Intraspecific growth and functional leaf trait responses to natural soil resource gradients for conifer species with contrasting leaf habit.

Interspecific relationships among species mean leaf traits, performance and species resource/climate distributions help provide the foundation for a predictive, functionally based plant ecology. Intraspecific responses of leaf traits and performance to resource gradients and how these vary among species may be equally important but have received less attention. Here, we examine relationships between proxies of soil resource availability, leaf traits and growth (height at 25 years, SI25) for winter deciduous Larix decidua Mill. and evergreen Pinus resinosa Ait. trees distributed over soil resource gradients in the Great Lakes region of North America. We predicted that (i) leaf trait responses to soil resources within species will be similar to reported distributions of mean leaf traits over soil resource gradients among species; (ii) soil resource-related variation in leaf traits can help explain SI25; and (iii) SI25 will be greater for Larix than Pinus at higher soil resources and greater for Pinus than Larix at lower soil resources and this pattern will be associated with species differences in leaf trait responses to soil resources. Among the measured leaf traits (live N, Mg, Ca, K, P, and Mn, litter N, N resorption, carbon isotope discrimination, specific leaf area, lifespan), soil resources only impacted live and litter N for both species and K for Pinus. In turn, only the leaf traits responsive to soil resources affected SI25 in the expected manner. Larix had greater SI25 than Pinus across soil resource gradients and both species had similar growth and leaf trait sensitivities to resources. In summary: (i) several leaf traits reported to be associated with performance and edaphic distributions across species were, within species, unresponsive to nitrogen and water availability and unrelated to growth; (ii) leaf N showed high plasticity to soil resources and this plasticity was functionally relevant to growth over its entire range of response; (iii) large species-level differences in leaf traits between Larix and Pinus did not translate into different leaf trait and growth responses to soil resources.

Optimal partitioning theory revisited: nonstructural carbohydrates dominate root mass responses to nitrogen.

Under optimal partitioning theory (OPT), plants preferentially allocate biomass to acquire the resource that most limits growth. Within this framework, higher root mass under low nutrients is often assumed to reflect an allocation response to build more absorptive surface. However, higher root mass also could result from increased storage of total nonstructural carbohydrates (TNC) without an increase in non-storage mass or root surface area. To test the relative contributions of TNC and non-storage mass as components of root mass responses to resources, we grew seedlings of seven northern hardwood tree species (black, red, and white oak, sugar and red maple, American beech, and black cherry) in a factorial light x nitrogen (N) greenhouse experiment. Because root mass is a coarse metric of absorptive surface, we also examined treatment effects on fine-root surface area (FRSA). Consistent with OPT, total root mass as a proportion of whole-plant mass generally was greater in low vs. high N. However, changes in root mass were influenced by TNC mass in all seven species and were especially strong in the three oak species. In contrast, non-storage mass contributed to increased total root mass under low N in three of the seven species. Root morphology also responded, with higher fine-root surface area (normalized to root mass) under low vs. high N in four species. Although biomass partitioning responses to resources were consistent with OPT, our results challenge the implicit assumption that increases in root mass under low nutrient levels primarily reflect allocation shifts to build more root surface area. Rather, root responses to low N included increases in: TNC, non-storage mass and fine-root surface area, with increases in TNC being the largest and most consistent of these responses. The greatest TNC accumulation occurred when C was abundant relative to N. Total nonstructural carbohydrates storage could provide seedlings a carbon buffer when respiratory or growth demands are not synchronized with photosynthesis, flexibility in responding to uncertain and fluctuating abiotic and biotic conditions, and increased access to soil resources by providing an energy source for mycorrhizae, decomposers in the rhizosphere, or root uptake of nutrients.

Senescence-related changes in nitrogen in fine roots: mass loss affects estimation.

The fate of nitrogen (N) in senescing fine roots has broad implications for whole-plant N economies and ecosystem N cycling. Studies to date have generally shown negligible changes in fine root N per unit root mass during senescence. However, unmeasured loss of mobile non-N constituents during senescence could lead to underestimates of fine root N loss. For N fertilized and unfertilized potted seedlings of Populus tremuloides Michx., Acer rubrum L., Acer saccharum Marsh. and Betula alleghaniensis Britton, we predicted that the fine roots would lose mass and N during senescence. We estimated mass loss as the product of changes in root mass per length and root length between live and recently dead fine roots. Changes in root N were compared among treatments on uncorrected mass, length (which is independent of changes in mass per length), calcium (Ca) and corrected mass bases and by evaluating the relationships of dead root N as a function of live root N, species and fertilization treatments. Across species, from live to dead roots, mass decreased 28-40%, N uncorrected for mass loss increased 10-35%, N per length decreased 5-16%, N per Ca declined 14-48% and N corrected for mass declined 12-28%. Given the magnitude of senescence-related root mass loss and uncertainties about Ca dynamics in senescing roots, N loss corrected for mass loss is likely the most reliable estimate of N loss. We re-evaluated the published estimates of N changes during root senescence based on our values of mass loss and found an average of 28% lower N in dead roots than in fine roots. Despite uncertainty about the contributions of resorption, leaching and microbial immobilization to the net loss of N during root senescence, live root N was a strong and proportional predictor of dead root N across species and fertilization treatments, suggesting that live root N alone could be used to predict the contributions of senescing fine roots to whole-plant N economies and N cycling.

Elevated CO(2) concentration affects leaf photosynthesis-nitrogen relationships in Pinus taeda over nine years in FACE.

To investigate whether long-term elevated carbon dioxide concentration ([CO(2)]) causes declines in photosynthetic enhancement and leaf nitrogen (N) owing to limited soil fertility, we measured photosynthesis, carboxylation capacity and area-based leaf nitrogen concentration (N(a)) in Pinus taeda L. growing in a long-term free-air CO(2) enrichment (FACE) facility at an N-limited site. We also determined how maximum rates of carboxylation (V(cmax)) and electron transport (J(max)) varied with N(a) under elevated [CO(2)]. In trees exposed to elevated [CO(2)] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity (A(net-Ca)) and N(a) was significantly reduced by 37% in 1-year-old needles, whereas it was unaffected in current-year needles. The slope of the relationships of both V(cmax) and J(max) with N(a) decreased in 1-year-old needles after up to 9 years of growth in elevated [CO(2)], which was accompanied by a 15% reduction in N allocation to the carboxylating enzyme. Nitrogen fertilization (110 kg N ha(-1)) in the ninth year of exposure to elevated [CO(2)] restored the slopes of the relationships of V(cmax) and J(max) with N(a) to those of control trees (i.e., in ambient [CO(2)]). The J(max):V(cmax) ratio was unaffected by either [CO(2)] or N fertilization. Changes in the apparent allocation of N to photosynthetic components may be an important adjustment in pines exposed to elevated [CO(2)] on low-fertility sites. We conclude that fundamental relationships between photosynthesis or its component processes with N(a) may be altered in aging pine needles after more than 5 years of exposure to elevated atmospheric [CO(2)].

Relationships of leaf dark respiration to leaf nitrogen, specific leaf area and leaf life-span: a test across biomes and functional groups.

Based on prior evidence of coordinated multiple leaf trait scaling, we hypothesized that variation among species in leaf dark respiration rate (R d) should scale with variation in traits such as leaf nitrogen (N), leaf life-span, specific leaf area (SLA), and net photosynthetic capacity (A max). However, it is not known whether such scaling, if it exists, is similar among disparate biomes and plant functional types. We tested this idea by examining the interspecific relationships between R d measured at a standard temperature and leaf life-span, N, SLA and A max for 69 species from four functional groups (forbs, broad-leafed trees and shrubs, and needle-leafed conifers) in six biomes traversing the Americas: alpine tundra/subalpine forest, Colorado; cold temperate forest/grassland, Wisconsin; cool temperate forest, North Carolina; desert/shrubland, New Mexico; subtropical forest, South Carolina; and tropical rain forest, Amazonas, Venezuela. Area-based R d was positively related to area-based leaf N within functional groups and for all species pooled, but not when comparing among species within any site. At all sites, mass-based R d (R d-mass) decreased sharply with increasing leaf life-span and was positively related to SLA and mass-based A max and leaf N (leaf N mass). These intra-biome relationships were similar in shape and slope among sites, where in each case we compared species belonging to different plant functional groups. Significant R d-mass-N mass relationships were observed in all functional groups (pooled across sites), but the relationships differed, with higher R d at any given leaf N in functional groups (such as forbs) with higher SLA and shorter leaf life-span. Regardless of biome or functional group, R d-mass was well predicted by all combinations of leaf life-span, N mass and/or SLA (r 2≥ 0.79, P < 0.0001). At any given SLA, R d-mass rises with increasing N mass and/or decreasing leaf life-span; and at any level of N mass, R d-mass rises with increasing SLA and/or decreasing leaf life-span. The relationships between R d and leaf traits observed in this study support the idea of a global set of predictable interrelationships between key leaf morphological, chemical and metabolic traits.