George L. Vourlitis

Effects of Dry-Season N Input on the Productivity and N Storage of Mediterranean-Type Shrublands

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 ambientxa0+xa050xa0kgN ha−1 y−1 (added N) that was added as NH4NO3 during the fall dry-season of each year. Plots exposed to added N had significantly higher accumulation of NH4 and NO3 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 NO3 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.

Nitrogen and carbon mineralization of semi-arid shrubland soil exposed to long-term atmospheric nitrogen deposition

Anthropogenic N-deposition represents a significant input of N into semi-arid chaparral and coastal sage scrub (CSS) shrublands of southern California. High levels of atmospheric N deposition have the potential to increase soil C and N mineralization, and we hypothesize that semi-arid shrubland soil exposed to long-term (decades) high N deposition will have significantly higher C and N mineralization potentials. This hypothesis was tested in a laboratory incubation where the inorganic N (NH4+NO3) and CO2 production of soils maintained at a constant temperature of 25°C and a soil moisture of 0.25xa0g H2O/g (65% water-filled pore space) were sampled sequentially over a 50-week period. The temporal trend in cumulative C and N mineralization was well described by a first- and zero-order model, respectively. Long-term atmospheric N deposition significantly increased potential N mineralization but not C mineralization, and both the rate and total N mineralization were significantly positively correlated with the surface (0–10xa0cm) soil δ15N natural abundance and negatively correlated with the surface soil C:N ratio. While the incubation techniques used here do not provide realistic estimates of in situ C or N mineralization, these assays indicate that atmospheric N deposition has significantly altered ecosystem N storage and cycling.

Seasonal Patterns of Evapotranspiration for a Vochysia divergens Forest in the Brazilian Pantanal

The recent and widespread expansion of the pioneer tree species Vochysia divergens Pohl into western Brazil has the potential to significantly alter the structure and function of the Pantanal—a UNESCO World Heritage Site and the World’s largest tropical wetland. Here we assess the seasonal pattern of evapotranspiration (ET) and micrometeorological variables of V. divergens (locally known as cambarazal), located in the Northeast of the Brazilian Pantanal. ET was calculated from a number of micrometeorological measurements recorded between January 2007 and January 2008. The results indicate that ET was the dominant sink for net radiation (Rn) during the wet and dry seasons, primarily because the forest was either flooded (December–May) or retained a high level of soil moisture. ET decreased during the dry season due to a decline in Rn and surface water availability, and an increase in atmospheric vapor pressure deficit. Based on this analysis we conclude that the spread of V. divergens into the Pantanal and the associated high rates of ET are due in part to high water availability, even during the dry season, and the consistently high leaf area index that increases the transpiration surface area when the water table is below the soil surface.

Plant and Soil N Response of Southern Californian Semi-arid Shrublands After 1 Year of Experimental N Deposition

Large inputs of atmospheric N from dry deposition accumulate on vegetation and soil surfaces of southern Californian chaparral and coastal sage scrub (CSS) ecosystems during the late-summer and early-fall and become available as a pulse following winter rainfall; however, the fate of this dry season atmospheric N addition is unknown. To assess the potential for dry season atmospheric N inputs to be incorporated into soil and/or vegetation N pools, an in situ N addition experiment was initiated in a post-fire chaparral and a mature CSS stand where 10xa0×xa010xa0m plots were exposed to either ambient N deposition (control) or ambient +50xa0kg Nxa0ha−1 (added N) added as NH4NO3 during a single application in October 2003. After 1 year of N addition, plots exposed to added N had significantly higher accumulation of extractable inorganic N (NH4−Nxa0+xa0NO3−N) on ion exchange resins deployed in the 0–10xa0cm mineral soil layer and higher soil extractable N in the subsurface (30–40xa0cm) mineral soil than plots exposed to ambient N. Chaparral and CSS shrubs exposed to added N also exhibited a significant increase in tissue N concentration and a decline in the tissue C:N ratio, and added N significantly altered the shrub tissue δ15N natural abundance. Leaching of inorganic N to 1xa0m below the soil surface was on average 2–3 times higher in the added N plots, but large within treatment variability cause these differences to be statistically insignificant. Although a large fraction of the added N could not be accounted for in the shrub and soil N pools investigated, these observations suggest that dry season N inputs can significantly and rapidly alter N availability and shrub tissue chemistry in Mediterranean-type chaparral and CSS shrublands of southern California.

Photosynthetic parameters of two invasive tree species of the Brazilian Pantanal in response to seasonal flooding

The Pantanal is the largest wetland in the world with extremely high plant and animal diversity, but large areas have been invaded by Vochysia divergens Pohl (Vochysiaceae), a tree that is native to the Amazon Basin, and Curatella americana L. (Dilleniaceae), a tree that is native to the Brazilian savanna (cerrado). V. divergens is reportedly floodadapted, thus its ability to invade the Pantanal may not be surprising, but the invasion of C. americana is counterintuitive, because this species is adapted to the well-drained soils of the cerrado. Thus, we were interested in comparing the photosynthetic capacity, in terms of CO2 conductance, carboxylation, and electron transport of these species over a seasonal flooding cycle. Given that V. divergens is reportedly flood-adapted, we predicted that this species would have a higher photosynthetic capacity than C. americana, especially under flooding. To test this hypothesis we measured the photosynthetic CO2 response (PN/Cc) of V. divergens and C. americana within 1 year to determine, if photosynthetic capacity varied systematically over time and between species. Contrary to our hypothesis, V. divergens did not always have a higher photosynthetic capacity than C. americana. Rather, species differences were influenced by temporal variations in flooding and the leaf age. Leaf CO2 assimilation and photosynthetic capacity of both species were lower during the flood period, but the differences were not statistically significant. The physiological performance of both species was strongly related to leaf N and P concentrations, but P limitation appeared to be more important than N limitation for these species and ecosystem. Photosynthetic capacity was higher and more stable for V. divergens, but such an advantage did not result in a statistically significant increase in PN. Our results suggest that both species are tolerant to flooding even though they are adapted to very different hydrological conditions. Such physiological plasticity, especially for C. americana, might be a key feature for the ability to survive and persist in the seasonally flooded Pantanal.

Ground and remote sensing-based measurements of leaf area index in a transitional forest and seasonal flooded forest in Brazil

Leaf area index (LAI) is a key driver of forest productivity and evapotranspiration; however, it is a difficult and labor-intensive variable to measure, making its measurement impractical for large-scale and long-term studies of tropical forest structure and function. In contrast, satellite estimates of LAI have shown promise for large-scale and long-term studies, but their performance has been equivocal and the biases are not well known. We measured total, overstory, and understory LAI of an Amazon-savanna transitional forest (ASTF) over 3xa0years and a seasonal flooded forest (SFF) during 4xa0years using a light extinction method and two remote sensing methods (LAI MODIS product and the Landsat-METRIC method), with the objectives of (1) evaluating the performance of the remote sensing methods, and (2) understanding how total, overstory and understory LAI interact with micrometeorological variables. Total, overstory and understory LAI differed between both sites, with ASTF having higher LAI values than SFF, but neither site exhibited year-to-year variation in LAI despite large differences in meteorological variables. LAI values at the two sites have different patterns of correlation with micrometeorological variables. ASTF exhibited smaller seasonal variations in LAI than SFF. In contrast, SFF exhibited small changes in total LAI; however, dry season declines in overstory LAI were counteracted by understory increases in LAI. MODIS LAI correlated weakly to total LAI for SFF but not for ASTF, while METRIC LAI had no correlation to total LAI. However, MODIS LAI correlated strongly with overstory LAI for both sites, but had no correlation with understory LAI. Furthermore, LAI estimates based on canopy light extinction were correlated positively with seasonal variations in rainfall and soil water content and negatively with vapor pressure deficit and solar radiation; however, in some cases satellite-derived estimates of LAI exhibited no correlation with climate variables (METRIC LAI or MODIS LAI for ASTF). These data indicate that the satellite-derived estimates of LAI are insensitive to the understory variations in LAI that occur in many seasonal tropical forests and the micrometeorological variables that control seasonal variations in leaf phenology. While more ground-based measurements are needed to adequately quantify the performance of these satellite-based LAI products, our data indicate that their output must be interpreted with caution in seasonal tropical forests.

Variations in evapotranspiration and climate for an Amazonian semi-deciduous forest over seasonal, annual, and El Niño cycles

Tropical forests exchange large amounts of water and energy with the atmosphere and are important in controlling regional and global climate; however, climate and evaportranspiration (E) vary significantly across multiple time scales. To better understand temporal patterns in E and climate, we measured the energy balance and meteorology of a semi-deciduous forest in the rainforest-savanna ecotone of northern Mato Grosso, Brazil, over a 7-year period and analyzed regional climate patterns over a 16-year period. Spectral analysis revealed that E and local climate exhibited consistent cycles over annual, seasonal, and weekly time scales. Annual and seasonal cycles were also apparent in the regional monthly rainfall and humidity time series, and a cycle on the order of 3–5.5xa0years was also apparent in the regional air temperature time series, which is coincident with the average return interval of El Niño. Annual rates of E were significantly affected by the 2002 El Niño. Prior to this event, annual E was on average 1,011xa0mm/year and accounted for 52xa0% of the annual rainfall, while after, annual E was 931xa0mm/year and accounted for 42xa0% of the annual rainfall. Our data also suggest that E declined significantly over the 7-year study period while air temperature significantly increased, which was coincident with a long-term, regional warming and drying trend. These results suggest that drought and warming induced by El Niño and/or climate change cause declines in E for semi-deciduous forests of the southeast Amazon Basin.

Is the dry season an important driver of phenology and growth for two Brazilian savanna tree species with contrasting leaf habits

Brazilian savanna (known as cerrado) has highly seasonal variation in rainfall yet trees have widely different phenological strategies ranging from evergreen to fully deciduous. While qualitative patterns of canopy phenology are well-known, few studies have quantitatively measured schedules of leaf and branch phenology. We measured the leaf and vegetative phenology of two widely distributed cerrado tree species Vochysia divergens Pohl., an evergreen species, and Curatella americana L., a semi-deciduous species, over a 1-year period and hypothesized that the dry season would represent a trigger for leaf abscission and leaf and branch growth. Leaf and branch emergence and leaf abscission for the semi-deciduous species were coincident with the end and beginning of the dry season, respectively, and were significantly correlated with dry season meteorology, but with time lags that varied depending on the meteorological variable. Leaf and branch emergence and leaf abscission for the evergreen species were also coincident with the dry season, but correlations with meteorological variables were weaker and seasonal patterns were more subtle. V. divergens leaves also suffered more from herbivory than C. americana, and there is evidence that herbivory may have altered patterns of leaf emergence for V. divergens. V. divergens leaves survived longer than C. americana leaves, and relative branch growth rates were significantly higher for C. americana. While our study was limited to only two tree species and 1xa0year, we demonstrated quantitatively that patterns of leaf and branch phenology were highly correlated with climatic variations. A strategy of leaf emergence and branch growth initiation during the dry season likely maximizes carbon gain by increasing rates of C assimilation by plants at the onset of the rainy season.

Nutrient resorption in tropical savanna forests and woodlands of central Brazil

AbstractNutrientn limitation in Brazilian savanna (known as cerrado) presumably causes trees to maximize nutrient resorption from senesced leaves to reduce their dependence on nutrient availability. To assess patterns between nutrient resorption and soil fertility, we measured community-level nitrogen (N), phosphorus (P), and potassium (K) concentrations in mature and senesced leaves and soil fertility in the upper 50xa0cm soil layer in structurally diverse cerrado ecosystems in the Cuiaba Basin (CB) and Pantanal (PAN) of Mato Grosso, Brazil. Foliar nutrient concentration data were used to estimate resorption efficiency and proficiency, and correlation was used to determine whether resorption efficiency and proficiency varied across soil fertility gradients. We found that N and P resorption proficiency (NRP and PRP, respectively) and P resorption efficiency (PRE) increased significantly as total soil N (NRP) and extractable P (PRP and PRE) declined. In contrast, K resorption efficiency (KRE) declined as soil sand content and bulk density increased, which was likely due to a reduction in soil water-holding capacity. Leaf N/P ratios indicate potential N limitation and/or Nxa0+xa0P co-limitation for ecosystems in the PAN and P limitation and/or Nxa0+xa0P co-limitation for ecosystems in the CB, while trends in leaf N/K ratios indicate possible K or Kxa0+xa0P co-limitation for the CB only. Our results illustrate that cerrado forests and woodlands have highly variable nutrient resorption capacities that vary predictably across soil fertility or textural gradients and indicate that cerrado communities have flexible nutrient resorption that can reduce their dependence on soil nutrientnavailability.

Variations in aboveground vegetation structure along a nutrient availability gradient in the Brazilian pantanal

Background and aimsForest expansion into seasonally flooded (hyperseasonal) savanna of the Brazilian Pantanal has been occurring for decades. Our goal was to evaluate how ecosystem physiognomy varied across a nutrient availability gradient and if hyperseasonal savanna had adequate nutrient stocks to support forest expansion.MethodsWe quantified soil properties, aboveground ecosystem structure, and nutrient stocks of three savanna and three forest stands in the Pantanal of Mato Grosso, Brazil, and used correlation analysis to assess how aboveground vegetation structure varied across a soil nutrient availability gradient.ResultsWood and foliage carbon storage and leaf area index were positively correlated with soil extractable phosphorus (P), calcium (Ca2+), and magnesium (Mg2+) concentrations but not soil organic matter or texture. Soil profiles indicated that vegetation enriched surface P and K+ availability but not Ca2+ and Mg2+. Savanna ecosystems had adequate K+, Ca2+, and Mg2+ to support gallery and riparian forests but not palm forest, while the savanna P stock was inadequate to support forest expansion.ConclusionsHyperseasonal savanna has adequate nutrients (except P) to support forest expansion. Forest trees likely invade P-deficient savanna by surviving in P-rich microsites. Over time, biotic enrichment of soil may accelerate forest expansion into P-poor savanna.