Relationships Between Climate and Flowering of Eight Herbs in a Swedish Deciduous Forest

Effects of annual variation in rainfall, temperature and humidity on flowering abundance of eight temperate woodland plants (Anemone nemorosa, Cardamine bulbifera, Lamiastrum galeobdolon,Oxalis acetosella , Ranunculus ficaria, Stellaria holostea, Viola reichenbachiana and Viola riviniana) were studied during 12 consecutive years (1989–2000) in a hornbeam (Carpinus betulus) forest in southeast Sweden. Above-average rainfall/humidity in late summer to early autumn of the preceding year increased flowering abundance in L. galeobdolon, O. acetosella, V. reichenbachiana, V. riviniana and, especially, in R. ficaria, but not in S. holostea and A. nemorosa. Moreover, flowering of R. ficaria and O. acetosella was positively related to rainfall/humidity during several parts of, or the entire, preceding year. On the contrary, flowering of S. holostea and A. nemorosa was closely related to low values of rainfall/humidity in autumn and/or winter of the preceding year and also to low humidity in the current year in A. nemorosa. Two long periods (3–4 years) of increasing rainfall deficit coincided with decreasing flowering abundance in most of the species, but not with decreasing vegetative development. Temperature variability was less consistently related to flowering. A cool period during the preceding summer or autumn seemed important for flowering in L. galeobdolon, O. acetosella and the Viola species, although these relations were, at least partly, caused by interactions with rainfall/humidity. No significant (P < 0.05) correlations were found between flowering and the conditions prevailing in April to May—the main flowering season—of the current year.