A dendro‐ecological study of forest overstorey productivity following the invasion of the non‐indigenous shrub Lonicera maackii

Question: Will a non‐indigenous, invasive, understorey shrub, such as Lonicera maackii (Amur honeysuckle) have an impact on the productivity of overstorey trees in hardwood forests? Location: Trees from 12 invaded and four non‐invaded sites were sampled in hardwood forests of southwestern Ohio, US. Methods: Changes in radial and basal area tree growth in the ten years prior to L. maackii invasion vs. ten years after invasion were examined using dendrochronological techniques. Intervention analysis was used to detect growth changes 25 years prior to and 25 years following invasion, and estimates of load impacts for L. maackii population and biomass were also calculated. Results: We found that the rate of radial and basal area growth of overstorey trees was reduced significantly in eleven out of twelve invaded sites. Non‐invaded sites did not exhibit this consistent pattern of reduced growth. For invaded vs. non‐invaded sites, the mean basal area growth was reduced by 15.8%, and the overall rate of basal area growth was reduced by 53.1%. Intervention analysis revealed that the first significant growth reductions were 6.25 ± 1.24 (mean ± SE) years after invasion with the greatest frequency of negative growth changes occurring 20 years after invasion. In invaded stands, 41% of trees experienced negative growth changes. In terms of invasive load estimates per 1000 L. maackii individuals, radial tree growth was reduced by 0.56 mm.a^?1, and basal area growth was reduced by 0.74 cm².a^?1, Given these findings, significant economic losses could occur in hardwood forests of Ohio. Conclusions: To our knowledge, this is the first study using dendrochronological techniques to investigate the impact of a non‐indigenous, understorey plant on overstorey tree growth. Active management will likely be needed to maintain forest productivity in L. maackii impacted landscapes.     

Forest history and the development of old‐growth characteristics in fragmented boreal forests

Questions: Can small and isolated high‐conservation value forests (e.g. designated woodland key habitats) maintain old‐growth forest characteristics and functionality in fragmented landscapes? To what extent have past disturbances (natural and anthropogenic) influenced the development of old‐growth characteristics of these forests? How long does it take for selectively cut stands to attain conditions resembling old‐growth forests? Location: Southern boreal zone of central Sweden. Methods: We linked multiple lines of evidence from historical records, biological archives, and analyses of current forest structure to reconstruct the forest history of a boreal landscape, with special emphasis on six remaining core localities of high‐conservation value forest stands. Results: Our reconstructions revealed that several of these stands experienced wildfires up to the 1890s; all had been selectively harvested in the late 1800s; and all underwent substantial structural and compositional reorganization over the following 100‐150 years. This time interval was sufficient to recover considerable amounts of standing and downed dead wood (mean 60.3 m³ ha^?1), a range of tree ages and sizes (mean basal area 32.6 m² ha^?1), and dominance of shade‐tolerant spruce. It was insufficient to obtain clearly uneven tree age structures and large (>45 cm diameter) living and dead trees. Thus, these forests contain some, but not all, important compositional and structural attributes of old‐growth forests, their abundance being dependent on the timing and magnitude of past natural and anthropogenic disturbances. Our landscape‐level analysis showed marked compositional and structural differences between the historical forest landscape and the present landscape, with the latter having a greater proportion of young forests, introduction of non‐native species, and lack of large trees and dead wood. Conclusions: The remnant high‐conservation value stands were not true representatives of the pre‐industrial forests, but represent the last vestige of forests that have regenerated naturally and maintained a continuous tree cover. These traits, coupled with their capacity for old‐growth recovery, make them valuable focal areas for conservation.     

Structural segregation and scales of spatial dependency in Abies amabilis forests

Question: Is a mosaic structure apparent in the spatial distribution of trees in old‐growth Abies amabilis forests? Location: Montane forests of the western Cascade Range, Washington, USA. Methods: Maps of tree locations were created for study areas located in two, 300‐year old stands and a single 600‐year old stand. Stand structure parameters were calculated using several subsample quadrats sizes (56.25 ‐ 306.25 m²), which were drawn randomly with replacement at a density of 250 quadrats per ha from the stem maps in the computing environment. Spatial cross‐covariance functions between different canopy strata were estimated using the spline cross‐correlogram. Results: Negative spatial correlation (segregation) between subcanopy tree density and areas of high overstorey occupancy was detected. Understorey and midstorey tree densities were positively spatially correlated. These general trends were apparent across the range of observational scales investigated. Significant spatial correlation between canopy strata was observed at spatial scales of 12 ‐ 44 m and extended to the largest scales in the 600‐year old stand. Conclusion: The observed spatial segregation between canopy strata supports the hypothesis that old A. amabilis forests form fine‐scale structural mosaics. Structural segregation at small scales may be due to competitive interactions as well as exogenous forcing of tree locations (e.g. by mortality due to pathogens or disturbance), however segregation at large scales in the 600‐year old stand is likely due to exogenous factors alone. This study reinforces the idea that horizontal heterogeneity is an emergent property of old‐growth forests.     

Fire disturbance and forest structure in an old‐growth Pinus ponderosa forest,southern Cascades,USA

Questions: Did fire regimes in old‐growth Pinus ponderosa forest change with Euro‐American settlement compared to the pre‐settlement period? Do tree age structures exhibit a pattern of continuous regeneration or is regeneration episodic and related to fire disturbance or fire‐free periods? Are the forests compositionally stable? Do trees have a clumped spatial pattern and are clumps even‐ or mixed‐age? How might information from this old‐growth forest inform current restoration and management practices? Location: A 235‐ha old‐growth forest in the Ishi Wilderness, southern Cascade Mountains, California. Methods: Age, size, and spatial pattern of trees were quantified in seven stands. Fire history was reconstructed using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing fire history with age, size, and spatial structure of trees and identifying and measuring trees killed by two recent fires. Results: Species composition in plots was similar but density and basal area of tree populations varied. Age structure for P. ponderosa and Quercus kelloggii showed periods of episodic recruitment that varied among plots. Fire disturbance was frequent before 1905, with a median period between fires of 12 years. Fire frequency declined after 1905 but two recent fires (1990, 1994) killed 36% and 41% of mostly smaller diameter P. ponderosa and Q. kelloggii. Clusters of similar age trees occurred at scales of 28‐1018 m² but patches were not even‐aged. Interactions between tree regeneration and fire promoted development of uneven age groups of trees. Conclusions: Fire disturbance strongly influenced density, basal area, and spatial structure of tree populations. Fire exclusion over the last 100 years has caused compositional and structural changes. Two recent fires, however, thinned stands and created gaps favorable for Q. kelloggii and P. ponderosa regeneration. The effects of infrequent 20th century fire indicate that a low fire frequency can restore and sustain structural characteristics resembling those of the pre‐fire suppression period forest.     

Structural characteristics and aboveground biomass of old‐growth spruce–fir stands in the eastern Carpathian mountains,Ukraine

Abstract

Temperate old‐growth forests are known to have ecological characteristics distinct from younger forests, but these have been poorly described for the remaining old‐growth Picea abies–Abies alba forests in the eastern Carpathian mountains. In addition, recent studies suggest that old‐growth forests may be more significant carbon sinks than previously recognized. This has stimulated interest in quantifying aboveground carbon stocks in primary forest systems. We investigated the structural attributes and aboveground biomass in two remnant old‐growth spruce–fir stands and compared these against a primary (never logged) mature reference stand. Our sites were located in the Gorgany Nature Reserve in western Ukraine. Overstory data were collected using variable radius plots; coarse woody debris was sampled along line intercept transects. Differences among sites were assessed using non‐parametric statistical analyses. Goodness‐of‐fit tests were used to evaluate the form of diameter distributions. The results strongly supported the hypothesis that old‐growth temperate spruce–fir forests have greater structural complexity compared to mature forests, including higher densities of large trees, more complex horizontal structure, and elevated aboveground biomass. The late‐successional sites we sampled exhibited rotated sigmoid diameter distributions; these may reflect natural disturbance dynamics. Old‐growth Carpathian spruce–fir forests store on average approximately 155–165 Mg ha^?1 of carbon in aboveground tree parts alone. This is approximately 50% higher than mature stands. Given the scarcity of primary spruce–fir forests in the Carpathian region, remaining stands have high conservation value, both as habitat for late‐successional species and as carbon storage reservoirs.     

Diversity and succession of epiphytic macrolichen communities in low‐elevation managed conifer forests in Western Oregon

Abstract. We examined epiphytic macrolichen communities in Pseudotsuga menziesii (Douglas‐fir) forests across the western Oregon landscape for relationships to environmental gradients, stand age and structure, and commercial thinning. We used a retrospective, blocked design through the Coast and the western Cascade ranges of Oregon. Each of our 17 blocks consisted of a young, unthinned stand (age 50–110 yr); an adjacent, thinned stand of equivalent age; and an old‐growth stand (age > 200 yr). We found 110 epiphytic macrolichen taxa in the stands. Forage‐providing alectorioid lichens and the nitrogen‐fixing cyanolichen Lobaria oregana associated strongly with old‐growth stands and remnant old trees in younger stands (unthinned + thinned). Relative to unthinned stands, thinned stands had a slightly higher abundance of alectorioid lichens and a greater presence of Hypogymnia imshaugii. However, thinned stands hosted a lower landscape‐level (γ) diversity, lacking many species that occurred infrequently in the unthinned stands. Patterns in the lichen community composition correlated strongly with climatic gradients; the greatest variation in composition was between the Coast and Cascade ranges. The difference in communities between mountain ranges was greatest among stands 70–110 yr old, suggesting a difference in lichen successional dynamics between the ranges.     

Conservation management of Pinus palustris ecosystems from a landscape perspective

Question: Do case studies from silvicultural and restoration studies and applied conservation management in second‐growth Pinus palustris stands provide unique insights for conservation models? Methods: A review of management paradigms that conserve the high biological diversity and rare species, drawn from characteristics in both second‐growth and old‐growth stands, is presented for fire‐maintained Pinus palustris (longleaf pine) forests. Results: A common assumption that old‐growth stands provide the primary information for the development of conservation management strategies de‐emphasizes lessons learned from second‐growth and restoration studies. Primary conservation management goals for the Pinus palustris ecosystem include the perpetual regeneration of the fire‐maintained forest and conservation of the characteristically high biological diversity and rare species. Several attributes, such as a sustained population of Picoides borealis (red‐cockaded woodpecker), Aristida stricta (wiregrass)‐dominated ground cover, and undisturbed upland‐wetland ecotones, can predict a diverse and ecologically functional ecosystem. Such indicators are linked to critical structural and functional features of the system and reflect previous land management histories that suggest sustainable approaches. Conclusions: A traditional definition of ‘old‐growth’ relying on overstorey may be limited in describing important features of healthy, diverse Pinus palustris ecosystems. Some characteristics are significantly more important for maintenance of diversity than age of the trees and these features may be present in old‐ or second‐growth forests. We advocate that the management history, structural characteristics and landscape context of stands that harbour desirable conservation attributes (red‐cockaded woodpeckers, wiregrass, gopher tortoises and undisturbed upland‐wetland ecotones) can be used as indicators to identify important conservation and forest management principles.     

Succession in sub‐boreal forests of West‐Central British Columbia

Abstract. Structural and compositional changes were analysed over the course of 400+ yr of post‐fire succession in the sub‐boreal forests of west‐central British Columbia. Using a chronosequence of 57 stands ranging from 11 to 438 yr in age, we examined changes in forest structure and composition with complementary PCA and DCA ordination techniques. To determine stand ages and timing of tree recruitment, approximately 1800 trees were aged. Most early successional forests were dominated by Pinus contorta, which established rapidly following fire. Abies lasiocarpa and Picea glauca × engel‐mannii were also able to establish quickly, but continued to establish throughout the sere. Few Pinus contorta survived beyond 200 yr, resulting in major changes in forest structure. In some stands P. contorta never established, which led to considerable variation among stands less than 200 yr old. The oldest forests converged on dominance by Abies lasiocarpa. Vascular plant diversity decreased during succession whereas canopy structure became more complex as gap dynamics developed. Although these sub‐boreal forests contain few tree species, successional changes were pronounced, with structure changing more than composition across the chronosequence.     

Vegetation structure and dynamics of Pinus gerardiana forests in Balouchistan,Pakistan

Abstract. 44 forest stands, including 42 stands with Pinus gerardiana Wall, ex Lamb dominant and two stands with broad-leaved trees, were sampled in the Suleiman Range in Balouchistan. Density oi Pinus gerardiana trees ranged from 24 to 930 trees / ha with a mean of 266 individuals / ha; the average basal area was 25.5 m² ha^-1. Adequate recruitment of Pinus seedlings was observed; higher seedling density is recorded from east-facing slopes, while tree density was higher on west-facing slopes. The average growth rate was estimated as 0.08 cm / yr radial growth. However, trees on high elevations and cooler slopes grow faster. Soil variables showed no correlation with density, basal area or importance values. It is suggested that the present degraded stage of the forests in the study area is of anthropogenic origin.     

Non‐native mangroves support carbon storage,sediment carbon burial,and accretion of coastal ecosystems

Mangrove forests play an important role in climate change adaptation and mitigation by maintaining coastline elevations relative to sea level rise, protecting coastal infrastructure from storm damage, and storing substantial quantities of carbon (C) in live and detrital pools. Determining the efficacy of mangroves in achieving climate goals can be complicated by difficulty in quantifying C inputs (i.e., differentiating newer inputs from younger trees from older residual C pools), and mitigation assessments rarely consider potential offsets to CO₂ storage by methane (CH₄) production in mangrove sediments. The establishment of non‐native Rhizophora mangle along Hawaiian coastlines over the last century offers an opportunity to examine the role mangroves play in climate mitigation and adaptation both globally and locally as novel ecosystems. We quantified total ecosystem C storage, sedimentation, accretion, sediment organic C burial and CH₄ emissions from ~70 year old R. mangle stands and adjacent uninvaded mudflats. Ecosystem C stocks of mangrove stands exceeded mudflats by 434 ± 33 Mg C/ha, and mangrove establishment increased average coastal accretion by 460%. Sediment organic C burial increased 10‐fold (to 4.5 Mg C ha^?1 year^?1), double the global mean for old growth mangrove forests, suggesting that C accumulation from younger trees may occur faster than previously thought, with implications for mangrove restoration. Simulations indicate that increased CH₄ emissions from sediments offset ecosystem CO₂ storage by only 2%–4%, equivalent to 30–60 Mg CO₂‐eq/ha over mangrove lifetime (100 year sustained global warming potential). Results highlight the importance of mangroves as novel systems that can rapidly accumulate C, have a net positive atmospheric greenhouse gas removal effect, and support shoreline accretion rates that outpace current sea level rise. Sequestration potential of novel mangrove forests should be taken into account when considering their removal or management, especially in the context of climate mitigation goals.     

Secondary forest growth deviation from chronosequence predictions in central Amazonia

Nearly all published rates of secondary forest (SF) regrowth for Amazonia are inferred from chronosequences. We examined SF regrowth on abandoned pastures over a 4‐year period to determine if measured rates of forest recovery differ from chronosequence predictions. We studied the emergence, development and death of over 1300 stems in 10 SFs representing three age classes (<1–5, 6–10 and 11–14 years old). Mean tree biomass accumulation in both the <1–5 and 6–10 years old (4.4 and 5.7 Mg ha⁻¹ yr⁻¹, respectively) abandoned pastures was lower than predicted and deviated significantly (57% and 41%) from rates estimated from the chronosequence. The older SFs, with a mean growth rate of 9.9 Mg ha⁻¹ yr⁻¹ followed the rate predicted by the chronosequence. Understocking was the primary cause of low biomass recovery rates in the youngest forests; although the youngest stands had a diameter at breast height increment three times the oldest stands, the youngest stands lacked sufficient density to cumulatively produce high biomass accumulation rates. Four years of measurement indicated that the youngest stands had developed 59% of the stems measured in the older stands during the same time period. The 6–10‐year‐old stands were rapidly self‐thinning and approached stem density values measured in the same aged stands at the onset of the study. Mortality was high for all stands, with 54% of the original stems remaining after 4 years in intermediate‐aged stands. The forests were dominated by the tree Vismia, which represented 55–66% of the biomass in all stands. The Vismia share of the biomass was decreasing over time, with other genera replacing the pioneer. Our measured rates of regrowth indicate that generalized estimates of forest regrowth through chronosequence studies will overestimate forest regrowth for the youngest forests that were under land use for longer time‐periods before abandonment. Certified Emission Reductions under the Clean Development Mechanism of the Kyoto protocol should consider these results when predicting and compensating for carbon sequestered under natural forest management.     

Natural and anthropogenic influences on coarse woody debris stocks in Nothofagus–Araucaria forests of northern Patagonia,Argentina

Coarse woody debris (CWD) is an important element driving ecological processes, strengthening ecosystem resilience and for biodiversity within forest ecosystems. However, the abundance and distribution of CWD and their relation to natural and human factors are poorly known in southern South America. In this work we studied the density and volume of CWD types in Nothofagus–Araucaria stands in northern Patagonia (Neuquén – Argentina) and relationships with forest composition and structure. We also studied their relationships with fire history, topography and human‐related variables. Twenty‐three stands with Nothofagus pumilio, Nothofagus antarctica and/or Araucaria araucana were sampled to estimate quantities of logs, snags and dead branches using the planar‐intersect method. CWD density and volume in these forests were moderate and varied across the landscape with a spatial pattern determined by biotic, abiotic and human use–related variables. Mean CWD volume was 52.9 m³ ha^?1 (range: 1.6–143.7) and significantly varied among forest types and watersheds. CWD was positively related to dbh, tree height and slope, but negatively related to tree density. CWD was clearly influenced by composition and structural characteristics of stands, where the tree species traits had an important role. As well, the observed amount and type of CWD, whereby most of the stands showed low levels of old (pre‐disturbance) logs/snags and poor new inputs of deadwood, may be explained by fire frequency. Firewood gathering and livestock grazing negatively affected deadwood stocks and topography counteracts this effect by limiting human access. Fire disturbance history, windthrow and dieback pulses produced by insect outbreaks and human access seemed to be the main causes that best explained CWD spatial distribution and abundance patterns in north‐western Patagonian forests.     

Effects of European colonization on indigenous ecosystems: post-settlement changes in tree stand structures in Eucalyptus–Callitris woodlands in central New South Wales, Australia

Aim There has been considerable debate about pre‐settlement stand structures in temperate woodlands in south‐eastern Australia. Traditional histories assumed massive tree losses across the region, whereas a number of recent histories propose that woodlands were originally open and trees regenerated densely after settlement. To reconcile these conflicting models, we gathered quantitative data on pre‐settlement stand structures in Eucalyptus–Callitris woodlands in central New South Wales Australia, including: (1) tree density, composition, basal area and canopy cover at the time of European settlement; and (2) post‐settlement changes in these attributes. Location Woodlands dominated by Eucalyptus species and Callitris glaucophylla, which originally occupied approximately 100,000 km² in central New South Wales, Australia. Methods We recorded all evidence of pre‐settlement trees, including stumps, stags and veteran trees, from 39 relatively undisturbed 1‐ha stands within 16 State Forests evenly distributed across the region. Current trees were recorded in a nested 900 m² quadrat at each site. Allometric relationships were used to estimate girth over bark at breast height, tree basal area, and crown diameter from the girth of cut stumps. A post‐settlement disturbance index was developed to assess correlations between post‐settlement disturbance and attributes of pre‐settlement stands. Results The densities of all large trees (> 60 cm girth over bark at breast height) were significantly greater in current stands than at the time of European settlement (198 vs. 39 trees ha^?1). Pre‐settlement and current stands did not differ in basal area. However, the proportional representation of Eucalyptus and Callitris changed completely. At the time of settlement, stands were dominated by Eucalyptus (78% of basal area), whereas current stands are dominated by Callitris (74%). On average, Eucalyptus afforded 83% of crown cover at the time of settlement. Moreover, the estimated density, basal area and crown cover of Eucalyptus at the time of settlement were significantly negatively correlated with post‐settlement disturbance, which suggests that these results underestimate pre‐settlement Eucalyptus representation in the most disturbed stands. Main conclusions These results incorporate elements of traditional and recent vegetation histories. Since European settlement, State Forests have been transformed from Eucalyptus to Callitris dominance as a result of the widespread clearance of pre‐settlement Eucalyptus and dense post‐settlement recruitment of Callitris. Tree densities did increase greatly after European settlement, but most stands were much denser at the time of settlement than recent histories suggest. The original degree of dominance by Eucalyptus was unexpected, and has been consistently underestimated in the past. This study has greatly refined our understanding of post‐settlement changes in woodland stand structures, and will strengthen the foundation for management policies that incorporate historical benchmarks of landscape vegetation changes.     

Litterfall of epiphytic macrolichens in Nothofagus forests of northern Patagonia,Argentina: Relation to stand age and precipitation

Abstract: The objective of this study was to analyse how stand age and precipitation influence abundance and diversity of epiphytic macrolichens in southern beech Nothofagus forests, estimated by lichen litter sampling. Five sites of Nothofagus dombeyi (Mirbel) Oersted were selected in Nahuel Huapi National Park, Argentina. At each site, lichen fragments from the forest floor were collected at 12.5 m² plots in pairs of young and mature N. dombeyi forest. Additionally, two sites with multi‐aged subalpine Nothofagus pumilio (Poepp. et Endl.) Krasser forest were investigated in a similar manner. Average litterfall biomass per stand varied from less than 1 kg ha^?1 in a young low‐precipitation stand to a maximum of 20 kg ha^?1 in a mature high‐precipitation stand. In places with higher precipitation, litterfall biomass in N. dombeyi forest was considerably higher in old stands as compared with young ones. In places with less than 2000 mm of precipitation, differences in biomass were less pronounced. Old humid stands contained about twice as many taxa in the litter as old low‐precipitation stands and young stands in general. Mature stands in low‐precipitation sites only contained 17% of the litter biomass as compared with mature stands in high‐precipitation sites. Epiphytic lichen composition changed from predominating fruticose lichens (Usnea spp. and Protousnea spp.) in low‐precipitation stands to Pseudocyphellaria spp., Nephroma spp. and other foliose lichens, in the high‐precipitation stands. There were no clear differences in the proportion of fruticose and foliose lichens between young and old stands. Fruticose lichens dominated litter biomass in both N. pumilio sites.     

A comparison of structural characteristics between old-growth and postfire second-growth hemlock–hardwood forests in Adirondack Park, New York, U. S. A.

1 This study compares the structural characteristics of 12 old‐growth and six postfire second‐growth hemlock–northern hardwood stands in north central Adirondack Park, New York, in order to test the null hypothesis that there are no differences in species composition, size structure, age structure and attributes such as dead wood and canopy gaps between old‐growth stands and this type of second‐growth forest. 2 The second‐growth forests of this study regenerated following widespread logging‐related fires in either 1903 or 1908; the old growth and second growth have similar environmental settings. 3 Estimates of stand ages, derived from an increment core of the oldest tree in each stand, range from 88 to 390 years. 4 Structural attributes are related to stand age (i.e. stage of development). In comparison with the second‐growth forests of this study, older stands are characterized as (a) a larger average diameter of canopy trees; (b) a greater basal area of trees; (c) a lower density of canopy trees and of all trees ≥ 10 cm d.b.h.; (d) a higher density of eastern hemlock (Tsuga canadensis (L.) Carrière) trees; (e) a higher density of large trees (≥ 50 cm d.b.h.); (f) larger canopy gaps; and (g) a greater volume of coarse woody debris (both logs ≥ 20 cm d.b.h. and snags ≥ 10 cm d.b.h.). 5 Despite differences between old growth and second growth, especially in species composition, it appears from observations of the 18 stands that second‐growth forests are developing some structural characteristics of old growth. 6 Structural attributes of the old‐growth forests are similar to characteristics of the same forest type in geographically distant areas in eastern USA.     

The carbon balance in natural and disturbed forests of the southern taiga in central Siberia

Abstract. We evaluated the balance of production and decomposition in natural ecosystems of Pinus sylvestris, Larix sibirica and Betula pendula in the southern boreal forests of central Siberia, using the Yenisei transect. We also investigated whether anthropogenic disturbances (logging, fire and recreation pressure) influence the carbon budget. Pinus and Larix stands up to age class VI act as a net sink for atmospheric carbon. Mineralization rates in young Betula forests exceed rates of uptake via photosynthesis assimilation. Old‐growth stands of all three forest types are CO₂ sources to the atmosphere. The prevalence of old‐growth Larix in the southern taiga suggests that Larix stands are a net source of CO₂. The CO₂ flux to the atmosphere exceeds the uptake of atmospheric carbon via photosynthesis by 0.23 t C.ha^‐1.yr^‐1 (47%). Betula and Pinus forests are net sinks, as photosynthesis exceeds respiration by 13% and 16% respectively. The total carbon flux from Pinus, Larix and Betula ecosystems to the atmosphere is 10 387 thousand tons C.yr^‐1. Net Primary Production (0.935 t‐C.ha^‐1) exceeds carbon release from decomposition of labile and mobile soil organic matter (Rh) by 767 thousand tons C (0.064 t‐C.ha^‐1), so that these forests are net C‐sinks. The emissions due to decomposition of slash (101 thousand tons C; 1.0%) and from fires (0.21%) are very small. The carbon balance of human‐disturbed forests is significantly different. A sharp decrease in biomass stored in Pinus and Betula ecosystems leads to decreased production. As a result, the labile organic matter pool decreased by 6–8 times; course plant residues with a low decomposition rate thus dominate this pool. Annual carbon emissions to the atmosphere from these ecosystems are determined primarily by decomposing fresh litterfall. This source comprises 40–79% of the emissions from disturbed forests compared to only 13–28% in undisturbed forests. The ratio of emissions to production (NPP) is 20–30% in disturbed and 52–76% in undisturbed forests.     

The role of anthropogenic influences in a mixed-conifer forest mortality episode

Abstract. Anthropogenic influences on recent tree mortality in mid-montane mixed conifer forests of southern California, USA, and northern Baja California, Mexico, were investigated. The Pinus jeffreyi-Abies concolor phase of the mixed-conifer montane community was sampled at three sites, characterized by: (1) low levels of air pollution and long-term fire suppression (Holcomb Valley in the San Bernardino Mountains, California); (2) chronic, severe air pollution and long-term fire suppression (Barton Flats in the San Bernardino Mountains, California); and (3) little air pollution and no fire suppression (La Corona Arriba in the Sierra San Pedro Martir in Baja California, Mexico). Similar percentages of dead standing trees, around 14 %, were found at both San Bernardino sites, but a comparatively low level, 4 %, was found in the Mexican forest. Anthropogenic effects, in particular fire suppression, may play an important role in enhancing the impact of natural stresses on the dynamics of mixed conifer forests.     

Patterns of growth,mortality and biomass change in a coastal Picea sitchensis - Tsuga heterophylla forest

Abstract. Ten years (1979-1989) of growth and mortality were determined in a 130-yr old stand on the Oregon coast based on periodic remeasurements in 441000 m² plots. Western hemlock (Tsuga heterophylla) constituted 90 % of the individuals and 57 % of the biomass. Wind is a major form of disturbance in this area, creating both small discrete and large diffuse disturbance patches; wind therefore has a direct effect on the location and extent of regeneration. Rates of tree mortality were high for this coastal stand (2.8 %/yr), especially compared to similar-aged stands in the western and eastern Cascade Ranges. Though low in absolute density, Sitka spruce (Picea sitchensis) persisted in competition with the more tolerant western hemlock. Net production of bole biomass (4.9 Mg ha^-1 yr^-1) did not equal mortality (8.7 Mg ha^-1 yr^-1), and total biomass declined over the 10-yr measurement period from 499 to 460 Mg/ha; this trend may have begun as early as the mid-1950's at a peak biomass of about 600 Mg/ha. The decline may have been due to a positive feedback in which new gaps and enlarging gap perimeters exposed more and more trees to potential wind damage.     

Carbon isotope discrimination and growth response of old Pinus ponderosa trees to stand density reductions

Stand density reductions have been proposed as a method by which old‐growth ponderosa pine (Pinus ponderosa) forests of North America can be converted back to pre‐1900 conditions, thereby reducing the danger of catastrophic forest fires and insect attacks while increasing the productivity of the remaining old‐growth individuals. However, the duration of productivity response of individual trees and the physiological mechanisms underlying such a response remain speculative issues, particularly in old trees. Tree‐ring measurements of carbon isotope ratios (δ¹³C) and basal area increment (BAI) were used to assess the response of intrinsic water‐use efficiency (the ratio of photosynthesis, A to stomatal conductance, g) and growth of individual> 250‐year‐old‐ponderosa pine trees to stand density reductions. It was hypothesized that reductions in stand density would increase soil moisture availability, thus decreasing canopy A/g and increasing carbon isotope discrimination (Δ). Cellulose‐δ¹³C of annual tree rings, soil water availability (estimated from pre‐dawn leaf water potential), photosynthetic capacity, stem basal growth and xylem anatomy were measured in individual trees within three pairs of thinned and un‐thinned stands. The thinned stands were treated 7 to 15 years prior to measurement. The values of δ¹³C and BAI were assessed for 20 consecutive years overlapping the date of thinning in a single intensively studied stand, and was measured for 3 years on either side of the date of thinning for the two other stands to assess the generality of the response. After thinning, Δ increased by 0.89‰ (± 0.15‰). The trees in the un‐thinned stands showed no change in Δ (0.00‰ ± 0.04‰). In the intensively studied trees, significant differences were expressed in the first growing season after the thinning took place but it took 6 years before the full 0.89‰ difference was observed. BAI doubled or tripled after disturbance, depending on the stand, and the increased BAI lasted up to 15 years after thinning. In the intensively studied trees, the BAI response did not begin until 3 years after the Δ response, peaked 1 year after the Δ peak, and then BAI and Δ oscillated in unison. The lag between BAI and Δ was not due to slow changes in anatomical properties of the sapwood, because tracheid dimensions and sapwood‐specific conductivity remained unchanged after disturbance. The Δ response of thinned trees indicated that A/g decreased after thinning. Photosynthetic capacity, as indexed by foliar nitrogen ([N]) and by the relationship between photosynthesis and internal CO₂ (A–C_i curves), was unchanged by thinning, confirming our suspicion that the decline in A/g was due to a relatively greater increase in g in comparison with A. Model estimates agreed with this conclusion, predicting that g increased by nearly 25% after thinning relative to a 15% increase in A. Pre‐dawn leaf water potential averaged 0.11 MPa (± 0.03 MPa) less negative for the thinned compared with the un‐thinned trees in all stands, and was strongly correlated with Δ post‐thinning (R² = 0.91). There was a strong relationship between BAI and modelled A, suggesting that changes in water availability and g have a significant effect on carbon assimilation and growth of these old trees. These results confirm that stand density reductions result in increased growth of individual trees via increased stomatal conductance. Furthermore, they show that a physiological response to stand density reductions can last for up to 15 years in old ponderosa pines if stand leaf area is not fully re‐established.