Seasonal Degree Days Research Articles

Climatic controls on the survival and loss of ancient types of barley on North Atlantic Islands

For ancient types of barley at sites in the Scottish Isles, Faroes, and Iceland, we calculated minimum temperature requirements for grain production (grain production threshold, GPT) as accumulated degree days over the cropping season. Site suitability for barley from AD 1200 to 2000 was investigated by comparing these thresholds with reconstructions of annual cropping season degree days (CSDD) using temperature and tree-ring data. In Iceland, between AD 1200 and 1500, reconstructed CSDD were more favorable in the southwest (Reykjavik), with fewer years below the GPT, than in the North, East and West, but there were two periods (1340–1389 and 1426–1475) with low average CSDD and several years below the GPT which possibly influenced the abandonment of barley cultivation around this time. Reconstructed CSDD for the Faroes (Tórshavn) had only one year below the GPT, but 15 periods of four or more consecutive years with low CSDD which would have challenged barley cultivation, especially in the thirteenth century. Reconstructed CSDD were highest for the Scottish Isles, allowing a more prominent role of barley in the farming system and economy. Nevertheless, years with poor harvests or famines were common and about half were associated with low CSDD, resulting in a significant temperature link but also demonstrating the important contribution of other factors. Despite frequent unfavorable years in both the Faroes and Scottish Isles, resilient production systems, well-adapted barley strains and socio-economic factors allowed barley cultivation to continue, and some ancient types to survive to the present day.

Applicability of multiple remotely sensed vegetation indices for extracting key phenological metrics of Tamarix chinensis shrubs based on CO2 flux observation and Sentinel-2 data

We analyzed the relationship between gross primary productivity (GPP) and environmental factors at Sidaoqiao Superstation of the Ejina Oasis in China's Gobi Desert, by combining eddy flux and meteorological data from 2018 to 2019 and Sentinel-2 remote sensing images from 2017 to 2020. We evaluated the applicability of 12 remote sensing vegetation indices to simulate the growth of Tamarix chinensis and extract key phenological metrics. A seven-parameter double-logistic function (DL-7) + global model function (GMF) was used to fit the growth curves of GPP and vegetation indices. Three key phenological metrics, i.e., the start of the growing season (SOS), the peak of the growing season (POS), and the end of the growing season (EOS), were extracted for each year. Growing season degree days (GDD) and soil water content were the main environmental factors affecting the phenological dynamics of T. chinensis. Compared with 2018, the lower temperatures in 2019 resulted in slower accumulation rate of accumulated temperature before the SOS. T. chinensis required longer heat accumulation to enter growing season, which might cause later SOS in 2019. The hydrothermal conditions between SOS and POS were similar for 2018 and 2019. Howe-ver, the POS in 2019 was 8 days later than that in 2018, because of the late SOS in 2019. Following the POS in 2019, high GDD and low soil water content caused the T. chinensis to suffer from water stress, resulting in a shortened late growing season. The linear regression between the standardized Sentinel-2 vegetation index and the average value of GPP between 10:00 and 14:00 indicated that the enhanced vegetation index of the broadband vegetation index and the chlorophyll red edge index, inverted red edge chlorophyll index, and red-edge normalized difference vegetation index (NDVI705) of the narrowband vegetation index were highly consistent with the GPP of T. chinensis. Remote sensing extraction of SOS and POS of T. chinensis suggested that the Sentinel-2 narrowband vegetation index was more accurate than the broadband vegetation index. The modified chlorophyll absorption in reflectance index provided the most accurate extraction of SOS, while the MERIS terrestrial chlorophyll index provided the most accurate extraction of EOS. Conversely, the Sentinel-2 broadband vegetation index was the most accurate for extracting POS, especially the 2-band enhanced vegetation index and the near-infrared reflectance of vegetation. Overall, NDVI705 was the best index to estimate phenological metrics.

An Analysis of Problems and Trends Relating to the Development of Heat-Supply Systems for Russian Cities

Problems associated with the development of heat-supply systems for large- and medium-size cities in Russia are analyzed. To this end, the approved heat-supply schemes of almost forty cities are reviewed and briefly analyzed taking into account their key territorial features. The main features of heat-supply systems for cities are analyzed, including the state of heat networks, excess capacities of sources, parameters of combined heat and power plants operating in the forced generation mode, and also the possibilities of using secondary and renewable energy sources for heat generation. The specific indicators characterizing heat consumption are calculated with taking into account, among other factors, the heating season degree days. The dependences of specific fuel consumption and energy generation efficiency on the climatic parameters, population density, and other factors are considered. For estimating the potential efficiency improvement reserves, consolidated data on the total installed capacity and heat-supply modes in cities are given. The aim of the study was to reveal the key problems and main trends associated with the development of heat-supply systems for cities. Large- and medium-size cities of Russia were distributed into a few typological groups, and their analysis has shown similarity of the problems occurring in them along with possible sets of typical measures and methods for solving them. The effectiveness of heat-supply systems as a whole for some cities depending on the population density and number and in different climatic zones is estimated. It is found that the obtained indicators characterizing the resulting efficiency of heat-supply systems are not directly linked with the fraction of industrial load, population density, and climate. In elaborating the heat-supply schemes, it is proposed to use the energy and environmental effectiveness indicators for revealing the key reserves for sustainable development of the most important life-support infrastructures of cities in Russia’s different climatic zones.

Seasonal and Interannual Variations of CO2 Fluxes Over 10 Years in an Alpine Wetland on the Qinghai‐Tibetan Plateau

AbstractAlpine wetlands play a sensitive function in global carbon cycle during the ongoing climate warming, yet the temporal patterns of carbon dynamics from in situ ground‐based long‐term observations remain unclear. Here, we analyzed the continuous net ecosystem CO2 exchange (NEE) measured with the eddy covariance technique over an alpine peatland on the northeastern Qinghai‐Tibetan Plateau from 2007 to 2016. The wetland acted as a net CO2 source with a positive NEE (120.4 ± 34.8 gC m−2 year−1, Mean ± SD), with the mean annual gross primary productivity (GPP) of 500.3 ± 59.4 gC m−2 year−1 and annual ecosystem respiration (RES) of 620.7 ± 74.2 gC m−2 year−1. At the seasonal scale, the classification and regression trees (CART) analysis showed that aggregated growing season degree days (GDDs) were the predominant determinant on variations in monthly NEE and monthly GPP. Variations in monthly RES were determined by soil temperature (Ts). Furthermore, nongrowing season Ts had a significant positive correlation with the following year annual GPP (p < 0.05). Nongrowing season RES only accounted for about 25% of annual RES but had significant correlation with annual RES and annual NEE (p < 0.05). The further partial correlation analysis showed that nongrowing season air temperature (Ta, p = 0.05), rather than precipitation (PPT, p = 0.25) was a predominant determinant on variations in annual NEE. Our results highlighted the importance in carbon dynamics of climate fluctuations and CO2 emission from the nongrowing season in alpine wetlands. We speculated that the vast peadlands would positively feedback to climate change on the Tibetan plateau where the nongrowing season warming was significant.

Planting Date Effects on Cotton Lint Yield and Fiber Quality in the U.S. Southern High Plains

Cotton planting date effects in the U.S. Southern High Plains (SHP) were evaluated based on 11 years of May-planted and June-planted irrigated variety trials. Multiple cultivars planted in each year’s trial allowed for the calculation of 153 yield effects and 162 effects in 5 fiber quality parameters. Yield and quality effects were considered in the context of related changes in total growing season degree days (GDDS) and total cool hours (CHRS) during a boll formation period 80 to 110 days after planting. May planting increased GDDS and significantly increased yields in 8 of 10 years that comparisons could be made. Micronaire and fiber elongation were the most sensitive quality parameters to planting date. June planting resulted in increased CHRS every year and a significantly higher incidence of low micronaire in 7 of 11 years. In 7 of 11 years May planting significantly reduced fiber elongation relative to June planting. Analysis of SHP temperature data show that late-April to early-May planting dates may increase yield and micronaire by maximizing GDDS and minimizing CHRS. Although this practice may be optimal to the SHP environment it may also require high-vigor seed and pre-planting irrigation. Adapting genetics to an early planting strategy might include selecting for improved seed vigor and cold germination with acceptable yield and fiber quality traits.

Estimating lake–climate responses from sparse data: An application to high elevation lakes

AbstractAlthough many studies demonstrate lake warming, few document trends from lakes with sparse data. Diel and seasonal variability of surface temperatures limit conventional trend analyses to datasets with frequent repeated observations. Thus, remote lakes, including many high elevation lakes, are underrepresented in trend analyses. We used a Bayesian technique to analyze sparse data that explicitly incorporated diel and seasonal variability. This approach allowed us to estimate lake warming in a region of limited knowledge: high elevation lakes (> 2100 m ASL) of the Southern Rocky Mountains, U.S.A. The analysis allowed for inclusion of lakes with few repeated measurements, and observations made before 1980 when more intensive lake monitoring began. We accumulated the largest dataset of high elevation lake temperatures analyzed to date. Data from 590 high elevation lakes in the Southern Rocky Mountains showed a 0.13°C decade−1 increase in surface temperatures and a 14% increase in seasonal degree days since 1955. This result is lower than other regional and global estimates of lake warming; however, it is similar to other high elevation lake studies. Our approach can be applied to other understudied regions, increasing our overall understanding of the effects of climate change on lakes and their temporal dynamics.

Growth stage-dependant variability in water vapor and CO2 exchanges over a humid alpine shrubland on the northeastern Qinghai-Tibetan Plateau

Large uncertainties exist in carbon-water-climate feedbacks in cold regions, partly due to an insufficient understanding of the simultaneous effects of climatic and biotic controls on water and carbon dynamics. The 10-year growing season flux data were analyzed to evaluate the relative contributions of climatic and biotic effects on the variability of water vapor (ET) and net ecosystem CO2 (NEE) exchanges over a humid alpine deciduous shrubland on the northeastern Qinghai-Tibetan Plateau. The results showed that the alpine shrubland ecosystem acted as a water source and a carbon sink during the growing season, and its potential ET and NEE ranged from 161.4 mm and –41.0 g C·m−2 to 408.0 mm and –278.4 g C·m−2 at a 95% confidence interval, respectively. The average 8-day ET and NEE during the early growing season (June to July) were both significantly (P < 0.05) more than those of the late growing season (August to September). And the slopes of ET and NEE against the Julian day during the two growth stages also changed significantly (P < 0.01). Such asymmetric manners of ET and NEE during the two growth stages were probably related to the seasonal variations of net radiation (Rn) and vegetation growth (satellite-derived enhanced vegetation index: EVI), respectively. The structural equation models showed that the seasonal variations of 8-day ET were jointly determined by Rn and vapor pressure deficit (VPD), as partly indicated by a modest decoupling coefficient (0.54 ± 0.03). The seasonal variability in 8-day NEE was controlled by the combinations of EVI and growing season degree days (GDD). The standardized coefficient of the direct effect of EVI on ET was 0.16, much less than the corresponding value (0.51) on NEE, suggesting that a weak coupling between ET and NEE arose likely because water vapor loss were about half controlled by surface evaporation, whereas CO2 flux were largely regulated by vascular plant activity. Our results highlighted the asymmetric sensitivities of ET and NEE during the early and the late growing season, and the weak coupling of water loss and carbon fixation during the whole growing season. These findings would provide a new sight to understand the growth stage-dependent responses of water budget and carbon sequestration to grazing management and climate change in humid alpine shrublands.

Recent warming across the North Atlantic region may be contributing to an expansion in barley cultivation

Although grass dominates most agricultural systems in the North Atlantic region (NAR), spring barley is the most important cereal and is used for animal feed and food and drink products. Recent changes in climate have resulted in warmer conditions across the NAR which have major implications for crop production. In this paper, we investigate the thermal requirement of spring barley in the region and use the results to examine the effects of recent trends in temperature and rainfall on barley cultivation, based on 11 regional meteorological sites. At these sites, between 1975 and 2015, we found significant warming trends for several months of the cropping season and significant trends for increases in the cropping season degree days (CSDD). In recent years, this has resulted in an increased proportion of years when the estimated minimum thermal requirement for barley has been met at sites above about 60°N. However, annual variations in CSDD are large and years still occur at these sites where this is insufficient. While warming could potentially allow an earlier start and later end to the cropping season, it is likely that high rainfall at maritime sites, and low rainfall at continental sites, will limit the ability of growers to benefit from this. Warming is considered to have been one of the main factors contributing to the large expansion of the area of barley cultivated in Iceland since the 1990s.

Analyzing Temperature and Precipitation Influences on Yield Distributions of Canola and Spring Wheat in Saskatchewan

AbstractThe IPCC indicates that global mean temperature increases of 2°C or more above preindustrial levels negatively affect such crops as wheat. Canadian climate model projections show warmer temperatures and variable rainfall will likely affect Saskatchewan’s canola and spring wheat production. Drier weather will have the greatest impact. The major climate change challenges will be summer water availability, greater drought frequencies, and crop adaptation. This study investigates the impact of precipitation and temperature changes on canola and spring wheat yield distributions using Environment Canada weather data and Statistics Canada crop yield and planted area for 20 crop districts over the 1987–2010 period. The moment-based methods (full- and partial-moment-based approaches) are employed to characterize and estimate asymmetric relationships between climate variables and the higher-order moments of crop yields. A stochastic production function and the focus on crop yield’s elasticity imply choosing the natural logarithm function as the mean function transformation prior to higher-moment function estimation. Results show that average crop yields are positively associated with the growing season degree-days and pregrowing season precipitation, while they are negatively affected by extremely high temperatures in the growing season. The climate measures have asymmetric effects on the higher moments of crop yield distribution along with stronger effects of changing temperatures than precipitation on yield distribution. Higher temperatures tend to decrease wheat yields, confirming earlier Saskatchewan studies. This study finds pregrowing season precipitation and precipitation in the early plant growth stages particularly relevant in providing opportunities to develop new crop varieties and agronomic practices to mitigate climate changes.

Identification of preharvest factors determining postharvest ripening behaviors in ‘Hass’ avocado under long term storage

A major challenge for the global avocado industry is to provide a homogenous product in terms of fruit-ripening behavior, especially considering the significant variability in quality that can be found within a box or pallet of the fruit. The broad range of conditions under which trees are grown, particularly with regard to climate, soil and agronomical management, can influence this ripening variability. The aims of this study were (i) to determine the variability in fruit ripening among ‘Hass’ avocado grown under different conditions in Chile and (ii) to understand the postharvest fruit-ripening behavior of ‘Hass’ avocado due to the combined effect of several preharvest variables. Preharvest variables were evaluated at 42 experimental sites in Chile during three consecutive seasons. In addition, avocados with over 21% dry matter were collected at each site during each season and stored for 35d at 5°C under normal atmospheric conditions before being ripened at 20°C. Indicators of ripening behavior, such as the softening rate (SOFTRATE), change in peel color (COLO35) and days at 20°C necessary to reach the ready-to-eat stage (RTE35), were evaluated. As expected, high fruit variability in terms of ripening behavior was observed among the experimental sites and seasons. Multivariate analysis showed that the seasonal mean minimum air temperature, seasonal degree-days, trunk diameter and fruit firmness at harvest had a proportional relationship with postharvest SOFTRATE and COLOR35 during storage and a significant inverse relationship with RTE35. Conversely, the leaf area index, number of plants per hectare, and irrigation management at the bloom stage had a proportional relationship with RTE35 and an inverse relationship with SOFTRATE and COLOR35. Moreover, all of the three postharvest ripening behavior indicators were significantly (p<0.05) estimated by predictive models considering preharvest variables. Therefore, attempting to predict postharvest behavior by considering only a single preharvest variable could be a misleading simplification of reality because several factors, including climate/environmental, agronomic management and physiological variables, influence the ripening behavior of ‘Hass’ avocado fruit.

Seasonal and inter-annual variations in CO2 fluxes over 10 years in an alpine shrubland on the Qinghai-Tibetan Plateau, China

Alpine ecosystems play an important role in the global carbon cycle, yet the long-term response of in situ ground-based observations of carbon fluxes to climate change remains not fully understood. Here, we analyzed the continuous net ecosystem CO2 exchange (NEE) measured with the eddy covariance technique over an alpine Potentilla fruticosa shrubland on the northeastern Qinghai-Tibetan Plateau from 2003 to 2012. The shrubland acted as a net CO2 sink with a negative NEE (−74.4±12.7gCm−2year−1, Mean±S.E.). The mean annual gross primary productivity (GPP) and annual ecosystem respiration (RES) were 511.8±11.3 and 437.4±17.8gCm−2year−1, respectively. The classification and regression trees (CART) analysis showed that aggregated growing season degree days (GDD) was the predominant determinant on variations in monthly NEE and monthly GPP, including its effect on leaf area index (LAI, satellite-retrieved data). However, variations in monthly RES were determined much more strongly by LAI. Non-growing season soil temperature (Ts) and growing season length (GSL) accounted for 59% and 42% of variations in annual GPP and annual NEE, respectively. Growing season soil water content (SWC) exerted a positive linear influence on variations in annual RES (r2=0.40, p=0.03). The thermal conditions and soil water status during the onset of the growing season are crucial for inter-annual variations of carbon fluxes. Our results suggested that an extended growing season and warmer non-growing season would enhance carbon assimilation capacity in the alpine shrubland.

Spatial and seasonal variations of CO2 flux and photosynthetic and respiratory parameters of larch forests in East Asia

Larch (Larix spp.) forests are predominantly distributed across high latitudes of Eurasia. They potentially have a strong influence on the terrestrial carbon and energy cycles, because of their vast area and the large carbon stocks in their peat soils in the permafrost. In this study, we elucidated intersite variation of ecosystem photosynthetic and respiratory parameters of eight larch forests in East Asia using the CarboEastAsia carbon flux and micrometeorology dataset. These parameters were determined using the empirical relationship between the carbon fluxes (photosynthesis and respiration) and micrometeorological variables (light and temperature). In addition, we examined leaf area index (LAI) determined by Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing data to explain the intersite variation. Linear or exponential relationships with annual mean temperature or seasonal maximum LAI at the study sites were found for the annual carbon fluxes (gross primary production [GPP] and total ecosystem respiration [RE]) as well as for four of the five seasonal maximum values of determined photosynthetic and respiratory parameters (maximum GPP at light saturation, initial slope of the light-response curve, daytime respiration, and RE at the reference temperature of 10°C). Phenological indices, such as start day of the growing season, growing season length and growing season degree days explained much of the intersite variation of GPP and RE of the studied larch forests; however, the relationship between MODIS LAI and photosynthetic or respiratory parameters implies that the intersite variation in GPP and RE was caused not only by the temperature variation (abiotic factor), but also by the variation in the photosynthetic and respiration activity by vegetation (biotic factor) through the change in leaf (or whole vegetation) biomass. Our analysis shows that MODIS LAI serves as a good index to explain the variation of the ecosystem photosynthetic and respiratory characteristics of East Asian larch forests.

Forest production dynamics along a wood density spectrum in eastern US forests

Key message Emerging plant economics spectrum theories were confirmed across temperate forest sys- tems of the eastern US where the use of a forest stand's mean wood density elucidated forest volume and bio- mass production dynamics integrating aspects of cli- mate, tree mortality/growth, and rates of site occupancy. Abstract As a tree's functional trait of wood density has been used to refine models of tree competition, it may also aid in evaluating hypotheses of forest production such as declining growth and mortality across a spectrum of increasing wood density. The goal of this study was to examine trends in aboveground live tree production as related to mean wood density using a region-wide repeated forest inventory across eastern US forests. Using quantile regression, the 90th percentile of volume and biomass accretion was negatively related to the mean wood density of a stand's constituent tree species. This relationship was strongest on forest sites with the highest number of growing season degree days, as growing season length influences the rates of stand development. For these sites, variations in volume and biomass accretion were most pronounced in stands with low mean tree wood density, which also demonstrated the highest rates of site occu- pancy and mortality. This study confirmed aspects of the emerging theory of ''fast-slow'' plant economics spec- trums across temperate forest ecosystems. Stands with relatively low wood density appear to occupy sites more rapidly leading to a concomitantly higher rate of tree mortality, but with less biomass accretion relative to vol- ume due to allocating biomass or carbon to a greater tree volume. In contrast, stands with higher wood density exhibited slower site occupancy due to high wood density construction costs, but with increased biomass relative to volume accretion. These findings highlight the potential application of the plant economics spectrum theory in refining our understanding of general patterns of forest stand production, the role of plant traits in forest man- agement, and knowledge gaps such a shifts in tree allom- etry during stand development.

Health Status of Alfalfa Leafcutting Bee Larvae (Hymenoptera: Megachilidae) in United States Alfalfa Seed Fields

We conducted a broad geographic survey in the northwestern United States to quantify production losses in the alfalfa leafcutting bee (Megachile rotundata (F.), Hymenoptera: Megachilidae), a solitary pollinator used extensively in alfalfa seed production. Viable larvae were found in only 47.1% of the nest cells collected at the end of the season. Most of the rest of the cells contained pollen balls (typified by a provision but no larva; 16.7%), unknown causes of mortality (15.5%), or larvae killed by chalkbrood (8.0%). Prevalence of pollen balls was correlated positively with bee release rates and negatively with alfalfa stand age. The unknown mortality was correlated with the U.S. Department of Agriculture-Plant Hardiness Zone, and thus, some of the mortality may be caused by high temperature extremes, although the nesting season degree-days were not correlated with this mortality. Chalkbrood prevalence was correlated with possible nesting-resource or crowding-related factors, such as the number of bees released per hectare and the number of shelters used, but not with nesting board disinfection practices. Vapona is used to control parasitoids when the parent bees are incubated before release, and use of this fumigant was associated with an increase in both chalkbrood and diapausing offspring, although any reason for these correlations are unknown. This survey quantifies the variation in the quality of alfalfa leafcutting bee cocoons produced across much of the U.S. alfalfa seed production area.

Fruit ripening in Vitis vinifera: light intensity before and not during ripening determines the concentration of 2‐methoxy‐3‐isobutylpyrazine in Cabernet Sauvignon berries

The roles of light and temperature in the accumulation of the vegetal impact compound 2-methoxy-3-isobutylpyrazine (MIBP) in grape (Vitis vinifera L.) berries were determined. Individual clusters were exposed to various light intensities using neutral density shade cloth before ripening, during ripening or throughout the season in three growing seasons. A recently developed method using headspace solid-phase microextraction combined with GC-MS in the selected ion-monitoring mode was employed to measure MIBP in berries. Berry MIBP concentration increased subsequent to berry set, reached a maximum prior to onset of ripening, and then decreased thereafter until harvest. Complete shading of clusters increased the concentration of MIBP more than 100% compared to unshaded controls in 2 out of 3 years. Light increasingly inhibited MIBP concentrations up to 25-50% of ambient light intensities (1500 µmol photons m(-2) s(-1) ). However, only changes in light intensity before ripening had any effect on MIBP accumulation or final MIBP concentration. Analyses of weather data showed that the 1 year in which shading was ineffective was unusually warm, warm early in the season, and had more hot days and higher early season degree days than the other 2 years. In controlled environment experiments, warm growth conditions reduced MIBP concentrations in fruit about as much as light exposure reduced MIBP concentrations in the field experiments. The results indicate that both light and temperature significantly affect MIBP in harvested fruit, but that the light environment during ripening does not significantly affect MIBP concentrations in the berries at harvest.

Carbon and Nitrogen Accumulation in Post‐Agricultural Forest Soils of Western New England

The agricultural history of New England is reflected in the species composition, biomass, and productivity of the second‐growth forests of the region. There are also effects of agricultural use on soil morphology, soil organic C (SOC) and N content, erosion status, and C stability, which can be specific to site‐level characteristics or prior agricultural use. We conducted a survey of SOC amounts in chronosequences comprising 25 post‐agricultural second‐growth forests. Time since abandonment was estimated from tree ages, and the chronosequences were bounded by sites currently in agricultural use (n = 6) and minimally disturbed old forest (n = 4). At sites currently used for pasture, hay, or row crops, the median amount of SOC (to a depth of 55 cm) was 6.5 kg C m−2 compared with 10.3 kg C m−2 in the soils of the oldest forest stands. The chronosequences indicated that nearly all of the SOC accumulation occurred in the first century of afforestation. There were significant trends of SOC accumulation in the forest floor, upper mineral horizons (0–10 or 10–20 cm), and B horizons (20–55 cm) of soils that had been used for pasture, hay, or row crops but not for former woodlots. Multivariate analysis showed that years since agricultural abandonment and growing season degree days were the most important variables related to SOC content (68% sum of squares explained). There was a significant trend of N accumulation (1.3 g N m−2 yr−1) in soils (to a depth of 55 cm) formerly used for cultivated crops but not for other former uses. Farms in this region were an important C sink during the last 150 yr; however, the capacity of former agricultural soils to gain SOC in the future is probably limited.

Controls on the Spatial Patterns of Carbon and Nitrogen in Adirondack Forest Soils along a Gradient of Nitrogen Deposition

We sampled 42 permanent vegetation plots in the Adirondack Mountains, NY to characterize the spatial patterns of, and controls on soil C and N contents. Average C (24 kg m−2) and N (1.1 kg m−2) contents of the combined organic and mineral horizons were high for northeastern U.S. forests. Contrary to our expectations, whole‐profile C and N amounts were not different among northern hardwood (NH), spruce‐fir (SF), and pine‐dominated (PW) plots. Most of the C and N were stored in organic horizons in the high‐elevation SF plots, and in the mineral horizons in NH and PW plots. Regression analyses of the pooled set of sites revealed that the factors that explained the most variability in soil C and N contents were different for organic and mineral horizons and differed between forest types. Overall, growing season degree‐days (GSDD) was the variable most closely correlated with C and N contents in both organic and mineral horizons, and varying combinations of N deposition, conifer importance, and soil texture were the principal secondary influences. Spruce‐fir plots received the most atmospheric N deposition, and multivariate regression tree (MRT) analysis indicated that N deposition rate was the environmental variable that explained the most variation in organic horizon C (sums of squares [SS] explained = 54%) and N (79% SS explained) amounts in these high‐elevation stands. However, even in the unlikely case of 100% retention of the atmospheric inputs of N deposited over the past 50 yr, this source only accounted for a small portion of soil N, and the differences in N among the plots along the deposition gradient. It is likely that differences in GSDD accounted for most of the differences in C and N amounts.

Nitrogen availability and forest productivity along a climosequence on Whiteface Mountain, New York

We studied broadleaf and needle-leaf forests along an elevation gradient (600–1200 m) at Whiteface Mountain, New York, to determine relationships among temperature, mineral N availability, and aboveground net primary productivity (ANPP) and controls on the latter two variables. We measured net N mineralization during the growing season, annual litterfall quantity and quality, aboveground woody biomass accumulation, and soil organic matter quality. Inorganic N deposition from cloudwater markedly increases mineral N availability above 1000 m in this region. Consequently, mineral N availability across the climosequence remains relatively constant because N mineralization decreases with increasing elevation. Across this climosequence, air temperature (as growing season degree-days) exerted the most control on ANPP. Nitrogen mineralization was most strongly related to soil growing season degree-days and less so to lignin to N ratios in litter. ANPP was correlated with N mineralization but not with mineral N availability. Combining our data with those from similar studies in other boreal and cool temperate forests shows that N mineralization and ANPP are correlated at local, regional, and interbiome scales. Regarding the persistent question concerning cause and effect in the N mineralization – forest productivity relationship, our data provide evidence that at least in this case, forest productivity is a control on N mineralization.

Snow Tussocks, Chaos, and the Evolution of Mast Seeding

One hitherto intractable problem in studying mast seeding (synchronous intermittent heavy flowering by a population of perennial plants) is determining the relative roles of weather, plant reserves, and evolutionary selective pressures such as predator satiation. We parameterize a mechanistic resource-based model for mast seeding in Chionochloa pallens (Poaceae) using a long-term individually structured data set. Each plant's energy reserves were reconstructed using annual inputs (growing degree days), outputs (flowering), and a novel regression technique. This allowed the estimation of the parameters that control internal plant resource dynamics, and thereby allowed different models for masting to be tested against each other. Models based only on plant size, season degree days, and/or climatic cues (warm January temperatures) fail to reproduce the pattern of autocovariation in individual flowering and the high levels of flowering synchrony seen in the field. This shows that resource-matching or simple cue-based models cannot account for this example of mast seeding. In contrast, the resource-based model pulsed by a simple climate cue accurately describes both individual-level and population-level aspects of the data. The fitted resource-based model, in the absence of environmental forcing, has chaotic (but often statistically periodic) dynamics. Environmental forcing synchronizes individual reproduction, and the models predict highly variable seed production in close agreement with the data. An evolutionary model shows that the chaotic internal resource dynamics, as predicted by the fitted model, is selectively advantageous provided that adult mortality is low and seeds survive for more than 1 yr, both of which are true for C. pallens. Highly variable masting and chaotic dynamics appear to be advantageous in this case because they reduce seed losses to specialist seed predators, while balancing the costs of missed reproductive events.

Snow Tussocks, Chaos, and the Evolution of Mast Seeding

One hitherto intractable problem in studying mast seeding (synchronous intermittent heavy flowering by a population of perennial plants) is determining the relative roles of weather, plant reserves, and evolutionary selective pressures such as predator satiation. We parameterize a mechanistic resource‐based model for mast seeding in Chionochloa pallens (Poaceae) using a long‐term individually structured data set. Each plant’s energy reserves were reconstructed using annual inputs (growing degree days), outputs (flowering), and a novel regression technique. This allowed the estimation of the parameters that control internal plant resource dynamics, and thereby allowed different models for masting to be tested against each other. Models based only on plant size, season degree days, and/or climatic cues (warm January temperatures) fail to reproduce the pattern of autocovariation in individual flowering and the high levels of flowering synchrony seen in the field. This shows that resource‐matching or simple cue‐based models cannot account for this example of mast seeding. In contrast, the resource‐based model pulsed by a simple climate cue accurately describes both individual‐level and population‐level aspects of the data. The fitted resource‐based model, in the absence of environmental forcing, has chaotic (but often statistically periodic) dynamics. Environmental forcing synchronizes individual reproduction, and the models predict highly variable seed production in close agreement with the data. An evolutionary model shows that the chaotic internal resource dynamics, as predicted by the fitted model, is selectively advantageous provided that adult mortality is low and seeds survive for more than 1 yr, both of which are true for C. pallens. Highly variable masting and chaotic dynamics appear to be advantageous in this case because they reduce seed losses to specialist seed predators, while balancing the costs of missed reproductive events.