It is known that woody encroachment is occurring in grassland worldwide. I want to know which factors will affect this process, please give me some relating references.
Van Auken, O.W. 2000. Shrub invasions of North American semiarid grasslands. Annu. Rev. Ecol. Syst. 31: 197-215.
1. Pärtel, M. & Helm, A. Invasion of woody species into temperate grasslands: Relationship with abiotic and biotic soil resource heterogeneity. J. Veg. Sci. 18, 63 (2007).
1. Hegedüšová, K. & Senko, D. Successional changes of dry grasslands in southwestern Slovakia after 46 years of abandonment. Plant Biosyst. - An Int. J. Deal. with all Asp. Plant Biol. 145, 666–687 (2011).
these are some that I know in my bibliography !
In the sub-tropical/semi-arid grasslands of southern Arabia we see decline in woodland due to overgrazing and subsequent loss of horizontal precipitation capture (http://web.mit.edu/eltahir/www/Publications_files/2006%20Hildebrandt%20Eltahir%20Oman%20forest%20GRL.pdf). However, on many hillsides unpalatable woody shrub layers dominate. On the seaward-facing slopes the optimum habitat is Dhofarian woodland (http://www.tandfonline.com/doi/abs/10.1080/00837792.2012.10670909#.VCLBcvldWSo) and then, moving inland towards the desert, the vegetation becomes increasingly sparse and species composition changes.
The processes of the woodland-grassland interface in S. Arabia are much more complex than in other parts of the world due to the short monsoon mist season and the increasing livestock populations. It is safe to say that woodland encroachment is definitely not occurring in Southern Arabia!
As for encroachment, try:
http://tropicalgrasslands.info/index.php/tgft/article/view/90/55
http://www.nature.com/nclimate/journal/v3/n9/full/nclimate1904.html
http://www.agu.org/books/gm/v153/153GM09/153GM09.pdf
Hi Hua,
Something previous answers may have neglected was fire suppression. One of the seminal papers in this field was by Steve Archer (see citation below). Anything more recently published by Archer or by Alan Knapp (Colorado St.) would be excellent to review. Much work has been at Konza prairie and Jornada experiment stations in SW US. I hope this helps.
Archer, S.; Schimel, D. S.; Holland, E. A. 1995. Mechanisms of shrubland expansion: land use, climate change or CO2? Climatic Change 29: 91-95
If you google "JP Bakker and grazing", you will find a lot of good references.
A nice book:
http://www.springer.com/environment/nature+conservation+-+biodiversity/book/978-94-010-5886-5
or his google scholar page:
http://scholar.google.com/citations?user=9hIE_H0AAAAJ&hl=en
One of the important factors is build up of nutrients and a well developed soi. Grazing suppresses the wood growth, as wood species cannot cope with the grazing pressure, and grasses can wel do it.
For north-eastern Australia, I have covered many of these issues in the attached publication (bit dated now).
Other authors to have written about the related issue of woodland thickening/grazing/fire in Queensland Australia are W H Burrows and also RJ Fensham .
Hello, Hua
The following articles may help you.
Acácio, V., Holmgren, M., Jansen, P. A., & Schrotter, O. (2007). Multiple recruitment limitation causes arrested succession in Mediterranean cork oak systems. Ecosystems, 10(7), 1220-1230.
Acácio, V., Holmgren, M., Rego, F., Moreira, F., & Mohren, G. M. (2009). Are drought and wildfires turning Mediterranean cork oak forests into persistent shrublands?. Agroforestry Systems, 76(2), 389-400.
The primary causes of woody encroachment have been overgrazing and fire suppression. In addition to Van Auken 2000 as Natalia suggested, I recommend reading:
Archer, S., D.S. Schimel, and E.A. Holland. 1995. Mechanisms of shrubland expansion: Land use, climate,or CO2? Clim. Change 29:91–99.
In arid and semi-arid tropics there are many possible causes, and the often advocated one, 'overgrazing' not always obvious. Changes in run-off/run-on balances in relation with landuse changes can explain some of the observed dynamics. It is important to consider the mode of dispersion of the woody species: the encroachment of pioneer shrubs that followed the droughts of the 70's and 80's in Sahel was in part due to wind dispersed seed species such as Calotropis procera and Leptadenia pyrotechnica...
Hiernaux P., Diarra L., Trichon V., Mougin, E., Baup F., 2009, Woody plant population dynamics in response to climate changes from 1984 to 2006 in Sahel (Gourma, Mali). Journal of Hydrology, 375 (1-2): 103-113
Arid plant species are rarely wood apart from a Acacia species and Combretum therefore the pressure of these few species can be interpreted as over exploitation. Apparently due to a lot CO and carbon dioxide in the atmosphere there is a school of thought alluding to the fact this scenario has triggered seed banks to support early colonizers the hers rather than mid or late colonizers like the Albizias and other hard grain woody species.
The study can take another twist when you skew results related to non timber forest plant species exploitation.
You may be interested in the attached, "A century of vegetation change on the Santa Rita Experimental Range." It provides a long-term look at changes in mesquite, burro weed, & cacti as well as perennial grasses.
At least two factors here in the arid western USA-
1.) -Overgrazing of the perennial native grasslands, and
2.) \The removal of soil nutrients, dropping the levels to below what the native grass seedlings need to survive.
See the paired photos of Dr. Humphrey at http://www.ecoseeds.com/desertgrass.html.
SOIL TESTS-- have been doing native grassland restorations here in California for 44 years, and the first thing I look at, is what are the soil nutrient levels in the shrub-invading area, N-P-K-Ca-Mg-Mn-Cu-Fe-S-plus soil organic matter percentage.
Then test the top 5 cm. of a REPRODUCING native grassland, where you see native grass seedlings being produced, and see what is missing from the shrub-invading area.
Then you can do ex-situ tests, by putting soil from the shrub-invading area into wood boxes or plastic flats about 30 by 60 cm. and 10 cm deep, and plant different species of the local native grasses, and see how they grow without any added fertilizer, then plant seeds in other boxes and adding what nutrients and organic matter you found was missing after doing your soil tests.
You can see the results of my tests, the left box no fertilizer or organic matter added, and to the right, the proper amount of fertilizer and organic matter added. To the left, the seeds sprouted grew 3 cm tall and died, to the right, live plants thriving. See http://www.ecoseeds.com/good.example.html which is one of the exercises from my Ecological Restoration classes that I teach.
Most of the arid Western USA is suffering from the lack of nutrients, caused by the grazing animals walking away with them in their stomachs, so that even if you could wave a magic wand and all of the woody plants disappear tomorrow, the soil nutrient levels and organic matter levels would be too low for native grassland seedlings to survive and restore most of the Western USA arid grasslands
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