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Hydrological indicators of flow in headwaters for assessing farm management impacts: Streamside forestry management case study
Ecological Indicators 98 (2019) 627–633
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Hydrological indicators of flow in headwaters for assessing farm management impacts: Streamside forestry management case study
T
Philip J. Smethurst CSIRO, Private Bag 12, Hobart, TAS 7001, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Plantation Water Environment Reforestation Riparian Buffer
Hydrological indicators have been used for interpreting stream flow patterns in large catchments. It would be useful to evaluate such indicators for use also in headwater catchments close to the source of on-farm management impacts. Reforestation of cleared farmland is encouraged internationally for water quality and biodiversity improvements. One practice option is to establish fast-growing plantations in streamside management zones (SMZs), but impacts on stream flows are not well understood. A eucalypt plantation was established in a steep headwater catchment in temperate Australia and stream flows compared 2 years pre- and 4 years postestablishment with an adjacent unplanted catchment. The flow ratio of SMZ to control was never significantly different from one (no SMZ effect) despite a significant decreasing trend during the six-years of monitoring. The decrease in flow ratio was attributable mainly to a reduction in some higher flows resulting from increased surface roughness and interception of overland flow. Nonparametric analyses also indicated a reduction of some low flows due to plantation establishment. Overall, changes in flow patterns appeared minor, but the value placed on such changes will depend on social and environmental contexts. Because of the rarity of such data and the importance of understanding water flows as affected by plantations established on farmland, similar studies should be conducted elsewhere. The range of flow indicators tested in this case were useful and should be considered for other similar applications. In addition, a simulation capability needs to be developed to enable the effects of SMZ plantations on stream flows to be predicted for a range of climate, landscape and management contexts and to contribute further to policy and practice discussions.
1. Introduction Hydrological indicators have been used at regional scales (Poff et al., 2010) to interpret stream flow patterns and infer impacts on environmental values. For example, Lynch et al. (2018) used the R package EflowStats to assess changes in catchments of 16–542 km2, and Sengupta et al. (2018) used an ensemble of hydrological models for catchments of 14–289 km2. However, in agricultural landscapes, management occurs at the farm and headwater catchment scales of less than 1 km2. Use of flow indicators at this finer scale is unusual, but is needed to understand the impacts of farm management. Improved stream water quality is commonly associated with a higher proportion of forested land in a catchment and a reduced proportion of intensive land uses like cropping or dairy (Lintern et al., 2018). Streams in Tasmania, Australia, are no exception (Magierowski et al., 2012). Many communities prefer to source their water from forested landscapes (e.g. Kuczera, 1987), and there is evidence that forests can increase water availability under some conditions compared to cleared landscapes (Ellison et al., 2017). In forests managed for wood
supply, the level of disturbance associated with harvesting and re-establishment presents some risks to water quality, but many countries mandate or encourage responsible practices through codes of practices, certification schemes, and best management practices. Similarly, reforestation of cleared farmland is encouraged by some authorities for water quality improvements, particularly in streamside management zones (Neary et al., 2011). However, some codes of practice in Australia discourage streamside management zone (SMZ) plantations (Smethurst, 2008). One concern is that conversion of farmland to a forest plantation (afforestation) has the potential to change ground water levels and stream flows in some situations (Almeida et al., 2016; Jones et al., 2017), although the effect is highly context-dependent and sometimes absent (Smethurst et al., 2015). Salemi et al. (2012) found that flows from catchments treated with SMZ plantations were 456 ± 125 mm yr−1 lower than areas without plantations pro-rated to the proportion of area treated. However, because the SMZ plantations occupied small proportions of each catchment, the actual effect measured for each catchment ranged from −104 to +170 mm yr.1. Over this wide range of measured effects, there were
E-mail address: [email protected]. https://doi.org/10.1016/j.ecolind.2018.11.048 Received 1 June 2018; Received in revised form 14 November 2018; Accepted 16 November 2018 1470-160X/ Crown Copyright © 2018 Published by Elsevier Ltd. All rights reserved.
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no strong indicators of relative flows, and additional research was encouraged to improve the understanding and predictability of stream flow outcomes of this practice. Amongst the research reviewed there were no sites that represented fast-growing plantations in a low-rainfall, cool-temperate environment like that found in some parts of Tasmania, or with annual rainfall similar to that in other parts of temperate Australia where plantations are grown (i.e. 600–1000 mm annual rainfall) and where stream flows are generally considered susceptible to increased water use by plantations compared to pasture (Zhang et al., 2001). The aim of the research reported here was to compare stream flows in adjacent headwater catchments in Tasmania that received c. 700 mm annual rainfall before and after establishing a fast-growing eucalypt plantation in the SMZ of one of these catchments. There is a focus here only on identifying changes to stream flows as a result of SMZ plantation establishment. Earlier reports identified positive effects of the SMZ plantation on several aspects of water quality, i.e. decreased concentrations of sediment, phosphate, and bacteria (Neary et al., 2010; Smethurst et al., 2011; Smethurst et al., 2012; Smethurst et al., 2014).
plantation on water quality. The pasture consisted of grasses and nonleguminous dicotyledons. Within 2 months of planting, all eucalypt seedlings received 200 g of diammonium phosphate (17.5% N 20.0% P; equivalent of 5.1 kg N and 5.8 kg P ha−1 for the total catchment), which was split between two spade slits in the soil about 15 cm on opposite sides of the planting position. Weeds (pasture species) were controlled using granulated Macspred Eucmix® in a 1 m radius around each eucalypt seedling until tree crowns suppressed weed growth after age 2 years. A few seedlings died soon after planting or were killed by browsers, all of which were replaced by new seedlings retained from the original nursery stock. Overall stocking of the plantation was 1419 trees per ha. A photo of the catchments and a close-up of the SMZ is provided (Photo 1). 2.2. Measurements An automatic weather station was installed on the boundary of the two catchments in 2007 that recorded rainfall. All tree heights and diameters (at 1.3 m over bark, DBH) in ten plots distributed throughout the SMZ were measured at 4–12 month intervals from age 1.5 years. A 60°, aluminium plate, V-notch weir was installed at the outlet of each catchment in early 2008 and included water level readings with a capacitance probe every 5 min. Water level was converted to flow using a standard rating curve equation. Anecdotally, the collaborating farmer did not express any subjective observation of or concern about SMZ plantation effects on stream flows, but stream flow in the study catchments was not critical as stock watering was provided at off-stream troughs that were supplied by water from outside these catchments. During plantation establishment, there were anecdotal concerns in the community about the possibility of broad-scale plantation establishment, but these concerns were allayed by the realisation that only SMZs were to contain plantations. However, farmer and community values in relation to SMZ plantations and potentially positive or negative effects on water yield and quality were not assessed objectively.
2. Materials and methods 2.1. Site The study area was two adjacent north-facing headwater catchments in the Forsters Rivulet catchment that flows into the Huon River in southern Tasmania. One 9.8 ha catchment supported grazed pasture (68%) in the lower part, and native forest (32%) in the upper part. The other catchment was 3.5 ha in area and entirely supported grazed pasture. Both catchments were managed as one grazing unit (paddock) prior to and during the study. Farmer records indicate average annual rainfall (1991–2006) was 722 mm (range 501–975 mm). Rain falls every month, with slight winter-spring dominance (June-November). The catchment is in steep terrain (average 30.6% slope), with proportionally more very high slopes (46.6–78.1%) in the larger catchment. Soils are c. 3 m deep, derived from interlaid and mixed slope deposits of Cretaceous syenite and Permian mudstone. Syenite outcrops at the top of the larger catchment, but not in the smaller catchment. Surface soil (0–10 cm depth; < 2 mm) properties were loam or clayloam textures with 0.7–1.0 g cm−3 bulk density, 0.081 dS m−1 electrical conductivity (1:5 soil:water), 5.5 pHwater, 0.46 mg kg−1 total N, 16 C:N, 40.8 mg kg−1 available-P (Colwell), 17.5 meq 100 g−1 effective cation exchange capacity, and 94% base saturation. The soil fraction < 2 mm was 51–99% in surface soil samples, but only 1% in some samples at the bottom of the profile (3 m). Soils are classified as a grey dermosol (Isbell, 2016) and a palexerult (Soil Survey Staff, 2014). For the year preceding this study (2007), the catchment was irregularly grazed within a general regime of moderate stocking density (c. 2–3 head per ha) for 2–6 week periods at 2–3 month intervals. Cattle had free access to all unfenced streams. A mixed-species plantation was established in 2008 in the 5–30 m variable-width streamside management zone (SMZ) of the larger catchment. The area of the SMZ (0.6 ha) was 6% of the catchment and 10% of the pasture area in the catchment. Within the SMZ, soil outside the intermittently saturated riparian zone was cultivated by a ‘scoop-and-mound’ method using a mini-excavator, which created a pit adjacent to a mound on which a tree seedling was planted (Photo 1). Tree seedlings were planted in August 2008. In the lower, northern half of the SMZ, Eucalyptus nitens (shining gum) was planted on each mound, and Acacia melanoxylon (blackwood) was planted in the saturated riparian zone, which was not cultivated. In the top, southern half of the SMZ, Eucalyptus globulus (blue gum) was planted on each mound top and there was no saturated riparian zone. Fertilizers were not used in the catchment for several years prior to or during the study, except during plantation establishment in the SMZ. This type of low-input catchment was targeted for the research to maximise the chance of detecting any deleterious effect of an SMZ
2.3. Data analysis Total flows and low-flows (control catchment < 0.05 mm d−1) were analysed as ratios between the SMZ and control catchments (log10transformed), and compared to the theoretical ratio of 1.0. Linear trends in these ratios were compared to the theoretical ratio by using 95% confidence limit of predictions provided by SigmaPlot©. The IHA© software (version 7.1, The Nature Conservancy, 2009; Richter et al., 1996) was used to calculate flow exceedance (duration) curves, low flows (< 50% of median flow) and minimums for various periods (1, 3, 7, 30 and 90 d). Non-parametric methods were used to calculate the significance of differences in environmental flows. 3. Results 3.1. Plantation growth and rainfall Planted seedlings of all species experienced heavy browsing by native animals for the first year after planting, which is common in the region (Miller et al., 2006) and delayed growth, but by the second year browsing pressure had decreased and browsing control measures (repellents, tree guards, and shooting) provided a better opportunity for the seedlings to grow. Survival by age 6 years (including replacements) was 98% for both eucalypt species and 68% for acacia. Both eucalypt species grew approximately linearly in height and diameter between 1 and 6 years of age, reaching 11 m height and 14 cm diameter at 1.3 m height (Fig. 1). Acacia height and diameter grew at about a third of the rate of the eucalypts. At this stage, the plantation was ready for pruning and thinning (which commenced the following year) as part of a plan to produce sawn and veneer products. 628
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Flow ratio (SMZ treated/control) was calculated daily, but equipment failure (data loggers and weir) reduced the number of days that this ratio could be calculated, and we could not calculate annual discharge ratios (runoff/rainfall) as had been originally planned. For all flows combined and low flows (control flow < 0.05 mm d−1), the expected flow ratio of 1 (no treatment effect) was within the 95% confidence limit of predictions of these trends (Fig. 3). This result suggests no significant change in flow patterns due to SMZ establishment. However, there was a significant decrease in flow ratio for all flows, but not when flows were low (control flow < 0.05 mm d−1), suggesting that the decrease trend in flow ratio for all flows could be attributed only to flows > 0.05 mm d−1. Flow duration curves (FDCs) pre- and post-establishment differed for both the control and SMZ treated catchments, presumably due to differences in rainfall patterns, but the pre- and post-establishment relativities were similar in both catchments (Fig. 4). For both catchments, pre-establishment flows were higher than post-establishment flows in the range 0.4 to 10 mm d−1, and the range of pre-establishment flows included more lower and more higher flows than post-establishment flows, reflecting the relative rainfall ranges pre- and post-establishment (Fig. 1). At higher flows, pre-establishment flows were higher than postestablishment in both catchments. As calculated by the IHA software, low flows (< 50% of median) were higher every month post-establishment in the control catchment, and significantly so for half the months (Fig. 5). In contrast, SMZ treated low flows were higher post-establishment only in August and September. Likewise, 1- and 90-day minimum flows were significantly higher post-establishment in the control catchment, but not in the SMZ treated catchment (Fig. 6). Together, these low flow analyses indicate that SMZ plantation reduced some low flows, but the effect was not consistent. As low flows make up a minor proportion of total annual flows generally, it is doubtful that there was a significant effect on total annual flow.
Fig. 1. Height and diameter growth of trees planted in the SMZ.
4. Discussion This research demonstrates the use of indicators of hydrological flow alternation at a headwater catchment scale for understanding the effects of farm management. It contributes a case study to a growing body of knowledge on the effects of SMZ plantations on stream flows (Newbold et al., 2010; Salemi et al., 2012). Proximity of native forests or introduced tree species to streams favours water use for evapotranspiration compared to vegetation that is more distant and higher on the landscape because of differences in access to groundwater (Doody et al., 2014; O’Grady et al., 2006; Scott-Shaw et al., 2017). In this case, the SMZ plantation caused a decrease in some high, medium and low flows. However, this body of data so far provides a limited basis on which to generalise the effects that range from substantial increases to substantial decreases. Salemi et al. (2012) concluded from a met-analysis that removal of trees from SMZs potentially resulted in reduced evapotranspiration from that zone and increased stream flow, but actual effects depended on the water use behaviour of the remaining vegetation. When replanted, suppressed forests resulted in increased water yield compared to decreased flows in more vigorously regenerating or planted forests, but there was a paucity of paired-catchment studies that examined these effects. Generalisation is difficult because there are many interacting factors that we currently do not have the capability to integrate quantitatively. Similar complexities prevail for catchments regardless of SMZ effects, but progress is apparent using process-based modelling and good quality data that enables the main effects to be accounted for of soil, climate, land use, and water use by vegetation in SMZs or elsewhere (Smethurst et al, 2015; Almeida et al., 2016). Further development of this modelling capability is encouraged along with an expansion of the database to cover a wider range of contexts in relation to SMZ plantations. After enduring initial browsing pressure, trees grew rapidly and
Fig. 2. Annual rainfall during the study period in relation to date of tree planting and pre- and post-establishment periods for hydrological comparison.
Annual rainfalls during the first two years of measurement (551 mm 2008; 1004 mm 2009) were the lowest and highest for the study period, with the remaining years being intermediate and relatively uniform (617–853 mm) (Fig. 2). As tree seedlings grew minimally during their first year (planted September 2008) and rainfall is lowest during summer at this site, the defined date separating pre- from and postestablishment hydrological comparisons was 1st January 2010. 3.2. Stream flow Pre-establishment, mean stream flow was 13 ML d−1 in the control catchment and 29.9 ML d−1 in the SMZ-treated catchment. The ratio of SMZ flows to control flows was therefore 2.3, rather than 2.8 as expected from the ratio of areas. However, conversion to mm d−1, which accounts for area, and observations of daily variability indicated that the flow ratio was within the 95% confidence limit of prediction of expectations (Fig. 3). Pre- and post-establishment, and in both catchments, daily stream flow reflected daily or recent rainfall, as expected. 629
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Fig. 3. Daily rainfall, stream flows and flow ratios in comparison to the expected flow ratio. A decline in flow ratio (bold solid line) indicates that flows reduced in the SMZ treated plantation compared to the control during the study period, but the expected flow ratio (broken line) remained within the 95% confidence band of prediction (non-bold solid lines). This decline had a significant slope for all flows, but not for low flows (control < 0.05 mm d−1).
was taken to produce each mound. Tree stocking (density of pits; 1419 ha−1) was probably high enough that all overland flow paths were intercepted by at least one pit (Photo 1; also see Fig. 5.2 in Smethurst et al., 2012). Each pit retained c. 8 L water. This change in hydrology decreases overland flow, increases soil water content and provides an opportunity for tree roots to access water that would otherwise have produced stream flow. By this mechanism, instead of increasing evapotranspiration, one could expect SMZ plantations with increased surface roughness to increase base flows or low flows, but there was no evidence for this effect (Fig. 5, Fig. 6). Although SMZ plantations can change stream flows in some contexts, the importance of a change in flow regime (increase or decrease) depends on the values placed on water retained in comparison to that flowing further down the stream network. These values depend on the socio-economic context, including the values placed by people on environment aspects. For example, individuals and communities within catchments and below catchments, if facing a shortage of good quality water, will place a high value on receiving it. Industrial or agricultural industries, likewise, place a high value on access to suitable quality water. In these cases, seasonality of flows might not be important if there is adequate storage from which supplies can be drawn during dry periods. However, seasonality and particular patterns of high or low flows might be very important within or down-stream of a catchment for the provision of suitable habitat of native plants and animals, e.g. fish. Where there is a lower net value placed on water below the
approached that of a fast-growing commercial Eucalyptus nitens plantation at 4.7 years (9 m height, compared to 11 m in Cromer et al., 2002). Such high growth rate plantations have a high leaf area index (LAI; Smethurst et al., 2003) and likely high rates of water use (Benyon et al., 2006) compared to other phases of the plantation with lower LAI values. Peak LAI in similar Australian eucalypt plantations occurred at 3–6 years of age (Whitehead and Beadle, 2004). As the SMZ plantation here was managed for high value sawn timber and veneer, pruning and thinning commenced soon after the data reported here were collected, which would have reduced LAI and could be expected to have also reduced tree water use. Hence, the measurement period reported probably includes the phase of highest-water-use by this SMZ plantation. By comparison, basal area of this eucalypt plantation was much higher (22 m2 ha−1 at age 6 years) than in a similar study using deciduous species in Pennsylvania, USA (3 m2 ha−1; Newbold et al., 2010). Increased surface roughness caused by pits and mounds is well known to mediate potential runoff (Valtera and Schaetzl, 2017), which is possibly the reason that flows significantly declined during the study period for total flows but not for low flows (control catchment flows < 0.05 mm d−1; Fig. 3). This result suggests that medium and higher flows were more affected than low flows by the SMZ plantation. A likely reason for this result is higher surface roughness in the SMZ plantation that increased infiltration, as spot cultivation produced mounds on which tree seedlings were planted and pits from which soil 630
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Fig. 4. Probability exceedance (flow duration) curves of the control and SMZ treated catchments before and after plantation establishment.
Fig. 6. Median of minimum daily flows (over 1, 3, 7, 30, and 90 d periods), and base flows, for the control and SMZ treated catchments. Asterisks indicate significance (P = 0.05; non-parametric).
(evapotranspiration) within a catchment and thereby maximise flows below the catchment. Water quality is also an important consideration that is often related to water quantity, and all stakeholders apply preferred water quality standards to available water. Hence, management for water yield should not be considered independent of water quality. Several factors need to be considered when extrapolating the current results to other catchments, apart from the different values placed on water as discussed above. Stream flows could potentially be reduced by using wider SMZs, higher growth rate plantations, and lower sloping landscapes. Similarly, water quality can be reduced if care is not taken with stock management, fencing, cultivation, fertilisers, herbicides, and harvesting. Codes of practice and the use of best management practices are useful methods for promoting careful practices in SMZs that protect water quality (Neary et al., 2010, 2011). If fenced to exclude livestock (e.g. cattle and sheep), SMZ buffers are a management action that can substantially improve water quality in grazed agricultural landscapes (Mayer et al., 2007; McDowell et al., 2017; Smethurst et al., 2012; Zhang et al., 2010). However, these buffers require management to control weeds, vermin and fire risks for which fencing and other management costs can be substantial. Commercial options for these zones or subsidies encourage SMZ adoption (Curtis and Robertson, 2003; Buckley et al., 2012). Using SMZs for wood production is an option for providing funds to offset those costs or even provide a profit (Robins, 2004; Stewart and Reid, 2006). Commercial non-wood options include occasional controlled grazing, seed, floriculture, honey, bush foods, essential oils, nuts and pharmaceuticals (Robins, 2002). Widespread adoption of SMZ plantations could substantially change wood supply patterns in regional Australia and
Fig. 5. Monthly low flows in the control and SMZ treated catchments. Asterisks indicate significance (P = 0.05; non-parametric).
catchment, it could be deemed appropriate to encourage activities that retain water in the catchment and use it for uses of higher value. In contrast, it may be a net benefit to minimise water use 631
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Acknowledgements I thank Craig Baillie, Keith Churchill, and Dale Worledge for managing instrumentation and other data collection, and Chris White for providing in-kind support and access to his farm. The project received financial and in-kind support from Private Forest Tasmania, CSIRO, the Landscape Logic CERF, and the CRC for Forestry. These organisations were consulted during the design phase of the research, but did not influence the interpretation of results. Comments on earlier drafts by Auro Almeida, Tanya Doody and two anonymous reviewers are much appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecolind.2018.11.048. These data include Google maps of the most important areas described in this article. References Almeida, A.C., Smethurst, A.S., Cavalcante, R.B.L., Borges, N., 2016. Quantifying the effects of Eucalyptus plantations and management on water resources at plot and catchment scales. Hydrol. Process. 30, 4687–4703. https://doi.org/10.1002/hyp. 10992. Benyon, R.G., Theiveyanathan, S., Doody, T.M., 2006. Impacts of tree plantations on groundwater in south-eastern Australia. Aust. J. Bot. 54, 181–192. https://doi.org/ 10.1071/BT05046. Buckley, C., Hynes, S., Mechan, S., 2012. Supply of an ecosystem service—Farmers’ willingness to adopt riparian buffer zones in agricultural catchments. Environ. Sci. Policy 24, 101–109. Cromer, R.N., Turnbull, C.R.A., LaSala, A.V., Smethurst, P.J., Mitchell, A.D., 2002. Eucalyptus growth in relation to combined nitrogen and phosphorus fertiliser and soil chemistry in Tasmania. Aust. For. 65, 256–264. https://doi.org/10.1080/00049158. 2002.10674877. Curtis, A., Robertson, A., 2003. Understanding landholder management of river frontages: the Goulburn Broken. Ecol. Manage. Restor. 4, 45–54. https://doi.org/10.1046/j. 1442-8903.2003.t01-1-00137.x. Doody, T.M., Benyon, R.G., Theiveyanathan, S., Koul, V., Stewart, L., 2014. Development of pan coefficients for estimating evapotranspiration from riparian woody vegetation. Hydrol. Process. 28, 2129–2149. https://doi.org/10.1002/hyp.9753. Ellison, D., Morris, C.E., Locatelli, B., Sheil, D., Cohen, J., Murdiyarso, D., Gutierrez, V., Van Noordwijk, M., Creed, I.F., Pokorny, J., Gaveau, D., 2017. Trees, forests and water: cool insights for a hot world. Glob. Environ. Chang. 43, 51–61. https://doi. org/10.1016/j.gloenvcha.2017.01.002. Isbell, R. 2016. The Australian soil classification. CSIRO publishing. ISBN: 9781486304646. http://www.publish.csiro.au/book/7428 (accessed 6.03.2018). Kuczera, G., 1987. Prediction of water yield reductions following a bushfire in ash-mixed species eucalypt forest. J. Hydrol. 94, 215–236. https://doi.org/10.1016/00221694(87)90054-0. Jones, J., Almeida, A., Cisneros, F., Iroumé, A., Jobbágy, E., Lara, A., Lima, W.D.P., Little, C., Llerena, C., Silveira, L., Villegas, J.C., 2017. Forests and water in South America. Hydrol. Process. 31, 972–980. https://doi.org/10.1002/hyp.11035. Lintern, A., Webb, J.A., Ryu, D., Liu, S., Bende-Michl, U., Waters, D., Leahy, P., Wilson, P., Western, A.W., 2018. Key factors influencing differences in stream water quality across space. Wiley Interdiscip. Rev. Water. 5. https://doi.org/10.1002/wat2.1260. Lynch, D.T., Leasure, D.R., Magoulick, D.D., 2018. The influence of drought on flowecology relationships in Ozark Highland streams. Freshwater Biol. Magierowski, R.H., Davies, P.E., Read, S.M., Horrigan, N., 2012. Impacts of land use on the structure of river macroinvertebrate communities across Tasmania, Australia: spatial scales and thresholds. J. Mar. Freshw. Res. 63, 762–776. https://doi.org/10. 1071/MF11267. Mayer, P.M., Reynolds, S.K., McCutchen, M.D., Canfield, T.J., 2007. Meta-analysis of nitrogen removal in riparian buffers. J. Environ. Qual. 36, 1172–1180. https://doi. org/10.2134/jeq2006.0462. McDowell, R.W., Monaghan, R.M., Dougherty, W., Gourley, C.J.P., Vibart, R., Shepherd, M., 2017. Balancing water-quality threats from nutrients and production in Australian and New Zealand dairy farms under low profit margins. Anim. Prod. Sci. 57, 1419–1430. https://doi.org/10.1071/AN16646. Miller, A.M., McArthur, C., Smethurst, P.J., 2006. Characteristics of tree seedlings and neighbouring vegetation have an additive influence on browsing by generalist herbivores. For. Ecol. Manage. 228, 197–205. https://doi.org/10.1016/j.foreco.2006. 03.003. Neary, D.G., Smethurst, P.J., Baillie, B., Petrone, K.C. 2011. Water quality, biodiversity, and codes of practice in relation to harvesting forest plantations in Streamside Management Zones. Canberra, Australia: CSIRO Special Report, 99. https://www.fs. fed.us/rm/pubs_other/rmrs_2011_neary_d004.pdf (accessed 6.03.2018). Neary, D.G., Smethurst, P.J., Baillie, B.R., Petrone, K.C., Cotching, W.E., Baillie, C.C., 2010. 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Photo 1. Top: This photo shows a view across the buffer with spot cultivation by a scoop-and-mound method that increased surface roughness and retained about 50% grass coverage. Note that the saturated riparian zone (indicated by tussocks) was planted to acacia seedlings, but not cultivated and on which no fertilizers or herbicides were applied. Eucalypt seedlings were planted on the mounds and white tree-guards were used to protect seedlings from wildlife browsing. The buffer was fenced to exclude stock. Middle: One pit full of water soon after an overland flow event. Bottom: View of both catchments November 2010: control catchment (right), SMZ treated catchment (middle). Also shown is a line of trees crossing the bottom of the photo that was below both catchments.
improve water quality. Hence SMZ plantations should be considered during policy and practice discussions of improved landscape management options at state and national levels.
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CRC Forestry Technical Report No. 178. http://www.crcforestry.com.au/ publications/downloads/TR178-Smethurst-FORPRINT.pdf (accessed 6.03.2018). Smethurst, P.J., Almeida, A.C., Loos, R.A., 2015. Stream flow unaffected by Eucalyptus plantation harvesting implicates water use by the native forest streamside reserve. J. Hydrol. Reg. Stud. 3, 187–198. https://doi.org/10.1016/j.ejrh.2014.11.002. Smethurst, P., Baillie, C., Cherry, M., Holz, G., 2003. Fertilizer effects on LAI and growth of four Eucalyptus nitens plantations. For. Ecol. Manage. 176, 531–542. https://doi. org/10.1016/S0378-1127(02)00226-8. Smethurst, P.J., Petrone, K.C., Baillie, C.C., Worledge, D., Langergraber, G. 2011. Streamside management zones for buffering streams on farms: observations and nitrate modelling. Landscape Logic Commonwealth Environmental Research Facility Technical Report No. 28, 31p. https://www.pc-progress.com/Documents/wetland/ Smethurst_et_al_2011.pdf (accessed 6/03/2018). Smethurst, P.J., Petrone, K.C., Langergraber, G., Baillie, C.C., Worledge, D., Nash, D., 2014. Nitrate dynamics in a rural headwater catchment: measurements and modelling. Hydrol. Process. 28, 1820–1834. https://doi.org/10.1002/hyp.9709. Soil Survey Staff 2014. Keys to Soil Taxonomy, 12th ed. USDA-Natural Resources Conservation Service, Washington, DC. https://www.nrcs.usda.gov/wps/portal/ nrcs/detail/soils/survey/class/taxonomy/?cid=nrcs142p2_053580 (accessed 6.03. 2018). Stewart, A., Reid, R. 2006. Ten percent multipurpose tree cover for every farm: A low risk, high opportunity first step. In Victorian Farmers Federation conference July 11th. https://www.researchgate.net/profile/Rowan_Reid/publication/254676571_Ten_ percent_multipurpose_tree_cover_for_every_farm_A_low_risk_high_opportunity_first_ step/links/5408c5ee0cf2718acd3bde90.pdf (accessed 6.03.2018). The Nature Conservancy 2009. Indicators of Hydrologic Alteration. Version 7.1 User's Manual. http://www.nature.org (accessed 6.03.2018). Valtera, M., Schaetzl, R.J., 2017. Pit-mound microrelief in forest soils: review of implications for water retention and hydrologic modelling. For. Ecol. Manage. 393, 40–51. https://doi.org/10.1016/j.foreco.2017.02.048. Whitehead, D., Beadle, C.L., 2004. Physiological regulation of productivity and water use in Eucalyptus: a review. For. Ecol. Manage. 193, 113–140. https://doi.org/10.1016/j. foreco.2004.01.026. Zhang, L., Dawes, W.R., Walker, G.R., 2001. Response of mean annual evapotranspiration to vegetation changes at catchment scale. Water Resour. Res. 37, 701–708. https:// doi.org/10.1029/2000WR900325. Zhang, X., Liu, X., Zhang, M., Dahlgren, R.A., Eitzel, M., 2010. A review of vegetated buffers and a meta-analysis of their mitigation efficacy in reducing nonpoint source pollution. J. Environ. Qual. 39, 76–84. https://doi.org/10.2134/jeq2008.0496.
Newbold, J.D., Herbert, S., Sweeney, B.W., Kiry, P., Alberts, S.J., 2010. Water quality functions of a 15-year-old riparian forest buffer system. J. Am. Water Resour. Assoc. 46, 299–310. https://doi.org/10.1111/j.1752-1688.2010.00421.x. O’Grady, A.P., Eamus, D., Cook, P.G., Lamontagne, S., 2006. Groundwater use by riparian vegetation in the wet–dry tropics of northern Australia. Aust. J. Bot. 54, 145–154. https://doi.org/10.1071/BT04164. Poff, N.L., Richter, B.D., Arthington, A.H., Bunn, S.E., Naiman, R.J., Kendy, E., Acreman, M., Apse, C., Bledsoe, B.P., Freeman, M.C., Henriksen, J., 2010. The ecological limits of hydrologic alteration (ELOHA): a new framework for developing regional environmental flow standards. Freshwater Biol. 55, 147–170. https://doi:10.1111/j. 1365-2427.2009.02204.x. Richter, B.D., Baumgartner, J.V., Powell, J., Braun, D.P., 1996. A method for assessing hydrologic alteration within ecosystems. Conserv. Biol. 10, 1163–1174. https://doi. org/10.1046/j.1523-1739.1996.10041163.x. Robins, L., 2002. Managing riparian land for multiple uses. Rural Industries Research and Development Corporation Publication No. 02/103, Canberra. ISBN 0 642 58502 4, ISSN 1440-6845, https://rirdc.infoservices.com.au/downloads/02-103 (accessed 6. 03/2018). Robins, L., 2004. A case for rethinking policy and research directions for the rehabilitation of riparian lands in Australia. Australas. J. Environ. Manage. 11, 190–200. https://doi.org/10.1080/14486563.2004.10648613. Salemi, L.F., Groppo, J.D., Trevisan, R., de Moraes, J.M., de Paula Lima, W., Martinelli, L.A., 2012. Riparian vegetation and water yield: a synthesis. J. Hydrol. 454, 195–202. https://doi.org/10.1016/j.jhydrol.2012.05.061. Sengupta, A., Adams, S.K., Bledsoe, B.P., Stein, E.D., McCune, K.S., Mazor, R.D., 2018. Tools for managing hydrologic alteration on a regional scale: Estimating changes in flow characteristics at ungauged sites. Freshwater Biol. 63, 769–785. https://doi.org/ 10.1111/fwb.13074. Scott-Shaw, B.C., Everson, C.S., Clulow, A.D., 2017. Water-use dynamics of an alien-invaded riparian forest within the Mediterranean climate zone of the Western Cape. South Africa. Hydrol. Earth Syst. Sci. 21, 4551. https://doi.org/10.5194/hess-214551-2017. Smethurst, P., Petrone, K., Neary, D., 2012. Understanding the effectiveness of vegetated streamside management zones for protecting water quality. In: Lefroy, T., Curtis, A., Jakeman, T., McKee, J. (Eds.), Landscape Logic: Pattern, People and Process in Landscape Management. CSIRO, Collingwood, Australia, pp. 51–67 http:// www.publish.csiro.au/pid/6769.htm (accessed 6.03.2018). Smethurst, P. 2008. Summary of Australian codes of forest practice as they pertain to managing commercial plantations in stream-side buffers on cleared agricultural land.
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Ecological Indicators journal homepage: www.elsevier.com/locate/ecolind
Hydrological indicators of flow in headwaters for assessing farm management impacts: Streamside forestry management case study
T
Philip J. Smethurst CSIRO, Private Bag 12, Hobart, TAS 7001, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Plantation Water Environment Reforestation Riparian Buffer
Hydrological indicators have been used for interpreting stream flow patterns in large catchments. It would be useful to evaluate such indicators for use also in headwater catchments close to the source of on-farm management impacts. Reforestation of cleared farmland is encouraged internationally for water quality and biodiversity improvements. One practice option is to establish fast-growing plantations in streamside management zones (SMZs), but impacts on stream flows are not well understood. A eucalypt plantation was established in a steep headwater catchment in temperate Australia and stream flows compared 2 years pre- and 4 years postestablishment with an adjacent unplanted catchment. The flow ratio of SMZ to control was never significantly different from one (no SMZ effect) despite a significant decreasing trend during the six-years of monitoring. The decrease in flow ratio was attributable mainly to a reduction in some higher flows resulting from increased surface roughness and interception of overland flow. Nonparametric analyses also indicated a reduction of some low flows due to plantation establishment. Overall, changes in flow patterns appeared minor, but the value placed on such changes will depend on social and environmental contexts. Because of the rarity of such data and the importance of understanding water flows as affected by plantations established on farmland, similar studies should be conducted elsewhere. The range of flow indicators tested in this case were useful and should be considered for other similar applications. In addition, a simulation capability needs to be developed to enable the effects of SMZ plantations on stream flows to be predicted for a range of climate, landscape and management contexts and to contribute further to policy and practice discussions.
1. Introduction Hydrological indicators have been used at regional scales (Poff et al., 2010) to interpret stream flow patterns and infer impacts on environmental values. For example, Lynch et al. (2018) used the R package EflowStats to assess changes in catchments of 16–542 km2, and Sengupta et al. (2018) used an ensemble of hydrological models for catchments of 14–289 km2. However, in agricultural landscapes, management occurs at the farm and headwater catchment scales of less than 1 km2. Use of flow indicators at this finer scale is unusual, but is needed to understand the impacts of farm management. Improved stream water quality is commonly associated with a higher proportion of forested land in a catchment and a reduced proportion of intensive land uses like cropping or dairy (Lintern et al., 2018). Streams in Tasmania, Australia, are no exception (Magierowski et al., 2012). Many communities prefer to source their water from forested landscapes (e.g. Kuczera, 1987), and there is evidence that forests can increase water availability under some conditions compared to cleared landscapes (Ellison et al., 2017). In forests managed for wood
supply, the level of disturbance associated with harvesting and re-establishment presents some risks to water quality, but many countries mandate or encourage responsible practices through codes of practices, certification schemes, and best management practices. Similarly, reforestation of cleared farmland is encouraged by some authorities for water quality improvements, particularly in streamside management zones (Neary et al., 2011). However, some codes of practice in Australia discourage streamside management zone (SMZ) plantations (Smethurst, 2008). One concern is that conversion of farmland to a forest plantation (afforestation) has the potential to change ground water levels and stream flows in some situations (Almeida et al., 2016; Jones et al., 2017), although the effect is highly context-dependent and sometimes absent (Smethurst et al., 2015). Salemi et al. (2012) found that flows from catchments treated with SMZ plantations were 456 ± 125 mm yr−1 lower than areas without plantations pro-rated to the proportion of area treated. However, because the SMZ plantations occupied small proportions of each catchment, the actual effect measured for each catchment ranged from −104 to +170 mm yr.1. Over this wide range of measured effects, there were
E-mail address: [email protected]. https://doi.org/10.1016/j.ecolind.2018.11.048 Received 1 June 2018; Received in revised form 14 November 2018; Accepted 16 November 2018 1470-160X/ Crown Copyright © 2018 Published by Elsevier Ltd. All rights reserved.
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no strong indicators of relative flows, and additional research was encouraged to improve the understanding and predictability of stream flow outcomes of this practice. Amongst the research reviewed there were no sites that represented fast-growing plantations in a low-rainfall, cool-temperate environment like that found in some parts of Tasmania, or with annual rainfall similar to that in other parts of temperate Australia where plantations are grown (i.e. 600–1000 mm annual rainfall) and where stream flows are generally considered susceptible to increased water use by plantations compared to pasture (Zhang et al., 2001). The aim of the research reported here was to compare stream flows in adjacent headwater catchments in Tasmania that received c. 700 mm annual rainfall before and after establishing a fast-growing eucalypt plantation in the SMZ of one of these catchments. There is a focus here only on identifying changes to stream flows as a result of SMZ plantation establishment. Earlier reports identified positive effects of the SMZ plantation on several aspects of water quality, i.e. decreased concentrations of sediment, phosphate, and bacteria (Neary et al., 2010; Smethurst et al., 2011; Smethurst et al., 2012; Smethurst et al., 2014).
plantation on water quality. The pasture consisted of grasses and nonleguminous dicotyledons. Within 2 months of planting, all eucalypt seedlings received 200 g of diammonium phosphate (17.5% N 20.0% P; equivalent of 5.1 kg N and 5.8 kg P ha−1 for the total catchment), which was split between two spade slits in the soil about 15 cm on opposite sides of the planting position. Weeds (pasture species) were controlled using granulated Macspred Eucmix® in a 1 m radius around each eucalypt seedling until tree crowns suppressed weed growth after age 2 years. A few seedlings died soon after planting or were killed by browsers, all of which were replaced by new seedlings retained from the original nursery stock. Overall stocking of the plantation was 1419 trees per ha. A photo of the catchments and a close-up of the SMZ is provided (Photo 1). 2.2. Measurements An automatic weather station was installed on the boundary of the two catchments in 2007 that recorded rainfall. All tree heights and diameters (at 1.3 m over bark, DBH) in ten plots distributed throughout the SMZ were measured at 4–12 month intervals from age 1.5 years. A 60°, aluminium plate, V-notch weir was installed at the outlet of each catchment in early 2008 and included water level readings with a capacitance probe every 5 min. Water level was converted to flow using a standard rating curve equation. Anecdotally, the collaborating farmer did not express any subjective observation of or concern about SMZ plantation effects on stream flows, but stream flow in the study catchments was not critical as stock watering was provided at off-stream troughs that were supplied by water from outside these catchments. During plantation establishment, there were anecdotal concerns in the community about the possibility of broad-scale plantation establishment, but these concerns were allayed by the realisation that only SMZs were to contain plantations. However, farmer and community values in relation to SMZ plantations and potentially positive or negative effects on water yield and quality were not assessed objectively.
2. Materials and methods 2.1. Site The study area was two adjacent north-facing headwater catchments in the Forsters Rivulet catchment that flows into the Huon River in southern Tasmania. One 9.8 ha catchment supported grazed pasture (68%) in the lower part, and native forest (32%) in the upper part. The other catchment was 3.5 ha in area and entirely supported grazed pasture. Both catchments were managed as one grazing unit (paddock) prior to and during the study. Farmer records indicate average annual rainfall (1991–2006) was 722 mm (range 501–975 mm). Rain falls every month, with slight winter-spring dominance (June-November). The catchment is in steep terrain (average 30.6% slope), with proportionally more very high slopes (46.6–78.1%) in the larger catchment. Soils are c. 3 m deep, derived from interlaid and mixed slope deposits of Cretaceous syenite and Permian mudstone. Syenite outcrops at the top of the larger catchment, but not in the smaller catchment. Surface soil (0–10 cm depth; < 2 mm) properties were loam or clayloam textures with 0.7–1.0 g cm−3 bulk density, 0.081 dS m−1 electrical conductivity (1:5 soil:water), 5.5 pHwater, 0.46 mg kg−1 total N, 16 C:N, 40.8 mg kg−1 available-P (Colwell), 17.5 meq 100 g−1 effective cation exchange capacity, and 94% base saturation. The soil fraction < 2 mm was 51–99% in surface soil samples, but only 1% in some samples at the bottom of the profile (3 m). Soils are classified as a grey dermosol (Isbell, 2016) and a palexerult (Soil Survey Staff, 2014). For the year preceding this study (2007), the catchment was irregularly grazed within a general regime of moderate stocking density (c. 2–3 head per ha) for 2–6 week periods at 2–3 month intervals. Cattle had free access to all unfenced streams. A mixed-species plantation was established in 2008 in the 5–30 m variable-width streamside management zone (SMZ) of the larger catchment. The area of the SMZ (0.6 ha) was 6% of the catchment and 10% of the pasture area in the catchment. Within the SMZ, soil outside the intermittently saturated riparian zone was cultivated by a ‘scoop-and-mound’ method using a mini-excavator, which created a pit adjacent to a mound on which a tree seedling was planted (Photo 1). Tree seedlings were planted in August 2008. In the lower, northern half of the SMZ, Eucalyptus nitens (shining gum) was planted on each mound, and Acacia melanoxylon (blackwood) was planted in the saturated riparian zone, which was not cultivated. In the top, southern half of the SMZ, Eucalyptus globulus (blue gum) was planted on each mound top and there was no saturated riparian zone. Fertilizers were not used in the catchment for several years prior to or during the study, except during plantation establishment in the SMZ. This type of low-input catchment was targeted for the research to maximise the chance of detecting any deleterious effect of an SMZ
2.3. Data analysis Total flows and low-flows (control catchment < 0.05 mm d−1) were analysed as ratios between the SMZ and control catchments (log10transformed), and compared to the theoretical ratio of 1.0. Linear trends in these ratios were compared to the theoretical ratio by using 95% confidence limit of predictions provided by SigmaPlot©. The IHA© software (version 7.1, The Nature Conservancy, 2009; Richter et al., 1996) was used to calculate flow exceedance (duration) curves, low flows (< 50% of median flow) and minimums for various periods (1, 3, 7, 30 and 90 d). Non-parametric methods were used to calculate the significance of differences in environmental flows. 3. Results 3.1. Plantation growth and rainfall Planted seedlings of all species experienced heavy browsing by native animals for the first year after planting, which is common in the region (Miller et al., 2006) and delayed growth, but by the second year browsing pressure had decreased and browsing control measures (repellents, tree guards, and shooting) provided a better opportunity for the seedlings to grow. Survival by age 6 years (including replacements) was 98% for both eucalypt species and 68% for acacia. Both eucalypt species grew approximately linearly in height and diameter between 1 and 6 years of age, reaching 11 m height and 14 cm diameter at 1.3 m height (Fig. 1). Acacia height and diameter grew at about a third of the rate of the eucalypts. At this stage, the plantation was ready for pruning and thinning (which commenced the following year) as part of a plan to produce sawn and veneer products. 628
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Flow ratio (SMZ treated/control) was calculated daily, but equipment failure (data loggers and weir) reduced the number of days that this ratio could be calculated, and we could not calculate annual discharge ratios (runoff/rainfall) as had been originally planned. For all flows combined and low flows (control flow < 0.05 mm d−1), the expected flow ratio of 1 (no treatment effect) was within the 95% confidence limit of predictions of these trends (Fig. 3). This result suggests no significant change in flow patterns due to SMZ establishment. However, there was a significant decrease in flow ratio for all flows, but not when flows were low (control flow < 0.05 mm d−1), suggesting that the decrease trend in flow ratio for all flows could be attributed only to flows > 0.05 mm d−1. Flow duration curves (FDCs) pre- and post-establishment differed for both the control and SMZ treated catchments, presumably due to differences in rainfall patterns, but the pre- and post-establishment relativities were similar in both catchments (Fig. 4). For both catchments, pre-establishment flows were higher than post-establishment flows in the range 0.4 to 10 mm d−1, and the range of pre-establishment flows included more lower and more higher flows than post-establishment flows, reflecting the relative rainfall ranges pre- and post-establishment (Fig. 1). At higher flows, pre-establishment flows were higher than postestablishment in both catchments. As calculated by the IHA software, low flows (< 50% of median) were higher every month post-establishment in the control catchment, and significantly so for half the months (Fig. 5). In contrast, SMZ treated low flows were higher post-establishment only in August and September. Likewise, 1- and 90-day minimum flows were significantly higher post-establishment in the control catchment, but not in the SMZ treated catchment (Fig. 6). Together, these low flow analyses indicate that SMZ plantation reduced some low flows, but the effect was not consistent. As low flows make up a minor proportion of total annual flows generally, it is doubtful that there was a significant effect on total annual flow.
Fig. 1. Height and diameter growth of trees planted in the SMZ.
4. Discussion This research demonstrates the use of indicators of hydrological flow alternation at a headwater catchment scale for understanding the effects of farm management. It contributes a case study to a growing body of knowledge on the effects of SMZ plantations on stream flows (Newbold et al., 2010; Salemi et al., 2012). Proximity of native forests or introduced tree species to streams favours water use for evapotranspiration compared to vegetation that is more distant and higher on the landscape because of differences in access to groundwater (Doody et al., 2014; O’Grady et al., 2006; Scott-Shaw et al., 2017). In this case, the SMZ plantation caused a decrease in some high, medium and low flows. However, this body of data so far provides a limited basis on which to generalise the effects that range from substantial increases to substantial decreases. Salemi et al. (2012) concluded from a met-analysis that removal of trees from SMZs potentially resulted in reduced evapotranspiration from that zone and increased stream flow, but actual effects depended on the water use behaviour of the remaining vegetation. When replanted, suppressed forests resulted in increased water yield compared to decreased flows in more vigorously regenerating or planted forests, but there was a paucity of paired-catchment studies that examined these effects. Generalisation is difficult because there are many interacting factors that we currently do not have the capability to integrate quantitatively. Similar complexities prevail for catchments regardless of SMZ effects, but progress is apparent using process-based modelling and good quality data that enables the main effects to be accounted for of soil, climate, land use, and water use by vegetation in SMZs or elsewhere (Smethurst et al, 2015; Almeida et al., 2016). Further development of this modelling capability is encouraged along with an expansion of the database to cover a wider range of contexts in relation to SMZ plantations. After enduring initial browsing pressure, trees grew rapidly and
Fig. 2. Annual rainfall during the study period in relation to date of tree planting and pre- and post-establishment periods for hydrological comparison.
Annual rainfalls during the first two years of measurement (551 mm 2008; 1004 mm 2009) were the lowest and highest for the study period, with the remaining years being intermediate and relatively uniform (617–853 mm) (Fig. 2). As tree seedlings grew minimally during their first year (planted September 2008) and rainfall is lowest during summer at this site, the defined date separating pre- from and postestablishment hydrological comparisons was 1st January 2010. 3.2. Stream flow Pre-establishment, mean stream flow was 13 ML d−1 in the control catchment and 29.9 ML d−1 in the SMZ-treated catchment. The ratio of SMZ flows to control flows was therefore 2.3, rather than 2.8 as expected from the ratio of areas. However, conversion to mm d−1, which accounts for area, and observations of daily variability indicated that the flow ratio was within the 95% confidence limit of prediction of expectations (Fig. 3). Pre- and post-establishment, and in both catchments, daily stream flow reflected daily or recent rainfall, as expected. 629
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Fig. 3. Daily rainfall, stream flows and flow ratios in comparison to the expected flow ratio. A decline in flow ratio (bold solid line) indicates that flows reduced in the SMZ treated plantation compared to the control during the study period, but the expected flow ratio (broken line) remained within the 95% confidence band of prediction (non-bold solid lines). This decline had a significant slope for all flows, but not for low flows (control < 0.05 mm d−1).
was taken to produce each mound. Tree stocking (density of pits; 1419 ha−1) was probably high enough that all overland flow paths were intercepted by at least one pit (Photo 1; also see Fig. 5.2 in Smethurst et al., 2012). Each pit retained c. 8 L water. This change in hydrology decreases overland flow, increases soil water content and provides an opportunity for tree roots to access water that would otherwise have produced stream flow. By this mechanism, instead of increasing evapotranspiration, one could expect SMZ plantations with increased surface roughness to increase base flows or low flows, but there was no evidence for this effect (Fig. 5, Fig. 6). Although SMZ plantations can change stream flows in some contexts, the importance of a change in flow regime (increase or decrease) depends on the values placed on water retained in comparison to that flowing further down the stream network. These values depend on the socio-economic context, including the values placed by people on environment aspects. For example, individuals and communities within catchments and below catchments, if facing a shortage of good quality water, will place a high value on receiving it. Industrial or agricultural industries, likewise, place a high value on access to suitable quality water. In these cases, seasonality of flows might not be important if there is adequate storage from which supplies can be drawn during dry periods. However, seasonality and particular patterns of high or low flows might be very important within or down-stream of a catchment for the provision of suitable habitat of native plants and animals, e.g. fish. Where there is a lower net value placed on water below the
approached that of a fast-growing commercial Eucalyptus nitens plantation at 4.7 years (9 m height, compared to 11 m in Cromer et al., 2002). Such high growth rate plantations have a high leaf area index (LAI; Smethurst et al., 2003) and likely high rates of water use (Benyon et al., 2006) compared to other phases of the plantation with lower LAI values. Peak LAI in similar Australian eucalypt plantations occurred at 3–6 years of age (Whitehead and Beadle, 2004). As the SMZ plantation here was managed for high value sawn timber and veneer, pruning and thinning commenced soon after the data reported here were collected, which would have reduced LAI and could be expected to have also reduced tree water use. Hence, the measurement period reported probably includes the phase of highest-water-use by this SMZ plantation. By comparison, basal area of this eucalypt plantation was much higher (22 m2 ha−1 at age 6 years) than in a similar study using deciduous species in Pennsylvania, USA (3 m2 ha−1; Newbold et al., 2010). Increased surface roughness caused by pits and mounds is well known to mediate potential runoff (Valtera and Schaetzl, 2017), which is possibly the reason that flows significantly declined during the study period for total flows but not for low flows (control catchment flows < 0.05 mm d−1; Fig. 3). This result suggests that medium and higher flows were more affected than low flows by the SMZ plantation. A likely reason for this result is higher surface roughness in the SMZ plantation that increased infiltration, as spot cultivation produced mounds on which tree seedlings were planted and pits from which soil 630
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Fig. 4. Probability exceedance (flow duration) curves of the control and SMZ treated catchments before and after plantation establishment.
Fig. 6. Median of minimum daily flows (over 1, 3, 7, 30, and 90 d periods), and base flows, for the control and SMZ treated catchments. Asterisks indicate significance (P = 0.05; non-parametric).
(evapotranspiration) within a catchment and thereby maximise flows below the catchment. Water quality is also an important consideration that is often related to water quantity, and all stakeholders apply preferred water quality standards to available water. Hence, management for water yield should not be considered independent of water quality. Several factors need to be considered when extrapolating the current results to other catchments, apart from the different values placed on water as discussed above. Stream flows could potentially be reduced by using wider SMZs, higher growth rate plantations, and lower sloping landscapes. Similarly, water quality can be reduced if care is not taken with stock management, fencing, cultivation, fertilisers, herbicides, and harvesting. Codes of practice and the use of best management practices are useful methods for promoting careful practices in SMZs that protect water quality (Neary et al., 2010, 2011). If fenced to exclude livestock (e.g. cattle and sheep), SMZ buffers are a management action that can substantially improve water quality in grazed agricultural landscapes (Mayer et al., 2007; McDowell et al., 2017; Smethurst et al., 2012; Zhang et al., 2010). However, these buffers require management to control weeds, vermin and fire risks for which fencing and other management costs can be substantial. Commercial options for these zones or subsidies encourage SMZ adoption (Curtis and Robertson, 2003; Buckley et al., 2012). Using SMZs for wood production is an option for providing funds to offset those costs or even provide a profit (Robins, 2004; Stewart and Reid, 2006). Commercial non-wood options include occasional controlled grazing, seed, floriculture, honey, bush foods, essential oils, nuts and pharmaceuticals (Robins, 2002). Widespread adoption of SMZ plantations could substantially change wood supply patterns in regional Australia and
Fig. 5. Monthly low flows in the control and SMZ treated catchments. Asterisks indicate significance (P = 0.05; non-parametric).
catchment, it could be deemed appropriate to encourage activities that retain water in the catchment and use it for uses of higher value. In contrast, it may be a net benefit to minimise water use 631
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Acknowledgements I thank Craig Baillie, Keith Churchill, and Dale Worledge for managing instrumentation and other data collection, and Chris White for providing in-kind support and access to his farm. The project received financial and in-kind support from Private Forest Tasmania, CSIRO, the Landscape Logic CERF, and the CRC for Forestry. These organisations were consulted during the design phase of the research, but did not influence the interpretation of results. Comments on earlier drafts by Auro Almeida, Tanya Doody and two anonymous reviewers are much appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecolind.2018.11.048. These data include Google maps of the most important areas described in this article. References Almeida, A.C., Smethurst, A.S., Cavalcante, R.B.L., Borges, N., 2016. Quantifying the effects of Eucalyptus plantations and management on water resources at plot and catchment scales. 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Photo 1. Top: This photo shows a view across the buffer with spot cultivation by a scoop-and-mound method that increased surface roughness and retained about 50% grass coverage. Note that the saturated riparian zone (indicated by tussocks) was planted to acacia seedlings, but not cultivated and on which no fertilizers or herbicides were applied. Eucalypt seedlings were planted on the mounds and white tree-guards were used to protect seedlings from wildlife browsing. The buffer was fenced to exclude stock. Middle: One pit full of water soon after an overland flow event. Bottom: View of both catchments November 2010: control catchment (right), SMZ treated catchment (middle). Also shown is a line of trees crossing the bottom of the photo that was below both catchments.
improve water quality. Hence SMZ plantations should be considered during policy and practice discussions of improved landscape management options at state and national levels.
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