The principal disturbance agents creating canopy gaps and opening growing space for forest regeneration at Marajoara can be divided into four interrelated categories: 1) wind; 2) excess precipitation during the rainy season leading to waterlogged soils, floods, and tip-ups; 3) soil water deficits during the dry season leading to drought mortality and (presumably) fire; and 4) insect pests & fungal pathogens.
WIND: Almost all structural damage by wind observed at Marajoara occurs in two waves during the early and mid rainy season, between October and February. Early season rains typically fall during highly localized convectional thunderstorms. Severe mid-season windstorms are associated with the early stages of the ITCZ’s northerly retreat from its southernmost position. The second wave of wind damage tends to be heaviest because soils are water saturated by the middle of the rainy season.
As an example, the 1996–1997 rainy season was heavier than average, and marked by severe windstorms one month apart in January and February which caused widespread canopy disturbance at Marajoara. In transect surveys covering more than 15 km of forest trails, nearly half (46%) of all storm-damaged trees and crowns fell between 241–300°, that is, westwards – the two windstorms, as most weather on this landscape does, blew in out of the east. Both storms felled many emergent trees, especially jatobá (Hymenaea courbaril, Caesalpinioideae) and mangue (Trattinickia burseraefolia, Burseraceae), and the majority of these trees stood on high ground. Most events were single treefalls with collateral damage, that is, smaller trees fallen beneath single large ones. From exposed roots of windthrown trees it appeared that few individuals at any topographic position formed taproots.
A third wave of tree- and branchfalls occurs each year during heavy late wet season rains with little or no wind. Again, soils and tree crowns are water saturated by this time, reducing stability. Vine-laden crowns are especially prone to tip-up or disintegration during this period, when a day’s walk in the forest was punctuated at intervals by the crack and boom of falling branches and trees.
The importance of vines in shaping disturbance regimes cannot be overemphasized. Up to 50% of adult mortality by mahogany is directly (by pulling trees over under heavy loads) or indirectly (through crown suppression) related to vines. During the late rainy season the sound of tree- and branchfalls is common in closed forest, with many trees falling or snapping under the weight of rain-soaked vines. One enormous high-ground tree species, mangue (Trattinickia burseraefolia), is particularly prone to heavy infestation and partial crown loss beneath the weight of rain-soaked vines, opening large gaps on one or more sides of the tree.
EXCESS PRECIPITATION: As demonstrated by water table and gravimetric moisture data, deep-soil water recharge occurs rapidly at Marajoara during the early rainy season, especially at low-ground positions, leading to prolonged ‘perching’ at the soil surface during average- to above-average rainfall years. Once soils are water-saturated, stream levels become highly responsive to heavy rainfalls (30+ mm), and consecutive events lead to localized flooding in adjacent flats with silt deposition and, in places, persistent standing water. In low-ground areas with micro-topographic relief, pits fill with water that may persist days or weeks depending on rainfall patterns. Thus water-saturated soils increase windthrow probabilities by restricting wet season rooting to surface horizons, and decrease the area available for seed and seedling regeneration by filling pit & mound micro-relief with standing water.
SOIL WATER DEFICITS: Conversely, once the rains stop falling in May or June and lateral sub-surface water flow across slopes begins to wane by the mid dry season, sandy low-ground soils drain excessively to 1 m depth, creating droughty conditions for trees forced into shallow rooting patterns by anaerobic wet season soil conditions. While drought-related mortality at low-ground positions occurs annually, we observed extensive drought mortality during the El Niño year of 1997–1998. Several small-statured midstory and overstory tree species appeared especially prone to water stress, including three louros (Lauraceae), tachí (Sclerolobium sp., Caesalpinioideae), tamanqueira (Zanthoxylum negneliana, Rutaceae), junçara, and burra leiteira (Sapium marmieri, Euphorbiaceae). This suggests that alternating feast-and-famine phreatic cycles on lower slopes entail high risks for drought intolerant species.
Charcoal’s presence in each of six soil pits dug to 50 cm, most of it in bits < 2 mm length and common at all depths, indicates that these forests may burn during excessively dry years, or following successive abnormally dry years. Spotting agricultural and pastoral fires threaten to ignite groundfires at Marajoara – in all forests on this landscape, for that matter – every late dry season. Groundfires have burned through extensive areas in forests bordering the project at Marajoara since 1995, but these fires were fueled in large extent by woody debris created by logging. Whether unlogged closed forests within the study region have historically burned under normal or abnormal drought cycles remains an open question. Soil charcoal indicates that the answer is yes.
Water table data indicate that compounding low-rainfall years could place drought intolerant species at risk of mortality, whose number would increase as the inter-annual drought interval lengthens, building fuel loads for fires at long return intervals. Radiocarbon-dated soil charcoal collected from forest soils throughout Amazonia have led other authors to conclude that successive strata were laid by burns occurring one to many centuries apart since the last glacial maximum. The presence of charcoal in soils at Marajoara suggests that fire, whether occurring at intervals decades or centuries wide, does indeed play some role in forest dynamics there. Whether this role includes opening growing space for mahogany’s regeneration at large spatial scales as reported in Central America remains to be seen.
Finally, INSECT PESTS & FUNGAL PATHOGENS contribute to forest disturbance patterns at Marajoara. Insects may attack selectively, as for example the butterfly whose larvae defoliate sub-populations of Eicherodendron sp. (Flacourtiaceae) at two- to three-year intervals, or generally, as for example the thrip (Thysanoptera) that appeared by the tens of millions during the 1997 dry season in a 500-m2 patch of forest which was completely defoliated. Root pathogens may account for frequent inexplicable ‘dead zones’ covering 200–400 m2 in closed, otherwise undisturbed forests.
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