Long-Term Trends and Influence of Climate and Land-Use Changes on Pollen Profiles of a Mediterranean Oak Forest

Abstract

Climate change is producing observable modifications in natural ecosystems, but other human actives such as alterations to land use are also having a direct impact, especially on species distribution, loss of biodiversity, phenology, and interaction patterns. The objective of this study is to evaluate the impact of climate and land-use changes on airborne pollen concentrations in a Mediterranean forest (South Iberian Peninsula) over a 23-year period (1998-2020). 61 different pollen types were identified during the monitoring period, mainly from trees and shrubs, such as Quercus, Olea, Pinus or Pistacia, but also from herbaceous plants, such as Poaceae, Plantago, Urticaceae or Rumex. A comparison of pollen data from the first years of the study (1998-2002) with those obtained more recently (2016-2020) showed that there has been a substantial decrease in the pollen from autochthonous species associated with natural areas, such as Quercus (-18.64%) or Plantago (-1.13%). However, the relative abundance of the pollen from Olea crops increased to 13.85%, while the concentration of that from Pinus, which is used for reforestation, doubled (0.94%). Significant changes in pollen phenology were also observed, with earlier flowering in some species of Urticaceae but a delay in that of Compositae and Cyperaceae. Meteorological trends in the area generally resulted in an increase in both minimum and maximum temperatures, along with a decrease in precipitations. Changes in pollen concentration and phenology were correlated with changes in air temperatures and precipitation. The results suggest that climate change is having an impact on the concentration of airborne pollen in the region and may have consequences for human health and biodiversity. Finally, land cover changes motivated by human decisions are crucial to the changes in the pollen spectrum, such as in the increase in hectares of forest and olive crops.

Keywords: airborne pollen, Mediterranean oak forest, climate change, land cover, natural areas