Tree-ring networks at multiple geographical scales: patterns of coordinated ecological responses to global warming = Xarxes d'anells de creixement en arbres a escales geogràfiques múltiples: patrons de respostes ecològiques coordinades en front a l'escalfament global

Abstract

Forest biomes are major reserves for terrestrial carbon and thus small shifts in the carbon balance of forests could have a large impact on the global carbon cycle. In spite of unambiguous evidence pointing to climate warming as modern factor increasing the risk of regional forest vulnerability, many uncertainties still remain preventing accurate predictions of forest productivity under future climate change. Improving our understanding of the diversity in tree performance at varying spatiotemporal scales must provide insights on the environmental responses of forests and their function as carbon holders in the coming years. To this end, we proposed a new conceptual framework to investigate tree-ring records by grouping chronologies into potentially homogeneous subsets based on mixed modeling principles, as illustrated for a Quercus spp. dataset from the northeastern Iberian Peninsula. Accordingly, we assessed spatiotemporal patterns of signal strength of tree-ring assemblages across environmental gradients and at multiple geographical scales. We used long-term tree ring-width (TRW) records to assess changes in the temporal coherence of radial growth in multispecies dendrochronological networks. In combination with stable isotope records (carbon isotope discrimination, Δ13C; oxygen isotope composition, δ18O) we characterized the physiological drivers of tree productivity under varying environmental conditions. Finally, we investigated shifts in climate sensitivity of tree growth and performance in response to warming-induced heat and drought effects. We found significant spatial autocorrelations for TRW, Δ13C and δ18O spreading over sites separated up to 1,000 km in Eurasia. However, growth synchrony was not uniform continent-wide, increasing along a latitudinal gradient and, thus, reflecting the extent of environmental coherence in the dominant climatic drivers of regional tree growth (i.e., rainfall patterns at mid latitudes vs. temperature patterns at high latitudes). Also, there were geographically-structured relationships between ring-width and either Δ13C (decreasing northwards and upwards) and δ18O (increasing northwards). These results pointed to limited CO2 diffusion impairing carbon uptake as main mechanism controlling the temporal coherence of forest growth signals at latitudes below 50°N in Europe and altitudes below ~1,600 m a.s.l. in Western Mediterranean. Notably, our results showed an increasing influence of climate on the spatial variability of tree growth over the twentieth century, ultimately leading to enhanced temporal coherence in ring-width records (spatial synchrony) along the local–continental continuum in most European

forests (except Fennoscandia) and also in central Siberia. Growth-climate analyses highlighted coherent shifts in vegetative activity towards more favorable periods depending on site location through an advanced onset of growth in the early season and temperature effects in summer (e.g., strengthening of drought-induced growth limitations in Mediterranean forests and relaxation of cold constraints in boreal forests). Increase in growth synchrony together with a tighter relationship between ring-width and Δ13C over the twentieth century suggested increasing drought effects on growth patterns in southern and central European forests and also in mountain Mediterranean forests. Particularly, our results indicated that warming-induced drought stress is spreading upwards in Mediterranean forests and northwards in temperate forests, overriding taxonomic imprints and local drivers of tree growth. However, uppermost and northernmost forests exhibited a diverging reaction to warming, which highlights the complexities of changing climate effects on peripheral forest populations, hence requiring further examination. Collectively, we demonstrated that broad-scale climatic variation influence ecophysiological traits in both conifers and hardwoods in ways that have previously gone unrecognized, which may help to understand the ecological implications of ecosystem functioning and make better predictions on future forest responses to the ongoing global warming. The evidence for substantial altitudinal and latitudinal variation in tree growth dependencies on leaf-level physiology could be valuable to forecast the extent by which future forests may progressively shift from temperature- to moisture-sensitive growth.

Actualmente existen muchas incertezas que impiden predecir con precisión futuros cambios en productividad de los bosques en respuesta al cambio global. Una mejor comprensión del comportamiento de los árboles a diversas escalas espaciotemporales debe proporcionar pistas sobre las respuestas de éstos frente al medio y su función como sumideros de carbono. Esta tesis propone un nuevo marco conceptual para investigar registros de anillos mediante la agrupación de cronologías utilizando modelos mixtos. Se han utilizado series de anchura de anillos (TRW) para determinar cambios en la coherencia temporal del crecimiento en redes multiespecíficas. En combinación con registros isotópicos, se han estudiado los controladores fisiológicos de la productividad. Finalmente, se han investigado los cambios en sensibilidad climática del crecimiento. Se hallaron autocorrelaciones espaciales relevantes para TRW, Δ13C y δ18O entre bosques separados hasta 1000 km en Eurasia. Sin embargo, la sincronía en crecimiento no resultó uniforme geográficamente, incrementando latitudinalmente y reflejando la importancia de ciertos factores climáticos dominantes. Igualmente, se observaron relaciones estructuradas entre el grosor de anillos y Δ13C (disminuyendo altitudinalmente y hacia el norte) o δ18O (incrementando hacia el norte). Ello apunta a una limitación en la difusión intercelular del CO2 como mecanismo de control de la coherencia temporal del crecimiento en latitudes <50°N y altitudes <1,600 m en el oeste mediterráneo. Asimismo, el clima ejerció una influencia creciente sobre la variabilidad espacial del crecimiento en el siglo XX, conduciendo a un aumento en la coherencia temporal de los registros de anchura de anillo (sincronía) en la mayoría de bosques europeos. El análisis de las relaciones crecimiento-clima destacó la presencia de cambios coherentes en la actividad vegetativa hacia periodos de crecimiento favorables. El incremento en crecimiento sincrónico y la relación más estrecha entre anchura de anillo y Δ13C indican que la sequía inducida por calentamiento se extiende altitudinalmente en bosques mediterráneos y septentrionalmente en bosques templados, encubriendo diferencias taxonómicas y factores locales. Colectivamente, se ha evidenciado que la variación climática influencia el comportamiento ecofisiológico en coníferas y angiospermas de forma desconocida hasta la fecha, lo que debería ayudar a realizar predicciones más fiables de la respuesta futura de nuestros bosques.