Phenocams as a tool to investigate leaf economic spectrum relations in Seasonally Dry Tropical Forests

Leaf construction can be costly to plants with a short leaf lifespan (LLS), with a necessity to pay back the investment in leaf deployment. Costs of leaf construction are often measured as leaf mass per area (LMA) and the deciduousness strategies (deciduous, semideciduous or evergreen) used as proxy to LLS (evergreen species having longest leaf duration compared to semideciduous and deciduous species). According to the leaf economic spectrum theory, a positive correlation between LMA and LLS is expected, with evergreen species having higher LMA than deciduous species. Nonetheless, aridity constraints increase leaf maintenance costs in plants, and the deciduous strategy turns to be the most common leaf exchange behavior in Seasonally Dry Tropical Forests (SDTF). In this study we are testing if the relation of LMA and LLS is influenced by aridity in SDTF, using the length of growing season (LOS) as a proxy of overall response for drought. We expect that LMA: LLS relationship will become stronger towards driest sites. The caatinga vegetation is the largest SDTF in the New World, covering an area of ca. 850,000 km² located in North-eastern Brazil. Although the region is characterized by having low amount of rainfall (<1100 mm per year), there is a gradient of aridity that affects plants living across these areas. We applied the near-surface remote method trough the usage of phenocams to simultaneously monitor leaf phenology of 27 tree species from four areas of Caatinga, in a gradient of aridity ranging from 387 mm to 800 mm total annual rainfall. For these species, we used the green chromatic coordinate (Gcc) time series to calculate the phenological transition dates of Start (SOS) and End (EOS) and the Length (LOS) of Growing Season, during two to four growing seasons, from 2017 to 2021. LOS presented high variability among species, ranging from 143 days for Manihot pseudoglaziovii and 314 days for Aspidosperma pyrifolium. In general, LOS tend to be shorter for species towards driest sites and analyes of the relation between LMA and LLS are suggesting trade-offs important to understand the acquisitive strategies of plants from semi-arid vegetation with implications for carbon fluxes.

Supported by FAPESP (#2019/11835-2); FAPESP-NERC (FAPESP #2015/50488-5; #2017/17380-1), by CNPq and FACEPE (Caatinga-FLUX, #483223/2011-5 and Caatinga-FLUX Fase 2, #0062-1.07/15); UNESP CAPES-PrInt Program (grant #88887.310463/2018-00; schoolarship ##88887.512218/2020-00) and CNPq productitivity fellowship (#428055/2018-4).