Photosynthetic and water transport strategies of plants along a tropical forest aridity gradient: a test of optimality theory

Abstract

The direct effect of aridity on photosynthetic and water-transport strategies is not easy to discern in global analyses because of large-scale correlations between precipitation and temperature. We analyze tree traits collected along an aridity gradient in Ghana, West Africa, that shows very little temperature variation, in an attempt to disentangle thermal and hydraulic influences on plant traits. Predictions derived from optimality theory of the variation of key plant traits along the gradient are tested with field measurements. Most photosynthetic traits show trends consistent with optimality-theory predictions, including higher photosynthetic rates in the drier sites, and an association of higher photosynthetic rates with greater respiration rates and greater water transport. Leaf economic and hydraulic traits show less consistency with theory, however. In particular, potential specific hydraulic conductivity (Kp) increases toward drier sites and positively correlates with sapwood-to-leaf-area ratio (AS/AL), different from that shown in a global dataset and different from predictions based on xylem efficiency-sfatety trafeoff. Nonetheless, the link between photosynthesis and water transport holds: species with both higher AS/AL and Kp (implying higher mid-day transpiration) (predominantly deciduous species found in drier sites) tend to have both higher photosynthetic capacity, and lower leaf-internal CO2. These results indicate that aridity is an independent driver of the spatial pattern of photosynthetic traits, while plants show a diversity of water-transport strategies along the aridity gradient.

Why models underestimate West African tropical forest productivity

“…Biometric GPP was measured in the field using the Global Ecosystem Monitoring protocol, which sums measurements of the components of NPP and autotrophic respiration to calculate GPP (26) and is described in detail in previous study (21). Biometric GPP was originally quantified at the plot scale.…”

“…For both TRENDY and FLUXCOM, we extracted the grid cells where the three study sites are located, as an average from 2009 to 2019. S3) (21). The grey areas denote measurement uncertainty, not standard error.…”

“…PFT noted as 'not classified probably bare soil' implies that frac of the pixel does not sum up to 1, the remaining was assumed bare soil as told by LPJ-GUESS user manual. Only a screenshot of part of the The following study sites description information is also available at (21).…”

“…The uncertainty in estimates of tropical forest productivity is particularly large in West Africa 20 . The first field quantification of GPP in African forests (by measuring each component of GPP in three study sites, termed 'biometric') reported possibly the highest GPP value recorded in intact forestsgreatly surpassing values measured in Amazonia 21 . However, the high GPP of the three study sites are underestimated by about 60% in both the MODIS and FLUXCOM products, two widely used global maps of tropical forest productivity 21 .…”

“…Figure 2 Intercomparison of gross primary productivity (GPP, MgC/ha/year) from various independent sources for study sites (a) Ankasa, (b) Bobiri and (c) Kogaye.. The red dotted lines denote in situ biometric GPP, as a mean of multiple one-hectare plots (TableS3)(21). The grey areas denote measurement uncertainty, not standard error.…”