The roles of trees In the Critical Zone

Abstract:

As trees extract water from a wide variety of potential subsurface reservoirs and transpire it into the forest boundary layer they link Earth and atmosphere and influence the critical zone (CZ). In seasonally dry climates there is growing evidence that some tree species derive their primary source of water from a deep weathered bedrock reservoir below the soil and saprolite while other species use a wide range of water sources in a more opportunistic manner. As trees extract water from these different sources they can change the behavior of stream baseflow, recharge dynamics in both the subsurface and streams, and impact water availability for evapotranspiration (ET). Each of these changes in turn change the site-specific hydrological balance in very different ways as well as the nature of how well buffered the CZ is in response to climate and climate change. I will present examples from the Critical Zone Observatory network on (1) where different tree species derive their water in the CZ ranging from less that a meter deep to many tens of meters deep, (2) how this compares to the diversity of known water reservoirs in the subsurface from the surface of the Earth to fresh bedrock, (3) how this impacts baseflow and recharge dynamics of the CZ, and (4) how the manner and magnitude of tree water use can feedback and feed forward to change local climate and the strength of the linkages between Earth and atmosphere as modulated by trees in the critical zone.