Tropical forest light regimes in a human‐modified landscape

Abstract

Light is the key energy input for all vegetated systems. Forest light regimes are complex, with the vertical pattern of light within canopies influenced by forest structure. Human disturbances in tropical forests impact forest structure and hence may influence the light environment and thus competitiveness of different trees. In this study, we measured vertical diffuse light profiles along a gradient of anthropogenic disturbance, sampling intact, logged, secondary, and fragmented sites in the biodiversity hot spot of the Atlantic forest, southeast Brazil, using photosynthetically active radiation sensors and a novel approach with estimations of vertical light profiles from hemispherical photographs. Our results show clear differences in vertical light profiles with disturbance: Fragmented forests are characterized by rapid light extinction within their low canopies, while the profiles in logged forests show high heterogeneity and high light in the mid‐canopy despite decades of recovery. The secondary forest showed similar light profiles to intact forest, but with a lower canopy height. We also show that in some cases the upper canopy layer and heavy liana infestations can severely limit light penetration. Light extinction with height above the ground and depth below the canopy top was highest in fragmented forest and negatively correlated with canopy height. The novel, inexpensive, and rapid methods described here can be applied to other sites to quantify rarely measured vertical light profiles.

Forest structure, diversity, and primary production in relation to disturbance severity

“…Disturbance severity modified the subcanopy light environment and exerted mixed effects on leaf traits. Deeper canopy light penetration is widely observed at higher disturbance severities (Fauset et al, 2017; Turton & Siegenthaler, 2004) as is increased light spatial homogeneity as canopies become more uniformly open (Chazdon & Fetcher, 1984). We incorrectly anticipated that at high disturbance severities, a more homogenous and enriched subcanopy light environment would consistently augment leaf trait uniformity and promote sun leaf physiology and morphology.…”

A Research Agenda for Microclimate Ecology in Human-Modified Tropical Forests

“…Related to this, we need to understand what physiological and behavioral strategies species adopt to cope with or evade unfavorable microclimates. For instance, mobile species might shift from diurnal to nocturnal activity to escape warmer temperatures (Davison et al, 2019;Levy et al, 2019), while sessile organisms such as plants can thermoregulate by increasing transpiration and adjusting leaf angles (Fauset et al, 2017). Alternatively, some species may be able to rapidly acclimate and/or adapt to novel microclimates, as was recently shown for a poison frog in Costa Rica that exhibits higher preferred body temperatures in forests that have been logged (Rivera-Ordonez et al, 2019).…”

“…But perhaps the most pervasive impact of logging is that by opening and thinning the canopy, it increases solar radiation and air flow in the understorey and decreases evapotranspiration-thereby altering the forest's microclimate (Breshears, 2006;Hardwick et al, 2015;Senior et al, 2017). As a result, not only do logged forests tend to be warmer, drier and brighter than old-growth ones (Hardwick et al, 2015;Fauset et al, 2017;Senior et al, 2017), but microclimatic conditions in these human-modified ecosystems are also more spatially and temporally heterogeneous (Hardwick et al, 2015;Blonder et al, 2018).…”

Extreme and Highly Heterogeneous Microclimates in Selectively Logged Tropical Forests

“…Disturbances can affect tropical forests' ability to buffer changes in their microclimate from changes in broader-scale climate (Ewers and Banks-Leite, 2013;De Frenne and Verheyen, 2016). For example, selective logging may cause reductions in canopy closure that in turn drive increased radiation and airflow, which then lead to decreased broad-scale climate buffering and higher variation in microclimate (Fauset et al, 2017). The extent of these effects remains controversial (Senior et al, 2017b).…”