Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Resumen:

Tropical dry forests (TDFs) are often degraded and fragmented through human impact, which is also the case in Southern Ecuador, where land-use pressure is high. In this context we studied tree species composition and forest structure in a protected and adjacent disturbed TDF at altitudes between 560-1080 m asl. Fabaceae and Malvaceae were identified as the most important tree families in both forest types. The disturbed forest displayed lower tree species richness than the protected forest, and the gap in species richness between the two forest types increased with increasing altitude. Ten species of the protected forest were not recorded in the disturbed forest, two of them endemic. The disturbed site was further characterized by a lower number of stems but with larger diameters, in comparison with the protected forest. The majority of the most abundant tree species in the disturbed forest had rather low wood densities, but also the combination of high wood density with browsing tolerance and high resprouting capacity was encountered, and seems to be advantageous for getting established in such sites. Although certain tree species were well represented in the disturbed forest, some endemic species with relatively low abundances (e.g. Simira ecuadorensis, Prockia crucis) should receive more conservation attention.

Resumen:

Lowered temperatures may reduce the root water uptake of tropical trees at high elevations through several mechanisms; however, field studies to test their relevance are lacking. We measured sap flux density (J) in small-diameter tree roots across a 2000-m elevation transect in a tropical mountain forest for quantifying the effects of temperature, VPD and soil moisture on root water flow and uptake at different elevations. Recently developed miniature heat balance-sap flow gauges were applied to roots of about 10mm in diameter in mountain forest stands at 1050, 1890 and 3060m a.s.l. in the Ecuadorian Andes and the measured flow was related to anatomical properties of the root xylem. Between 1050 and 3060 m, mean J decreased to about a third. VPD was the most influential environmental factor controlling J at 1050 and 1890 m, while Ta was the key determinant at 3060 m. Large vessels were absent in the root xylem of high-elevation trees which resulted in a 10-fold decrease of theoretical hydraulic conductivity (kh theor) between 1050 and 3060 m. We conclude that both physical limitations (reduced VPD, increased viscosity of water) and biological constraints (large decrease of kh theor) result in a significantly reduced J and root water uptake of the trees in high-elevation tropical forests.