the theoretical gas‐exchange scenarios as a response to increasing C_a were found: generally, C_i tended to increase proportionally to C_a, except for trees at the driest sites in which C_i remained constant. C_i from the oak sites displaying higher water availability tended to increase at a comparable rate to C_a. Multiple linear models fitted at site level to predict basal area increment (BAI) using iWUE and climatic variables better explained tree growth in pines (31.9%–71.4%) than in oak stands (15.8%–46.8%). iWUE was negatively linked to pine growth, whereas its effect on growth of oak differed across sites. Tree growth in the western and central oak stands was negatively related to iWUE, whereas BAI from the easternmost stand was positively associated with iWUE. Thus, some Q. petraea stands might have partially benefited from the “fertilizing” effect of rising C_a, whereas P. sylvestris stands due to their strict closure of stomata did not profit from increased iWUE and consequently showed in general growth reductions across sites. Additionally, the inter‐annual variability of BAI and iWUE displayed a geographical polarity in the Mediterranean.     

Associations Among Breeding Birds and Gambel Oak in Southwestern Ponderosa Pine Forests

Abstract: Ponderosa pine (Pinus ponderosa) forests with Gambel oak (Quercus gambelii) are associated with higher bird abundance and diversity than are ponderosa pine forests lacking Gambel oak. Little is known, however, about specific structural characteristics of Gambel oak trees, clumps, and stands that may be important to birds in ponderosa pine-Gambel oak (hereafter pine-oak) forests. We examined associations among breeding birds and structural characteristics of Gambel oak at a scale similar in size to individual bird territories in pine-oak forests in northern Arizona and western New Mexico, USA. Avian species richness and occurrence of some bird species were associated with specific growth forms of Gambel oak. Estimated probability of Virginia's warblers (Vermivora virginiae), black-headed grosbeaks (Pheucticus melanocephalus), and red-faced warblers (Cardellina rubrifrons) occurring at points increased with increasing density of pole-sized Gambel oak 7–15 cm in diameter at breast height. We also found evidence that large Gambel oak trees (≥23 cm dbh) were associated with increased occurrence of yellow-rumped warblers (Dendroica coronata) at points. Some avian associations with oak were influenced by characteristics of ponderosa pines. For example, bird species richness was positively associated with the abundance of large Gambel oak when density of large pine trees ≥23 cm in diameter at breast height was low. Because large oak trees are rare and their numbers are thought to be declining, efforts should be made to retain and promote growth of additional oaks in this size class. Forest management practices that maintain forest openings, such as prescribed burning, could promote growth of pole-sized Gambel oak, which appears important to some bird species in pine-oak forests.     

Factors affecting cork oak growth under dry conditions: local adaptation and contrasting additive genetic variance within populations

Increased drought severity is expected in the Mediterranean Basin over the twenty-first century, but our understanding of the potential of most forest tree species to cope with it remains uncertain. In this study, (1) we examined the potential effects of long-term selection and the capacity to respond to future changes in selective pressures in three populations of cork oak (Quercus suber L.). For this purpose, we evaluated the response to dry conditions of 45 open-pollinated trees originating from populations in Morocco, Portugal, and Spain. Growth, leaf size, specific leaf area (SLA), carbon isotope discrimination (Δ¹³C), leaf nitrogen content (N_mass), and total chlorophyll content (Chl_mass) were measured in 9-year-old plants. (2) We also investigated the relationships between functional traits and aboveground growth by regression models. Plants presenting larger and more sclerophyllous leaves (low SLA and high leaf thickness) exhibited higher growths, with results suggesting that these traits are subjected to divergent selection in this species. Heritability estimates were moderately high for Δ¹³C (0.43 ± 0.25–0.83 ± 0.31) and stem diameter (0.40 ± 0.15–0.71 ± 0.28) for the tree populations. For the rest of the traits (except for annual growth), heritability values varied among populations, particularly for height, leaf size, leaf thickness, and N_mass. Our results suggest that natural selection has led to local adaptations and has also affected the genetic variance intrapopulation in these cork oak populations, although studies with a higher number of populations should be carried out across different years. Additionally, the absence of significant genetic correlations and the fact that correlated traits did not undergo opposing selection provided little evidence for constraints on evolution caused by genetic correlations.     

Effects of Predation Risk on Habitat Selection by Water Column Fish, Benthic Fish and Crayfish in Stream Pools

Predation risk can affect habitat selection by water column stream fish and crayfish, but little is known regarding effects of predation risk on habitat selection by benthic fish or assemblages of fish and crayfish. I used comparative studies and manipulative field experiments to determine whether, (1) habitat selection by stream fish and crayfish is affected by predation risk, and (2) benthic fish, water column fish, and crayfish differ in their habitat selection and response to predation risk. Snorkeling was used to observe fish and crayfish in, (1) unmanipulated stream pools with and without large smallmouth bass predators (Micropterus dolomieui >200 mm total length, TL) and (2) manipulated stream pools before and after addition of a single large smallmouth bass, to determine if prey size and presence of large fish predators affected habitat selection. Observations of microhabitat use were compared with microhabitat availability to determine microhabitat selection. Small fish (60–100 mm TL, except darters that were 30–100 mm TL) and crayfish (40–100 mm rostrum to telson length; TL) had significantly reduced densities in pools with large bass, whereas densities of large fish and crayfish (> 100 mm TL) did not differ significantly between pools with and without large bass. Small orangethroat darters (Etheostoma spectabile), northern crayfish (Orconectes virilis), and creek chubs (Semotilus atromaculatus) showed significantly greater densities in pools without large bass. The presence of large smallmouth bass did not significantly affect depths selected by fish and crayfish, except minnows, which were found significantly more often at medium depths when bass were present. Small minnows and large and small crayfish showed the greatest response to additions of bass to stream pools by moving away from bass locations and into shallow water. Small darters and sunfish showed an intermediate response, whereas large minnows showed no significant response to bass additions. Response to predation risk was dependent on prey size and species, with preferred prey, crayfish and small minnows, showing the greatest response. Small benthic fish, such as darters, are intermediate between small water column fish and crayfish and large water column fish in their risk of predation from large smallmouth bass.     

Common climatic signals affecting oak tree-ring growth in SE Central Europe

A network of 41 local tree-ring chronologies of oak (Quercus petraea and Quercus robur) in Austria, Hungary, Slovenia, Croatia and Serbia (latitudes 45.00–48.00N, longitudes 13.14–21.63E, altitudes 80–800 m a.s.l.) was constructed and used to establish common climatic signals in oak tree rings in the region. Co-variation of residual chronologies could be resumed in 11 significant principal components (PC), explaining 79 % of common variability. Three of them, PC1, PC2 and PC3, made it possible to identify similarities among the sites. PC1, significantly correlated with all 41 chronologies, indicated a common positive response to precipitation in spring and summer (March and June) and a negative response to temperature in spring and summer (April and June). PC2, significantly correlated with 12 chronologies, indicated a common positive response to precipitation especially in spring (May) and a negative one to high summer temperatures (especially in August) with a pronounced north to south gradient. PC3, significantly correlated with ten chronologies, indicated that a warm previous December and warm current September have a positive effect on tree growth, especially in the south-western part of the study area. The obtained climate–growth relationships will help to understand better the variability of oak growth, to fill palaeoclimatic gaps and to improve dendrochronological research in the region.     

Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought

Drought extent and severity have increased and are predicted to continue to increase in many parts of the world. Understanding tree vulnerability to drought at both individual and species levels is key to ongoing forest management and preparation for future transitions in community composition. The influence of subsurface hydrologic processes is particularly important in water‐limited ecosystems, and is an under‐studied aspect of tree drought vulnerability. With California's 2013–2016 extraordinary drought as a natural experiment, we studied four co‐occurring woodland tree species, blue oak (Quercus douglasii), valley oak (Quercus lobata), gray pine (Pinus sabiniana), and California juniper (Juniperus californica), examining drought vulnerability as a function of climate, lithology and hydrology using regional aerial dieback surveys and site‐scale field surveys. We found that in addition to climatic drought severity (i.e., rainfall), subsurface processes explained variation in drought vulnerability within and across species at both scales. Regionally for blue oak, severity of dieback was related to the bedrock lithology, with higher mortality on igneous and metamorphic substrates, and to regional reductions in groundwater. At the site scale, access to deep subsurface water, evidenced by stem water stable isotope composition, was related to canopy condition across all species. Along hillslope gradients, channel locations supported similar environments in terms of water stress across a wide climatic gradient, indicating that subsurface hydrology mediates species’ experience of drought, and that areas associated with persistent access to subsurface hydrologic resources may provide important refugia at species’ xeric range edges. Despite this persistent overall influence of the subsurface environment, individual species showed markedly different response patterns. We argue that hydrologic niche segregation can be a useful lens through which to interpret these differences in vulnerability to climatic drought and climate change.     

Climatic responses of tree-ring width and δ13C signatures of sessile oak (Quercus petraea Liebl.) on soils with contrasting water supply

We investigated climate–growth relationships (in terms of tree-ring width, basal area increment (BAI), and tree-ring δ¹³C signatures) of Quercus petraea in Central Europe (Luxembourg). Tree responses were assessed for 160 years and compared for sites with contrasting water supply (i.e. Cambisols vs. Regosols with 175 and 42 mm available water capacity, respectively). Oak trees displayed very low climate sensitivity, and climatic variables explained only 24 and 21 % of variance in tree-ring width (TRW) (Cambisol and Regosol sites, respectively). Contrary to our expectations, site-related differences in growth responses (i.e. BAI, δ¹³C signatures) to climate shifts were not significant. This finding suggests a high plasticity of oak trees in the study area. Despite a distinct growth depression found for all trees in the decade 1988–1997 (attributable to increasing annual mean temperatures by 1.1 °C), oak trees completely recovered in subsequent years. This indicates a high resilience of sessile oak to climate change. Shifts in δ¹³C_corr signatures were mainly affected by temperature, and peaks in δ¹³C_corr values (corrected for the anthropogenic increase in atmospheric CO₂) coincided with decadal maximum temperatures. Correlations between δ¹³C signatures and TRW (mainly affected by precipitation) were not significant. This finding suggests that wood growth often was disconnected from carbon assimilation (e.g. due to carbon storage in the trunk or allocation to seeds). Since the selection of drought-resistant tree species gains importance within the context of adaptive forest management strategies, Q. petraea proves to be an adaptive tree species in Central Europe’s forests under shifting climatic conditions.     

中国北方蒙古栎林起源和发展的初步探讨

古生态学的研究显示 ,栎林 (包括蒙古栎林 )在华北地区和东北地区的分布在地质历史时期不是一成不变的 ,其优势度也呈现上下波动 ,栎树优势度的增减是由于地质历史时期气候的变化造成的 ;蒙古栎的起源时间可能不早于上新世中期 ,蒙古栎林的起源也应不早于上新世中期。蒙古栎林的起源原因有 2种 ,即火与人类的反复砍伐。火是原生性质蒙古栎林在地质历史时期存在和扩散的根本原因 ,过度的人类活动 (主要是反复砍伐 )是促成现在蒙古栎次生林占优势地位的主要原因 ,这是由蒙古栎这个树种的生物学特性决定的 ,在没有外界因素的干扰和火减少的情况下 ,蒙古栎在演替过程中逐渐被耐荫的树种所取代 ,但是在中国北方 ,由于人类活动的加剧 ,蒙古栎林的优势度有增加的趋势。