Showing papers by "Nicole Estrella published in 2009"

Influence of altitude on phenology of selected plant species in the Alpine region (1971-2000)

Abstract: We present a quantitative and qualitative analysis of the influence of altitude on plant phenology over an area, delimited by 46 to 49°N latitude and 5 to 15°E longitude, that includes major parts of the Alpine region for the period 1971-2000. Our attention is focused on flowering of widely distributed plants in Europe, including some important allergenic species. We calculated the dependence of phenological mean dates on altitude for different phenophases as a linear regres- sion. Results show a statistically significant linear dependence (p < 0.0001) for each phenophase, with regression coefficients that range from 0.92 ± 0.21 d (100 m) -1 altitude for beginning of flower- ing of Norway spruce to 4.56 ± 0.21 d (100 m) -1 for beginning of flowering of common hazel. Pheno- logical temporal trends and thermal temporal trends show a less strong dependence on altitude, predominantly non-significant. The most significant regression coefficients are negative for pheno- logical trends (beginning of flowering of common alder, -0.065 ± 0.028 d yr -1 (100 m) -1 , p = 0.0187, and Norway spruce, -0.049 ± 0.020 d yr -1 (100 m) -1 , p = 0.0167) and positive for temperature trends (February, 0.0017°C yr -1 (100 m) -1 , p < 0.0001). The significant linear dependence of February tem- perature trends on altitude induces an inverse significant linear dependence for phenological trends of flowering of common alder (occurring in March). The prevalence of negative mean pheno- logical trends suggests a stronger advance of flowering phases at higher altitude. Finally, a regional analysis suggests a tendency towards a stronger altitudinal response in the northern than in the southern Alps.

Effects of temperature, phase type and timing, location, and human density on plant phenological responses in Europe

Abstract: Phenological onset dates are closely linked to temperature. In this study, we analysed a phenological dataset collected during the COST 725 Action ‘Establishing a European phenological data platform for climatological applications’, which contained more than 36000 phenological time series for Europe covering 1971–2000. We analysed the temperature response of the phenological phases, their regional differences, and the relationship between the sizes of the local temperature and phenology trends in connection with a high-resolution climate grid of Europe. As an external factor, we examined the influence of human population density on phenology. Our analyses confirm differences in behaviour between annual and perennial plants in Europe. The average temperature response of perennial plants was significantly greater (–4.2 d °C–1) than that of annual agricultural crops (–3.0 d °C–1). The correlation between temperature and phenology trends was greatest for leaf unfolding of fruit trees and deciduous trees (r = –0.63 and –0.46, respectively). The geographic coordinates (latitude and longitude) had only a modest influence on the mean onset of the groups of phases; however, inclusion of altitude improved the models for some groups.

Spatial variation in onset dates and trends in phenology across Europe

Abstract: This study presents an approach based on the Bayesian paradigm to identify and com- pare observed changes in the timing of phenological events in plants. Previous studies have been based mostly on linear trend analyses. Our comprehensive phenological dataset consists of long-term observational records (>30 yr) within the 1951-1999 period across central Europe, from which we selected 2600 quality-checked records of 90 phenophases (mostly in spring and summer). We esti- mated the model probabilities and rates of change (trends) of 3 competing models: (1) constant (mean onset date), (2) linear (constant trend over time) and (3) change point (time-varying change). The change point model involves the selection of 2 linear segments which match at a particular time. The matching point is estimated by an examination of all possible breaks weighted by their respective change point probability. Generally we found more pronounced changes in maritime Western and Central Europe. The functional behaviour of all 2600 time series was best represented by the change point model (62%), followed by the linear model (24%); the constant model was the least preferred alternative. Therefore, non-linear phenological changes were by far the most commonly observed feature, especially in Western Europe. Regression analyses of change point model probabilities against geographic coordinates and altitude resulted in some significant negative regression coeffi- cients with longitude; in contrast, the constant model probabilities increased with longitude. Even when differences between locations across Europe existed, an overall trend towards earlier flowering was determined at most locations. Multiple regressions confirmed that mean advancing trends in the 1990s were stronger in the northwestern part of the study area.

Effects of recent warm and cold spells on European plant phenology

Abstract: Climate change is already altering the magnitude and/or frequency of extreme events which will in turn affect plant fitness more than any change in the average. Although the fingerprint of anthropogenic warming in recent phenological records is well understood, the impacts of extreme events have been largely neglected. Thus, the temperature response of European phenological records to warm and cold spells was studied using the COST725 database. We restricted our analysis to the period 1951–2004 due to better spatial coverage. Warm and cold spells were identified using monthly mean ENSEMBLES temperature data on a 0.5° grid for Europe. Their phenological impact was assessed as anomalies from maps displaying mean onsets for 1930–1939. Our results clearly exhibit continental cold spells predominating in the period 1951–1988, especially during the growing season, whereas the period from 1989 onwards was mainly characterised by warm spells in all seasons. The impacts of these warm/cold spells on the onset of phenological seasons differed strongly depending on species, phase and timing. “False” phases such as the sowing of winter cereals hardly reacted to summer warm/cold spells; only the sowing of summer cereals mirrored spring temperature warm/cold spells. The heading dates of winter cereals did not reveal any consistent results probably due to fewer warm/cold spells identified in the relevant late spring months. Apple flowering and the harvest of winter cereals were the best indicators of warm/cold spells in early spring and summer, also being spatially coherent with the patterns of warm/cold spells.