Plants are integrators of climate influences over the entire growing season. So plants serve as indicators of climate change that can be compared with temperature measurements. In some cases, certain phases of a plant's growth and reproductive cycle are very sensitive to specific climatic factors. The change from one point in the plant's life cycle to another is called its phenological development, and it depends on accumulated heat energy as measured by day to day values of temperature. Penuelas and Filella (2001) provide numerous examples of plant phenological changes due to a warmer environment. For instance, they quote a research study documenting that deciduous plants in the area of the Mediterranean Sea produce leaves 16 days earlier and drop them 13 days later than they did 50 years ago. Changes in arrival of biological spring from 1959 to 1993 showed an 8-day earlier arrival in Europe and 6 days earlier in North America. Satellite data show that this lengthening of the growing season has produced more vigorous plant growth across the northern part of the Northern Hemisphere. Plant flowering at many locations is reported to be about a week earlier than 50 years ago.
Animals and insects also are affected, as are some of their relationships to their plant-based food supply. For instance, birds that overwinter in tropical areas experiencing less phenological shift may migrate to northern latitudes at historically appropriate times only to find the food conditions have been altered in ways that may adversely affect their survival.
Penuelas and Filella (2001) conclude that "Many ecological (carbon sequestration, nutrient and water cycles, species competition, pests and diseases, bird migration and reproduction, and species interactions), agricultural (crop suitability, yield potential, length of growing season, risk of frost damage, epidemiology of pests and diseases, timing and amount of pesticide use, and food quality), and socioeconomic and sanitary (duration of the pollen season and distribution and population size of disease vectors) factors depend strongly on plant and animal phenology." And these, in turn, are increasingly influenced by climate change.
Reference
Penuelas, J., and I. Filella, 2001: Responses to a warming world. Science, 294, 793-795.