Forest restoration in abandoned cattle pastures in Costa Rica: Leaf traits determine light dependent tradeoffs between survival and growth in native and rare rainforest tree species

Background/Question/Methods: As a result of government policy and a system of environmental service payments for protecting existing forest cover and reforesting agricultural land, Costa Rica has reversed its historical trend of deforestation to achieve a net afforestation of 2% per year in 2011 according to the United Nations Food and Agriculture Organization. As more native species are being utilized in new plantations, foresters are faced with a fundamental lack of information on performance in the full sun environment of abandoned pastures and fields. Our objectives were to evaluate the suitability of 18 old-growth native tree species for reforestation projects in Costa Rica, 13 of which have not previously been examined in restoration efforts. We have evaluated responses to light environment and quantified how leaf traits, survival and relative growth rates vary between treatments and across species.  Tree seedlings were planted under 10-12 year old secondary forest cover and in abandoned cattle pastures in 2009.  In 2010-11we measured  a range of morphological and physiological traits: relative growth rate (RGR), survival, specific leaf area (SLA), maximum photosynthesis (Amax), dark respiration (DResp), Light compensation point (LCP), chlorophyll content, and leaf toughness.

Results/Conclusions: As predicted, shade grown individuals had lower RGR, SLA, Amax, DResp, and LCP and higher chlorophyll A and B and leaf toughness than pasture seedlings. However, species-specific performance was extremely varied across a shade tolerance spectrum, with rare shade tolerant species such as Brosimum lactenscens, Minquartia guianensis and Schlerolobium costaricense showing low RGR, survival and physiological performance in both light treatments, but with slightly improved performance in shade. Light tolerant species such as Ceiba pentandra, Hyeronima alchorneoides, Inga oerstediana, Terminalia amazonia and Vochysia guatemalensis showed high RGR and survival in the pasture treatment but high mortality and poor physiological performance under forest canopy.  The remaining 10 species showed intermediate levels of shade tolerance and high intraspecific variability in responses.  In addition to providing the first basic morphological and physiological measurements for many of these rare and threatened native tree species, this experiment supports a key hypothesis, that there are growth-survival tradeoffs across native rainforest species driven by light environment.  This study also shows that the number of old growth species able to tolerate high light environments is much higher than expected. Widespread dissemination of these results should result in a broader pool of native species becoming available for use in reforestation activities that promote biodiversity goals and ecosystem services.