Plant and Ecosystem Memory

Of course plants do not have brains and, thus, cannot actually remember what happened to them in the past. Although plants cannot remember, however, we use “memory” as a metaphor to refer to the effect of the past on current and future plant and ecosystem functioning. Such memory effects have been repeatedly shown in ecosystems (such as deserts) that are often defined by highly variable environmental conditions, where air temperatures, humidity, and soil water availability can differ greatly from one day to the next, week to week, among seasons, and year to year. For example, the amount of carbon that an ecosystem “releases” to the atmosphere through plant respiration and microbial decomposition of organic matter (e.g., dead plant leaves and roots) is strongly controlled by past (antecedent) temperature and soil water content. Similarly, annual tree growth, as recorded in tree-ring widths (see Figure 2), is strongly controlled by temperature conditions and precipitation received months and years before the initiation of new growth. On shorter time scales, the rate at which a plant leaf takes up CO2 (via photosynthesis) and releases water vapor (via transpiration) to the atmosphere is regulated by past humidity, temperature, and soil moisture availability. Thus, plants and ecosystems do remember. That is, past environmental conditions are imprinted on current and future plant and ecosystem functions. Of course, memories of our past are important for who we are and for what we will become, and ecologists have recognized their potential for understanding and predicting plant and ecosystem behavior.