Old-growth Douglas-fir Forests Research Articles

Historical pyrodiversity in Douglas-fir forests of the southern Cascades of Oregon, USA

Our understanding of forest dynamics and successional pathways in coastal Douglas-fir (Pseudotsuga menziesii var menziesii) forests with relatively frequent mixed-severity fires is limited by a lack of annually precise dendroecological reconstructions that combine records of historical fires and tree establishment. The processes by which old-forest heterogeneity developed under historical fire regimes with recurrent low- and moderate-severity fires has not been well studied at fine temporal scales and across spatial scales. We developed crossdated multi-century records of fire and tree establishment histories in old forests (170 – 550 years) at 34 plots distributed across six sites. Study sites include warm-dry to cool-moist Douglas-fir forest types found in the southern west Cascades of Oregon, USA. Spatial variability in historical fire frequency and fire effects resulted in tremendous diversity in forest developmental histories, age structure, and forest conditions. Most historical fire intervals were very frequent (<10 years) to frequent (<25 years) in dry Douglas-fir forests. Exceptionally high fire frequency and an abrupt decrease in fire frequency after European colonization in dry Douglas-fir forests adds to growing evidence and recognition of Indigenous fire stewardship in montane Douglas-fir forests. In moist forests where Douglas-fir is seral to western hemlock, fire intervals were frequent to moderately frequent (<50 years), but intervals varied substantially over time. Relatively young moist forests burned frequently while mature moist forests had long fire intervals (50–160 years). Nearly all tree establishment cohorts were preceded by either stand-replacing (28 %) or non-stand-replacing fires (64 %). However, tree cohorts only provided evidence of 16 % of historical fire events that we reconstructed from cambial fire scars. This study demonstrates that frequent fire can be an important driver of forest development and in some contexts shapes the structure of coastal old-growth Douglas-fir forests, which are often characterized as developing from endogenous disturbances during long fire-free periods. The high level of pyrodiversity we observed was associated with variation in and interactions of micro-climate, topography, fuels, and Indigenous fire stewardship. We recommend rigorous dendroecological reconstructions across the coastal Douglas-fir region to refine our understanding of the geography of fire-mediated forest developmental dynamics in this important forest type, to inform forest management, conservation, and ecocultural restoration.

A Place for Inquiry, A Place for Wonder: The Andrews Forest. By William G. Robbins

William Robbins’s latest study makes an important contribution to the environmental history of the Pacific Northwest through its thorough, careful scholarship that documents a significant, as-yet little recognized site in our nation’s forest science history. In A Place for Inquiry, A Place for Wonder: The Andrews Forest, Robbins argues that the 15,800-acre experimental forest—“a cloistered place, unknown to the vast majority of Oregon citizens” (p. 192)—is the site of significant contributions to ecosystem science, as well as the environmental humanities, at regional, national, and international scales. The H.J. Andrews (named posthumously for Horace J. Andrews, Regional Forester of the U.S. Forest Service’s Pacific Northwest Region (1943–1951)), located west of the Cascade Mountains within the Willamette National Forest, was established in 1948. Over its seven-decade history, this Experimental Forest of the U.S. Forest Service has generated insights about old-growth Douglas fir forests and plantations and the drainage basin’s watershed processes....

Evaluating the role of active management in mature Douglas-fir (Pseudotsuga menziesii) stands for songbird conservation

Forest birds, particularly those associated with late-successional forests, are of widespread conservation interest. Although birds are among the more widely studied taxa of forest wildlife, relatively few studies have examined the long-term effects of active management (i.e., intentional stand density reduction) on the forest bird assemblage. This is an important omission, as changes in stand structure and composition over the decades following harvesting may influence wildlife utilization of forest stands. We developed an observational study to evaluate forest bird response to stand structural development multiple decades following harvesting with differing levels of overstory density reduction, and in unmanaged stands. We focused, in particular, on songbirds and cavity excavating species associated with old-growth Douglas-fir forests, and the active management strategies that we examined – thinning and structural retention harvesting – were selected based on their potential to accelerate stand structural development. Our 18 mature stands (age range: 107–187 years) were located in the western hemlock zone of western Oregon, and bird surveys were conducted an average of 41 years and 22 years post-harvest in thinned and retention harvest stands, respectively. Poisson generalized linear models were formulated to evaluate the effects of management condition on forest birds. Although five species were associated with specific management conditions, relative abundance was not statistically different between unmanaged, thinned and retention harvest stands for a majority of the species in our analysis. Species richness was also relatively invariant across management conditions. Our results do suggest that retention harvesting adversely affects some songbird species associated with old-growth forests – Pacific-slope flycatcher (Empidonax difficilis) and Pacific wren (Troglodytes pacificus) for example - but our findings also indicate that retention harvesting has long-term benefits for birds associated with early-successional forests. This includes migrant species that have experienced significant population declines in Western North America over recent decades, such as Wilson’s and MacGillivray’s warblers (Cardellina pusilla and Geothlypis tolmiei, respectively). Overall, our results imply that for many species of forest birds, including those associated with old-growth forests, managers have some flexibility in overstory density management where long-term species persistence is an objective. Equally significant, our results provide strong support for the application of variable retention harvesting as a tool for the conservation of bird species associated with early-successional forests, while also reinforcing the value of mature structural legacies for bird species associated with late-successional forests. While also reinforcing the value of mature structural legacies for birds associated with late-successional forests.

Dissolved organic carbon production and flux under long-term litter manipulations in a Pacific Northwest old-growth forest

Dissolved organic carbon (DOC) flux is an important mechanism to convey soil carbon (C) from aboveground organic debris (litter) to deeper soil horizons and can influence the formation of stable soil organic C compounds. The magnitude of this flux depends on both infiltration and DOC production rates which are functions of the climatic, soil, topographic and ecological characteristics of a region. Aboveground litter quantity and quality was manipulated for 20 years in an old-growth Douglas fir forest under six treatments to study relationships among litter inputs, DOC production and flux, and soil C dynamics. DOC concentrations were measured at two depths using tension lysimeters, and a hydrologic model was created to quantify water and DOC flux through the soil profile. DOC concentrations ranged from 3.0–8.0 and 1.5–2.5 mg C/L among treatments at 30 and 100 cm below the soil surface, respectively. Aboveground detrital inputs did not have a consistent positive effect on soil solution DOC; the addition of coarse woody debris increased soil solution DOC by 58% 30 cm belowground, while doubling the mass of aboveground leaf litter decreased DOC concentrations by 30%. We suggest that high-quality leaf litter accelerated microbial processing, resulting in a “priming” effect that led to the lower concentrations. Annual DOC flux into groundwater was small (2.7–3.7 g C/m2/year) and accounts for < 0.1% of estimated litter C at the site. Therefore, direct DOC loss from surface litter to groundwater is relatively negligible to the soil C budget. However, DOC flux into the soil surface was much greater (73–210 g C/m2/year), equivalent to 1.4–2.4% of aboveground litter C. Therefore, DOC transport is an important source of C to shallow soil horizons.

Projections of water, carbon, and nitrogen dynamics under future climate change in an old-growth Douglas-fir forest in the western Cascade Range using a biogeochemical model

Statistically downscaled climate change scenarios from four General Circulation Models for two Representative Concentration Pathways (RCP) were applied as inputs to a biogeochemical model, PnET-BGC, to examine potential future dynamics of water, carbon, and nitrogen in an old-growth Douglas-fir forest in the western Cascade Range. Projections show 56% to 77% increases in stomatal conductance throughout the year from 1986–2010 to 2076–2100, and 65% to 104% increases in leaf carbon assimilation between October and June over the same period. However, future dynamics of water and carbon under the RCP scenarios are affected by a 49% to 86% reduction in foliar biomass resulting from severe air temperature and humidity stress to the forest in summer. Important implications of future decreases in foliar biomass include 1) 20% to 71% decreases in annual transpiration which increase soil moisture by 7% to 15% in summer and fall; 2) decreases in photosynthesis by 77% and soil organic matter by 62% under the high radiative forcing scenario; and 3) altered foliar and soil carbon to nitrogen stoichiometry. Potential carbon dioxide fertilization effects on vegetation are projected to 1) amplify decreases in transpiration by 4% to 9% and increases in soil moisture in summer and fall by 1% to 2%; and 2) alleviate decreases in photosynthesis by 4%; while 3) having negligible effects on the dynamics of nitrogen. Our projections suggest that future decrease in transpiration and moderate water holding capacity may mitigate soil moisture stress to the old-growth Douglas-fir forest. Future increases in nitrogen concentration in soil organic matter are projected to alleviate the decrease in net nitrogen mineralization despite a reduction in decomposition of soil organic matter by the end of the century.

Combustion and Spectral Methods for Quantifying Carbon and Nitrogen Concentrations in Pacific Northwest Douglas-Fir Forest Soils

&lt;p&gt;Traditional combustion methods for assessing soil carbon (C) and nitrogen (N) stocks are time consuming and expensive; visible and near-infrared (VNIR) methods offer a quick and inexpensive alternative for establishing soil C and N concentrations. We compared combustion and spectral methods for quantifying soil carbon and nitrogen concentrations. We sampled organic and mineral soil horizons in managed and old-growth Douglas-fir (&lt;em&gt;Pseudotsuga menziesii&lt;/em&gt;) forests in western Oregon. We applied combustion methods to determine total soil carbon and nitrogen concentrations of these samples. We then applied VNIR methods to derive a reference spectral library for analyzing Pacific Northwest (PNW) andesitic forest soils. Our spectral analysis confirmed that visible range spectra (especially in the 650-750 nm range) are the most useful for distinguishing differences in sample soil organic matter content. Our results provide a robust calibration model for applying spectral analysis combined with Partial Least Square Regression (PLSR) to quantify carbon and nitrogen stocks in PNW Douglas-fir forest soils. Model validation resulted in R&lt;sup&gt;2&lt;/sup&gt; values ranging from 0.92 to 0.95 for C and from 0.73 to 0.84 for N.&lt;/p&gt;

Structure of early old-growth Douglas-fir forests in the Pacific Northwest

Few empirical studies exist on structural transitions from mature forests dominated by Douglas-fir to structurally complex older forests. Stand structure (live tree diameter, height distributions, snag and log metrics) and composition of nine early old-growth (200–350years old) forests were quantified and compared with those of fully developed old-growth (400–600years old) forests. We investigated Douglas-fir and western hemlock-dominated forests of the Pacific Northwest where Douglas-fir established following a single stand-replacing disturbance. Stand-level attributes were summarized using descriptive statistics, nonlinear regression, and old-growth indices. Variability in individual structural features was large between sites but broadly consistent with models of natural Douglas-fir forest development. Compared to older (>450years old) forests, diameter distributions exhibited similar reverse J-shapes. Tree height distributions showed that shade tolerant species occupy lower canopy positions. Coarse woody debris was abundant in early old-growth forests for both snags (42–140m3ha−1) and logs (172–584m3ha−1). Early old-growth stands scored high enough on old-growth indices to qualify as old growth but their scores were significantly lower than older forests (400–600years-old). Early old-growth structure fits well with the natural conceptual model of development providing a basis for ecologically focused management. Furthermore, structural conditions and variability of early old-growth forests provides a closer temporal target for managers seeking to accelerate the development of structure in younger stands or retain natural elements such as spatial patterning of trees.

Spatial Relationships between Canopy Structure and Understory Vegetation of an Old-Growth Douglas-Fir Forest

Canopy structure and spatial distributions of understory vegetation are known to be closely correlated in forest community. The primary objective of this study is to examine the spatial relationships between the canopy heterogeneity and understory vegetation at multiple scales in an old-growth Douglas fir forest. A 12 ha plot located in the T.T. Munger Experimental Forest of Washington was stem-mapped, and the understory vegetation was surveyed along two 400 m transects by species and life form groups. Canopy structure of the plot was modeled utilizing a stem map and a crown geometry model. The three-dimensional canopy characteristics derived from the models were then used to assess the effects of canopy structure on understory vegetation at multiple resolutions using correlation and wavelet analysis techniques. Correlation results showed significant associations of understory species with the three-dimensional canopy structure. The majority of dominant herb species were highly associated with canopy opening, and they were more severely affected by lower canopy layers (heights of 10-35 m) than by dominant-codominant layers (heights of 40-50 m). The saplings of western hemlock had a significant negative association with lower canopy layers, while vine maples and the saplings of Pacific silver fir were positively correlated with dominant-codominant canopy layers. Wavelet analysis indicated that the relationship between canopy structure and understory vegetation was highly scale-dependent, i.e., understory variables responded differently to the heterogeneity of canopy patterns at different scales.

Fate of Decomposed Fungal Cell Wall Material in Organic Horizons of Old-Growth Douglas-fir Forest Soils

Organic horizons of old-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forests are colonized by ectomycorrhizal fungi, which can form large patches or mats. Respiration, N mineralization, chitinase activity, and amino sugar C and N turnover are higher in soils colonized by ectomycorrhizal mats. To test if microbial activity in mat soil is driven by decomposition of fungal biomass, we incubated mat and non-mat soil with ¹³C¹⁵N-labeled, chitin-enriched fungal cell wall material. Cell wall material was readily decomposed during 22-d incubations in both soils, and cell-wall-derived ¹³C and ¹⁵N were traced into mineralized pools, extractable organic pools, microbial biomass, and microbial phospholipid fatty acids (PLFAs). Basal microbial activity was higher in mat soils. Although total respiration was higher in mat soils with cell walls added, more cell wall C was mineralized in non-mat than mat soils; this was inferred to reflect greater preferential respiration of cell wall C over soil organic matter (SOM). Mineralization of N from cell wall material was low, with no difference between soils, and positive priming of N mineralization from SOM occurred after 3 wk. Both the proportion of cell wall N assimilated and N growth yield efficiency (GYE) were higher than for cell wall C. The relative abundance of fungi was higher in cell-wall-amended soils, and cell-wall-derived ¹³C was found in fungal and Gram-negative bacterial PLFAs. Cell wall C assimilation and C GYE were higher in mat soils, which could be linked to greater mineralization of SOM or to differential activity of select microbial taxonomic groups. Chitin-rich fungal cell wall material was utilized as both a microbial N and C source; N was predominantly assimilated and C was predominantly metabolized. It is possible that fungal necromass may contribute to N stabilization in these soils.

Contributions of ectomycorrhizal fungal mats to forest soil respiration

Abstract. Distinct aggregations of fungal hyphae and rhizomorphs, or "mats", formed by some genera of ectomycorrhizal (EcM) fungi are common features of soils in coniferous forests of the Pacific Northwest. We measured in situ respiration rates of Piloderma mats and neighboring non-mat soils in an old-growth Douglas-fir forest in western Oregon to investigate whether there was higher respiration from mats, and to estimate mat contributions to total soil respiration. We found that areas where Piloderma mats colonized the organic horizon often had higher soil surface flux than non-mats, with the relative increase in respiration averaging 16% across two growing seasons. Both soil physical factors and biochemistry were related to the higher surface flux of mat soils. When soil moisture was high, soil CO2 production was concentrated into near-surface soil horizons where mats tend to colonize, resulting in greater apparent differences in respiration between mat and non-mat soils. Respiration rates were also correlated with the activity of chitin-degrading soil enzymes. This finding supports the notion that the abundance of fungal biomass in EcM mats is an important driver of C and N cycling. We found Piloderma mats present across 57% of the exposed soil, and use this value to estimate a respiratory contribution from mats at the stand-scale of about 9% of total soil respiration. The activity of EcM mats, which includes both EcM fungi and microbial associates, appeared to constitute a substantial portion of total soil respiration in this old-growth Douglas-fir forest.

Light on the Devils: Coming of Age on the Klamath by Louise Wagenknecht

 Reviews Richard Louv writes in the introduction, “A growing body of scientific evidence strongly suggests that all of us are starved for nature time” and that spending time outdoors is a central part of healthy living (as proof of that, look no further than North Portland’s new Randall Children’s Hospital on the Emanuel campus, which provides outdoors spaces and plants and trees for patients precisely for their therapeutic benefits).The new edition also contains several walks and essays focused on green infrastructure,which has become a much moreprominentpartof thecity’sbuiltenvironment .A stroll down North Mississippi Avenue can be a lesson in stormwater management, nicely leavened by the thriving food-drinkand -shopping scene. It should be noted that the new edition of the book was supported financially by seemingly every government agency in town with an interest in this subject,from Portland’s Bureau of Environmental Services to the U.S. Fish & Wildlife Service. Many critics of the region’s governmental establishment will no doubt look with suspicion at the values trumpeted in this book. But for readers who simply want a fuller appreciation of what they are seeing as they amble along the Willamette or march determinedly to the top of Mt. Tabor, Wild in the City will be a valuable guide for years to come — until the third edition, no doubt. Jeff Mapes Portland, Oregon Light on the Devils: Coming of Age on the Klamath by Louise Wagenknecht Oregon State University Press, Corvallis, 2011. Illustrations, bibliography. 232 pages. $19.95 paper. Light on the Devils is a memoir set in the remote timber town of Happy Camp, California, in the early 1960s. The book will appeal to readers interested in the history of the Northwest timber industry and particularly in the role of the U.S.Forest Service.The author’s stepfather was a Forest Service forester who vigorously practiced the ethic of scientific “sustained yield”forestry, and who spent his career liquidating the last big tracts of old-growth Douglas fir forest in the Pacific Northwest (pp. 180–81). As Wagenknecht recounts, even her stepfather had his doubts about sustained-yield forestry in the end (pp. 210–11). Her tale is animated by the tension between her early,naïve belief in the rightness and goodness of her way of life,which was dependent on the timber industry,and the dawning realization that she and her close-knit community inhabited a vanishing reality. She begins the book with an essay called “Where it Ended,” an account of the suicide of a man she knew in high school, an outof -work millhand whose wife had left him. This dramatic scene sets up what I took to be the book’s premise: that the implosion of the timber economy was responsible for the man’s death. So I was expecting some sort of hardhitting exposé. To my relief, what emerged instead was a tender, funny autobiography of a bookish, unconsciously charming tomboy, a family-challenged eldest girl perfectly at home in the woods and less at ease in the befuddling thickets of domestic tension and the high school caste system. The first chapter, then, strikes a major false note. It is a cliché — private-tragedy-asmetaphor -for-social-decline is an overworked trope,especiallyinthiscontext.Moreimportant, Wagenknecht does not need it.Her own private perplexities, observations, and awakenings distill the larger story she is trying to tell. There are other rough spots, mostly occurring when Wagenknecht turns from her own experience and begins to explain things. She tells, for example, of a glorious solitary ramble through woods near her house. “My parents didn’t know exactly where I was,”she confides. “I hugged my knees, delighted.”She sits on the ground, “picking deerbrush twigs out of my  OHQ vol. 113, no. 4 hair.”Then she writes:“Deerbrush (Ceanothus integerrimus) is a fast-growing,nitrogen-fixing deciduous shrub that can reach over six feet in height” (p. 57). The shift of voice is jarring. I pictured the adult author sitting at her computer , reaching for her field guide to check the spelling of the binomial. Fortunately, Wagenknecht does not abandon her storytelling voice for long.She lyrically depicts the astonishing...

Vegetation Response to Prescribed Fire in Douglas-Fir Forests, Olympic National Park

Abstract Fire exclusion for more than a century in Olympic National Park changed old-growth Douglas-fir forest stand structure and species composition from an open structure of fire-adapted species to a more crowded, complex structure formed by fire-avoiding species. A previous study identified an historical mean fire return interval of 21 yr for these forests in the eastern Olympics prior to fire exclusion, with frequent, small surface fires maintaining an open forest. We tested whether Douglas-fir/salal forests would support low intensity prescribed fires, and monitored community responses for 3 yr post-fire in a randomized complete block ANOVA. Compared to pre-fire values, total fuels, 1000-hr fuels, 1-hr fuels, duff depth, total tree density, tree species density, sapling density, understory cover, and understory frequency of five prominent species were significantly lower one month post-fire. Differences in tree basal area, Douglas-fir sapling density, and western redcedar tree density were not signi...

Soil Properties in Old-Growth Douglas-Fir Forest Gaps in the Western Cascade Mountains of Oregon

Abstract This was a study of vegetation and soil properties in tree-fall gaps in a coniferous forest of the Pacific Northwest. It had three objectives: (1) to determine if there are correlations between above-ground vegetation and below-ground soil properties within large 50 m diameter gaps, (2) to determine how large gaps influence forest soils compared with non-gap soils, and (3) to measure the effects of differently sized gaps on gap soils. To address these objectives, circular canopy gaps were created in old-growth Douglas-fir forests of the H. J. Andrews Experimental Forest in the western Oregon Cascade Mountains. To address the first objective, within-gap soil spatial patterns were compared with above ground distributions of both vegetation and large woody debris in two large gaps. Spatial and Pearson correlation analyses showed no consistent correlations between soil characteristics and above ground vegetation and coarse woody debris. With reference to the second objective, statistically significan...

Tree microhabitat structures as indicators of biodiversity in Douglas-fir forests of different stand ages and management histories in the Pacific Northwest, U.S.A.

The importance of structural complexity in forest ecosystems for ecosystem diversity has been widely acknowledged. Tree microhabitat structures as indicators of biodiversity, however, have only seldom been the focus of diversity research although their occurrence is highly correlated with the abundance of forest species and ecosystem functions. In this study, microhabitat structures in Douglas-fir ( Pseudotsuga menziesii) forests were defined and their frequency and abundance in natural stands and stands of varying active management histories and stand ages was compared. Indicator microhabitat structures for natural forests were determined and the relationship of the abundance of microhabitat structures with tree diameter of Douglas-fir trees was analysed. Most of the investigated microhabitats are indeed indicators of natural mature and natural old-growth stands, e.g., broken tree top, bayonet top, crack or scar, bark loss, hollow chamber, stem cavity with decay, bark pocket with and without decay, bark bowl, burl, heavy resinosis, and bark burst. In Douglas-fir trees, resin drops and heavy resinosis were the dominant microhabitats in trees with >20.0–40.0 cm diameter at breast height (dbh), whereas bark structures such as bowls in the bark, bark pockets, and bark pockets with decay were the most abundant microhabitats in Douglas-fir trees >80.0 cm. Both management history (including no treatment in natural stands) and stand age determined the abundance of microhabitats and microhabitat composition of stands in our study. The observed microhabitat variability was highest in stands that had not been harvested or otherwise treated silviculturally in many years (low treatment history) and the natural stands and lowest in the recently managed stands. Recently managed stands had, on average, 115 microhabitats/ha, stands with a low treatment history had 520 microhabitats/ha, and natural mature and natural old-growth stands had 745 microhabitats/ha. Active management for microhabitats in silviculturally-treated stands is important if the aim is to create structural complexity for a variety of organisms and ecosystem functions in even-aged Douglas-fir stands. Although the management of microhabitats with respect to biodiversity and economic objectives often seem to be in conflict, we suggest silvicultural measures to reduce the current homogenization of forest stands with relatively minor losses of wood production especially if the reduced timber output is compared with the expected longterm social, economic, and ecological benefits. It may, however, take many decades to obtain stands that approximate the criteria for old-growth according to the interim minimum standards for old-growth Douglas-fir forests in their native western Washington and Oregon.

Litter decomposition contrasts in second- and old-growth Douglas-fir forests of the Pacific Northwest, USA

Litterfall and its subsequent decomposition are important feedback mechanisms in the intrasystem cycling of nutrients in forest ecosystems. The amount of litterfall and the rate of decomposition are expected to vary with stand age and climate. Over a 2-year period, decomposition of five litter types were measured in two second-growth forest stands and one old-growth stand in the Cascade Mountains of southern Washington state, USA. Both second-growth stands were dominated by Douglas-fir [Pseudotsuga menziesii (Mirb.,) Franco] but one had a significant proportion of red alder (Alnus rubra Bong.), a nitrogen (N) fixer. The old-growth stand was dominated by Douglas-fir and western hemlock [Tsuga heterophylla (Raf.) Sarg.]. All stands had a relatively shallow layer of forest floor mass. The five litter types were placed in each stand to evaluate decomposition patterns. Despite significant differences in stand age, microclimate and mean residence times for carbon (C) and N, the rates of litter mass loss varied only slightly between sites. The relative order of species litter mass loss was: vine maple ≫ salal = western hemlock > Douglas-fir (from the youngest stand) > Douglas-fir (from the N rich stand with red alder). The initial litter lignin concentration, not lignin:N, was the primary determinant of decomposition rates, although the initial N concentration was the predictor for mass loss after 2 years in the N rich Douglas-fir-alder stand. All litter types showed immobilization of N for nearly 2 years. Data for Douglas-fir litter suggest that higher levels of N may retard decomposition of tissues with greater amounts of lignified material. The retention of N by the litter appeared influenced by the nutrient capital of the stands as well as the forest floor C:N ratio. Decomposition was minimal during the cold winter months, but displayed a definitive peak period during early Fall with wet weather, warm soils, and fungal activity. Thus, long-term climatic change effects on forest floor C storage may depend more on changes in seasonality of precipitation changes than just temperature changes.

Overstory and understory development in thinned and underplanted Oregon Coast Range Douglas-fir stands

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests managed for timber in western Oregon frequently lack structure and diversity associated with old-growth forests. We examined thinning effects on overstory and understory development for 8 years after treatment. Three 30- to 33-year-old Oregon Coast Range plantations were partitioned into four overstory treatments: unthinned (~550 trees/ha) and lightly (~250 trees/ha), moderately (~150 trees/ha), and heavily (~75 trees/ha) thinned. Within each overstory treatment, two understory treatments were established: underplanted with Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.) or not underplanted. Thinning increased overstory stem growth, crown expansion, and retained crown length. Thinned overstory canopies began to close rapidly the third year after thinning, decreasing % skylight by approximately 2%/year, whereas % skylight in unthinned stands increased slightly. All seedlings planted in unthinned stands died, whereas eighth year survival in thinned stands averaged 88%. Natural regeneration densities and distributions were highly variable. Understory shrub cover was reduced by harvesting disturbance but recovered by the fifth year. Thinning increased understory plant species diversity, and no shrub species were lost. Thinning to low densities and underplanting has the potential to accelerate development of multilayered stands characteristic of old-growth Douglas-fir forests.

The importance of canopy structure in controlling the interception loss of rainfall: Examples from a young and an old-growth Douglas-fir forest

The canopy water storage capacity ( S), direct throughfall fraction ( p), the ratio of evaporation to rainfall intensity ( E ¯ / R ¯ ) and interception loss ( I n), of a Douglas-fir forest are influenced by short (seasonal) and long-term (decades to centuries) changes in the forest canopy. Gross precipitation (P G) and net precipitation (P n) were measured in a young (25-year-old) Douglas-fir forest and the results compared with measurements previously made in a nearby old-growth (>450-year-old) Douglas-fir forest [Link, T.E., Unsworth, M.H., Marks, D., 2004. The dynamics of rainfall interception by a seasonal temperate rainforest. Agric. Forest Meteorol. 124, 171–191.]. Canopy rainfall variables were estimated using a regression-based method that estimates S, p and E ¯ / R ¯ for individual storms using the relationship between P G and P n. The individual storm estimates of S, p and E ¯ / R ¯ for the young forest were applied to a rainfall interception model (Gash model [Gash, J.H.C., 1979. An analytical model of rainfall interception by forest. Q. J. R. Meteorol. Soc. 105, 43–55.]) to determine the effect of seasonal changes in canopy hydrologic variables have on estimates of I n (young forest only). The Gash model was previously applied to the old-growth forest [Link, T.E., Unsworth, M.H., Marks, D., 2004. The dynamics of rainfall interception by a seasonal temperate rainforest. Agric. Forest Meteorol. 124, 171–191.]. The young forest had significantly different S (1.40 mm ± 0.27) and p (0.12 ± 0.07) relative to the old-growth forest ( S = 3.32 ± 0.35; p = 0.42 ± 0.07). Seasonal variation in canopy structure, such as deciduous leaf senescence and coniferous needle drop, were correlated with decreases in S. The differences in S and p between the two forests resulted in an I n that was only slightly larger in the old-growth forest because the E ¯ / R ¯ for the two forests were similar (young = 0.18 ± 0.06; old-growth = 0.17 ± 0.08). E ¯ / R ¯ in the young and old-growth forests may have been similar because developmental changes associated with old-growth forest may alter the external resistance ( r a) and the effective area for evaporation. The Gash model successfully predicted I n for the young forest on a seasonal basis (3.29% error), but experienced larger errors (range = −91 to 36% error) for individual storms. The seasonal error and the error for individual storms improved when seasonal variations in canopy characteristics were incorporated in the model (seasonal error = 2.37%; individual storm error range = −12.0 to 21.7%). Therefore, short-term (seasonal) changes in phenology and long-term (decades to centuries) horizontal and vertical development of the forest canopy influence S, p, I n and E ¯ / R ¯ of Douglas-fir forests.

Vertical stratification of soil water storage and release dynamics in Pacific Northwest coniferous forests

We characterized vertical variation in the seasonal release of stored soil moisture in old-growth ponderosa pine (OG-PP, xeric), and young and old-growth Douglas-fir (Y-DF, OG-DF, mesic) forests to evaluate changes in water availability for root uptake. Soil water potential ( ψ) and volumetric water content ( θ) were measured concurrently at 10 cm intervals to 1 m depth to create in situ soil water retention curves (SWRC) under drying conditions. Non-linear regression was used to fit SWRC specific to each depth and site. We also quantified root biomass, soil texture, and hydraulic redistribution (HR) of soil water by roots to identify factors affecting the seasonal dynamics of root water uptake and depletion from the soil profile. Soil θ measured at a particular ψ increased with soil depth, and was strongly dependent upon soil texture. For example, when ψ was −0.1 MPa, θ ranged from 13% at 20 cm to 35% at 100 cm for the OG-DF forest. Soil texture and bulk density accounted for 60–90% of the variation in the SWRC. As the summer drought progressed, water extraction shifted to the deeper layers, and recharge from HR approached 0.15 mm day −1 in the upper 60 cm for all sites. Total water use from the upper 2 m at all sites peaked between 1.5–2.5 mm day −1 in mid-July and then declined to 0.5–1.0 mm day −1 by the end of the dry season. Total fine root biomass in the upper 1 m was 0.77 kg m −2 (OG-PP), 1.08 kg m −2 (OG-DF) and 1.15 kg m −2 (Y-DF), with 40% (PP) to 60% (DF) of fine roots located in the upper 20 cm. However, the upper 20 cm only accounted for 20% of total water depletion from the upper 2 m at peak water uptake, declining to 4–6% later in the season, illustrating the contribution of deeper roots to water uptake. Nevertheless, daily water uptake from the entire 2 m profile was strongly dependent on water potential at 20 cm, indicating that fine roots in the upper soil may play an important role in regulating water uptake through hydraulic effects on stomatal conductance.

Tree species and size structure of old-growth Douglas-fir forests in central western Oregon, USA

We characterized the structure of 91 old-growth forests dominated by Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco), using inventory data from recent (1985–1991) old-growth timber sales in western Oregon. The data were complete counts (i.e., censuses) of all live trees >20 cm diameter at breast height (dbh, measured at 1.4 m above the ground) over a mean area of 17.1 ha at each site. Across all sites, Douglas-fir accounted for 79% of the total basal area (m 2/ha) of all species. The average density of trees >100 cm dbh was 19 trees/ha and 90% of these trees were Douglas-fir. Species other than Douglas-fir constituted only about 20% of the total basal area at each old-growth site, on average, but largely accounted for the structural variation between sites. We used multivariate techniques such as cluster analysis, indicator species analysis, and ordination with non-metric multidimensional scaling (NMS) to identify and characterize six structural groups in terms of basal area in different species–diameter classes. Almost 97% of the structural information was captured by the first ( r 2 = 0.841) and second ( r 2 = 0.128) NMS ordination axis. Geographic information systems (GIS) analysis and NMS indicated that the structural differences among groups of sites were associated with moisture, temperature, and elevation gradients within the study area. This type of analysis can be used to help define differences among old-growth forests and to set local structural goals for growing forests with old-growth characteristics.

Three-Dimensional Canopy Structure of an Old-Growth Douglas-Fir Forest

Abstract In this study, the structurally heterogeneous canopy of a 12-ha plot in an old-growth Douglas-fir forest was analyzed using three-dimensional (3-D) canopy modeling, geographic information system (GIS), and spatial statistics. Using this approach, we were able to depict how species composition and spatial distribution affect the 3-D structure of canopies. We were also able to slice the canopy and calculate the canopy coverage at various heights, and therefore were able to calculate the cumulative canopy volume at individual crown bases. Information generated by GIS allowed us to calculate how much area canopies or canopy openings occupy. Total canopy volume (243,641 m3/ha) was dominated by western hemlocks (67.1%) and Douglas-firs (23.7%) in this plot. Western hemlocks and Douglas-firs also dominated the canopy coverage, with 65.7% and 25.5% average coverage, respectively. Average total canopy coverage was 84.3% with variations between 78.7% and 89.9% among the 12 1-ha subplots, implying that 15.7% of the stand consisted of canopy openings. Coverage of the canopy projection changed greatly from the lower to the upper canopy layers, with the maximum at about 20 m high. We also examined spatial relationships among groups of trees at different heights and at various scales using bivariate Ripley's K analysis. We found that different species of the understory layer showed different spatial relationships relative to the overstory canopy layer. Likewise the same species in the understory layer showed different spatial relationships when the overstory species differed. This approach provided a useful tool for characterizing 3-D forest canopies, and the results will be very helpful for examining leaf distribution, understory light environment, understory vegetation, and microclimate. FOR. SCI. 50(3):376–386.