Abstract
Recent studies have shown directional changes in old-growth tropical forests, but changes are complex and diverse, and their drivers unclear. Here, we report rapid net structural and compositional changes in an old-growth subtropical forest and we assess the functional nature of these changes to test hypothetical drivers including recovery from past disturbances, reduction in ungulate browsing, CO2 fertilization, and increases in rainfall and temperature. The study relies on 15 years of demographic monitoring within 8 ha of subtropical montane forest in Argentina. Between 1992 and 2007, stem density markedly increased by 50% (12 stems ha−1 y−1) and basal area by 6% (0.13 m2 ha−1 y−1). Increased stem density resulted from enhanced recruitment of understory treelets (Piper tucumanum, Eugenia uniflora, Allophylus edulis) into small size classes. Among 27 common tree species, net population growth was negatively correlated with maximum tree size and longevity, and positively correlated with leaf size and leaf nutrient content, especially so when initial population size was controlled for. Changes were inconsistent with predictions derived from past disturbances (no increase in shade-tolerant or long-lived late-succesional species), rainfall or temperature increase (no increase in evergreen or deciduous species, respectively). However, the increase in nutrient-rich soft-leaved species was consistent with exclusion of large herbivores two decades before monitoring started; and CO2 fertilization could help explain the disproportionate increase in small stems. Reductions in populations of large vertebrates have been observed in many otherwise undisturbed tropical forests, and our results suggest they can have important structural and functional repercussions in these forests.
Highlights
The long-held assumption that old-growth forests are in longterm dynamic and structural steady state is currently under question for tropical forests, with indications that they are experiencing directional changes, e.g. Laurance et al [1], Feeley et al [2], Lewis et al [3]
Temporal Changes at Stand-level Between 1992 and 2007, there was a steep increase in both density of distinct individual trees (10 individuals ha21 y21), and density of stems (12 stems ha21 y21) (57 and 55%, respectively) (Fig. 2a, b), and a comparatively minor (6%) increase in basal area (0.13 m2 ha21 y21) (Fig. 2c)
The 115–130 cm dbh size class experienced a basal area reduction of more than 50% (Fig. 3), but this was due to the death of a single individual of C. porphyrium while other size classes remained relatively stable
Summary
The long-held assumption that old-growth forests are in longterm dynamic and structural steady state is currently under question for tropical forests, with indications that they are experiencing directional changes, e.g. Laurance et al [1], Feeley et al [2], Lewis et al [3]. Analyses combining large monitoring plots from the American, African and Asian tropics indicate a widespread acceleration in tree recruitment and mortality between the 1980s and early 2000s [3]. Potential explanations for the changing dynamics and biomass increase in tropical forests include: (a) long-term recovery from unrecorded past disturbances, (b) reduced ungulate herbivory, mainly due to hunting pressure, or changes in environmental drivers including: (c) increasing air temperatures (but see Clark et al [5]), (d) rising atmospheric CO2 concentrations, (e) changes in rainfall, and (f) nutrient deposition from industrial and agricultural sources, see extensive reviews by Lewis et al [6] and [7]. The relative contribution of these potential factors is still uncertain, but dissecting the responses among tree diameter classes, habitats, and the demography and plant attributes of species with changing populations should contribute to falsify hypotheses and narrow down on potential drivers, e.g. Valencia et al [9]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
