e_moon60 (e_moon60) wrote,

80 acres: Carbon sequestration & climate change

I've been reading articles in Science and Nature (various issues, but Science had a lot of articles about a month ago) on forestry management in this time of climate change, with particular interest in temperate forest management in terms of carbon sequestration and fire problems.

According to research done here in Texas, 1 acre of Hill  Country thicket (what developers call brush and bulldoze off) sequesters enough carbon in a year to balance a car (average car, I'm assuming) driving 26,000 miles.   According to the science journals, fire suppression in US forests increased the carbon sequestration 15% in the 20th century...possibly delaying awareness of the growing increase of carbon dioxide and climate warming.    However, climate warming and changes in rainfall patterns has affected those forests (in British Columbia, for instance, which due to a bark, beetle infestation has turned from a carbon sink into a carbon source, and in California and other areas by increasing fire danger so that the stored carbon is burning rapidly.) 

As a small-scale land manager with some "thicket" and some actual riparian woods, I would like our land to be a carbon sink, not a carbon source.   Long-lived woody plants sequester more carbon for longer than other plants.  We are on the edge of possibility when it comes to growing trees that live >50 years: we don't have (didn't have before the warming and drying of our climate) the water to become a forest.   Brush, however, could invade (is working on invading) the grassland, even though the original vegetation was tallgrass prairie.    Human use and abuse has lowered the water table, so trees can't reach it in most place.  Springs have dried up.  Our land--the 80 acres--has no permanent water source.  

Research has shown that second-growth woodland sequesters more carbon per year than mature woodland (as you'd expect: fast-growing plants are laying down carbon faster than the discarded leaves and twigs are decaying; slow-growing or mature plants are just maintaining stasis.)  OTOH, the need for carbon sinks will continue into the next century...planting fast-growing poplars (for instance) even if we had the water for them, would mean a release of carbon dioxide in 20-40 years when the trees died, just when carbon dioxide levels are expected to peak.   That's pushing a cart partway up a hill and then letting it roll back down, while saying to oneself "Well, I did my bit!"   Ideally, trees planted now should be long-lived, trees that will survive 75+ years and be harvested incrementally. 

But it's not just about carbon sequestration, and in the minds of most people water and fire issues come first.  For managers who think about stream-flow and surface water accumulation, woody plants are an easy villain.  Woody plants take up water.  (The rabid attacks on Texas brush, including Ashe juniper, arise from the notion that these plants are "stealing" water.)   Streamside woods are also accused of "causing floods" (the woods slow the water, so it spreads sideways...the flood is actually caused by surface runoff, of course.) 

 I just read praise of another Rice grad in the alumni newspaper--he won an award from Texas Parks & Wildlife for clearing brush and "saving" water and contends that if only other ranchers in the Concho Valley would do the same, they could "save" enough water to fill a reservoir and supply San Angelo.  (Not when it evaporates as fast as it does out there, I thought to myself. )   Leaving aside my feelings for San Angelo,  I'll agree that water resource management is a huge issue and must be dealt with--but it can't be dealt with in isolation.   Water-in-plants--especially water-in-woody-plants--can't be considered "wasted" water...it's water that's sustaining plants that are a) making oxygen and b) sequestering carbon.   By doing the latter, they're mitigating global warming, and global warming is one of the things making water scarcer in many areas and causing floods in others. 

Then there's the fire issue.  There's been a lot of negative talk about fire suppression, how unnatural it is, etc.  But fire returns carbon dioxide to the atmosphere in big lumps (along with smoke and other chemicals)...it undoes the work of carbon sequestration.  Without fire suppression, it will be impossible to grow enough trees in anything but a rainforest to do any good...and though the tropical rainforests are incredibly important for carbon sinks, the temperate forests and woodlands are far from negligible.   Some writers advocate removing everything but canopy trees.  This creates other problems--some trees grow best (healthiest, fastest) in community with their understory vegetation--the understory helps produce a ground-level climate favorable to the canopy and also supports the animal life that assists the canopy trees in surviving other challenges.  

And so on and so on.  Every time a tree is cut down--at the moment it's cut down--a big wodge of carbon dioxide is released.  Before the tree decays, before it is burned, that carbon dioxide is released (with more to come after decay or burning, of course.)   Why?  Because the soil biota that were in balance with the tree's living underground parts--the roots and rootlets and root hairs--die, and the decomposers come in and quickly convert those once-living parts to--among other things--carbon dioxide.  

So I look at the 80 acres and think and think.  Restoration alone won't work--it would require the climate that used to be here--that many hours below freezing in the winter, that many inches of rain on average, etc--and we don't have that now and won't have that in the next century.   The southernmost big bluestem used to live just east of San Antonio--over 100 miles south and much hotter--but  is it there now?   Will our big blue survive?  What about the other tallgrasses?   Should I shift the emphasis in prairie restoration to mid- and short-grasses even though this was originally tallgrass?   

Already we're choosing to bring in plants that grow south and west of here--hotter and drier areas--in anticipation of hotter and drier conditions here.  But--how much brush should we allow to encroach on the grass (if any) and what species of it?  The much-despised Ashe juniper can survive on very low rainfall and in fairly high heat; the much-despised mesquite (and the rest of the south Texas brush community) does even better in those conditions.  Though their habitat structure is different, they both support a wide variety of wildlife in their original locations.   They will use all the water they can get, but they don't *need* it in the same way that a cottonwood or black willow does.  Both are long-lived (over 100 years) but the Ashe juniper will support a very high-heat, explosive fire.  Mesquite burns hot, but not explosively.  Cedar elm is a long-lived tree if it gets the water for it.  We have a healthy-looking osage orange about 50 years old.   The riparian woods...take out the undergrowth in the hopes that a rogue grass fire wouldn't take out the big trees?  But some of the big trees are known not to tolerate even a ground fire.  And the wildlife utilize the undergrowth for food and cover.   The wildlife then fertilize the trees and protect them from some pests (when a big flock of robins spends a few days in the creek woods in migration, it sounds like rain pattering down.  Don't look up!)

We plant trees.  We planted trees this spring and a lot of them died for lack of water.  A few may live.  The "blue oaks" from the southern edge of the Hill  Country are doing fine so far.  (One has been in three summers now, even though we're 1000 feet lower than its preferred range.)    We look at the land and try to guess where it wants to go next and how it will respond to the change in climate (running 10 degrees F above what was "normal" in the early 1980s.)   We're constantly tinkering with our original plan...and can only hope that we're tinkering in the right direction.

Tags: carbon sequestration, climate, walking the land

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