Latitude, solar elevation angles and gap‐regenerating rain forest pioneers

Abstract

Summary 1. Tropical rain forests have more species‐rich tree assemblages than forests at higher latitudes, but is this because they comprise a wider array of niches or functional types? We address this by considering one tree functional type – light‐demanding canopy trees with fast foliage turnover and growth – that is common in the tropics and subtropics, but virtually absent from mid‐latitude rain forests. Although often referred to as ‘tall pioneers’ or ‘large pioneers’, they are by no means confined to early‐successional stages, also recruiting directly to the canopy in old‐growth stands by rapid growth beneath tree‐fall gaps. 2. We also explored the influence of latitude on tree‐fall gap light environments as a possible constraint on the geographic distribution of this functional type, using the YPLANT program to simulate light interception and potential carbon gain by seedlings of the Australian rain forest pioneer Polyscias murrayi beneath idealized gaps at tropical, subtropical and cool temperate sites (latitudes 17, 29 and 42°S, respectively). P. murrayi grows quickly to heights of 20–25 m, has high photosynthetic capacity and respiration rates, and a leaf life span of 6–9 months. 3. Simulated light interception and potential carbon gain were strongly influenced by latitude, and by the interaction of latitude with position within an idealized tree‐fall gap of 100 m2. Potential net daily carbon gain of P. murrayi was strongly positive beneath the gap centre at latitude 17, and beneath the poleward (i.e. southern) gap margin at latitude 29, but negative beneath both the gap centre and margin at latitude 42. Light interception and carbon gain were also influenced by geographic variation in sunshine hours, which were highest at latitude 29 and lowest at latitude 42. A larger gap of 300 m2 permitted positive net carbon gain at all latitudes, although rates were again predicted to be highest beneath the gap centre in the tropics. 4. Synthesis. YPLANT simulations supported the hypothesis that sun angles could prevent trees with high metabolic rates from invading old‐growth mid‐latitude rain forests, where light environments suitable for their establishment will be scarce. Geographic variation in forest light environments is therefore likely to influence the range of viable functional types at different latitudes.