Competition and facilitation between tree species change with stand development

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability.     

Mechanical Resistance of Different Tree Species to Rockfall in the French Alps

In order to determine the mechanical resistance of several forest tree species to rockfall, an inventory of the type of damage sustained in an active rockfall corridor was carried out in the French Alps. The diameter, spatial position and type of damage incurred were measured in 423 trees. Only 5% of trees had sustained damage above a height of 1.3 m and in damaged trees, 66% of broken or uprooted trees were conifers. Larger trees were more likely to be wounded or dead than smaller trees, although the size of the wounds was relatively smaller in larger trees. The species with the least proportion of damage through stem breakage, uprooting or wounding was European beech (Fagus sylvatica L.). Winching tests were carried out on two conifer species, Norway spruce (Picea abies L.) and Silver fir (Abies alba Mill.), as well as European beech, in order to verify the hypothesis that beech was highly resistant to rockfall and that conifers were more susceptible to uprooting or stem breakage. Nineteen trees were winched downhill and the force necessary to cause failure was measured. The energy (E _fail) required to break or uproot a tree was then calculated. Most Silver fir trees failed in the stem and Norway spruce usually failed through uprooting. European beech was either uprooted or broke in the stem and was twice as resistant to failure as Silver fir, and three times more resistant than Norway spruce. E _fail was strongly related to stem diameter in European beech only, and was significantly higher in this species compared to Norway spruce. Results suggest that European beech would be a better species to plant with regards to protection against rockfall. Nevertheless, all types of different abiotic stresses on any particular alpine site should be considered by the forest manager, as planting only broadleaf species may compromise the protecting capacity of the forest e.g. in the case of snow avalanches.     

Detecting snow-related signals in radial growth of Pinus uncinata mountain forests

Climate warming is responsible for observed reduction in snowpack depth and an earlier and faster melt-out in many mountains of the Northern Hemisphere. Such changes in mountain hydroclimate could negatively affect productivity and tree growth in high-elevation forests, but few studies have investigated how and where recent warming trends and changes in snow cover influence forest growth. A network comprising 36 high-elevation Pinus uncinata forests was sampled in the NE Iberian Peninsula, mainly across the Spanish Pyrenees, using dendrochronology to relate tree radial growth to a detailed air temperature and snow depth data. Radial growth was negatively influenced by a longer winter snow season and a higher late-spring snowpack depth. Notably, the effect of snow on tree growth was found regardless the widely reported positive effect of growing-season air temperatures on P. uncinata growth. No positive influence of moisture from spring snowmelt on annual growth of P. uncinata was detected in sampled forests. Tall trees showed a lower growth responsiveness to snow than small trees. Decreasing trends in winter and spring snow depths were detected at most Pyrenean forests, suggesting that the growth of high-elevation P. uncinata forests can beneficiate for a shallower and of shorter duration snowpack associated with warmer conditions. However, water-limited sites located on steep slopes or on rocky substrates, with poor soil-water holding capacity, could experience drought stress because of early depleted snow-related soil moisture.     

Stability in the patterns of long‐term development and growth of the Canadian spruce–moss forest

Aim The spruce–moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long‐term development and growth of the spruce–moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re‐establishment of black spruce (Picea mariana) trees. Location The study area occurs in Québec, Canada, and extends from 70°00′ to 72°00′ W and 47°30′ to 56°00′ N. Methods A spatial inventory of spruce–moss forest stands was performed along 34 transects. Nineteen spruce–moss forests were selected. A 500 m² quadrat at each site was used for radiocarbon and tree‐ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co‐dominant tree species [black spruce and balsam fir (Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp . Forests < 325 cal. yr bp are dominated by trees of the first post‐fire cohort and are not yet at equilibrium, whereas older forests show a reverse‐J diameter distribution typical of mature, old‐growth stands. The younger forests display faster height and radial growth‐rate patterns than the older forests, due to factors associated with long‐term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce–moss forests are able to self‐regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325–4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce–moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type.     

Treeline advances along the Urals mountain range – driven by improved winter conditions?

High‐altitude treelines are temperature‐limited vegetation boundaries, but little quantitative evidence exists about the impact of climate change on treelines in untouched areas of Russia. Here, we estimated how forest‐tundra ecotones have changed during the last century along the Ural mountains. In the South, North, Sub‐Polar, and Polar Urals, we compared 450 historical and recent photographs and determined the ages of 11 100 trees along 16 altitudinal gradients. In these four regions, boundaries of open and closed forests (crown covers above 20% and 40%) expanded upwards by 4 to 8 m in altitude per decade. Results strongly suggest that snow was an important driver for these forest advances: (i) Winter precipitation has increased substantially throughout the Urals (~7 mm decade^?1), which corresponds to almost a doubling in the Polar Urals, while summer temperatures have only changed slightly (~0.05 °C decade^?1). (ii) There was a positive correlation between canopy cover, snow height and soil temperatures, suggesting that an increasing canopy cover promotes snow accumulation and, hence, a more favorable microclimate. (iii) Tree age analysis showed that forest expansion mainly began around the year 1900 on concave wind‐sheltered slopes with thick snow covers, while it started in the 1950s and 1970s on slopes with shallower snow covers. (iv) During the 20th century, dominant growth forms of trees have changed from multistemmed trees, resulting from harsh winter conditions, to single‐stemmed trees. While 87%, 31%, and 93% of stems appearing before 1950 were from multistemmed trees in the South, North and Polar Urals, more than 95% of the younger trees had a single stem. Currently, there is a high density of seedlings and saplings in the forest‐tundra ecotone, indicating that forest expansion is ongoing and that alpine tundra vegetation will disappear from most mountains of the South and North Urals where treeline is already close to the highest peaks.     

Morphological plasticity of regeneration subject to different levels of canopy cover in mixed-species, multiaged forests of the Romanian Carpathians

Morphological plasticity was studied for advanced regeneration trees in different light environments of the mountainous, mixed-species forests in the Carpathian Mountains of Romania. The primary species in these mixtures were very shade tolerant silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.), and midtolerant Norway spruce (Picea abies (L.) Karst). Seedlings/saplings of these species were selected for measurements in different stands from two different geographical locations. Various morphological traits (specific leaf area, live crown ratio, crown width to length ratio, terminal to lateral ratio, number of internodal shoots, number of shoots in terminal whorl, stem symmetry, stem orientation, stem forking) for each regenerating tree were measured during summers of 2001 and 2002. Percentage of above canopy light and stand basal area measures were used to assess the available growing space for each seedling/sapling. Regression relationships were developed for the different morphological indicators as a function of these two variables. All species adapted their morphology along the gradient in light and basal area. Spruce seemed to be less adapted to low light conditions than both fir and beech. However, no significant differences in terms of shade tolerance were detected using the above indicators. In really dense stand conditions (less than 20% above canopy light and stand basal area above 36 m² ha⁻¹), probability for stem forking in beech increased. In open, all three species adapted their morphology for vigorous growth. Under such conditions, spruce was better adapted than fir.     

Spruce-fir growth form changes in the forest-tundra ecotone of Rocky Mountain National Park, Colorado, USA

Tree regeneration has traditionally been used as a measure of the response of treeline to climate Changes in growth form of krummholz trees may also indicate whether treeline is responding to changes in climate The purpose of this study was to determine whether krummholz trees m the forest-tundra ecotone of Rocky Mountain National Park, Colorado have experienced significant vertical stem growth, in the absence of mortality, and if this growth occurred in response to recent changes in climate We sampled and dated Engelmann spruce and subalpine fir krummholz leaders stratified by height class at three sampling locations to determine the dates leaders initiated growth above mean snow depth At one sampling location, 215 additional leaders were sampled to construct an age structure of leader release dates Dates of leader release taken from the age structure were compared with seasonal temperatures, seasonal precipitation, winter snow depths, and annual runoff using t-tests Dates of leader release were also compared to proxy climate records for the southern Rocky Mountain region Based on historical photos as well as the data presented here, both spruce and fir krummholz trees experienced significant height growth as early as the 1850's and continued to grow vertically, at least through the 1970's This vertical stem growth occurred in the absence of significant mortality Running mean annual temperature and May snow depth are both positively associated with years of leader release, suggesting that a warmer, wetter climate, possibly following the end of the Little Ice Age ca 1850, may have induced these changes in the ecotone     

Changes in crown development patterns and current-year shoot structure with light environment and tree height in Fagus crenata (Fagaceae)

The relative effects of light and tree height on the architecture of leader crowns (i.e., the leading section of the main trunk, 100 cm in length) and current-year shoots for a canopy species, Fagus crenata, occupying both the ridge top and the valley bottom in a cool-temperate forest in Japan were investigated. For leader crowns, the number of current-year shoots and leaves increased with increasing tree height, whereas the mean length of current-year shoots increased with increasing relative photon flux density (PFD). The leader crown area decreased, and the depth and leaf area index of leader crowns increased, with increasing relative PFD. The mass of current-year shoots increased with relative PFD. However, this total mass was allocated differently between stems and leaves depending on tree height, such that the relative allocation to stems increased with increasing tree height. Furthermore, stem structures within current-year shoots also changed with height, such that taller trees produced thicker and shorter stems of the same volume. In contrast, leaf structure and leaf biomass allocations changed with relative PFD. Specific leaf area decreased with increasing relative PFD. In addition, leaf number increased more rapidly with increasing individual leaf mass for trees exposed to greater relative PFD. Consequently, the total leaf area supported by a stem of a given diameter decreased with increasing tree height and relative PFD. Thus, the architecture of leader crowns and current-year shoots were related differently to light and tree height, which are considered important for efficient light capture and the growth of small and tall trees in different environments.     

Biases in RCS tree ring chronologies due to sampling heights of trees

The Regional Curve Standardization (RCS) is one of the most employed standardization methods to remove biological signals in long tree ring chronologies. The approach assumes that an overall age-related growth trend typify all tree ring series to be included in a standardized tree ring chronology. Although several potential problems of the method have been examined, the influence of varying the sampling height along tree stems has not been evaluated. Considering that age-related growth trends may vary with stem height, biases may arise when combining samples from unknown or variable sampling heights, a frequent situation with subfossil logs. In this study we perform a detailed stem analysis of 15 lakeshore black spruce (Picea mariana Mill. B.S.P.) trees in the taiga of eastern Canada to describe how the age-related growth trend varies with stem height and evaluate associated biases in RCS chronologies built from living and subfossil trees. Results show that the age-related growth trends vary markedly and systematically along stems, potentially generating large methodological biases in RCS chronologies, especially near the recent chronology end. These biases may lead to erroneous reconstructions of recent climatic trends and cause false divergence between tree ring and climate series. We have developed a correction procedure that appears efficient in removing these biases from chronologies built with the lakeshore trees and associated subfossil logs.     

Influence of experimental snow removal on root and canopy physiology of sugar maple trees in a northern hardwood forest

Due to projected increases in winter air temperatures in the northeastern USA over the next 100 years, the snowpack is expected to decrease in depth and duration, thereby increasing soil exposure to freezing air temperatures. To evaluate the potential physiological responses of sugar maple (Acer saccharum Marsh.) to a reduced snowpack, we measured root injury, foliar cation and carbohydrate concentrations, woody shoot carbohydrate levels, and terminal woody shoot lengths of trees in a snow manipulation experiment in New Hampshire, USA. Snow was removed from treatment plots for the first 6 weeks of winter for two consecutive years, resulting in lower soil temperatures to a depth of 50 cm for both winters compared to reference plots with an undisturbed snowpack. Visibly uninjured roots from trees in the snow removal plots had significantly higher (but sub-lethal) levels of relative electrolyte leakage than trees in the reference plots. Foliar calcium: aluminum (Al) molar ratios were significantly lower, and Al concentrations were significantly higher, in trees from snow removal plots than trees from reference plots. Snow removal also reduced terminal shoot growth and increased foliar starch concentrations. Our results are consistent with previous research implicating soil freezing as a cause of soil acidification that leads to soil cation imbalances, but are the first to show that this translates into altered foliar cation pools, and changes in soluble and structural carbon pools in trees. Increased soil freezing due to a reduced snowpack could exacerbate soil cation imbalances already caused by acidic deposition, and have widespread implications for forest health in the northeastern USA.     

Latitudinal response of subarctic tree lines to recent climate change in eastern Canada

Aim The predictions from biogeographical models of poleward expansion of biomes under a warmer 2 × CO2 scenario might not be warranted, given the non‐climatic influences on vegetation dynamics. Milder climatic conditions have occurred in northern Québec, Canada, in the 20th century. The purpose of this study was to document the early signs of a northward expansion of the boreal forest into the subarctic forest‐tundra, a vast heterogeneous ecotone. Colonization of upland tundra sites by black spruce (Picea mariana (Mill.) BSP.) forming local subarctic tree lines was quantified at the biome scale. Because it was previously shown that the regenerative potential of spruce is reduced with increasing latitude, we predicted that tree line advances and recent establishment of seedlings above tree lines will also decrease northwards. Location Black spruce regeneration patterns were surveyed across a > 300‐km latitudinal transect spanning the forest‐tundra of northern Québec, Canada (55°29′–58°27′ N). Methods Elevational transects were positioned at forest–tundra interfaces in two regions from the southern forest‐tundra and two regions from the northern forest‐tundra, including the arctic tree line. The surroundings of stunted black spruce, forming the species limit in the shrub tundra, were also examined. Position, total height and origin (seed or layer) of all black spruce stems established in the elevational transects were determined. Dendrochronological and topographical data allowed recent subarctic tree line advances to be estimated. Age structures of spruce recently established from seed (< 2.5 m high) were constructed and compared between forest‐tundra regions. Five to 20‐year heat sum (growing degree‐days, > 5 °C) and precipitation fluctuations were computed from regional climatic data, and compared with seedling recruitment patterns. Results During the 20th century, all tree lines from the southern forest‐tundra rose slightly through establishment of seed‐origin spruce, while some tree lines in the northern forest‐tundra rose through height growth of stunted spruce already established on the tundra hilltops. However, the rate of rise in tree lines did not slow down with latitude. The density of < 2.5‐m spruce established by seed declined exponentially with latitude. While the majority of < 2.5‐m spruce has established since the late 1970s on the southernmost tundra hilltops, the regeneration pool was mainly composed of old, suppressed individuals in the northern forest‐tundra. Spruce age generally decreased with increasing elevation in the southern forest‐tundra stands, therefore indicating current colonization of tundra hilltops. Although spruce reproductive success has improved over the twentieth century in the southern forest‐tundra, there was hardly any evidence that recruitment of seed‐origin spruce was controlled by 5‐ to 20‐year regional climatic fluctuations, except for winter precipitation. Main conclusions Besides the milder 20th century climate, local topographic factors appear to have influenced the rise in tree lines and recent establishment by seed. The effect of black spruce's semi‐serotinous cones in trapping seeds and the difficulty of establishment on exposed, drought‐prone tundra vegetation are some factors likely to explain the scarcity of significant correlations between tree establishment and climatic variables in the short term. The age data suggest impending reforestation of the southernmost tundra sites, although the development of spruce seedlings into forest might be slowed down by the harsh wind‐exposure conditions.     

Restoration of Old Forest Features in Coast Redwood Forests Using Early‐stage Variable‐density Thinning

To accelerate development of old forest features in coast redwood, two thinning treatments and an unthinned control were compared in three treatment areas in north coastal California. One thinning treatment was designed to restore old forest densities of 125 trees/ha and the other 250 trees/ha representing a one‐step and partial treatments to the desired stand density. Four years after treatment, numbers of trees had increased in the thinning treatments due to recruitment of new trees, but had decreased in the control due to self‐thinning. Residual trees increased in stem volume following thinning by 128% in low‐density thinning compared to 70% in the controls indicating thinning accelerated stand development. The thinning treatments also moved the species composition of these stands to a greater proportion of redwood. Considerable slash was produced by the thinning treatments but was decomposing rapidly. Black bears damaged approximately 15% of all trees and more than 38% of residual trees in the thinned treatments compared to less than 2% of all trees in the control. This damage included killing some trees and damaging other trees that survived. Decisions over restoration densities in these stands are complicated by prolonged stand development, and balancing risks and costs. In this case, the bears represent a stochastic factor that dramatically increases risk. Thinning appears to be an effective means of enhancing old forest development by accelerating tree growth, modifying species composition, and increasing stand‐level variability. Continued monitoring will be necessary to evaluate long‐term trends in density relative to effects of bear damage.     

Production of Seeds and Cones and Consequences for Wood Radial Increment in Norway Spruce (Picea Abies (L.) Karst.)

Abstract

The presence of a trade–off between growth and reproduction was tested in four sites in a subalpine Norway spruce (Picea abies (L.) Karst.) forest by measuring annual stem diameter increments at breast height and seed and cone productions during the periods 1962–1985 and 1983–1990, respectively. Trees growing in forest stands near the timber line (about 1900 m above sea level) had the greatest reduction in annual stem diameter increment during mast years; while trees growing at about 1300–1500 m above sea level did not show any reduction. Trees growing at about 1700 m showed only a limited reduction. At the same elevation, trees growing within closed forest stands suffered a greater reduction in stem growth when compared with trees growing at the edge of a cutting.     

Stable carbon isotope analysis reveals widespread drought stress in boreal black spruce forests

Unprecedented rates of climate warming over the past century have resulted in increased forest stress and mortality worldwide. Decreased tree growth in association with increasing temperatures is generally accepted as a signal of temperature‐induced drought stress. However, variations in tree growth alone do not reveal the physiological mechanisms behind recent changes in tree growth. Examining stable carbon isotope composition of tree rings in addition to tree growth can provide a secondary line of evidence for physiological drought stress. In this study, we examined patterns of black spruce growth and carbon isotopic composition in tree rings in response to climate warming and drying in the boreal forest of interior Alaska. We examined trees at three nested scales: landscape, toposequence, and a subsample of trees within the toposequence. At each scale, we studied the potential effects of differences in microclimate and moisture availability by sampling on northern and southern aspects. We found that black spruce radial growth responded negatively to monthly metrics of temperature at all examined scales, and we examined ?¹³C responses on a subsample of trees as representative of the wider region. The negative ?¹³C responses to temperature reveal that black spruce trees are experiencing moisture stress on both northern and southern aspects. Contrary to our expectations, ?¹³C from trees on the northern aspect exhibited the strongest drought signal. Our results highlight the prominence of drought stress in the boreal forest of interior Alaska. We conclude that if temperatures continue to warm, we can expect drought‐induced productivity declines across large regions of the boreal forest, even for trees located in cool and moist landscape positions.     

How does the root system inhibit windthrow in thinned black spruce sites in the boreal forest?

Key message

The root shape and the angle between roots play an important role to prevent windthrow occurrence.

Abstract

Partial cutting is frequently applied to increase the volume growth of residual stems. However, the opening of the forest increases the wind speed within the site, and consequently, the risk of windthrow. In the case of black spruce, uprooted trees are normally characterized by a lifting of the root plate. This research was conducted to compare the root systems of standing and uprooted black spruces, after commercial thinning, by looking at root architecture, volume and radial growth. For this purpose, data from a pool of 18 standing and 18 uprooted trees from three study areas were analyzed. The distribution of roots around the stump was compared between both types of trees, standing and uprooted. The radial growth was measured at 30 cm in the stem, 10 cm and 60 cm in the roots. The shape (I and T-beam) and volume were recorded for each root system. The structure of the roots was also mapped to obtain a spatial overview of the angle between roots. The root shape (at 10 and 60 cm) and the angle between roots combined with the diameter of the stem at stump height seem to determine the vulnerability of black spruce to windthrow. Uprooted trees developed fewer roots, with a large sector around the stump without lateral roots which suggests its major implication in the resistance to windthrow.     

Combining tree-ring analyses on stems and coarse roots to study the growth dynamics of forest trees: a case study on Norway spruce (Picea abies [L.] H. Karst)

We show the potential of a new method combining tree-ring analyses on stems and on coarse roots of individual trees in order to advance the understanding of growth dynamics in forest trees. To this end, we studied the root–shoot allometry of trees and its dependence on site conditions. Along a gradient in water supply in Southern Germany from dry to moist sites we selected 43 Norway spruce trees (Picea abies [L.] H. Karst.) aged 65–100 years. Increment cores were taken from stem and main roots revealing aboveground and belowground growth course over the last 34 years. Annual growth rates in roots and stems and their allometric relationships were applied as surrogate variables for tree resource allocation to aboveground and belowground organs. The mean sensitivities of both stem and root chronologies were found to be site-specific, and increased from the moist through the dry sites. No temporal offset between aboveground and belowground growth reactions to climate conditions was found in Norway spruce at any of the sites. These results suggest that the root–shoot allometry depends on the specific site conditions only at the driest site, following the optimal biomass partitioning theory (the more restricted the water supply, the more organic matter allocation into the belowground organs).     

Stand structure, variability in growth and intraspecific competition in a peatland stand of black spruce Picea mariana

Age and size distributions and growth rates of individual trees were examined in a black spruce stand growing on a peatland in Alberta, Canada, 38 years after fire. Both height and basal girth frequency distributions were positively-skewed, with many small and few large individuals. Age distributions were negatively-skewed; most trees were recruited five to seven years after fire and there was little recruitment after 25 years. Stem analysis data indicated that increment in height was nearly constant over the life of the trees but larger trees grew faster than smaller trees. The trees which started growth immediately after the fire maintained faster growth than those recruited some years later. At the time of study, above ground competition and canopy shading were not considered to have a large effect on growth rates of individual trees. Several hypotheses are presented to explain these differences in growth of individual trees.     

辽东山区天然次生林雪／风灾害成因及分析

对2003年春发生在辽东山区的森林雪/风灾害的成因、过程、受灾情况、造成的危害、与林分结构特征的关系以及对未来次生林生态系统的影响等进行了调查分析。结果表明,雪/风的发生是在一个大的降水天气过程基础上,由于气温的异常变化形成的,受灾严重区多分布于海拔高、坡度大,林型比较单一的桦树、柞树、色树、胡桃树和杨树等林分,林分密度和受灾率及土层厚度和受害株数均呈显著的线性负相关;受灾数量与径级和树高分别呈指数负相关和指数正相关,同时探讨了雪/风害对天然次生林生态系统内病虫害发生、林下植被、生境因子和建群种变化产生的可能影响。     

Enhanced growth and ethylene increases spiral grain formation in Picea abies and Abies balsamea trees

Spiral grain angle in Norway spruce (Picea abies) trees and balsam fir (Abies balsamea) seedlings was investigated in relation to growth rate, endogenous and applied ethylene. Trees from stands of Norway spruce, which were irrigated and fertilised in order to enhance growth, and trees having different growth rates in non-treated stands were studied. Stem growth rate at the stand level (m³ ha^-1 year^-1) was measured annually, or by means of microscopy on stem sections as the number and size of tracheids produced. Enhanced growth increased ethylene evolution and maintained a high level of left-handed spiral grain angle in comparison to slower-growing trees. An increased number of earlywood tracheids in fast growing trees was correlated to a more left-handed spiral grain angle. Ethrel, applied to stems of balsam fir seedlings, increased the internal ethylene levels in parallel with increased left-handed spiral grain angle. The results indicate that ethylene regulates the extent of spiral grain angle.     

Predicting Ecosystem Resilience to Fire from Tree Ring Analysis in Black Spruce Forests

Climate change has increased the occurrence, severity, and impact of disturbances on forested ecosystems worldwide, resulting in a need to identify factors that contribute to an ecosystem’s resilience or capacity to recover from disturbance. Forest resilience to disturbance may decline with climate change if mature trees are able to persist under stressful environmental conditions that do not permit successful recruitment and survival after a disturbance. In this study, we used the change in proportional representation of black spruce pre- to post-fire as a surrogate for resilience. We explored links between patterns of resilience and tree ring signals of drought stress across topographic moisture gradients within the boreal forest. We sampled 72 recently (2004) burned stands of black spruce in interior Alaska (USA); the relative dominance of black spruce after fire ranged from almost no change (high resilience) to a 90% decrease (low resilience). Variance partitioning analysis indicated that resilience was related to site environmental characteristics and climate–growth responses, with no unique contribution of pre-fire stand composition. The largest shifts in post-fire species composition occurred in sites that experienced the compounding effects of pre-fire drought stress and shallow post-fire organic layer thickness. These sites were generally located at warmer and drier landscape positions, suggesting they are less resilient to disturbance than sites in cool and moist locations. Climate–growth responses can provide an estimate of stand environmental stress to climate change and as such are a valuable tool for predicting landscape variations in forest ecosystem resilience.