Tree Recruitment Research Articles

Response of tree species in cypress (Taxodium spp.) domes of central Florida to a hydrologic gradient

Cypress domes are isolated wetlands dominated by cypress (Taxodium spp.) with multiple subdominant species. Although one of the most common wetlands in the southeastern United States, the number and extent of cypress domes are progressively declining. While tree communities must be resilient to survive both extended periods of flooding and desiccation associated with climate cycles, ongoing changes in land use and climate are altering hydrologic extremes and potentially exceeding the resilience of species. Upland species such as pines and oaks can increase in density in response to persistent shortened hydroperiod, while mature wetland trees including cypress and tupelo can persist for extended periods. This study investigated tree community structure in cypress domes across a gradient of hydroperiods and water depths potentially experienced by cypress domes with altered hydrology. Three decades of water level data were used to describe cypress dome tree community responses to long-term, chronic alterations to hydrology.The tree community structure of cypress dome was correlated with average annual hydroperiod and water depth, but maximum water depth was not significant. Nyssa sylvatica was correlated with extended hydroperiods/deeper water depths, while both Quercus laurifolia and Pinus elliottii correlated with reduced hydrology. The presence of cypress was not correlated with hydrology, presumably because of its dominance in all sample wetlands. Mature (>6 m) wetland trees remained dominant in all sample wetlands with non-wetland trees showing increased abundance when hydroperiods and water depths were below 3 months and 0.075 m, respectively. Sub-adult (<6 m) non-wetland trees became increasingly important when hydroperiods fell below 6 months and were dominant below 3 months. Wetland tree recruitment was reduced when hydroperiods were altered to less than three months and longer than nine months; and average water depths were altered to <0.075 m and>0.225 m. Our study indicates that cypress dome communities may not be as resilient as previously thought. The long-term persistence of cypress domes is not dependent on the ability of mature wetland trees to survive extended periods of flood and drought, but instead maintaining hydrology and recruitment of wetland trees. By utilizing a long-term hydrologic database, a tipping point for cypress domes was observed for water depth and duration of inundation, beyond which the long-term persistence of these systems is in jeopardy.

Hardwoods influence effect of climate and intraspecific competition on growth of woodland longleaf pine trees

AbstractLongleaf pine woodlands of the North American Coastal Plain are proposed to be resilient to climate change impacts, but little is known about changes in limiting factors to longleaf pine growth as climate has changed in late 20th century and early 21st century. Moreover, the role that neighborhood trees play in the context of climate change remains largely unexplored. We used static and moving window tree ring and climatic analyses to measure the effects of climate on longleaf pine growth at a site in southwest Georgia, USA. We then performed maximum likelihood analysis to examine the influence of neighboring hardwoods on the response of longleaf pine growth to the joint effects of competition and climate. Analysis of climate data from local stations in southwest Georgia over six decades indicated that mean air temperature decreased until the late 20th century then began to rise, and that the variability of spring and summer precipitation has increased. Tree ring and climate analyses indicated longleaf pine radial growth is sensitive to precipitation and air temperature, and that the strength of correlation of longleaf pine growth to summer air temperature and summer precipitation increased since the 1950s. Likelihood models, which were applied over a shorter (23‐year) period and explicitly incorporated competition, did not support a link between summer temperature and growth but did indicate summer precipitation increased growth. Furthermore, basal area (BA) of neighboring hardwoods was correlated with greater pine growth per millimeter of precipitation. BA of neighboring longleaf pine negatively affected growth of conspecific trees; the presence of hardwoods increased the competitive effect when BA of neighboring pine trees was low (&lt;10 m2 ha−1) but decreased the competitive effect when BA of neighboring pine trees was high (≥10 m2 ha−1). These results suggest that retention or recruitment of hardwood trees when restoring longleaf pine woodlands may contribute to increased ability to withstand dry summers and may help to allay concerns of managers that retention of hardwoods will unduly affect growth of residual mature longleaf pines.

Long-term exclusion of invasive ungulates alters tree recruitment and functional traits but not total forest carbon.

Forests are major carbon (C) sinks, but their ability to sequester C and thus mitigate climate change, varies with the environment, disturbance regime, and biotic interactions. Herbivory by invasive, nonnative ungulates can have profound ecosystem effects, yet its consequences for forest C stocks remain poorly understood. We determined the impact of invasive ungulates on C pools, both above- and belowground (to 30 cm), and on forest structure and diversity using 26 paired long-term (>20 years) ungulate exclosures and adjacent unfenced control plots located in native temperate rainforests across New Zealand, spanning 36-41° S. Total ecosystem C was similar between ungulate exclosure (299.93 ± 25.94 Mg C ha-1 ) and unfenced control (324.60 ± 38.39 Mg C ha-1 ) plots. Most (60%) variation in total ecosystem C was explained by the biomass of the largest tree (mean diameter at breast height [dbh]: 88 cm) within each plot. Ungulate exclusion increased the abundance and diversity of saplings and small trees (dbh ≥2.5, <10 cm) compared with unfenced controls, but these accounted for ~5% of total ecosystem C, demonstrating that a few, large trees dominate the total forest ecosystem C but are unaffected by invasive ungulates at a timescale of 20-50 years. However, changes in understory C pools, species composition, and functional diversity did occur following long-term ungulate exclusion. Our findings suggest that, although the removal of invasive herbivores may not affect total forest C at the decadal scale, major shifts in the diversity and composition of regenerating species will have longer term consequences for ecosystem processes and forest C.

Dynamic Fire Regimes and Forest Conditions Across Three Centuries in a Shortleaf Pine-Oak Forest in the Ouachita Mountains, Arkansas, USA

In the interior highlands of the eastern United States, there is evidence that fire was frequent in some forests during the eighteenth and nineteenth centuries before declining drastically in the twentieth century. To better understand past fire regimes and how they shaped forest dynamics during periods of change, we conducted a dendroecological study at Lake Winona Research Natural Area (LWRNA), a 110-ha unlogged forest dominated by shortleaf pine (Pinus echinata Mill.) in the Ouachita Mountains of Arkansas. We used remnant wood and living tree cores to construct a multicentury record of fire occurrence and tree recruitment. Our results indicate the forest at LWRNA passed through multiple fire regime transitions that altered forest dynamics. During the protohistoric period (1701–1834), prior to widespread European American settlement (EAS), fire was frequent, but limited sample depth results in greater uncertainty regarding fire frequency and forest conditions. During EAS (1835–1929), fire was very frequent and tree establishment was dominated by shortleaf pine. After 1930, effective fire protection led to establishment shifting toward increasingly fire-intolerant hardwoods. Evidence of temporal variations in the fire regime, age structure, and contemporary composition broaden our understanding of reference conditions in this pine-oak forest and demonstrate that fire management could be used to restore a range of vegetation conditions from frequently burned pine- and oak-dominated woodlands to fire-excluded, closed-canopy mesophytic communities.

Asymmetric effects of daytime and nighttime warming on alpine treeline recruitment.

Trees at their upper range limits are highly sensitive to climate change, and thus alpine treelines worldwide have changed their recruitment patterns in response to climate warming. However, previous studies focused only on daily mean temperature, neglecting the asymmetric influences of daytime and nighttime warming on recruitments in alpine treelines. Here, based on the compiled dataset of tree recruitment series from 172 alpine treelines across the Northern Hemisphere, we quantified and compared the different effects of daytime and nighttime warming on treeline recruitment using four indices of temperature sensitivity, and assessed the responses of treeline recruitment to warming-induced drought stress. Our analyses demonstrated that even in different environmental regions, both daytime and nighttime warming could significantly promote treeline recruitment, and however, treeline recruitment was much more sensitive to nighttime warming than to daytime warming, which could be attributable to the presence of drought stress. The increasing drought stress primarily driven by daytime warming rather than by nighttime warming would likely constrain the responses of treeline recruitment to daytime warming. Our findings provided compelling evidence that nighttime warming rather than daytime warming could play a primary role in promoting the recruitment in alpine treelines, which was related to the daytime warming-induced drought stress. Thus, daytime and nighttime warming should be considered separately to improve future projections of global change impacts across alpine ecosystems.

The effects of the invasive species, Lantana camara, on regeneration of an African rainforest

AbstractInvasive plants adversely affect native communities by altering ecosystem function and disrupting natural regeneration. We investigate the effect of invasive Lantana camara L. (Verbenaceae) on forest regeneration in Kibale National Park, Uganda. We appraise the efficacy of cutting and uprooting Lantana for promoting native tree recruitment. Sample plots comprised three types: (i) currently invaded by Lantana; (ii) cleared of Lantana and now managed; and (iii) forest reference plots uninvaded by Lantana. Tree species numbering 51, 19 shrubs, and 17 herb species were identified. Lantana reduced tree, shrub, and herb cover and diversity, and suppressed tree regeneration. The short‐term management of Lantana did not promote tree establishment. The tree community in cleared areas was not converging on uninvaded adjacent forest. Lantana is known to allelopathically suppress tree seedling establishment, but even at sites cleared of Lantana, tree species recruitment was poor. While insufficient time may have passed for tree recruitment, we argue that an increase in shrub and herb cover and diversity arrested forest tree regeneration. Sustained follow‐up clearing of dense secondary shrubs and herbs and resprouted L. camara in cleared areas is key to ensuring long‐term recovery of the forest tree community.

Using a demographic model to project the long‐term effects of fire management on tree biomass in Australian savannas

AbstractTropical savannas are characterized by high primary productivity and high fire frequency, such that much of the carbon captured by vegetation is rapidly returned to the atmosphere. Hence, there have been suggestions that management‐driven reductions in savanna fire frequency and/or severity could significantly reduce greenhouse gas emissions and sequester carbon in tree biomass. However, a key knowledge gap is the extent to which savanna tree biomass will respond to modest shifts in fire regimes due to plausible, large‐scale management interventions. Here, we: (1) characterize relationships between the frequency and severity of fires and key demographic rates of savanna trees, based on long‐term observations in vegetation monitoring plots across northern Australia; (2) use these relationships to develop a process‐explicit demographic model describing the effects of fire on savanna tree populations; and (3) use the demographic model to address the question: to what extent is it feasible, through the strategic application of prescribed burning, to increase tree biomass in Australian tropical savannas? Our long‐term tree monitoring dataset included observations of 12,344 tagged trees in 236 plots, monitored for between 3 and 24 years. Analysis of this dataset showed that frequent high‐severity fires significantly reduced savanna tree recruitment, survival, and growth. Our demographic model suggested that: (1) despite the negative effects of frequent high‐severity fires on demographic rates, savanna tree biomass appears to be suppressed by only a relatively small amount by contemporary fire regimes, characterized by a mix of low‐ to high‐severity fires; and (2) plausible, management‐driven reductions in the frequency of high‐severity fires are likely to lead to increases in tree biomass of about 11.0 t DM ha−1 (95% CI: −1.2–20.8) over a century. Accounting for this increase in carbon storage could generate significant carbon credits, worth, on average, three times those generated annually by current greenhouse gas (methane and nitrous oxide) abatement projects, and has the potential to significantly increase the economic viability of fire/carbon projects, thereby promoting ecologically sustainable management of tropical savannas in Australia and elsewhere. This growing industry has the potential to bring much‐needed economic activity to savanna landscapes, without compromising important natural and cultural values.

The current fire regime in northern Andean shrublands hinders tree recruitment and arrests forest succession

The northern Andes is a biodiversity hotspot. However, historical land use and the current increase in wildfires have profoundly altered the landscape. There is an urgent need to understand how successional processes and wildfires operate to support biodiversity management in this region. We studied the effect of wildfire frequency and time since the last fire on vegetation regeneration patterns in a heavily transformed northern Andean landscape. In the past, Andean Forests were common in this region. Quercus once dominated humboldtii, the only Quercus species native to South America, which was frequent in these forests. Using the fire map of the Iguaque Mountains (Colombian Andes), we selected areas with different fire frequencies (three and five fires in thirty years) and the time since the last fire (one to six years) to study post-fire vegetation structure and composition. Resprouting herbs and small shrubs dominated post-fire communities. Their composition was closer to subpáramo vegetation (greater richness and density of herbaceous and shrub species than tree species) than to Andean forests. Indeed, forest species were virtually absent from burned sites, as were obligate seeders. Fire frequency did not affect the composition and structure of the herbaceous layer.Conversely, we found a substantial reduction in species richness and cover of woody species with the increase in fire frequency, a reduction of the basal area of large shrubs and trees, and a trend towards a decrease in the proportion of obligate tree resprouters. The richness, density, and cover of herbaceous species was high the first year after the fire and still showed a small increase until year 6, as the number of obligate resprouters decreased. The basal area of woody species barely increased between years 1 and 6, but the richness and density of shrubs and trees increased gradually during this period. Exotic species were largely herbaceous and dominated by Pteridium aquilinum and Andropogon lehmannii. Their cover was not affected by fire frequency or time since the last fire, although its composition changed. While the cover of Pteridium aquilinum was not affected by fire frequency, it decreased from year 1 to year 6. Our results show that current wildfire regimes in the Iguaque Mountains favor the persistence of fire-tolerant herbs and shrubs, compromise tree recruitment, and arrest forest succession. New integrated fire management strategies are needed to protect remnant forests and establish oak forests in burned shrublands.

Stand, plot characteristics, and tree species diversity jointly dominate the recruitment biomass of subtropical forests

Forest dynamics is largely affected by the recruitment of trees. Despite several investigations on forest recruitment biomass (FRB), it is still not well explored how tree species diversity, stand characteristics, climate, and topography regulate FRB. Moreover, there have been limited studies and discussions on FRB in typical subtropical forests in karst areas. We examined the relationship between influencing factors and forest biomass from the perspective of tree recruitment using data from 2334 Chinese National Forest Inventory (CNFI) plots in Guizhou Province. The best predictors within each group were chosen using a stepwise regression model, and the effects of five categories, including tree species diversity, stand factors (e.g., stand density), climatic factors (e.g., mean annual temperature and annual precipitation), topographic factors (e.g., elevation and slope), and plot factors (e.g., canopy density and forest composition) on FRB were determined using structural equation models (SEM). Besides this, the relative importance of each variable among five categories in determining FRB was also quantified by hierarchical partitioning analysis (HPA). Our results indicated that diversity of tree species had the highest relative importance in determining FRB (28.85 %), followed by average DBH (21.06 %), canopy density (10.29 %) based on HPA. While for five categories, stand factors contributed the most to FRB (37.04 %), followed by plot factors (29.01 %), tree species diversity (28.85 %), climatic factors (3.81 %), and topographic factors (1.29 %). Moreover, SEM showed FRB was strongly directly and significantly controlled by tree species diversity (β = 0.63, P < 0.05), while stand factors had a strong direct effect (β = 0.45, P < 0.05) and indirect effect (β = -0.78, P < 0.05). We conclude that niche complementarity is the primary mechanism driving this forest's FRB. Our findings highlight the significance of tree recruitment in maintaining subtropical forests' carbon storage capacity. Furthermore, increasing tree species diversity, particularly by combining conifers and broad-leaved trees, will encourage the recruitment of young trees.

Masting is shaped by tree-level attributes and stand structure, more than climate, in a Rocky Mountain conifer species

Masting describes the spatiotemporal variability in seed production by a population of plants. Both abiotic and biotic factors drive masting, but the importance of these factors can vary among individuals and populations. To better understand how a changing climate, altered disturbance regimes, or novel management strategies might affect future seed production, we quantified the joint influence of multiple factors on annual cone production in a widespread conifer species, Rocky Mountain ponderosa pine (Pinus ponderosa var. scopulorum). We reconstructed individual-level annual cone production across climatic gradients using the cone abscission scar method, and explored the causes and drivers of masting in this species. Site-level responses between weather and masting were highly variable, notably differing in the strength of their response to either summer or spring weather. In addition, the relationship to summer precipitation during cone initiation, a primary driver of annual seed production in this species, was strongest at hotter and drier sites. Additionally, we found that masting was strongly influenced by tree- and stand-level factors such as diameter, age, and local neighborhood density, all of which were associated with the mean, interannual variability, and between-tree synchrony of cone production at the individual-level. Larger and older trees produced more cones, more frequently, and with less synchrony than smaller and younger trees. Open grown trees experiencing lower levels of neighborhood competition also produced more cones with less interannual variability, but with higher between-tree synchrony. Because tree- and stand-level traits appear to regulate seed production more strongly than climate or weather in this species, management interventions targeting these factors could be powerful tools to manage future tree recruitment. Thus, current efforts to reduce stand density and conserve large trees in some ponderosa pine forests may enhance tree-level seed production and reduce variability in seed crops among years.

Spatial determinants of tree recruitment in mixed stands in southeastern Poland- silver fir's interaction with lowland tree species

Silver fir (Abies alba Mill.) is a typical mountain tree species, but it can also grow in lowlands below 300 m a.s.l., where it must coexist with a variety of competitors. Much less were studied in the northeastern part of its natural range, where annual precipitation varies between 500 and 600 mm, distinctly less than in the mountains. The general objective of this study was to investigate the regeneration process of silver fir in mixed lowland forests in regions with water shortage. The evaluation of spatial relationships between old, dominant and newly established trees was used to clarify the mechanisms of tree species coexistence. The study was conducted in managed and protected stands in Janów Forests in the southeastern part of Poland. We selected mature forest stands (oldest trees older than 100 years) representing three types of mixed stands in nature reserves, as well as in comparable managed forests. Data was collected on 500 m2 sample plots established randomly in each stand category (20 sample plots × 6 stands). Silver fir as the target tree showed statistically significant segregation from other species in 15 cases. Small silver fir trees were associated with large pines in two forest types and segregated form large firs in one. The spatial relationships between fir and black alder were negative. Black alder was the second species with the most statistically significant spatial relationships with other tree species (11 cases). Alders separated from large pines and large firs and from medium and small silver firs. Strong segregation of medium-sized hornbeam from the small class of silver fir was observed. Our study showed that silver fir natural regeneration and colonization of new areas at the northeastern limit of its range are limited by low seed rain density, interactions with other competing species, and local site conditions. Areas with moderately high water table are potentially suitable for fir regeneration, but black alder has a competitive advantage in wetter parts of forest stand. The major advantage of such sites is the absence of beech populations, which are an important competitor to fir in the central European mountains, but a likely equally important limitation would be the presence of hornbeam populations. Our results suggest that less intensive cuttings at carefully selected places are more conducive to fir regeneration than more intensive salvage cuttings.

Impact of Fire History on the Structure of a Temperate Forest in Northern Mexico

Understanding the ecological role of fire in forests is essential for proper management and conservation programs. The objectives of this study were: (1) to reconstruct the history of fires in a temperate forest in Sierra Madre Occidental; and (2) to interpret the impacts of fire and climate on forest structure. Sixty tree cross-sections with fire scars were analyzed, and descriptive statistics of fire history were generated. Additionally, growth cores were analyzed, and the ages of trees of different diameter categories were calculated. The synchrony between fire history and tree establishment was determined, and precipitation and Palmer Drought Severity Index (PDSI) values were correlated with the number of trees established per year. The presence of 137 fire scars was determined, which allowed the reconstruction of 41 fire events over the period 1855–2019; however, only the period 1940–2015 was used to compare tree recruitment, as tree establishment was detected in this period. The mean fire interval (MFI) was 2.28 years in general, and 12.17 years for extensive fires. As regards vegetation, a continuous recruitment pattern was observed, typical of a frequent low-intensity fire regime, although peak regeneration occurred after extensive fires. The correlation analysis showed that the number of trees established per year was influenced by the wet conditions that occurred in December of the previous year and the dry conditions in September and October of the previous year. This finding demonstrates the historical influence of fire and climate on the structure of the current stand in the study area. Therefore, the present study highlights the importance of including fire in forest management programs, considering the natural fire regime to which the species in this ecosystem are already adapted.

Seasonal snow cover patterns explain alpine treeline elevation better than temperature at regional scale

Unprecedented modern rates of warming are expected to advance alpine treelines to higher elevations, but global evidence suggests that current treeline dynamics are influenced by a variety of factors. Seasonal snow cover has an essential impact on tree recruitment and growth in alpine regions, which may in turn influence current treeline elevation; however, little research has been conducted on its role in regional treeline formation. Based on 11,804 treeline locations in the eastern Himalayas, we extracted elevation, climate, and topographic data for treeline and snowline. Specifically, we used linear and structural equation modelling to assess the relationship between these environmental factors and treeline elevation, and the climate-snow-treeline interaction mechanism. The results showed that the treeline elevation increased with summer temperature and permanent or seasonal snowline elevation, but decreased with snow cover days and spring temperature at the treeline positions (P ​< ​0.001). Importantly, spring snowline elevation (33.4%) and seasonal snow cover days (21.1%) contributed the most to treeline elevation, outperforming the permanent snowline, temperature, precipitation, and light. Our results support the assertion that the temperature-moisture interaction affects treeline elevation in the eastern Himalayas, but we also found that the effects were strongly mediated by seasonal snow cover patterns. The increasing tendency of snow cover governed by climate humidification observed in the eastern Himalayas, is likely to limit future treeline advancement and may even cause treeline decline due to the mortality of the remaining old trees. Together, our findings highlight the role of seasonal snow cover patterns in determining treeline elevation in the eastern Himalayas, which should be considered when assessing the potential for treeline ascent in snow-mediated alpine systems elsewhere.

Processes underpinning natural capital account compilation highlight the potential for low-input grazing to mitigate farm carbon emissions while also improving biodiversity outcomes

Remnant woodland areas are commonly part of livestock grazing operations in temperate Australia. These remnant areas can store substantial amounts of carbon and have important biodiversity value, but are at risk of ongoing decline due to nutrient enrichment, overgrazing, clearing of woody debris and lack of tree recruitment. The process of compiling experimental farm-scale natural capital accounts (NCA) for 11 wool-growing businesses in temperate Australia demonstrated that some wool growers may be managing these areas in ways that sustained, and at times regenerated, carbon and other natural values. When managed sensitively, these largely native areas provide forage and shelter for livestock production while carbon and associated biodiversity is also protected, and in some cases regenerated. The farm-scale NCA process highlighted that for some farm businesses net sequestration of carbon can occur at a farm-scale, substantially owing to the management approach applied to these remnant areas. The process highlighted the potential for livestock grazing approaches underpinned by the ecosystem services provided by native grasses, forbs and woodlands (i.e. low-input rather than nutrient enrichment and pasture modification) to contribute to balancing carbon emissions from other areas of more intensive management within a farm business. With the current momentum worldwide aimed at achieving net zero emissions, there is an opportunity that has not existed previously to conserve, and sometimes regenerate, these remnant woodland areas on farmland. This could contribute to reversing a crisis of biodiversity loss in this threatened ecosystem while also helping farm businesses to reduce overall carbon emissions.

Small-Scale Environmental Heterogeneity Enhances Tree Recruitment through Carbon Recharge and Water Use Diversification

Studying regeneration processes in oak-dominated forests requires a multi-faceted approach that considers local factors, disturbances, management actions, and tree responses. Our aims were to quantify the carbon and water-use responses of saplings and evaluate ecological consequences at the early tree regeneration phase of a pedunculate oak-hornbeam forest. We measured plant eco-physiological parameters using an open-chamber IRGA equipment in large experimental canopy gaps (instantaneous field data) and greenhouse (climate-controlled reference data) conditions. We used the non-parametric Kruskal–Wallis ANOVA test to analyze differences and similarities in the gas-exchange response. Functional fingerprints indicated shared resource use and efficiency functions at species-specific performance levels with temporal variations. Medium-level and seasonally balanced carbon uptake and water-use functions characterized pedunculate oak (Quercus robur L.) and European hornbeam (Carpinus betulus L.). In contrast, the response patterns in wild cherry (Prunus avium (L.) L.) and green ash (Fraxinus pennsylvanica Marshall) were dominated by water use. Goat willow (Salix caprea L.) had consistently elevated gas-exchange levels with the largest seasonal variation among the study species. We found that trees could be ranked on a relative isohydric-to-anisohydric scale regarding their species–environment interaction. According to the carbon-gain response pattern coupling with a non-structural carbohydrate exchange scheme, we were able to classify tree species as having medium- and long-term carbon resource management. In conclusion, spatially heterogeneous and temporally balanced canopy gaps facilitate tree species’ development and mixed-stand regeneration by providing a functionally diversifying recruiting environment.

Recruitment, mortality and growth in semi-arid conifer-eucalypt forest: Small trees insure against fire and drought.

Well-managed semi-arid forests help offset global change by storing significant amounts of carbon above- and belowground and maintaining hydrological cycles. Larger trees have been the focus of many studies due to their carbon storage and habitat quality, yet recruitment and small trees are important components of ecosystem resilience and recovery. Here, we study the impacts of disturbances (including harvesting) on recruitment, mortality and growth for a mixed conifer-broadleaf semi-arid forest type using long-term data. Pilliga Forest in New South Wales, inland eastern Australia. Callitris-Eucalyptus forests. We used data from permanent sample plots (PSPs) spanning 55 years, calculated stand structure, gains and losses and determined reasons for tree death (harvesting, fire, wind, drought and other effects). We extracted climate and fire data for the PSP locations using spatial analysis. Stocking of studied forests remained stable (modest increase in basal area and stem density), despite harvesting and wildfires over 6 decades. Compared to stands in the 1940s and prior to European settlement, current forests are composed of more trees per unit area, and these trees have smaller diameters. Recruitment and sustained presence of small trees have buffered impacts of recurring drought, fire and harvesting. Fires are a common feature of the studied ecosystems and fire impacts have increased in the past 20 years, especially in unmanaged stands, where fires have reduced tree carbon by >50%. Recruitment and growth of small trees are critical to offset carbon losses due to fire, drought and harvesting. All size classes have important ecological values in semi-arid forests and must be included in long-term monitoring programmes. Long-term data offer unique insights into combined effects of climate change, management and disturbances, especially for fire-prone ecosystems, where small trees are often susceptible to fire.

Recruitment of European beech, Norway spruce and silver fir in uneven-aged forests: optimal and critical stand, site and climatic conditions

Tree recruitment models are important for predicting the dynamics of uneven-aged forests. Previous studies of recruitment of European beech, Norway spruce and silver fir have shown different ecological amplitudes of these species. However, recruitment in uneven-aged stands and the values of environmental factors at which the greatest and poorest recruitment can be expected remain poorly explained. The main objectives of this study were to 1) explain how 39 stand, site and climatic factors and their interactions influence the number of recruited trees in uneven-aged forests; 2) determine the optimal and critical ranges of influential factors, including stand basal area, number of trees, proportion of tree species, shade casting, soil pH, site productivity, temperature and precipitation; and 3) estimate the maximum expected response of recruitment to changes in stand density while controlling for the effect of other limiting factors. A Tobit censored regression model was used to consider that the observed range of the number of recruited trees is censored at zero. The models were parametrized and validated using 30,963 forest inventory plots (200 m2 each) in uneven-aged forests in Slovenia. The models, which used 9 stand, 6 site and 3 climatic factors, explained 15 %, 10 % and 8 % of the total variation of the number of recruited spruce, fir and beech, respectively. Stand structure was the most important factor, with stand basal area (BA) and the proportion of the studied species having the greatest effect. Site factors including soil pH and rockiness were important for fir recruitment. The number of recruited beech and spruce was positively influenced by decadal precipitation. Higher temperatures decreased recruitment of spruce. Beech was the only species sensitive to shade casting. Recruitment of beech was higher if shade was imposed by tree species other than beech. The optimal and critical ranges of limiting factors differ between species. The model suggests that the optimal stand basal area for recruitment of beech is ≤ 19 m2/ha, which is higher than that for spruce (≤ 16 m2/ha) but lower than that for fir (≤ 28 m2/ha). The maximum predicted response of the studied species to changes in stand basal area shows that stand density control is efficient for regulating recruitment of spruce and beech, but not for fir. The suggested sensitivities and threshold values may be used in individual tree growth models or simulation–optimization studies in support of forest management decisions.

Wildfire affects boreal forest resilience through post-fire recruitment in Northeastern China

Climate change has increased the severity, frequency, and impact of fire disturbance on boreal forests worldwide. Unusual fire events can trigger a shift from needle-leaf dominated forests to broad-leaf dominated forests, altering the original succession trajectory. Thus, it is necessary to identify how altered fire regimes affect boreal forest resilience or capacity to recover after fire disturbance. In this study, we used the post-fire tree recruitment density, the aboveground biomass, the understory shrub cover, and the herb cover as indicators of forest resilience. We explored the links between patterns of resilience and burn severity across topographic positions in two post-fire forests with different time since fire. We quantified how burn severity, time since fire, and topographic position affect resilience indicators related to post-fire overstory and understory recruitment in the Great Xing’an Mountains of Northeastern China. We found that burn severity and topographic position exhibited strong influences on post-fire overstory tree recruitment and aboveground biomass. Severe fire promoted deciduous needle-leaf tree (larch) recruitment in lowland sites by removing a thick organic layer. In contrast, severe fire promoted broad-leaf tree (birch and aspen) recruitment in south-facing upland sites with relatively warmer and drier microclimates. Therefore, severe fire disturbance may trigger a post-fire species shift from larch dominated forest to broad-leaf forest, especially in south-facing upland sties. We also found that the understory may be more resilient to high severity fire than the overstory, because severity had short-term effects on post-fire understory recruitment. Our study suggested that burn severity was a crucial factor that affects boreal forest resilience through substantially altering resilience indicators related to post-fire overstory tree recruitment.

Significance of seed dispersal by the largest frugivore for large-diaspore trees

How do large-bodied frugivores contribute to seed dispersal of large-diaspore plants? This study examined seed dispersal effectiveness for two large-diaspore tree species, Astrotrichilia asterotricha (AA) and Abrahamia deflexa (AD), in a Madagascan forest. I evaluated fruit removal rates through focal tree observations and factors affecting seedling recruitment up to the 2-year-old seedling stage. I confirmed brown lemur (Eulemur fulvus) as the sole disperser, removing 58.8% and 26.0% of fruits produced by AA and AD. Brown lemurs frequently visited large-crowned AA trees with high density of fallen fruits and more adjacent fruiting trees during seasons with low fruit diversity. Most AA seedlings were removed by predators, although canopy openness slightly improved seedling establishment. Although AD seeds were severely attacked by predators under mother trees, the seedlings survived under dispersal conditions distant from the mother trees, and with low density of diaspores. AD had a higher cumulative probability from fruit removal to seedling recruitment (6.5%) than AA (1.5%) in the first rainy season. This study clarifies the significance of seed dispersal to tree recruitment strategies, which vary among different combinations of tree species and large frugivores, i.e. quantitative dispersal to reach suitable microhabitats, and qualitative dispersal to escape from dangerous zones near mother trees.

A climate-sensitive mixed-effects tree recruitment model for oaks (Quercus spp.) in Hunan Province, south-central China

Tree recruitment is an essential component and plays a fundamental role in forest management decision-making. However, the existing tree recruitment models for oak (Quercus spp.) species have been developed using traditional modeling functions, and have neglected the effects of climate, which do not apply to long-term projections of future forest composition under a climatic change scenario. In this study, we developed tree recruitment models based on the data of 548 permanent sample plots distributed in Hunan Province, south-central China. Five commonly used recruitment approaches (negative binomial model, zero-inflated Poisson, zero-inflated negative binomial model, Hurdle-Poisson model, and Hurdle-negative binomial model) were employed to analyze the data. Then the random effects of the regions were included to develop the nonlinear mixed-effects models and account for spatial and temporal correlations. Among the various candidate predictors tested including stand characteristics, site conditions, and climate, stand basal area (BA), quadratic mean diameter (QMD), basal area of the species (Bai), cosine of the slope combined with the natural logarithm of elevation (CIE), and mean annual temperature (MAT) had significant effects on oak recruitment. The zero-inflated negative binomial model was selected as the optimal model compared to the other four basic models. The performance of the models improved significantly when the random effects were added. These results and knowledge of adaptive management will be useful for developing practical forest management plans.