Ideas and perspectives: how coupled is the vegetation to the boundary layer?

Abstract

Abstract. Understanding the sensitivity of transpiration to stomatal conductance is critical to simulating the water cycle. This sensitivity is a function of the degree of coupling between the vegetation and the atmosphere and is commonly expressed by the decoupling factor. The degree of coupling assumed by models varies considerably and has previously been shown to be a major cause of model disagreement when simulating changes in transpiration in response to elevated CO 2 . The degree of coupling also offers us insight into how different vegetation types control transpiration fluxes, which is fundamental to our understanding of land-atmosphere interactions. To explore this issue, we combined an extensive literature summary from 41 studies with estimates of the decoupling coefficient estimated from FLUXNET data. We found some notable departures from the values previously reported in single-site studies. There was large variability in estimated decoupling coefficients (range 0.05-0.51) for evergreen needleleaf forests. This is a result that was broadly supported by our literature review but contrasts with the early literature which suggests that evergreen needleleaf forests are generally well coupled. Estimates from FLUXNET indicated that evergreen broadleaved forests were the most tightly coupled, differing from our literature review and instead suggesting that it was evergreen needleleaf forests. We also found that the assumption that grasses would be strongly decoupled (due to vegetation stature) was only true for high precipitation sites. These results were robust to assumptions about aerodynamic conductance and, to a lesser extent, energy balance closure. Thus, these data form a benchmarking metric against which to test model assumptions about coupling. Our results identify a clear need to improve the quantification of the processes involved in scaling from the leaf to the whole ecosystem. Progress could be made with targeted measurement campaigns at flux sites and greater site characteristic information across the FLUXNET network.

Dry‐Season Greening and Water Stress in Amazonia: The Role of Modeling Leaf Phenology

“…The small sensitivity of ET to leaf phenology is explained by the fact that changes in the maximum Rubisco capacity ( ) due to seasonality (i.e., e r e l ) have direct effects on carbon assimilation ( A n ) and GPP according to the Farquhar model A n is proportional to in light‐rich environments; Bonan et al, ; Collatz et al, ; Farquhar et al, ; see the supporting information) but only an indirect impact on ET through changes in the stomatal conductance ( g s ) of sunlit and shaded leaves (modeled according to Leuning, , in T&C). In particular, while e r e l affects g s , the impact of leaf phenology on transpiration is buffered by canopy‐atmosphere decoupling (De Kauwe et al, ), significant for tall broadleaf tropical forests, and concomitant LAI changes, which reduce the changes in ET as compared to Δ G P P (see the supporting information for details).…”

Applying the Concept of Ecohydrological Equilibrium to Predict Steady State Leaf Area Index

“…In addition, nutrient limitation may be more important in regions with high LAI, and thus, consideration of nutrient availability may be necessary in these areas (McMurtrie & Dewar, ). Furthermore, the model does not take into account the decoupling between the vegetation and boundary layer, which may be significant in high‐LAI systems (De Kauwe et al, ). These limitations of our simple, parsimonious approach could potentially be overcome if the theory were implemented in a TBM which treats these processes in more detail.…”

“…The goodness of fit of SDGVM L equ to observations suggested that such a simplification may be reasonable and necessary considering the limitation in computational capacity. However, some recent applications of the SDGVM model have found it to significantly overestimate LAI at specific sites (De Kauwe et al, ; Medlyn et al, ).…”

When Does Vapor Pressure Deficit Drive or Reduce Evapotranspiration?

“…First, we assume that VPD at the leaf surface is the same as VPD at measurement height; physically, this implies that leaves are perfectly coupled to the atmosphere. In reality, for some conditions and plant types, the leaves can become decoupled from the boundary layer (De Kauwe et al., ; Medlyn et al., ). Therefore, our derivation will be most applicable in times like the growing season (when we also expect uWUE to be most valid), when relatively high insolation induces instability and convective boundary layers, and we would expect the surface to be generally well coupled.…”