The influence of nitrogen on rain forest dipterocarp seedlings exposed to a large increase in irradiance

Dipterocarps dominate the canopy of lowland tropical rain forest in South-e ast Asia. Seedlings of these species form diverse assemblages on the forest floor where low irradiance severely limits their growth. Further growth de pends largely upon the increased irradiance that can occur with the creatio n of canopy gaps. However, the response of dipterocarp seedlings to increas ed irradiance and their subsequent establishment in the canopy may be influ enced by the availability of other resources, such as nutrient availability . We investigated the influence of nitrogen supply on aspects of the photos ynthetic physiology and growth of seedlings of four dipterocarp species (Sh orea leprosula, Shorea johorensis, Shorea oleosa and Dryobalanops lanceolat a) growing under low irradiance, during transfer from low to high irradianc e, and during subsequent growth at high irradiance. All four species increa sed growth and photosynthetic capacity in response to N-supply at high irra diances but not at low irradiance approximating that which can be expected to occur in the forest understorey. When seedlings grown at low irradiances and varying N-supply were exposed to a large increase in irradiance, all s pecies showed some degree of initial photodamage (measured through chloroph yll fluorescence), the extent of which was similar between species but diff ered markedly depending on the pre-exposure growth irradiance and N-supply. Greater photodamage occurred in seedlings grown at lower compared with hig her N-supply and irradiance. Despite these initial difference in the extent of this photodamage, all seedlings demonstrated a similar capacity to reco ver from damage. However, the alterations in the photosynthetic physiology of leaves during this recovery differed between species and depended on N-s upply. Under high N-supply all species apart from S. oleosa increased photo synthetic capacity per unit chlorophyll following exposure to high irradian ce by increasing photosynthetic capacity per unit leaf area while, under lo w N-supply, an increase in photosynthetic capacity per unit leaf only occur red in D. lanceolata. Our results suggest that variations in N-availability may have a much greater impact on the relative competitiveness of dipteroc arp seedlings during the regenerative phase following canopy gap formation than physiological differences between seedlings. Our results demonstrate a potentially significant role for N-availability in the regeneration dynami cs and distribution of canopy-dominating dipterocarp species.