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Old Trees: Extraction, Conservation Can Coexist
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BECAUSE LARGE OLD TREES ARE ESSENTIAL FOR FOREST ECOSYSTEM INTEGRITY AND BIODIVERsity,
timber extraction in managed forests should preferentially be concentrated where large old
trees are least likely to develop (“Global decline in large old trees,” D. B. Lindenmayer et al.,
Perspectives, 7 December 2012, p. 1305). However, timber extraction and the conservation of
large old trees are not necessarily mutually exclusive.
Current forest policy and management practices in Flanders, Belgium, aim to convert
even-aged stands (areas in which trees are all the same age) to stands with trees of varying
ages in an effort to increase forest ecosystem stability and resilience and to allow trees to
grow old. As part of their ecologically sustainable forest management, public forest managers
have adopted a large-tree retention approach [see also (1, 2)]. Tree islands within
stands managed for production of high-quality timber are reserved for conservation, and
trees within these islands will never be extracted. Large old trees of commercially valuable
species that have grown beyond the commercially optimal dimensions will
not be logged either. And no tree beyond a threshold diameter [currently set at
dbh (diameter at breast height) of more than 102 cm] will ever be logged. The
strip-shelterwood system (in which trees are cut in linear strips and surrounding
trees are given time to grow old) and the coppice-with-standards system
(in which some trees are left to grow while others around them are cut) are
two examples of forest management that allows the combination of sustainable
forest exploitation and conservation of large old trees
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RPCCR Tutorial
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Old-growth forests as global carbon sinks
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Old-growth forests remove carbon dioxide from the atmosphere1,2 at rates that vary with climate and nitrogen deposition3. The seques- tered carbon dioxide is stored in live woody tissues and slowly decomposing organic matter in litter and soil4. Old-growth forests therefore serve as a global carbon dioxide sink, but they are not protected by international treaties, because it is generally thought that ageing forests cease to accumulate carbon5,6. Here we report a search of literature and databases for forest carbon-flux estimates. We find that in forests between 15 and 800 years of age, net ecosys- tem productivity (the net carbon balance of the forest including soils) is usually positive. Our results demonstrate that old-growth forests can continue to accumulate carbon, contrary to the long- standing view that they are carbon neutral. Over 30 per cent of the global forest area is unmanaged primary forest, and this area con- tains the remaining old-growth forests7. Half of the primary forests (6 3 108 hectares) are located in the boreal and temperate regions of the Northern Hemisphere. On the basis of our analysis, these forests alone sequester about 1.3 6 0.5 gigatonnes of carbon per year. Thus, our findings suggest that 15 per cent of the global forest area, which is currently not considered when offsetting increasing atmospheric carbon dioxide concentrations, provides at least 10 per cent of the global net ecosystem productivity8. Old-growth forests accumulate carbon for centuries and contain large quantities of it. We expect, however, that much of this carbon, even soil carbon9, will move back to the atmosphere if these forests are disturbed.
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Old-Growth Forests Can Accumulate Carbon in Soils
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1st paragraph: ld-growth forests have traditionally been considered negligible as carbon sinks because carbon uptake has been thought to be balanced by respiration (1). We show that soils in the top 20-cm soil layer in preserved old-growth forests in southern China accumulated atmospheric carbon at an unexpectedly high rate from 1979 to 2003. This phenomenon indicates the need for future research on the complex responses and adaptation of belowground processes to global environmental change.
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Oligocene CO2 Decline Promoted C4 Photosynthesis in Grasses
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C4 photosynthesis is an adaptation derived from the more common C3 photosynthetic pathway that con- fers a higher productivity under warm temperature and low atmospheric CO2 concentration [1, 2]. C4 evolution has been seen as a consequence of past atmospheric CO2 decline, such as the abrupt CO2 fall 32–25 million years ago (Mya) [3–6]. This relationship has never been tested rigorously, mainly because of a lack of accurate estimates of divergence times for the different C4 lineages [3]. In this study, we inferred a large phylogenetic tree for the grass family and es- timated, through Bayesian molecular dating, the ages of the 17 to 18 independent grass C4 lineages. The first transition from C3 to C4 photosynthesis occurred in the Chloridoideae subfamily, 32.0–25.0 Mya. The link between CO2 decrease and transition to C4 pho- tosynthesis was tested by a novel maximum likeli- hood approach. We showed that the model incorpo- rating the atmospheric CO2 levels was significantly better than the null model, supporting the importance of CO2 decline on C4 photosynthesis evolvability. This finding is relevant for understanding the origin of C4 photosynthesis in grasses, which is one of the most successful ecological and evolutionary innovations in plant history.
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Olson 1969.pdf
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Olson Unionidae.pdf
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On a collision course: competition and dispersal differences create no-analogue communities and cause extinctions during climate change
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Most climate change predictions omit species interactions and interspecific variation in dispersal. Here, we develop a model of multiple competing species along a warming climatic gradient that includes temperature- dependent competition, differences in niche breadth and interspecific differences in dispersal ability. Competition and dispersal differences decreased diversity and produced so-called ‘no-analogue’ commu- nities, defined as a novel combination of species that does not currently co-occur. Climate change altered community richness the most when species had narrow niches, when mean community-wide dispersal rates were low and when species differed in dispersal abilities. With high interspecific dispersal variance, the best dispersers tracked climate change, out-competed slower dispersers and caused their extinction. Overall, competition slowed the advance of colonists into newly suitable habitats, creating lags in climate tracking. We predict that climate change will most threaten communities of species that have narrow niches (e.g. tropics), vary in dispersal (most communities) and compete strongly. Current forecasts probably underestimate climate change impacts on biodiversity by neglecting competition and dispersal differences.
Keywords: climate change; competition; dispersal; community ecology; movement ecology; thermal performance breadth
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On the forest cover–water yield debate: from demand- to supply-side thinking
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Several major articles from the past decade and beyond conclude the impact of reforestation or afforestation on water yield is negative: additional forest cover will reduce and removing forests will raise downstream water availability. A second group of authors argue the opposite: planting additional forests should raise downstream water availability and intensify the hydrologic cycle. Obtaining supporting evidence for this second group of authors has been more dif- ficult due to the larger scales at which the positive effects of forests on the water cycle may be seen. We argue that for- est cover is inextricably linked to precipitation. Forest-driven evapotranspiration removed from a particular catchment contributes to the availability of atmospheric moisture vapor and its cross-continental transport, raising the likelihood of precipitation events and increasing water yield, in particular in continental interiors more distant from oceans. Sea- sonal relationships heighten the importance of this phenomenon. We review the arguments from different scales and perspectives. This clarifies the generally beneficial relationship between forest cover and the intensity of the hydro- logic cycle. While evidence supports both sides of the argument – trees can reduce runoff at the small catchment scale – at larger scales, trees are more clearly linked to increased precipitation and water availability. Progressive deforesta- tion, land conversion from forest to agriculture and urbanization have potentially negative consequences for global precipitation, prompting us to think of forest ecosystems as global public goods. Policy-making attempts to measure product water footprints, estimate the value of ecosystem services, promote afforestation, develop drought mitigation strategies and otherwise manage land use must consider the linkage of forests to the supply of precipitation.
Keywords: afforestation, climate change adaptation, forest ecosystem services, precipitation recycling, water yield
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On the Hydrologic Adjustment of Climate-Model Projections: The Potential Pitfall of Potential Evapotranspiration
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Hydrologic models often are applied to adjust projections of hydroclimatic change that come from climate models. Such adjustment includes climate-bias correction, spatial refinement (‘‘downscaling’’), and consideration of the roles of hydrologic processes that were neglected in the climate model. Described herein is a quantitative analysis of the effects of hydrologic adjustment on the projections of runoff change associated with projected twenty-first-century climate change. In a case study including three climate models and 10 river basins in the contiguous United States, the authors find that relative (i.e., fractional or percentage) runoff change computed with hydrologic adjustment more often than not was less positive (or, equivalently, more negative) than what was pro- jected by the climate models. The dominant contributor to this decrease in runoff was a ubiquitous change in runoff (median 211%) caused by the hydrologic model’s apparent amplification of the climate-model-implied growth in potential evapotranspiration. Analysis suggests that the hydrologic model, on the basis of the empirical, temperature-based modified Jensen–Haise formula, calculates a change in potential evapotranspiration that is typically 3 times the change implied by the climate models, which explicitly track surface energy budgets. In com- parison with the amplification of potential evapotranspiration, central tendencies of other contributions from hydrologic adjustment (spatial refinement, climate-bias adjustment, and process refinement) were relatively small. The authors’ findings highlight the need for caution when projecting changes in potential evapotranspiration for use in hydrologic models or drought indices to evaluate climate change impacts on water.
KEYWORDS: Hydrologic model; Climate change; Potential evapotranspi- ration
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