## 5 Conclusions  
Seasonality plays a critical role in the variability of R\({}_{\rm S}\) along forest succession in TDFs from northwestern Mexico. The patterns of R\({}_{\rm S}\) responded differently to precipitation inputs according to the forest succession, where the early- (ES) and mid-secondary (MS) forests showed a faster response at the early wet season, but with an ephemeral flux with rapid decreasing towards the end of the wet season compared to the old-growth forest (OG). Mean annual rates of R\({}_{\rm S}\) did not show significant differences among sites, although the OG forest sustained the highest rates along the study period.  
Our results revealed through structural equation models (SEM) that as the forest transitions to advanced successional stages, the path interactions between R\({}_{\rm S}\) and the biophysical factors become more complex. We found a variable control of GPP on R\({}_{\rm S}\) across succession, suggesting that the complexity of the rooting systems plays an important role on the soil respiration flux of the TDF.  
Finally, soils of the TDF in northwestern Mexico showed annual net emissions between 618 to 1006 g C m\({}^{-2}\), varying according to environmental conditions and forest succession but within the range observed in other tropical dry forest.  
Conceptualization, E.A.Y. and M.L.V.-T.; methodology, M.L.V.-T., D.F.-R., B.C.-V. and Z.M.S.-M.; formal analysis, M.L.V.-T.; D.F.-R., B.C.-V. and Z.M.S.-M.; investigation, M.L.V.-T., N.E.R.-R. and M.S.-A.; supervision, E.A.Y., Z.M.S.-M., B.C.-V., D.F.-R., A.R.-M. and J.G.-P.; writing--original draft preparation, M.L.V.-T., D.F.-R., B.C.-V. and E.A.Y.; writing--review and editing, Z.M.S.-M., N.E.R.-R., M.S.-A., A.R.-M., J.G.-P. All authors have read and agreed to the published version of the manuscript.  
Financial support for this project came from CONACYT Mexico project CB-2013-01:221014 and a graduate scholarship to MLVT (542178). The PROFAPI-ITISON program provided additional support  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ecosystem Type** & **Country** & **Annual Cumulative R\({}_{\rm S}\) (g C m\({}^{-2}\))** & **Mean Annual Precipitation (mm)** & **Reference** \\ \hline Subtropical monsoon & China & OG-1163 & 1956 & [PERSON] et al. [87] \\ evergreen broadleaved & & MS-592 & & \\ forest & & ES-1023 & & \\ \hline Tropical forest & Malaysia & OG-2242 & 1895 & [PERSON] et al. [84] \\  & & MS-1672 & & \\ \hline Subtropical evergreen & China & OG-1861 \(\pm\) 214 & 1664 & [PERSON] et al. [131] \\ broadleaved & & MS-1259 \(\pm\) 84 & & \\ forest & & & & \\ \hline Tropical dry forest & Mexico & 1032 & 1650 & [PERSON] and [PERSON], [132] \\ \hline Tropical dry forest & Costa Rica & MS-742 \(\pm\) 121 & 1500 & [PERSON] et al. [56] \\  & & ES-856 \(\pm\) 137 & & \\  & & AP-722 \(\pm\) 142 & & \\ \hline Tropical dry & Thailand & 996 \(\pm\) 56 & 1253 & [PERSON] et al. [130] \\ dipterocarp forest & & & & \\ \hline
**Tropical dry forest** & **Mexico** & **OG-869 \(\pm\) 142** & **724** & **This study** \\  & & **MS-719 \(\pm\) 97** & & \\  & & **ES-732 \(\pm\) 53** & & \\ \hline Semiarid loess plateau & China & 647 \(\pm\) 169 & 560 & [PERSON] et al. [133] \\ \hline Oak-grass savannah & USA & 488 & 559 & [PERSON] and [PERSON], [134] \\ \hline \hline \end{tabular} AP = abandoned pasture, ES = early-secondary forest, MS = mid-secondary forest, OG = old-growth forest. Bolded text highlights the results from the present study.  
\end{table}
Table 7: Annual cumulative R\({}_{\rm S}\) estimates along a precipitation gradient from tropical and subtropical ecosystems. Study sites are displayed according to the mean annual precipitation. Mean \(\pm\) standard deviation is expressed in multiyear studies.