Predawn water-potential measurements indicate neutral, positive, or even negative effects of the trees over the grasses (Figure 9.4), depending on soil water content and evaporative demand (Fernández et al., 2002; Fernández, 2003). In both grass species, in periods with high soil water content, the net effect over plant water status was neutral or positive, particularly in treatments with higher tree covers. This may be due to a similar soil water availability but a lower evaporative demand under trees than the grassland. On the other hand, when soil water content was low (less than 13% Vol) and evaporative demand was high, neutral to negative effects were detected in plants growing under trees compared to those in the grassland or BTC (Figure 9.4). This may result from root competition between trees and grasses for scarce water resources, and in the case of the position Under Crowns, a relatively high evaporative demand because of high radiation levels compared to position BTC (e.g., Fernández, 2003). This was due to the movement of shadows at these high
Soil depth=0−20cm
21/9 21/10 21/11 21/12 21/1 21/2 21/3 21/4 21/5
21/9 21/10 21/11 21/12 21/1 21/2 21/3 21/4 21/5
21/9 21/10 21/11 21/12 21/1 21/2 21/3 21/4 21/5
21/9 21/10 21/11 21/12 21/1 21/2 21/3 21/4 21/5
Soil water content (% vol)
FIGURE 9.1 (a) Soil water content measured at 0–20 cm of soil depth during two growing seasons with a TDR equipment (Imko GmbH, Germany). (b) Soil water content measured at 120–140 cm of soil depth during two growing seasons. 350=500 ¼ number of trees ha1. Significant differences between all forested plots and the open grassland are indicated with asterisks. (Data from Fernández, M.E., Influencia del Componente Arbóreo Sobre Aspectos Fisiológicos Determinantes de la Productividad Herbácea en Sistemas Silvopastoriles de la Patagonia Argentina, Doctoral Thesis, Universidad Nacional del Comahue, Bariloche, Argentina, 2003.)
174 Ecological Basis of Agroforestry
latitudes, which are displaced with respect to the object that produces them. However, considering the Integral of Water Potential over the whole growing season (Myers, 1988), trees in the more dense treatments showed a cumulative positive effect over grass water status (Table 9.2).
Contrary to similar results of both species in relation to water status, relative growth (evaluated through a Growth Index, which considers tiller and leaf production, see Fernández et al. (2002) for more details) showed a different pattern between both species. Growth of S. speciosa decreased as
0
FIGURE 9.2 Mean EVT (in mm per day) of different treatments during the growing season 1999–2000 estimated from water balances. 350=500 PP ¼ number of pines ha1; UC¼ under canopy; BTC ¼ between tree crowns. Potential EVT (mm per day) for each period is also indicated. (Reprinted from Gyenge, J.E., M.E.
Fernández, T.M. Schlichter and D. Dalla Salda, Agroforest. Syst., 55, 47, 2002. With permission of Kluwer Academic Publishers.)
0:25 2:25 4:25 6:25 8:25 10:25 12:25 14:25 16:25 18:25 20:25
Solar time (h) u (mL cm−2min−1)
Ypd=−0.69 MPa Ymdv=−1.51 MPa
0:25 3:25 6:25 9:25 12:25 15:25 18:25 21:25
Ypd=−0.93 MPa Ymd=−1.7 MPa
0:25 3:25 6:25 9:25 12:25 15:25 18:25 21:25
Ypd=−0.87 MPa Ymd=−1.77 MPa
FIGURE 9.3 Sap-flow density (u ± S.D.) of Pinus ponderosa in three bright days during the season 1999–2000. (a) 23 November 1999, (b) 26 January 2000, and (c) 11 March 2000. Filled lines represent the average u of treatment with 500 trees ha1, and dashed lines represent trees from the treatment with 350 trees ha1. Mean predawn water potential (Cpd) and midday water potential (Cmd) of trees in each treatment and date are also indicated. (Reprinted from Gyenge, J.E., M.E. Fernández and T.M. Schlichter, Trees, 17, 417, 2003. With permission of Springer-Verlag.)
Tree–Grass Interactions and Water Use in Silvopastoral Systems in N.W. Patagonia 175
tree cover increased (Figure 9.5; Fernández et al., 2002). In contrast, growth of F. pallescens was similar in all treatments until relatively high tree-cover level (75%–80%) (Figure 9.5). In this species, growth was measured in two growing seasons contrasting in climate conditions: a wet season (2000–2001) and a dry one (2001–2002). The magnitude of growth was higher in the first wetter season (see maximum values in Figure 9.5), but a trend (not statistically significant) of a higher positive effect of trees over grass growth was detected in the driest season. In 2000–2001, mean growth of plants in the grassland was intermediate of that of plants in forested plots.
However, mean growth of plants in the grassland was lower than in forested systems in the dry year.
Stipa speciosa, season 1999−2000
0
Festuca pallescens, season 2000−2001
0
Festuca pallescens, season 2001−2002
0
FIGURE 9.4 Predawn water potential (in MPa) of Stipa speciosa and Festuca pallescens tussocks growing in different treatments. Significant differences between plants of any forested treatment and those of the open grassland are indicated with asterisks. 350=500 ¼ number of pinesha1; UC¼ under canopy; BTC ¼ between tree crowns (Data from Fernández, M.E., Influencia del Componente Arbóreo Sobre Aspectos Fisiológicos Determinantes de la Productividad Herbácea en Sistemas Silvopastoriles de la Patagonia Argentina, Doctoral Thesis, Universidad Nacional del Comahue, Bariloche, Argentina, 2003.)
TABLE 9.2
Integral of Predawn Water Potential along the Whole Growing Season (October–April, in MPa Days): Higher Values Indicate Higher
350 Between tree crowns 265.82 67.7
500 Under canopy 235.02 53.6
500 Between tree crowns 252.95 60.7 Note: 350=500 ¼ number of pines ha1. Each number is the
average of 3–4 plants.
176 Ecological Basis of Agroforestry
These results agree with those of natural ecosystems in which facilitation effects are more intense under more stressful conditions (e.g., Bertness and Ewanchuk, 2002).
In the case of S. speciosa, facilitation or neutral effects over its water status were detected under trees (Gyenge et al., 2002). However, growth results indicate that the net balance of interactions was negative (Fernández et al., 2002). In this drought tolerant species, radiation had a higher relative limitation than water, thus competition for this resource was more important than any amelioration in water conditions under trees.
Considering results of F. pallescens, plant water status in thefirst wetter season showed that plants in all treatments were in the same good conditions. For this reason, net tree effects over grasses were neutral to positive, specially considering that grasses in forested plots have propor-tionally much less roots than in the open (Fernández et al., 2004). Growth values agreed with these results, that is, there were no differences between treatments, and in some forested treat-ments, mean values were even higher than in the open (but not statistically different). From these results, we can infer that in relatively wet summers, facilitative interactions are more important than competition for resources, resulting in a positive net balance. On the contrary, in a very dry summer, competition for soil water between trees and grasses appeared to be more important than any amelioration in environmental conditions under trees. These results support Ong and Leakey (1999) ideas about ecological interactions in agroforestry systems. In February 2002, plants growing in the treatment with lower tree density had more negative water potentials than plants in the open, and had even lower water potentials than plants growing in the densest treatment (Figure 9.4). In plots with 350 pines ha1, plants growing under tree crowns were those which experienced the highest water stress, probably due to high evaporative demand under a relatively low tree cover, and at the same time, high competition for soil water with tree roots. In the plots with 500 pines ha1, plants also experienced competition for soil water with trees, that is, they had water potentials lower than in the open grassland. However, they were exposed to lower evap-orative demand due to shading than in plots with 350 trees ha1. Therefore, the net balance had a different result (less negative) than in lower tree densities. In this case, the nature of ecological
Stipa speciosa
Nov Dec Jan Feb Mar Apr
FIGURE 9.5 Relative growth index estimated for Stipa speciosa and Festuca pallescens tussocks growing in different treatments. 350=500 ¼ number of pines ha1; UC¼ under canopy; BTC ¼ between tree crowns. The only significant differences were observed between plants of Stipa speciosa growing in the grassland respect to those in forested treatments with 500 pines ha1, in January and March. (Data from Fernández, M.E., Influencia del Componente Arbóreo Sobre Aspectos Fisiológicos Determinantes de la Productividad Herbácea en Sistemas Silvopastoriles de la Patagonia Argentina, Doctoral Thesis, Universidad Nacional del Comahue, Bariloche, Argentina, 2003.)
Tree–Grass Interactions and Water Use in Silvopastoral Systems in N.W. Patagonia 177
interactions—their net balance—was the same, but its strength was different. This same response was also found in other plant associations depending on environmental or species characteristics (Bertness and Ewanchuk, 2002). On the other hand, considering only the wetter periods within the second growing season, results of water status agreed with those of the former year; plants in forested treatments showed equal or even better hydric conditions than in the open. Based on these results of plant water status, we can conclude that in dry seasons or periods the net balance of tree–grass interactions is negative, similar to what was described in other silvopastoral systems (e.g., De Montard et al., 1999), but opposite to what happens in natural ecosystems (e.g., Frost and McDougald, 1989;
Callaway and Walker, 1997). However, growth results suggest a contrary conclusion: in the drier growing season, positive effects are also higher than negative ones, that is, silvopastoral systems based on the studied species in Patagonia behave as tree–grass associations in savannas.
How can we reconcile the opposite results of water status and growth in dry periods, considering also that F. pallescens is a species vulnerable to water deficits (Fernández, 2003)? One possibility is that, in spite of plants in the open grassland showing a better water status, high evaporative demand probably forced stomata to be closed early during the day, decreasing carbon (C)fixation. Stomatal conductance of this species is linearly related to relative humidity (RH) of the air for values below 50% (Fernández, 2003). Despite the fact that we did notfind statistical differences in this environ-mental variable (measured 15 cm above plant canopies) between open and forested plots (Fernández, 2003), leaf temperature under direct radiation was probably higher in plants of the open grassland, thus decreasing the RH of the layer of air close to the leaf surface. In addition to this hypothesis, it is also possible that a higher Cfixation in plants of the open (due to their better water status), could have been counterbalanced by high respiration losses by roots. As mentioned earlier, root:shoot ratios of F. pallescens plants of the open were significantly higher than in forested plots (Fernández et al., 2004), and therefore, respiration was expected to be higher. Moreover, root respiration rate of plants in the open could have been higher due to higher soil temperatures in the open than in shaded treatments (e.g., Kitzberger, 1995). High water potentials of plants in the open were probably maintained with a high C allocation to root production, while in shaded treatments, biomass allocation to aboveground structures was increased. These changes are expected to be a primary response to radiation decrease in forested plots, as was described for a great number of species growing under shade conditions (e.g., Allard et al., 1991; Cruz, 1997; Valladares et al., 2002). Biomass allocation changes could confer these plants a lower competitive capacity when water reserves are low, but also would imply less maintenance costs of belowground structures. Finally, it is important to note that F. pallescens has a typical bimodal aboveground growth pattern, with one growth peak in early spring and the other in autumn (Defossé et al., 1990), coinciding with periods of high water availability. For this reason, worse hydric conditions in the driest month do not necessarily have to imply a reduction in the overall seasonal growth. In spite of this being the common pattern in thefield, this species is able to take advantage of rainfall events during the summer as was seen in thefirst growing season and also under irrigation conditions (Fernández, 2003).
In addition to better water status of F. pallescens plants in periods with high soil-water content and a different biomass allocation under trees, other morphological variables changed in plants growing under shade. Whole plant architecture (leaf angle distribution) as well as specific leaf area changed in a way that allow the plants better light capture in radiation-limited microenvironments (Fernández et al., 2004).
‘‘Results from both studied species agree with the hypothesis that radiation being a more limiting resource than water in drought-tolerant species we can expect a different balance between facilitation–competition interactions in different species growing in the same environment.’’