•Tree species, crowding, and diameter were top predictors of growth and survival.•Tree growth improved if neighbor species differed from that of the focal tree.•Crowding from saplings did not ...influence canopy tree growth.•Within a stand, tree success may be limited by insufficient and excess moisture.
Individual tree growth and mortality drive forest stand dynamics and are universally important metrics of tree success. Studying factors that affect growth and mortality is particularly challenging in mixed-species, uneven-aged systems due to their defining heterogeneity and strong temporal and spatial variability. The goal of this study was to determine the relative importance of individual tree attributes (i.e., species, size, neighborhood crowding, crown position) and environmental characteristics (i.e., soil moisture) in driving tree growth and survival in an uneven-aged, mixed species forest. In particular we tested if the factors regulating growth were the same as those regulating mortality, as is often assumed. Due to its large size and intensive sampling, the 3-ha, stem-mapped plot (established in 1989) at Howland Research Forest in central Maine, USA, allowed us to address additional questions regarding the influence of sapling crowding, neighbor species identity, and legacies of past disturbance. Growth and survival of over 3000 plot trees was assessed after 25 years and modeled using multiple linear regression (growth) and binary logistic regression (survival). As expected, species, neighborhood crowding, and tree diameter were top predictors of growth and survival. Specifically, growth and survival decreased with greater crowding, and increased with larger diameters. We also found that the identity of neighbors influenced focal tree growth: growth generally improved in neighborhoods comprised of species different from that of the focal. However, this general finding did not hold for all species: eastern hemlock grew better in hemlock neighborhoods, and northern white-cedar showed no response related to neighbor identities. In contrast to growth, neighborhood identity was not related to survival. Crowding from saplings did not explain any additional variability in growth; however, unexpectedly, individuals with greater sapling crowding were more likely to survive. For both growth and survival, we found an interaction between crowding and soil moisture, suggesting that within a single stand, individual success can be limited by both excess and insufficient moisture. We found no relationship between neighborhood cut stumps (legacy of past disturbance) and recent growth or survival. These results highlight the many variables driving growth and survival in uneven-aged, mixed-species forests. The top predictors for growth were identical to those for survival; however, other predictors differed in their relative importance. Given the recent emphasis on promoting uneven-aged, mixed-species forests, we suggest that studies addressing a full range of predictors of individual tree success are necessary to better manage and maintain these complex systems.
Conserving large mammals on small islands poses a great challenge, given their high resource demand within the limited space available. The endangered Togean Islands babirusa (
Babyrousa togeanensis
...) is one of these species, with a distribution range limited to four small islands in the Togean Archipelago, Indonesia. Despite being listed as endangered, very little information is available on the distribution and ecology of this species. To address this critical knowledge gap, we here report the first field-based ecological study of the Togean Islands babirusa across its entire distribution range. Following a stratified random sampling procedure, we distributed camera traps at 103 stations across four islands to collect data on the species distribution from July-October 2022. We performed an occupancy modeling analysis to assess the species’ habitat use, with various habitat features estimated through remote sensing and field measurements as covariates. We found that forest and mangrove availability over a large area positively influenced babirusa habitat selection. Babirusas only made use of agricultural areas when large forest areas were available nearby. Our results highlight the benefits of redesigning the national park area to accommodate babirusa habitat requirements, specifically by reassigning the non-forested park areas (about 30% of the park area) to non-protected forests currently outside the park boundary (about 50% of total forested area). Our case study exemplifies key challenges associated with conserving large mammals on small islands and highlights the importance of following an adaptive management approach, which in this case implies shifting 30% of the current protected area.
Increasing heat and aridity in coming decades is expected to negatively impact tree growth and threaten forest sustainability in dry areas. Maintaining low stand density has the potential to mitigate ...the negative effects of increasingly severe droughts by minimizing competitive intensity.
However, the direct impact of stand density on the growing environment (i.e. soil moisture), and the specific drought metrics that best quantify that environment, are not well explored for any forest ecosystem. We examined the relationship of varying stand density (i.e. basal area) on soil moisture and stand‐level growth in a long‐term (multi‐decadal), ponderosa pine Pinus ponderosa, forest management experiment. We accounted for the influence of stand‐level density on moisture availability by measuring and modelling soil moisture using an ecosystem water balance model.
To quantify the growing environment, we developed metrics of ecological drought that integrate the influence of moisture availability in the soil with moisture demand by the atmosphere. We paired these results with stand‐level dendrochronological data, avoiding the potential bias introduced from individual tree‐based assessments, and used critical climate period analysis to identify the timing and duration of these drought metrics that most relate to forest growth.
We found that stand‐level growth is highly responsive to the combination of high temperature and low soil moisture. Growth in all stands was negatively related to temperature and positively related to moisture availability, although the sensitivity of growth to those conditions varied among stand density treatments. Growth enhancement during cool years is greatest in low density stands. In addition, low density stands displayed substantially higher long‐term average growth than higher density stands and maintained higher growth even when temperatures were high. Growth in low density stands also increased more than higher density stands in response to greater long‐term moisture availability.
Synthesis and applications. We quantified the influence of stand‐level density on the environmental conditions that determine tree growth and related forest growth to patterns of moisture supply and demand. Our drought metrics, and analytical approach for quantifying drought impacts on forest growth, are a novel approach for assessing forest vulnerability to drought under climate change. These results provide new perspective on the potential for density management to mitigate drought stress and maintain forest stand growth during and after drought events in water‐limited forests.
We quantified the influence of stand‐level density on the environmental conditions that determine tree growth and related forest growth to patterns of moisture supply and demand. Our drought metrics, and analytical approach for quantifying drought impacts on forest growth, are a novel approach for assessing forest vulnerability to drought under climate change. These results provide new perspective on the potential for density management to mitigate drought stress and maintain forest stand growth during and after drought events in water‐limited forests.
•63% of cedars contained internal decay, precluding the use of age distributions.•Mean disturbance rate was 6.4% of canopy area per decade; pulses rarely exceeded 25%.•Most canopy trees showed one to ...three growth release before achieving canopy status.•Structure similar to other regional old-growth forests, but with abundant woody debris.
Natural disturbance histories and stand structures derived from old-growth forests are increasingly used to guide forest management prescriptions. Although such information is readily available for a number of forest types, it is lacking for others, such as northern white-cedar (Thuja occidentalis) forests, despite this forest type’s wide distribution, ecological value, and economic importance in northeastern North America. We applied standard dendrochronological methods to six old-growth northern white-cedar stands within the Big Reed Forest Reserve of northern Maine, USA, to reconstruct the frequency and severity of past natural disturbances. The prevalence of internal decay (well-known for this species) precluded the construction of robust age-class distributions. Overall, 63% of cedar trees contained internal decay, and the probability of rot increased with increasing diameter. Evidence from growth releases reveals sporadic pulses of low- to moderate-severity canopy disturbances. The mean disturbance rate was 6.4% of canopy area disturbed per decade, and pulses rarely exceeded 25% per decade. Based on the subset of complete cores (i.e., those without internal decay), 44% of current canopy trees showed one growth release before achieving canopy status, 26% showed two releases, and 7% showed three releases. Of the 23% that showed no release, most showed persistent slow growth that eventually placed them in the canopy. However, an apparent hiatus in cedar recruitment in recent decades (albeit based on a subset of complete cores), as well as low cedar abundance in the sapling layer, suggest that cedar may not maintain dominance in the future overstory. Current structure in these stands is similar to that reported from other old-growth conifer forests in the region: mean living tree basal area was 44.3 m2 ha−1, density of large (>40 cm dbh) living trees was 130 trees ha−1, and coarse woody debris volume was 183 m3 ha−1. Taken together, these findings suggest that multi-aged silvicultural treatments incorporating periodic harvests of low to moderate intensity, retention trees or patches, and protection of coarse woody debris would be appropriate for sustaining or restoring lowland northern white-cedar forests.
Montane Norway spruce forests of Central Europe have a very long tradition of use for timber production; however, recently there has been increasing concern for their role in maintaining biological ...diversity. This concern, coupled with recent severe windstorms that led to wide-spread bark beetle outbreaks, has brought the management of montane spruce forests to the forefront of public policy discussions in Central Europe. In order to shed light on the natural development and current structure of mature montane spruce forests, we established four 0.25
ha research plots in a semi-natural montane spruce forest in the Šumava Mountains (The Bohemian Forest), Czech Republic. We mapped all trees, extracted increment cores for age and growth-pattern analyses, and inventoried all current tree regeneration, including the substrates on which it was found. Stands were characterized by uni-modal tree diameter distributions and high basal areas (56.6
m
2
ha
−1 on average), indicating a natural transition from the stem exclusion phase towards the understory reinitiation phase. The stands showed largely single-cohort recruitment age structures, however, with recruitment spanning seven decades. Our analyses suggest that this cohort existed as advance regeneration prior to major disturbances in the late 1800s, which included post-bark beetle salvage logging. Spatial pattern analyses of living and dead stems combined, showed an increase in uniformity of living trees, pointing to the role of natural density-dependent mortality. However, past growth patterns and historical documentation suggest that low intensity canopy disturbances (wind and snow) also caused mortality and diversified canopy structure. Because the stands developed naturally over the past 120+ years and thus escaped thinning operations, high volumes of coarse woody debris (94
m
3
ha
−1) and snag densities (546
stems
ha
−1) have accrued. Advance spruce regeneration was quite abundant and existed primarily on deadwood substrates, even though these occupied only a small percent of stand area. Because of salvage logging in the late 1880s, these stands do not qualify, according to the traditional paradigm, as natural spruce forests. As a result, they are recently subject to active management practices including salvage logging that remove dead and dying trees. Given the importance of deadwood for forest regeneration and recovery from disturbance, as demonstrated in this study, we argue that dead wood removal may limit future natural regeneration in these stands. Thus, the purported benefits of removing dead and dying trees from semi-natural forests must be carefully weighed against the potential detrimental impacts on natural spruce forest regeneration and biodiversity.
Multi-year drought and climate change can impact tree growth, especially in California's Mediterranean climate where growing season rainfall is limited or absent. Active forest restoration has the ...potential to mitigate climate impacts by reducing stand density and conversion towards more resilient species' composition. We used dendrochronology methods to examine climate-growth relationships for coast Douglas-fir (Pseudotsuga menziesii var. menziesii) trees in mixed multiaged stands near the species' natural southern range margin. We found positive correlations of ring width with spring-early summer and prior October precipitation and an evapotranspiration index. Additionally, cooler spring temperature was negatively correlated with growth. We also studied tree resistance, resilience, and recovery from two multi-year drought events. Restoration treatments enhanced resistance and resilience to drought relative to trees growing in untreated plots. We did not detect differences in drought resistance and resilience between two common restoration methods, giving managers options for restoration to lessen drought impacts on tree growth.
Several aspects of the forest carbon cycle have not been examined in detail, including sources of variation in carbon dioxide (CO
2
) emissions from coarse woody material (CWM). To address this ...knowledge gap, we examined CO
2
emissions from Acer saccharum Marshall logs within four harvesting treatments, using closed chambers fitted to the logs. We found that CO
2
emissions were highest for logs in small (31.8 ± 20.4 µmol·CO
2
·m
−3
·s
−1
) and large gaps (29.6 ± 24.4 µmol·CO
2
·m
−3
·s
−1
) compared to those in control (13.9 ± 8.3 µmol·CO
2
·m
−3
·s
−1
) and thinned matrix (13.6 ± 8.0 µmol·CO
2
·m
−3
·s
−1
) treatments. CO
2
flux rates did not differ between gap sizes, but they increased with temperature, which was higher in the small gap treatment. In addition, two individual logs fitted with multiple closed chambers revealed significant within-log variability in CO
2
emissions. On a subset of logs repeatedly sampled throughout the day, we found that log surface temperature generally peaked at midday and was positively correlated with CO
2
emissions, although this relationship was weak in one log. This study provides insight into sources of variation in CO
2
emissions from CWM while improving our understanding of the forest carbon cycle.
•Northern white-cedar is not recruiting well from seedling to sapling stage in northeastern lowland forests.•Substrate moisture in lowland sites varied predicably among microtopographic ...features.•Cedar regeneration was found more often than expected on mounds, and less often in pits.•Canopy openness was most strongly associated with cedar sapling success.•Findings support managing for diverse microtopography, expanding gaps, and control of browsing.
Throughout much of its range, northern white-cedar (Thuja occidentalis L., hereafter cedar) has experienced a bottleneck in recruitment: cedar seedlings are often abundant in these stands, particularly in lowland settings, yet cedar sapling densities are quite low, leading to concerns that cedar canopy trees are not being replaced. Several barriers to cedar recruitment have been suggested; however, findings from previous studies have been inconsistent with regard to limiting factors. Our objective was to characterize the microsite conditions associated with the establishment of cedar seedlings and saplings. We achieved this objective by mapping the location of seedlings, saplings, and overstory trees in 15 lowland cedar stands at five sites in Maine, USA, and examining the fine-scale site conditions (microtopographic features, canopy openness) in which cedar seedlings and saplings occurred. In particular, we recorded the occurrence of seedlings and saplings on microtopographic mounds, pits (small depressions), and flats (transitional features between mounds and pits). Substrate moisture content in these features decreased in the order pits > flats > mounds. Contingency-table results demonstrated that live cedar seedlings, and to a slightly lesser extent saplings, were found more often than expected by chance on mounds, and less often on flats and pits. Logistic regressions using status (live vs. dead) as the response variable generally supported these findings: dead seedlings were strongly associated with pits; however, the occurrence of live seedlings and saplings did not differ between flats and mounds. A companion planted-seedling experiment strongly supported these results, showing that after two growing seasons, survival was significantly lower in pits (12%) when compared to flats and mounds, which had similarly high levels of survival (62 and 80% respectively). Logistic regressions also showed live seedlings and saplings to be prevalent under more open canopy conditions (mean 19%, range 6 to 42% openness); though results from a planted-seedling experiment suggested that greater canopy openness (mean 32%, range 13 to 57%) was detrimental to survival. Regressions also showed that browsed seedlings were more likely to be found dead. These findings point to management prescriptions that maintain microtopographic diversity, create moderately open canopy conditions, and protect stands from browsing to promote viable cedar populations in these ecologically and economically important forests.
Increasing evidence indicates that forest disturbances are changing in response to global change, yet local variability in disturbance remains high. We quantified this considerable variability and ...analyzed whether recent disturbance episodes around the globe were consistently driven by climate, and if human influence modulates patterns of forest disturbance. We combined remote sensing data on recent (2001-2014) disturbances with in-depth local information for 50 protected landscapes and their surroundings across the temperate biome. Disturbance patterns are highly variable, and shaped by variation in disturbance agents and traits of prevailing tree species. However, high disturbance activity is consistently linked to warmer and drier than average conditions across the globe. Disturbances in protected areas are smaller and more complex in shape compared to their surroundings affected by human land use. This signal disappears in areas with high recent natural disturbance activity, underlining the potential of climate-mediated disturbance to transform forest landscapes.
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