Is Climate Change a Possible Explanation for Woody Thickening in Arid and Semi-Arid Regions?

Increased woody plant density (woody encroachment or woody thickening) is a globally observed phenomenon. Similarly, increased atmospheric carbon dioxide concentrations and decreased pan evaporation rates are globally observed phenomena. In this paper, we propose that the former (increased woody plant density) is a product of the latter. We propose that decreased stomatal conductance and increased rates of carbon ﬁxation arising from an enriched atmospheric carbon dioxide concentration, in con-junction with reduced rates of pan evaporation, result in increased woody plant density. We suggest that this is analogous to the increased woody plant density that is observed along rainfall gradients that span arid to mesic environments. From this conceptual model, we make three predictions, namely, that (a) long-term trends in tree water-use-e ﬃ ciency should reveal increased values; (b) run-o ﬀ data should show an increase where woody thickening is occurring; (c) enriched CO 2 experiments should reveal an enhanced plant water status. These three predictions are discussed and shown to be supported by experimental data.

Potential causes of woody thickening have been extensively discussed [8,9].The Walther model invokes competition for water and nutrients [10][11][12] where the deeper roots of woody plants access deep stores of water.Alternatively, changes in the timing, intensity, and frequency of fire [13][14][15] and changes in herbivory by large mammals [14,16] have also been considered important factors in causing an increase in woody plant density [17,18].
We propose an alternative biophysical mechanism to explain woody thickening based upon changes in the soilplant-atmosphere continuum resulting from a change in global atmospheric conditions.This mechanism is global in reach, appears consistent with a number of observations of trends in evaporation rates, run-off and soil moisture, and has several testable predictions, which we briefly discuss.

THE MODEL
The following observations constitute the a priori foundations for the model.
(1) The concentration of CO 2 in the atmosphere ([CO 2 ] a ) has been increasing since the start of the industrial revolution [19].(2) An increase in [CO 2 ] a has two effects.(a) it increases rates of photosynthesis of woody plants, typically of about 30% to 50% [20,21].Photosynthesis is enhanced more in woody shrubs (+45%) than grasses (+38%) or trees (+25%) in response to CO 2 enrichment [22]; and (b) leaf stomatal conductance of woody plants is decreased by about 20% [23].C4 grasses show a smaller stomatal and photosynthetic response to CO 2 enrichment than C3 plants such as shrubs and trees.
(3) The growth rate of trees and shrubs is increased by enrichment of [CO 2 ] a because of an enhanced rate of photosynthesis [21].Importantly, the proportional increase in tree growth is larger under xeric than mesic conditions [20].(4) Pan evaporation rates have declined globally [24].Across Australia, for example, the decline in pan evaporation rates for the past four decades has been between 1 and 3 mm per year [25,26].(5) Because of decreased rates of evaporation and g s , soil moisture content is larger in the short-to-mediumterm and this leads to increased tree density.
We propose that these observations may explain the phenomenon of woody thickening.It is useful to note that there are three key predictions from this conceptual model.First, long-term trends in tree water-use-efficiency should reveal increased values.Second, run-off data should show an increase where woody thickening is occurring.Third, enriched CO 2 experiments should reveal an enhanced plant water status.These predictions are discussed later.

MATERIALS AND METHODS
We explored long-term climate data sets for southern Africa to determine whether site wetness has been increasing.We are not aware of any analyses of such data from southern Africa to-date and therefore chose this location to add to the analyses available for Europe, America, Australia, and New Zealand [25,27].We analysed records from evaporation pans, rainfall, and run-off at these sites.Using reports from the literature and aerial photography interpretation, we tested whether woody cover has been increasing at selected sites throughout the arid and semiarid regions of southern Africa, in agreement with trends reported globally.

MODIS LAI product
We explored trends in the leaf area index for large areas (>10 km 2 ) with a known history of woody biomass increase using the MODIS LAI product.We used 7 years of the MODIS 8-day 1 km Collection 4 (MOD15A2) composite LAI/FPAR product [28] for the period from February 2000 to December 2006 from the NASA Distributed Active Archive Centre.These data were extracted using the MODIS reprojection tool and imported into IDRISI image processing package to create a 7-year data stack.The geocodes for the USA (Jornada LTER, NM) site were entered into the GIS, 8-day LAI values for the region were extracted, and a single 8-day mean was calculated for each year.

RESULTS AND DISCUSSION
In contrast to the expectation that global warming would result in increased evaporation, a decrease in pan evaporation rates has been recorded for the northern hemisphere, Australia, and New Zealand [24,25].This result is confirmed in the present study for several arid and semiarid regions of southern Africa (Figure 1) in the absence of any discernable trend in annual precipitation (Figure 2).In water-limited ecosystems, decreasing pan evaporation rates can best be explained by decreased wind speed [29] and decreased solar radiation receipt at ground level because of increased cloud cover and atmospheric aerosol content.
Vapour pressure deficit has decreased for water-limiting ecosystems of Africa, Australia, and the Indian subcontinent [30]; this is another measure of a decreasing pan evaporation rate.Because of a decrease in evaporative demand, we must predict that in the absence of a change in rainfall, either soil moisture content must be larger in the short-term or excess moisture is being lost through a larger rate of run-off.In the longer-term, we expect enhanced rates of vegetation water use that has increased its density in response to the earlier rise in soil moisture content.The prediction that increased run-off as a consequence of climate change is supported from both observational and simulation studies [31,32].The high [CO 2 ] a , of recent decades, compared to the levels observed in the 18th century, has also been recently invoked to explain this increased run-off [33] through the observed response of g s to [CO 2 ] a , [20,22].
There is evidence of global soil moisture increasing [34,35], with positive soil moisture trends observed during the 20th century in the Ukraine, Mongolia and the western USA, for example [36,37].While models of global warming predict summer soil desiccation, there is no evidence for this even in regions that have been warming over the past 50 years [35].For arid and semiarid regions of the southern hemisphere, however, evidence of increases in soil moisture is limited.Elevated moisture levels across land-use gradients have been recorded in grassed deep sand-dunes of the southern Kalahari [38] and this has promoted the success of C3 shrubs and trees at these locations.Similarly in an encroached area at the Jornada LTER site in New Mexico, where soil moisture has been recorded monthly since 1982, soil moisture at a depth of 1.3 m has increased in the non-wooded playa but decreased in the areas encroached by Prosopis glandulosa with a concomitant increase in MODIS LAI (Figure 3).Such increased soil moisture where woody thickening has yet to occur is expected in the short-term, whilst a decline in soil moisture where woody thickening has occurred is expected in the longerterm.
In many field CO 2 enrichment studies, the soil moisture content below woody vegetation is increased in response to CO 2 enrichment even when the leaf area index was increased [39,40].Such a response is predicted from the model proposed.
A second prediction from this model is that a reduced g s in response to CO 2 enrichment and a concomitant increase of soil moisture stores (for at least some months of the year) will result in a more positive plant water status.There is ample evidence that an improved water status is observed under CO 2 enriched conditions [41].Reference [42] reviews much of the literature and shows that improved water status is frequently observed in CO 2 enrichment studies.These differences were larger during drought than wetter conditions, indicating the importance of this effect during periods of low water availability [43].
A final prediction from our conceptual model is that water-efficiency should have been increasing in woody plants for the past century because of the increase in photosynthesis and the decline in g s arising from increased [CO 2 ] a .In support of this prediction is the long-term increase in WUE of trees that has been reported over the past 100 years [44].In addition, drier sites will be more sensitive to inter-annual rainfall than wetter sites and the response of above-ground net primary production (and hence woody thickening) will be larger in low rainfall sites, so we might expect a significantly larger degree of woody thickening here than in mesic regions [45].
Clearly, we have a simple yet powerful explanation for the trend of increasing woody thickening observed in arid and semiarid regions over the past 50-100 years.As pan evaporation rates have declined, the availability of soil moisture has increased, effectively equivalent to increased rainfall.This has resulted in an increased ecosystem-scale woody thickening.Superimposed on this is the decrease in g s resulting from increased atmospheric [CO 2 ] a [21,22] which can also reduce leaf-scale transpiration rates.We conclude that this conceptual model and its three predictions are strongly supported by both experimental and simulation studies.

Figure 1 :Figure 2 :
Figure 1: Trends in evaporation for two stations in South Africa.Data were provided by the Hydrological Information System of the South African Department of Water Affairs and Forestry.

DFigure 3 :
Figure 3: Trends in deep soil moisture at the Jornada LTER site.Access tube 1(AT1) is located in a playa which has low woody shrub cover, while the area around access tube 2 (AT2) is encroached by Prosopis glandulosa.Increased MODIS LAI was observed at sites where woody encroachment was occurring.