Hafiz Maherali

Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (AL/AS) in desert versus montane trees could increase leaf-specific hydraulic conductance (KL). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (JS) and shoot water potential during the summer (June–July) and fall (August–September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and JS. In the summer, mean maximum JS was 31.80±5.74 and 24.34±3.05xa0g m–2 s–1 for desert and montane trees (a 30.6% difference), respectively. In the fall, JS was 25.33±8.52 and 16.36±4.64xa0g m–2 s–1 in desert and montane trees (a 54.8% difference), respectively. JS was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher KL than montane trees in the summer (2.41×10–5 versus 1.05×10–5 kg m–2 s–1 MPa–1, P<0.001) and a 162% higher KL in the fall (1.97×10–5 versus 0.75×10–5 kg m–2 s–1 MPa–1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum gC was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11xa0kg (11%) and 10.6xa0kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high KL in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase KL and not to increase the contribution of stored water to transpiration.

Hydraulic adjustment of maple saplings to canopy gap formation

Abstract The leaf-specific hydraulic conductivity (KL) of plant stems can control leaf water supply. This property is influenced by variation in leaf/sapwood area ratio (AL/AS) and the specific hydraulic conductivity of xylem tissue (KS). In environments with high atmospheric vapor pressure deficit (VPD), KL may increase to support higher transpiration rates. We predicted that saplings of Acerrubrum and A.pensylvanicum grown in forest canopy gaps, under high light and VPD, would have higher KL and lower AL/AS than similar sized saplings in the understory. Leaf-specific hydraulic conductivity and KS increased with sapling size for both species. In A. rubrum, KS did not differ between the two environments but lower AL/AS (P=0.05, ANCOVA) led to higher KL for gap-grown saplings (P < 0.05, ANCOVA). In A.pensylvanicum, neither KS, AL/AS, nor KL differed between environments. In a second experiment, we examined the impact of sapling size on the water relations and carbon assimilation of A.pensylvanicum. Maximum stomatal conductance for A.pensylvanicum increased with KL (r2=0.75, P <u20090.05). A hypothetical large A.pensylvanicum sapling (2u2009m tall) had 2.4 times higher KL and 22 times greater daily carbon assimilation than a small (1u2009m tall) sapling. Size-related hydraulic limitations in A.pensylvanicum caused a 68% reduction in daily carbon assimilation in small saplings. Mid-day water potential increased with A.pensylvanicum sapling size (r2=0.69, Pu2009<u20090.05). Calculations indicated that small A.pensylvanicum saplings (low KL) could not transpire at the rate of large saplings (high KL) without reaching theoretical thresholds for xylem embolism induction. The coordination between KL and stomatal conductance in saplings may prevent xylem water potential from reaching levels that cause embolism but also limits transpiration. The KS of the xylem did not vary across environments, suggesting that altering biomass allocation is the primary mechanism of increasing KL. However, the ability to alter aboveground biomass allocation in response to canopy gaps is species-specific. As a result of the increase in KL and KS with sapling size for both species, hydraulic limitation of water flux may impose a greater restriction on daily carbon assimilation for small saplings in the gap environment.

Interactive effects of elevated CO2 and temperature on water transport inponderosa pine

Many studies report that water flux through trees declines in response to elevated CO(2), but this response may be modified by exposure to increased temperatures. To determine whether elevated CO(2) and temperature interact to affect hydraulic conductivity, we grew ponderosa pine seedlings for 24 wk in growth chambers with one of four atmospheric CO(2) concentrations (350, 550, 750, and 1100 ppm) and either a low (15°C nights, 25°C days) or high (20°C nights, 30°C days) temperature treatment. Vapor pressure deficits were also higher in the elevated temperature treatment. Seedling biomass increased with CO(2) concentration but was not affected by temperature. Rootu200a:u200ashoot ratio was unaffected by CO(2) and temperature. Leafu200a:u200asapwood area ratio (A(L)/A(S)) declined in response to elevated temperature but was not influenced by CO(2). Larger tracheid diameters at elevated temperature caused an increase in xylem-specific hydraulic conductivity (K(S)). The increase in K(S) and decrease in A(L)/A(S) led to higher leaf-specific hydraulic conductivity (K(L)) at elevated temperature. Stomatal conductance (g(S)) was correlated with K(L) across all treatments. Neither K(S), K(L), nor g(S) were affected by elevated CO(2) concentrations. High K(L) in response to elevated temperature may support increased transpiration or reduce the incidence of xylem cavitation in ponderosa pine in future, warmer climates.

Sapling biomass allocation and growth in the understory of a deciduous hardwood forest

Above- and belowground tissues of co-occurring saplings (0.1-1 m height) of Acer saccharum Marsh. (very shade tolerant), Acer rubrum L. (shade tolerant), Fraxinus americana L. (intermediate shade tolerant), and Prunus serotina Ehrh. (shade intolerant) were harvested from a forest understory to test the hypothesis that the pattern of biomass allocation varied predictably with shade-tolerance rank. The placement and length of branches along the main axis were consistent with the formation of a monolayer of foliage for the tolerant and intermediate species. Other morphological characteristics did not vary predictably with shade-tolerance rank. The maintenance of high specific leaf area (SLA; leaf area/leaf mass) and leaf area ratio (LAR; leaf area/sapling mass) is considered important for growth under extreme shade, yet these traits were not clearly related to the shade-tolerance rank of these species. Fraxinus americana, an intermediate species, had the highest LAR and growth rate in the understory, and with the exception of P. serotina, the very shade-tolerant A. saccharum had the lowest LAR. Prunus serotina maintained a large starch-rich tap root and shoot dieback was common, yielding the largest root/shoot ratio for these species. The observed allocation patterns were not similar to the long-standing expectation for the phenotypic response of juvenile trees to shade, but were consistent with three hypothetical growth strategies in the understory: (1) the low SLA and LAR of A. saccharum may provide a measure of defense against herbivores and pathogens and thus promote persistence in the understory, (2) the high SLA for F. americana and high LAR for F. americana and A. rubrum may enable these species to achieve high growth rates in shade, and (3) the large carbohydrate stores of P. serotina may poise this species for opportunistic growth following disturbance. The relative importance of resistance to herbivores and pathogens vs. the maintenance of high growth rates may be important in evaluating the patterns of biomass allocation in the understory.

Influence of nutrient availability on the mechanisms of tolerance to herbivory in an annual grass, Avena barbata (Poaceae)

Tolerance, or the capacity of a genotype to survive and reproduce following herbivore damage, varies widely across the plant kingdom. One proximate cause of this variation is resource availability, which can influence tolerance through mechanisms such as growth rate and photosynthesis. We examined the effect of high and low soil nutrient levels on the relationship between tolerance and two of its underlying mechanisms, biomass regrowth and photosynthetic upregulation, among genotypes of the Mediterranean annual grass Avena barbata. Although defoliated plants did not reach the same biomass as controls, biomass regrowth was higher at high nutrients. However, increased seed abortion at high nutrients caused tolerance to be the same in both nutrient treatments. Increased seed abortion also uncoupled biomass regrowth from tolerance at high nutrients. We found no evidence for photosynthetic upregulation in defoliated compared to control plants in either nutrient treatment. However, tolerance was positively correlated with predefoliation photosynthetic efficiency at high nutrients. Thus, constitutive photosynthetic efficiency may be a better predictor of tolerance than photosynthetic responses following herbivory in A. barbata. More generally, our results highlight the possibility that the mechanisms of tolerance can differ across resource environments even if tolerance is the same.

Xylem function and climate adaptation in Pinus

UNLABELLEDnnnnPREMISE OF THE STUDYnThe distribution of species is determined in part by their functional traits. One important function is the ability of xylem to supply water to leaves and withstand water-stress-induced cavitation. These hydraulic traits are hypothesized to have evolved in response to selection by precipitation and temperature. •nnnMETHODSnWe grew 26 species in the genus Pinus in a common environment and used phylogenetic comparative methods to examine whether the evolution of seedling hydraulic and wood density traits were associated with the climate of the extant geographic range of the species. We also examined whether these traits were correlated with each other, with integrated water-use efficiency (WUE), and with plant growth. •nnnKEY RESULTSnContrary to predictions from a hydraulic model, we found no association between stem hydraulic conductivity (K(S)) and precipitation, even though there was substantial variation for K(S) in the genus. Nevertheless, K(S) was positively correlated with temperature, plant biomass, and WUE. Wood density was infrequently associated with climate or correlated with other traits except for plant biomass. •nnnCONCLUSIONSnReduced K(S) in cold climates, if associated with reduced conduit diameter, likely evolved to increase resistance to freezing-induced xylem cavitation. The absence of a K(S)-precipitation relationship among Pinus seedlings suggests that associations between hydraulic traits and precipitation found in adult trees arise through plastic responses to moisture availability and/or develop over ontogeny. The weak association among wood density, climate, and other traits suggest that this trait does not contribute to climate adaptation in Pinus.

Mechanisms and Consequences of Water Stress–Induced Parental Effects in an Invasive Annual Grass

Premise of research.u2003Tests of the adaptive value of parental effects have generally focused on offspring fitness. However, the evolution of parental effects depends on their consequences for both offspring and parental fitness. Whether parental effects are adaptive can also depend on the mechanism of these effects. Parental effects caused by differences in the quality rather than quantity of resources provisioned to offspring may be more likely to be adaptive because they can persist through the life cycle. Methodology.u2003We estimated parental effects in response to water stress in the invasive annual Avena barbata. To test whether these effects were adaptive, we reciprocally transplanted offspring of wet- and dry-grown parents into wet and dry environments. We also tested whether seed size and nitrogen content, which represent the quantity and quality of parental investment, were mechanisms of parental effects in A. barbata. Pivotal results.u2003We found evidence of parental effects in response to water stress in A. barbata; dry-grown parents produced offspring with significantly higher germination, longer radicles, and earlier emergence than wet-grown parents. The offspring of dry-grown parents had higher biomass and seed production than the offspring of wet-grown parents. However, when cumulative fitness was calculated across parental and offspring generations, dry-grown parents had significantly lower fitness than wet-grown parents because of trade-offs between seed size and number. Although dry-grown parents provisioned their offspring with more nitrogen than wet-grown parents, offspring performance was primarily explained by variation in seed mass. Conclusions.u2003Water stress–induced parental effects were adaptive from the offspring but not the parental perspective, suggesting that the evolution of these effects may be constrained. In addition, water stress–induced parental effects were primarily caused by differences in seed mass, suggesting that the quantity of resources provisioned to offspring is a more important mechanism of parental effects than resource quality.

Why Are Trade-Offs between Flower Size and Number Infrequently Detected? A Test of Three Hypotheses

Although the flower is the unit of sexual reproduction in angiosperms, a plant’s reproductive success is determined by its entire floral display. A trade-off between the size and number of flowers produced is an assumption of models of floral display evolution, but this trade-off is often not observed. We tested three hypotheses for why a trade-off between flower size and number is not observed, using 32 populations of Mimulus guttatus and 83 genera from the California flora. We found support for the hypothesis that high variance in resource acquisition masks a trade-off between flower size and number. In contrast, we did not find support for the hypothesis that trade-offs between current and future reproduction mask negative correlations between flower size and number in lineages with a perennial life history. We also did not find support for the hypothesis that negative flower size-number correlations are less likely to be observed in lineages with lower variation in flower number because the correlation between two variables is weaker when the range of values is narrower. Overall, our results support the life history evolution literature in suggesting that high variance in resource acquisition relative to allocation can often explain why traits that are predicted to trade off are instead positively correlated.

Increased Photosynthetic Capacity as a Mechanism of Drought Adaptation in C3 Plants

Premise of research.u2003Plant available soil moisture can influence the evolution of C3 photosynthesis in multiple ways. Water limitation could select for enhanced photosynthetic capacity in order to overcome stomatal limitations to CO2 supply. Alternatively, moisture-limited soils may select for reduced photosynthetic capacity because of the high costs of investment in N-rich Rubisco under chronic CO2 limitation. Methodology.u2003Using literature data on photosynthetic capacity, phylogenetic information, and georeferenced climate records, we assessed the magnitude and direction of the relationship between photosynthetic capacity, as described by the carboxylation capacity of Rubisco (Vcmax) and annual precipitation (MAP). We also examined the association between leaf nitrogen content expressed on leaf area and leaf mass bases (Nmass and Narea, respectively) and MAP. Pivotal results.u2003Both Vcmax and Nmass increased with decreasing MAP, a finding that was relatively consistent across growth forms, including deciduous and evergreen angiosperms. There was no association between Narea and MAP. Conclusions.u2003Selection by dry environments may be responsible for the evolution of increased photosynthetic capacity and leaf N content in C3 plants despite potential metabolic costs of maintaining high investment in N-rich Rubisco. Greater photosynthetic capacity can maximize photosynthetic carbon gain per unit water transpired, making it an important adaptation to arid/semiarid environments.