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Browsing by Author "Walters, M B"

Now showing 1 - 3 of 3
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    The evolution of plant functional variation: traits, spectra, and strategies
    (University of Chicago Press, 2003) Reich, Peter B; Wright, I J; Cavender‐Bares, J; Craine, J M; Oleksyn, J; Westoby, M; Walters, M B
    Variation in plant functional traits results from evolutionary and environmental drivers that operate at a variety of different scales, which makes it a challenge to differentiate among them. In this article we describe patterns of functional trait variation and trait correlations within and among habitats in relation to several environmental and trade‐off axes. We then ask whether such patterns reflect natural selection and can be considered plant strategies. In so doing we highlight evidence that demonstrates that (1) patterns of trait variation across resource and environmental gradients (light, water, nutrients, and temperature) probably reflect adaptation, (2) plant trait variation typically involves multiple‐correlated traits that arise because of inevitable trade‐offs among traits and across levels of whole‐plant integration and that must be understood from a whole‐plant perspective, and (3) such adaptation may be globally generalizable for like conditions; i.e., the set of traits (collections of traits in syndromes) of taxa can be considered as “plant strategies.”
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    Growth of Acer saccharum seedlings in deeply shaded understories of northern Wisconsin: Effects of nitrogen and water availability
    (NRC Research Press, 1997) Walters, M B; Reich, Peter B
    Availability of soil nitrogen, soil moisture, and light were examined, along with the growth, biomass allocation, and leaf nitrogen concentration of naturally established Acer saccharum Marsh. seedlings, in the understories of 12 forest sites in northern Wisconsin. The sites represented a nutrient and moisture gradient (poor to rich) according to a habitat classification system. We asked (1) Are seedling growth rates, biomass allocation patterns, and leaf nitrogen related to soil water and nitrogen availability? and (2) Do soil resource rankings predicted by habitat classifications mirror our direct observations? Across sites compared in a low-light data set (plots with <5% canopy openness), rich sites had 2- to 4-fold higher seedling growth, percent leaf nitrogen, nitrogen mineralization rates, and nitrification rates than poor sites. Seedling growth in low light correlated (P ≤ 0.05) positively with nitrification, total nitrogen mineralization, percent leaf nitrogen, soil moisture, and organic carbon, and negatively with fine root density. In multiple regression, soil moisture (P = 0.033) and nitrification (P = 0.015) together explained 79% of the variation in growth. Thus, seedling growth in shade was enhanced on richer sites in part because of higher nitrate N and water availability. This has potential implications for forest dynamics, since the probability of sugar maple becoming the dominant woody regeneration in any given understory may be partially dependent upon the level of soil resources.
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    Leaf life-span in relation to leaf, plant, and stand characteristics among diverse ecosystems
    (1992) Reich, Peter B; Walters, M B; Ellsworth, D S
    Variation in leaf life-span has long been considered of ecological significance. Despite this, quantitative evaluation of the relationships between leaf life-span and other plant and ecosystem characteristics has been rare. In this paper we ask whether leaf lifespan is related to other leaf, plant, and stand traits of species from diverse ecosystems and biomes. We also examine the interaction between leaf, plant, and stand traits and their relation to productivity and ecological patterns. Among all species, both mass- (Amass) and area-based (Aarea) maximum net photosynthesis decreased with- increasing leaf life-span, but the relationship was stronger on a mass (P < .001, r2 = 0.70) than an area (P < .05, r2 = 0.24) basis. Similarly, mass-based leaf nitrogen (leafNmass) decreased (P < .001, r2 = 0.52) with leaf life-span, but area-based leaf N (leaf Narea) did not (P > .25, r2 = 0.01). Specific leaf area (SLA, leaf area/leaf dry mass) and leaf diffusive conductance also decreased with increasing leaf life-span. Decreasing Amass with increasing leaf life-span results from the impact of decreasing Nmass and SLA on Amass· Variation in leaf traits as a function of leaflife-span was similar for broad-leaved and needle-leaved subsets of the data. These leaf-scale data from several biomes were compared to a data set from a single biome, Amazonia. For several leaf traits (e.g., SLA, Nmass• and Amass) the quantitative relationship with leaf life-span was similar in the two independent data sets, suggesting that these are fundamental relations applicable to all species. Amass was a linear function of Nmass (P < .001, r2 = 0.74) with a regression similar to previous analyses, while Aarea was not significantly related to Narea· These results suggest that the photosynthesis-leafN relationship among species should be considered universal when expressed on a mass, but not on a leaf arna, basis. Relative growth rates (RGR) and leaf area ratio (LAR, the whole-plant ratio of leaf area to total dry mass) of seedlings decreased with increasing leaflife-span (P < .001, r2 = 0.61 and 0.89, respectively). LAR was positively related to both RGR and Amass (r2 = 0.68 and 0.84, respectively), and Amass and RGR were also positively related (r2 = 0.55). Absolute height growth rates of young trees decreased with increasing leaf life-span (P < .001, r2 = 0.72) and increased with Amass (P < .001, r2 = 0.78). It appears that a suite of traits including short leaf life-span and high leafNmas"' SLA, LAR, and Amass interactively contribute to high growth rates in open-grown individuals. These traits interact similarly at the stand level, but stands differ from individuals in one key trait. In closed-canopy forests, species with longer lived foliage (and low LAR as seedlings) have greater foliage mass per unit ground area (P < .001, r2 = 0.74) and a greater proportion of total mass in foliage. The above ground production efficiency (ANPP /foliar biomass) of forest stands decreased markedly with increasing leaf life-span or total foliage mass (P < .001, r2 = 0.78 and 0.72, respectively), probably as a result of decreasing Amass• Nmam and SLA, all of which were positively related with production efficiency and negatively related to total foliage mass. However, high foliage mass of species with extended leaf life-spans appears to compensate for low production per unit foliage, since above ground net primary production (ANPP, in megagrams per hectare per year) of forest stands was not related to leaf life-span. Extended leaf life-span also appears to compensate for lower potential production per unit leaf N per unit time, with the result that stand-level N use efficiency is weakly positively related to leaf life-span. We hypothesize that co-variation among species in leaflife-span, SLA, leafNmam Amass• and growth rate reflects a set of mutually supporting traits that interact to determine plant behavior and production, and provide a useful conceptual link between processes at short term leaf scales and longer term whole plant and stand-level scales. Although this paper has focused on leaf life-span, this trait is so closely interrelated with several others that this cohort of leaf traits should be viewed as causally interrelated. Generality in the relationships between leaf life-span and other plant traits across diverse communities and ecosystems suggests that they are universal in nature and thus can provide a quantitative link and/ or common currency for ecological comparisons among diverse systems.