Light and water interactions in silvopastoral systems in a seasonally dry region of Costa Rica Hernán J. Andrade C.1 Robert M. Brook2 Muhammad Ibrahim3 1 Ecofriendly Production of Tropical Crops Research Group , Faculty of Agronomy, University of Tolima. Ibagué, Colombia, E-mail: hjandrade@ut.edu.co; 2 School of the Environment, Natural Resources & Geography, Bangor University, Wales, UK; 3 Livestock & Environmental Management Group, CATIE, Costa Rica.

Introduction Deforestation: Around 13 million ha year-1 , most for agriculture (FAO 2005). Permanent pastures: 63% agricultural land in Central America and 82% in Costa Rica. Increasing adoption of Brachiaria pasture species in milk and beef production in Costa Rica Silvopastoral systems (SPS): trees in pastures, low adoption due mainly to perceived competition for water in dry periods in semi-arid environments Main goal: To understanding light and water interactions in silvopastoral systems with native timber tree species in dry tropics: key for the appropriate management: radiation, water, fine root dynamics.

Description of experimental site Materials and methods Description of experimental site (Cañas, Costa Rica)   Tropical dry forest zone: 50 m altitude Rainfall: 1500 mm year-1 Reference evapotranspiration: 1800 mm year-1 Temp: 28ºC (23-36ºC) Soils: Lithic Ustorthent – Lithic Dystropept Complex Treatments: indigenous trees Pithecellobium saman, Diphysa robinioides Dalbergia retusa planted in double lines in pastures: Hyparrhenia rufa (‘juaragua’) or Brachiaria (syn. Urochloa) brizantha (palisade grass) Flexible grazing system with 2.0 AU ha-1 and free access to all experiment Source: Google Earth 2006

Evaluated variables Radiation and water use efficiency Photosynthetically active radiation (PAR) Hemispherical photographs analyzed by Gap Light Analyzer Sunscan PAR sensors (Delta T Devices) Meteorological data Dry matter production Water use Granier sap flow gauges (heat dissipation method) on trees Simplified water balance (gravimetric) for pasture and trees Radiation and water use efficiency 1 m 0.5 m

Transmission of PAR (PARt) Results Transmission of PAR (PARt) Hemispherical photography Sunscan measurements Mean values of PARt: P. saman 65.5% D. retusa 47.5% D. robinioides 39.4%

Dynamics of water use of trees at stand level Tree water use: D. robinioides: Bb = Hr D. retusa: Bb > Hr P. saman: Hr > Bb

Water use per unit leaf area (from sap flow)

Biomass accumulation rates (t DM ha-1 y-1) Trees Diphysa robinoides 5.2 Dalbergia retusa 1.7 Pithecellobium saman 0.8 Pastures Hyparrhenia rufa 5.3 Brachiaria brizantha 14.7

Pasture resource use efficiency Water use efficiency (no tree plots) g DM kg-1 H20-1 1.2 3.7 Radiation use efficiency g DM incident MJ-1 + trees -trees 0.20 0.10 0.55 0.27 Hyparrhenia rufa Brachiaria brizantha

Total carbon storage by biomass components (51 months after establishment) From: Andrade et al. (2008) Growth, production and carbon sequestration of silvopastoral systems with native timber species in the dry lowlands of Costa Rica. Plant & Soil 308:11–22

Conclusions P. saman: highest PAR transmission, which allows a high initial tree density in SPS. D. robinioides had the lowest PAR transmission & highest LAI D. robinoides fastest growing, P. saman the slowest Grasses did affect trees, B. brizantha inhibited P. saman growth, H. rufa inhibited D. retusa growth However, no effect of tree treatments on grass production B. brizantha productivity was almost three times that of H. rufa: water and radiation use efficiency of B. brizantha was three times than that of H. rufa Tree-grass interactions in tree water use Trees have different water use strategies: P. saman had the highest water use by leaf area D. robinioides: Bb = Hr D. retusa: Bb > Hr P. saman: Hr > Bb

A final word of warning Pithecellobium saman is the ‘rain tree’, and commonly grows to a large size, as seen here in Lae, Papua New Guinea

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