Análisis de las posibles variaciones climáticas y bioclimáticas en california (EEUU) y su relación con la vegetación

Abstract

The global mean surface temperature has shown a perceptible increase since the late 19th century. However, this increase has not been constant in space or time, and not many studies are known to have delved into this peculiarity in the State of California. In addition, drought, a natural phenomenon worldwide, has been exacerbated by global warming, which has led to an increase in affected areas. The study of drought involves the analysis of various indicators and indices that provide insight into changes in the hydrological cycle of a given region. Rising temperatures and changes in precipitation patterns in climate change scenarios are accelerating soil water depletion and increasing the risk of drought. This, in turn, alters the conditions that many plant species need to survive. In this sense, Bioclimatology is an interdisciplinary scientific discipline that explains the complex relationships between climatic conditions and the distribution patterns of living organisms and their communities across the planet, especially in plant communities. In this context, the assessment of climatic and bioclimatic parameters and indices at specific locations can serve as a basis for decision-making processes related to the potential impacts of climate change. One of the objectives of this doctoral thesis is to deepen our knowledge of the spatiotemporal evolution of mean temperatures and precipitation in California on monthly, seasonal and annual scales. In addition, it aims to analyse the relationships between the atmospheric teleconnection patterns which influence California and the aforementioned variables. Another objective of this research is to study trends in bioclimatic parameters and indices over the period 1980-2019. In addition, we conducted, for the first time, the qualitative and quantitative characterisation of native conifer taxa which are of economic, scientific and cultural interest in California. As a final objective, we set out to analyse possible changes in habitat suitability for the taxa studied under a future climate change scenario. To achieve these objectives, we initially conducted a detailed analysis of data from 350 weather stations with an observation period spanning from 1980 to 2019. To ensure the reliability of the results, the homogeneity of the data was verified and advanced statistical techniques, such as modified Sen’s slope and Mann-Kendall tests, were applied to each weather station to determine the direction and magnitude of the trends, as well as their statistical significance. The spatial distribution of monthly, seasonal and annual trends was analysed using the geostatistical Empirical Bayesian Kriging (EBK) technique. In order to establish relationships between climate variables and teleconnection patterns, a robust correlation analysis was performed using Spearman’s partial non-parametric test with a 95% confidence level. Regionalisation of the correlations between atmospheric teleconnection patterns and climate variables was carried out by applying principal component analysis (PCA) and agglomerative hierarchical cluster analysis (HCA). For the establishment of the qualitative and quantitative diagnoses of the conifers studied, an exhaustive search of points of presence in the Gbif database was carried out and the bioclimatic proposal by Rivas-Martínez et al (2011) was followed. Robust linear regression models were applied to determine the future climate of California (year 2050). Boolean logic, specifically the conditional logical operator (CON), was also used to establish the possible presence (1) or absence (0) of each species for the 15 variables analysed and to determine habitat suitability for the taxa under study. In this way, suitable areas (based on the maximum and minimum values of each species for the bioclimatic variables calculated) and optimal areas (derived from the values between the first quartile (Q1) and the third quartile (Q3) of each species and for each variable) were defined for the present time (1980-2019) and their future projection (2050). Analyses of gains, losses and maintenance of habitat suitability were also carried out comparing current areas and those derived from future projections. The results of the trend analyses are illustrated mainly on contour maps and reveal that local temperatures showed statistically significant increases, being higher in southern California than in northern areas. Increases in precipitation were observed in the mountainous areas of Klamath and decreases mainly in the southern areas of the Sierra Nevada. In addition, negative trends in ombrothermal indices (Io, Ios2, Ios3, Ios4) were observed in the south and centre, and increases in bioclimatic drought were detected throughout the territory. These were particularly noticeable in summer and spring, in the central and southern regions of the State. We observed significant correlations between temperatures in February, March, April and May in most of the teleconnection patterns studied. In particular, the Western Pacific Oscillation (WPO) teleconnection pattern showed the highest negative correlation. Furthermore, regionalisation of the State of California based on the correlation values between precipitation and teleconnection patterns has shown 4 distinct areas. Qualitative bioclimatic diagnoses at the isobioclimatic level showed that most of the studied species develop in the Mediterranean macro-bioclimate. The results of the habitat suitability analyses revealed that all species are experiencing a reduction of their range in California due to climate change, as well as a shift to more northerly and/or higher altitudes. Abies magnifica and Pinus albicaulis would be the least affected taxa in terms of loss of habitat suitability in 2050. Finally, the research undertaken in this Doctoral Thesis has once again demonstrated the usefulness and predictive nature of Bioclimatology, which can help managers to apply strategic measures to mitigate the possible impacts of climate change on flora and vegetation.