Over-yielding in Temperate Silvopastures: A Meta-Analysis

Over-yielding in Temperate Silvopastures

• Over-yielding occurs in an intercropping system when the intercrop's productivity surpasses the overall productivity of systems managed separately.
• Objective: systematically review and calculate the over-yielding of silvopastures compared to open pastures, forests, timber plantations, or orchards managed separately in temperate regions.
• Silvopasture practices improve land productivity by 42–55% compared to separate management of pasture or trees.
• Over-yielding often occurs despite reduced individual productivity of forage, livestock, or trees in silvopastures.

Keywords

• Silvopasture, Yield, Production, Forage, Livestock, Tree

Introduction

• Integrating trees with pastures for livestock is an intensive practice.

• Managers must protect trees from livestock damage and prune/thin trees to maintain forage production.

• Trees may compete with forages for water during droughts, and competition for light may reduce forage growth.

• Managers must adjust livestock stocking to suit silvopastures for long-term forage production.

• Silvopastures present an opportunity for greater land productivity through managing interactions and complexity.

• Interactions in silvopastures can be competitive (resource competition) or facilitative (cooling effect of trees, nitrogen contribution).

• Niche partitioning allows for greater system productivity through improved resource use efficiency.

• Land Equivalency Ratio (LER) is a mathematical tool to understand the value of intercropping practices relative to segregated production practices (Vandermeer 1981).

  • The LER is calculated by the summation of two ratios:

    • A. The productivity of forages or livestock in the silvopasture to the productivity of forages or livestock in a comparable open pasture.
    • B. The ratio of productivity of timber or tree crops in the silvopasture to the productivity of timber or tree crops in a comparable forest, woodlot, or orchard.

• Objective: Summarize the LERs of silvopastures in temperate regions by comparing productivity in silvopastures to similar pastures and forests/orchards managed separately.

Materials and Methods

• A systematic review was conducted using the PRISMA methodology.

• Silvopasture Definition: Trees and livestock combined with improved pasture plants, integrating animal husbandry, silviculture, and forage agronomy.

• Forest grazing and similar rangeland-management practices were excluded.

• The review compared the productivity of forages/livestock in open pastures & timber/non-timber forest products in forests, timber plantations, or orchards to the productivity of those components in comparable silvopastures.

• Studies had to report productivity data of forages/livestock in both silvopastures and open pastures, and productivity data of trees in silvopastures or conventional silvicultural systems.

• Studies were included if comparisons to conventional systems were provided or reasonable assumptions could be made through site descriptions or alternative manuscripts.

• Only productivity in temperate silvopastures was compared.

• A comprehensive search was completed using CAB Direct (Centre for Agriculture and Bioscience International).

• Search terms included silvopasture, agricultural/forestry production, livestock/forage production, and exclusionary terms for tropical regions and plant species (Table 1).

• Studies were screened through a stepwise process: titles, abstracts, keywords, and full-text review.

• Peer-reviewed journal articles or book chapters written in English were selected.

• Production data was extracted from tables or digitized from figures using GetData Graph Digitizer.

• Information recorded: study site location, tree species, predominant forage species, and livestock species.

• Mean productivity values of pasture or silvicultural production metrics were collected.

• Mean standard errors were also collected if available.

  • Reported standard deviations (SD) were converted to standard error (SE) by: $SE = SD / "sqrt(n)$, where n is the sample size.
  • Reported LSD values ($\alpha$ = 0.05) were converted to SE by dividing the LSD by three.

• Metrics related to tree productivity varied widely (tree height/diameter, basal area per hectare, trees per hectare, biomass yield per hectare).

• Metrics related to forage production were reported/converted to dry matter basis yield per hectare or a percent of conventional open pasture productivity.

• Metrics related to animal production included individual animal yield (weight gains, fleece weights) and stocking rates/carrying capacities per year.

• All comparisons were converted to the same metric of productivity.

• A brief description of how controls were utilized and assumptions made was included for each study.

Data analysis

• LERs were compared to determine system productivity of silvopastures compared to open pastures/forests managed separately.

• The production of each component (tree, forage, and/or livestock) within a silvopasture was compared to production in open pasture/forest using a ratio.

• These ratios were summed to calculate separate LERs for forage/animal production:

  • $LER<em>{Forage} = \frac{Tree \ yield</em>{SP}}{Tree \ yield<em>F} + \frac{Forage \ yield</em>{SP}}{Forage \ yield_P}$

  • $LER<em>{animal} = \frac{Tree \ yield</em>{SP}}{Tree \ yield<em>F} + \frac{Animal \ yield</em>{SP}}{Animal \ yield_P}$

    • Where F signifies productivity in a forest, timber plantation, or orchard comparison, P signifies productivity in an open pasture comparison, and SP signifies productivity in the silvopasture treatment.

Results

• A total of 862 records were identified through the search, plus six additional articles based on prior knowledge.
• Most records were excluded as they didn't report actual production numbers or were from tropical regions.
• 38 full-text articles were assessed for eligibility, and 22 were included in the quantitative synthesis.
• The general approach was taken to standardize the metrics reported by the selected studies. This required some unit conversion as well as some assumptions described as follows for each study.

Study selection process

• Systematic screening process for eligibility of articles included in the review based on the PRISMA Flow Diagram (Moher et al. 2009).
• Characteristics and assumptions of selected records.

Results and synthesis of studies

• The compiled studies report production characteristics from a wide variety of locations across the temperate regions of the globe, but the majority of the work in this analysis was collected within the United States (Fig. 2).

• The trees used in the studies were primarily coniferous.

• Comparisons of hardwood trees grown traditionally for timber were more common than comparisons of hardwood trees grown for non-timber forest products, such as nuts (e.g. black walnut) or pods (e.g. honeylocust)

• The forages in half of the studies consisted of perennial, cool season forage species.

• Systems utilizing annual forages were less common than pastures managed for perennial forage species.

• Sheep were the dominant livestock present in the comparisons that measured animal productivity.

• Seven studies made direct, replicated comparisons between both open pasture productivity and forest productivity to productivity of forages or livestock and trees in silvopastures (Sharrow et al. 1996; Orefice et al. 2016; Ford et al. 2017; Bird et al. 2010; Clason 1999; Lewis et al. 1983; Teklehaimanot et al. 2002).

• The tree productivity estimates for two studies (Burner and Brauer 2003; McAdam and Hoppe 1997) came from additional publications of data collected on the same sites (Burner et al. 2011; Sibbald and Daiziel 2000).

• Three studies had no open pasture control.

• Tree productivity metrics were variable between studies (Table 2), and included measures of trees per unit of area (trees ha-1), volume production per unit of area ($m^3 ha^{-1}$), basal area per unit of area ($m^2 ha^{-1}$), diameter at breast height per unit of area ($m ha^{-1}$), and predicted total biomass per unit of area ($kg \ ha^{-1}$).

• Forage productivity metrics were generally reported as total yield per unit of year ($kg \ ha^{-1}$), although this may have been defined as either mean yield per cutting or per year.

• Livestock production was reported as liveweight gains of animal per unit of area, animal, year, or day, except in a single case where livestock production was reported as fleece weight per unit of area (Anderson and Moore 1987).

• There were only two studies that reported that the yield of forages harvested from a silvopasture exceeded the yield of forages harvested from an open pasture (Pent and Fike 2018; Buergler et al. 2005).

• Several studies reported livestock productivity in some silvopastures to be greater than livestock productivity in open pastures, although these were marginal differences and in most cases were not reported as significantly different (Fannon et al. 2017 ; Pent and Fike 2018; McAdam and Hoppe 1997 ; Teklehaimanot et al. 2002).

• Three studies reported tree productivity in some silvopastures to be greater than tree productivity in control timber plantations (Shar- row et al. 1996; Clason 1999; Lewis et al. 1983).

• All of the LERs based on forage and tree production exceeded one (Table 3).

• All but one study reported LERs based on animal and tree production that were greater than one.

• The mean LER based on forage and tree production was 1.52 (SE = 0.04), and the mean LER based on animal and tree production was 1.44 (SE = 0.07).

  • Based on this review, it would take around 0.71 ha of forest, timber plantation, or orchard plus 0.80 ha of pasture to equal the production from one hectare of silvopasture (Fig. 3).
  • Similarly, one hectare of sil- vopasture would produce the same amount of live- stock and silvicultural products as 0.74 ha of pasture and 0.70 ha of forest (Fig. 3).

• The distribution of LER values was greater when livestock production was used as the metric of open pasture productivity than when forage yield was used as the metric of open pasture productivity (Fig. 4).

Discussion

Land productivity of silvopastures

• Temperate silvopasture practices warrant consideration due to their significantly greater production potential.
• The improved productivity of silvopastures is similar to or better than other intercropping practices.
• Silvopastures improve land productivity by 42–55%.
• Agroforestry practices may be more environmentally beneficial and culturally acceptable.
• Improved resource utilization results in less nutrient loss and better water quality.
• Silvopastures sequester more carbon.
• Trees buffer climatic extremes for livestock and forages.
• Greater biodiversity within silvopastures is appreciated.
• Greater productivity may come at a cost of higher labor demands, managerial considerations, and inputs.
• Trees in silvopastures may need regular pruning.
• Landowners cite uncertainty in markets and the risk of long-term investments as concerns.
• Product diversity can stabilize farm income and improve cash flow.
• Silvopastures may be more or less profitable than conventional practices.
• Profitability depends on the valuation of silvicultural and agricultural products and inputs.

Tree productivity in silvopastures

• Coniferous trees were the most common in reviewed practices due to their fast growth.
• Coniferous trees may be less sensitive to damage from grazing livestock.
• They may have more upright canopies that intercept less light than hardwood canopies.
• Hardwood trees may be selected for valuable timber and non-timber forest products like honeylocust pods.
• Hardwoods may be selected for their adaptation to a site.
• The manager’s ability to select trees is limited by the species of trees present in the stand, while the manager has more trees available for selection when planting trees into an existing pasture.
• The productivity per unit area of land of the trees in the silvopasture was about 70% the productivity of trees managed in conventional silvicultural compar- isons.
• Such reductions must often be made to permit enough light to reach the forage canopy for adequate growth.
• Comparisons of tree numbers rely on the assumption that individual tree growth is similar between prac- tices.
• The growth of individual pines in silvopastures has been shown to be greater than individual tree growth in timber-only plantations (Oswald et al 2017; Clason 1999).
• For hardwood trees, height and diameter growth have been shown to be less when grown in a pasture than when grown free of competition with grass (Houx et al. 2012).
• Regarding production of nuts from trees, there is a lack of data available for individual trees in silvopastures as compared to trees in orchards.

Forage productivity in silvopastures

• Warm-season forages are more productive in warm environments than cool-season forages due to the photosynthetic pathway utilized.
• Cool-season grasses may be just as productive under 50% shading as when exposed to full sunlight, compared to warm- season grasses which are much less productive in the shade than in full sunlight (Lin et al. 2001).
• Yields of cool-season forages may be less affected by the presence of trees in silvopastures than yields of warm- season forages.
• The selection of forages for a silvopasture will often be dictated by the edaphic and climatic conditions of the site. However, trees will alter the microclimate of the silvopasture (Karki and Goodman 2015).
• The reduction in forage growth in silvopastures compared to open pastures was less than the reduction in tree productivity in the same sites.
• The land manager may presumably shift this balance towards tree production or forage production depending on which is more desirable.
• Over time as tree canopies close, forage production will decrease in silvopastures if the trees are not thinned or pruned (Lewis 1989; Bird et al. 2010).
• The nutri- tional value and the intake of the forage by the animal are of primary importance in determining the quality of a forage (Ball et al. 2001).
• Forages grown in silvopasture conditions in general have greater protein or non- protein nitrogen levels and lower total non-structural carbohydrates (Kephart and Buxton 1993; Neel et al. 2008; Buergler et al. 2006).

Livestock productivity in silvopastures

• Sheep are well suited to silvopastures.
• Sheep will consume browse from woody shrubs or stump regrowth more readily than cattle.
• Small ruminants also disturb and compact soils to a lesser degree than cattle (Betteridge et al. 1999).
• However, cattle are more prevalent than sheep in pasture production systems in the temperate regions of the globe.
• While sheep may be suited to silvopasture practices, there are opportunities to manage cattle effectively in silvopastures, as demonstrated by some high, long-term production values reported in studies with cattle managed in silvopastures (Lewis et al. 1983).
• In general, LERs were greater when forage yield was used as the metric of pasture productivity than when livestock production was used as the metric of productivity
• Forage characteristics as well as animal health and well-being are significant drivers of animal production.
• Some studies have reported lower nutritive value of forages in silvopastures compared to open pastures, largely a function of reduced total non- structural carbohydrate levels and greater concentra- tion of fiber (Belesky 2005a, b; Neel et al. 2008; Buergler et al. 2006).
• The cooler microclimate and reduced radiation in silvopas- tures can reduce core body temperatures of livestock compared to livestock without access to shade
• Determining in which seasons and conditions silvopastures will support the greatest level of animal performance will help farmers under- stand the potential value and function of silvopasture practices on their farms.

Conclusions

• Silvopastures are 55% or 42% more productive than conventional practices, depending on whether pasture productivity is measured by forage yield or livestock output.
• Coupled with environmental and social benefits, this can improve the sustainability of agricultural and silvicultural production practices.
• Profitability depends on productivity, inputs, and market valuation.
• Future work should focus on financial implications given these systems' high levels of productivity.