Mature Forest Sites Research Articles

Interannual variations and trends of gross primary production and transpiration of four mature deciduous broadleaf forest sites during 2000–2020

The interannual variations of gross primary production (GPP) and transpiration (T) in deciduous broadleaf forests reflect how the forest responds to climate change. However, our knowledge remains limited due to lack of multi-decadal data. In this study, we selected four mature deciduous broadleaf forest sites in the United States of America and Canada from 2000 to 2020 to investigate decadal trends in atmospheric CO2 concentrations, climate, vegetation indices, phenology, and carbon and water fluxes. The Vegetation Photosynthesis Model (VPM v3.0) and the Vegetation Transpiration Model (VTM, v2.0) were used to estimate GPP and T at the four sites. The GPP from the VPM simulations (GPPVPM) is highly consistent with that from the eddy flux tower sites (GPPEC) (R2 range from 0.77 to 0.89). The interannual trends of carbon and water fluxes during the period from 2000 to 2020 varied by sites, ranging from increases (CA-Cbo) to no trend (US-Ha1, US-MMs, US-WCr sites), dependent upon the temporal changes in atmospheric CO2 concentration and climate. Spring phenology at these sites had no significant trends due to the lack of an interannual trend of air temperature during 2000–2020. In those years with hotter winter and early spring season (WESS), the start of the season (SOS) advanced and the growing season length (GSL) extended, but there were little changes in the end of the season (EOS) and annual GPP. This study highlights the value of long-term measurements at forest eddy flux tower sites and the skill of VPM v3.0 and VTM v2.0 models and the ERA5 climate dataset for simulations over the past two decades, which could be used to do regional and global simulations of deciduous broadleaf forests and assess their responses to climate change.

Direct topsoil transfer to already planted reforestation sites increases native plant understory and not ruderals

Forests restored passively or by tree planting can take many decades to be recolonized by native forest understory plant species, if at all. Our study tested (1) the ability of forest topsoil transfer to accelerate the recovery of native forest plant communities in post‐agricultural reforestation sites after tree‐planting and without previous topsoil removal and (2) the effect of adding combinations of woody debris (WD), shrub plantings, and shade shelters (SS) on top of the transferred topsoil. Five 12.5 × 10 m treatment blocks were established in each of three recipient sites, which included two post‐agricultural reforestation sites, and one abandoned gravel pit site. Each treatment blocks received forest topsoil and a combination of additional treatments. Treatment and control plots were sampled for all vascular species in spring and summer. Native forest plant species richness in topsoil recipient plots was similar (p > 0.05) to that of mature donor forest sites, and significantly higher (p < 0.05) than that of passive control plots in the recipient sites. The plant community composition of all topsoil recipient plots had also become more like the donor forests and less like recipient site controls. Only the unplanted gravel pit‐site increased in non‐native ruderal plant species after topsoil transfer. The addition of WD, shrub plantings, or SS had no significant effect after two growing seasons. We recommend that topsoil should when possible be added where trees have already been planted, allowing for shorter time to canopy closure and thereby higher survival of shade‐adapted understory species.

Response of birds that rely on mature Piedmont Forests of Northwestern Argentina to fire severity

Fire is an integral part of the ecology of Seasonal Dry Forests. However, fire severity could be a determining factor in the resilience of these forests. The Piedmont Forest is part of the Neotropical Seasonal Dry Forests, concentrates high species biodiversity and endemism and is one of the most threatened ecosystems of the Andean region. In recent decades there has been an increasing recurrency of fires of high-severity in the Piedmont Forest. Our objective was to assess the effects of fire severity on birds that rely on mature Piedmont Forest of northwestern Argentina. To achieve this, we selected a mature unburnt forest (reference) site and two sites disturbed by fires in the year 2013 of low- and high- severity. During the wet (breeding) and dry (non-breeding) seasons of the year 2016, we carried out 30 bird point counts at each site and recorded birds at different distance categories. We only considered in the analyses bird species that rely on mature forests. To determine bird density at each site we carried out Distance Sampling models. To determine if sites had different bird assemblages, we evaluated the beta diversity through Sorensen dissimilarity index. Our results show that in the wet season, birds that rely on mature forests and resident birds had a lower density in the high-severity burnt site than in the reference site. Also in the wet season, the bird assemblage that rely on mature forests and resident birds had a greater dissimilarity in high-severity burnt site compared to the reference site. In the dry season, only the density of resident birds had a significant lower density in high-severity burnt site than in the reference site. However, no dissimilarity was found for any bird group among sites in the dry season. Birds that rely on mature forests were adversely influenced by high-severe fire in the breeding season, but not in the non-breeding season. This suggests that high-severe fires in the Piedmont Forest probably reduce available nesting resources and diminishes the breeding habitat quality. High-severe fires change the vegetation structure and reduce the habitat quality for resident birds in both seasons. An important management option would be to prevent human-caused high-severity fire and consider prescribed burning to limit the fire severity. In a context where fires are becoming more severe, it is of vital importance the inclusion of information on bird response to fire severity to ensure the conservation of biodiversity.

Biogeographical patterns of rhizosphere microbial communities in Robinia pseudoacacia forests along a north–south transect in the Loess Plateau

Rhizosphere microbiota play an essential role in plant productivity; however, their biogeographical patterns in restored forest ecosystems across large spatial scales remain poorly understood. We used the Ion S5TM XL sequencing platform to explore the distributions of microbial communities in the rhizosphere and bulk soil of Robinia pseudoacacia at 11 mature forest sites along a north–south precipitation gradient in the Loess Plateau, China. Microbial alpha-diversity mostly peaked in the south zone at lower latitudes, and significant distance-decay patterns were observed in both bacterial and fungal communities. Fungal communities exhibited more distinct spatial patterns in the rhizosphere than bacterial communities. The relative influence of geographic factors on bacterial/fungal community turnover in the rhizosphere (12.4/16.7%) and bulk soil (16.1/17.1%) was greater than that of local environmental factors. Microbial community assembly was mainly governed by deterministic processes (98% of nearest taxon index > 2), despite their relative importance decreasing from bulk soil to the rhizosphere in the case of bacteria. Co-occurrence patterns of sensitive taxa were more complex in the bacterial network than in the fungal network owing to a marked regional effect. Interactions of bacterial taxa differentially enriched between soil compartments were much closer than fungal interactions, indicating a stronger rhizosphere effect on bacteria. The results provide novel insights into the assembly of rhizosphere microbial communities in large-scale forest ecosystems under vegetation restoration.

Leaf-litter frog abundance increases during succession of regenerating pastures

Abstract The extensive clearing and modification of forests by anthropogenic activities is a major driver of biodiversity loss. Declines of common species are especially concerning because of the potentially large cascading effects they might have on ecosystems. Regrowth of secondary forests may help reverse population declines by restoring habitats to similar conditions prior to land conversion but the value of these secondary forests to fauna is not well understood. We compared the abundance of a direct-developing terrestrial frog, Craugastor stejnegerianus, in riparian and upland habitats of pasture, secondary forest, and mature forest sites. Mean abundance per transect was lower in upland pasture compared to mature forest. Secondary forest had similar abundance to mature forest regardless of age. We show that conversion of forest habitat to pasture represents a conservation threat to this species. However, riparian buffers help mitigate the negative effect of conversion of forest to pasture, and regrowth of secondary forest is an effective management strategy for restoring the abundance of this common leaf-litter species.

Assessing and modeling diurnal temperature buffering and evapotranspiration dynamics in forest restoration using ECOSTRESS thermal imaging

Restoration of temperate forests alters the diurnal dynamics of land surface temperature (LST) and evapotranspiration (ET) through increases in biomass, diversity, and complexity. The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) is the first space-based thermal imaging instrument that allows for a study of diurnal LST and ET dynamics at a moderate spatial resolution (70 × 70 m).We quantified, compared, and modeled the LST and ET dynamics of two groups of forest restoration sites (43 sites in total, between 2 and 75 ha) in Southern Ontario, Canada that were initially restored from mainly agriculture between 2007 and 2019. We included all useable LST (n = 29) and ET (n = 9) ECOSTRESS image products from a full growing season in 2020 (June 1st to September 30th). Forest restoration sites were compared by age and against agriculture (pre-restoration state), mature forest (post-restoration state), and suburban residential sites (competing land use).The ability of forest restoration sites to buffer temperatures was highest in the afternoon, around 14:00 local time (EDT). As predicted, restoration sites were significantly cooler (x̄ 4.4–7.4 °C) than residential and agricultural areas and significantly warmer (x̄1.6–2.9 °C) compared to mature reference forest sites at both groups of sites. Relative diurnal (24-h) LST variability of forest and restoration sites was also significantly lower (0.9–2.9 °C) compared to agriculture and residential sites (3.3–5.2 °C). Daytime LST decreased significantly by 0.1 °C (3.1%), per year since restoration for one of the groups of sites relative to nearby mature reference forest sites as per a linear mixed effects regression model. It would take these sites ~30 years to reach the same buffering as mature forests. In characterizing ET dynamics of a subset of sites, we found that more recently restored sites had a statistically significant higher overall ET than older ones and that daytime relative instantaneous ET decreased with years since restoration. The variation explained by the ET model was however low.Our study provides insights into how diurnal forest ecosystem energy conversion and storage dynamics changes over time after restoration. These diurnal thermal dynamics impact wildlife habitat as well as human wellbeing. The change in thermal buffering over time could be used to assess the pace and trajectory of restoration by managers. The thermal buffering provided by restored forest can also be quantified as an ecosystem service. As such, the study demonstrates the utility and also limitations of novel thermal remote sensing methods, using free and publicly available imagery from ECOSTRESS.

Riparian buffers provide refugia during secondary forest succession

AbstractAimSecondary forests regenerating from human disturbance are increasingly becoming a predominant forest type in many regions, and they play a significant role in forest community dynamics. Understanding the factors that underlie the variation in species responses during secondary succession is important for understanding community assembly and biodiversity monitoring and management. Because species vary in ecology and behaviour, responses to ecosystem change should vary among species. Here, we show that habitat type (riparian, upland), phylogeny, and species traits mediate anuran and lizard probability of occurrence and species richness in pasture and secondary forest.LocationSarapiquí and Osa Peninsula, Costa Rica.MethodsWe used phylogenetic occupancy models to estimate assemblage‐level and species‐specific responses to forest succession in 30 chronosequence sites that include pasture, secondary forest regenerating from pasture, and mature forest sites.ResultsFor the majority of species, we found increasing probability of occurrence in upland habitats as forest regenerated from pasture to secondary forest and similar probability of occurrence in riparian habitats across pasture, secondary forest, and mature forest sites. Species' responses to forest stage were phylogenetically correlated, and the trend was especially strong for anuran response to pasture sites. Anurans with lentic larval habitat had a positive occupancy response to pasture upland habitat, and anurans with lotic larval habitat had a variable response to different forest stages compared to mature forest.Main ConclusionsOur study, which focuses on sites that are minimally isolated from mature forest reference sites, indicated that anuran and lizard occupancy rapidly recovered to a level similar to mature forest in a relatively short time span (approximately 20 years). Riparian habitats are key ecosystem features in our system and provide refugia for organisms in early successional stages. Maintenance of vegetation along streams shows that we can mitigate forest conversion by maintaining riparian buffers.

Tree species richness as an important biotic factor regulates the soil phosphorus density in China's mature natural forests

Tree species richness has been recognized as an underlying driving factor for regulating soil phosphorus (P) status in many site-specific studies. However, it remains poorly understood whether this is true at broad scales where soil P strongly rely on climate, soil type and vegetation type. Here, based on the data of 946 mature natural forest sites from a nationwide field survey in China, we analyzed the impact of tree species richness on soil P density of China's mature natural forests (deciduous coniferous forest, DCF; evergreen coniferous forest, ECF; deciduous broad-leaved forest, DBF; evergreen broad-leaved forest, EBF; and mixed coniferous and broad-leaved forest, MF). Our results showed that tree species richness had a negative effect on soil P density in China's mature natural forests. The Random Forest regression model showed that the relative importance of tree species richness to soil P density was second only to the climate factors (mean annual temperature, MAT; mean annual precipitation, MAP). In addition, the structural equation model (SEM) results showed that the goodness fit of SEM increased when the tree species richness was included into the model. These results suggested that tree species richness was an important factor in regulating the China's mature natural forests soil P density. Furthermore, the SEM results showed that the decreased soil P density was related to the increase in ANPP and the decrease in litter P concentration induced by tree species richness. This result indicates that tree species richness could facilitate plant P absorption and inhibit plant P return into the soil, and thus reducing the soil P density in China's mature natural forests. In conclusion, we found tree species richness was an important biotic factor in regulating soil P density at broad scales, which should be fully considered in Earth models that represent P cycle.

Metabarcoding reveals landscape drivers of beetle community composition approximately 50 years after timber harvesting

Landscape conservation planning in managed forests requires information on the relative importance of different aspects of mature forests. Regeneration forest sites with greater access to source populations for re-establishment of biodiversity are expected to have greater similarity in species composition to unharvested mature areas, i.e., when sites are in close proximity to mature forest (forest influence), and/or have a high percentage of mature forest in the surrounding landscape (landscape context). We investigated how recovery of ground-active beetle biodiversity in the mid-late successional stage (40–58 years) of previously harvested regeneration forests is affected by forest influence, landscape context, and other characteristics of the surrounding landscape. We used DNA metabarcoding to characterise beetle communities in 12 mature forest sites and 64 regeneration forest sites. Generalized dissimilarity modelling (GDM), constrained redundancy analysis, and ordinations evaluated the contribution of predictors (i.e., bioclimate, landscape configuration, regeneration age, spatial position and topography) to beetle composition. Beetle composition was significantly different between different forest ages and landscape context classes. GDM of all predictors explained 34.1% of total variance in beetle community turnover. While the geographic locations of sites accounted for most (75.1%) of composite ecological gradients, the beetle community is subtly influenced by the effects of landscape context (1.8%), forest influence (1.2%) and other variables relating to landscape configuration (collectively 5.2%). Long-term conservation of local biodiversity in managed forests requires maintaining a certain amount of mature forest, but their importance as beetle source populations declines as the regeneration forests mature.

DNA metabarcoding captures subtle differences in forest beetle communities following disturbance

DNA metabarcoding is an emerging approach for monitoring biodiversity, but uncertainties remain about its capacity to detect subtle differences in invertebrate community composition comparable to those achievable based on conventional morphological identification. In this study, DNA metabarcoding and morphology‐based approaches were compared as tools for investigating whether logging history impacted beetle communities in Tasmanian wet eucalypt forests. We compared 12 unlogged mature forest sites with 12 neighboring regeneration sites that had been logged approximately 55 years previously. The number of species identified based on morphology (173) was close to the number of zero‐radius operational taxonomic units (ZOTUs) identified by DNA metabarcoding of cytochrome c oxidase subunit I (COI, 176) and 16S ribosomal RNA (16S, 156) markers. Subtle but significant differences in beetle species composition between regeneration and unlogged mature forests were captured by both morphology‐based and COI DNA metabarcoding approaches, but not by 16S DNA metabarcoding. Our results support the suitability of mitochondrial COI for studying invertebrate biodiversity. A slight loss of signal compared to the morphology‐based approach may be resolved by developing more comprehensive DNA reference databases. While confirming forest recovery of 48–58 years did not fully restore mature forest beetle communities, we suggest that DNA metabarcoding can be used for monitoring biodiversity and probing subtle differences in community composition.

A comparison between reclamation stockpile and boreal forest seed banks and plant communities

Soil stockpiles are essential to the reclamation of large‐ and small‐scale mining and other industrial sites. However, stockpiling soils can lead to the degradation of seed banks. This study examines the diversity, composition, depth of seed storage, and relationships between the aboveground and seed bank plant communities in stockpiles and compares them to the nearby boreal forest. The seed bank and aboveground vegetation sampled at eight stockpiles and six mature forest sites were near Fort McMurray (57.337°N, 111.755°W) and Cold Lake (54.695°N, 110.730°W), Alberta, Canada. Seed bank samples were taken from the forest floor (LFH) and depths of 0–5, 5–10, 10–20, 20–30, >50 cm. Aboveground vegetation cover was also estimated at these locations. The seed bank composition was determined using the seedling emergence method in a greenhouse. Stockpile seed banks had higher seedling abundance and species richness than nearby forested sites but were dominated by grasses and non‐native forbs. Most seeds germinated from the surface layer, with 92% of seeds germinating from the LFH layers in the forested sites, and 68% from the 0 to 5 cm layer in the stockpiles. Mature forest sites had more similar aboveground and seed bank communities than the stockpiles. Overall, integrating information on seed bank and aboveground plant communities would improve reclamation decisions, rather than relying on aboveground vegetation alone.

Climate more important than soils for predicting forest biomass at the continental scale

Above‐ground biomass in forests is critical to the global carbon cycle as it stores and sequesters carbon from the atmosphere. Climate change will disrupt the carbon cycle hence understanding how climate and other abiotic variables determine forest biomass at broad spatial scales is important for validating and constraining Earth System models and predicting the impacts of climate change on forest carbon stores. We examined the importance of climate and soil variables to explaining above‐ground biomass distribution across the Australian continent using publicly available biomass data from 3130 mature forest sites, in 6 broad ecoregions, encompassing tropical, subtropical and temperate biomes. We used the Random Forest algorithm to test the explanatory power of 14 abiotic variables (8 climate, 6 soil) and to identify the best‐performing models based on climate‐only, soil‐only and climate plus soil. The best performing models explained ~50% of the variation (climate‐only: R2 = 0.47 ± 0.04, and climate plus soils: R2 = 0.49 ± 0.04). Mean temperature of the driest quarter was the most important climate variable, and bulk density was the most important soil variable. Climate variables were consistently more important than soil variables in combined models, and model predictive performance was not substantively improved by the inclusion of soil variables. This result was also achieved when the analysis was repeated at the ecoregion scale. Predicted forest above‐ground biomass ranged from 18 to 1066 Mg ha−1, often under‐predicting measured above‐ground biomass, which ranged from 7 to 1500 Mg ha−1. This suggested that other non‐climate, non‐edaphic variables impose a substantial influence on forest above‐ground biomass, particularly in the high biomass range. We conclude that climate is a strong predictor of above‐ground biomass at broad spatial scales and across large environmental gradients, yet to predict forest above‐ground biomass distribution under future climates, other non‐climatic factors must also be identified.

Canopy and understory nitrogen additions did not significantly change the community structure of soil fauna under a mature subtropical forest

Increasing nitrogen (N) deposition has seriously harmed the structure and function of ecosystems throughout the world and this problem has been increasing. How N deposition affects soil faunal communities is poorly understood, compared to plant and microbial communities. Canopy and understory N additions of 25 and 50 kg ha−1 year−1 were employed to determine whether the effects of N addition on the soil fauna differ between N released to the canopy or to the understory. Specifically, we examined how the soil fauna survives when N additions produce desynchronized and complex impacts on the soil, microbes and litter quantity under mature subtropical forest ecosystems. Our results showed that no significant differences were observed between the soil faunal communities receiving canopy and understory N additions. This is consistent with our observation that the concentrations of ammonia nitrogen and nitrate nitrogen in the soil did not significantly differ under the two different methods of applying N. There were no observed effects on the litter quantity, soil microbial Phospholipid Fatty Acids or soil physical-chemical properties; therefore, it is not surprising that N treatments for 4 years did not significantly alter the community structure of soil fauna under the mature subtropical forest sites. However, the shifts in seasonal differences in the microbial communities under the N treatments had a positive effect on soil microbial development compared to control, which might also produce a time-delay influence on the relative development of the soil fauna under mature subtropical forest in the future. Further dynamic monitoring is needed to illustrate the possible effects and mechanisms by which increasing N deposition may alter soil faunal development in the future.

Songbird community response to regeneration of reclaimed wellsites in the boreal forest of Alberta

The boreal forest of Alberta, Canada is important breeding habitat for North American songbirds. Thousands of oil and gas wellsites exist in this region that have been actively reclaimed since the 1960s. Limited information exists on how songbirds respond to regeneration of wellsites following reclamation. Methods that provide spatially accurate data are required to determine impacts of these small disturbances characteristic of energy sector on songbirds. Acoustic localization can be used to determine singing locations, based on time of arrival differences of songs to an array of microphones. We used acoustic localization to determine the assemblage of songbirds on 12 reclaimed wellsites ranging from 7 to 49 years since reclamation, and how the similarity of this assemblage to 12 control mature forest sites (greater than 80 years old) changed with increasing canopy cover on the wellsite. Songbird community composition became more similar to mature forest as canopy cover increased on reclaimed wellsites. Results from this study suggest that wellsite reclamation practices are allowing for initial suitable vegetation recovery, however more research on the effectiveness of different strategies at promoting regeneration of wellsites and subsequent impact on songbird communities is required.

Energy and CO2 exchange in an undisturbed spruce forest and clear-cut in the Southern Taiga

Effects of clear cutting and other forest disturbances on surface radiative properties and the energy and CO2 fluxes between land surface and the atmosphere can vary significantly depending on local climatic and moisture conditions, forest structure and species composition, soil properties and many other factors. In this study we analyzed the influence of clear-cutting on the energy, water vapor and CO2 fluxes in the still very poorly investigated part of the boreal forest community in the European part of Russia. This issue has become particularly relevant due to intensified logging in the region during recent decades. The sensible (H) and latent (LE) heat, as well as CO2 fluxes were continuously measured at recently clear-cut and undisturbed mature spruce forest sites using eddy covariance technique during the first growing season following harvest. Because of their close location they are characterized by similar meteorological conditions. The results of our field measurements showed that the clear-cut strongly influenced the energy balance and CO2 fluxes between the land surface and atmosphere. Energy fluxes (LE and H) at the undisturbed forest site were consistently larger than at the clear-cut throughout the period of measurements. The Bowen ratio (β=H/LE) varied significantly over time, though was similar at both sites. Whereas H was almost equal to LE at both sites in spring, the LE significantly exceeded H over the summer (β≈0.2 - for mature spruce forest and β = 0.4 - for clear-cut). The mean β for the entire period was similar (β≈0.5) at both sites. Analysis of CO2 fluxes showed that the clear-cut was a consistent source of CO2 to the atmosphere. Net ecosystem exchange (NEE) at the clear-cut averaged 3.3 ± 1.3 gC∙m−2∙d-1 (±1 SD), while average NEE at the undisturbed mature forest was close to zero (0.1 ± 1.9 gC∙m−2∙d-1). Differences in NEE were mainly governed by differences in gross primary productivity (GPP) between sites (7.0 ± 4.1 gC∙m−2∙d-1 and 4.1 ± 3.0 gC∙m−2∙d-1, for the undisturbed forest and clear-cut, respectively). Total ecosystem respiration (TER) did not significantly (p < 0.05) differ between sites (7.1 ± 3.6 gC∙m−2∙d-1 at the undisturbed mature forest and 7.4 ± 3.4 gC∙m−2∙d-1 at clear-cut). TER at the undisturbed forest showed higher sensitivity to changes in soil temperature, whereas GPP at the clear-cut was characterized by higher light-use efficiency. Our measurements showed that TER rates were relatively high in the southern taiga in comparison with other boreal sites where CO2 fluxes were previously investigated.

Examining mineral-associated soil organic matter pools through depth in harvested forest soil profiles.

Mineral-associated organic matter is associated with a suite of soil minerals that can confer stability, resulting in the potential for long-term storage of carbon (C). Not all interactions impart the same level of protection, however; evidence is suggesting that C in certain mineral pools is dynamic and vulnerable to disturbance in the decades following harvesting. The objective of this research was to describe and characterize organic matter-mineral interactions through depth in horizons of soils of contrasting stand age. Sequential selective dissolutions representing increasingly stable mineral-associated organic matter pools from water soluble minerals (deionized water), organo-metal complexes (Na-pyrophosphate), poorly-crystalline minerals (HCl hydroxylamine), and crystalline secondary minerals (Na-dithionite HCl)) were carried out for Ae, Bf and BC horizons sampled from a Young and Mature forest site (35 and 110 years post-harvest) in Mooseland, Nova Scotia, Canada. Sequential selective dissolution extracts were analyzed for C, δ13C, iron (Fe) and aluminum (Al). Organo-metal complexes (OMC) were the largest mineral-associated OM pool in all horizons. This pool dominated the C distribution in B horizons (~60–70% of Bf bulk C), with a minor contribution from poorly-crystalline (PCrys), crystalline (Crys) minerals and water soluble (WS) associations. C in OMC and PCrys pools explained the variation in bulk C in horizons through depth at both sites. Twice as much C in OMC pools was measured at the Mature site compared to the Young site in the Bf horizons, supported by higher C:(Fe+Al) ratios. Isotopic analysis indicated that this extraction procedure isolated distinct mineral-associated OM pools. δ13C signatures of pyrophosphate-extracted OMC pools ranged from -27‰ to -28‰, similar to δ13C of bulk C and to plant-derived humic acids and associated biomass. The water soluble phase (mean δ13C = -29 ‰) was up to 2 ‰ more depleted, whereas the δ13C of Crys pools were more enriched in 13C (-13‰ to -16 ‰) compared to bulk soil. The results from this study suggest that association with minerals does not necessarily confer stability: organo-metal pools dominate in podzol horizons through depth, and contribute most to C storage, but are potentially susceptible to destabilization following the physical changes resulting from forest harvesting disturbance.

Response of ground and rove beetles (Coleoptera: Carabidae, Staphylinidae) to operational oil sands mine reclamation in northeastern Alberta, a case study

Species loss caused by anthropogenic disturbance threatens forest ecosystems globally. Until 50 years ago, the major sources of boreal forest disturbance in western Canada were a combination of forest wild fire events, pest insect outbreaks, and forest timber harvesting. However, in the 1960s, when the oil boom started in Alberta, oil and gas development along with oil sands mining quickly became another major forest disturbance agent. In this case study we report the effects of operational oil sands mine reclamation on terrestrial arthropod communities and compare them with nearby burned and mature forest sites as a way to provide a benchmark from which to understand the long-term trajectory of recovery for these groups. During the summer of 2016 over 6700 epigaeic beetles were collected using pitfall traps. A total of 43 species of ground beetles and 118 species of rove beetles were collected. Epigaeic beetle assemblages differed between the reclaimed, burned, and mature forest sites. Partitioning of beta diversity in the reclaimed, burned areas and mature forests indicated that species turnover formed the largest component of diversity. Species richness patterns were similar among sites; however, cluster analysis indicated that epigaeic beetle assemblages were only 20% similar between the reclaimed and natural sites. Although ground beetles of the reclaimed area showed positive spatial autocorrelation among treatments, both ground and rove beetles showed responses to the reclamation treatments. The reclaimed areas were dominated by small- to medium-sized open-habitat eurytopic species, whereas the fire and mature forest sites were dominated by larger forest species. The reclaimed area of this case study constitutes a novel, reconstructed ecosystem that is clearly not equivalent in species assemblage to burnt stands of similar age or to mature forest stands.

Avian community characteristics and demographics reveal how conservation value of regenerating tropical dry forest changes with forest age.

Expansion of secondary forests following the abandonment of agriculture may have important implications for bird conservation, but few studies have examined the dynamics of this process. We studied bird use of a chronosequence of differently-aged abandoned pastures regenerating to dry forest to better understand how the value of these habitats to birds changes over time. In a five year study on Hispaniola, we recorded 7,315 net captures of 60 species of landbirds in sites that began the study at two, five, 10, and 20 years post-abandonment, and in mature native dry forest. Twenty-five species made up 97% of all net captures. Highest capture rates were in the two youngest sites. These early-successional habitats had many over-wintering Neotropical migrants; among residents, granivores and frugivores predominated. In contrast, both the twenty-year-old and mature forest sites had few migrants, more resident insectivores and omnivorous species, and a greater proportion of endemics. Age and sex ratios, body condition and site persistence suggest early successional sites were sub-optimal for most over-wintering migrants, but habitat improved with age for three migratory species; results for permanent residents varied among species. Remnant trees and understory shrubs in the agroecological matrix likely contributed to avian diversity in regenerating dry forest sites, and proximity to mature forest also likely affected the diversity and abundance of birds in regenerating habitat. Our study shows that regenerating forests do not fully compensate for loss of mature dry forest habitat, even after 24 years of regeneration; natural restoration of complex microhabitats in dry forest sites converted to agriculture may take decades or longer. The highest value of regenerating forests may be as habitat for some over-wintering Neotropical migrants, and in creating a buffer zone that enhances biodiversity conservation by re-integrating these lands into the protected tracts of mature forest needed by the islands more unique and endemic bird species.

Manipulative experiments demonstrate how long-term soil moisture changes alter controls of plant water use

Tree transpiration depends on biotic and abiotic factors that might change in the future, including precipitation and soil moisture status. Although short-term sap flux responses to soil moisture and evaporative demand have been the subject of attention before, the relative sensitivity of sap flux to these two factors under long-term changes in soil moisture conditions has rarely been determined experimentally. We tested how long-term artificial change in soil moisture affects the sensitivity of tree-level sap flux to daily atmospheric vapor pressure deficit (VPD) and soil moisture variations, and the generality of these effects across forest types and environments using four manipulative sites in mature forests. Exposure to relatively long-term (two to six years) soil moisture reduction decreases tree sap flux sensitivity to daily VPD and relative extractable water (REW) variations, leading to lower sap flux even under high soil moisture and optimal VPD. Inversely, trees subjected to long-term irrigation showed a significant increase in their sensitivity to daily VPD and REW, but only at the most water-limited site. The ratio between the relative change in soil moisture manipulation and the relative change in sap flux sensitivity to VPD and REW variations was similar across sites suggesting common adjustment mechanisms to long-term soil moisture status across environments for evergreen tree species. Overall, our results show that long-term changes in soil water availability, and subsequent adjustments to these novel conditions, could play a critical and increasingly important role in controlling forest water use in the future.

Water limitations on forest carbon cycling and conifer traits along a steep climatic gradient in the Cascade Mountains, Oregon

Abstract. Severe droughts occurred in the western United States during recent decades, and continued human greenhouse gas emissions are expected to exacerbate warming and drying in this region. We investigated the role of water availability in shaping forest carbon cycling and morphological traits in the eastern Cascade Mountains, Oregon, focusing on the transition from low-elevation, dry western juniper (Juniperus occidentalis) woodlands to higher-elevation, wetter ponderosa pine (Pinus ponderosa) and grand fir (Abies grandis) forests. We examined 12 sites in mature forests that spanned a 1300 mm yr−1 gradient in mean growing-year climate moisture index (CMIgy ), computed annually (1964 to 2013) as monthly precipitation minus reference evapotranspiration and summed October to September. Maximum leaf area, annual aboveground productivity, and aboveground live tree biomass increased with CMIgy (r2 = 0.67–0.88, P &lt; 0.05), approximately 50-, 30-, and 10-fold along this drier to wetter gradient. Interannual fluctuations in CMI affected the annual radial growth of 91 % of juniper, 51 % of pine, and 12 % of fir individuals from 1964 to 2013. The magnitude of the site-average growth–CMI correlations decreased with increased CMIgy (r2 = 0.53, P &lt; 0.05). All three species, particularly fir, experienced pronounced declines in radial growth from c. 1985 to 1994, coinciding with a period of sustained below-average CMIgy and extensive insect outbreak. Traits of stress-tolerant juniper included short stature, high wood density for cavitation resistance, and high investment in water transport relative to leaf area. Species occupying wetter areas invested more resources in height growth in response to competition for light relative to investment in hydraulic architecture. Consequently, maximum tree height, leaf area : sapwood area ratio, and stem wood density were all correlated with CMIgy . The tight coupling of forest carbon cycling and species traits with water availability suggests that warmer and drier conditions projected for the 21st century could have significant biogeochemical, ecological, and social consequences in the Pacific Northwest.