Species-Specific Growth Responses to Climate Variations in Understory Trees of a Central African Rain Forest

Basic knowledge of the relationships between tree growth and environmental variables is crucial for understanding forest dynamics and predicting vegetation responses to climate variations. Trees growing in tropical areas with a clear seasonality in rainfall often form annual growth rings. In the understory, however, tree growth is supposed to be mainly affected by interference for access to light and other resources. In the semi-deciduous Mayombe forest of the Democratic Republic of Congo, the evergreen species Aidia ochroleuca, Corynanthe paniculata and Xylopia wilwerthii dominate the understory. We studied their wood to determine whether they form annual growth rings in response to changing climate conditions. Distinct growth rings were proved to be annual and triggered by a common external factor for the three species. Species-specific site chronologies were thus constructed from the cross-dated individual growth-ring series. Correlation analysis with climatic variables revealed that annual radial stem growth is positively related to precipitation during the rainy season but at different months. The growth was found to associate with precipitation during the early rainy season for Aidia but at the end of the rainy season for Corynanthe and Xylopia. Our results suggest that a dendrochronological approach allows the understanding of climate–growth relationships in tropical forests, not only for canopy trees but also for evergreen understory species and thus arguably for the whole tree community. Global climate change influences climatic seasonality in tropical forest areas, which is likely to result in differential responses across species with a possible effect on forest composition over time.     

Role of nurse plants in Araucaria Forest expansion over grassland in south Brazil

Abstract Araucaria Forest expansion over grassland takes place under wet climate conditions and low disturbance and it is hypothesized that isolated trees established on grassland facilitate the establishment of forest woody species beneath their canopies. Forest with Araucaria angustifolia is a particular type of Brazilian Atlantic Forest and the main forest type on the highland plateau in south Brazil, often forming mosaics with natural Campos grassland. The objectives of this paper were to evaluate the role of isolated shrubs and trees as colonization sites for seedlings of Araucaria Forest woody species on grassland, to determine which species function as preferential nurse plants in the process and the importance of vertebrate diaspore dispersal on the structure of seedling communities beneath nurse plants. The study was conducted in São Francisco de Paula, Rio Grande do Sul State, where we sampled isolated shrubs and trees in natural grassland near Araucaria Forest edges. Seedlings were counted and identified, and seedling diaspore dispersal syndromes, size and colour were registered. We detected 11 woody species with a potential role in nucleating grassland colonization by forest species. Beneath the canopies of nurse plants more forest species seedlings were found compared with open field grassland and the seedlings had diaspores mostly dispersed by vertebrates. Also, more seedlings were found under the canopy of A. angustifolia than beneath other nurse plant species. We conclude that A. angustifolia trees established on grassland act as nurse plants, by attracting disperser birds that promote colonization of the site by other forest species seedlings, and that under low level of grassland disturbance, conservation of frugivorous vertebrate assemblages may increase forest expansion over natural grassland and also facilitate the regeneration of degraded forest areas.     

Effect of climate on the growth of annual rings in the main roots of perennial forbs in an Inner Mongolian semi‐arid grassland,China

Question: Is there a pattern in growth of annual rings in roots of perennial forbs in relation to climate and climate extremes in grassland ecosystems? Location: Semi‐arid grassland in Duolun (42°27′N, 116°41′E, 1380 m a.s.l.), central Inner Mongolia, China. Methods: Main roots of three perennial species, Potentilla anserina L., Cymbaria dahurica L. and Lespedeza daurica Schindl., were sampled. Cross‐sections (10–15‐μm thick) were produced from the proximal end of sampled roots using a sledge microtome. Annual growth rings in the main roots were identified and measured by differentiating between earlywood and latewood in the secondary xylem. Relationships between annual growth rings and monthly mean temperature and total monthly precipitation were identified using correlation analysis. Differences in an annual ring width to the previous and following years were examined by calculating a distinctness score. Results: The three perennial forbs showed clearly demarcated annual growth rings in all individuals and the same fluctuation patterns. Their ring widths were generally positively correlated with precipitation from April to October (except for August) and with temperature from February to June (except June for L. daurica), September to October, and the annual mean. Strong deviations of annual ring widths from their neighbour rings were observed in 1998 and 2000. The trend of absolute distinctness scores (D_m) increased significantly from 1988 to 2003, indicating an increase in the frequency of annual ring width variation. Conclusions: Annual growth rings in the main roots of three perennial forb species can be used as an indicator of the influence of climate on below‐ground grassland growth. The change in below‐ground conditions and effects on the functioning of grassland should receive more attention in future studies.     

Annual Growth Ring Patterns in Brachystegia spiciformis Reveal Influence of Precipitation on Tree Growth1

The availability of exactly dated tree‐ring chronologies is limited in tropical regions. However, these chronologies could contribute widely to studies of the influence of natural and human‐induced factors on tropical forests. We examine the potential for building a chronology based on three sites in the miombo woodland of western Zambia. Brachystegia spiciformis Benth., a dominant species from this vegetation type, is used. Response of the chronology to several climatic factors is examined. All specimens showed very clear growth rings, and cross‐dating between radii of a tree was successful for all trees. Site chronologies could be constructed after cross‐dating of growth ring series of individual trees. The mean growth ring curves of the three sites were significantly similar, allowing for the construction of a regional chronology. Correlation function analysis between the tree‐ring chronology and regional climatic variables revealed that climate at the core of the rainy season, in December and January, has an explicit influence on tree growth. Where precipitation and relative humidity in these months influence tree growth positively, temperature correlates in a negative way. Some 20 percent of the variance in the B. spiciformis tree‐ring chronology is accounted for by wet season rainfall. The successful cross‐dating and correlation between a tree‐ring chronology and climate demonstrated in this study indicate annual ring formation in B. spiciformis trees and sensitivity to climatic conditions.     

Age and Long-term Growth of Trees in an Old-growth Tropical Rain Forest, Based on Analyses of Tree Rings and 14C1

In an old‐growth tropical wet forest at La Selva, Costa Rica, we combined radiocarbon (¹⁴C) dating and tree‐ring analysis to estimate the ages of large trees of canopy and emergent species spanning a broad range of wood densities and growth rates. We collected samples from the trunks of 29 fallen, dead individuals. We found that all eight sampled species formed visible growth rings, which varied considerably in distinctiveness. For five of the six species for which we combined wood anatomical studies with ¹⁴C‐dates (ring ages), the analyses demonstrated that growth rings were of annual formation. The oldest tree we found by direct ring counting was a Hymenolobium mesoamericanum Lima (Papilionaceae) specimen, with an age of ca. 530 years at the time of death. All other sampled individuals, including very large trees of slow‐growing species, had died at ages between 200 and 300 years. These results show that, even in an everwet tropical rain forest, tree growth of many species can be rhythmic, with an annual periodicity. This study thus raises the possibility of extending tree‐ring analyses throughout the tropical forest types lacking a strong dry season or annual flooding. Our findings and similar measurements from other tropical forests indicate that the maximum ages of tropical emergent trees are unlikely to be much greater than 600 years, and that these trees often die earlier from various natural causes.     

Taxonomy,together with ontogeny and growing conditions,drives needleleaf species' sensitivity to climate in boreal North America

Currently, there is no consensus regarding the way that changes in climate will affect boreal forest growth, where warming is occurring faster than in other biomes. Some studies suggest negative effects due to drought‐induced stresses, while others provide evidence of increased growth rates due to a longer growing season. Studies focusing on the effects of environmental conditions on growth–climate relationships are usually limited to small sampling areas that do not encompass the full range of environmental conditions; therefore, they only provide a limited understanding of the processes at play. Here, we studied how environmental conditions and ontogeny modulated growth trends and growth–climate relationships of black spruce (Picea mariana) and jack pine (Pinus banksiana) using an extensive dataset from a forest inventory network. We quantified the long‐term growth trends at the stand scale, based on analysis of the absolutely dated ring‐width measurements of 2,266 trees. We assessed the relationship between annual growth rates and seasonal climate variables and evaluated the effects of various explanatory variables on long‐term growth trends and growth–climate relationships. Both growth trends and growth–climate relationships were species‐specific and spatially heterogeneous. While the growth of jack pine barely increased during the study period, we observed a growth decline for black spruce which was more pronounced for older stands. This decline was likely due to a negative balance between direct growth gains induced by improved photosynthesis during hotter‐than‐average growing conditions in early summers and the loss of growth occurring the following year due to the indirect effects of late‐summer heat waves on accumulation of carbon reserves. For stands at the high end of our elevational gradient, frost damage during milder‐than‐average springs could act as an additional growth stressor. Competition and soil conditions also modified climate sensitivity, which suggests that effects of climate change will be highly heterogeneous across the boreal biome.     

Age-specific responses to climate identified in the growth of Quercus alba

The objective of this research was to determine whether the dendroclimatic responses of young Quercus alba (aged 29–126 years) differ from those of old Q. alba (149–312 years). We collected Q. alba increment cores across a range of size classes from Buffalo Mountain Natural Area Preserve, an oak-hickory forest in southcentral Virginia, USA. Tree cores were crossdated and raw ring widths were detrended to remove the influence of increasing circumference with age, microsite, and local stand dynamics. Standardized ring widths were averaged to develop two master chronologies from the 20 oldest and youngest trees. Ring-width indices were correlated with temperature, precipitation, and Palmer Drought Severity Index (PDSI). Annual tree-ring growth in old and young Q. alba was significantly correlated with precipitation from the previous growing season, but was not significantly correlated with temperature. Only the old trees showed a significant correlation between annual ring width and PDSI. These results may indicate that growth in old trees is more sensitive to drought than in young trees. If future climate change includes the predicted increase in mid-growing season droughts, tree-level responses are likely to be age-dependent with older trees experiencing relatively greater reductions in growth.     

Dendrochronological analyses and climatic signals of Alchornea triplinervia in subtropical forest of southern Brazil

The predominance of secondary forest‐species in Brazilian subtropical forests highlights the importance of understanding the ecology of these taxa, and dendrochronology provides valuable information about the growth and climate response of tree species. The wide distribution of Alchornea triplinervia (Spreng.) Mull. Arg. (Euphorbiaceae), and the presence of growth rings in its wood, leds to its selection for this study. Samples were collected from 34 trees growing in rainy dense forest fragments in the cities of Blumenau and Brusque, in southern Brazil, and subjected to standard dendrochronological techniques. The local chronology spanned from 1889 to 2013 and the age of the trees ranged from 32 to 125 years. The mean age was 73 years and annual increment was 1.13 mm year^?1; the diameter and age showed a weak correlation. The results suggest that A. triplinervia growth responded negatively to hot summers and increased rainfall in the previous spring and winter. The high perdiodicity trend characteristic of ENSO activity was reflected in wavelet power spectra of A. triplinervia chronology, affecting positively its growth after strong ENSO events (1970–2009). Negative influences occurred before 1970 during normal ENSO events. Also, there is evidence of the existence of a connection between Pacific and Atlantic sea surface temperatures influencing on tree growth.     

Spatial and temporal variation in Nothofagus pumilio growth at tree line along its latitudinal range (35°40'–55° S) in the Chilean Andes

Aim To identify the dominant spatial and temporal patterns of Nothofagus pumilio radial growth over its entire latitudinal range in Chile, and to find how these patterns relate to temperature and precipitation variation from instrumental records. Location This study comprises 48 tree line or high elevation N. pumilio sites in the Chilean Andes between 35° 36′ and 55° S. Nothofagus pumilio is a deciduous tree species that dominates the upper tree line of the Chilean and Argentinean Andes in this latitudinal range. Methods At each of the sampled sites, two cores from 15 to 40 living trees were collected using increment borers. Cores were processed, tree rings were measured and cross‐dated, using standard dendrochronological procedures. Radii from nearby sites were grouped into 13 study regions. A composite tree‐ring width chronology was developed for each region in order to capture and integrate the common growth patterns. For the identification of the dominant patterns of growth, as well as temperature and precipitation variation, we used principal components (PCs) analysis. Correlation analysis was used for the study of the relationship of N. pumilio tree‐ring growth with temperature and precipitation records. Results Nothofagus pumilio tree line elevation is 1600 m in the northernmost region and gradually decreases to 400 m in the southernmost region. Despite local differences along the transect, the decrease in tree line elevation is fairly constant, averaging c. 60 m per degree of latitude (111 km). Tree growth at the northernmost regions shows a positive correlation with annual precipitation (PC1‐prec) and negative correlation with mean annual temperature (PC2‐temp), under a Mediterranean‐type climate where water availability is a major limiting factor. Conversely, tree growth is positively correlated with mean annual temperature (PC1‐temp) in the southern portion of the gradient, under a relatively cooler climate with little seasonality in precipitation. Main conclusions Our findings indicate that temperature has a spatially larger control of N. pumilio growth than precipitation, as indicated by a significant (P < 0.05) either positive or negative correlation of tree growth and PC1‐temp and/or PC2‐temp for nine of the 13 regional chronologies (69.2% of the total), whereas precipitation is significantly correlated with only two chronologies (15.4% of the total). Temporal patterns of N. pumilio tree growth reflected in PC1‐growth for the period between 1778 and 1996 indicate an increasing trend with above the mean values after 1963, showing high loadings in the southern part of the gradient. This trend may be explained by a well‐documented increase in temperature in southern Patagonia. Ongoing and future research on N. pumilio growth patterns and their relationship to climate covering the Chilean and Argentinean Andes will improve the understanding of long‐term climate fluctuations of the last three to four centuries, and their relationship to global change at a wide range of spatial and temporal scales.     

Identification of anatomically non-distinct annual rings in tropical trees using stable isotopes

Annual rings are generally not anatomically distinct in trees growing in the humid tropics. The possibility to use radial variation in stable isotopes (δ¹⁸O and δ¹³C) for the identification of annual rings in these trees was investigated in two species growing in the tropical rainforest of Central Guyana, Carapa guianensis and Goupia glabra. The climate is characterised by an annual precipitation of 2,700 mm that is distributed over two rainy and two dry seasons. Cores were taken from trees of measured diameter increment rates. High-resolution tangential sections in radial direction were dissected from these cores and isotopic ratios were measured on whole wood. Variation in δ¹³C was about 1‰ at an annual scale, whereas δ¹⁸O showed two to four times larger annual excursions. The minima in δ¹⁸O were selected as primary indicators of annual boundaries at the main wet season when also δ¹⁸O of precipitation water has its minimum. The minima in δ¹³C coincided often with these. The simultaneous occurrence is consistent with increased discrimination against ¹³C at high water availability. They were used as secondary criteria. Annual rings could thus be identified with reasonable certainty in both species from radial variation in isotopic ratios as verified with measured diameter increment rates. The short sequence covered in the analysis did not show clear correlation with the available precipitation data for the area. The method supplemented with other dating methods may prove to be practically useful for identifying annual rings and applying classical dendrochronology when more cost effective automatic sampling devices become available.     

The effect of missing rings on stand-age estimation of even-aged forests in northern Hokkaido,Japan

The effectiveness of estimating stand age from increment core samples taken at stump height (20 cm above ground) was tested in an even-aged stand ofBetula ermanii that had regenerated after a forest fire in 1945 in Hokkaido, Japan. Careful cross-dating revealed that annual rings were missing in 11 cores out of a total of 42 cores sampled, and that all these missing rings occurred in the outermost part of the core. These facts indicate that precaution has to be taken in selecting trees from which cores are to be sampled. The present work also revealed that those trees with missing rings have a characteristic appearance, with a thinner stem and less crown foliage than normal trees with complete rings. It was also found that even if this appearance test failed, the possibility of missing rings could be detected from a declining growth pattern, with extremely narrow rings on the increment core which normal trees did not show.     

Predicting tree biomass growth in the temperate–boreal ecotone: Is tree size,age, competition,or climate response most important?

As global temperatures rise, variation in annual climate is also changing, with unknown consequences for forest biomes. Growing forests have the ability to capture atmospheric CO₂ and thereby slow rising CO₂ concentrations. Forests’ ongoing ability to sequester C depends on how tree communities respond to changes in climate variation. Much of what we know about tree and forest response to climate variation comes from tree‐ring records. Yet typical tree‐ring datasets and models do not capture the diversity of climate responses that exist within and among trees and species. We address this issue using a model that estimates individual tree response to climate variables while accounting for variation in individuals’ size, age, competitive status, and spatially structured latent covariates. Our model allows for inference about variance within and among species. We quantify how variables influence aboveground biomass growth of individual trees from a representative sample of 15 northern or southern tree species growing in a transition zone between boreal and temperate biomes. Individual trees varied in their growth response to fluctuating mean annual temperature and summer moisture stress. The variation among individuals within a species was wider than mean differences among species. The effects of mean temperature and summer moisture stress interacted, such that warm years produced positive responses to summer moisture availability and cool years produced negative responses. As climate models project significant increases in annual temperatures, growth of species like Acer saccharum, Quercus rubra, and Picea glauca will vary more in response to summer moisture stress than in the past. The magnitude of biomass growth variation in response to annual climate was 92–95% smaller than responses to tree size and age. This means that measuring or predicting the physical structure of current and future forests could tell us more about future C dynamics than growth responses related to climate change alone.     

Seasonal cambium activity in the subtropical rain forest tree Araucaria angustifolia

Information on the timing and dynamics of tree ring formation is essential to assess the seasonal behavior of secondary wood growth and its associated environmental influences. Araucaria angustifolia is a dominant species in highland pluvial ecosystems of southeastern South America. Previous investigations indicated that their growth rings are formed annually, but no information exists about the timing of growth ring formation and the environmental triggers influencing cambium activity. In this paper we examine inter- and intra-annual cambial activity in A. angustifolia, through anatomical and dendrochronological evidence at two study sites, and model the relationships between regional climate variation and intra-annual tree ring formation. The results confirm the annual growth ring formation in A. angustifolia and indicate that its growth season extends from October to April. Day length and temperature were the main environmental factors influencing the seasonal cambium activity. Our results evidence the dendrochronological potential of A. angustifolia for ecological and climatological studies in southeastern South America.     

Annual tree rings revealed by radiocarbon dating in seasonally flooded forest of the Mapire River,a tributary of the lower Orinoco River,Venezuela

The occurrence of seasonal growth rings in the wood of Campsiandra laurifolia, Acosmiun nitens, Pouteria orinocoensis and Psidium ovatifolium, common species growing in the flooding forest of the Mapire river, was analyzed using wood anatomy and ring- width analysis. The test of the annual ring formation was performed using radiocarbon analysis based on the nuclear weapon effect. All species showed growth rings visible to the naked eye. The ring boundaries in all cases were marked by bands of marginal parenchyma. The index ring-width curves of the four studied species showed a strong relationship with the fluctuation of the water river level during the non flooded months, suggesting that an increase in the water level during these months positively influenced the growth indicating that the rings were formed on an annual basis. The content of radiocarbon in the wood of anatomically predated rings of Campsiandra laurifolia and Pouteria orinocoensis confirm these results. All studied trees are slow growing with less than 2.5 mm annual increment.     

Detecting invisible growth rings of trees in seasonally dry forests in Thailand: isotopic and wood anatomical approaches

We measured radial variation of carbon isotope composition and vessel traits in tree species in seasonally dry forests of Northeast Thailand to explore a more reliable and amenable method of tropical dendrochronology for trees that lack visually detectable and consistent growth rings. Six Dipterocarpaceae species (3 Shorea, 2 Dipterocarpus, and 1 Hopea species) with indistinct or irregular growth rings and teak (Tectona grandis), a species which forms distinct growth rings, were examined. The δ¹³C value variations in all species showed annual cyclicity. Dipterocarpaceae species usually marked the lowest values of δ¹³C in the middle of the growing season, whereas teak had the lowest values at nearly the end of the growing season. Since the growing season of the species examined almost corresponds to the rainy season in the study area, the δ¹³C variation was likely caused by the change in moisture availability. The different variation pattern of teak was attributable to its stronger dependence on ¹³C-enriched reserved material early in the growing season. Changes in tree vessel traits for all species examined also showed annual cyclicity. Dipterocarpaceae species showed significant correlation between δ¹³C values and vessel measurements. Vessel lumen (mean area, tangential and radial diameter, and proportion of total area) had a negative correlation, whereas vessel frequency showed a positive correlation. The correlations indicated that changes in vessel traits were caused by the seasonal variation of moisture available to the trees. Thus, we concluded that methods using wood anatomy, as well as δ¹³C, have great potential for use as tools in tropical dendrochronology within the context of seasonal climate.     

Periodicity of growth rings in Juniperus procera from Ethiopia inferred from crossdating and radiocarbon dating

African pencil cedar (Juniperus procera Hochst. ex Endlicher 1847) is a tropical, irregularly growing species that can produce annual growth rings in response to an annual cycle of wet and dry seasons. In this paper, we assess the periodicity of growth-ring formation for 13 stem discs from a site in Central-Northern Ethiopia by crossdating and radiocarbon dating. The crossdating process is described more transparently than usual to allow open discussion of the methodology employed. Although the ring-width series could be tentatively matched, radiocarbon dating revealed that the growth rings of the junipers from the studied site are neither annual nor represent a common periodicity. It was found that the trees are exceptionally sensitive and respond individually to the complex local climate. For future research, it is recommended to select more mesic sites with an unambiguously unimodal rainfall regime and to gain external evidence to support assumptions about the periodicity of growth-ring formation in Juniperus procera.     

Variation in stem radial growth of the Australian conifer, Callitris columellaris, across the world’s driest and least fertile vegetated continent

Climate change could alter the biogeography of many tree species. However, there have been few studies of tree growth across climatic gradients at a continental scale. Callitris columellaris is a widespread conifer that spans many climates and landscape positions across Australia. Our aim was to determine how stem radial growth of C. columellaris varies with tree size and with the biogeographic factors of rainfall, temperature, soil fertility and inter-tree competition. We sampled cores from trees at 85 sites in biomes ranging from tropical savanna to arid desert and temperate forest, and measured widths of the 100 outermost growth rings. We analysed ring width in relation to changes in tree age and diameter, and also evaluated the influence of the biogeographic factors on the width of the ten most recently formed rings. The average width of outermost rings varied only slightly with stem diameter, because the decrease in ring width with age and diameter within trees is offset by an increase with diameter among trees. Our analyses thus explain the weak, inconsistent relationships often observed between stem diameter and growth rate amongst trees. The most important biogeographic factors were the climatic ones: across Australia, ring width increased with both mean annual rainfall and mean annual temperature. These relationships were largely driven by continental scale differences between the tropical and the southern (arid plus temperate) sites, while relationships within climate zones were comparatively weak. Ring width decreased with intense inter-tree competition but showed little correlation with available soil nitrogen or phosphorus.     

Nonannual tree rings in a climate‐sensitive Prioria copaifera chronology in the Atrato River,Colombia

In temperate climates, tree growth dormancy usually ensures the annual nature of tree rings, but in tropical environments, determination of annual periodicity can be more complex. The purposes of the work are as follows: (1) to generate a reliable tree‐ring width chronology for Prioria copaifera Griseb. (Leguminoceae), a tropical tree species dwelling in the Atrato River floodplains, Colombia; (2) to assess the climate signal recorded by the tree‐ring records; and (3) to validate the annual periodicity of the tree rings using independent methods. We used standard dendrochronological procedures to generate the P. copaifera tree‐ring chronology. We used Pearson correlations to evaluate the relationship of the chronology with the meteorological records, climate regional indices, and gridded precipitation/sea surface temperature products. We also evaluated 24 high‐precision ¹⁴C measurements spread over a range of preselected tree rings, with assigned calendar years by dendrochronological techniques, before and after the bomb spike in order to validate the annual nature of the tree rings. The tree‐ring width chronology was statistically reliable, and it correlated significantly with local records of annual and October–December (OND) streamflow and precipitation across the upper river watershed (positive), and OND temperature (negative). It was also significantly related to the Oceanic Niño Index, Pacific Decadal Oscillation, and the Southern Oscillation Index, as well as sea surface temperatures over the Caribbean and the Pacific region. However, ¹⁴C high‐precision measurements over the tree rings demonstrated offsets of up to 40 years that indicate that P. copaifera can produce more than one ring in certain years. Results derived from the strongest climate–growth relationship during the most recent years of the record suggest that the climatic signal reported may be due to the presence of annual rings in some of those trees in recent years. Our study alerts about the risk of applying dendrochronology in species with challenging anatomical features defining tree rings, commonly found in the tropics, without an independent validation of annual periodicity of tree rings. High‐precision ¹⁴C measurements in multiple trees are a useful method to validate the identification of annual tree rings.     

天山东部西伯利亚落叶松树轮生长对气候要素的响应分析

天山东部西伯利亚落叶松的树木年轮学研究可以看出：森林上限树轮宽度年表之间相关性较高而下限年表间相关稍低,表明下限小生境要素对树木生长干扰较大。森林上下限树轮年表中样本的总解释量（ESP）和信噪比（SNR）都比较高,说明树木中都含有较多的环境信息;但标准年表中平均敏感度（M．S．）和轮宽指数的标准差（S．D．）都是森林上限数值低于下限,这表明森林上限树木生长对环境变化响应的敏感性降低;相关分析和响应分析也发现森林下限生长的树木对气候因子的响应较为显著。就温度而言,森林上限和下限表现基本一致,树木生长多与温度负相关,其中下限树木生长与春季均温和3．6月份均温显著负相关;降水表现出一定的差别,上限树木生长与春季、夏季及年降水量有较高的负相关,而对下限树木生长影响最大的则是冬季和3—6月份降水。湿润指数与降水基本一致即上限呈负相关而下限正相关,温暖指数全为负相关,寒冷指数下限负相关显著;显然该地区森林上下限树木生长的生态模式存在着一定的差异。研究发现,冬春季节的不同水热组合则是形成树木年轮宽窄的限制因素;同时,前期生长的滞后效应对年轮形成有重要的影响。     

芦芽山华北落叶松(Larix principis-rupprechtii)树轮宽度年表对气候因子的响应

在芦芽山地区采集3个不同海拔的华北落叶松(Larix principis-rupprechtii),在传统去趋势的基础上,采用"signal-free"方法对拟合曲线进行修正,避免了中等频率的气候信息引起的拟合偏差,最终建立3个不同海拔树轮宽度标准年表(STD)。同时以10a为界对上述年表进行滤波处理,得到3个低频年表。年表特征值表明,随着海拔升高,年轮平均轮宽变窄,敏感性和高频信息增强,低频信息减弱,这可能与逐渐恶劣的生境有关。中、低海拔年表的低频信息更一致,中、高海拔的高频信息更接近,而高、低海拔无论是标准年表还是高频、低频年表相似性均较差。树轮气候响应分析显示,低海拔STD年表与5月最低温负相关最为显著,STD和低频年表均与5、6月份土壤温度显著负相关,说明生境暖干,树木主要受生长季的干旱胁迫;中海拔STD年表与当年5月最高温正相关最为显著,STD和低频年表与土壤温度相关均不显著,说明生境逐渐变得冷湿,生长季的低温成为树木生长的限制因子;高海拔STD年表与气象要素相关不显著,低频年表与当年4月土壤温度正相关,说明高海拔最为冷湿,并有季节性冻土分布,生长季的土壤低温成为树木生长的限制因子。因此,全球变暖的趋势将更有利于高海拔树木的生长,而低海拔树木的干旱胁迫进一步加剧。