Previous work demonstrates conflicting evidence regarding the influence of snowmelt timing on forest net ecosystem exchange (NEE). Based on 15years of eddy covariance measurements in Colorado, years with earlier snowmelt exhibited less net carbon uptake during the snow ablation period, which is a period of high potential for productivity. Earlier snowmelt aligned with colder periods of the seasonal air temperature cycle relative to later snowmelt. We found that the colder ablation-period air temperatures during these early snowmelt years lead to reduced rates of daily NEE. Hence, earlier snowmelt associated with climate warming, counterintuitively, leads to colder atmospheric temperatures during the snow ablation period and concomitantly reduced rates of net carbon uptake. Using a multilinear-regression (R-2=0.79, P<0.001) relating snow ablation period mean air temperature and peak snow water equivalent (SWE) to ablation-period NEE, we predict that earlier snowmelt and decreased SWE may cause a 45% reduction in midcentury ablation-period net carbon uptake.

Changes in the productivity of temperate forests have important implications for atmospheric carbon dioxide (CO2) concentrations, and many efforts have focused on methods to monitor both gross and net primary productivity in temperate forests. Remotely sensed vegetation indices provide spatially extensive measures of vegetation activity, and the Enhanced Vegetation Index (EVI) has been widely linked to photosynthetic activity of vegetation. Networks of tree ring width (TRW) chronologies provide ground-based estimates of annual net carbon (C) uptake in forests, and time series of EVI and TRW may capture common productivity signals. Robust correlations between mean TRW and EVI may enhance spatial extrapolations of TRW-based productivity estimates, ultimately improving understanding of spatio-temporal variability in forest productivity. The research presented in this thesis investigates potential empirical relationships between networks of TRW chronologies and time series of Landsat EVI and Landsat-based phenological metrics in the Northeast United States. We hypothesized that mean TRW is positively correlated with mean monthly EVI during the growing season, EVI integrated over the growing season, and growing season length. Results indicate that correlations between TRW and EVI are largely not significant in this region. The complex response of tree growth to a variety of limiting climatic factors in temperate forests may decouple measures of TRW growth and canopy reflectance. However, results also indicate that there may be important lag effects in which EVI affects mean TRW during the following year. These findings may improve understanding of links between C uptake and growth of tree stems over large spatial scales.

The encroachment of rangelands by woody plants causes an imbalance in the grass:bush ratio leading to decline in ecosystem services including grazing capacity, biodiversity, and water yield. This study assessed the historical changes in woody cover, and the effects of bush clearing on water loss, atmospheric carbon uptake, herbaceous vegetation, and soil seed bank (SSB) dynamics. The study was conducted at Makapanstad (Radi and Maseding) and Kgomokgomo rangelands, North-West Province, South Africa. Landsat imagery was used to assess woody cover over 34 years (1984 - 2018) in a total area of 16 397 ha. Evapotranspiration (ET) and gross primary productivity (GPP) were assessed in cleared sites and their adjacent uncleared sites from 2013 to 2018. Woody plant densities, herbaceous composition, ground cover and biomass production were assessed in 24, 5×5-m plots distributed equally in three 2500-m2 bush-encroached blocks in each rangeland in February 2016. Three samples for soil nutrient analysis were collected 20-cm deep in each plot. Thereafter, half (1250-m2) of each block was mechanically cleared to make three replicates of cleared and uncleared microsites. Post-treatment survey of vegetation was conducted in February 2017. SSB was assessed for samples collected in April 2016 before bush clearing and in August, October and December 2016, and April and August 2017 in paired microsites. In 1984, herbaceous cover (humid + arid grasslands) dominated, accounting for 36% of the total area followed by woody cover (shrublands + bushlands) and unvegetated cover (bare soils + abandoned croplands), with respective proportions of 33 and 31%. There were substantial land cover changes over time, with shrub cover increasing linearly (r2 = 0.94, p < 0.05) at a rate of 0.26% year-1, increasing the total woody cover (TWC) to 38% in 2018. Cropland abandonment was the main driver of increase in shrub cover in the study area. Herbaceous cover declined with increase in TWC (r2 = 0.69), whereas bare soil cover increased (r2 = 0.70) with TWC over time. The main encroachers were Vachellia tenuispina at Radi, Vachellia tortilis at Maseding and Dichrostachys cinerea and V. tortilis at Kgomokgomo. Most of the woody species correlated with total N and clay content (r = 0.96; p < 0.05) in black vertic clay soils of Radi and Maseding, whereas non-leguminous species correlated with total N and C:N in the red-yellow apedal soils of Kgomokgomo. ET and GPP varied interannually in all rangelands and both declined significantly (p < 0.05) after bush clearing at Radi and Kgomokgomo but not at Maseding. ET increased with GPP in cleared (r2 = 0.50-0.59) and uncleared sites (r2 = 0.82-0.93) at Radi and they showed a strong relationship (r2 >0.70) in both sites at Maseding and Kgomokgomo. ET ranged from 0.26-0.46 and 0.48-0.97 Kg H2O m-2 day-1 in cleared and uncleared sites, respectively, at Radi and from 0.44-0.98 and 0.63-1.24 Kg H2O m-2 day-1 at Kgomokgomo. Total SSB densities (constituted mostly by early successional species) increased significantly (p < 0.05) to 2470, 1872 and 693 seeds m-2 at Radi, Maseding and Kgomokgomo, respectively, in cleared microsites in August 2017. Similarly, biomass production (BP) and basal cover (BC) were highest in cleared relative to uncleared microsites. Overall, increase in woody cover reduced herbaceous cover and this calls for bush control in the studied rangelands. The decline in ET after bush clearing suggested that bush clearing could be used to increase rangeland water yield. Increased SSB densities, BP and BC indicated that bush clearing facilitates passive restoration. / Thesis (PhD (Pasture Science))--University of Pretoria, 2020. / Agricultural Research Council / National Research Foundation / Plant Production and Soil Science / PhD (Pasture Science) / Unrestricted

Das Ziel der vorliegenden Arbeit war es, zu untersuchen, ob sich mit der Düngung von Stroh, Gehölzhäcksel, frischem Pferdemist und Grüngutkompost die Ertragsleistung und Nährstoffaufnahme von Leguminosen sowie der Folgefrucht Winterweizen steigern lässt. Darüber hinaus sollte die Aufnahme von düngebürtigem Kohlenstoff durch Rotklee mittels 13-C-angereichertem Stroh quantifiziert werden. Die Verfügbarkeit von Kalium, teilweise auch von Schwefel und Molybdän, wurde vor allem nach der Düngung von Grüngutkompost und von Pferdemist für die untersuchten Leguminosen verbessert, während nach der Düngung von Gehölzhäcksel und Stroh keine signifikanten Effekte auf die Nährstoffaufnahme der Pflanzen festgestellt werden konnten. Die scheinbare Ausnutzung des gedüngten Stickstoffs durch die Leguminosen lag in Abhängigkeit von der Düngung zwischen 0 und 9 % (Gehölzhäcksel < Grüngutkompost < Pferdemist < Stroh). Nur nach der Düngung von Pferdemist zu Saatplatterbse und Ackerbohne verringerte sich der Anteil an symbiotisch fixiertem Luftstickstoff am Spross-N gegenüber der Kontrolle, nicht jedoch die symbiotisch fixierte N-Menge. Der Vorrat an mineralischem Stickstoff im Boden in einer Tiefe von 0 bis 30 cm war unter Ackerbohne und Rotklee ca. 40 Tage nach der Düngung und Einarbeitung von Gehölzhäcksel gegenüber den Vergleichsvarianten und der Kontrolle deutlich vermindert. Ein Einfluss der durch die Düngung bedingten temporären N-Festlegung im Boden auf den Anteil an fixiertem Luftstickstoff am Spross-N konnte bei keiner der untersuchten Leguminosen festgestellt werden. Der nachfolgende Winterweizen reagierte in einem Jahr mit signifikant höherem Kornertrag auf die Düngung von Grüngutkompost zu Ackerbohne und erhöhtem Rohproteingehalt im Korn auf die Düngung von Pferdemist zu Saatplatterbse. Mit Hilfe von 13-C-markiertem Stroh wurde im Freiland eine Assimilation von 0,5 % der mit dem Stroh gedüngten Kohlenstoffmenge durch Rotklee ermittelt. / The aim of this study was to examine, whether the application of wheat straw, pruning, horse manure and green compost improves the yield formation and nutrient uptake of legumes and the succeeding crop winter wheat. Furthermore it should be quantified the amount of carbon in red clover derived from the organic fertilizer by carbon dioxid assimilation. An improved uptake of potassium, partially of sulphur and molybdenum for the legumes could be detected after the application of green compost and horse manure. No effects were found on plant nutrient uptake after the application of straw and pruning. The apparent utilization of the applied nitrogen by organic fertilizer was 0 to 9 %, depending on the fertilization (pruning < green compost < horse manure < straw). The proportion of symbiontically fixed nitrogen decrease in faba bean and in chick-pea after manuring with horse manure compared with the control. However the total amount of symbiotically fixed nitrogen did not decreased. The amount of mineral nitrogen in the soil (depth: 0 to 0,3 m) was significant reduced under faba bean and red clover, 40 days after application and incorporation of pruning. No effect on the symbiotic nitrogen fixation of the legumes was found because of the organic fertilizer induced temporary immobilization of nitrogen in this soil layer. An increased grain yield was determined in the succeeding winter wheat after fertilizing green compost to faba bean and a higher content of crude protein in grain of the wheat was determined after fertilizing horse manure to chick-pea. With the help of a 13C-tracer method it could be estimated, that about 0,5 % of the added carbon with straw was assimilated by red clover under field conditions.