Papers by Sarah C Pasquini
Ecology, 2018
We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, ... more We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level analyses of tree growth rates for a 15-year factorial nitrogen (N), phosphorus (P) and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10 years of fertilization (Wright et al. 2011) but not after 15 years (this study). Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora.
Journal of the Torrey Botanical Society, 2017
American beech (Fagus grandifolia Ehrh.) regenerates both from seed and also clonally via root sp... more American beech (Fagus grandifolia Ehrh.) regenerates both from seed and also clonally via root sprouts. Regenerating beech saplings often form dense and depauperate understories that cast deep shade and displace cooccurring species. The relative proportion of saplings that originate from seed versus root sprouts varies widely throughout the range of beech. Although the cause for that variation remains unclear, it may be linked to canopy or soil disturbances, the spread of beech bark disease (BBD), or overabundant deer. Here, we asked whether the longterm exclusion of deer and the absence of BBD would favor the regeneration of saplings (20-150 cm tall) of seed origin versus those of sprout origin. We addressed this question using deer exclosures (16 and 60þ yr old) and paired controls in one forest in Pennsylvania where BBD had caused major adult mortality and another where BBD was not present. We found that excluding deer significantly decreased the relative proportion of root sprouts from approximately 60% to approximately 25% in each forest, regardless of stand age, exclosure age, soil type, and presence or absence of BBD. Our findings provide evidence that deer, acting as herbivores, seed predators, agents of physical disturbance, or all of those simultaneously, create forest understories where root sprouts predominate. Although speculative, our results may apply to large regions because deer have been overabundant throughout much of the geographic range of American beech.
Functional Ecology, 2016
1. Drought-induced mortality and regional dieback of woody vegetation are reported from numerous ... more 1. Drought-induced mortality and regional dieback of woody vegetation are reported from numerous locations around the world. Yet within any one site, predicting which species are most likely to survive global change-type drought is a challenge. 2. We studied the diversity of drought survival traits of a community of 15 woody plant species in a desert-chaparral ecotone. The vegetation was a mix of chaparral and desert shrubs, as well as endemic species that only occur along this margin. This vegetation boundary has large potential for drought-induced mortality because nearly all species are at the edge of their range. 3. Drought survival traits studied were vulnerability to drought-induced xylem cavitation, sapwood capacitance, deciduousness, photosynthetic stems, deep roots, photosynthetic responses to leaf water potential and hydraulic architecture. Drought survival strategies were evaluated as combinations of traits that could be effective in dealing with drought. 4. The large variation in seasonal predawn water potential of leaves and stem xylem ranged from À6Á82 to À0Á29 MPa and À6Á92 to À0Á27 MPa, respectively. The water potential at which photosynthesis ceases ranged from À9Á42 to À3Á44 MPa. Architecture was a determinant of hydraulic traits, with species supporting large leaf area per sapwood area exhibiting high rates of water transport, but also xylem that is vulnerable to drought-induced cavitation. Species with more negative midday leaf water potential during the growing season also showed access to deeper water sources based on hydrogen isotope analysis. 5. Drought survival mechanisms comprised of drought deciduousness, photosynthetic stems, tolerance of low minimum seasonal tissue water potential and vulnerability to drought-induced xylem cavitation thus varied orthogonally among species, and promote a diverse array of drought survival strategies in an arid ecosystem of considerable floristic complexity.
Ecology, 2015
bstract. Lianas are a prominent growth form in tropical forests, and there is compelling evidence... more bstract. Lianas are a prominent growth form in tropical forests, and there is compelling evidence that they are increasing in abundance throughout the Neotropics. While recent evidence shows that soil resources limit tree growth even in deep shade, the degree to which soil resources limit lianas in forest understories, where they coexist with trees for decades, remains unknown. Regardless, the physiological underpinnings of soil resource limitation in deeply shaded tropical habitats remain largely unexplored for either trees or lianas. Theory predicts that lianas should be more limited by soil resources than trees because they occupy the quick-return end of the ''leaf economic spectrum,'' characterized by high rates of photosynthesis, high specific leaf area, short leaf life span, affinity to high-nutrient sites, and greater foliar nutrient concentrations. To address these issues, we asked whether soil resources (nitrogen, phosphorus, and potassium), alone or in combination, applied experimentally for more than a decade would cause significant changes in the morphology or physiology of tree and liana seedlings in a lowland tropical forest. We found evidence for the first time that phosphorus limits the photosynthetic performance of both trees and lianas in deeply shaded understory habitats. More importantly, lianas always showed significantly greater photosynthetic capacity, quenching, and saturating light levels compared to trees across all treatments. We found little evidence for nutrient 3 growth form interactions, indicating that lianas were not disproportionately favored in nutrient-rich habitats. Tree and liana seedlings differed markedly for six key morphological traits, demonstrating that architectural differences occurred very early in ontogeny prior to lianas finding a trellis (all seedlings were self-supporting). Overall, our results do not support nutrient loading as a mechanism of increasing liana abundance in the Neotropics. Rather, our finding that lianas always outperform trees, in terms of photosynthetic processes and under contrasting rates of resource supply of macronutrients, will allow lianas to increase in abundance if disturbance and tree turnover rates are increasing in Neotropical forests as has been suggested.
Oecologia, 2012
We investigated how photosynthesis by understory seedlings of the lowland tropical tree species A... more We investigated how photosynthesis by understory seedlings of the lowland tropical tree species Alseis blackiana responded to 10 years of soil nutrient fertilization with N, P and K. We ask whether nutrients are limiting to light and CO 2 acquisition in a low light understory environment. We measured foliar nutrient concentrations of N, P and K, isotopic composition of carbon (d 13 C) and nitrogen (d 15 N), and light response curves of photosynthesis and chlorophyll fluorescence. Canopy openness was measured above each study seedling and included in statistical analyses to account for variation in light availability. Foliar N concentration increased by 20% with N addition. Foliar P concentration increased by 78% with P addition and decreased by 14% with N addition. Foliar K increased by 8% with K addition. Foliar d 13 C showed no significant responses, and foliar d 15 N decreased strongly with N addition, matching the low d 15 N values of applied fertilizer. Canopy openness ranged from 0.01 to 6.71% with a mean of 1.76 ± 0.14 (±1SE). Maximum photosynthetic CO 2 assimilation rate increased by 9% with N addition. Stomatal conductance increased with P addition and with P and K in combination. Chlorophyll fluorescence measurements revealed that quantum yield of photosystem II increased with K addition, maximum electron transport rate trended 9% greater with N addition (p = 0.07), and saturating photosynthetically active radiation increased with N addition. The results demonstrate that nutrient addition can enhance photosynthetic processes, even under low light availability.
American Journal of Botany, 2011
Journal of arid environments, 2010
High levels of atmospheric nitrogen (N) deposition to southern California chaparral shrublands ma... more High levels of atmospheric nitrogen (N) deposition to southern California chaparral shrublands may interact with fire to affect biomass production and plant species composition during secondary succession. To determine the potential interactions between postfire recovery and N deposition we compared rates of aboveground net primary production (ANPP), shrub growth, and the relative abundance of Adenostoma fasciculatum, other sub-dominant shrubs, and herbaceous species of three chaparral stands exposed to different levels of atmospheric N deposition over the first 3 years of post-fire succession. Our data suggest that rates of ANPP (gdw m À2 month À1 ) and aboveground N storage (gN m À2 month À1 ) for these chaparral stands were not related to N deposition even though sites exposed to high levels of N deposition had significantly higher rates of shrub growth (gdw plant À1 month À1 ) and N uptake (gN plant À1 month À1 ). However, high-N stands were composed of larger shrubs with a lower density, and this trade-off between shrub size and density may explain the low correlation between N deposition and post-fire ANPP. Differences in relative plant species abundance between sites were significantly correlated with N deposition exposure, where stands exposed to high N deposition had a lower relative abundance of A. fasciculatum and a higher relative abundance of other shrub and herbaceous species. While many factors can affect rates and patterns of post-fire recovery, these results suggest that chronic exposure to N deposition may significantly alter plant growth and species composition in successional chaparral stands.
Ecology, 2009
Nitrogen (N) deposition in heavily polluted southern Californian shrublands is estimated to be 20... more Nitrogen (N) deposition in heavily polluted southern Californian shrublands is estimated to be 20-45 kg NÁha À1 Áyr À1 , but more exposed locales can receive as much as 145 kg NÁha À1 Áyr À1 . This large anthropogenic N input has the capacity to alter the composition of plant communities. We conducted N-fertilization experiments in chaparral and coastal sage scrub (CSS) stands over a five-year period to test the hypothesis that plant community composition would change in response to dry-season N addition because of an increase in the relative abundance of herbaceous plant species. Our results indicate that dry-season addition of N significantly altered the community composition of CSS but not chaparral. Contrary to our original hypothesis, changes in community composition were due to changes in the relative abundance of dominant shrubs and not herbaceous plant species. Given that community-level responses to changes in resource availability may take years to decades in order to fully materialize, our results suggest that continued dry-season input of N will cause even larger changes in community composition over time. These results have implications for plant species composition and diversity of mediterranean-type shrublands as N deposition increases with population growth and fossil-fuel use.
Ecosystems, 2009
Anthropogenic nitrogen (N) deposition is a globally important source of N that is expected to inc... more Anthropogenic nitrogen (N) deposition is a globally important source of N that is expected to increase with population growth. In southern California, N input from dry deposition accumulates on vegetation and soil surfaces of chaparral and coastal sage scrub (CSS) ecosystems during the summer and fall and becomes available as a pulse following winter rainfall. Presumably, N input will act to stimulate the productivity and N storage of these Mediterranean-type, semi-arid shrublands because these ecosystems are thought to be N limited. To assess whether dry-season N inputs alter ecosystem productivity and N storage, a field experiment was conducted over a 4-year period where plots were exposed to either ambient N deposition (control) or ambient + 50 kgN ha -1 y -1 (added N) that was added as NH 4 NO 3 during the fall dry-season of each year. Plots exposed to added N had significantly higher accumulation of NH 4 and NO 3 on ion exchange resins that was due in part to direct fertilization and N mineralization, and the increase in N availability lead to a significant increase in NO 3 leaching in chaparral but not CSS. Nitrogen addition also lead to an increase in litter and tissue N concentration and a decline in the C:N ratio, but failed to alter the ecosystem productivity and N storage of the chaparral and CSS shrublands over the 4-year study period. The reasons for the lack of a treatment response are unknown; however, it is possible that these semi-arid shrublands are not N limited, cannot respond rapidly enough to capture the ephemeral N pulse, are limited by other nutrients, or the N response is dependent on the amount and/or distribution of rainfall. These results have important implications for understanding the potential effects of anthropogenic N deposition on the C and N cycling and storage of Mediterranean-type, semi-arid shrublands.
Journal of Arid Environments, 2008
Fire and atmospheric N deposition have the capacity to alter the N and C cycling and storage of s... more Fire and atmospheric N deposition have the capacity to alter the N and C cycling and storage of semi-arid shrublands. Thus, we measured the pre-and post-fire soil and tissue C and N dynamics of three southern California chaparral stands exposed to varying levels of N deposition. Total soil N was positively correlated with N deposition exposure before and after fire, indicating that fire did little to alter patterns of soil N enrichment from atmospheric N deposition. Fire caused a significant increase in soil extractable N, and this increase was positively correlated with N deposition indicating that fire enhanced soil N availability more at sites exposed to high N deposition. Fire also caused the tissue N concentration of regenerating Adenostoma fasciculatum (chamise) shrubs to be significantly correlated with N deposition; however, the correlation was negative during the first year of fire recovery (r 2 ¼ 0.62) and positive during the second (r 2 ¼ 0.94) indicating complex and transient dynamics in shrub growth and N concentration. Overall, our results suggest that periodic fire in chaparral may not reduce the potential for N enrichment that develops over decades of N deposition and have important implications for the propensity of these semi-arid shrublands to become ''N-saturated''. r
Journal of Arid Environments, 2007
Semi-arid shrublands of southern California, including chaparral and coastal sage, are found in w... more Semi-arid shrublands of southern California, including chaparral and coastal sage, are found in widely varying elevation and microclimatic regimes and are subjected to disturbance such as fire and atmospheric N deposition that have the capacity to alter soil and litter C and N storage. Here we present a case study where soil and litter C and N were measured over 19 months in post-fire chaparral and mature coastal sage stands to assess whether differences in soil and litter C and N between these diverse shrublands could be attributed to differences in elevation, stand age, rainfall, and/or estimated N deposition exposure. Our results indicate that atmospheric N deposition exposure, either alone or in conjunction with other environmental variables (elevation, rainfall, and/or stand age), was the most frequent predictor of the spatial pattern in the soil and litter N and C variables observed. These results are consistent with those reported for high-elevation coniferous forests arrayed along an N deposition gradient in southern California, suggesting that N deposition may affect the soil N and C storage of semiarid shrublands and woodlands in a qualitatively similar manner. r
Soil Science Society of America Journal, 2007
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Papers by Sarah C Pasquini