H umans have long been fascinated by the dynamism of free-flowing waters. Yet we have expended great effort to tame rivers for transportation, water supply, flood control, agriculture, and power generation. It is now recognized that harnessing of streams and rivers comes at great cost: Many rivers no longer support socially valued native species or sustain healthy ecosystems that provide important goods and services (Naiman et al. 1995, NRC 1992).

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The Natural Flow Regime

We report the draft genome of the black cottonwood tree, Populus trichocarpa. Integration of shotgun sequence assembly with genetic mapping enabled chromosome-scale reconstruction of the genome. More than 45,000 putative protein-coding genes were identified. Analysis of the assembled genome revealed a whole-genome duplication event; about 8000 pairs of duplicated genes from that event survived in the Populus genome. A second, older duplication event is indistinguishably coincident with the divergence of the Populus and Arabidopsis lineages. Nucleotide substitution, tandem gene duplication, and gross chromosomal rearrangement appear to proceed substantially more slowly in Populus than in Arabidopsis. Populus has more protein-coding genes than Arabidopsis, ranging on average from 1.4 to 1.6 putative Populus homologs for each Arabidopsis gene. However, the relative frequency of protein domains in the two genomes is similar. Overrepresented exceptions in Populus include genes associated with lignocellulosic wall biosynthesis, meristem development, disease resistance, and metabolite transport.

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The genome of black cottonwood, Populus trichocarpa (Torr. & Gray)

Summary Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, pre- diction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric

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Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?

Plant functional traits are the features (morphological, physiological, phenological) that represent ecological strategies and determine how plants respond to environmental factors, affect other trophic levels and influence ecosystem properties. Variation in plant functional traits, and trait syndromes, has proven useful for tackling many important ecological questions at a range of scales, giving rise to a demand for standardised ways to measure ecologically meaningful plant traits. This line of research has been among the most fruitful avenues for understanding ecological and evolutionary patterns and processes. It also has the potential both to build a predictive set of local, regional and global relationships between plants and environment and to quantify a wide range of natural and human-driven processes, including changes in biodiversity, the impacts of species invasions, alterations in biogeochemical processes and vegetation–atmosphere interactions. The importance of these topics dictates the urgent need for more and better data, and increases the value of standardised protocols for quantifying trait variation of different species, in particular for traits with power to predict plant- and ecosystem-level processes, and for traits that can be measured relatively easily. Updated and expanded from the widely used previous version, this handbook retains the focus on clearly presented, widely applicable, step-by-step recipes, with a minimum of text on theory, and not only includes updated methods for the traits previously covered, but also introduces many new protocols for further traits. This new handbook has a better balance between whole-plant traits, leaf traits, root and stem traits and regenerative traits, and puts particular emphasis on traits important for predicting species’ effects on key ecosystem properties. We hope this new handbook becomes a standard companion in local and global efforts to learn about the responses and impacts of different plant species with respect to environmental changes in the present, past and future.

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New handbook for standardised measurement of plant functional traits worldwide

Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient β to the data for each biome (Gale and Grigal 1987). β is a simple numerical index of rooting distribution based on the asymptotic equation Y=1-βd, where d = depth and Y = the proportion of roots from the surface to depth d. High values of β correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (β=0.913, 0.943, and 0.943, respectively), with 80-90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (β=0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m-2. Tropical evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil. (β=0.952), while shrubs had only 21% in the same depth increment (β=0.978). The rooting distribution of all temperate and tropical trees was β=0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was β=0.966 (r 2=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.

A global analysis of root distributions for terrestrial biomes

Poplar is increasingly recognized as an excellent model tree for the study of tree growth and its underlying physiology and genetics By studying trees of the genus Populus (poplars, cottonwoods, aspens), which in their native ecosystems play a major role in the re-colonization of sites after disturbances, new insights have been gained into plantation culture and the development of improved cultivars Of the 20 chapters in this publication, authored by an international group of researchers, one section deals with systematics, genetics, genetic manipulation and biotic interactions of Populus, while the other deals with stress response and the physiology of growth and productivity

Biology of Populus and its implications for management and conservation

The past 15 years have seen a surge of interest in modularity in plants; that is, in the implications of the fact that plants are composed of repetitive modules that may in some ways behave as in independent units (32). Much has been written about the importance of modularity in plant population biology (e.g. 33, 34, 113, 114), and also about the advantages and disadvantages of independence or interdependence among separate but connected modules (7, 31, 69). The interest in modularity is mainly at two scales. At the smallest scale, interest has focused on the "nutritional unit" (1) or "physiologically independent subunit" (111, 112), comprising a unit of foliage, the section of stem to which it is attached, and the subtending axillary bud. At the opposite end of the spectrum, research has focused on clonal herbs (3, 81, 82, 113), in which each module (ramet) contains all of the structural parts and physiological processes necessary for independent existence. However, this upsurge of interest in modularity has also led to renewed speculation about other, intermediately scaled, functional units that may also behave semiautonomously (112). For woody plants, considerable interest has focused on the degree of autonomy of individual branches' on a tree or shrub (107, 112, 117).

https://www.jstor.org/stable/info/2097264

The theory and practice of branch autonomy

In tall old forests, limitations to water transport may limit maximum tree height and reduce photosynthesis and carbon sequestration. We evaluated the degree to which tall trees could potentially compensate for hydraulic limitations to water transport by increased use of water stored in xylem. Using sap flux measurements in three tree species of the Pacific Northwest, we showed that reliance on stored water increases with tree size and estimated that use of stored water increases photosynthesis. For Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), water stored in xylem accounted for 20 to 25% of total daily water use in 60-m trees, whereas stored water comprised 7% of daily water use in 15-m trees. For Oregon white oak (Quercus garryana Dougl. ex Hook.), water stored in xylem accounted for 10 to 23% of total daily water use in 25-m trees, whereas stored water comprised 9 to 13% of daily water use in 10-m trees. For ponderosa pine (Pinus ponderosa Dougl. ex Laws.), water stored in xylem accounted for 4 to 20% of total daily water use in 36-m trees, whereas stored water comprised 2 to 4% of daily water use in 12-m trees. In 60-m Douglas-fir trees, we estimated that use of stored water supported 18% more photosynthesis on a daily basis than would occur if no stored water were used, whereas 15-m Douglas-fir trees gained 10% greater daily photosynthesis from use of stored water. We conclude that water storage plays a significant role in the water and carbon economy of tall trees and old forests.

Reliance on stored water increases with tree size in three species in the Pacific Northwest.

Temporal and Spatial Variations in the Water Status of Forest Trees

The leaf area to sapwood area ratio ( Al:As) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that A l :A s must decrease to maintain leaf-specific hydraulic sufficiency as path length, gravity, and tortuosity constrain whole-plant hy- draulic conductance. We tested the hypothesis that A l :A s declines with tree height. Whole-tree Al:As was measured on 15 individuals of Douglas-fir ( Pseudotsuga menziesii var. menziesii) ranging in height from 13 to 62 m (aged 20-450 years). A l :A s declined substantially as height in- creased (P=0.02). Our test of the hypothesis that A l :A s declines with tree height was extended using a combina- tion of original and published data on nine species across a range of maximum heights and climates. Meta-analysis of 13 whole-tree studies revealed a consistent and signif- icant reduction in A l :A s with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in Al:As with height, although the reason for this is not clear. The slope of the relationship between A l :A s and tree height (∆ A l :A s /∆ h) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with ∆ A l :A s /∆ h. The decrease in A l :A s with increasing tree size that we observed in the majority of species may be a homeostatic mechanism that partially compensates for decreased hydraulic con- ductance as trees grow in height.

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Thomas M. Hinckley

Papers

Biology of Populus and its implications for management and conservation

The theory and practice of branch autonomy

Reliance on stored water increases with tree size in three species in the Pacific Northwest.

Temporal and Spatial Variations in the Water Status of Forest Trees

The relationship between tree height and leaf area: sapwood area ratio.