Coastal Temperate Rainforest Research Articles

Comparison of airborne laser scanning and digital stereo imagery for characterizing forest canopy gaps in coastal temperate rainforests

Forest canopy gaps play an important role in forest dynamics. Airborne laser scanning (ALS) data provide demonstrated capacity to systematically and accurately detect and map canopy gaps over large forest areas. Digital aerial photogrammetry (DAP) is emerging as an alternative, lower-cost source of three-dimensional information for characterizing forest structure and modelling forest inventory attributes. In this study we compared the relative capacities of ALS and DAP data to map canopy gaps in a complex coastal temperate rainforest on Vancouver Island, British Columbia, Canada. We applied fixed- and variable-height threshold approaches for gap detection using both ALS and DAP data, and validated outcomes using independent data derived via visual image interpretation. Overall accuracies for ALS-derived gaps were 96.50% and 89.50% for the fixed- and variable-height threshold approaches respectively, compared to 59.50% and 50.00% for the DAP-derived gaps, with DAP data having large errors of omission (>88%). We found that 70% of ALS-derived gaps were identified in old seral stage stands (age > 250 years), while 65% of DAP-derived gaps were located in early seral stage stands (age < 40 years). For the DAP data, gap detection accuracy was 80% in early seral stands, compared to 50% in old seral stands. In contrast, ALS detection accuracy varied by only ~6% between early and old seral stages. We compared detected gaps using a variety of metrics and found significant differences in the number and average size of gaps detected using ALS and DAP data. Using the fixed-height threshold, the ALS data identified 16 times more gaps and 6.5 times more gap area than the DAP data, with a mean ALS-derived gap size that was half that of the DAP data. The average amount of overlap between ALS- and DAP-detected gaps was 13.26% and 42.90% for the variable and fixed thresholds, respectively. We attribute these differences in gap detection to the nature of the DAP data itself, which characterizes primarily the outer canopy envelope, as well as to the confounding effects of canopy complexity and related occlusions and shadows on image matching algorithms. We conclude that DAP data do not provide analogous results to ALS data for canopy gap detection and mapping in coastal temperate rainforests, and that ALS data enable markedly superior accuracy and detailed gap characterizations.

Nitrogen dynamics vary across hydrologic gradients and by forest community composition in the perhumid coastal temperate rainforest of southeast Alaska

Nitrogen (N) limitation constrains plant growth, but complex interactions among species and ecosystems hinder our ability to identify primary drivers of N availability. Hydrologic, biogeochemical, and ecological processes interact spatially and temporally, requiring measurements of N across diverse ecosystem types and as a function of both site conditions and vegetation composition. We measured initial exchangeable and mineralized N along a hydrologic gradient in the Alaskan perhumid coastal temperate rainforest to test a conceptual model of linkages between N availability and landscape, hydrologic, and ecosystem characteristics in temperate forests. Mineralization was closely associated with inorganic N concentrations. Inorganic N as NH4+ generally increased with increasing depth to groundwater but was strongly determined by plant–water interactions. Exchangeable and mineralized N were closely linked to tree species, forest biomass, and hydrologic regime regardless of ecosystem type. The emergence of tree species as indicators of N cycling highlights the effect that species have on nutrient dynamics, while the trend of increasing inorganic N with increasing soil saturation points to the role of hydrology in driving N availability. Our research quantified N dynamics for an understudied, yet critical, system and provides a framework for exploring feedbacks among soil saturation, forest composition, and nutrient cycling in temperate forests.

Limited Effects of Variable-Retention Harvesting on Fungal Communities Decomposing Fine Roots in Coastal Temperate Rainforests.

Fine root litter is the principal source of carbon stored in forest soils and a dominant source of carbon for fungal decomposers. Differences in decomposer capacity between fungal species may be important determinants of fine-root decomposition rates. Variable-retention harvesting (VRH) provides refuge for ectomycorrhizal fungi, but its influence on fine-root decomposers is unknown, as are the effects of functional shifts in these fungal communities on carbon cycling. We compared fungal communities decomposing fine roots (in litter bags) under VRH, clear-cut, and uncut stands at two sites (6 and 13 years postharvest) and two decay stages (43 days and 1 year after burial) in Douglas fir forests in coastal British Columbia, Canada. Fungal species and guilds were identified from decomposed fine roots using high-throughput sequencing. Variable retention had short-term effects on β-diversity; harvest treatment modified the fungal community composition at the 6-year-postharvest site, but not at the 13-year-postharvest site. Ericoid and ectomycorrhizal guilds were not more abundant under VRH, but stand age significantly structured species composition. Guild composition varied by decay stage, with ruderal species later replaced by saprotrophs and ectomycorrhizae. Ectomycorrhizal abundance on decomposing fine roots may partially explain why fine roots typically decompose more slowly than surface litter. Our results indicate that stand age structures fine-root decomposers but that decay stage is more important in structuring the fungal community than shifts caused by harvesting. The rapid postharvest recovery of fungal communities decomposing fine roots suggests resiliency within this community, at least in these young regenerating stands in coastal British Columbia.IMPORTANCE Globally, fine roots are a dominant source of carbon in forest soils, yet the fungi that decompose this material and that drive the sequestration or respiration of this carbon remain largely uncharacterized. Fungi vary in their capacity to decompose plant litter, suggesting that fungal community composition is an important determinant of decomposition rates. Variable-retention harvesting is a forestry practice that modifies fungal communities by providing refuge for ectomycorrhizal fungi. We evaluated the effects of variable retention and clear-cut harvesting on fungal communities decomposing fine roots at two sites (6 and 13 years postharvest), at two decay stages (43 days and 1 year), and in uncut stands in temperate rainforests. Harvesting impacts on fungal community composition were detected only after 6 years after harvest. We suggest that fungal community composition may be an important factor that reduces fine-root decomposition rates relative to those of above-ground plant litter, which has important consequences for forest carbon cycling.

“Legalizing” the Great Bear Rainforest Agreements: Colonial Adaptations Toward Reconciliation and Conservation

The Great Bear Rainforest (GBR) agreements are heralded as one of the most important conservation initiatives in the world. They are intended to result in the protection of eighty-five per cent of the coastal temperate rainforest landscape on the British Columbia coast and to see seventy percent of the rainforest returned to old-growth forest. A clear terrestrial environmental success, the negotiation process and agreements are equally important for their enlivenment of Aboriginal rights and the governance authority of the Indigenous communities of the central and north coasts within a colonial law context. After stakeholders wrangled largely over the details of ecosystem-based management, First Nations and the provincial government engaged in government-to-government negotiations that are yielding agreement on the exercise of Aboriginal rights across an intact landscape, funding and priority access for First Nations’ ventures as part of a conservation economy, and enhanced roles in decision making. In the absence of treaties and in a common law Aboriginal rights and title context, these agreements are a robust example of the movement toward reconciliation. The purpose of this article is to describe how the protection of the GBR and the expression of Aboriginal rights in that process has manifested in colonial law, and to examine these agreements in the context of reconciliation in Canada. While unique and ongoing, as all reconciliation efforts will be, the GBR agreements locate land-based protection and governance at their core. As an applied, ongoing initiative, these agreements give life to the concepts of joint decision making and underscore the nation- and place-specific context of any reconciliation process that must adapt over time.

Limited stand expansion by a long‐lived conifer at a leading northern range edge, despite available habitat

Abstract In an era of rapid climate change, understanding the natural capacity of species' ranges to track shifting climatic niches is a critical research and conservation need. Because species do not move across the landscape through empty space, but instead have to migrate through existing biotic communities, basic dispersal ecology and biotic interactions are important considerations beyond simple climate niche tracking. Yellow‐cedar (Callitropsis nootkatensis), a long‐lived conifer of the North Pacific coastal temperate rainforest region, is thought to be undergoing a continued natural range expansion in southeast Alaska. At the same time, yellow‐cedar's trailing edge is approaching its leading edge in the region, due to climate‐induced root injury leading to widespread mortality over the past century. To examine the current dispersal capacity of yellow‐cedar at its leading range edge, and potential for the species' leading edge to stay ahead of its trailing edge, we characterized recent yellow‐cedar stand development near Juneau, Alaska, and surveyed the spread of yellow‐cedar seedlings just beyond existing stand boundaries. Despite suitable habitat beyond stand edges, stand expansion appears limited in recent decades to centuries. Large quantities of seed are germinating within stands and just beyond boundaries, but seedlings are not developing to maturity. Furthermore, c. 100–200‐year‐old yellow‐cedar trees are located abruptly at stand boundaries, indicating stand expansion is in a period of stasis with a last pulse at the end of the Little Ice Age climate period. Vegetative regeneration is common across stands and may be an adaptive strategy for this long‐lived tree to persist on the landscape until conditions are favourable for successful seedling recruitment, leading to an overall punctuated migration and colonization of new landscapes. Synthesis. Species ranges do not always respond linearly to shifting climatic conditions. Instead, successful colonization of new habitat may be tied to episodic, threshold‐related landscape phenomena, dispersal ability, and competition with existing plant communities.

Ecological legacies of anthropogenic burning in a British Columbia coastal temperate rain forest

AbstractAimFew long‐term fire histories have been reconstructed in coastal temperate rain forests, and little is known regarding the spatial and temporal characteristics of lightning and human ignitions. We use a multidisciplinary approach to assess the impact, scale and ecological legacies of historic fires.LocationWe focus on perhumid temperate rain forests located on the central coast of British Columbia, Canada.MethodsWe reconstructed 700 years of temporal and spatial aspects of fire activity with 30 plots on Hecate Island using fire scars and forest‐stand establishment. We then conducted a paired study of 20 former indigenous habitation and control sites on 15 islands to relate fire activity to patterns of human settlement. We mapped 15 years of lightning strike densities and use mixed‐effects modelling to assess whether fire activity predicted the distribution and abundance of traditional plants.ResultsSixteen low‐ and mixed‐severity fires were recorded from 1376 to 1893. The abundance of traditional plants and the density of western redcedar trees were best predicted by the location of former habitation sites and shorter mean fire intervals. Lightning is too rare to explain the pattern of fire activity in the study area. No fire activity was detected after 1893, coinciding with the relocation of indigenous groups from the study area.Main conclusionsFire was strongly associated with former indigenous habitation sites during the periods of occupation. People likely utilized fire as a tool for resource management to influence the densities of specific plants by creating mosaics of vegetation in different stages of succession. By assessing the ecological impacts of historic fire events, we gain a better understanding of the abrupt changes that occurred in the 20th century. Our ability to understand present‐day temperate rain forest ecosystems may be compromised if we underestimate the role of humans in driving historic fire activity.

A global hotspot for dissolved organic carbon in hypermaritime watersheds of coastal British Columbia

Abstract. The perhumid region of the coastal temperate rainforest (CTR) of Pacific North America is one of the wettest places on Earth and contains numerous small catchments that discharge freshwater and high concentrations of dissolved organic carbon (DOC) directly to the coastal ocean. However, empirical data on the flux and composition of DOC exported from these watersheds are scarce. We established monitoring stations at the outlets of seven catchments on Calvert and Hecate islands, British Columbia, which represent the rain-dominated hypermaritime region of the perhumid CTR. Over several years, we measured stream discharge, stream water DOC concentration, and stream water dissolved organic-matter (DOM) composition. Discharge and DOC concentrations were used to calculate DOC fluxes and yields, and DOM composition was characterized using absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC). The areal estimate of annual DOC yield in water year 2015 was 33.3 Mg C km−2 yr−1, with individual watersheds ranging from an average of 24.1 to 37.7 Mg C km−2 yr−1. This represents some of the highest DOC yields to be measured at the coastal margin. We observed seasonality in the quantity and composition of exports, with the majority of DOC export occurring during the extended wet period (September–April). Stream flow from catchments reacted quickly to rain inputs, resulting in rapid export of relatively fresh, highly terrestrial-like DOM. DOC concentration and measures of DOM composition were related to stream discharge and stream temperature and correlated with watershed attributes, including the extent of lakes and wetlands, and the thickness of organic and mineral soil horizons. Our discovery of high DOC yields from these small catchments in the CTR is especially compelling as they deliver relatively fresh, highly terrestrial organic matter directly to the coastal ocean. Hypermaritime landscapes are common on the British Columbia coast, suggesting that this coastal margin may play an important role in the regional processing of carbon and in linking terrestrial carbon to marine ecosystems.

Quantifying Coastal Rain Forest Carbon Transport

Aquatic Carbon Biogeochemistry of the Pacific Coastal Temperate Rainforest Region Workshop; Seattle, Washington, 7–10 February 2017

Spatial and Temporal Variation in the Diets of Pacific Staghorn Sculpins Related to Hydrological Factors in a Glacially Influenced Estuary

AbstractIn southeastern Alaska, a large volume of freshwater flows into estuaries from thinning and retreating glaciers and from high rainfall in the surrounding coastal temperate rainforest. Freshwater runoff to the coast affects the physical and chemical properties of estuarine habitats and, in turn, the structure and dynamics of biological communities. This study quantified temporal and spatial variation in the diets of Pacific Staghorn Sculpins Leptocottus armatus as an important step toward understanding how climate‐driven changes in freshwater runoff may translate into shifts in the feeding ecology of a highly abundant consumer in estuaries along the west coast of North America. Stomach contents of Staghorn Sculpins were collected monthly (April–September 2014) from three intertidal sites at the mouths of rivers that differ in their headwater hydrology. Staghorn Sculpins (N = 402) consumed a variety of marine prey items including epibenthic invertebrates, juvenile fishes, and terrestrial and aquatic insects. Based on a multivariate analysis of similarity, diet composition showed weak to moderate differences across sampling months and sites. Diet differences reflected seasonal shifts in the contributions of freshwater‐tolerant marine invertebrates and young of the year fishes. The greatest site differences were observed between the Cowee Creek estuary and Mendenhall River estuary, the watersheds with the lowest and highest glacial coverage, respectively. Overall, the generalist feeding behavior of Staghorn Sculpins and their consumption of prey items that are well adapted to brackish and freshwater conditions suggest that these fish are resilient to changes in estuarine conditions associated with increasing freshwater runoff.

Carbon stocks and timber harvest. Alternative policy approaches for the Great Bear rainforest and their consequences

Forest policy-making can be dramatically enhanced by implementing computer based decision support systems (DSS) during the development phase. While stakeholders usually agree on general policy objectives, the best course of action for achieving them is often unclear. Forest DSS can support the assessment of alternative forest policy frameworks with respect to their impact on multiple variables of interest for stakeholders and decision makers, yet, these tools are often underutilised.In January 2016 a new land use order policy for the Great Bear Rainforest (GBR) in Canada, the largest coastal temperate rainforest in the world, was legally established. The GBR order aims to support ecosystem-based management and legally binding objectives to ensure the long term health of this unique area. This new policy framework will lead to a dramatic decrease in timber harvest levels. We developed 5 alternative forest management scenarios with a less abrupt decline in harvest levels, including carbon sequestration as an option to economically buffer the expected declines in yield. We modelled the development of the forest resources in the area under each scenario over a timeframe of 350years. In doing so we were able to show how the use of computer based DSS is able to assist policy making by providing a comprehensive assessment of consequences associated with the alternatives, to ensure that long term targets are met.Our results show that currently, the GBR stores around 1.73Bt carbon, a number that will increase to 1.84Bt under the GBR order. While it is possible to increase carbon storage by up to 8% when lowering overall timber harvest by 36% over a timeframe of 350years, naturally, those scenarios with a less dramatic decline in timber harvest were related to an overall lower level of carbon stocks. Combined approaches though, which allow balancing timber harvests with ecosystem integrity, are a powerful alternative to a dramatic and abrupt decrease in harvest levels. The success of combined approaches does heavily depend on timber and carbon markets though. With notoriously cyclical markets the long term economy related to the implementation of the new GBR order remains uncertain when looking at carbon and timber markets only, strongly suggesting to rely on diversification and the creation of added value products.

Linking LiDAR with streamwater biogeochemistry in coastal temperate rainforest watersheds

The goal of this study was to use watershed characteristics derived from light detection and ranging (LiDAR) data to predict stream biogeochemistry in Perhumid Coastal Temperate Rainforest (PCTR) watersheds. Over a 2-day period, we sampled 37 streams for concentrations of dissolved C, N, P, major cations, and measures of dissolved organic matter quality (specific ultraviolet absorbance, SUVA254) and bioavailability. Random forest – classification and regression tree analysis showed that aboveground biomass and structure and watershed characteristics, inclusive of mean watershed slope and elevation, watershed size, and topographic wetness, explained more than 60% of the variation in concentration for most measured constituents. These results indicate this approach may be particularly useful for predicting stream biogeochemistry in small forested watersheds where fine resolution is needed to resolve subtle differences in forest biomass, structure, and topography. Overall, we suggest that the use of LiDAR in many of the small and remote watersheds across the Southeast Alaskan PCTR as well as other forested regions could help inform land management decisions that have the potential to alter ecosystems services related to watershed biogeochemical fluxes.

Vulnerability of wetland soil carbon stocks to climate warming in the perhumid coastal temperate rainforest

The perhumid coastal temperate rainforest (PCTR) of southeast Alaska has some of the densest soil organic carbon (SOC) stocks in the world (>300 Mg C ha−1) but the fate of this SOC with continued warming remains largely unknown. We quantified dissolved organic carbon (DOC) and carbon dioxide (CO2) yields from four different wetland types (rich fen, poor fen, forested wetland and cedar wetland) using controlled laboratory incubations of surface (10 cm) and subsurface (25 cm) soils incubated at 8 and 15 °C for 37 weeks. Furthermore, we used fluorescence characterization of DOC and laboratory bioassays to assess how climate-induced soil warming may impact the quality and bioavailability of DOC delivered to fluvial systems. Soil temperature was the strongest control on SOC turnover, with wetland type and soil depth less important in controlling CO2 flux and extractable DOC. The high temperature incubation increased average CO2 yield by ~40 and ~25% for DOC suggesting PCTR soils contain a sizeable pool of readily biodegradable SOC that can be mineralized to DOC and CO2 with future climate warming. Fluxes of CO2 were positively correlated to both extractable DOC and percent bioavailable DOC during the last few months of the incubation suggesting mineralization of SOC to DOC is a strong control of soil respiration rates. Whether the net result is increased export of either carbon form will depend on the balance between the land to water transport of DOC and the ability of soil microbial communities to mineralize DOC to CO2.

Long-term recovery of epiphytic communities in the Great Bear Rainforest of coastal British Columbia

The recent Great Bear Rainforest agreement recognises the high biodiversity values of this large intact area of coastal temperate rainforest by calling for old forest targets to be met by 2264. Recruiting young stands has joined conserving existing old stands as a strategy for achieving targets, but the point at which second growth stands recover oldgrowth attributes remains uncertain. We examined the recovery of epiphytes towards oldgrowth conditions by comparing community composition, richness and abundance between young (55–100year old), mature (101–250years old) and oldgrowth stands (>250year old). We felled 77 western redcedar, amabilis fir, western hemlock and Sitka spruce trees, identified all epiphytes, and examined effects of stand age, region, tree species, site nutrient status and presence of residual trees on the epiphyte community. We found 229 taxa, including 49 bryophytes, 98 macrolichens and 82 crustose lichens. Epiphyte community varied by region and among tree species, but not by site productivity or presence of residual trees. In the northern region, trees in oldgrowth supported twice as many epiphyte species, seven times as many unique species, and a significantly different community composition for all functional groups (bryophytes, crustose lichens, hair lichens, cyanolichens and other macrolichens) relative to trees in stands younger than 200years. Overall similarity between second growth and oldgrowth was about 50%. Young and mature stands overlapped considerably in richness, abundance, and community composition, indicating little recovery between 55 and 200years. Our study suggests that in the northern region of the Great Bear Rainforest, epiphyte communities need more than 200years to recover to oldgrowth conditions.

Assessing the long-term ecological effects of riparian management practices on headwater streams in a coastal temperate rainforest

Our understanding of how effective forestry Best Management Practices (BMPs) can mitigate logging impacts on the structure and functioning of temperate headwater streams tends to be based upon studies conducted within the first few years post-harvest. Little is known about how the affected biological parameters of streams change during riparian and upland forest regeneration, particularly the time taken to return to pre-harvest levels, or to within the range of reference conditions. By following up on an earlier study in southwestern British Columbia, Canada, we examined the post-logging recovery trajectories of stream attributes under multiple forest treatments. These treatments provided a gradient of riparian protection, including streamside clear-cutting with no reserves, and with fixed-width reserves (10m and 30m), and thinning with 50% basal area removal of riparian trees, which were compared with unharvested control sites. About 15years after clear-cutting and 9years after thinning, slower decomposition of red alder leaves (Alnus rubra) was reported in streams affected by forest harvesting, which was attributed to lower density of shredder invertebrates. Thinned reaches had lower litter decomposition rate than, but similar shredder density and richness to, unharvested reference sites. After a further 7years of forest regrowth (time elapsed between the previous and present study), we found that litter decomposition rate and shredder density in logging-affected reaches converged with values in reference reaches. Stream functional integrity, as reflected by litter decomposition, increased across all forest treatments. Some differences in rarefied shredder richness among forest treatments emerged in this study. The legacy effects of thinning on some biotic variables likely diminished more quickly (2–9years post-harvest) than those of clear-cutting with and without riparian reserves (8–15years). Stream recovery from thinning to reference conditions should continue to warrant attention, as wider, larger-scale partial harvesting practices are recommended within emerging paradigms of forest management. Overall, our findings indicate time scales towards recovery of an important ecological process of streams under common forestry practices, which could be longer than other stream properties, such as water quality. Such differences in recovery time frames should be considered for the planning of forestry operations and monitoring of harvesting legacies.

Seven hundred years of human-driven and climate-influenced fire activity in a British Columbia coastal temperate rainforest.

While wildland fire is globally most common at the savannah-grassland ecotone, there is little evidence of fire in coastal temperate rainforests. We reconstructed fire activity with a ca 700-year fire history derived from fire scars and stand establishment from 30 sites in a very wet (up to 4000 mm annual precipitation) temperate rainforest in coastal British Columbia, Canada. Drought and warmer temperatures in the year prior were positively associated with fire events though there was little coherence of climate indices on the years of fires. At the decadal scale, fires were more likely to occur after positive El Niño-Southern Oscillation and Pacific Decadal Oscillation phases and exhibited 30-year periods of synchrony with the negative phase of the Arctic Oscillation. Fire frequency was significantly inversely correlated with the distance from former Indigenous habitation sites and fires ceased following cultural disorganization caused by disease and other European impacts in the late nineteenth century. Indigenous people were likely to have been the primary ignition source in this and many coastal temperate rainforest settings. These data are directly relevant to contemporary forest management and discredit the myth of coastal temperate rainforests as pristine landscapes.

Seeing the forest for its multiple ecosystem services: Indicators for cultural services in heterogeneous forests

The ecosystem service (ES) framework is gaining traction in ecosystem management as a means to recognize the multiple benefits that ecosystems provide. In forested ecosystems, many structural attributes (trees, understory plants and woody debris) create heterogeneous ecosystems that provide numerous ecosystem services, including many that are culturally important. However, application of the ES framework to forest management is challenged by difficulties measuring and comparing multiple ES across diverse and heterogeneous forest conditions. Indicators can help bring the ES approach to forest management by providing a means for accurate ES inventory and mapping. We measured 10 forest ES in contrasting forest types to investigate the effects of past forest harvesting in coastal temperate rainforest of Vancouver Island, BC, Canada. Our objectives were to build a systematic set of ES indicators for coastal temperate forests based on forest structural features, including trees, coarse woody debris, and understory plants. To achieve this, we 1) analyzed field data to compare the effects of forest age (old-growth vs. second-growth) and ecological site conditions (riparian vs. upland forest) on the bundle of ES provided by different forest types; and 2) worked with a local indigenous wood carver to identify attributes of cedar trees (Thuja plicata) essential for traditional uses, including canoe carving. Forest age and forest type had significant and major effects on bundles of ES. Old-growth forests provided three times higher carbon storage, nine times higher wood volume, and eighteen times higher canopy habitat services than recovering forests. Within old-growth forests, the proportion of trees suitable for traditional indigenous wood carving was significantly higher in riparian stands. Yet of 456 trees measured, only 17 were cedar with potential traditional uses. Of those, trees for canoe carving were the least frequent (n=3), which we identified as large (>110cm DBH) trees of exceptional quality. In general, old-growth riparian forests were a hotspot of ES, providing for example nearly three times as much carbon storage as old-growth forests on upland sites and 12 times the amount of carbon storage as found in second-growth forests on upland sites. These results indicate that typical inventories of forest ES, which usually generalize across heterogeneity, may oversimplify dramatic variations in ES bundles in forested landscapes. Our novel set of stand-level ES indicators can improve the accuracy of ES assessments, incorporate important cultural ES, and help address the role of landscape heterogeneity in influencing ES.

13,000 years of fire history derived from soil charcoal in a British Columbia coastal temperate rain forest

AbstractLittle is known regarding the fire history of high‐latitude coastal temperate rain forests in the Pacific Northwest (PNW) of North America. While reconstructing historical fire regimes typically requires dendrochronological records from fire‐scarred trees or stratigraphically preserved lake sediment data, this type of information is virtually non‐existent in this region. To describe the long‐term fire history of a site on the central coast of British Columbia, Canada, we radiocarbon‐dated 52 pieces of charcoal. Charcoal ages ranged from 12,670 to 70 yr BP. Fires occurred regularly since 12,670 yr BP, with the exception of a distinct fire‐free period at 7500–5500 yr BP. Time since fire (TSF) estimates from soil charcoal and fire‐scarred trees ranged from 12,670 to 100 yr BP (median = 327 yr), and 70% of the sites examined had burned within the past 1000 yr. An increase in fire frequency in the late Holocene is consistent with the widely held hypothesis that anthropogenic fires were common across the PNW. We evaluate TSF distributions and discuss the difficulties in assigning actual fire dates from charcoal fragments with large inbuilt ages in a coastal temperate rain forest setting. We determine that a comprehensive approach using soil charcoal and fire scar analyses is necessary to reconstruct general trends in fire activity throughout the Holocene in this region.

Microbial and Environmental Controls of Methane Fluxes Along a Soil Moisture Gradient in a Pacific Coastal Temperate Rainforest

Most studies of greenhouse gas fluxes from forest soils in the coastal rainforest have considered carbon dioxide (CO2), whereas methane (CH4) has not received the same attention. Soil hydrology is a key driver of CH4 dynamics in ecosystems, but the impact on the function and distribution of the underlying microbial communities involved in CH4 cycling and the resultant net CH4 exchange is not well understood at this scale. We studied the growing season variations of in situ CH4 fluxes, microbial gene abundances of methanotrophs (CH4 oxidizers) and methanogens (CH4 producers), soil hydrology, and nutrient availability in three typical forest types across a soil moisture gradient. CH4 displayed a spatial variability changing from a net uptake in the upland soils (3.9–46 µmol CH4 m−2 h−1) to a net emission in the wetter soils (0–90 μmol CH4 m−2 h−1). Seasonal variations of CH4 fluxes were related to soil hydrology in both upland and wet soils. Thus, in the upland soils, uptake rates increased with the decreasing soil moisture, whereas CH4 emission was inversely related to the water table depth in the wet soils. Spatial variability of CH4 exchange was related to the abundance of genes involved in CH4 oxidation and production, but there was no indication of a temporal link between microbial groups and CH4 exchange. Our data show that the abundances of genes involved in CH4 oxidation and production are strongly influenced by soil moisture and each other and grouped by the upland–wetland classification but not forest type.

Disturbance Legacy on Soil Carbon Stocks and Stability within a Coastal Temperate Forest of Southwestern British Columbia, Canada

Although it has been recognized that soils play a critical role in carbon storage and that coastal temperate forests have considerable potential to sequester soil organic carbon (SOC), studies related to SOC stocks and stability are scarce in these ecosystems. Forest disturbances may leave legacies on SOC properties and may further compromise SOC storage capacity of these ecosystems. In the Pacific Spirit Regional Park of southwestern British Columbia, we compared SOC stocks and stability among three second-growth forests that have been affected by disturbances of different magnitudes. We collected data on soil chemical and physical properties to estimate SOC content and assess SOC stability. We found that SOC stocks in the forest characterized by low magnitude disturbance were greater than those of the forest characterized by high magnitude disturbance (8.2 ± 1.3 kg·Cm-2 versus 5.3 ± 0.1 kg·Cm-2 to 30 cm depth). SOC was less stable in the highly disturbed forest and subsequent vegetation changes might have further reduced SOC stability. Our results provide insight into the role of disturbance history in the current SOC storage capacity of coastal temperate rainforests of British Columbia.

Warm summer nights and the growth decline of shore pine in Southeast Alaska

Shore pine, which is a subspecies of lodgepole pine, was a widespread and dominant tree species in Southeast Alaska during the early Holocene. At present, the distribution of shore pine in Alaska is restricted to coastal bogs and fens, likely by competition with Sitka spruce and Western hemlock. Monitoring of permanent plots as part of the United States Forest Service Forest Inventory and Analysis program identified a recent loss of shore pine biomass in Southeast Alaska. The apparent loss of shore pine is concerning, because its presence adds a vertical dimension to coastal wetlands, which are the richest plant communities of the coastal temperate rainforest in Alaska. In this study, we examined the shore pine tree-ring record from a newly established plot network throughout Southeast Alaska and explored climate-growth relationships. We found a steep decline in shore pine growth from the early 1960s to the present. Random Forest regression revealed a strong correlation between the decline in shore pine growth and the rise in growing season diurnal minimum air temperature. Warm summer nights, cool daytime temperatures and a reduced diurnal temperature range are associated with greater cloud cover in Southeast Alaska. This suite of conditions could lead to unfavorable tree carbon budgets (reduced daytime photosynthesis and greater nighttime respiration) and/or favor infection by foliar pathogens, such as Dothistroma needle blight, which has recently caused widespread tree mortality on lodgepole pine plantations in British Columbia. Further field study that includes experimental manipulation (e.g., fungicide application) will be necessary to identify the proximal cause(s) of the growth decline. In the meantime, we anticipate continuation of the shore pine growth decline in Southeast Alaska.