Is there a latitudinal gradient in sapling growth strategies?

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Jiahui Li and Carly E. Pomeroy, EEB, UCLA, Los Angeles, CA, Nuria Apaza, Biological Station Cocha Cashu, San Diego Zoo Global, Peru, Cuzco, Peru, Nathan Kraft and Marcel Carita Vaz, Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA

Background/Question/Methods
Light is one of the most limiting resources for plants growing in forests globally. Yet, latitudinal variation in the angle of incidence of light may limit the availability of this resource even further, thus affecting the growth strategies of trees. Canopy trees, e.g., tend to have thicker crowns in higher latitudes to intercept light more efficiently. Combined with lower angles of light incidence, thicker crowns greatly limit light penetration into the forest understory, also limiting the number of foliage strata of the forest and, thus, the complexity of its light regime. This variation in light regime, therefore, must impact the regenerative strategies of tree species, including sapling growth strategies, which can range from shade-avoidance (optimistic) to shade-tolerance (pessimistic). Because pessimistic saplings (short-stature, large crowns) are believed to outperform optimistic (tall, small crowns) ones in shadier areas and vice-versa, we predict that there will be a latitudinal gradient in sapling architecture and allometry, with optimists dominating the tropics and pessimists becoming more common polewards. To test this prediction we used a published global database on sapling architecture (height and crown width and depth), together with fresh data from two study sites in the Central and Western Amazon.
Results/Conclusions
We found that the sapling architectures from higher latitudes were a subset of those present in tropical forests, which had several different growth strategies. As expected, saplings from the similar latitudes tended to have similar architectures (Mantel r = 0.19; p = 0.001). Moreover, sapling optimism did decrease polewards, especially due to increasing crown thickness relative to sapling height (R2 = 0.20; p < 0.001). This agrees with the notion that plants in higher latitudes have to make deeper crowns in order to intercept light at lower incidence angles. Interestingly, deep-crowned saplings are also common in the tropics, maybe because light incidence angle varies along a single day in the lower latitudes as much as it varies year-round in higher latitudes. The diversity in light incidence angles might thus explain the large diversity of sapling growth strategies in tropical forests, which in turn helps to explain the great number of plant species found there. Next, we will take into account environmental variables that may affect light regimes inside forests, such as topography or forest disturbance. We will also account for sampling effects in our results, as higher latitude forests have only a small fraction of the richness of tropical ones.