Scaling from Whole-tree Water Use to Stand Level Transpiration- Examining Relationships between Stand Age, Tree Size, Stand Variability, and Seasons within a Complex Tropical Montane Region

Abstract:

Quantifying transpiration rates across stands and landscapes is a critical component for quantifying forest hydrology. One method for quantifying stand transpiration requires installing sap flow probes in a subset of trees, deriving a relationship with an easily measured variable (such as DBH) and measuring DBH in all trees across a stand. Some studies have highlighted how scaling relationships can vary across species, forests types, or climates while others have found remarkable similarity, suggesting a “functional convergence” of transpiration rates. These scaling relationships are not trivial and can be particularly challenging in tropical regions that consist of various land uses resulting in higly variable forest structure across the landscape. In the tropical montane region of Veracruz, Mexico we quantified annual sap flow rates in forty trees across two regenerating forests (20 and 40 years), one mature forest, and one coffee plantation and examined tree-to-stand scaling relationships across species, stands (land cover types), and seasons.

We found remarkable convergence of the sap flow-DBH relationship within each stand. R-squared values within sites were greater than 0.93 across all sites in wet and dry seasons. Relationships were more variable during the winter fog season when semi-deciduousness occurs. When sap flow-DBH relationships were culled across stands, there were consistent relationships across stands despite trees of various ages and species. Using the within-site relationships to quantify stand transpiration rates resulted in similar rates between the 40-yr regenerating forest and the mature forest. The shade-grown coffee and 20-yr regenerating forest sites had transpiration rates approximately 75% and 25% of the mature forest, respectively. Estimates of stand transpiration using the universal relationship varied by less than 5% from estimates using site-specific relationships and fell within natural ranges of variation across stands. These results suggest a strong convergence in plant functioning while also highlighting the importance of capturing sources of annual variation. We further show that by taking these factors into account, relationships can be reliably extrapolated across sites, thereby allowing for reliable scaling of transpiration.