It is well known and understood that forests play a critical role in the global hydrological cycle, the never-ending process in which water circulates from clouds, to land, to water bodies and back to the clouds over again. When it rains, precipitation is partially intercepted by the canopy of trees and absorbed by the soil. Trees pull water from this soil through their roots to uphold their life processes. Trees also release water into the atmosphere (i.e. transpiration), which is lost through pores in their leaves or needles. This water, together with the water evaporated from other system surfaces (i.e. evapotranspiration) comprises the total amount of water vapour which returns to the atmosphere to keep the cycle running. (Check this nice video form the National Science Foundation for a deeper yet simple explanation on the main flows and storage processes of the Water Cycle).
Let’s dig deeper into the transpiration process to better understand water flows and needs in trees. I’ll try to keep it simple, as Prof. del Campo admirably does when he shares his wealth of knowledge with the GUARDIAN partners and stakeholders.
Liquid water flows from the roots to the leaves driven by capillarity and by water potential differences (measured in MPa), which is the magnitude used in plant physiology to measure the tendency of water to move from one area to another due to diverse physical or chemical phenomena like osmosis, gravity, mechanical pressure, etc. Overall, the energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. As such, water always moves from the system with a higher water potential (soil) to the system with a lower water potential (atmosphere), as can be seen in the figure (watch out! water potential is a negative variable in our system at hand!).
The atmosphere to which the leaf is exposed drives transpiration, causing massive water loss from the plant under severe drought periods. Dehydratation in plants may cause physiology changes (e.g. trees may stop carrying out photosynthesis and stop growing) ending up to irreversible effects causing death. You may want to know more about how drought kills plants (if so check out this outstanding piece of work from CREAF scientists) but, the topic covered in here deals with how to keep trees alive and vigorous!
In the following paragraphs I’ll try to summarize the efforts taken at the Guardian project to improve fire resilience and ecosystem services in la Vallesa through ecohydrological-based forest management practices. What a complex name, when we can call it proactive silviculture!