## 4 Discussion
### Responses of R\({}_{\rm S}\) to Biophysical Controls  
In this study, structural equation models (SEM) were applied to integrate multiyear observations of R\({}_{\rm S}\), ecosystem flux and meteorological variables, in order to identify the main biophysical controls of the variability of R\({}_{\rm S}\) and their relative importance along a three TDF succession sites. Overall, we hypothesized that the relative importance of biophysical controls in the interannual variability of R\({}_{\rm S}\), such as global radiation (K), air temperature (T\({}_{\rm air}\)), vapor pressure deficit (VPD), evapotranspiration (ET) and gross primary production (GPP), would be different among OG, MS and ES TDF sites. At the TDF of northwestern Mexico, R\({}_{\rm S}\) is mainly driven by soil volumetric water content (SWC\({}_{\rm site}\)), and soil temperature (TS\({}_{\rm site}\)) represents a lesser role in this flux, similar to other studies in seasonally dry ecosystems [91, 92, 93, 94]. According to our results, the influence of SWCsite was higher in the OG than the MS site; even though SWCsite and ET showed the highest mean annual values in the ES site during the study period, there was no significant effect on R5 from these controls (Figure 2). The strong influence of SWCsite observed in the OG and MS sites is consistent with observations in other forests and seasonally dry ecosystems [11,95,96]. Contrary to our expectations, R5 was consistently influenced by TSsite, regardless of forest succession on the TDF sites. However, the relative importance over R5 was different along the forest successions since the influence of TSsite on the ES site was higher and decreased as the forest transitioned to more advanced successional stages at MS and OG and as observed in other types of forests [89]. The contrasting effects of GPP on R5 in our TDF succession gradient suggests that legume pioneer tree species may be exerting a strong influence on R5 in the ES site due to a high metabolic activity (high GPP) as a consequence of a fast canopy and root system development during these early stages of succession in TDF [97]. Pioneering trees species maintain an active metabolism under water-stressed and drought conditions to maximize their net photosynthetic rates and resource acquisition efficiency [98, 99, 100]. At the TDF in northwest Mexico, the pioneer species _A. cochliacantha_ forms a dense canopy and displays functional traits for high and rapid water use, such as an extensive lateral shallow-rooted system with no tap roots [52,102]. This trait combination suggests an active autotrophic activity in the ES site that is reflected in a strong positive effect of GPP on R5 in our SEM analysis (SRW = 0.70, Figure 2c). It is worth mentioning that the effect of GPP over R5 in the MS site is absent (Figure 2b), having been significant in the OG site but not as strong as in the ES forest (SRW = 0.41, Figure 2a). The complexity of the rooting systems might explain these contrasting effects of GPP over R5 along the TDF succession as vegetation structure and composition changes with forest succession [103,104]. In the mid-secondary forest (MS), the dominance of _A. cochliacantha_ decreases as tree species with deeper root systems increase [52]. In contrast, the OG site presents mature vegetation and a dense herbaceous and shrub understory with a well-developed shallow-rooted system that might contribute to the effect of GPP over R5. Other biophysical controls should be considered in order to improve our understanding in mechanisms involving the variability of R5. First, vapor pressure deficit (VPD) and gross primary production (GPP) were negatively correlated at all sites (Figure 2). Gas exchange and carbon isotope studies have discussed the link between tree photosynthesis, respiration and VPD [105, 106, 107, 108, 109].