For this week’s Why Trees Matter topic, we look at the important role trees play in soil conservation. Soil conservation deals with methods or processes used to prevent soil erosion and maintain soil health and productivity. Trees help conserve soil by reducing soil erosion, increasing soil organic matter, enhancing soil structure, and aiding in nutrient cycling. As a third of the world’s land surface is threaten by desertification, planting and caring for trees to help retain fertile soil is crucial not only to the health of the land but to over 850 million people who rely on agriculture as their livelihood.
The most fertile part of soil is found in its first couple centimeters, or topsoil. It is vital that topsoil is maintained and prevented from being washed away as it is necessary for crop and pasture growth. The more topsoil removed, the more fertilizer will be needed and the quicker desertification occurs. Trees help conserve soil by holding it in place with tree roots (pictured below) and by creating a rougher soil surface with tree stumps, fallen branches, and foliage litter. A rougher surface area slows the rate of water flow that erodes soil and a rougher surface improves water infiltration into the soil which increases soil fertility and decreases the need to provide additional watering, something important as water becomes scarcer. Moist soil also prevents it from being eroded by wind.
Another way trees conserve soil is by creating soil organic matter from their leaf litter and decaying roots. Trees and plants perform the first trophic, or nutritional, level of organic matter needed in the soil food web (refer to the diagram below). Soil organic matter increases the activity of microorganisms found in the soil which help increase soil fertility. As organisms decompose various organic matter or consume other organisms, nutrients are converted from one form to another and made available to plants and other soil organisms. All plants depend on the food web for their nutrition and soil plays a vital role in the process.
Trees also strengthen soil structure by providing biopores—pores left behind from decayed roots. These openings help water and nutrients travel through compacted soils to crops. While benefits from biospores may not be seen for several years, their effects remain important as crops in compacted soils can use biopore pathways left by trees many years after that area has been cleared of trees.
Lastly, trees conserve soil by aiding in nutrient cycling. Trees recycle nutrients by moving them from underground to the soil surface, where they decompose to form soil organic matter. Tree litter such as leaves then decompose on the ground, releasing nutrients back to the soil for the tree to take back up (refer to the diagram below). Thick tree canopies are great sources of free, natural fertilizer not only by shedding leaves but also by capturing nutrients on leaf surfaces which are then washed from the leaves to the soil by rain.
So far through our Why Trees Matter series we have shown the range of benefits that trees provide which we often take for granted or don’t even realize. These benefits include energy savings, carbon sequestration, air pollution reduction, and soil conservation. Next week we will introduce another tree benefit which is very relevant to our Arizona desert landscape—water conservation.
http://www.dnr.state.md.us/forests/publications/roots.html (picture 1)
http://www.nrcs.usda.gov/Internet/FSE_MEDIA/nrcs142p2_049822.jpg (diagram 1)
http://gratefultreesandbees.com/harvest-fall-leaves-to-feed-your-trees-and-garden/nutrient-cycle/ (diagram 2)
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